Orthopaedic Implant Mechanics & Materials Flashcards

Question

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

Answer 1

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

Answer 2

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.

Answer 3

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.

Answer 4

Provide structural support

Answer 5

Yes, on the most part

Answer 6

1. Load transfer | 2. Load sharing

Answer 7

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

Answer 8

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

Answer 9

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).

Answer 10

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

Answer 11

1. Compressive loading | 2. Shear loading

Answer 12

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

Answer 13

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.

Answer 14

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.

Answer 15

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

Answer 16

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

Answer 17

It depends how much load is shared in the load sharing region. This in turn depends on the stiffness of each component.

Answer 18

1. its material stiffness | 2. its geometric stiffness

Answer 19

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)

Answer 20

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

Answer 21

k = EA / L

Answer 22

1. cross-sectional shape 2. length (based on the equation for stiffness)

Answer 23

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

Answer 24

R = E x A where E = young's modulus A = area

Answer 25

R = E x I where E = young's modulus I = second moment of area

Answer 26

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.

Answer 27

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

Answer 28

A measure of how resistant a structure is to torsion.

Answer 29

Bone-implant interface Must be fixed and free from movement to prevent loosening and ultimate failure

Answer 30

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

Answer 31

Mainly for fracture fixation

Answer 32

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

Answer 33

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.

Answer 34

The bone will split

Answer 35

Cementless joint replacements

Answer 36

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

Answer 37

In stems of joint replacements

Answer 38

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

Answer 39

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

Answer 40

1. using beads of the same material as the metallic implant | 2. using a ceramic, eg hydroxyapatite (HAp), the main mineral constituent of bone

Answer 41

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

Answer 42

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.

Answer 43

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

Answer 44

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.

Answer 45

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

Answer 46

Interacts well with the body.

Answer 47

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

Answer 48

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.

Answer 49

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.

Answer 50

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.

Answer 51

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

Answer 52

Stainless steel alloys Cobalt chrome alloys Titatanium alloys

Answer 53

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.

Answer 54

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

Answer 55

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

Answer 56

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

Answer 57

Edges of bone plates and between screws and plates

Answer 58

1. Nitric acid immersion | 2. Titanium nitride coating

Answer 59

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

Answer 60

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

Answer 61

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

Answer 62

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

Answer 63

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

Answer 64

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

Answer 65

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

Answer 66

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.

Answer 67

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

Answer 68

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

Answer 69

Stellite 21 - 65% cobalt - 30% chromium - 6% molybdenum

Answer 70

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.

Answer 71

The replacement part is large enough to provide sufficient strength.

Answer 72

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

Answer 73

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

Answer 74

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

Answer 75

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

Answer 76

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

Answer 77

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

Answer 78

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

Answer 79

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.

Answer 80

A "Thompson" hemi-arthroplasty: | Replaced the femoral head with steel. Had a long stem driven down into the femoral canal ("press fit")

Answer 81

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.

Answer 82

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.

Answer 83

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.

Answer 84

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

Answer 85

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

Answer 86

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

Answer 87

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.

Answer 88

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.

Answer 89

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

Answer 90

Generally - 90% will last 10 years.

Answer 91

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.

Answer 92

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.

Answer 93

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

Answer 94

1. External loads | 2. Muscle forces acting at the joint

Answer 95

1. Strain gauges (experimental method) | 2. Finite Element Analysis (computerised method)

Answer 96

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.

Answer 97

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

Answer 98

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

Answer 99

ascending stairs (around 7.2x BW)

Answer 100

rising from a chair (around 3x BW)

Answer 101

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).

Answer 102

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.

Answer 103

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

Answer 104

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

Answer 105

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

Answer 106

The prosthesis will loosen and sink into the medullary cavity.

Answer 107

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

Answer 108

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

Answer 109

- 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

Answer 110

- using a collar at the proximal stem | - tapering the stem

Answer 111

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

Answer 112

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

Answer 113

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.

Answer 114

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

Answer 115

(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.

Answer 116

(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.

Answer 117

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

Answer 118

- 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

Answer 119

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

Answer 120

- select a suitable rigidity for the stem

Answer 121

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.

Answer 122

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

Answer 123

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

Answer 124

- if the stem is too short | - if there isn't a good fit of the stem in the medullary cavity

Answer 125

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

Answer 126

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

Answer 127

To reduce the rotational shear stress.

Answer 128

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.

Answer 129

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

Answer 130

Polymethyl methacrylate (PMMA) - a polymer

Answer 131

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

Answer 132

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

Answer 133

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

Answer 134

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

Answer 135

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

Answer 136

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.

Answer 137

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.

Answer 138

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.

Answer 139

Its thickness and its stiffness.

Answer 140

1. very high cement stresses | 2. bone resorption at the proximal femur (due to cement debris causing an adverse tissue reaction)

Answer 141

3-7mm proximally, 2mm distally

Answer 142

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.

Answer 143

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.

Answer 144

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

Answer 145

Fully coated stems promote stress shielding of the bone.

Answer 146

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

Answer 147

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

Answer 148

contact load and material properties of both surfaces

Answer 149

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

Answer 150

Adhesive wear and Abrasive wear

Answer 151

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.

Answer 152

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

Answer 153

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

Answer 154

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

Answer 155

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

Answer 156

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

Answer 157

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

Answer 158

- 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

Answer 159

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

Answer 160

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.

Answer 161

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

Answer 162

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

Answer 163

- ligaments - integrity of the posterior joint capsule - musculature

Answer 164

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

Answer 165

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

Answer 166

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

Answer 167

Origin - lateral femoral condyle Insertion - lateral fibular head Resists adduction of the joint

Answer 168

Origin - medial femoral condyle Insertion - medial tibia surface Resists abduction of the joint

Answer 169

- limit distraction of the knee | - limit long axis rotation of the joint

Answer 170

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

Answer 171

- the LCL is longer than the MCL. | - the MCL and the LCL can both become lax.

Answer 172

They don't! The ligaments move nearly isometrically.

Answer 173

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.

Answer 174

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

Answer 175

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

Answer 176

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.

Answer 177

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.

Answer 178

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.

Answer 179

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).

Answer 180

Femoral component - cobalt chrome Tibial component - HDP

Answer 181

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

Answer 182

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

Answer 183

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)

Answer 184

Must be dissected off the back of the femur. To allow the replacement knee to fully extend.

Answer 185

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

Answer 186

Knee costs around 5x more than hip replacement.

Answer 187

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.

Answer 188

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

Answer 189

a surface replacement - tibial or femoral.

Answer 190

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

Answer 191

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

Answer 192

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

Answer 193

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

Answer 194

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.

Answer 195

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

Answer 196

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

Answer 197

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

Answer 198

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

Answer 199

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

Answer 200

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

Answer 201

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

Answer 202

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.

Answer 203

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

Answer 204

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

Answer 205

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

Answer 206

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)

Answer 207

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

Answer 208

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

Answer 209

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

Answer 210

(See pg 20 in notes) Contact area moves posterior from extension to flexion

Answer 211

Central part of contact area - always in compression Periphery of contact area - always in tension

Answer 212

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

Answer 213

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

Answer 214

The thinner the HDP component, the greater it is stressed. Non-linear relationship - below 8mm the contact stresses rise steeply.

Answer 215

Allows for ligament re-balancing.

Answer 216

(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.

Answer 217

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.

Answer 218

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

Answer 219

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

Answer 220

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

Answer 221

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.

Answer 222

Prevents the required inventory from doubling in size. No need for matched instruments, which keep costs down.

Answer 223

The surface shape design of the tibial component

Answer 224

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.

Answer 225

Cemented (PMMA)

Answer 226

Inserting projections built into the undersurfaces of the tibial components

Answer 227

HDP wear and wear particles

Answer 228

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.

Answer 229

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.

Answer 230

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

Answer 231

Rheumatoid arthritis

Answer 232

1) stem in the tibial component | 2) press fit and pegs in the femoral component

Answer 233

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

Answer 234

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

Answer 235

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

Answer 236

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.

Answer 237

To resist axial rotation

Answer 238

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.

Answer 239

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

Answer 240

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

Answer 241

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

Answer 242

- 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

Answer 243

A metal (CoCr or stainless steel) and HDP.

Answer 244

Walking and rising from a chair

Answer 245

Meniscal bearings

Answer 246

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.

Answer 247

Tibial bone - mainly soft cancellous bone which rapidly widens then narrows Talar bone - area for fixation is very small

Answer 248

No - can only be justified in specialist centres

Answer 249

Tibia and the talus. Tibio-talar or talocrural joint.

Answer 250

25-30 degrees in both dorsiflexion and plantarflexion

Answer 251

10 degrees dorsiflexion | 15 degrees plantarflexion

Answer 252

A hinge joint - has a single axis of rotation

Answer 253

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

Answer 254

The calcaneus and the talus Talo-calcaneal joint

Answer 255

Inversion and eversion

Answer 256

A few degrees

Answer 257

It's at right angles to the ankle joint

Answer 258

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

Answer 259

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

Answer 260

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.

Answer 261

Vertical: 4-5 xBW Fore-aft: 0.4-0.7 xBW

Answer 262

Congruent - matching bearing surfaces Incongruent - non-matching bearing surfaces

Answer 263

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

Answer 264

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

Answer 265

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.

Answer 266

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

Answer 267

Planterflexion-dorsiflexion and inversion-eversion. No axial rotation because its curvature is different in the sagittal and frontal planes.

Answer 268

Single axis of plantarflexion/dorsiflexion rotation. | Some medio-lateral resistance

Answer 269

Requires more bone resection than the cylindrical shape

Answer 270

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

Answer 271

Cylindrical

Answer 272

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.

Answer 273

1) Trochlear (saddle) shape | 2) Convex-concave shape

Answer 274

plantarflexion-dorsiflexion, inversion-eversion and axial rotation

Answer 275

1) cylindrical | 2) spherical

Answer 276

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

Answer 277

65% failure at 5 years

Answer 278

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

Answer 279

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

Answer 280

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

Answer 281

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.

Answer 282

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

Answer 283

Relieve pain Secondary aim is to restore function!

Answer 284

``` RA OA Osteonecrosis Post-traumatic arthritis Fractures ```

Answer 285

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).

Answer 286

- most painful replaced first | - if all equal in pain, then work distally to proximally

Answer 287

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

Answer 288

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.

Answer 289

Similar to other orthopaedic implants: - stainless steel - titanium alloys - cobalt chrome alloys - vitallium alloys - HDP Fixation - PMMA cement

Answer 290

Silicone elastomer (rubber) - has excellent biocompatability and biodurability

Answer 291

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

Answer 292

Total arthroplasty

Answer 293

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

Answer 294

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

Answer 295

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

Answer 296

No - too complex, so only performed at specialist centres

Answer 297

Relatively more expensive than hip. Difference is due to lower frequency of upper limb joint replacements.

Answer 298

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

Answer 299

1950s by Charles Neer

Answer 300

According to the amount of movement constraint: 1) unconstrained (Neer prosthesis) 2) semi-constrained (Gristina prosthesis) 3) constrained (Reese prosthesis)

Answer 301

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

Answer 302

90-135 degrees of abduction

Answer 303

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

Answer 304

To allow the hand to be positioned in space

Answer 305

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

Answer 306

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.

Answer 307

Shallow glenoid fossa

Answer 308

The surrounding soft tissues - mainly rotator cuff

Answer 309

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.

Answer 310

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

Answer 311

Intact and functioning rotator cuff mechanism

Answer 312

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)

Answer 313

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

Answer 314

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.

Answer 315

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

Answer 316

Ball-in-socket designs Can be normal anatomy: humeral ball and glenoid socket Or reverse anatomy: humeral socket and glenoid ball

Answer 317

1) Stanmore 2) Michael Reese 3) Trispherical

Answer 318

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

Answer 319

- unsnapping of the components - instability - glenoid component loosening

Answer 320

- 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

Answer 321

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

Answer 322

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

Answer 323

Greater range of motion and avoids impingement

Answer 324

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

Answer 325

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

Answer 326

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

Answer 327

Pain relief

Answer 328

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

Answer 329

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

Answer 330

6-7 xBW in dynamic activities 3 xBW in static loading

Answer 331

Humeroulnar - humeroradial and proximal radiocapitellar provide additional stability

Answer 332

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.

Answer 333

140 degrees flexion 70 degrees pronation 80 degrees supination

Answer 334

30-130 degrees flexion 50 degrees pronation 50 degrees supination

Answer 335

They provide stability equally!!!

Answer 336

Medial collateral ligament - over 50% of the joint stability

Answer 337

Resists valgus stress

Answer 338

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

Answer 339

"constrained" or "hinged" designs - they were based upon single axis hinges

Answer 340

Dee - 1968

Answer 341

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

Answer 342

Around 40% loose in 3-5 years post-op Constrained prostheses are rarely used now

Answer 343

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

Answer 344

Designs that also resurface the radial head

Answer 345

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

Answer 346

Give varying degrees of side-to-side laxity

Answer 347

Semi-constrained: more stability than unconstrained

Answer 348

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

Answer 349

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

Answer 350

Ewald (capitellocondylar) Kudo Souter-Strathclyde

Answer 351

Humeral component - Vitallium Ulnar component - HDP

Answer 352

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%)

Answer 353

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

Answer 354

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

Answer 355

1) flexible hinge | 2) total wrist

Answer 356

Pain associated with RA or OA

Answer 357

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

Answer 358

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

Answer 359

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

Answer 360

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

Answer 361

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

Answer 362

Flexion 10 degrees | Extension 35 degrees

Answer 363

Radiocarpal joint (condyloid joint)

Answer 364

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

Answer 365

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.

Answer 366

High performance silicone elastomer (rubber)

Answer 367

Titanium bone liners (grommets) Not really - results were similar either way

Answer 368

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

Answer 369

No - they slide in and out of the intramedullary canals

Answer 370

1) Meuli design: ball-ad-socket type | 2) Voltz: non-spherical "ball" and shallower socket

Answer 371

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

Answer 372

Advantage: ball-and-socket avoids possibility of rotational failures Disadvantage: relies on adequate and proper soft tissue balance to prevent undesirable rotary motion

Answer 373

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

Answer 374

Not a true sphere - a segment of a torus with a larger radius of curvature for abduction-adduction. this mirrors the normal radiocarpal joint

Answer 375

1) Loosening | 2) Stress shielding

Answer 376

Migration into the bone, loosening, | mechanical problems

Answer 377

1) flexible hinge | 2) total MCP

Answer 378

Joint capsule collateral ligaments fibrocartilaginous palmar plate muscle tendons

Answer 379

Sagittal - up to 90 degrees of flexion

Answer 380

A flexible silicone elastomer

Answer 381

Encapsulation

Answer 382

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.

Answer 383

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

Answer 384

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

Answer 385

1) implant fracture 2) migration 3) loosening

Answer 386

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

Answer 387

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

Answer 388

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

Answer 389

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

Answer 390

a) compressive loading | b) shear loading

Answer 391

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

Answer 392

The cross-sectional area of the bone. The larger the area, the stronger and stiffer the bone.

Answer 393

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

Answer 394

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

Answer 395

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.

Answer 396

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.

Answer 397

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.

Answer 398

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.

Answer 399

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.

Answer 400

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

Answer 401

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

Answer 402

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

Answer 403

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

Answer 404

Cartilage is laid down rather than bone cells. Pseudoarthrosis can occur - formation of a false joint bewteen rapidly proliferating cartilage cells at either end.

Answer 405

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.

Answer 406

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

Answer 407

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

Answer 408

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

Answer 409

Micromovement and loading along the bone's long axis.

Answer 410

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

Answer 411

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

Answer 412

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

Answer 413

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

Answer 414

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

Answer 415

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

Answer 416

Closed reduction Open (surgical) reduction

Answer 417

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

Answer 418

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

Answer 419

to minimise deformation or movement between the fracture fragments

Answer 420

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.

Answer 421

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

Answer 422

Retarders - alum and borax.

Answer 423

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.

Answer 424

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

Answer 425

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

Answer 426

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

Answer 427

Above knee Cylinder Below knee

Answer 428

Stiff muscles and wasting, prolonging overall rehabilitation. Impairment caused by immobilisation of joints, leading to dependency and prolong hospital stay

Answer 429

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

Answer 430

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

Answer 431

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

Answer 432

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

Answer 433

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

Answer 434

1) Isoprene rubbers or polycaprolactone sheets | 2) Glass fibre or artificial fibre and polyurethane composites

Answer 435

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.

Answer 436

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

Answer 437

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.

Answer 438

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

Answer 439

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

Answer 440

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

Answer 441

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

Answer 442

Reduction =controlled use of force on a relaxed limb to position bones Holding = altering muscle tone to maintain a position achieved at reduction

Answer 443

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.

Answer 444

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

Answer 445

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

Answer 446

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

Answer 447

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

Answer 448

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

Answer 449

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

Answer 450

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

Answer 451

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

Answer 452

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

Answer 453

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.

Answer 454

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

Answer 455

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

Answer 456

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

Answer 457

1) Internal fixation - bone screws - bone screws and plates - intramedullary nails 2) External fixation - external fixators

Answer 458

Biocompatability

Answer 459

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

Answer 460

Galvanic corrosion can occur if the materials are different

Answer 461

They don't! They have the same mechanical principles

Answer 462

A mechanism that produces linear motion as it is rotated.

Answer 463

A helix-shaped thread on a shaft.

Answer 464

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

Answer 465

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

Answer 466

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

Answer 467

Hexagonal Crosshead/Philips Star

Answer 468

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 .

Answer 469

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.

Answer 470

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

Answer 471

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

Answer 472

- shape - depth - pitch

Answer 473

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.

Answer 474

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

Answer 475

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.

Answer 476

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

Answer 477

- blunt - corkscrew - trocar - self-tapping

Answer 478

self-tapping

Answer 479

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.

Answer 480

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.

Answer 481

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

Answer 482

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

Answer 483

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.

Answer 484

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.

Answer 485

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.

Answer 486

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

Answer 487

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

Answer 488

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

Answer 489

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

Answer 490

The reconstruction of a fractured bone by surgical and mechanical means

Answer 491

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.

Answer 492

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

Answer 493

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

Answer 494

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

Answer 495

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

Answer 496

Bone graft

Answer 497

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

Answer 498

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.

Answer 499

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

Answer 500

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

Answer 501

0.5 to 2-3 mm

Answer 502

- 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

Answer 503

Flexible wires

Answer 504

To induce compression

Answer 505

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

Answer 506

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.

Answer 507

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

Answer 508

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

Answer 509

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

Answer 510

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

Answer 511

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

Answer 512

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

Answer 513

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

Answer 514

They can be both!!

Answer 515

So that the nails are easier to insert

Answer 516

The femur and tibia - transverse and short oblique fractures - comminuted fractures - pathological shaft fractures - delayed or non-union shaft fractures - selected open fractures

Answer 517

1) a simple nail with no additions 2) nails used with cross screws 3) nails used in combo with plates

Answer 518

Increases the working length of the bone, as nails only work if in contact with the bone, which is only in the middle part.

Answer 519

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.

Answer 520

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

Answer 521

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.

Answer 522

1) Orthopaedic use (non-trauma) | 2) post-trauma use

Answer 523

- limb lengthening - limb shortening - joint fusion - angulatory or rotational deformity correction - bone segment transportation

Answer 524

1) Temporary - open fractures with extensive soft tissue damage 2) Definitive - planned us, possibly up until fracture healing

Answer 525

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

Answer 526

1) the bone/frame construct should be stable | 2) pin placement must not tether soft tissues or restrict access to wounds

Answer 527

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

Answer 528

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

Answer 529

Pins can be sited at right angles to each other through the same side of the limb to construct an A or V frame

Answer 530

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

Answer 531

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

Answer 532

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

Answer 533

1) fixator complications - modular components can come loose 2) pin loosening 3) pin infection 4) soft tissue tethering