Orthopaedic Implant Mechanics & Materials

Question 1

What are orthopaedic implant devices made from?

Answer 1

Non-biologic materials, usually metals (stainless steel, titanium alloy, cobalt chrome alloy)

Question 2

What are the 2 general reasons for an implant device?

Answer 2

1. provide structural support after injury

2. replace diseased bone

Question 3

What materials are used in joints for implant devices, and why?

Answer 3

Metal and plastic materials (usually HDP).

Metal to metal contact has been unsuccessful as a bearing surface.

Question 4

What is the main problem with implant devices and why?

Answer 4

Infection - bacteria is attracted to and adheres to metal and cement surfaces. The immune system works less efficiently in the presence of implants

Question 5

What are the 5 criteria for a successful orthopaedic implant?

Answer 5

1. but not have any short-term and little long-term adverse toxic effects e.g. carcinogenesis
2. relieve pain and provide sufficient mobility
3. function without failure until it is no longer required
4. have a design which predict a guaranteed outcome
5. cost-effective

Question 6

2 main structural requirements for an orthopaedic implant

Answer 6

strength & stability

Question 7

What structural factors must be considered during implant design?

Answer 7

1. strength
2. stiffness
3. lubrication
4. wear
5. fatigue

Question 8

What kinematic factor must be considered during implant design?

Answer 8

Motion - must enable daily living functions, whilst being controlled to enable stability

Question 9

What are the requirements of an orthopaedic implant that are essential for biocompatability?

Answer 9

1. Biological integration - the implant must not react with body tissues at unsafe levels, and body tissues should not corrode the implant at an excessive rate
2. Functional integration - the implant shouldn’t affect the function of other body parts

Question 10

What does the term ‘composite structure’ mean in terms of orthopaedic implants and their structural behaviour?

Answer 10

Most orthopaedic implants attach themselves to bone and form a composite structure.

A composite structure therefore is made up of more than one material.

The structural behaviour of a composite material depends on the mechanical properties of both individual materials.

Question 11

Describe the difference in shape at the end regions of bones, and explain why?

Answer 11

The end regions are wider - this accommodates for the joint

Question 12

What type of bone is found in the end regions of bones, and explain why?

Answer 12

Cancellous bone - more porous and flexible, so has desirable shock absorbing qualities

Question 13

How are the trabeculae arranged in cancellous bone?

Answer 13

Aligned along the directions of greatest stress (the directions depend on the directions of natural load placed on the bone)

Question 14

What is the composition of the bone directly below articular surfaces, and explain why?

Answer 14

More dense than the cancellous bone below it, in order to provide a dense underlying surface for the joint so it doesn’t deform under loading.

Question 15

Is the Young’s Modulus for a structural material always constant?

Answer 15

It depends - only if the material is not deformed near or over its elastic limit.

Question 16

Non-biologic structural materials are isotropic - what does this mean?

Answer 16

The mechanical properties of the material are the same no matter what direction it is loaded.

Question 17

Bone is anisotropic - what does this mean?

Answer 17

Its Young’s modulus is dependent on the direction it is loaded.

Question 18

Under what direction of loading is cortical bone stiffest and strongest - longitudinal or transverse?

Answer 18

Longitudinal (for all types of loading - tensile, compressive, shear)

Question 19

Compare the strength of bone in the diaphysis (shaft of a long bone) to the metaphysis (near the ends of bone)?

Answer 19

Bone strength at the metaphysis is around half of that of the diaphysis.

Question 20

Under what type of loading is bone strongest and weakest?

Answer 20

Strongest - compressive loading

Weakest - shear loading

Question 21

Bone is viscoelastic - what does this mean?

Answer 21

The stiffness of bone changes with the rate it is loaded. The faster it is loaded the stiffer it becomes.

Question 22

Does bone have the same value of maximum stress for different types of loading?

Answer 22

No - it has different values of ultimate stress depending on the type of loading (compressive - strongest, shear - weakest)

Question 23

In terms of implant design, what ways of loading bone are sought to be avoided?

Answer 23

Avoid shear stresses especially, as well as tensile forces.

Aim to load the bone under compressive forces.

Question 24

Compare the Young’s modulus of cancellous bone to that of cortical bone?

Answer 24

Cancellous bone has a very variable Young’s modulus - from a maximum of 50% of that of cortical bone to less than 0.5%.

Question 25

Why is it not desirable to create an implant with such a wide range of stiffness like cancellous bone?

Answer 25

1. Increased infection risk with such an increase in surface material
2. a more flexible material may not provide adequate firmness for attaching or bonding an artificial joint

Question 26

Explain the concept of ‘stress shielding’ in terms of artificial implants?

Answer 26

After insertion of implants, parts of the bone which the implant is attached to experience a reduction in loading as the weight is now shared between the bone and the implant.

Due to the bone being able to alter its mechanical properties in response to loading, bone is resorbed where it is not needed.

Question 27

What are the consequences of stress shielding in orthopaedic implants?

Answer 27

Stress protection and consequential resorption of bone results in a breakdown between implant and bone, and ultimate loosening of the implant.

Replacement of the implant then becomes an issue, as the original bone has been lost due to resorption so fixation is a problem.

Question 28

What is the most major role of all orthopaedic implants?

Answer 28

Provide structural support

Question 29

Do all types of implants have the same principals/mechanisms of loading bearing?

Answer 29

Yes, on the most part

Question 30

What are the 2 main types of load support mechanisms in orthopaedic implants?

Answer 30

1. Load transfer

2. Load sharing

Question 31

Explain the term ‘load transfer’?

Answer 31

Part of the applied load (e.g. body weight) in the bone above is transferred to the plate/stem of implant.

Question 32

Explain the term ‘load sharing’?

Answer 32

Occurs in a middle region, where there is load sharing between the bone and the plate/stem.

Question 33

Does load transfer occur along the whole of the bone-implant interface?

Answer 33

No!

Load is transferred only at screw regions (where there is a plate device), and at the end regions of a stem (in a stem implant).

Question 34

When load transfer is occurring in composite materials, what are the 2 possible outcomes that can occur at the interface?

Answer 34

1. Interface stresses - occurs when the materials are bonded together.
2. Relative movement - occurs when the materials are not bonded together, or the bond has come loose.

Question 35

Load transfer can occur under which types of loading?

Answer 35

1. Compressive loading

2. Shear loading

Question 36

Explain load transfer under compressive loading when the bottom material is more flexible than the top material?

Answer 36

The bottom material compresses and expands more laterally than the top material.

If the materials are bonded - lateral strain at the interface is the same for both materials, and a shear stress is generated at the interface because one material is trying to expand more than the other.

If the materials are not bonded - sliding occurs

Question 37

Explain load transfer under compressive loading when the top material is more flexible than the top material?

Answer 37

The top material compresses more than the bottom material.

The highest levels of stress at the interface occur under the region of the applied load.

Question 38

When shear stresses are generated at interfaces, is the shear stress constant across the whole length of the interface?

Answer 38

No - only at areas where there is load transfer (generally at the ends of the bar). The middle portion is where load sharing occurs, so no shear stresses are generated here.

Question 39

Why can interface stresses be much higher than theoretical calculations?

Answer 39

1. Non-uniformity of contact

2. Non-uniformity of mechanical properties - stiffness, stress concentrations

Question 40

What is the most common loading type of implant situation to cause osteopenia (bone loss)

Answer 40

Situations where stress shielding occurs i.e. when load transfer occurs in 2 stages (in between is a portion of load sharing).

Question 41

What factors determine how much load is transferred from bone to plate and vice versa?

Answer 41

It depends how much load is shared in the load sharing region.

This in turn depends on the stiffness of each component.

Question 42

What 2 factors determine the structural stiffness of a structural component involved in orthopaedic implantation?

Answer 42

1. its material stiffness

2. its geometric stiffness

Question 43

What is the ‘material stiffness’ of a structural component?

Answer 43

Stiffness of the material under an axial or bending load - indicated by Young’s modulus (E)

Stiffness of the material under shear loading - indicated by Shear Modulus (G)

Question 44

What is the ‘geometric stiffness’ of a structural component?

Answer 44

The force required to produce a unit deflection (k) - concerned with the cross-section of the shape.

Question 45

What is the formula for calculating stiffness (k) of a structural component?

Answer 45

k = EA / L

Question 46

What are the geometrical factors that affect the axial stiffness of a structural component?

Answer 46

1. cross-sectional shape
2. length

(based on the equation for stiffness)

Question 47

When is it appropriate to use Rigidity as a method for indication of stiffness of a structural component?

Answer 47

When length is not a consideration for indicating stiffness, for example when comparing two intramedullary nails or plates of the same length.

Question 48

What is the equation for axial rigidity?

Answer 48

R = E x A

where
E = young’s modulus
A = area

Question 49

What is the equation for bending rigidity?

Answer 49

R = E x I

where
E = young’s modulus
I = second moment of area

Question 50

What is the second moment of area?

Answer 50

A measure of how resistant a structure is to bending.

The further away a material is from the neutral axis the more rigid it is when bent.

i.e. the higher I, the more rigid the structure.

Question 51

What is the equation for torsional rigidity?

Answer 51

R = G x J

where
G = shear modulus
J = polar second moment of area

Question 52

What is the polar second moment of area?

Answer 52

A measure of how resistant a structure is to torsion.

Question 53

What is the name of the contact area between the implant fixator and bone?

What is the most important quality that this contact area must have?

Answer 53

Bone-implant interface

Must be fixed and free from movement to prevent loosening and ultimate failure

Question 54

How is fixation between implant and bone achieved?

Answer 54

By different methods, depending on whether the implant is to be removed at a later date.

Question 55

What is the main use for screws in the fixation of orthopaedic devices?

Answer 55

Mainly for fracture fixation

Question 56

Why are nuts and bolts rarely used for attaching implants to bone?

Answer 56

Causes more trauma, as needs to be access to bone from both sides (one for the bolt, the other for the nut)

Question 57

Explain the fixation technique of ‘interference fit’?

Answer 57

Doesn’t have a specific fixation device, and relies on a tight fit between the implant and bone to give enough friction to prevent movement. The inner component is usually slightly larger than the outer component.

Question 58

What is the consequence of an interference fit being made too tight?

Answer 58

The bone will split

Question 59

In what fixation situation is ‘interference fit’ used as a method?

Answer 59

Cementless joint replacements

Question 60

What is the main purpose of using bone cement in fixation?

Answer 60

To act as a filling material between bone and implant, so perfect match is not required. Does not act as an adhesive!

Question 61

When is bone cement most commonly used?

Answer 61

In stems of joint replacements

Question 62

Why is bone cement currently used instead of adhesives?

Answer 62

It is too difficult to apply adhesives - the bones are wet and it would be too difficult to prepare the bones prior to application of adhesive

Question 63

What is ‘biological fixation’?

Answer 63

Materials coated onto the surface of implants which are porous, or mesh-like and therefore encourage bone to grow into this coating and form a lock between the bone and the implant

Question 64

What are the 2 main methods of biological fixation?

Answer 64

1. using beads of the same material as the metallic implant

2. using a ceramic, eg hydroxyapatite (HAp), the main mineral constituent of bone

Question 65

Explain the method of using metallic beads in biological fixation?

Answer 65

There are small pores between spherical beads which encourage bone to grow into them.

Question 66

What is a disadvantage of using metallic beads in biological fixation, and what is done to overcome this?

Answer 66

The metal is very exposed, and is susceptible to corrosion, especially crevice corrosion.

The technique is mainly used with titanium stems, as titanium is least corrosive.

Question 67

Explain the method of using ceramics such as hydroxyapatite in biological fixation?

Answer 67

Hydroxyapatite can be sprayed directly onto the metal implant using a technique called plasma spray coating.

Question 68

What is a disadvantage of using ceramics in biological fixation?

Answer 68

Only a short term solution - after 1 or 2 years there is loosening, which is thought to be due to some of the coating disappearing.

Question 69

Define a ‘biomaterial’?

Answer 69

A non-biological material, which is used in the body normally to repair/replace failed body parts

Question 70

It is important that a biomaterial has a high biocompatability - what does this mean?

Answer 70

Interacts well with the body.

Question 71

What are the 2 main factors which affect biocompatibility?

Answer 71

1. body fluids interaction with the material - usually corrosion, which ultimately leads to failure
2. effect of the material on body tissues - tends to cause abnormal changes, like allergy, ulceration or cancer.

Question 72

Define corrosion?

Answer 72

Progressive unwanted removal of a material by an electrochemical process.

Occurs when 2 solid materials which conduct electricity (electrodes) are placed in a liquid which conducts electricity (electrolyte). An electric current flows from one metal to the other through the liquid, allowing a chemical reaction to take place between the electrode and the electrolyte –> galvanic corrosion.

Question 73

How does corrosion occur in orthopaedic implants?

Answer 73

Implants are electrodes because they are either metal, or some other conductive material (like carbon in carbon fibre reinforced plastics).

Body tissues act as electrolytes, as they contain salts which are very corrosive.

The corrosion causes loss of the implant material, which will cause high stress concentrations and ultimate fatigue failure of the implant.

Question 74

When is a corrosion reaction most severe - when two metal electrodes are the same or different?

Answer 74

Different

Question 75

Under what circumstances can an implant made of one alloy behave as two electrodes and lead to corrosion?

Answer 75

If there is a non-homogenous region within the component, which can arise if there is inclusion of impurities in the metal, or a non-uniform distribution of alloy material.

Question 76

What is the main step taken to reduce corrosion of metal implant?

Answer 76

Mixing metals together to form alloys, rather than using pure metals (apart from titanium, which is corrosion resistant in its pure form).

Question 77

What are the only 3 alloys used in implants?

Answer 77

Stainless steel alloys
Cobalt chrome alloys
Titatanium alloys

Question 78

Why are metal alloys and titanium metal good for resisting corrosion?

Answer 78

They form a thin passivation layer composed of metal oxide which forms as a product of corrosion, but acts to seal the underlying material from further corrosion.

Question 79

What is ‘fretting corrosion’?

Answer 79

Corrosion which occurs as the result of abrasion between materials in contact, which removes the protective metal oxide layer.

Question 80

In what situation does fretting corrosion occur?

Answer 80

It occurs when there is repetitive rubbing of materials which are not meant to have any movement between them
e.g. between screws and plates, and in interference fits

Question 81

What is ‘crevice corrosion’?

Answer 81

Corrosion which occurs in crevices between implants, where body fluids get trapped and loses its oxygen supply. As a result there is a build-up of acid which corrodes the implant

Question 82

What areas are prone to crevice corrosion?

Answer 82

Edges of bone plates and between screws and plates

Question 83

What are the 2 methods used on metal implants to improve corrosion resistance?

Answer 83

1. Nitric acid immersion

2. Titanium nitride coating

Question 84

What is a recent development in metal corrosion resistance of implants?

Answer 84

The effect of niobium reducing corrosion on 316L stainless steel.

Question 85

Give 7 adverse effects of implanted materials on the human body?

Answer 85

1. Growth of a thin fibrous layer between the implant and body tissue.
   - results due to micromotion
   - means bone and implant are not fixed together
2. Local infection.
   - presence of foreign material suppress’s body’s ability to fight infection
3. Body sensitisation to metals.
4. Inflammation in regions where corrosion has occurred.
   - metal oxide layer is lost and there is contact between metal particles and body tissues.
5. Tissue necrosis in regions of bone cement.
6. Immune reaction due to wear particles from surfaces of joint replacements.
   - leads to cell-mediated bone resorption (poorly understood)
7. Tumours at implant site (long-term)

Question 86

What is the most common type of stainless steel used for implants?

Answer 86

316L grade stainless steel - a low carbon steel (0.3% carbon)

Question 87

Why is a low-carbon steel preferred for implants?

Answer 87

To reduce sensitisation of tissues and therefore make it more resistant to corrosion.

Question 88

What are the main elements making up 316L grade stainless steel?

Answer 88

Iron 
Chromium 
Nickel 
Molybdenum 
Manganese 
Silicon, Sulphur, Phosphorus (very small amounts)

Question 89

Is 316L grade stainless steel completely corrosion resistant?

Answer 89

No - can corrode and crack and high stress and is prone to corrosion resistance

Question 90

What type of implants is 316L grade steel most suitable for?

Answer 90

Temporary implants, like in fracture fixation (screws and plates)

Question 91

How are 316L grade stainless steel implants usually made, and why is this method chosen?

Answer 91

They are forged - heated metal is forced into shape by hammering.

Used because it makes the metal up to 4 times stronger than steel produced by casting.

Question 92

What is a disadvantage of forging 316L grade stainless steel?

Answer 92

Steel becomes less ductile, so has a lower fatigue strength.

Question 93

Why are cobalt chrome alloys preferred to stainless steel for permanent implants?

Answer 93

They have better corrosion resistance, although they are not as strong

Question 94

What was the original cobalt chrome alloy used in orthopaedics? What is its composition?

Answer 94

Stellite 21

• 65% cobalt
• 30% chromium
• 6% molybdenum

Question 95

What is a disadvantage of cobalt chrome alloys? What has been done in an attempt to overcome this?

Answer 95

They are not as strong as stainless steel

• other alloys with superior strength (MP35N) have been tried, but they don’t have as good corrosion resistance.

Question 96

Despite cobalt chrome alloys being less strong than stainless steel alloys, why are they preferred for large permanent implants?

Answer 96

The replacement part is large enough to provide sufficient strength.

Question 97

Why are stainless steel alloys preferred to cobalt chrome alloys for temporary fixtures, like fracture fixation plates?

Answer 97

The cross-sections of the implants are very small, and cobalt chrome alloys are not strong enough to support.

Question 98

In pure titanium implants, what process is carried out before implantation?

Answer 98

Anodisation (increasing the thickness of the anti-corrosion layer on the surface)

Question 99

What mechanical properties of titanium make it the most suitable for use in implants?

Answer 99

Less dense (lighter) and half as stiff than stainless steel or cobalt chrome alloys.

Higher fatigue strength than stainless steel.

Question 100

What part of a joint replacement is titanium not used for and why?

Answer 100

The bearings in joint replacements - has a low wear resistance.

Question 101

What are fibre-reinforced plastics?

Answer 101

Composite materials used in fracture fixation, which use very stiff, high strength, but brittle fibres embedded in a flexible resin material.

Question 102

What is the main advantage to using fibre-reinforced plastics in orthopaedic implants?

Answer 102

Have a very high strength, but a very low stiffness, so are very mechanically compatible to bone.

Question 103

When did the first attempts at joint replacement begin?

Answer 103

In the 19th century, when prolonged anaesthesia became possible.

Question 104

Why did the first attempts at joint replacements fail?

Answer 104

There was only an attempt to replace the surfaces of joints - which couldn’t support the large load placed on them. The materials used incl. gold and leather caused high infection and rejection rates.

Question 105

What was the first recorded successful hip replacement?

Answer 105

A “Thompson” hemi-arthroplasty:

Replaced the femoral head with steel. Had a long stem driven down into the femoral canal (“press fit”)

Question 106

Explain the work by McKee and Farrar in the 1950s in the UK?

Answer 106

Were among the first to carry out a “total arthroplasty”.
Developed a metal acetabulum which had small projections on its outer surface.
This was “press fitted” together with a Thompson like femoral stem.

Question 107

Why did the work by McKee and Farrar ultimately fail?

Answer 107

They developed a metal-on-metal total arthroplasty, which had high levels of friction as a result. There were excessive shear forces transferred to the bone prosthesis which caused loosening.

Question 108

In the 1950’s, what were the Judet brothers from France known for?

Answer 108

They were the first to use polymethylmethacrylate (PMMA) in an arthroplasty. This plastic was used to make a femoral head, which was held in place with a small peg in the femur.

Question 109

What is the importance of John Charnley in hip joint replacement?

Answer 109

He was the first to develop a low friction total joint arthroplasty in the early 60s.

Question 110

What were the most important contributions to joint replacement design by Jon Charnley?

Answer 110

1. Designed a smaller femoral head:
   - reduce bearing friction and loosening
2. Introduced the use of bone cement (first was PMMA) between bone and prosthesis:
   - help to distribute load
3. Introduced high-density polyethylene (HDP) as a bearing material:
   - lower friction bearing
4. Produced a system of instrumentation to match his prosthesis

Question 111

List the 5 performance criteria for an orthopaedic implant?

Answer 111

1. Tolerated within the body and provide no short-term and little long-term adverse effects.
2. Give pain relief and allow return of normal daily activities.
3. Adequate life-span, ideally out-living the patient.
4. Should be insertable by a competent surgeon of average ability such that a predictable outcome can be reasonably guaranteed.
5. Should be of acceptable cost.

Question 112

To what extent do hip arthroplasty’s meet this performance criterion:
tolerated within the body and provide no short-term and little long-term adverse effects?

Answer 112

Most hip joints are made from cobalt chrome or titanium, which are fairly corrosion resistant.
HDP provides a good bearing surface, but it does give undesirable tissue reaction when fragmented.

Question 113

To what extent do hip arthroplasty’s meet this performance criterion:
give pain relief and allow the return of normal daily activities?

Answer 113

There is no actual cure or long-term pain relief that is suitable without causing a high risk of side effects, therefore hip replacement is the most effective way of relieving pain and restoring function.

Question 114

What type and range of motion are required from the hip in order to be able to stand, walk and sit down?

Answer 114

• slight extension
• min flexion of 30 degrees
• abduction when weight bearing
• rotate when in full extension

Question 115

To what extent do hip arthroplasty’s meet this performance criterion:
have adequate life-span, ideally out-living the patient?

Answer 115

Generally - 90% will last 10 years.

Question 116

To what extent do hip arthroplasty’s meet this performance criterion:
should be insertable by a competent surgeon of average ability such that a predictable outcome can be reasonably guaranteed?

Answer 116

An advantage with hip arthroplasty is that only an approximate reciprocal shape is needed as the remaining space is filled with PMMA to act as a filler between bone and prosthesis, meaning that this op is well within the ability of the surgeon.

Question 117

To what extent do hip arthroplasty’s meet this performance criterion:
Should be of acceptable cost?

Answer 117

Raw material costs are low, as are manufacturing costs.

However, the product is a low volume production item and labour intensive, but not over-expensive.

Question 118

Why is it important to determine the forces acting on the normal hip structure in designing a hip prosthetic implant?

Answer 118

Knowing the forces allows for the stresses to be calculated, which can then be used in the design process of the prosthesis.

Question 119

What are the 2 main contributors to force in a joint?

Answer 119

1. External loads

2. Muscle forces acting at the joint

Question 120

What are the 2 main experimental methods of estimating stress?

Answer 120

1. Strain gauges (experimental method)

2. Finite Element Analysis (computerised method)

Question 121

The Finite Element Analysis has mostly replaced experimental methods in estimating stresses. What is involved in this method?

Answer 121

Creation of a 2D or 3D model of the structure made up of small elements. Loads are then applied to the model and the computer calculates the stresses.

Question 122

What are the advantages of using the Finite Element Analysis in calculating stresses?

Answer 122

Ease of calculation of stresses.
Designs can be compared.
Regions of high stress can be easily identified.

Question 123

Give 2 reasons why it is so difficult to determine accurately the stresses in the components of a replacement hip?

Answer 123

1. Joint loading varies according to the physical activity being undertaken.
   - magnitude varies greatly because the hip has a wide range of movement.
2. The magnitude of muscle forces for different activities cannot be determined accurately.
   - if more than one muscle is active, there are more unknown forces than equations to solve them.

Question 124

Which type of activity generally generates the largest joint reaction force in the hip:
walking 
ascending stairs 
descending stairs 
rising from a chair

Answer 124

ascending stairs (around 7.2x BW)

Question 125

Which type of activity generally generates the smallest joint reaction force in the hip:
walking 
ascending stairs 
descending stairs 
rising from a chair

Answer 125

rising from a chair (around 3x BW)

Question 126

What is meant by an indeterminate structure?

Answer 126

When it is not possible to calculate forces because there are more unknown forces than equations to solve them (like in the hip joint if more than one muscle is active).

Question 127

What activity can be used to help analyse stresses in hip prostheses?

Answer 127

Standing on one leg - some muscles are not active at all (so their force values are 0), leaving the abductor muscles to be calculated.
It also generates high bending stresses.

Question 128

What types of stresses arise in the hip joint?

Answer 128

1. Compressive stresses
2. Bending stresses
3. Hoop stresses due to bending
4. Torsional stresses
5. Stresses in the acetabulum

Question 129

How does compressive stress in the femur arise?

How is it calculated?

Answer 129

The hip joint force has a component which causes a compressive force in the femur, giving rise to a compressive stress.

Compressive stress = F/A,

where F = compressive force, A = cross-sectional area

Question 130

What happens to compressive stresses in the femur when a prosthesis is present?

Answer 130

The compressive force from the stem is transferred to the femur as a shear force.

Question 131

What is the consequence of a stem-bone bond not being sufficiently strong enough?

Answer 131

The prosthesis will loosen and sink into the medullary cavity.

Question 132

How are compressive stresses in a hip prosthesis stem calculated?

Answer 132

F/A

where F = compressive load at any section
A = area of that cross-section

Question 133

Is the compressive load taken by a hip prosthesis stem uniform along its length?

Answer 133

No, it varies (due to load transfer and stress shielding)

Question 134

What methods are utilised to prevent a hip prosthesis stem from sinking distally into the medullary canal?

Answer 134

• tapering the stem
• using a collar at the proximal stem
• fixing the bone to the stem, using bone ingrowth or adhesion
• using a cement strong enough to withstand shear forces

Question 135

What methods are utilised to reduce interface shear stresses in a hip prosthesis by converting shear loads to compressive loads?

Answer 135

• using a collar at the proximal stem

- tapering the stem

Question 136

What methods are utilised to avoid fracture of the hip prosthesis stem?

Answer 136

• use a stem with a large enough cross-section to resist the stresses
• use a high-strength material for the stem

Question 137

What method is utilised to prevent excess stress shielding in a hip prosthesis?

Answer 137

Careful selection of the rigidity of the stem under axial loading.

Question 138

What causes a bending stress in the femur?

Answer 138

The direction of the joint force vector is not along the neutral axis, so the femur is subjected a bending moment and therefore also a bending stress.

Question 139

What is the equation for calculating bending stress?

Answer 139

Bending stress = My / I

where
M = the applied bending moment
y = the distance from the neutral axis to the section of interest
I = second moment of area

Question 140

Draw a normal femur showing the main forces acting on the bone and how they contribute to producing a bending stress in the bone?

Answer 140

(diagram - draw)

The force required by the abductor muscles is around 2BW. This creates a bending moment, which produces tension on the lateral side of the femur and medial compression.

Question 141

Draw a femur with an implanted stem to illustrate the main forces acting and how they contribute to produce an bending stress in the stem?

Answer 141

(diagram - draw)

To keep the stem in static equilibrium, the applied load due to the joint force must be balanced by reaction forces due to contact between the stem and the femur.

The proximal area on the medial side of the femur provides one main contact point and the lateral distal side provides another, which counteracts the tendency for the stem to rotate due to the bending action of the joint force.

The max bending moment (M) occurs in the proximal stem, and equals joint force (J) x d. Moving down the stem, the moment decreases to zero at the distal end.

Question 142

How does the presence of a femoral stem affect the magnitude of the bending stresses in the femur?

Answer 142

Reduces the stresses in the proximal end of the femur as the stem takes some of the bending load from the bone

Question 143

List the important design features of a femoral stem that are important to ensure the stem does not fail under a bending load?

Answer 143

• design it with a large enough second moment of area

- design its shape to limit the magnitude of the bending moment due to the joint force

Question 144

List the important design features of a femoral stem that are important to prevent the stem loosening under a bending load?

Answer 144

• provide a good enough strong bond between bone and stem or cement
• provide a good press fit

Question 145

List the important design features of a femoral stem that are important to minimise stress shielding of the bone under bending loads?

Answer 145

• select a suitable rigidity for the stem

Question 146

Describe what is meant by a ‘hoop stress’?

Answer 146

A hoop stress is also known as a circumferential stress, which is generated under the act of a bending load.

It results from radial stresses, which are stresses that directed out radially from a central point. Hoop stresses in the bone are primary tensile stresses that act in a direction that tend to split the bone.

Question 147

Where are radial stresses greatest?

Answer 147

points of bone-stem contact at the proximal and distal ends

Question 148

It has been shown that radial stresses are:
a) proportional
b) inversely proportional
to the square of the length of contact of the stem with the bone?

What does this mean for radial stresses in stems of short length?

Answer 148

Inversely proportional.

Stems of short length are prone to high radial stresses on the bone.

Question 149

Give 2 examples of how hoop stresses occur?

Answer 149

• if the stem is too short

- if there isn’t a good fit of the stem in the medullary cavity

Question 150

How do torsional stresses within the femur occur?

Answer 150

When the ankle is restrained and the upper body is rotated, the femur experiences a torsional load like the lower leg and knee.

Question 151

Give 4 examples of how shear stresses in stems can be converted into compressive stresses?

Answer 151

• use non-circular stems
• high shear strength of cement (if used)
• good bonding at interfaces
• surface treatments of the stem to improve interface bonding

Question 152

Why is it desirable to use non-circular sections for the stem of a femoral component?

Answer 152

To reduce the rotational shear stress.

Question 153

Why does the normal acetabulum have a high bending strength?

Answer 153

Due to its structure - it is a sandwich of cancellous bone in between two layers of cortical bone, one of which is covered with articular cartilage and forms the joint bearing surface.

Therefore it a lightweight but with good rigidity.

Question 154

List 5 important design factors for an acetabulum replacement?

Answer 154

1. size and conformity of joint replacement surfaces
   - affects contact areas, which affects contact stresses
2. ways to maintain the subchondral cortical bone
   - if this broken, the cancellous bone, which isn’t normally loaded, takes most of the load
3. stiffness and thickness of the cup
4. whether or not to use a cup with a metal backing plate
5. the technique used to fix the cup to the remaining acetabular bone.

Question 155

What is bone cement made from?

Answer 155

Polymethyl methacrylate (PMMA) - a polymer

Question 156

What is the benefit of coating a prosthesis component with PMMA during manufacturing?

Answer 156

The cement filler adheres better to the implant during surgery, and forms a stronger bond therefore a greater resistance to shear forces.

Question 157

Give two advantages of using cement in a hip replacement?

Answer 157

1. The surfaces between bone and implant do not need to be an exact fit, as the gaps can be filled with cement.
2. Cement fills in all the gaps between bone and prosthesis, preventing areas of high stress concentrations.

Question 158

List 3 problems with PMMA cement?

Answer 158

1. As the cement polymer sets, the temperatures are high enough to destroy body tissue next to the cement, to a depth of a few cm.
2. There is always some monomer left after the chemical reaction, which is very toxic and these small fragments cause intense inflammatory reactions.
3. Cement is a filler, and doesn’t chemically bond to bone or stem, so any tensile loading parts the cement from the bone and stem, ultimately leading to cement fatigue failure.

Question 159

Why is a non-bonded interface undesirable?

Answer 159

1. Still a significant amount of abrasive wear as bone rubs against metal.
2. There will still be a layer of fibrous tissue forming between the bone and the interface, unless the bonding is absolutely perfect, which is hard to achieve with a press fit.
3. There is still a small amount of motion between the surfaces, which causes wear particles to be released into tissue.

Question 160

List 3 ways to improve the integrity of the cement interfaces with bone and metal stems?

Answer 160

1. Roughen the stem surface or coat with beads -
   improve interlocking of cement to the prosthesis.
2. Coat the prosthesis with PMMA -
   bonds to the inserted cement to give an intimate fit.
3. Combine PMMA surface coating with a cement that bone can bond to -
   cement containing hydroxyapatite.

Question 161

Explain briefly the mechanism of load transfer from the stem to the femur, with particular reference to the regions of high interface shear stresses?

Answer 161

Load is transferred from stem to bone as a shear force.

Shear stress is highest at the ends of the stem (where load transfer occurs). The magnitude of shear stress is dependent on the magnitude of the shear force, which in turn depends on how much load is transferred to the bone proximally and distally e.g. the great the proximal load transfer from stem to bone, the greater the proximal shear stress.

Question 162

Explain the influence of stem rigidity on the magnitude of the proximal interface shear stresses?

Answer 162

Using a stiffer stem reduces the proximal shear stress (because less load is transferred from the stem to the bone proximally), however this increases stress shielding of the bone.
It also increases distal shear stress, because more load will be transferred to the bone distally.

Question 163

What is an ‘isoelastic stem’?

Why can they not be used in hip prostheses?

Answer 163

A stem with the same stiffness as bone.

They are good for stress shielding, however they increase the shear stress too much at the proximal end of load transfer, and can be high enough to cause failure of bonding at the interface.

Question 164

What is the magnitude of stresses in cement dependent on?

Answer 164

Its thickness and its stiffness.

Question 165

What risks are associated with using a cement layer that is too thin?

Answer 165

1. very high cement stresses

2. bone resorption at the proximal femur (due to cement debris causing an adverse tissue reaction)

Question 166

What is considered to be the optimal thickness for a cement layer?

Answer 166

3-7mm proximally, 2mm distally

Question 167

What effect does the use of a proximal collar have on load transfer from the stem to the femur?

Answer 167

Allows compressive load transfer from the stem to the bone, reducing stress shielding and lowering the stresses in the cement in the proximal medial region.

Question 168

What is the main argument against using a collar in femoral stems?

Answer 168

The collar-calcar contact area acts a pivot, about which the stem can rotate.
This means the distal end of the stem is prone to high stress concentrations.

Question 169

Cementless stems are now surface coated - usually with hydroxyapatite. What is the main advantage of completely coating the surface of a stem, as opposed to just partial coating?

Answer 169

1. Helps bone ingrowth and potentially eliminates metal debris from bone-metal abrasion.
2. Gives opportunity for the bone to bond to a larger area.

Question 170

Cementless stems are now surface coated - usually with hydroxyapatite. What is the main disadvantage of completely coating the surface of a stem, as opposed to just partial coating?

Answer 170

Fully coated stems promote stress shielding of the bone.

Question 171

How does a ‘tapered wedge’ stem help proximal load transfer?

Answer 171

Allows transfer of a significant amount of load in compression, rather than shear.

Question 172

Name the 2 ways that the neck of a femoral stem can be modified in order to reduce the bending moment at the stem?

What effect do these modifications have on the hip joint force?

Answer 172

1. Reduce the length of the neck of the stem
2. Increase the angle between the long axis and the axis of the neck
   - These increase the hip joint force, giving rise to greater wear and acetabular stresses.

Question 173

Which of the following affect the frictional force on a bearing:

a) contact load
b) contact area
c) material properties of the surface to which the load is applied
d) material properties of both surfaces?

Answer 173

contact load and material properties of both surfaces

Question 174

Why is high-density polyethylene (HDP) used as a bearing surface in replacement joints, when there are other polymers which have a much lower coefficient of friction?

Answer 174

It is one of the least toxic and best wearing as a bearing surface.

Question 175

What are the two types of wear that occur between bearing surfaces?

Answer 175

Adhesive wear and Abrasive wear

Question 176

When does adhesive wear occur between bearing surfaces?

Answer 176

The two bearing surfaces stick together when they are pressed together, and the softer one is torn off by the harder one.
Bearing surfaces should be made of materials with low levels of adhesion.

Question 177

When does abrasive wear occur between bearing surfaces?

Answer 177

When surfaces are not perfectly smooth, therefore highly polished surfaces are very important

Question 178

What are the 3 factors which affect the amount of wear that takes place in a bearing?

Answer 178

v = c.N.s / p

where
v = volume of wear
c = constant (the coefficient of wear)
N = applied load of the bearing surfaces
s = the distance of the bearing slides
p = the hardness of the surface being worn

Question 179

What is HDP’s main disadvantage as a bearing material?

Answer 179

HDP wear fragments can migrate considerably within an implant, which can cause intense inflammatory tissue reactions and is associated with aseptic loosening.

Question 180

Give 2 reasons why small diameter heads are used in hip replacements?

Answer 180

1. They reduce cup-bone interface shear stresses, which lessens the risk of loosening.
2. They produce less volume wear of HDP than large diameter heads

Question 181

What are the 2 main disadvantages of using a small diameter head in hip replacements?

Answer 181

1. The rate of depth of wear is greater than what it would be for a larger head because contact area is less.
2. There is an increased likelihood of dislocation in post-op period because there is increased likelihood of neck impingement on the edge of the cup

Question 182

What is the acetabular component usually made of?

Answer 182

HDP, +/- a metal backing between it and its interface with cement or bone.

Question 183

Give some important features in the design of modern acetabular cups?

Answer 183

• size of femoral head and acetabular cup
• HDP may or may not be lined with a metal backing plate
• thickness of the HDP layer
• outer dimension of the acetabulum

Question 184

Give 3 acetabular design features that affect the contact pressure at the bearing surface of the hip joint?

Answer 184

1. diameter of the cup
2. the radial clearance
3. thickness of HDP

Question 185

Why is there a minimum recommended thickness to the HDP cup?

Answer 185

This means the point contact load between the femoral head and the cup is spread out over a greater area and avoids excessive contact stress on the HDP.

Question 186

What is the advantage of using an acetabular component with a metal backing plate?

Answer 186

Holds the plastic in place and reduces its tendency to creep and distort, avoiding high contact stresses on the HDP.

Question 187

What are the 3 steps that lead to acetabular component loosening due to HDP fragments?

Answer 187

1. HDP fragments come into contact with bone
2. Tissue reaction to the HDP leads to bone resorption.
3. The HDP migrates further along the interface, causing further resorption.

Question 188

The knee joint has a surface which does not provide much stability. What does the joint rely on for stability?

Answer 188

• ligaments
• integrity of the posterior joint capsule
• musculature

Question 189

List the names of the 4 principal knee ligaments?

Answer 189

Anterior cruciate ligament (ACL)
Posterior cruciate ligament (PCL)
Medial collateral ligament (MCL)
Lateral collateral ligament (LCL)

Question 190

What is the origin and insertion of the ACL?

What is its main stabilising role?

Answer 190

Origin - the anterior intercondylar region of the tibia
Insertion - posterior femur, in the intercondylar fossa.

Resists anterior subluxation of the tibia over the femur.

Question 191

What is the origin and insertion of the PCL?

What is its main stabilising role?

Answer 191

Origin - the posterior intercondylar region of the tibia
Insertion - anterior femur, in the intercondylar fossa

Resists posterior subluxation of the tibia onto the femur.

Question 192

What is the origin and insertion of the LCL?

What is its main stabilising role?

Answer 192

Origin - lateral femoral condyle
Insertion - lateral fibular head

Resists adduction of the joint

Question 193

What is the origin and insertion of the MCL?

What is its main stabilising role?

Answer 193

Origin - medial femoral condyle
Insertion - medial tibia surface

Resists abduction of the joint

Question 194

The four ligaments of the knee act together to provide what 2 general stabilsing roles?

Answer 194

• limit distraction of the knee

- limit long axis rotation of the joint

Question 195

What is the posterior capsule of the knee joint?

What is its role?

Answer 195

a band of tendinous material running across the posterior surface of the knee.

resists hyperextension

Question 196

How can the LCL and the MCL contribute to knee instability?

Answer 196

• the LCL is longer than the MCL.

- the MCL and the LCL can both become lax.

Question 197

In terms of knee joint motion, how do the lengths of the ligaments change with flexion/extension?

Answer 197

They don’t! The ligaments move nearly isometrically.

Question 198

Describe how the axis of rotation of the knee changes with flexion?

Answer 198

The vertical line which passes through the centre of rotation moves horizontally.

As the knee flexes, the centre of rotation moves posteriorly, as does the point of surface contact of the femur and tibia.

Question 199

The centre of rotation in the knee joint is also known as…?

Answer 199

Instantaneous centre of rotation. - it changes at every instant of motion.

Question 200

The cruciate ligaments are responsible for the four-bar linkage mechanism the knee joint. What is this?

Answer 200

This constrains the motion of the femur on the tibia so that there is a combination of rolling and sliding motion.

Question 201

Explain how the cruciate ligaments act as a four-bar linkage mechanism?

Answer 201

The rolling distance required by the knee joint to reach its maximum flexion of 140 degrees is considerably high (>55 mm).

However, the range of rolling motion actually achieved by the knee is constrained within around 20mm (the distance the instantaneous centre of rotation moves from extension to full flexion).

This limit to the rolling distance is provided by the cruciate ligaments, and has the effect of controlling the position of the most posterior point of the centre of rotation, to allow the knee to fully flex without rolling up against the posterior capsule.

Question 202

Why is the joint reaction force at the knee much greater than body weight?

Answer 202

The external forces are mostly compressive, and the contacting femoral/tibial surfaces have the combined effects of:

• gravitational forces,
• contracting forces of the muscles and
• the balancing loads of the ligaments.

Question 203

Explain why the medial knee compartment is more heavily loaded during activities such as walking?

Answer 203

As well as a vertical component of GRF which acts compressively, there is also a horizontal component.

During gait the horizontal component is directed medially to the knee, creating an adduction moment which must be balanced by muscles and ligaments.

At a low magnitude of horizontal GRF, the quadriceps acting via the patellar tendon can hold the joint together.

At higher magnitudes, the hamstrings are also used which increases the joint reaction force. They dont have the strength to maintain contact at both condylar surfaces.

There is loss of contact from the lateral side, and the load is then taken by the medial condyle. Stability is now relied upon from the LCL to balance the moment.

Question 204

What 2 adverse effects could a high contact force in the medial compartment of the knee have on a joint replacement?

Answer 204

1) the tibial component needs to be able to transfer medial compartment loads to the bone without causing stresses that could cause the bone to fail.
2) If the LCL is absent or can’t be retained during surgery, then the knee prosthesis has to provide all lateral stability (usually in form of a hinge).

Question 205

Nearly all knee replacements are made of which materials?

Answer 205

Femoral component - cobalt chrome

Tibial component - HDP

Question 206

Compare the results of knee to hip in terms of longevity without failure?

Answer 206

Knee replacements now have almost the same results as those for hip replacements - this has happened in the last 10-15 years.

90-95% ten year survival rate

Question 207

List the 3 important functional kinematic requirements of a knee prosthesis?

Answer 207

1. Should extend to 180 degrees so that person can stand without effort by quadriceps.
2. Should flex to 90 degrees.
3. Should permit slight axial rotation to maintain natural ligament tension throughout flexion/extension.

Question 208

Why is the femur cut at about 7 degrees for proper alignment of the femoral component?

Answer 208

The 2 bearing surfaces in the knee must be cut parallel.
The femur has to be cut at an angle to compensate for the natural angulation of the femur relative to the tibia. (see diagram on pg 9)

Question 209

What must happen to the posterior capsule of the knee in a knee replacement, and why is this done?

Answer 209

Must be dissected off the back of the femur.

To allow the replacement knee to fully extend.

Question 210

Why must the collateral ligaments be balanced in equal tension in a knee replacement?

Answer 210

To keep the joint cuts in parallel and prevent any excess medial or lateral opening of the joint.

Question 211

Compare the cost of hip and knee replacement?

Answer 211

Knee costs around 5x more than hip replacement.

Question 212

Define the term ‘constraint’ in the context of knee replacements?

Answer 212

Refers to the relationship between the tibial and femoral bearing surface geometries.

The more constrained they are, the less freedom of movement they have to slide and rotate.

Question 213

What is a ‘fully constrained’ knee prosthesis?

Answer 213

a replacement with linked prostheses, like a hinge between the tibial and femoral component.

Question 214

What is an ‘unconstrained’ knee prosthesis?

Answer 214

a surface replacement - tibial or femoral.

Question 215

What is a ‘semi-constrained’ knee prosthesis?

Answer 215

a prosthesis with a constraining mechanism only active in certain degrees of extension

Question 216

Why is it not helpful to describe modern knee replacements in terms of constraint?

Answer 216

All replacements have some degree of constraint in their movement, and most modern knee replacements fall in to the semi-constrained category.

Question 217

When is it suitable for a hinged knee prosthesis to be used?

Answer 217

When there are no ligaments intact - the hinge constrains the knee to a single axis of motion so ligaments are not required.

Question 218

What is the main problem with hinged prostheses?

Answer 218

Prone to loosening - there is no give under lateral loading and rotational loading.

Question 219

If the PCL is not to be retained in knee replacement, what design consideration for the prosthesis must be taken into account?

Answer 219

Since the PCL controls the rolling motion of the tibia, a mechanism must be built into the prosthesis to enable the femur to rotate on the tibial plateau without sliding too posterior.

Question 220

What are the advantages to retaining the PCL in knee arthroplasty?

Answer 220

1) provides some antero-posterior stability and proprioceptive activity
2) walking on stairs is more stable

Question 221

What are the disadvantages to retaining the PCL in knee arthroplasty?

Answer 221

1) Prevents a free surgical dissection of the posterior capsule, which therefore may limit full extension
2) Encourages the femoral component to slide over the tibial bearing which can cause wearing

Question 222

What are the advantages to removing the PCL in knee arthroplasty?

Answer 222

1) allows for more congruent joint surfaces, which reduces HDP wear.
2) allows for any deformity correction

Question 223

Why does a replacement knee need to have a fairly flat tibial plateau when the PCL is retained?

Answer 223

A flat tibial plateau helps provides a kinematic design that allows the PCL to function properly.

Question 224

In a PCL retaining prosthesis, what are the consequences if the PCL is too loose?

Answer 224

Allows forward movement of the femur on the tibia so the rolling back motion no longer works

Question 225

In a PCL retaining prosthesis, what are the consequences if the PCL is too tight?

Answer 225

1) restricted flexion, and excessive rolling back of the femur on the tibia.
2) compression of the joint surfaces, leading to contact stresses.

Question 226

What are the main problems with high density polyethylene (HDP) material in knee arthroplasty?

Answer 226

1) its wear debris has an adverse effect on bone tissue, leading to resorption
2) oxidises over time leading to increase in density which causes increased stiffness
3) greater stiffness increases joint contact stresses, leading to wear
4) prone to fatigue failure under loading

Question 227

What is the purpose of using stems and pegs on femoral and tibial components of a knee prosthesis?

Answer 227

To maintain a large area of contact to prevent loads producing stresses large enough to loosen the interface
- long stems constrain the motion of the prosthesis to do this.

Question 228

List the factors that affect the rate of wear of a knee replacement bearing surface?

Answer 228

• the applied load across the bearing surfaces (the normal load)
• the distance the bearing slides
• the hardness of the surface being worn

Question 229

What is the formula that links the factors that affect the rate of wear of a prosthesis?

Answer 229

v = c.N.s / p

where
v = volume of wear
c = coefficient of wear (constant)
N = the applied load across the bearing surface (the normal load)
s = the distance the bearing slides
p = the hardness of the surface being worn

Question 230

The volume of wear of a knee prosthesis material is proportional to which particular factor, and why?

Answer 230

sliding distance moved.

From the formula, c and p are fixed, and for a given activity N is defined.

The formula can be simplified to
v = K x s

Question 231

What effect does diameter of a bearing have on the volume of wear material, and why?

Answer 231

Smaller diameter - reduces the volume of wear material.

Reason - The sliding distance to achieve a certain degree of rotation is proportional to the radius of the bearing. (s = r x degrees) (v = K x s)

Question 232

What factor can be used to determine the useful life of a knee prosthesis?

Answer 232

The rate of wear of the HDP component (it is less stiff so wears more than CoCr femoral component)

Question 233

Since it is the rate of wear of the HDP component that determines the useful life of a knee prosthesis, what equation links this and volume of wear?

Answer 233

volume of wear (v) = area of contact (A) x depth of wear (t)

Question 234

In order to reduce wear, why is it preferable to increase bearing width rather than to increase bearing diameter?

Answer 234

Increasing W and d will increase area of contact, however increasing d increases the volume rate of wear, so it is preferred increase W.

Question 235

How does the shape of the contact area vary in a knee prosthesis as the joint moves from flexion to extension?

Answer 235

(See pg 20 in notes)

Contact area moves posterior from extension to flexion

Question 236

What is the loading pattern within the contact area of the tibial plateau in a knee prosthesis?

Answer 236

Central part of contact area - always in compression

Periphery of contact area - always in tension

Question 237

How does the area of contact and associated loading pattern result in fatigue loading of a tibial component?

Answer 237

Contact points change between compression and tension and loading and unloading, which make a material susceptible to fatigue failure if loads are high.

Question 238

List the main design features that affect load transfer from the tibial plateau?

Answer 238

1) thickness of HDP component
2) whether the HDP component has a backing plate
3) whether the tibial component has a stem
4) the stiffness of the HDP material

Question 239

What is the relationship between thickness of the HDP component and load transfer from the tibial plateau?

Answer 239

The thinner the HDP component, the greater it is stressed.

Non-linear relationship - below 8mm the contact stresses rise steeply.

Question 240

What is the advantage of the ability to vary the tibial height in a prosthesis?

Answer 240

Allows for ligament re-balancing.

Question 241

What is the effect of using a metal backing plate on load transfer from the tibial plateau?

Answer 241

(See page 23)

The high stiffness of a metal plate underneath the tibial component held in place by a tibial plate distributes the high contact stresses under the condyles to evenly load the bone underneath.

Question 242

If a metal tray redistributes load well over the tibial plateau, why isn’t this method always used instead of a thick HDP component?

Answer 242

Results have not confirmed that it is better. If the knee is loaded unevenly, there can be problems with a tray:

1) Stress concentrations are greater on the medial side of the knee than they would be with all-HDP component, as metal is more stiff.
2) tensile stresses between plate and bone laterally.
3) metal causes higher tensile forces than all-HDP component.

Question 243

What is the effect of a tibial stem on load transfer from the tibial plateau?

Answer 243

Prevents sinkage into bone, which is the main cause of loosening.

Question 244

What effect does stiffness of the HDP material have on load transfer from the tibial plateau?

Answer 244

The higher the Young’s modulus, the greater the contact stress - doubling E increases stress by about 40%

Question 245

How can a tibial peg reduce the tendency for a tibial plateau to sink medially?

Answer 245

The peg takes about 25% of the load which relieves the tibial plateau and the underlying bone of some of the total load

Question 246

Describe the structure of the femoral component of a modern knee prosthesis?

Answer 246

Both sides of prosthesis are SYMMETRICAL - contrasts to normal knee where medial condyle is around 1.5mm radius larger than lateral.

Symmetrical condyles when looked at in profile vary in shape according to the design of the tibial plateau.

Very anterior part of femoral component curvature accommodates movement of patella during flexion/extension.

Question 247

What are the advantages to designing the femoral component of a prosthesis as symmetrical, rather than copying the natural asymmetry?

Answer 247

Prevents the required inventory from doubling in size.

No need for matched instruments, which keep costs down.

Question 248

The degree of constraint in a knee prosthesis is largely determined by what factor?

Answer 248

The surface shape design of the tibial component

Question 249

What is the function of a posterior stabilised knee prosthesis?

Answer 249

Has a surface shape which is dished in all directions, with a posterior stop

Provides a partially constrained shape to provide required stability, by preventing posterior femoral subluxation over the tibia, whilst providing the required degree of motion, by causing the femur to ‘roll back’ as it flexes.

Question 250

Are most knee replacements cemented or cementless?

Answer 250

Cemented (PMMA)

Question 251

How can additional rotary control be added to knee replacements to prevent tensile stresses arising due to lateral turning moments?

Answer 251

Inserting projections built into the undersurfaces of the tibial components

Question 252

What is the ultimate mode of failure for a knee replacement?

Answer 252

HDP wear and wear particles

Question 253

What is the main advantage of not replacing the patella?

Answer 253

patella provides a better leverage for patella tendon, so reduces the patellar force needed to provide a flexion moment, so reducing component wear and loading.

Question 254

What structural design factors must be taken into account for a patellar resurfacing?

Answer 254

1) must not fail due to stress as the reaction force of the patella against the femur can be 4-5 xBW
2) the anterior part of the femoral component needs to be grooved to better accommodate the patella (the shape of the contact surface affects wear rate)
3) replacement patella is made of HDP so excessive wear needs to be taken into account.

Question 255

Why is a metal-backed patella replacement generally undesirable?

Answer 255

Wears worse - the HDP is not thick enough to distribute the loads so is prone to higher contact stresses because of the metal.

Question 256

In what condition is it preferable to replacement all knee joint surfaces?

Answer 256

Rheumatoid arthritis

Question 257

What additional fixation methods are required for cementless knee prostheses?

Answer 257

1) stem in the tibial component

2) press fit and pegs in the femoral component

Question 258

Meniscal bearing prostheses have a meniscus made of what material?

Answer 258

HDP

Question 259

How do meniscal bearing designs differ from the design of most knee prostheses?

Answer 259

The meniscus slides at the lower bearing surface for long axis rotation and lateral movement

Question 260

What is the main disadvantage of meniscal bearing designs?

Answer 260

Increased technical difficulty in achieving ligament balance and overall alignment without risking dislocation of the moving bearing

Question 261

What is a hemi-arthroplasty?

Answer 261

Replacement of one side of the tibio-femoral joint - either the medial or the lateral side

Question 262

What is the purpose of hemi-arthroplasty?

Answer 262

Used as an alternative to osteotomy where there is a painful and deformed joint that isn’t severe enough to warrant total replacement, but with too advanced disease to permit osteotomy.

Question 263

Why are two pegs commonly used in unicompartmental surface replacement?

Answer 263

To resist axial rotation

Question 264

What are augmentation blocks and what is their function?

Answer 264

Used in knee revision, where there may be considerable bone loss and contact needs to be achieved between bone and prosthesis.

Augmentation blocks fill the gaps and allow prosthesis to rest on the bone.

Question 265

Give 4 reasons why there has been a lack of success of ankle joint replacements?

Answer 265

1) Ankle isn’t usually involved in primary osteoarthritic processes.
2) subtalar joint is often affected too (e.g. in RA) so just ankle replacement wouldn’t be sufficient.
3) the motion of the subtalar joint has to be taken into account as they function in association
4) athrodesis is available

Question 266

Arthrodesis of the ankle can only be carried out when the subtalar joint is
a) healthy
b) unhealthy?
Why?

Answer 266

Healthy!

Fusion of the ankle joint means the subtalar joint will be able to act as a dorsiflexor of the foot.

Question 267

What are the advantages of arthrodesis?

Answer 267

Relieves pain in a stiff ankle joint, without causing loss of movement (as subtalar joint can take over)

Question 268

What are the disadvantages of arthrodesis?

Answer 268

• Abnormal loading on the knee and subtalar joint of the same leg (can cause long term damage)
• Shortening of stride
• Patient will walk out-toed

Question 269

What are the main materials used for ankle joint replacements?

Answer 269

A metal (CoCr or stainless steel) and HDP.

Question 270

What activities of daily living can be restored by ankle replacement, but not by fusion?

Answer 270

Walking and rising from a chair

Question 271

The short and medium term results of ankle replacement are:

a) good
b) poor?

Answer 271

Poor

Question 272

What new design of ankle replacement is significantly improving clinical performance?

Answer 272

Meniscal bearings

Question 273

Give 3 reasons for ankle replacements currently not lasting a reasonable amount of time without revision?

Answer 273

1) The loads across the ankle joint were larger than anticipated for early designs
2) The bony areas where ankle replacement components can be fixed aren’t adequate to provide support for current cement techniques
3) If there is stiffness in the subtalar joint, it will add to the loosening process as large torques will be transmitted across the ankle with no modulation by thigh muscle controlled subtalar motion.

Question 274

Where are ankle replacement components fixed, and why dont they provide enough support?

Answer 274

Tibial bone - mainly soft cancellous bone which rapidly widens then narrows

Talar bone - area for fixation is very small

Question 275

Can all competent surgeons carry out ankle replacement?

Answer 275

No - can only be justified in specialist centres

Question 276

The ankle joint is the articulation between which bones?

What is it also known as?

Answer 276

Tibia and the talus.

Tibio-talar or talocrural joint.

Question 277

What is the normal range of motion of the ankle joint?

Answer 277

25-30 degrees in both dorsiflexion and plantarflexion

Question 278

What is the range of ankle motion during walking?

Answer 278

10 degrees dorsiflexion

15 degrees plantarflexion

Question 279

What type of joint is the ankle joint?

Answer 279

A hinge joint - has a single axis of rotation

Question 280

Is the axis of rotation of the ankle joint perpendicular to the sagittal plane?

Answer 280

No - it is inclined downwards and posteriorly on the lateral side.

Question 281

The subtalar joint is the articulation between which bones?

What is it also known as?

Answer 281

The calcaneus and the talus

Talo-calcaneal joint

Question 282

What movement does the subtalar joint permit?

Answer 282

Inversion and eversion

Question 283

What is the range of motion of the subtalar joint?

Answer 283

A few degrees

Question 284

What is the axis of rotation of the subtalar joint in relation to the ankle joint?

Answer 284

It’s at right angles to the ankle joint

Question 285

In RA, is it usual for one or both the ankle and subtalar joints to be affected?

What is the consequence of this?

Answer 285

Both!

A replacement needs to compensate for both joints, which is difficult to achieve.

Question 286

What is the importance of the subtalar joint in motion of the foot?

Answer 286

Helps to allow the foot to stand on level and uneven surfaces that the ankle joint alone can’t achieve.

The combined motion of the two joints is important for functional activities

Question 287

Describe the role of the ankle joint in getting out of a chair?

What happens if ankle motion isn’t sufficient?

Answer 287

Dorsiflexion of the ankle is needed to move the trunk forwards.

If one ankle cannot dorsiflex, then the good ankle can be placed at the rear.

If both ankles are affected and can’t dorsiflex sufficiently, then a greater upper limb effort is required to stand.

Question 288

What are the appropriate maximum vertical and fore-aft loads on the ankle joint?

Answer 288

Vertical: 4-5 xBW

Fore-aft: 0.4-0.7 xBW

Question 289

What are the 2 types of classification of ankle joint replacement?

Answer 289

Congruent - matching bearing surfaces

Incongruent - non-matching bearing surfaces

Question 290

Congruent ankle joint replacements only allow which type of movement?

Answer 290

Rotational - and the number of axes of rotation can be limited according to the design

Question 291

List the 4 types of congruent ankle joint replacement design?

Answer 291

1) spherical
2) spheroidal
c) conical
d) cylindrical
(page 5 of notes)

Question 292

Describe the design of the spherical congruent ankle joint replacement?

Why does care need to be taken in its positioning?

Answer 292

Allows for freedom of rotation, therefore can compensate for a degenerate subtalar joint.

Care needs to be taken because it has a specific centre of rotation.

Question 293

Compare the spherical and cylindrical designs of a congruent ankle joint replacement, in terms of angles of rotation?

Answer 293

For the same medio-lateral width of bearing surface, a cylindrical shape gives a greater angle of plantarflexion-dorsiflexion rotation

Question 294

What rotational movements does the spheroidal design of a congruent ankle joint replacement permit?

Answer 294

Planterflexion-dorsiflexion and inversion-eversion.

No axial rotation because its curvature is different in the sagittal and frontal planes.

Question 295

Does a congruent spheroidal design have any advantages over a spherical ankle joint replacement?

Answer 295

No

Question 296

What rotational movements does the conical design of a congruent ankle joint replacement permit?

Answer 296

Single axis of plantarflexion/dorsiflexion rotation.

Some medio-lateral resistance

Question 297

What is the main disadvantage of a congruent conical shape of ankle replacement?

Answer 297

Requires more bone resection than the cylindrical shape

Question 298

What are the 2 the main disadvantage of a congruent cylindrical shape of ankle replacement?

Answer 298

1) Cannot compensate for subtalar dysfunction because it only provides a basic single axis replication of the ankle joint.
2) Creates an area of concentrated stress under asymmetrical medio-lateral loading

Question 299

Most congruent ankle joint replacements have been which shape?

Answer 299

Cylindrical

Question 300

What is the main feature of incongruent ankle joint replacements?

Why is this feature seen as advantageous?

Answer 300

Less constraint in movement, so that some horizontal motion is possible.

Reduces load transmission to bone-cement prosthesis interfaces, by transferring some of the load to the soft tissues.

Question 301

Give 2 examples of types incongruent ankle joint replacement designs?

Answer 301

1) Trochlear (saddle) shape

2) Convex-concave shape

Question 302

What movements does a trochlear shape of incongruent ankle joint replacement permit?

Answer 302

plantarflexion-dorsiflexion, inversion-eversion and axial rotation

Question 303

The convex-concave type of incongruent ankle joint replacement can be which two shapes?

Answer 303

1) cylindrical

2) spherical

Question 304

List 3 disadvantages with incongruent shapes of ankle joint replacements?

Answer 304

1) higher rate of depth of wear than congruent types
2) higher contact stresses due to a lower contact area than the congruent types
3) less stability than congruent types due to their greater freedom of movement

Question 305

What is the current failure rate of ankle joint replacement?

Answer 305

65% failure at 5 years

Question 306

Give 6 reasons for failure of ankle joint replacement?

Answer 306

1) aseptic loosening
2) lateral or medial subluxation of the joint
3) subsidence of the talar component
4) impingement of the joint
5) wound healing problems
6) infection

Question 307

The most successful ankle joint protheses have what features?

Answer 307

• cementless with porous coated surfaces
• hollow tbial bearing surface
• have a meniscal bearing to provide congruent surfaces

Question 308

What is an advantage of a meniscal bearing ankle prostheses over cylindrical bearing design?

Answer 308

Meniscal bearings provide congruence, which gives lower wear, whilst still giving freedom of motion

Question 309

What is the role of finite element analysis in contributing to ankle joint prostheses designs?

Answer 309

Only 2 designs have been derived from this method in the 80’s.

Since then finite element analysis has become more sophisticated, so could help in the future.

Question 310

Upper limb joint replacement encompasses which joints?

Answer 310

The bearing surfaces of the shoulder, elbow, wrist and finger joints

Question 311

What is the main aim of upper limb joint replacement?

Answer 311

Relieve pain

Secondary aim is to restore function!

Question 312

What are the typical conditions patients have who are undergoing upper limb joint replacement?

Answer 312

RA 
OA 
Osteonecrosis 
Post-traumatic arthritis 
Fractures

Question 313

If a patient requiring upper limb joint replacement has RA, why might priority be given to other operative interventions, such as spine or lower limb?

Answer 313

In RA, several joints will be affected by the disease.

1) RA of the spine causes cervical instability is ass. with neurological problems
2) lower limb joint replacement will lessen the need for the upper body to support the body weight during other activities (crutches).

Question 314

When several upper limb joints are affected and require replacement how is the decision of priority given?

Answer 314

• most painful replaced first

- if all equal in pain, then work distally to proximally

Question 315

If several upper limb joints require replacements, why is it advised to work distally to proximally? Give 3 reasons.

Answer 315

1) primary objective of upper limb joint replacement after pain relief, is restoration of hand function
2) impairments in distal joints might affect the physio of proximal joints after replacements
3) it is argued that more functional improvement is gained in the distal joints

Question 316

Why do some surgeons argue to work proximally to distally in upper limb joint replacements? Give 3 reasons.

Answer 316

1) Shoulder pain more troublesome at night and can radiate to the elbow
2) An immobile shoulder ma cause abnormal loadings at the elbow and cause early failure of an elbow prosthesis.
3) rehab of other upper limb joints can be simplified if the shoulder is pain-free.

Question 317

What materials are used for upper limb joint replacements?

Answer 317

Similar to other orthopaedic implants:

• stainless steel
• titanium alloys
• cobalt chrome alloys
• vitallium alloys
• HDP

Fixation - PMMA cement

Question 318

What material has been used successfully in flexible wrist and finger joint replacements? Why has it been successful?

Answer 318

Silicone elastomer (rubber) - has excellent biocompatability and biodurability

Question 319

Generally, have upper limb joint replacements been shown to reduce pain?

Answer 319

Yes - can be eliminated or at least reduced to a tolerable level

Question 320

Which type of shoulder joint replacement is better at pain relief and providing a good range of motion:

a) total arthroplasty?
b) hemi-arthroplasty?

Answer 320

Total arthroplasty

Question 321

Compare the survival rates of upper limb joint replacements to lower limb?

Give the main reasons for the difference?

Answer 321

Have not reached same survival rate yet

Reasons for difference:

1) lower frequency of upper limb joint replacements
2) upper limb joint replacements are more complex than lower limb

Question 322

Which upper limb joint replacement has been most successful?

Answer 322

Shoulder, then elbow, then wrist and fingers with the worst survival rates

Question 323

Give a general estimate of survival rates for upper limb joint replacements, and compare this to hip replacements?

Answer 323

80% after ten years (hip around 90% after 10 years)

Question 324

Can upper limb joint replacements be performed by all competent surgeons with a predictable outcome?

Answer 324

No - too complex, so only performed at specialist centres

Question 325

Compare the cost of upper limb joint replacements to hip and knee replacements, and explain why there are differences?

Answer 325

Relatively more expensive than hip.

Difference is due to lower frequency of upper limb joint replacements.

Question 326

When was the first shoulder joint replacement performed?

Answer 326

1893 in Paris by Jules Pean - removed after 2 years due to infection

Question 327

When were the first successful shoulder joint prostheses developed?

Answer 327

1950s by Charles Neer

Question 328

What 3 categories are shoulder prostheses designs generally divided into?

Answer 328

According to the amount of movement constraint:

1) unconstrained (Neer prosthesis)
2) semi-constrained (Gristina prosthesis)
3) constrained (Reese prosthesis)

Question 329

What are ‘reversed’ or ‘inverted anatomy designs’ of shoulder prostheses?

Answer 329

A design of shoulder prostheses which does not conform to the anatomy of the normal joint: the humeral head component is a socket instead of a ball e.g. Cavendish prosthesis

Question 330

Although the primary aim of shoulder replacement is pain relief, what improvement in range of motion can be achieved with an unconstrained design?

Answer 330

90-135 degrees of abduction

Question 331

The type of shoulder joint replacement design chosen for a patient will depend largely on what factor, and why?

Answer 331

The quality of the soft tissues surrounding the shoulder joint - these are what determines the stability of the shoulder

• intact stability of rotator cuff: unconstrained prosthesis
• no stability of rotator cuff: constrained prosthesis

Question 332

What is the primary function of the shoulder?

Answer 332

To allow the hand to be positioned in space

Question 333

What are 4 articulations at the shoulder joint?

Answer 333

1) glenohumeral
2) acromioclavicular
3) sternoclavicular
4) scapulothoracic

Question 334

Which shoulder joint articulation is most important and why?

Answer 334

Glenohumeral

• has the largest range of motion and most load bearing
• this is the joint replaced in a total shoulder replacement
• a prosthesis must be able to withstand loads up to several xBW.

Question 335

Why is the glenohumeral joint inherently unstable?

Answer 335

Shallow glenoid fossa

Question 336

How is the instability of the shoulder joint compensated for?

Answer 336

The surrounding soft tissues - mainly rotator cuff

Question 337

Why is the stability of the soft tissues surrounding the shoulder joint so important for shoulder joint prostheses?

Answer 337

Must be inherently intact as these muscles give the stability to the shoulder joint

The soft tissues must be intact to be able to compensate for removal of bone stock to make way for prosthetic replacement.

Question 338

What is the most significant complication of total shoulder joint replacement, and what is the reason for this complication?

Answer 338

Loosening of the glenoid component - occurs because there is a limited amount of bone to which a prosthetic component can be attached.

Question 339

What are the use of unconstrained shoulder prostheses dependent on?

Answer 339

Intact and functioning rotator cuff mechanism

Question 340

List 4 advantages of an unconstrained shoulder prosthesis?

Answer 340

1) nearly anatomical in shape
2) allows max potential function
3) achieves good predictable pain relief
4) only requires minimal removal of bone (ensures soft tissues are preserved)

Question 341

What are the use of semi-constrained shoulder prostheses dependent on?

Answer 341

Intact and functioning rotator cuff mechanism, even though some constraint is built into its design

Question 342

How is some constraint built into a semi-constrained shoulder prosthesis?

Answer 342

The glenoid component is shaped so that it roofs over the superior aspect of the humeral component (hooded glenoid)

Resists the upwards shear force produced when the arm is elevated. Also prevents the upward subluxation of the humerus that can occur when the rotator cuff is absent.

Question 343

What are the disadvantages of semi-constrained shoulder prostheses?

Answer 343

1) motion is limited compared to unconstrained designs
2) greater forces are transmitted to the glenoid component bone-cement junction, resulting in more frequent loosening of the glenoid component

Question 344

What type of design are constrained total shoulder replacements usually?

Answer 344

Ball-in-socket designs

Can be normal anatomy: humeral ball and glenoid socket

Or reverse anatomy: humeral socket and glenoid ball

Question 345

What are the 3 types of constrained total shoulder replacements used today?

Answer 345

1) Stanmore
2) Michael Reese
3) Trispherical

Question 346

Describe the design of The Stanmore total shoulder replacement?

Answer 346

• metal on metal design
• cup-shaped glenoid socket
• glenoid socket fixed with 3 pegs and cement
• 2 components are snapped together

Question 347

List some complications of the Stanmore design total shoulder replacement?

Answer 347

• unsnapping of the components
• instability
• glenoid component loosening

Question 348

Describe the design of the Michael Reese total shoulder replacement?

Answer 348

• cobalt chromium humeral head
• polyethylene socket
• metal glenoid cup
• small diameter of lip so that the ball is captive
• humeral head dislocates when large torque is reached to prevent # of scapula

Question 349

What is the main disadvantage of a Michael Reese total shoulder replacement design?

Answer 349

impingement of the humeral component on the glenoid component, which restricts range of motion

Question 350

Why is the design of the Trispherical total shoulder replacement unusual?

Answer 350

It has 3 balls, instead one one - both humeral and glenoid components have a metal ball, which is contained within a 3rd larger polyethylene ball

Question 351

What are the advantages of a trispherical design for total shoulder replacement?

Answer 351

Greater range of motion and avoids impingement

Question 352

What are the disadvantages of constrained shoulder replacement designs to unconstrained designs?

Answer 352

• higher frequency of loosening
• dislocations are more common
• mechanical failure of components
• disassembly of components

Question 353

What methods have been used for glenoid component fixation?

Answer 353

• triangular shaped keel
• extended keel
• pegs
• a stem
• a wedge
• a large screw
• flanges bolted to the base of the spine of the scapula

Question 354

Constrained shoulder replacements tend to have more elaborate glenoid fixation, why is this?

Answer 354

To secure the component against the larger loads present in constrained designs.

Question 355

What is the primary indication for elective replacement of the elbow joint?

Answer 355

Pain relief

Question 356

What condition to most patients for elective elbow replacement suffer from?

Answer 356

RA

Question 357

What are the primary functions of the elbow?

Answer 357

Allow the positioning of the hand in space and allow the forearm to act as a lever

Question 358

What are the 3 articulations at the elbow joint?

Answer 358

1) humeroulnar (trochleoulnar)
2) humeroradial (radiocapitellar)
3) proximal radioulnar

Question 359

What loads must the elbow joint be able to sustain?

Answer 359

6-7 xBW in dynamic activities

3 xBW in static loading

Question 360

Which articulation at the elbow joint supports most of the load?

Answer 360

Humeroulnar

• humeroradial and proximal radiocapitellar provide additional stability

Question 361

Compare the coronal plane angle between the upper arm and forearm in a:

a) normal elbow
b) uniaxial hinge prostheses?

Answer 361

In a normal elbow, the coronal plane angle varies.
Seen when in anatomical position, the forearm is slightly valgus. As the elbw flexes this reduces to a few degrees.

In a uniaxial hinge prosthesis, the coronal plane angle is fixed, giving rise to excessive shearing forces at the bone-cement interfaces.

Question 362

What max ranges of motion is permitted at the elbow joint?

Answer 362

140 degrees flexion
70 degrees pronation
80 degrees supination

Question 363

What range of motion is required at the elbow joint for daily living activities?

Answer 363

30-130 degrees flexion
50 degrees pronation
50 degrees supination

Question 364

Is the stability of the elbow joint provided more by the:

a) congruity of the joint surfaces
b) the soft tissues?

Answer 364

They provide stability equally!!!

Question 365

During most activities of daily living the elbow is flexed. Which ligament provides the most joint stability in this case?

Answer 365

Medial collateral ligament - over 50% of the joint stability

Question 366

What is the role of the MCL in the elbow?

Answer 366

Resists valgus stress

Question 367

What relevance does a stiff shoulder have to the prosthetic replacement of the elbow?

Answer 367

Loadings on the elbow are dependent to some degree on shoulder stiffness.

So if a person with a total shoulder replacement and a stiff shoulder attempts internal or external rotation, the stiff shoulder will increase rotational stresses at the bone-cement interface

Question 368

What are First generation elbow prostheses also known as, and why?

Answer 368

“constrained” or “hinged” designs

• they were based upon single axis hinges

Question 369

Who was the first to design a successful total elbow prosthesis, and when did this occur?

Answer 369

Dee - 1968

Question 370

What are the 2 main problems with uniaxial hinged elbow prostheses?

Answer 370

1) Loosening
- restricted single-axis motion is unnatural and gives rise to excessive shearing forces on the elbow by the prosthesis
- metal wear debris from metal-on-metal articulations

2) Large amount of bone stock removal
- puts additional stress on the bone-cement interface
- difficult to salvage joint when failure occurs

Question 371

What are the failure rates for constrained hinged elbow prostheses, and what does this mean for their use today?

Answer 371

Around 40% loose in 3-5 years post-op

Constrained prostheses are rarely used now

Question 372

What are Second generation elbow prostheses also known as?

Answer 372

2 main types:

1) semi-constrained metal-to-polyethylene hinge types
2) unconstrained metal-to-polyethylene resurface types

Question 373

What is a recent variation of the two main types of second generation elbow prostheses?

Answer 373

Designs that also resurface the radial head

Question 374

What do semi-constrained elbow joint replacements generally consist of?

Answer 374

Stemmed humeral and ulnar components with a hinged-like metal-to-polyethylene articulation

Question 375

What is an advantage of a semi-constrained elbow joint replacement?

Answer 375

Give varying degrees of side-to-side laxity

Question 376

Which type of elbow joint replacement is most suited when there is some soft-tissue insufficiency or loss of bone stock?

Answer 376

Semi-constrained: more stability than unconstrained

Question 377

Give an example of a semi-constrained elbow joint prosthesis?

Answer 377

Tri-Axial (outcomes are dependent on patient’s condition)

Question 378

What do unconstrained elbow joint replacements generally consist of?

Answer 378

Resurfacing the lower end of the humerus and olecranon of the ulna, in order to reproduce anatomical structure

Question 379

Give an example of unconstrained elbow joint prostheses?

Answer 379

Ewald (capitellocondylar)
Kudo
Souter-Strathclyde

Question 380

What materials do an unconstrained elbow joint replacement usually consist of?

Answer 380

Humeral component - Vitallium

Ulnar component - HDP

Question 381

Compare semi-constrained and unconstrained elbow replacement designs in terms of:

a) Dislocation
b) Aseptic loosening

Answer 381

a) Dislocation in unconstrained designs is slightly higher (3-8% compared to 1-5%)
b) Aseptic loosening lower in unconstrained designs (1-3% compared to 8%)

Question 382

What is the purpose of resurfacing the radial head in elbow replacements?

Answer 382

To gain benefits of load-transmission stability that are afforded by the humeroradial articulation

Question 383

How successful has resurfacing of the radial head been in elbow replacements?

Answer 383

Variable - difficult to balance the 3 articulations at the time of operation.

Question 384

What are the 2 types of wrist joint prostheses?

Answer 384

1) flexible hinge

2) total wrist

Question 385

What is the main indication for prosthetic wrist replacement?

Answer 385

Pain associated with RA or OA

Question 386

When is arthrodesis preferred to wrist replacement?

Answer 386

When only the radiocarpal joint is affected, especially for younger patients

Question 387

What bones/structures does the wrist joint incorporate?

Answer 387

The wrist joint extends from the metaphyseal portion of the radius to the carpo-metacarpal joints and incorporates the 8 carpal bones

Question 388

Why must the distal radio-ulnar joint be considered in total wrist joint replacement, even though anatomically the joint is considered separately to the wrist?

Answer 388

Its primary function is supination-pronation, so most diseases involving the wrist also affect this joint.

Question 389

Where is considered to be the composite centre of rotation of the wrist joint? Why?

Answer 389

A fixed point on the capitate - the axes of rotation for both flexion-extension and abduction-adduction pass .

Question 390

What is the max range of motion of the wrist joint?

Answer 390

Flexion 80-90 degrees
Extension 70-80 degrees
Adduction 35 degrees
Abduction 15-20 degrees

Question 391

What ranges of motion of the wrist is required for daily activities of living?

Answer 391

Flexion 10 degrees

Extension 35 degrees

Question 392

Which motion of the wrist is particularly important?

Answer 392

Extension

Question 393

Which joint is an important contributor to the overall motion of the wrist?

Answer 393

Radiocarpal joint (condyloid joint)

Question 394

How does the motion of the radiocarpal joint differ in a wrist prosthesis?

Answer 394

Normal joint has a greater radius of curvature for abduction -adduction than for flexion-extension to provide more stability in the abduction-adduction plane. The smaller radius for flexion-extension provides less stability but a greater arc of motion.

Converse in prosthesis - makes it difficult to maintain stability in abduction-adduction plane

Question 395

Describe the structure of a flexible hinge wrist prosthesis?

Answer 395

Proximal and distal stem with a barrel-shaped midsection.

One stem is inserted into the medullary canal of the distal radius, and the other inserted into the 3rd metacarpal through the partially resected capitate.

Question 396

What material are flexible hinge prostheses made from?

Answer 396

High performance silicone elastomer (rubber)

Question 397

What percentage of flexible wrist prostheses have been found to tear on the sharp bony edges of the medullary canal?

Answer 397

20%

Question 398

What method was adopted in flexible wrist prostheses to attempt to reduce tearing from sharp bony edges? Was it successful?

Answer 398

Titanium bone liners (grommets)

Not really - results were similar either way

Question 399

What is the purpose of using a spacer (the barrel-shaped midsection) in a flexible wrist prosthesis?

Answer 399

Helps maintain adequate joint space and overall wrist alignment. After time a new capsulo-ligamentous system develops around the midsection

Question 400

Are the stems in a flexible wrist prosthesis fixed?

Answer 400

No - they slide in and out of the intramedullary canals

Question 401

What are the 2 main designs of cemented total wrist prostheses?

Answer 401

1) Meuli design: ball-ad-socket type

2) Voltz: non-spherical “ball” and shallower socket

Question 402

Describe the design of the Meuli cemented total wrist prosthesis?

Answer 402

3 parts

Distal component - eccentric prongs that insert into the 2nd and 3rd metacarpals

Radial component - twin pronged

Polyethylene ball - fitted onto radial component and articulates with distal component

Question 403

Give an advantage and a disadvantage of the Meuli wrist prosthesis ball-and-socket design?

Answer 403

Advantage:
ball-and-socket avoids possibility of rotational failures

Disadvantage:
relies on adequate and proper soft tissue balance to prevent undesirable rotary motion

Question 404

Describe the design of the Voltz cemented total wrist prosthesis?

Answer 404

3 parts:

Distal component: single stem that inserts into the 3rd metacarpal

Radial component: single stem

Polyethylene cup: mounted onto the radial component and articulates with the metacarpal component

Question 405

Describe the shape of the polyethylene cup in the Voltz total wrist prosthesis design and explain its significance?

Answer 405

Not a true sphere - a segment of a torus with a larger radius of curvature for abduction-adduction.

this mirrors the normal radiocarpal joint

Question 406

What are the 2 main problems associated with total wrist prostheses?

Answer 406

1) Loosening

2) Stress shielding

Question 407

Why were original finger prosthesis designs unsuccessful?

Answer 407

Migration into the bone, loosening,

mechanical problems

Question 408

What are the 2 types of MCP joint replacement?

Answer 408

1) flexible hinge

2) total MCP

Question 409

What type of joint is the MCP?

Answer 409

Condyloid

Question 410

What provides stability to the MCP joints?

Answer 410

Joint capsule
collateral ligaments
fibrocartilaginous palmar plate
muscle tendons

Question 411

In what plane does major motion of the MCP joint occur?

Answer 411

Sagittal - up to 90 degrees of flexion

Question 412

What material is a flexible hinge MCP joint replacement made from?

Answer 412

A flexible silicone elastomer

Question 413

What does a flexible hinge MCP joint replacement rely upon for stability?

Answer 413

Encapsulation

Question 414

Describe the design of a flexible hinge MCP joint replacement?

Answer 414

The implant is inserted into the hollowed medullary canals of the metacarpal and proximal phalanx once adequate soft tissue and bone are resected. The stems are not fixed.

Question 415

What is the role of grommets in flexible hinge MCP joint replacement?

Answer 415

They shield the implant from sharp bone edges - they are press fitted into position

Question 416

Describe the design of a total MCP joint prostheses?

Answer 416

Incorporates a metallic component that articulates with a polyethylene component, which are cemented into the medullary canals.

Question 417

List 3 problems associated with total MCP replacements?

Answer 417

1) implant fracture
2) migration
3) loosening

Question 418

Why is arthroplasty of the DIPs rarely indicated, and what procedure is usually done instead?

Answer 418

The IP joints hardly contribute to overall finger motion, so implant arthroplasty is rarely indicated.

Arthrodesis is done more often, as this only results in a minimal functional deficit

Question 419

What are the 2 types of IP joint replacements?

Answer 419

Flexible hinge and total IP (basically the same as MCPs, except with smaller dimensions)

Question 420

Functionally which is the most important IP joint, and why?

Answer 420

The PIP

the PIP joint contributes 85% of IP motion, and the DIP only contributes 15%.

Question 421

Bone is anisotropic. What does this mean?

Answer 421

It displays different mechanical behaviour under different types and directions of loading.

Question 422

What type of loading is a bone
a) strongest
b) weakest
in?

Answer 422

a) compressive loading

b) shear loading

Question 423

What 3 factors determine the location and mode of a fracture?

Answer 423

1) the geometry and structure of the bone
2) the loading mode (compression, bending, torsion)
3) the loading rate

Question 424

In tension and compression loading, the load required to cause failure in a bone is proportional to which feature

Answer 424

The cross-sectional area of the bone.

The larger the area, the stronger and stiffer the bone.

Question 425

In bending loading, which factors affect bone’s mechanical behaviour?

Answer 425

Cross-sectional area and distribution of bone tissue around a neutral axis (second moment of area)

Question 426

In torsional loading, what factors affect bone’s mechanical behaviour?

Answer 426

Cross-sectional area and the distribution of one tissue around a neutral axis (polar second moment of area)

Question 427

In what section of the tibia does torsional fracture commonly occur, and why?

Answer 427

Distal section
- although the distal section has a larger cross-sectional area of bone compared to the proximal section, more of the bone is located nearer to the neutral axis. This increases the magnitude of the torsional shear stress.

Question 428

Why is the structure of bone important in fractures?

Answer 428

The mid-diaphyses is made of cortical bone which is much stronger than the cancellous bone found at the metaphyses.
The cancellous bone is weaker under axial compressive loading and will fail before cortical bone.

Question 429

A bending load to a bone causes what type of fracture, and why?

Answer 429

Transverse -

A bending load results in a convex side of the bone which is loaded in tension, and a concave side which is loaded in compression.

The side in tension will fail first since bone is stronger in compression than tension.

Question 430

A pure compression load to a bone will cause what type of fracture?

Answer 430

Oblique

Question 431

A bending load superimposed on axial compression will cause what type of fracture, and why?

Answer 431

Butterfly

• combination of transverse and oblique fracture.
• bending produces a transverse crack on the tension side of the bone, whilst the compression results in an oblique fracture.
• Under the combined load the bone deforms, resulting in butterfly segment on the compressed side of the bone.

Question 432

A pure torsional load will cause what type of fracture.

Answer 432

Spiral

Question 433

What causes a high energy fracture?

Answer 433

When a bone is loaded in impact, its energy absorption capacity can be twice as high when it is loaded slowly. When the bone fails this energy is released suddenly and results in a high energy fracture.

Question 434

Summarise the fracture process?

Answer 434

1. Energy delivered to the limb
2. Energy is transferred via the soft tissues to the bone which absorbs the energy
3. The bone breaks and energy is released back to the soft tissues
4. The broken bone and damaged soft tissues bleed and haematoma builds up
5. Acute inflammatory response occurs, causing pain.

Question 435

Describe the process of natural (secondary) bone healing?

Answer 435

Weeks 0-2:
Macrophages invade haematoma and “mop up” dead and damaged tissue

Weeks 2-6:
New capillaries grow into the haematoma, bringing healing cells with them - fibroblasts and osteoblasts. The surviving periosteum begins to regenerate and grow between bone fragments

Weeks 6-12:
New bone tissue laid down in endosteal space. Two ends united as a provisional callus

Up to 12 months:
provisional callus continues to form woven bone which gradually remdoels to form a cortex

Up to 2 years:
callus matures

Question 436

How long does natural bone healing take?

Answer 436

Depends on the degree of damage and the time it takes for a new blood supply to be re-established.

Most heal in 6-12 weeks

Question 437

If blood supply is not re-established after fracture, what is the consequence?

Answer 437

No bony union will occur - described as an atrophic or fibrous union

Question 438

If there is excessive movement at a fracture site, what is the consequence?

Answer 438

Cartilage is laid down rather than bone cells.

Pseudoarthrosis can occur - formation of a false joint bewteen rapidly proliferating cartilage cells at either end.

Question 439

Define ‘primary bone healing’?

Answer 439

When a fracture heals without external callus formation due to there being no micromovement between fracture fragments during the healing process. New Haversian systems grow directly across the fracture gap.

Question 440

Compare the strength of healed bone that has occurred through secondary healing and primary healing?

Answer 440

Bone that has healed by secondary healing is stronger than that of primary healing - in primary healing the bone is deprived of the physical loading it normally bears

Question 441

What is Wolff’s law?

Answer 441

Describes the adaptation of bone to meets the mechanical demands placed on it

Question 442

What mechanical properties does a callus confer to a broken bone as it heals?

Answer 442

The callus compensates for its lower strength and rigidity of the material by significantly increasing its second moment of area.

(R = E x I)

At the later stage of bone healing, with the increase of strength and stiffness of the callus, its cross-section decreases until bone regains its original shape

Question 443

What is the main factor which promotes bone healing?

Answer 443

Micromovement and loading along the bone’s long axis.

Question 444

Give 3 possible factors which may explain why movement at the fracture site influences bone healing?

Answer 444

1. Electrical effects caused by moving crystals of hydroxyapatite (the basic mineral constituents of bone)
2. Hormonal factors
3. Electro-magnetic effects produced through electron flow away from the fracture site

Question 445

List 3 aims of fracture management?

Answer 445

1. save life
2. treat pain
3. restore function safely and reasonably

Question 446

Why can a fracture be life threatening?

Answer 446

Blood loss - broken bones bleed! Femoral and pelvic fractures are worst for bleeding

Question 447

Why does placing a fracture in a splint diminish pain?

Answer 447

Reduces the muscle spasm that occurs around a fracture when it is mobile

Question 448

What 3 factors determine restoration of function after a fracture?

Answer 448

1. severity of injury
2. general health of the patient
3. location of injury

Question 449

What are the 2 main processes in fracture treatment?

Answer 449

1. Reduction
   - restoration of anatomical shape.
2. Holding
   - the bone is held in reduced position until healing has taken place.

Question 450

What are the 2 types of fracture reduction?

Answer 450

Closed reduction

Open (surgical) reduction

Question 451

What are the types of fracture holding?

Answer 451

1. Plaster of paris
2. Traction
3. External fixation
4. Internal fixation

Question 452

What are the 3 methods of internal fixation?

Answer 452

1. plates and screws
2. pins and wires
3. rods and nails

Question 453

Why is fracture fixation necessary?

Answer 453

to minimise deformation or movement between the fracture fragments

Question 454

What does plaster of Paris consist of?

Answer 454

Calcium sulphate

• extracted in crystal form and heated to remove water, producing powdery compound called calcium sulphate hemihydrate.
• then mixed with water to form crystals again and sets to a solid. Results in heat production
• the more hemihydrate in the bandage, the more heat is produced.

Question 455

How are plaster of Paris casts made?

Answer 455

Calcium sulphate hemihydrate is dissolved in an organic solvent, eg ether, which has no water.

Starch added to mixture and whole paste is spread out on a cotton bandage. The wet bandage is dried and the solvent is collected for re-use.

The bandage is therefore coated with calcium sulphate held on by starch.

The bandage doesn’t add to the strength, but acts as a good vehicle to getting the plaster onto the part to be splinted.

The starch doesn’t add to the strength of the cast but speeds up setting - it is an accelerator

Question 456

What substances can be added to a plaster of Paris mixture to slow down setting?

Answer 456

Retarders - alum and borax.

Question 457

plaster of Paris itself is made up of two types of…?

Answer 457

crystals - long and short

Long crystals - occur naturally as alabaster. Give a finished cast a hard quality.

Short crystals - give the cast a softer quality.

Question 458

What factor mainly determines the properties of the material making up a final splint?

Answer 458

The physical interlocking of the long and short crystals - influenced by how wet the plaster is at the time of application

Question 459

What are the 2 functions of plaster cast, and which function predominates?

Answer 459

1. provide support to the soft tissues to support the broken bone by encasing the limb in rigid exoskeleton - hydraulic theory
2. provide 3-point fixation system
   no. 1 usually predominates, except in childhood when the tough periosteum provides the 3 point fixation

Question 460

What 3 factors of a broken bone does a plaster cast control?

Answer 460

1. length - prevent shortening
2. position - prevent tilt and shift
3. rotation - about the long axis of the bone

Question 461

What are the 3 different types of cast?

Answer 461

Above knee
Cylinder
Below knee

Question 462

What are the disdvantages of casting?

Answer 462

Stiff muscles and wasting, prolonging overall rehabilitation.

Impairment caused by immobilisation of joints, leading to dependency and prolong hospital stay

Question 463

How can the problem of immobility of joints caused by plaster casts be overcome?

Answer 463

By using functional braces - Careful moulding and applying hinges into the cast

Question 464

Describe the design of a functional femoral brace?

Answer 464

the upper 1/3 of the femoral component is squared off - slightly distorts the soft tissues but not enough to raise high points of pressure.

the upper tibia component is triangulated.

the knee is freed by hinges to allow the knee to move normally

Question 465

Describe the design of a functional tibial brace?

Answer 465

The upper third of the tibia is moulded, which achieves rotary control. Extensions to the cast encapture the femoral condyles in knee flexion.

Question 466

How soon after fracture should functional braces be applied?

Answer 466

2-3 weeks after injury - soft tissue and swelling has settled down

Question 467

Describe two advantages of a function brace, compared to a full-leg cast?

Answer 467

1) A functional brace allows movement at the joints and helps to reduce muscle wasting due to immobility
2) Functional braces are adjustable

Question 468

What are the two new classes of materials used in casts instead of plaster of Paris?

Answer 468

1) Isoprene rubbers or polycaprolactone sheets

2) Glass fibre or artificial fibre and polyurethane composites

Question 469

What advantageous properties do polycaprolactone/isoprene sheets have as a casting material?

Answer 469

Become ductile at low temperatures so can be moulded directly onto the skin achieving a reciprocal shape to the limb.
Become firm at room temp, but can still be gently adjusted.

Question 470

What are the disadvantages of polycaprolactone/isoprene sheets as a casting material?

Answer 470

1) expensive
2) require purchase of an oven
3) require considerable skills to use well

Question 471

How has the disadvantage or requiring considerable skills for applying polycaprolactone/isoprene sheet casts been overcome?

Answer 471

The basic shapes have been preformed in various sizes which can be finely adjusted during fitting, but this is still expensive despite it being easier to use.

Question 472

What do fibre/polyurethane composite casts consist of?

Answer 472

glass fibre or fabric woven bandages that are impregnated with a urethane monomer and catalyst

Question 473

What are the advantages of fibre/polyurethane composite as a casting material?

Answer 473

1) Light and extremely strong, yet flexible (this is because when exposed to warmth/moisture the materials form a true composite)

Question 474

In what type of fracture situation are fibre/polurethane composite ideal as a casting material?

Answer 474

A reasonably stable, healing fracture as they can form sophisticated shapes and interface well with hinge materials, giving firm anchorage.

Question 475

In what type of fracture situation are fibre/polyurethane composite not good as a casting material?

Answer 475

As a primary splintage material - difficult to use on unstable swollen limbs because they are conforming rather than being mouldable.

Question 476

Traction is used as a method of reduction and holding fractures. Although they have the same name, what is the difference between the methods?

Answer 476

Reduction =controlled use of force on a relaxed limb to position bones

Holding = altering muscle tone to maintain a position achieved at reduction

Question 477

Explain the principle of traction as a method for holding fractures?

Answer 477

Using traction along the axis of the limb induces an increase in tone of all muscle groups, which maintains alignment of the broken fragments.

As the bone heals and pain disappears, the patient will move the joints a little, which increase muscle contraction in an irregular way, helping to induce callus formation.

Question 478

What size of load is required for holding traction of the lower limb?

Answer 478

10N per 100N of body weight - must be countered by an equal force (tilting the bed) or the patient will be pulled out of bed

Question 479

What are the two ways which a load can be applied for traction?

Answer 479

1) Skin traction - load is applied via foam or sticky bandage to the skin
2) Skeletal traction - load is applied via a pin inserted through the bone

Question 480

What are the advantages and disadvantages of skin traction?

Answer 480

Advantages - convenient

Disadvantages -

1) dependent on the adhesiveness of the bandage or frictional resistance of the foam
2) only be used for loads up to 50N or can damage the skin

Question 481

What are the advantages and disadvantages of skeletal traction?

Answer 481

Advantages -

1) can apply large loads
2) load can be precisely relative to the long axis of the bone

Disadvantages -
1) risk of bone infection at bone pin interface

Question 482

What are the 3 ways of using the principle of traction?

Answer 482

1) static (fixed) traction
2) dynamic traction
3) balanced traction

Question 483

Describe the set-up of static traction?

Answer 483

The load is applied to the limb and attached to a splint so the splint itself provides the counter force

Question 484

What is a disadvantage of static traction?

Answer 484

The immobility prevents joint movement and does not induce axial movement at the fracture site, so leads to muscle disuse.

Only acceptable for 1-2 weeks

Question 485

What type of patient is static traction most suited for?

Answer 485

Children - they don’t cope well with complicated traction and their fractures heal quickly

Question 486

Describe the set-up of dynamic traction?

Answer 486

The load is arranged so that the net pull is maintained along the axis of the bone - irrespective of limb position.

Question 487

What are the 2 functions of the pulleys used in dynamic traction?

Answer 487

1) alter the direction of the force by being statically mounted on a surrounding bed frame
2) alter the magnitude of the traction force by being mounted on the limb or “free floating” within the traction cord system.

Question 488

Describe the set-up of balanced traction?

Answer 488

A small load is applied to the splint as a whole to draw the pressure off the groin area

Question 489

What are the clinical complications of traction?

Answer 489

• lying in bed for prolonged period of time
• bed sores
• chest and urinary infections
• disuse atrophy of muscles and bone

Question 490

Name the factors taken into consideration when deciding on the method that will be used to hold a fracture?

Answer 490

1) the patient
2) the injury
3) the facilities available
4) the skills of the operator

Question 491

What are the 2 methods of fixing fractures that require surgical intervention?

Answer 491

1) Internal fixation
- bone screws
- bone screws and plates
- intramedullary nails

2) External fixation
- external fixators

Question 492

What is the main limiting factor when choosing a suitable material for use in surgical fracture fixation?

Answer 492

Biocompatability

Question 493

What are the 2 main materials use in surgical fracture fixation? Give their advantages and disadvantages?

Answer 493

Stainless steel
A - strong, inexpensive and easy to manufacture
D - don’t tolerate stress reversals well

Titanium
A - strong, inexpensive, biologically more inert than stainless steel, less likely to cause allergies
D - difficult to machine

Question 494

Why is it important to remember that plates and screws must be of the same material in surgical fixation?

Answer 494

Galvanic corrosion can occur if the materials are different

Question 495

How is a bone screw different from a normal screw?

Answer 495

They don’t! They have the same mechanical principles

Question 496

What is a screw?

Answer 496

A mechanism that produces linear motion as it is rotated.

Question 497

What type of screw is used in orthopaedics?

Answer 497

A helix-shaped thread on a shaft.

Question 498

A screw can only fix 2 objects together if…?

Answer 498

1) the head of the screw is wider than the diameter of the shaft so that it pushes one block against the other.
2) the thread doesn’t grip block 1. can be achieved in 2 ways: - either the screw must have no thread on the section nearest to the head or if a screw thread is present, block 1 must have a pre-dilled hole in it

Question 499

What are the 3 factors that determine the strength of a screw fixation?

Answer 499

1) the strength of the screw material
2) the strength of the object material
3) the design of the screw thread

Question 500

What are the 2 functions of the head of a screw?

Answer 500

1) provides a buttress to stop the whole screw sinking into the bone
2) provides a connection with the screwdriver

Question 501

What are the different shapes of the drive connection in the head of a screw?

Answer 501

Hexagonal
Crosshead/Philips
Star

Question 502

Why do bone screws commonly use a hexagonal shaped drive connection?

Answer 502

1) it gives an effective coupling unlikely to be damaged in the screwing process.
2) the very positive interlock between screwdriver and screw makes it easy to use. No axial force is required to retain the driver in the head .

Question 503

Other than hexagonal, what shape of the drive connection in the head of a screw is also very successful?

Answer 503

Star

Question 504

Why is the shape of the undersurface of a screw head usually rounded?

Answer 504

So there is maximum area of contact between screw head and bone after countersinking, reducing the risk of a zone of excessive stress (stress raisers) which may crack the bone.

Question 505

In a screw, what are the different screw diameters to consider?

Answer 505

1) The core diameter - the smallest diameter of the threaded section of the shaft
2) The shaft diameter - the diameter of the shaft where there is no thread
3) The thread diameter - the diameter of the widest part of the threaded section

Question 506

What determines the strength of a screw?

Answer 506

The smallest diameter - the greater the smallest diameter, the stronger the screw will be

Question 507

What are the 3 important aspects of the thread of a bone screw?

Answer 507

• shape
• depth
• pitch

Question 508

What is the shape of the thread of most bone screws? What is the consequence of this shape?

Answer 508

Most are asymmetrical - flat on the upper surface in contact with bone and rounded underneath.

Provides a wide surface on the pulling side and little frictional resistance on the underside. More of the torque is therefore used in pulling two objects together and less wasted on overcoming friction during insertion of the screw.

Question 509

What is the depth of bone screw?

Answer 509

Half the difference between the thread diameter and the core diameter.

Question 510

What is implication of having a smaller/larger bone screw depth?

Answer 510

The amount of thread in contact with the bone determines how well the screw resists being pushed out of the bone. A larger screw depth is desirable in cancellous bone as this captures more material between the threads and so increases resistance of the screw to pulling out.

Question 511

What is the pitch of a thread of a bone screw?

Answer 511

The linear distance travelled by the screw for a complete turn (360 degrees) of the screw

Question 512

List 4 different types of screw tip?

Answer 512

• blunt
• corkscrew
• trocar
• self-tapping

Question 513

All cancellous screws have what type of tip?

Answer 513

self-tapping

Question 514

Describe the properties of a self-tapping screw?

Answer 514

the screw has a cutting tip which enables it to cut its own “female” thread track to match the “male” thread on the screw. The screw can drill a hole in the cancellous bone without the need to use a separate drill bit.

Question 515

Why are cortical screws not-self tapping?

Answer 515

Too much torque would be required, which risks jamming or breaking the screw.

The flutes (channels which provide a route for cuttings to escape) which are present on self-tapping screws are not suitable for cortical bone because bone can grow into the flute and make removal difficult.

Question 516

What is ‘lagging’?

Answer 516

The process of compressing two objects together (achieved by screws)

Question 517

In terms of screw thread, what is the only way that lagging can be achieved?

Answer 517

If the screw thread only grips one of the objects being compressed together.

Question 518

In what 2 ways can screws achieve a lag effect?

Answer 518

1) The screw can be designed as a lag screw, which are partially threaded only in the section nearest the tip
2) They can be inserted in a way that any screw can act as a lag screw, even if it is threaded all along its shaft.

Question 519

How does a screw which is designed as a lag screw, achieve a lagging effect?

Answer 519

Where there are 2 fragments of bone (A and B) which are to be compressed together…

Fragment A has a hole drilled which is the size of the core of the screw, and the screw firstly cuts its own thread in A and then in B using the corkscrew tip. The unthreaded shaft then slides though the hole in A until the head touches the surface of A. As the screw advances the head pulls A towards B.

Question 520

How can a screw be inserted in a way to act as a lag screw, even if it is not designed as one?

Answer 520

Where there are 2 fragments of bone (A and B) which are to be compressed together …

Fragment A has a hole drilled into it which is slightly larger in diameter than the screw thread diameter, then the screw slips through the hole without any twist from a screw driver. The hole in B is equivalent to the screw core diameter, the screw will advance by gripping the bone of B.

Question 521

Give 5 situations where screws may be used?

Answer 521

1) to prevent sideways displacement of fragments
2) to hold a plate against bone
3) to increase the grip of an intramedullary nail on the bone
4) to permit displacement in an axial direction
5) as part of an external fixator assembly

Question 522

Why are screw and plate combinations only recommended to fix upper limb forearm fractures now?

Answer 522

Improvements in intramedullary nail and external fixator designs have become more reliable methods to fix lower limb fractures

Question 523

In what situations are screw-plate combinations used around joints?

Answer 523

Where fragments of bone can’t be held by screws alone e.g.

• violent fractures
• soft bone found after a delay of a few days between injury and surgery
• if bone is unnaturally soft like in old age

Question 524

What is the use of plates in fracture fixation?

Answer 524

To achieve LOAD SHARING between plate and bone until bone is strong enough.

To achieve or enhance compression of bones at fracture site by forcing bone fragments together, where initial fixation by screws isn’t possible

Question 525

What is osteosynthesis?

Answer 525

The reconstruction of a fractured bone by surgical and mechanical means

Question 526

What are bridging plates used for?

Answer 526

In complex fractures with many fragments where incorporating all pieces in the fixation would compromise blood supply.

The 2 main bony shaft fragments are linked by a plate to restore length and alignment and the intervening small fragments are left unfixed and their blood supply undisturbed.

Question 527

Plates must be as compact as possible and malleabe to allow shaping. What is a disadvantage of this?

Answer 527

They have limited capabilities to resist an applied load when stressed in certain directions

Question 528

What is the main cause of fatigue failure of plates?

Answer 528

Backwards and forwards bending of the fracture plate (stress reversal) as the construct is loaded, due to the fractured bone being inadequately reassembled.

Question 529

What factors should be considered when using a plate in order to minimise stress reversal?

Answer 529

1) the whole fixation system should be as stable as possible
2) as little damage as possible to the blood supply
3) plate should be placed in a position relative to the broken bone so it is minimally stressed
4) plate should be placed so that it doesn’t damage un-injured soft tissues
5) plate should be made of strong material

Question 530

When is the use of plates in fracture fixation indicated?

Answer 530

1) when anatomical alignment must be restored accurately.
2) where the use of screws is inadequate
3) when load sharing can be achieved with confidence

Question 531

What would the management of a fracture involve when load sharing couldn’t be achieved by a plate, but anatomical alignment must be restored accurately, and screws aren’t strong enough on their own to hold the fracture?

Answer 531

Bone graft

Question 532

Name 4 areas of the body where plates are commonly used?

Answer 532

Around joints
In the bones of the forearm
On the pelvis
On the face and jaw

Question 533

Why are plates placed on the tension side of a bone that is eccentrically loaded (uneven loading along its axis)?

Answer 533

The tension side of the bone is the side tending to open (bone is weaker in tension than compression), and placing the plate on this side will counteract the eccentric load and compress the fragments together at the side under the plate.

Question 534

Why is contouring a fracture fixation plate before use beneficial?

Answer 534

Contouring the plate so it slightly more concave than the bone will encourage compression of the bone opposite the site of attachment of the plate

Question 535

What is a main disadvantage of plating techniques?

Answer 535

There must be a lot of soft tissue stripping, which further damages the blood supply to already damaged areas. Contributes to healing delay and risk of infection

Question 536

What thicknesses do pins come in?

Answer 536

0.5 to 2-3 mm

Question 537

What are pins also known as? How are pins designed?

Answer 537

• K-wires

They have sharp ‘trochar points’ self-tapping ends and are used in pairs to minimise the rotatory element in the final bone/pin construct

Question 538

What can pins be used in conjunction with to achieve compression between small bone surfaces?

Answer 538

Flexible wires

Question 539

What is the use of flexible wires?

Answer 539

To induce compression

Question 540

In what 2 ways do flexible wires induce compression?

Answer 540

1) can be used statically by encircling or crossing the fragments (cerclage), pushing them together
2) can be used dynamically as a tension band and utilising the power of surrounding muscles to produce compression at the fracture site

Question 541

How do intramedullary nails achieve fracture fixation?

Answer 541

The nail is placed in the medullary canal of a fractured bone and functions as an internal splint which stabilises long bone fractures with minimal damage to surrounding soft tissues.

Question 542

Describe the modern intramedullary nail insertion technique?

Answer 542

Antegrade technique -

the nail is inserted into the bone from one end whilst not disturbing the fracture site at all

Question 543

When planning an intramedullary nail insertion, what are design developments centred around?

Answer 543

Reaming technique -

Reaming = widening the intramedullary canal through paring off the inner surface of the bone

or

Unreamed technique = use nails which are solid and thinner so they dont damage the inner blood supply

Question 544

What are 3 important design considerations in determining the effectiveness of an intramedullary nail?

Answer 544

1) material it is made from
2) how much of the nail is in contact with the bone
3) the dimensions and shape of the nail

Question 545

What is the most common material for intramedullary nails, and why?

Answer 545

Stainless steel - good strength and stiffness and easy to handle and well-tolerated by body tissues

Question 546

Why is titanium not as good a material for intramedullary nails?

Answer 546

Susceptible to weakening if a hole is drilled across it, or abraded during insertion or locking (notch sensitivity)

Question 547

What is the ‘working length’ of an intramedullary nail?

Answer 547

The length of a nail that transmits load from one main fragment of a fractured bone to the other

Question 548

Why is the working length of an intramedullary nail important?

Answer 548

The stiffness of a nail in both rotation and bending is inversely related to its working length

Question 549

Intramedullary nails are

a) solid
b) hollow?

Answer 549

They can be both!!

Question 550

Why are some nails made less stiff by the use of a longitudinal slot in the wall off a nail?

Answer 550

So that the nails are easier to insert

Question 551

Where are intramedullary nails mostly used? What are their indications for use?

Answer 551

The femur and tibia

• transverse and short oblique fractures
• comminuted fractures
• pathological shaft fractures
• delayed or non-union shaft fractures
• selected open fractures

Question 552

In what 3 configurations can intramedullary nails be used in?

Answer 552

1) a simple nail with no additions
2) nails used with cross screws
3) nails used in combo with plates

Question 553

What is the purpose of adding cross screws to intramedullary nails?

Answer 553

Increases the working length of the bone, as nails only work if in contact with the bone, which is only in the middle part.

Question 554

What is the main use of plates in combination with intramedullary nails?

Answer 554

Femoral neck fractures (esp old ladies with osteoporosis) - restoration is technically difficult, so a plate is added to the lateral side of the femur to add support.

Question 555

What are the complications of intramedullary nails?

Answer 555

Mainly insertion problems -

1) reamers used to widen the intramedullary canal can get stuck
2) nail can be inserted in wrong orientation
3) rotatory misalignment
4) infection

Question 556

Describe the design of an external fixation device?

Answer 556

Pins drilled into the bone to which a metal beam is attached in parallel to the long axis of the bone. The beams and pins provide a stabilising support for the fracture, whilst permitting access to the soft tissues.

Question 557

The use of external fixation can be split into …?

Answer 557

1) Orthopaedic use (non-trauma)

2) post-trauma use

Question 558

List the uses of external fixation in orthopaedics (ie non trauma)?

Answer 558

• limb lengthening
• limb shortening
• joint fusion
• angulatory or rotational deformity correction
• bone segment transportation

Question 559

Post-trauma usage of external fixation can be divided into what 2 groups?

Answer 559

1) Temporary - open fractures with extensive soft tissue damage
2) Definitive - planned us, possibly up until fracture healing

Question 560

What is dynamisation of an external fixator?

Answer 560

A construction consideration of an external fixator, which means it can perform a function of sliding of one fracture fragment relative to another to stimulate callus formation

Question 561

What are the 2 main design requirements of an external fixator?

Answer 561

1) the bone/frame construct should be stable

2) pin placement must not tether soft tissues or restrict access to wounds

Question 562

What are bilateral external fixator frames, and why aren’t they used anymore?

Answer 562

the bone pins cross both cortices and pass through the skin and soft tissues on both sides of the limb - not sued because they cause unacceptable soft tissue tethering and limit limb motion

Question 563

What is the design of unilateral external fixator frames?

Answer 563

The pins pass through the skin on ones side of the limb and enter the proximal cortex and end by passing through the opposite cortex

Question 564

If a unilateral external fixator frame doesn’t confer adequate stability, what compromise can be made?

Answer 564

Pins can be sited at right angles to each other through the same side of the limb to construct an A or V frame

Question 565

Post-fracture, how can external fixator stability be determined?

Answer 565

1) the configuration of the frame
2) the degree of contact between bone ends
3) the extent of soft tissue injury
4) the quality of the bone/pin interface
5) the degree to which the clamps have been tightened
6) the total number of pins used

Question 566

What are the advantages of using external fixators?

Answer 566

1) can be assembled quickly, so good in emergencies
2) can be adjusted later
3) beam of the fixator can be removed to take clear x-rays
4) can be used in many sites without changing the basic model
5) excellent access to soft tissues

Question 567

What are the disadvantages of using external fixators?

Answer 567

1) bone/pin interfaces are site of infection
2) pin loosening - high bending stresses and strains at the bone/pin interfaces
3) inevitable soft tissue tethering by pins between skin and bone, resulting in inhibited movement and discomfort

Question 568

Give 4 complications that can occur with external fixation?

Answer 568

1) fixator complications - modular components can come loose
2) pin loosening
3) pin infection
4) soft tissue tethering