Orthopaedic Implant Mechanics & Materials Flashcards
What are orthopaedic implant devices made from?
Non-biologic materials, usually metals (stainless steel, titanium alloy, cobalt chrome alloy)
What are the 2 general reasons for an implant device?
- provide structural support after injury
2. replace diseased bone
What materials are used in joints for implant devices, and why?
Metal and plastic materials (usually HDP).
Metal to metal contact has been unsuccessful as a bearing surface.
What is the main problem with implant devices and why?
Infection - bacteria is attracted to and adheres to metal and cement surfaces. The immune system works less efficiently in the presence of implants
What are the 5 criteria for a successful orthopaedic implant?
- but not have any short-term and little long-term adverse toxic effects e.g. carcinogenesis
- relieve pain and provide sufficient mobility
- function without failure until it is no longer required
- have a design which predict a guaranteed outcome
- cost-effective
2 main structural requirements for an orthopaedic implant
strength & stability
What structural factors must be considered during implant design?
- strength
- stiffness
- lubrication
- wear
- fatigue
What kinematic factor must be considered during implant design?
Motion - must enable daily living functions, whilst being controlled to enable stability
What are the requirements of an orthopaedic implant that are essential for biocompatability?
- 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
- Functional integration - the implant shouldn’t affect the function of other body parts
What does the term ‘composite structure’ mean in terms of orthopaedic implants and their structural behaviour?
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.
Describe the difference in shape at the end regions of bones, and explain why?
The end regions are wider - this accommodates for the joint
What type of bone is found in the end regions of bones, and explain why?
Cancellous bone - more porous and flexible, so has desirable shock absorbing qualities
How are the trabeculae arranged in cancellous bone?
Aligned along the directions of greatest stress (the directions depend on the directions of natural load placed on the bone)
What is the composition of the bone directly below articular surfaces, and explain why?
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.
Is the Young’s Modulus for a structural material always constant?
It depends - only if the material is not deformed near or over its elastic limit.
Non-biologic structural materials are isotropic - what does this mean?
The mechanical properties of the material are the same no matter what direction it is loaded.
Bone is anisotropic - what does this mean?
Its Young’s modulus is dependent on the direction it is loaded.
Under what direction of loading is cortical bone stiffest and strongest - longitudinal or transverse?
Longitudinal (for all types of loading - tensile, compressive, shear)
Compare the strength of bone in the diaphysis (shaft of a long bone) to the metaphysis (near the ends of bone)?
Bone strength at the metaphysis is around half of that of the diaphysis.
Under what type of loading is bone strongest and weakest?
Strongest - compressive loading
Weakest - shear loading
Bone is viscoelastic - what does this mean?
The stiffness of bone changes with the rate it is loaded. The faster it is loaded the stiffer it becomes.
Does bone have the same value of maximum stress for different types of loading?
No - it has different values of ultimate stress depending on the type of loading (compressive - strongest, shear - weakest)
In terms of implant design, what ways of loading bone are sought to be avoided?
Avoid shear stresses especially, as well as tensile forces.
Aim to load the bone under compressive forces.
Compare the Young’s modulus of cancellous bone to that of cortical bone?
Cancellous bone has a very variable Young’s modulus - from a maximum of 50% of that of cortical bone to less than 0.5%.
Why is it not desirable to create an implant with such a wide range of stiffness like cancellous bone?
- Increased infection risk with such an increase in surface material
- a more flexible material may not provide adequate firmness for attaching or bonding an artificial joint
Explain the concept of ‘stress shielding’ in terms of artificial implants?
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.
What are the consequences of stress shielding in orthopaedic implants?
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.
What is the most major role of all orthopaedic implants?
Provide structural support
Do all types of implants have the same principals/mechanisms of loading bearing?
Yes, on the most part
What are the 2 main types of load support mechanisms in orthopaedic implants?
- Load transfer
2. Load sharing
Explain the term ‘load transfer’?
Part of the applied load (e.g. body weight) in the bone above is transferred to the plate/stem of implant.
Explain the term ‘load sharing’?
Occurs in a middle region, where there is load sharing between the bone and the plate/stem.
Does load transfer occur along the whole of the bone-implant interface?
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).
When load transfer is occurring in composite materials, what are the 2 possible outcomes that can occur at the interface?
- Interface stresses - occurs when the materials are bonded together.
- Relative movement - occurs when the materials are not bonded together, or the bond has come loose.
Load transfer can occur under which types of loading?
- Compressive loading
2. Shear loading
Explain load transfer under compressive loading when the bottom material is more flexible than the top material?
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
Explain load transfer under compressive loading when the top material is more flexible than the top material?
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.
When shear stresses are generated at interfaces, is the shear stress constant across the whole length of the interface?
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.
Why can interface stresses be much higher than theoretical calculations?
- Non-uniformity of contact
2. Non-uniformity of mechanical properties - stiffness, stress concentrations
What is the most common loading type of implant situation to cause osteopenia (bone loss)
Situations where stress shielding occurs i.e. when load transfer occurs in 2 stages (in between is a portion of load sharing).
What factors determine how much load is transferred from bone to plate and vice versa?
It depends how much load is shared in the load sharing region.
This in turn depends on the stiffness of each component.
What 2 factors determine the structural stiffness of a structural component involved in orthopaedic implantation?
- its material stiffness
2. its geometric stiffness
What is the ‘material stiffness’ of a structural component?
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)
What is the ‘geometric stiffness’ of a structural component?
The force required to produce a unit deflection (k) - concerned with the cross-section of the shape.
What is the formula for calculating stiffness (k) of a structural component?
k = EA / L
What are the geometrical factors that affect the axial stiffness of a structural component?
- cross-sectional shape
- length
(based on the equation for stiffness)
When is it appropriate to use Rigidity as a method for indication of stiffness of a structural component?
When length is not a consideration for indicating stiffness, for example when comparing two intramedullary nails or plates of the same length.
What is the equation for axial rigidity?
R = E x A
where
E = young’s modulus
A = area
What is the equation for bending rigidity?
R = E x I
where
E = young’s modulus
I = second moment of area
What is the second moment of area?
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.
What is the equation for torsional rigidity?
R = G x J
where
G = shear modulus
J = polar second moment of area
What is the polar second moment of area?
A measure of how resistant a structure is to torsion.
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?
Bone-implant interface
Must be fixed and free from movement to prevent loosening and ultimate failure
How is fixation between implant and bone achieved?
By different methods, depending on whether the implant is to be removed at a later date.
What is the main use for screws in the fixation of orthopaedic devices?
Mainly for fracture fixation
Why are nuts and bolts rarely used for attaching implants to bone?
Causes more trauma, as needs to be access to bone from both sides (one for the bolt, the other for the nut)
Explain the fixation technique of ‘interference fit’?
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.
What is the consequence of an interference fit being made too tight?
The bone will split
In what fixation situation is ‘interference fit’ used as a method?
Cementless joint replacements
What is the main purpose of using bone cement in fixation?
To act as a filling material between bone and implant, so perfect match is not required. Does not act as an adhesive!
When is bone cement most commonly used?
In stems of joint replacements
Why is bone cement currently used instead of adhesives?
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
What is ‘biological fixation’?
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
What are the 2 main methods of biological fixation?
- using beads of the same material as the metallic implant
2. using a ceramic, eg hydroxyapatite (HAp), the main mineral constituent of bone
Explain the method of using metallic beads in biological fixation?
There are small pores between spherical beads which encourage bone to grow into them.
What is a disadvantage of using metallic beads in biological fixation, and what is done to overcome this?
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.
Explain the method of using ceramics such as hydroxyapatite in biological fixation?
Hydroxyapatite can be sprayed directly onto the metal implant using a technique called plasma spray coating.
What is a disadvantage of using ceramics in biological fixation?
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.
Define a ‘biomaterial’?
A non-biological material, which is used in the body normally to repair/replace failed body parts
It is important that a biomaterial has a high biocompatability - what does this mean?
Interacts well with the body.
What are the 2 main factors which affect biocompatibility?
- body fluids interaction with the material - usually corrosion, which ultimately leads to failure
- effect of the material on body tissues - tends to cause abnormal changes, like allergy, ulceration or cancer.
Define corrosion?
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.
How does corrosion occur in orthopaedic implants?
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.
When is a corrosion reaction most severe - when two metal electrodes are the same or different?
Different
Under what circumstances can an implant made of one alloy behave as two electrodes and lead to corrosion?
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.
What is the main step taken to reduce corrosion of metal implant?
Mixing metals together to form alloys, rather than using pure metals (apart from titanium, which is corrosion resistant in its pure form).
What are the only 3 alloys used in implants?
Stainless steel alloys
Cobalt chrome alloys
Titatanium alloys
Why are metal alloys and titanium metal good for resisting corrosion?
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.
What is ‘fretting corrosion’?
Corrosion which occurs as the result of abrasion between materials in contact, which removes the protective metal oxide layer.
In what situation does fretting corrosion occur?
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
What is ‘crevice corrosion’?
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
What areas are prone to crevice corrosion?
Edges of bone plates and between screws and plates
What are the 2 methods used on metal implants to improve corrosion resistance?
- Nitric acid immersion
2. Titanium nitride coating
What is a recent development in metal corrosion resistance of implants?
The effect of niobium reducing corrosion on 316L stainless steel.
Give 7 adverse effects of implanted materials on the human body?
- Growth of a thin fibrous layer between the implant and body tissue.
- results due to micromotion
- means bone and implant are not fixed together - Local infection.
- presence of foreign material suppress’s body’s ability to fight infection - Body sensitisation to metals.
- Inflammation in regions where corrosion has occurred.
- metal oxide layer is lost and there is contact between metal particles and body tissues. - Tissue necrosis in regions of bone cement.
- Immune reaction due to wear particles from surfaces of joint replacements.
- leads to cell-mediated bone resorption (poorly understood) - Tumours at implant site (long-term)
What is the most common type of stainless steel used for implants?
316L grade stainless steel - a low carbon steel (0.3% carbon)
Why is a low-carbon steel preferred for implants?
To reduce sensitisation of tissues and therefore make it more resistant to corrosion.
What are the main elements making up 316L grade stainless steel?
Iron Chromium Nickel Molybdenum Manganese Silicon, Sulphur, Phosphorus (very small amounts)
Is 316L grade stainless steel completely corrosion resistant?
No - can corrode and crack and high stress and is prone to corrosion resistance
What type of implants is 316L grade steel most suitable for?
Temporary implants, like in fracture fixation (screws and plates)
How are 316L grade stainless steel implants usually made, and why is this method chosen?
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.
What is a disadvantage of forging 316L grade stainless steel?
Steel becomes less ductile, so has a lower fatigue strength.
Why are cobalt chrome alloys preferred to stainless steel for permanent implants?
They have better corrosion resistance, although they are not as strong
What was the original cobalt chrome alloy used in orthopaedics? What is its composition?
Stellite 21
- 65% cobalt
- 30% chromium
- 6% molybdenum
What is a disadvantage of cobalt chrome alloys? What has been done in an attempt to overcome this?
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.
Despite cobalt chrome alloys being less strong than stainless steel alloys, why are they preferred for large permanent implants?
The replacement part is large enough to provide sufficient strength.
Why are stainless steel alloys preferred to cobalt chrome alloys for temporary fixtures, like fracture fixation plates?
The cross-sections of the implants are very small, and cobalt chrome alloys are not strong enough to support.
In pure titanium implants, what process is carried out before implantation?
Anodisation (increasing the thickness of the anti-corrosion layer on the surface)
What mechanical properties of titanium make it the most suitable for use in implants?
Less dense (lighter) and half as stiff than stainless steel or cobalt chrome alloys.
Higher fatigue strength than stainless steel.
What part of a joint replacement is titanium not used for and why?
The bearings in joint replacements - has a low wear resistance.
What are fibre-reinforced plastics?
Composite materials used in fracture fixation, which use very stiff, high strength, but brittle fibres embedded in a flexible resin material.
What is the main advantage to using fibre-reinforced plastics in orthopaedic implants?
Have a very high strength, but a very low stiffness, so are very mechanically compatible to bone.
When did the first attempts at joint replacement begin?
In the 19th century, when prolonged anaesthesia became possible.
Why did the first attempts at joint replacements fail?
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.
What was the first recorded successful hip replacement?
A “Thompson” hemi-arthroplasty:
Replaced the femoral head with steel. Had a long stem driven down into the femoral canal (“press fit”)
Explain the work by McKee and Farrar in the 1950s in the UK?
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.
Why did the work by McKee and Farrar ultimately fail?
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.
In the 1950’s, what were the Judet brothers from France known for?
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.
What is the importance of John Charnley in hip joint replacement?
He was the first to develop a low friction total joint arthroplasty in the early 60s.
What were the most important contributions to joint replacement design by Jon Charnley?
- Designed a smaller femoral head:
- reduce bearing friction and loosening - Introduced the use of bone cement (first was PMMA) between bone and prosthesis:
- help to distribute load - Introduced high-density polyethylene (HDP) as a bearing material:
- lower friction bearing - Produced a system of instrumentation to match his prosthesis
List the 5 performance criteria for an orthopaedic implant?
- Tolerated within the body and provide no short-term and little long-term adverse effects.
- Give pain relief and allow return of normal daily activities.
- Adequate life-span, ideally out-living the patient.
- Should be insertable by a competent surgeon of average ability such that a predictable outcome can be reasonably guaranteed.
- Should be of acceptable cost.
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?
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.
To what extent do hip arthroplasty’s meet this performance criterion:
give pain relief and allow the return of normal daily activities?
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.
What type and range of motion are required from the hip in order to be able to stand, walk and sit down?
- slight extension
- min flexion of 30 degrees
- abduction when weight bearing
- rotate when in full extension
To what extent do hip arthroplasty’s meet this performance criterion:
have adequate life-span, ideally out-living the patient?
Generally - 90% will last 10 years.
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?
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.
To what extent do hip arthroplasty’s meet this performance criterion:
Should be of acceptable cost?
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.
Why is it important to determine the forces acting on the normal hip structure in designing a hip prosthetic implant?
Knowing the forces allows for the stresses to be calculated, which can then be used in the design process of the prosthesis.
What are the 2 main contributors to force in a joint?
- External loads
2. Muscle forces acting at the joint
What are the 2 main experimental methods of estimating stress?
- Strain gauges (experimental method)
2. Finite Element Analysis (computerised method)
The Finite Element Analysis has mostly replaced experimental methods in estimating stresses. What is involved in this method?
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.
What are the advantages of using the Finite Element Analysis in calculating stresses?
Ease of calculation of stresses.
Designs can be compared.
Regions of high stress can be easily identified.
Give 2 reasons why it is so difficult to determine accurately the stresses in the components of a replacement hip?
- Joint loading varies according to the physical activity being undertaken.
- magnitude varies greatly because the hip has a wide range of movement. - 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.
Which type of activity generally generates the largest joint reaction force in the hip: walking ascending stairs descending stairs rising from a chair
ascending stairs (around 7.2x BW)
Which type of activity generally generates the smallest joint reaction force in the hip: walking ascending stairs descending stairs rising from a chair
rising from a chair (around 3x BW)
What is meant by an indeterminate structure?
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).
What activity can be used to help analyse stresses in hip prostheses?
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.
What types of stresses arise in the hip joint?
- Compressive stresses
- Bending stresses
- Hoop stresses due to bending
- Torsional stresses
- Stresses in the acetabulum
How does compressive stress in the femur arise?
How is it calculated?
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
What happens to compressive stresses in the femur when a prosthesis is present?
The compressive force from the stem is transferred to the femur as a shear force.
What is the consequence of a stem-bone bond not being sufficiently strong enough?
The prosthesis will loosen and sink into the medullary cavity.
How are compressive stresses in a hip prosthesis stem calculated?
F/A
where F = compressive load at any section
A = area of that cross-section
Is the compressive load taken by a hip prosthesis stem uniform along its length?
No, it varies (due to load transfer and stress shielding)
What methods are utilised to prevent a hip prosthesis stem from sinking distally into the medullary canal?
- 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
What methods are utilised to reduce interface shear stresses in a hip prosthesis by converting shear loads to compressive loads?
- using a collar at the proximal stem
- tapering the stem
What methods are utilised to avoid fracture of the hip prosthesis stem?
- use a stem with a large enough cross-section to resist the stresses
- use a high-strength material for the stem
What method is utilised to prevent excess stress shielding in a hip prosthesis?
Careful selection of the rigidity of the stem under axial loading.
What causes a bending stress in the femur?
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.
What is the equation for calculating bending stress?
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
Draw a normal femur showing the main forces acting on the bone and how they contribute to producing a bending stress in the bone?
(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.
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?
(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.
How does the presence of a femoral stem affect the magnitude of the bending stresses in the femur?
Reduces the stresses in the proximal end of the femur as the stem takes some of the bending load from the bone
List the important design features of a femoral stem that are important to ensure the stem does not fail under a bending load?
- 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
List the important design features of a femoral stem that are important to prevent the stem loosening under a bending load?
- provide a good enough strong bond between bone and stem or cement
- provide a good press fit
List the important design features of a femoral stem that are important to minimise stress shielding of the bone under bending loads?
- select a suitable rigidity for the stem
Describe what is meant by a ‘hoop stress’?
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.
Where are radial stresses greatest?
points of bone-stem contact at the proximal and distal ends
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?
Inversely proportional.
Stems of short length are prone to high radial stresses on the bone.
Give 2 examples of how hoop stresses occur?
- if the stem is too short
- if there isn’t a good fit of the stem in the medullary cavity
How do torsional stresses within the femur occur?
When the ankle is restrained and the upper body is rotated, the femur experiences a torsional load like the lower leg and knee.
Give 4 examples of how shear stresses in stems can be converted into compressive stresses?
- use non-circular stems
- high shear strength of cement (if used)
- good bonding at interfaces
- surface treatments of the stem to improve interface bonding
Why is it desirable to use non-circular sections for the stem of a femoral component?
To reduce the rotational shear stress.
Why does the normal acetabulum have a high bending strength?
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.
List 5 important design factors for an acetabulum replacement?
- size and conformity of joint replacement surfaces
- affects contact areas, which affects contact stresses - ways to maintain the subchondral cortical bone
- if this broken, the cancellous bone, which isn’t normally loaded, takes most of the load - stiffness and thickness of the cup
- whether or not to use a cup with a metal backing plate
- the technique used to fix the cup to the remaining acetabular bone.
What is bone cement made from?
Polymethyl methacrylate (PMMA) - a polymer
What is the benefit of coating a prosthesis component with PMMA during manufacturing?
The cement filler adheres better to the implant during surgery, and forms a stronger bond therefore a greater resistance to shear forces.
Give two advantages of using cement in a hip replacement?
- The surfaces between bone and implant do not need to be an exact fit, as the gaps can be filled with cement.
- Cement fills in all the gaps between bone and prosthesis, preventing areas of high stress concentrations.
List 3 problems with PMMA cement?
- 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.
- There is always some monomer left after the chemical reaction, which is very toxic and these small fragments cause intense inflammatory reactions.
- 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.
Why is a non-bonded interface undesirable?
- Still a significant amount of abrasive wear as bone rubs against metal.
- 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.
- There is still a small amount of motion between the surfaces, which causes wear particles to be released into tissue.
List 3 ways to improve the integrity of the cement interfaces with bone and metal stems?
- Roughen the stem surface or coat with beads -
improve interlocking of cement to the prosthesis. - Coat the prosthesis with PMMA -
bonds to the inserted cement to give an intimate fit. - Combine PMMA surface coating with a cement that bone can bond to -
cement containing hydroxyapatite.
Explain briefly the mechanism of load transfer from the stem to the femur, with particular reference to the regions of high interface shear stresses?
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.
Explain the influence of stem rigidity on the magnitude of the proximal interface shear stresses?
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.
What is an ‘isoelastic stem’?
Why can they not be used in hip prostheses?
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.
What is the magnitude of stresses in cement dependent on?
Its thickness and its stiffness.
What risks are associated with using a cement layer that is too thin?
- very high cement stresses
2. bone resorption at the proximal femur (due to cement debris causing an adverse tissue reaction)
What is considered to be the optimal thickness for a cement layer?
3-7mm proximally, 2mm distally
What effect does the use of a proximal collar have on load transfer from the stem to the femur?
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.
What is the main argument against using a collar in femoral stems?
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.
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?
- Helps bone ingrowth and potentially eliminates metal debris from bone-metal abrasion.
- Gives opportunity for the bone to bond to a larger area.
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?
Fully coated stems promote stress shielding of the bone.
How does a ‘tapered wedge’ stem help proximal load transfer?
Allows transfer of a significant amount of load in compression, rather than shear.
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?
- Reduce the length of the neck of the stem
- 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.
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?
contact load and material properties of both surfaces
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?
It is one of the least toxic and best wearing as a bearing surface.
What are the two types of wear that occur between bearing surfaces?
Adhesive wear and Abrasive wear
When does adhesive wear occur between bearing surfaces?
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.
When does abrasive wear occur between bearing surfaces?
When surfaces are not perfectly smooth, therefore highly polished surfaces are very important
What are the 3 factors which affect the amount of wear that takes place in a bearing?
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
What is HDP’s main disadvantage as a bearing material?
HDP wear fragments can migrate considerably within an implant, which can cause intense inflammatory tissue reactions and is associated with aseptic loosening.
Give 2 reasons why small diameter heads are used in hip replacements?
- They reduce cup-bone interface shear stresses, which lessens the risk of loosening.
- They produce less volume wear of HDP than large diameter heads
What are the 2 main disadvantages of using a small diameter head in hip replacements?
- The rate of depth of wear is greater than what it would be for a larger head because contact area is less.
- 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
What is the acetabular component usually made of?
HDP, +/- a metal backing between it and its interface with cement or bone.
Give some important features in the design of modern acetabular cups?
- 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
Give 3 acetabular design features that affect the contact pressure at the bearing surface of the hip joint?
- diameter of the cup
- the radial clearance
- thickness of HDP
Why is there a minimum recommended thickness to the HDP cup?
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.
What is the advantage of using an acetabular component with a metal backing plate?
Holds the plastic in place and reduces its tendency to creep and distort, avoiding high contact stresses on the HDP.
What are the 3 steps that lead to acetabular component loosening due to HDP fragments?
- HDP fragments come into contact with bone
- Tissue reaction to the HDP leads to bone resorption.
- The HDP migrates further along the interface, causing further resorption.
The knee joint has a surface which does not provide much stability. What does the joint rely on for stability?
- ligaments
- integrity of the posterior joint capsule
- musculature
List the names of the 4 principal knee ligaments?
Anterior cruciate ligament (ACL)
Posterior cruciate ligament (PCL)
Medial collateral ligament (MCL)
Lateral collateral ligament (LCL)
What is the origin and insertion of the ACL?
What is its main stabilising role?
Origin - the anterior intercondylar region of the tibia
Insertion - posterior femur, in the intercondylar fossa.
Resists anterior subluxation of the tibia over the femur.
What is the origin and insertion of the PCL?
What is its main stabilising role?
Origin - the posterior intercondylar region of the tibia
Insertion - anterior femur, in the intercondylar fossa
Resists posterior subluxation of the tibia onto the femur.
What is the origin and insertion of the LCL?
What is its main stabilising role?
Origin - lateral femoral condyle
Insertion - lateral fibular head
Resists adduction of the joint
What is the origin and insertion of the MCL?
What is its main stabilising role?
Origin - medial femoral condyle
Insertion - medial tibia surface
Resists abduction of the joint
The four ligaments of the knee act together to provide what 2 general stabilsing roles?
- limit distraction of the knee
- limit long axis rotation of the joint
What is the posterior capsule of the knee joint?
What is its role?
a band of tendinous material running across the posterior surface of the knee.
resists hyperextension
How can the LCL and the MCL contribute to knee instability?
- the LCL is longer than the MCL.
- the MCL and the LCL can both become lax.
In terms of knee joint motion, how do the lengths of the ligaments change with flexion/extension?
They don’t! The ligaments move nearly isometrically.
Describe how the axis of rotation of the knee changes with flexion?
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.
The centre of rotation in the knee joint is also known as…?
Instantaneous centre of rotation. - it changes at every instant of motion.
The cruciate ligaments are responsible for the four-bar linkage mechanism the knee joint. What is this?
This constrains the motion of the femur on the tibia so that there is a combination of rolling and sliding motion.
Explain how the cruciate ligaments act as a four-bar linkage mechanism?
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.
Why is the joint reaction force at the knee much greater than body weight?
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.
Explain why the medial knee compartment is more heavily loaded during activities such as walking?
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.
What 2 adverse effects could a high contact force in the medial compartment of the knee have on a joint replacement?
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).
Nearly all knee replacements are made of which materials?
Femoral component - cobalt chrome
Tibial component - HDP
Compare the results of knee to hip in terms of longevity without failure?
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
List the 3 important functional kinematic requirements of a knee prosthesis?
- Should extend to 180 degrees so that person can stand without effort by quadriceps.
- Should flex to 90 degrees.
- Should permit slight axial rotation to maintain natural ligament tension throughout flexion/extension.
Why is the femur cut at about 7 degrees for proper alignment of the femoral component?
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)
What must happen to the posterior capsule of the knee in a knee replacement, and why is this done?
Must be dissected off the back of the femur.
To allow the replacement knee to fully extend.
Why must the collateral ligaments be balanced in equal tension in a knee replacement?
To keep the joint cuts in parallel and prevent any excess medial or lateral opening of the joint.
Compare the cost of hip and knee replacement?
Knee costs around 5x more than hip replacement.
Define the term ‘constraint’ in the context of knee replacements?
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.
What is a ‘fully constrained’ knee prosthesis?
a replacement with linked prostheses, like a hinge between the tibial and femoral component.
What is an ‘unconstrained’ knee prosthesis?
a surface replacement - tibial or femoral.
What is a ‘semi-constrained’ knee prosthesis?
a prosthesis with a constraining mechanism only active in certain degrees of extension
Why is it not helpful to describe modern knee replacements in terms of constraint?
All replacements have some degree of constraint in their movement, and most modern knee replacements fall in to the semi-constrained category.
When is it suitable for a hinged knee prosthesis to be used?
When there are no ligaments intact - the hinge constrains the knee to a single axis of motion so ligaments are not required.
What is the main problem with hinged prostheses?
Prone to loosening - there is no give under lateral loading and rotational loading.
If the PCL is not to be retained in knee replacement, what design consideration for the prosthesis must be taken into account?
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.
What are the advantages to retaining the PCL in knee arthroplasty?
1) provides some antero-posterior stability and proprioceptive activity
2) walking on stairs is more stable
What are the disadvantages to retaining the PCL in knee arthroplasty?
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
What are the advantages to removing the PCL in knee arthroplasty?
1) allows for more congruent joint surfaces, which reduces HDP wear.
2) allows for any deformity correction
Why does a replacement knee need to have a fairly flat tibial plateau when the PCL is retained?
A flat tibial plateau helps provides a kinematic design that allows the PCL to function properly.
In a PCL retaining prosthesis, what are the consequences if the PCL is too loose?
Allows forward movement of the femur on the tibia so the rolling back motion no longer works
In a PCL retaining prosthesis, what are the consequences if the PCL is too tight?
1) restricted flexion, and excessive rolling back of the femur on the tibia.
2) compression of the joint surfaces, leading to contact stresses.
What are the main problems with high density polyethylene (HDP) material in knee arthroplasty?
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
What is the purpose of using stems and pegs on femoral and tibial components of a knee prosthesis?
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.
List the factors that affect the rate of wear of a knee replacement bearing surface?
- the applied load across the bearing surfaces (the normal load)
- the distance the bearing slides
- the hardness of the surface being worn
What is the formula that links the factors that affect the rate of wear of a prosthesis?
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
The volume of wear of a knee prosthesis material is proportional to which particular factor, and why?
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
What effect does diameter of a bearing have on the volume of wear material, and why?
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)
What factor can be used to determine the useful life of a knee prosthesis?
The rate of wear of the HDP component (it is less stiff so wears more than CoCr femoral component)
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?
volume of wear (v) = area of contact (A) x depth of wear (t)
In order to reduce wear, why is it preferable to increase bearing width rather than to increase bearing diameter?
Increasing W and d will increase area of contact, however increasing d increases the volume rate of wear, so it is preferred increase W.
How does the shape of the contact area vary in a knee prosthesis as the joint moves from flexion to extension?
(See pg 20 in notes)
Contact area moves posterior from extension to flexion
What is the loading pattern within the contact area of the tibial plateau in a knee prosthesis?
Central part of contact area - always in compression
Periphery of contact area - always in tension
How does the area of contact and associated loading pattern result in fatigue loading of a tibial component?
Contact points change between compression and tension and loading and unloading, which make a material susceptible to fatigue failure if loads are high.
List the main design features that affect load transfer from the tibial plateau?
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
What is the relationship between thickness of the HDP component and load transfer from the tibial plateau?
The thinner the HDP component, the greater it is stressed.
Non-linear relationship - below 8mm the contact stresses rise steeply.
What is the advantage of the ability to vary the tibial height in a prosthesis?
Allows for ligament re-balancing.
What is the effect of using a metal backing plate on load transfer from the tibial plateau?
(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.
If a metal tray redistributes load well over the tibial plateau, why isn’t this method always used instead of a thick HDP component?
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.
What is the effect of a tibial stem on load transfer from the tibial plateau?
Prevents sinkage into bone, which is the main cause of loosening.
What effect does stiffness of the HDP material have on load transfer from the tibial plateau?
The higher the Young’s modulus, the greater the contact stress - doubling E increases stress by about 40%
How can a tibial peg reduce the tendency for a tibial plateau to sink medially?
The peg takes about 25% of the load which relieves the tibial plateau and the underlying bone of some of the total load
Describe the structure of the femoral component of a modern knee prosthesis?
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.
What are the advantages to designing the femoral component of a prosthesis as symmetrical, rather than copying the natural asymmetry?
Prevents the required inventory from doubling in size.
No need for matched instruments, which keep costs down.
The degree of constraint in a knee prosthesis is largely determined by what factor?
The surface shape design of the tibial component
What is the function of a posterior stabilised knee prosthesis?
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.
Are most knee replacements cemented or cementless?
Cemented (PMMA)
How can additional rotary control be added to knee replacements to prevent tensile stresses arising due to lateral turning moments?
Inserting projections built into the undersurfaces of the tibial components
What is the ultimate mode of failure for a knee replacement?
HDP wear and wear particles
What is the main advantage of not replacing the patella?
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.
What structural design factors must be taken into account for a patellar resurfacing?
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.
Why is a metal-backed patella replacement generally undesirable?
Wears worse - the HDP is not thick enough to distribute the loads so is prone to higher contact stresses because of the metal.
In what condition is it preferable to replacement all knee joint surfaces?
Rheumatoid arthritis
What additional fixation methods are required for cementless knee prostheses?
1) stem in the tibial component
2) press fit and pegs in the femoral component
Meniscal bearing prostheses have a meniscus made of what material?
HDP
How do meniscal bearing designs differ from the design of most knee prostheses?
The meniscus slides at the lower bearing surface for long axis rotation and lateral movement
What is the main disadvantage of meniscal bearing designs?
Increased technical difficulty in achieving ligament balance and overall alignment without risking dislocation of the moving bearing
What is a hemi-arthroplasty?
Replacement of one side of the tibio-femoral joint - either the medial or the lateral side
What is the purpose of hemi-arthroplasty?
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.
Why are two pegs commonly used in unicompartmental surface replacement?
To resist axial rotation
What are augmentation blocks and what is their function?
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.
Give 4 reasons why there has been a lack of success of ankle joint replacements?
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
Arthrodesis of the ankle can only be carried out when the subtalar joint is
a) healthy
b) unhealthy?
Why?
Healthy!
Fusion of the ankle joint means the subtalar joint will be able to act as a dorsiflexor of the foot.
What are the advantages of arthrodesis?
Relieves pain in a stiff ankle joint, without causing loss of movement (as subtalar joint can take over)
What are the disadvantages of arthrodesis?
- 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
What are the main materials used for ankle joint replacements?
A metal (CoCr or stainless steel) and HDP.
What activities of daily living can be restored by ankle replacement, but not by fusion?
Walking and rising from a chair
The short and medium term results of ankle replacement are:
a) good
b) poor?
Poor
What new design of ankle replacement is significantly improving clinical performance?
Meniscal bearings
Give 3 reasons for ankle replacements currently not lasting a reasonable amount of time without revision?
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.
Where are ankle replacement components fixed, and why dont they provide enough support?
Tibial bone - mainly soft cancellous bone which rapidly widens then narrows
Talar bone - area for fixation is very small
Can all competent surgeons carry out ankle replacement?
No - can only be justified in specialist centres
The ankle joint is the articulation between which bones?
What is it also known as?
Tibia and the talus.
Tibio-talar or talocrural joint.
What is the normal range of motion of the ankle joint?
25-30 degrees in both dorsiflexion and plantarflexion
What is the range of ankle motion during walking?
10 degrees dorsiflexion
15 degrees plantarflexion
What type of joint is the ankle joint?
A hinge joint - has a single axis of rotation
Is the axis of rotation of the ankle joint perpendicular to the sagittal plane?
No - it is inclined downwards and posteriorly on the lateral side.
The subtalar joint is the articulation between which bones?
What is it also known as?
The calcaneus and the talus
Talo-calcaneal joint
What movement does the subtalar joint permit?
Inversion and eversion
What is the range of motion of the subtalar joint?
A few degrees
What is the axis of rotation of the subtalar joint in relation to the ankle joint?
It’s at right angles to the ankle joint
In RA, is it usual for one or both the ankle and subtalar joints to be affected?
What is the consequence of this?
Both!
A replacement needs to compensate for both joints, which is difficult to achieve.
What is the importance of the subtalar joint in motion of the foot?
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
Describe the role of the ankle joint in getting out of a chair?
What happens if ankle motion isn’t sufficient?
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.
What are the appropriate maximum vertical and fore-aft loads on the ankle joint?
Vertical: 4-5 xBW
Fore-aft: 0.4-0.7 xBW
What are the 2 types of classification of ankle joint replacement?
Congruent - matching bearing surfaces
Incongruent - non-matching bearing surfaces
Congruent ankle joint replacements only allow which type of movement?
Rotational - and the number of axes of rotation can be limited according to the design
List the 4 types of congruent ankle joint replacement design?
1) spherical
2) spheroidal
c) conical
d) cylindrical
(page 5 of notes)
Describe the design of the spherical congruent ankle joint replacement?
Why does care need to be taken in its positioning?
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.
Compare the spherical and cylindrical designs of a congruent ankle joint replacement, in terms of angles of rotation?
For the same medio-lateral width of bearing surface, a cylindrical shape gives a greater angle of plantarflexion-dorsiflexion rotation
What rotational movements does the spheroidal design of a congruent ankle joint replacement permit?
Planterflexion-dorsiflexion and inversion-eversion.
No axial rotation because its curvature is different in the sagittal and frontal planes.
Does a congruent spheroidal design have any advantages over a spherical ankle joint replacement?
No
What rotational movements does the conical design of a congruent ankle joint replacement permit?
Single axis of plantarflexion/dorsiflexion rotation.
Some medio-lateral resistance
What is the main disadvantage of a congruent conical shape of ankle replacement?
Requires more bone resection than the cylindrical shape
What are the 2 the main disadvantage of a congruent cylindrical shape of ankle replacement?
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
Most congruent ankle joint replacements have been which shape?
Cylindrical
What is the main feature of incongruent ankle joint replacements?
Why is this feature seen as advantageous?
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.
Give 2 examples of types incongruent ankle joint replacement designs?
1) Trochlear (saddle) shape
2) Convex-concave shape
What movements does a trochlear shape of incongruent ankle joint replacement permit?
plantarflexion-dorsiflexion, inversion-eversion and axial rotation
The convex-concave type of incongruent ankle joint replacement can be which two shapes?
1) cylindrical
2) spherical
List 3 disadvantages with incongruent shapes of ankle joint replacements?
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
What is the current failure rate of ankle joint replacement?
65% failure at 5 years
Give 6 reasons for failure of ankle joint replacement?
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
The most successful ankle joint protheses have what features?
- cementless with porous coated surfaces
- hollow tbial bearing surface
- have a meniscal bearing to provide congruent surfaces
What is an advantage of a meniscal bearing ankle prostheses over cylindrical bearing design?
Meniscal bearings provide congruence, which gives lower wear, whilst still giving freedom of motion
What is the role of finite element analysis in contributing to ankle joint prostheses designs?
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.
Upper limb joint replacement encompasses which joints?
The bearing surfaces of the shoulder, elbow, wrist and finger joints
What is the main aim of upper limb joint replacement?
Relieve pain
Secondary aim is to restore function!
What are the typical conditions patients have who are undergoing upper limb joint replacement?
RA OA Osteonecrosis Post-traumatic arthritis Fractures
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?
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).
When several upper limb joints are affected and require replacement how is the decision of priority given?
- most painful replaced first
- if all equal in pain, then work distally to proximally
If several upper limb joints require replacements, why is it advised to work distally to proximally? Give 3 reasons.
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
Why do some surgeons argue to work proximally to distally in upper limb joint replacements? Give 3 reasons.
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.
What materials are used for upper limb joint replacements?
Similar to other orthopaedic implants:
- stainless steel
- titanium alloys
- cobalt chrome alloys
- vitallium alloys
- HDP
Fixation - PMMA cement
What material has been used successfully in flexible wrist and finger joint replacements? Why has it been successful?
Silicone elastomer (rubber) - has excellent biocompatability and biodurability
Generally, have upper limb joint replacements been shown to reduce pain?
Yes - can be eliminated or at least reduced to a tolerable level
Which type of shoulder joint replacement is better at pain relief and providing a good range of motion:
a) total arthroplasty?
b) hemi-arthroplasty?
Total arthroplasty
Compare the survival rates of upper limb joint replacements to lower limb?
Give the main reasons for the difference?
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
Which upper limb joint replacement has been most successful?
Shoulder, then elbow, then wrist and fingers with the worst survival rates
Give a general estimate of survival rates for upper limb joint replacements, and compare this to hip replacements?
80% after ten years (hip around 90% after 10 years)
Can upper limb joint replacements be performed by all competent surgeons with a predictable outcome?
No - too complex, so only performed at specialist centres
Compare the cost of upper limb joint replacements to hip and knee replacements, and explain why there are differences?
Relatively more expensive than hip.
Difference is due to lower frequency of upper limb joint replacements.
When was the first shoulder joint replacement performed?
1893 in Paris by Jules Pean - removed after 2 years due to infection
When were the first successful shoulder joint prostheses developed?
1950s by Charles Neer
What 3 categories are shoulder prostheses designs generally divided into?
According to the amount of movement constraint:
1) unconstrained (Neer prosthesis)
2) semi-constrained (Gristina prosthesis)
3) constrained (Reese prosthesis)
What are ‘reversed’ or ‘inverted anatomy designs’ of shoulder prostheses?
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
Although the primary aim of shoulder replacement is pain relief, what improvement in range of motion can be achieved with an unconstrained design?
90-135 degrees of abduction
The type of shoulder joint replacement design chosen for a patient will depend largely on what factor, and why?
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
What is the primary function of the shoulder?
To allow the hand to be positioned in space
What are 4 articulations at the shoulder joint?
1) glenohumeral
2) acromioclavicular
3) sternoclavicular
4) scapulothoracic
Which shoulder joint articulation is most important and why?
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.
Why is the glenohumeral joint inherently unstable?
Shallow glenoid fossa
How is the instability of the shoulder joint compensated for?
The surrounding soft tissues - mainly rotator cuff
Why is the stability of the soft tissues surrounding the shoulder joint so important for shoulder joint prostheses?
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.
What is the most significant complication of total shoulder joint replacement, and what is the reason for this complication?
Loosening of the glenoid component - occurs because there is a limited amount of bone to which a prosthetic component can be attached.
What are the use of unconstrained shoulder prostheses dependent on?
Intact and functioning rotator cuff mechanism
List 4 advantages of an unconstrained shoulder prosthesis?
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)
What are the use of semi-constrained shoulder prostheses dependent on?
Intact and functioning rotator cuff mechanism, even though some constraint is built into its design
How is some constraint built into a semi-constrained shoulder prosthesis?
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.
What are the disadvantages of semi-constrained shoulder prostheses?
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
What type of design are constrained total shoulder replacements usually?
Ball-in-socket designs
Can be normal anatomy: humeral ball and glenoid socket
Or reverse anatomy: humeral socket and glenoid ball
What are the 3 types of constrained total shoulder replacements used today?
1) Stanmore
2) Michael Reese
3) Trispherical
Describe the design of The Stanmore total shoulder replacement?
- metal on metal design
- cup-shaped glenoid socket
- glenoid socket fixed with 3 pegs and cement
- 2 components are snapped together
List some complications of the Stanmore design total shoulder replacement?
- unsnapping of the components
- instability
- glenoid component loosening
Describe the design of the Michael Reese total shoulder replacement?
- 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
What is the main disadvantage of a Michael Reese total shoulder replacement design?
impingement of the humeral component on the glenoid component, which restricts range of motion
Why is the design of the Trispherical total shoulder replacement unusual?
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
What are the advantages of a trispherical design for total shoulder replacement?
Greater range of motion and avoids impingement
What are the disadvantages of constrained shoulder replacement designs to unconstrained designs?
- higher frequency of loosening
- dislocations are more common
- mechanical failure of components
- disassembly of components
What methods have been used for glenoid component fixation?
- triangular shaped keel
- extended keel
- pegs
- a stem
- a wedge
- a large screw
- flanges bolted to the base of the spine of the scapula
Constrained shoulder replacements tend to have more elaborate glenoid fixation, why is this?
To secure the component against the larger loads present in constrained designs.
What is the primary indication for elective replacement of the elbow joint?
Pain relief
What condition to most patients for elective elbow replacement suffer from?
RA
What are the primary functions of the elbow?
Allow the positioning of the hand in space and allow the forearm to act as a lever
What are the 3 articulations at the elbow joint?
1) humeroulnar (trochleoulnar)
2) humeroradial (radiocapitellar)
3) proximal radioulnar
What loads must the elbow joint be able to sustain?
6-7 xBW in dynamic activities
3 xBW in static loading
Which articulation at the elbow joint supports most of the load?
Humeroulnar
- humeroradial and proximal radiocapitellar provide additional stability
Compare the coronal plane angle between the upper arm and forearm in a:
a) normal elbow
b) uniaxial hinge prostheses?
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.
What max ranges of motion is permitted at the elbow joint?
140 degrees flexion
70 degrees pronation
80 degrees supination
What range of motion is required at the elbow joint for daily living activities?
30-130 degrees flexion
50 degrees pronation
50 degrees supination
Is the stability of the elbow joint provided more by the:
a) congruity of the joint surfaces
b) the soft tissues?
They provide stability equally!!!
During most activities of daily living the elbow is flexed. Which ligament provides the most joint stability in this case?
Medial collateral ligament - over 50% of the joint stability
What is the role of the MCL in the elbow?
Resists valgus stress
What relevance does a stiff shoulder have to the prosthetic replacement of the elbow?
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
What are First generation elbow prostheses also known as, and why?
“constrained” or “hinged” designs
- they were based upon single axis hinges
Who was the first to design a successful total elbow prosthesis, and when did this occur?
Dee - 1968
What are the 2 main problems with uniaxial hinged elbow prostheses?
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
What are the failure rates for constrained hinged elbow prostheses, and what does this mean for their use today?
Around 40% loose in 3-5 years post-op
Constrained prostheses are rarely used now
What are Second generation elbow prostheses also known as?
2 main types:
1) semi-constrained metal-to-polyethylene hinge types
2) unconstrained metal-to-polyethylene resurface types
What is a recent variation of the two main types of second generation elbow prostheses?
Designs that also resurface the radial head
What do semi-constrained elbow joint replacements generally consist of?
Stemmed humeral and ulnar components with a hinged-like metal-to-polyethylene articulation
What is an advantage of a semi-constrained elbow joint replacement?
Give varying degrees of side-to-side laxity
Which type of elbow joint replacement is most suited when there is some soft-tissue insufficiency or loss of bone stock?
Semi-constrained: more stability than unconstrained
Give an example of a semi-constrained elbow joint prosthesis?
Tri-Axial (outcomes are dependent on patient’s condition)
What do unconstrained elbow joint replacements generally consist of?
Resurfacing the lower end of the humerus and olecranon of the ulna, in order to reproduce anatomical structure
Give an example of unconstrained elbow joint prostheses?
Ewald (capitellocondylar)
Kudo
Souter-Strathclyde
What materials do an unconstrained elbow joint replacement usually consist of?
Humeral component - Vitallium
Ulnar component - HDP
Compare semi-constrained and unconstrained elbow replacement designs in terms of:
a) Dislocation
b) Aseptic loosening
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%)
What is the purpose of resurfacing the radial head in elbow replacements?
To gain benefits of load-transmission stability that are afforded by the humeroradial articulation
How successful has resurfacing of the radial head been in elbow replacements?
Variable - difficult to balance the 3 articulations at the time of operation.
What are the 2 types of wrist joint prostheses?
1) flexible hinge
2) total wrist
What is the main indication for prosthetic wrist replacement?
Pain associated with RA or OA
When is arthrodesis preferred to wrist replacement?
When only the radiocarpal joint is affected, especially for younger patients
What bones/structures does the wrist joint incorporate?
The wrist joint extends from the metaphyseal portion of the radius to the carpo-metacarpal joints and incorporates the 8 carpal bones
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?
Its primary function is supination-pronation, so most diseases involving the wrist also affect this joint.
Where is considered to be the composite centre of rotation of the wrist joint? Why?
A fixed point on the capitate - the axes of rotation for both flexion-extension and abduction-adduction pass .
What is the max range of motion of the wrist joint?
Flexion 80-90 degrees
Extension 70-80 degrees
Adduction 35 degrees
Abduction 15-20 degrees
What ranges of motion of the wrist is required for daily activities of living?
Flexion 10 degrees
Extension 35 degrees
Which motion of the wrist is particularly important?
Extension
Which joint is an important contributor to the overall motion of the wrist?
Radiocarpal joint (condyloid joint)
How does the motion of the radiocarpal joint differ in a wrist prosthesis?
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
Describe the structure of a flexible hinge wrist prosthesis?
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.
What material are flexible hinge prostheses made from?
High performance silicone elastomer (rubber)
What percentage of flexible wrist prostheses have been found to tear on the sharp bony edges of the medullary canal?
20%
What method was adopted in flexible wrist prostheses to attempt to reduce tearing from sharp bony edges? Was it successful?
Titanium bone liners (grommets)
Not really - results were similar either way
What is the purpose of using a spacer (the barrel-shaped midsection) in a flexible wrist prosthesis?
Helps maintain adequate joint space and overall wrist alignment. After time a new capsulo-ligamentous system develops around the midsection
Are the stems in a flexible wrist prosthesis fixed?
No - they slide in and out of the intramedullary canals
What are the 2 main designs of cemented total wrist prostheses?
1) Meuli design: ball-ad-socket type
2) Voltz: non-spherical “ball” and shallower socket
Describe the design of the Meuli cemented total wrist prosthesis?
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
Give an advantage and a disadvantage of the Meuli wrist prosthesis ball-and-socket design?
Advantage:
ball-and-socket avoids possibility of rotational failures
Disadvantage:
relies on adequate and proper soft tissue balance to prevent undesirable rotary motion
Describe the design of the Voltz cemented total wrist prosthesis?
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
Describe the shape of the polyethylene cup in the Voltz total wrist prosthesis design and explain its significance?
Not a true sphere - a segment of a torus with a larger radius of curvature for abduction-adduction.
this mirrors the normal radiocarpal joint
What are the 2 main problems associated with total wrist prostheses?
1) Loosening
2) Stress shielding
Why were original finger prosthesis designs unsuccessful?
Migration into the bone, loosening,
mechanical problems
What are the 2 types of MCP joint replacement?
1) flexible hinge
2) total MCP
What type of joint is the MCP?
Condyloid
What provides stability to the MCP joints?
Joint capsule
collateral ligaments
fibrocartilaginous palmar plate
muscle tendons
In what plane does major motion of the MCP joint occur?
Sagittal - up to 90 degrees of flexion
What material is a flexible hinge MCP joint replacement made from?
A flexible silicone elastomer
What does a flexible hinge MCP joint replacement rely upon for stability?
Encapsulation
Describe the design of a flexible hinge MCP joint replacement?
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.
What is the role of grommets in flexible hinge MCP joint replacement?
They shield the implant from sharp bone edges - they are press fitted into position
Describe the design of a total MCP joint prostheses?
Incorporates a metallic component that articulates with a polyethylene component, which are cemented into the medullary canals.
List 3 problems associated with total MCP replacements?
1) implant fracture
2) migration
3) loosening
Why is arthroplasty of the DIPs rarely indicated, and what procedure is usually done instead?
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
What are the 2 types of IP joint replacements?
Flexible hinge and total IP (basically the same as MCPs, except with smaller dimensions)
Functionally which is the most important IP joint, and why?
The PIP
the PIP joint contributes 85% of IP motion, and the DIP only contributes 15%.
Bone is anisotropic. What does this mean?
It displays different mechanical behaviour under different types and directions of loading.
What type of loading is a bone
a) strongest
b) weakest
in?
a) compressive loading
b) shear loading
What 3 factors determine the location and mode of a fracture?
1) the geometry and structure of the bone
2) the loading mode (compression, bending, torsion)
3) the loading rate
In tension and compression loading, the load required to cause failure in a bone is proportional to which feature
The cross-sectional area of the bone.
The larger the area, the stronger and stiffer the bone.
In bending loading, which factors affect bone’s mechanical behaviour?
Cross-sectional area and distribution of bone tissue around a neutral axis (second moment of area)
In torsional loading, what factors affect bone’s mechanical behaviour?
Cross-sectional area and the distribution of one tissue around a neutral axis (polar second moment of area)
In what section of the tibia does torsional fracture commonly occur, and why?
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.
Why is the structure of bone important in fractures?
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.
A bending load to a bone causes what type of fracture, and why?
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.
A pure compression load to a bone will cause what type of fracture?
Oblique
A bending load superimposed on axial compression will cause what type of fracture, and why?
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.
A pure torsional load will cause what type of fracture.
Spiral
What causes a high energy fracture?
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.
Summarise the fracture process?
- Energy delivered to the limb
- Energy is transferred via the soft tissues to the bone which absorbs the energy
- The bone breaks and energy is released back to the soft tissues
- The broken bone and damaged soft tissues bleed and haematoma builds up
- Acute inflammatory response occurs, causing pain.
Describe the process of natural (secondary) bone healing?
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
How long does natural bone healing take?
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
If blood supply is not re-established after fracture, what is the consequence?
No bony union will occur - described as an atrophic or fibrous union
If there is excessive movement at a fracture site, what is the consequence?
Cartilage is laid down rather than bone cells.
Pseudoarthrosis can occur - formation of a false joint bewteen rapidly proliferating cartilage cells at either end.
Define ‘primary bone healing’?
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.
Compare the strength of healed bone that has occurred through secondary healing and primary healing?
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
What is Wolff’s law?
Describes the adaptation of bone to meets the mechanical demands placed on it
What mechanical properties does a callus confer to a broken bone as it heals?
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
What is the main factor which promotes bone healing?
Micromovement and loading along the bone’s long axis.
Give 3 possible factors which may explain why movement at the fracture site influences bone healing?
- Electrical effects caused by moving crystals of hydroxyapatite (the basic mineral constituents of bone)
- Hormonal factors
- Electro-magnetic effects produced through electron flow away from the fracture site
List 3 aims of fracture management?
- save life
- treat pain
- restore function safely and reasonably
Why can a fracture be life threatening?
Blood loss - broken bones bleed! Femoral and pelvic fractures are worst for bleeding
Why does placing a fracture in a splint diminish pain?
Reduces the muscle spasm that occurs around a fracture when it is mobile
What 3 factors determine restoration of function after a fracture?
- severity of injury
- general health of the patient
- location of injury
What are the 2 main processes in fracture treatment?
- Reduction
- restoration of anatomical shape. - Holding
- the bone is held in reduced position until healing has taken place.
What are the 2 types of fracture reduction?
Closed reduction
Open (surgical) reduction
What are the types of fracture holding?
- Plaster of paris
- Traction
- External fixation
- Internal fixation
What are the 3 methods of internal fixation?
- plates and screws
- pins and wires
- rods and nails
Why is fracture fixation necessary?
to minimise deformation or movement between the fracture fragments
What does plaster of Paris consist of?
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.
How are plaster of Paris casts made?
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
What substances can be added to a plaster of Paris mixture to slow down setting?
Retarders - alum and borax.
plaster of Paris itself is made up of two types of…?
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.
What factor mainly determines the properties of the material making up a final splint?
The physical interlocking of the long and short crystals - influenced by how wet the plaster is at the time of application
What are the 2 functions of plaster cast, and which function predominates?
- provide support to the soft tissues to support the broken bone by encasing the limb in rigid exoskeleton - hydraulic theory
- provide 3-point fixation system
no. 1 usually predominates, except in childhood when the tough periosteum provides the 3 point fixation
What 3 factors of a broken bone does a plaster cast control?
- length - prevent shortening
- position - prevent tilt and shift
- rotation - about the long axis of the bone
What are the 3 different types of cast?
Above knee
Cylinder
Below knee
What are the disdvantages of casting?
Stiff muscles and wasting, prolonging overall rehabilitation.
Impairment caused by immobilisation of joints, leading to dependency and prolong hospital stay
How can the problem of immobility of joints caused by plaster casts be overcome?
By using functional braces - Careful moulding and applying hinges into the cast
Describe the design of a functional femoral brace?
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
Describe the design of a functional tibial brace?
The upper third of the tibia is moulded, which achieves rotary control. Extensions to the cast encapture the femoral condyles in knee flexion.
How soon after fracture should functional braces be applied?
2-3 weeks after injury - soft tissue and swelling has settled down
Describe two advantages of a function brace, compared to a full-leg cast?
1) A functional brace allows movement at the joints and helps to reduce muscle wasting due to immobility
2) Functional braces are adjustable
What are the two new classes of materials used in casts instead of plaster of Paris?
1) Isoprene rubbers or polycaprolactone sheets
2) Glass fibre or artificial fibre and polyurethane composites
What advantageous properties do polycaprolactone/isoprene sheets have as a casting material?
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.
What are the disadvantages of polycaprolactone/isoprene sheets as a casting material?
1) expensive
2) require purchase of an oven
3) require considerable skills to use well
How has the disadvantage or requiring considerable skills for applying polycaprolactone/isoprene sheet casts been overcome?
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.
What do fibre/polyurethane composite casts consist of?
glass fibre or fabric woven bandages that are impregnated with a urethane monomer and catalyst
What are the advantages of fibre/polyurethane composite as a casting material?
1) Light and extremely strong, yet flexible (this is because when exposed to warmth/moisture the materials form a true composite)
In what type of fracture situation are fibre/polurethane composite ideal as a casting material?
A reasonably stable, healing fracture as they can form sophisticated shapes and interface well with hinge materials, giving firm anchorage.
In what type of fracture situation are fibre/polyurethane composite not good as a casting material?
As a primary splintage material - difficult to use on unstable swollen limbs because they are conforming rather than being mouldable.
Traction is used as a method of reduction and holding fractures. Although they have the same name, what is the difference between the methods?
Reduction =controlled use of force on a relaxed limb to position bones
Holding = altering muscle tone to maintain a position achieved at reduction
Explain the principle of traction as a method for holding fractures?
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.
What size of load is required for holding traction of the lower limb?
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
What are the two ways which a load can be applied for traction?
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
What are the advantages and disadvantages of skin traction?
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
What are the advantages and disadvantages of skeletal traction?
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
What are the 3 ways of using the principle of traction?
1) static (fixed) traction
2) dynamic traction
3) balanced traction
Describe the set-up of static traction?
The load is applied to the limb and attached to a splint so the splint itself provides the counter force
What is a disadvantage of static traction?
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
What type of patient is static traction most suited for?
Children - they don’t cope well with complicated traction and their fractures heal quickly
Describe the set-up of dynamic traction?
The load is arranged so that the net pull is maintained along the axis of the bone - irrespective of limb position.
What are the 2 functions of the pulleys used in dynamic traction?
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.
Describe the set-up of balanced traction?
A small load is applied to the splint as a whole to draw the pressure off the groin area
What are the clinical complications of traction?
- lying in bed for prolonged period of time
- bed sores
- chest and urinary infections
- disuse atrophy of muscles and bone
Name the factors taken into consideration when deciding on the method that will be used to hold a fracture?
1) the patient
2) the injury
3) the facilities available
4) the skills of the operator
What are the 2 methods of fixing fractures that require surgical intervention?
1) Internal fixation
- bone screws
- bone screws and plates
- intramedullary nails
2) External fixation
- external fixators
What is the main limiting factor when choosing a suitable material for use in surgical fracture fixation?
Biocompatability
What are the 2 main materials use in surgical fracture fixation? Give their advantages and disadvantages?
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
Why is it important to remember that plates and screws must be of the same material in surgical fixation?
Galvanic corrosion can occur if the materials are different
How is a bone screw different from a normal screw?
They don’t! They have the same mechanical principles
What is a screw?
A mechanism that produces linear motion as it is rotated.
What type of screw is used in orthopaedics?
A helix-shaped thread on a shaft.
A screw can only fix 2 objects together if…?
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
What are the 3 factors that determine the strength of a screw fixation?
1) the strength of the screw material
2) the strength of the object material
3) the design of the screw thread
What are the 2 functions of the head of a screw?
1) provides a buttress to stop the whole screw sinking into the bone
2) provides a connection with the screwdriver
What are the different shapes of the drive connection in the head of a screw?
Hexagonal
Crosshead/Philips
Star
Why do bone screws commonly use a hexagonal shaped drive connection?
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 .
Other than hexagonal, what shape of the drive connection in the head of a screw is also very successful?
Star
Why is the shape of the undersurface of a screw head usually rounded?
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.
In a screw, what are the different screw diameters to consider?
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
What determines the strength of a screw?
The smallest diameter - the greater the smallest diameter, the stronger the screw will be
What are the 3 important aspects of the thread of a bone screw?
- shape
- depth
- pitch
What is the shape of the thread of most bone screws? What is the consequence of this shape?
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.
What is the depth of bone screw?
Half the difference between the thread diameter and the core diameter.
What is implication of having a smaller/larger bone screw depth?
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.
What is the pitch of a thread of a bone screw?
The linear distance travelled by the screw for a complete turn (360 degrees) of the screw
List 4 different types of screw tip?
- blunt
- corkscrew
- trocar
- self-tapping
All cancellous screws have what type of tip?
self-tapping
Describe the properties of a self-tapping screw?
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.
Why are cortical screws not-self tapping?
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.
What is ‘lagging’?
The process of compressing two objects together (achieved by screws)
In terms of screw thread, what is the only way that lagging can be achieved?
If the screw thread only grips one of the objects being compressed together.
In what 2 ways can screws achieve a lag effect?
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.
How does a screw which is designed as a lag screw, achieve a lagging effect?
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.
How can a screw be inserted in a way to act as a lag screw, even if it is not designed as one?
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.
Give 5 situations where screws may be used?
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
Why are screw and plate combinations only recommended to fix upper limb forearm fractures now?
Improvements in intramedullary nail and external fixator designs have become more reliable methods to fix lower limb fractures
In what situations are screw-plate combinations used around joints?
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
What is the use of plates in fracture fixation?
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
What is osteosynthesis?
The reconstruction of a fractured bone by surgical and mechanical means
What are bridging plates used for?
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.
Plates must be as compact as possible and malleabe to allow shaping. What is a disadvantage of this?
They have limited capabilities to resist an applied load when stressed in certain directions
What is the main cause of fatigue failure of plates?
Backwards and forwards bending of the fracture plate (stress reversal) as the construct is loaded, due to the fractured bone being inadequately reassembled.
What factors should be considered when using a plate in order to minimise stress reversal?
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
When is the use of plates in fracture fixation indicated?
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
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?
Bone graft
Name 4 areas of the body where plates are commonly used?
Around joints
In the bones of the forearm
On the pelvis
On the face and jaw
Why are plates placed on the tension side of a bone that is eccentrically loaded (uneven loading along its axis)?
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.
Why is contouring a fracture fixation plate before use beneficial?
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
What is a main disadvantage of plating techniques?
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
What thicknesses do pins come in?
0.5 to 2-3 mm
What are pins also known as? How are pins designed?
- 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
What can pins be used in conjunction with to achieve compression between small bone surfaces?
Flexible wires
What is the use of flexible wires?
To induce compression
In what 2 ways do flexible wires induce compression?
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
How do intramedullary nails achieve fracture fixation?
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.
Describe the modern intramedullary nail insertion technique?
Antegrade technique -
the nail is inserted into the bone from one end whilst not disturbing the fracture site at all
When planning an intramedullary nail insertion, what are design developments centred around?
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
What are 3 important design considerations in determining the effectiveness of an intramedullary nail?
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
What is the most common material for intramedullary nails, and why?
Stainless steel - good strength and stiffness and easy to handle and well-tolerated by body tissues
Why is titanium not as good a material for intramedullary nails?
Susceptible to weakening if a hole is drilled across it, or abraded during insertion or locking (notch sensitivity)
What is the ‘working length’ of an intramedullary nail?
The length of a nail that transmits load from one main fragment of a fractured bone to the other
Why is the working length of an intramedullary nail important?
The stiffness of a nail in both rotation and bending is inversely related to its working length
Intramedullary nails are
a) solid
b) hollow?
They can be both!!
Why are some nails made less stiff by the use of a longitudinal slot in the wall off a nail?
So that the nails are easier to insert
Where are intramedullary nails mostly used? What are their indications for use?
The femur and tibia
- transverse and short oblique fractures
- comminuted fractures
- pathological shaft fractures
- delayed or non-union shaft fractures
- selected open fractures
In what 3 configurations can intramedullary nails be used in?
1) a simple nail with no additions
2) nails used with cross screws
3) nails used in combo with plates
What is the purpose of adding cross screws to intramedullary nails?
Increases the working length of the bone, as nails only work if in contact with the bone, which is only in the middle part.
What is the main use of plates in combination with intramedullary nails?
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.
What are the complications of intramedullary nails?
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
Describe the design of an external fixation device?
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.
The use of external fixation can be split into …?
1) Orthopaedic use (non-trauma)
2) post-trauma use
List the uses of external fixation in orthopaedics (ie non trauma)?
- limb lengthening
- limb shortening
- joint fusion
- angulatory or rotational deformity correction
- bone segment transportation
Post-trauma usage of external fixation can be divided into what 2 groups?
1) Temporary - open fractures with extensive soft tissue damage
2) Definitive - planned us, possibly up until fracture healing
What is dynamisation of an external fixator?
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
What are the 2 main design requirements of an external fixator?
1) the bone/frame construct should be stable
2) pin placement must not tether soft tissues or restrict access to wounds
What are bilateral external fixator frames, and why aren’t they used anymore?
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
What is the design of unilateral external fixator frames?
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
If a unilateral external fixator frame doesn’t confer adequate stability, what compromise can be made?
Pins can be sited at right angles to each other through the same side of the limb to construct an A or V frame
Post-fracture, how can external fixator stability be determined?
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
What are the advantages of using external fixators?
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
What are the disadvantages of using external fixators?
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
Give 4 complications that can occur with external fixation?
1) fixator complications - modular components can come loose
2) pin loosening
3) pin infection
4) soft tissue tethering
