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AY BMSc Implant Technology > Unit 2: Hip Joint Replacement > Flashcards

Unit 2: Hip Joint Replacement Flashcards

1
Q

What materials are generally used for hip joint replacement?

A

Cobalt chrome or titanium snd HDP

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2
Q

What is the essential functional range of the hip for daily living?

A

Extend slightly
Flex to min 30 degrees
Abduct when weight bearing
Rotate when in full extension

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3
Q

What are the two methods for estimating stresses in the hip?

A

Measuring (strain gauge)

Finite Element Analysis

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4
Q

How many groups of muscles act across the hip joint?

A

7

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5
Q

What does it mean when it is said that the hip joint as a structure is indeterminate?

A

The forces acting on the femur and the pelvis and across the joint cannot be calculated precisely and must be approximated

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6
Q

Why is standing on one leg used to calculated hip joint forces?

A

Some muscles are not active at all leaving mainly the abductor muscle forces to calculate

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7
Q

In which plane do the highest moments occur in the hip?

A

Coronal plane

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8
Q

How is compressive stress calculated in the femur?

A

Compressive force / area

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9
Q

How is the compressive joint force transferred from the stem to the femur?

A

As a shear force

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10
Q

Name 4 methods of preventing the stem from sinking distally in the medullary canal?

A

Tapering the stem
Using a collar at the proximal end of the stem
Fix the bone to the stem (bone ingrowth or adhesion)
Using cement strong enough to withstand shear stresses

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11
Q

Name 2 ways to reduce interface shear stresses by converting shear loads to compressive loads?

A

Using a support (e.g. proximal collar on the stem)

By tapering the stem

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12
Q

Name 2 ways to avoid fracture of the stem

A

Stem with large enough cross section to resist the stresses

Use high strength material for the stem

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13
Q

How can stress shielding of the bone be avoided as much as possible?

A

By careful selection of the rigidity of the stem under axial loading

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14
Q

What equation can be used to calculate bending stress in the femur?

A

stress = My / I

M = bending moment 
y = distance from neutral axis 
I = second moment of area
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15
Q

What effect does inserting a femoral stem have on the bending stresses in the femur?

A

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

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16
Q

What is the main likelihood of stem failure?

A

If it loosens proximally ( bending moment at the distal end increases drastically leading to failure)

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17
Q

Why is the stem more stressed than the adjacent bone at any point along the stem?

A

The value of I is smaller (because it’s cross-sectional dimensions are smaller)

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18
Q

What 2 design factors should be considered to ensure that the stem does not fail under a bending load?

A

Design stem with a large enough second moment of area

Design stem shape to limit the magnitude of the bending moment sue to the joint force

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19
Q

In what ways can you avoid stem loosening?

A

Providing sufficiently strong between the bone and the stem or cement
Providing good press fit of the stem in the medullary canal

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20
Q

How can stress shielding e minimised under bending loads?

A

Select suitable rigidity for the stem

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21
Q

What are radial stresses and when are they greatest?

A

Stresses that are directed radially outwards from a central point - greatest at points of bone-stem contact at the proximal and distal ends

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22
Q

How do hoop stresses occur?

A

Under the action of a bending load radial stresses are generated which in turn cause hoop stresses (tensile stresses that act in a direction that tends to split the bone )

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23
Q

How does the length of the stem affect the radial stresses?

A

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

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24
Q

What design factors can help in avoiding excessive hoop stresses?

A

Ensuring stem is long enough

Provide a good fit of the stem in the medullary cavity

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25
Q

Why must a stem be secure against torsional loads?

A

Stem will loosen and rotate relative to the bone once loose stem may also sink under the action of a compressive load

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26
Q

What are the important design factors to consider when thinking about torsional shear forces?

A

Use non circular sections
Shear strength of cement
Good bonding at the bone-cement and cement-implant interfaces
Surface treatments of the stem to improve interface bonding

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27
Q

What is the structure of the acetabulum?

A

Sandwich of cancellous bone between 2 layers of cortical bone - one covered with articular cartilage forming the joint bearing surface

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28
Q

What type of stress is the acetabulum most subjected to?

A

Compressive stress

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29
Q

Why are their higher concentrations if the cortical shell is broken in the acetabulum of a replacement hip?

A

Replacement femoral head and cup usually have a smaller diameter than the natural components

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30
Q

What are the important design factors to consider for stresses in the acetabulum?

A

Size and conformity of replacement joint surfaces
Ways to maintain the integrity of subchondral cortical bone
Stiffness and thickness of the cup
Thickness of the cement
To use cup with a metal backing plate or not
technique used to fix the cup to the remaining acetabular bone

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31
Q

What is bone cement made from?

A

Polymethyl methacrylate (PMMA)

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32
Q

How is PMMA made?

A

Monomer (powder) is mixed with volatile agent containing a catalyst - it is mixed to a doughy consistency and remains plastic long enough to be inserted into the appropriate cavity

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33
Q

What additives are often included in cement?

A

Antibiotics

Radio-opaque material (usually barium sulphate)

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34
Q

Why are some prostheis components coated with a layer of PMMA cement during the manufacturing process?

A

If surface remains clean during implantation cement filler inserted during surgery will adhere to the implant forming a stronger bond and greater resistance to shear forces

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35
Q

When can a physical bond form with PMMA cement?

A

Can form with small trabeculae of the cancellous bone of the femur
On icroscopic scratches and scores of machining and polishing marks on the mettal of the prosthesis

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36
Q

What are the advantages of cemented prostheses?

A

Do not need exact geometrical match (no need to cut bone)

Allows even stress distribution

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37
Q

What are the disadvantages of cement?

A

Exothermic reaction
Small fragments can cause intense inflammatory reactions or fall into joints
Low tensile and shear strength (as does not bond chemically micromotion can cause loosening)

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38
Q

Why have their been very few advancements in types of cement?

A

Classed as drug - v expensive

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39
Q

What tends to form at an interface unless there is good bonding?

A

Layer of fibrous tissue

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40
Q

What problems does a layer of fibrous tissue between the implant and bone cause?

A

It has no tensile or shear strength

It prevents proper ingrowth of bone into hydroxyapatite coated prostheses

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41
Q

What can be done to minimise the fibrous layer?

A

Micromotion kept to a minimum after surgery (accurate fit of prosthesis at surgery followed by controlled weight bearing)

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42
Q

What methods can be used to improve the longevity of cemented prostheses?

A

Increase keying of the cement to the prosthesis
Coat metal components with PMMA
Combine PMMA surface coating with a cement that bone can bond to
Make stem smooth and allow it to sink into the canal and fit

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43
Q

How can the keying of the cement to the prosthesis be increased?

A

Roughen prosthesis surface or coat with beads or wires (but can cause abrasive wear if unsuccessful)

44
Q

In very simple terms what in the load transfer mechanism for an intramedullary stem?

A

The stem transfers some of its load to the bone proximally, the rest distally with a central region of load sharing

45
Q

How does the rigidity of the stem affect how the load is shared?

A

The more rigid the stem the more load it takes proximally - causing stress shielding

46
Q

What is the downside of a less stiff stem?

A

Generates higher shear stresses at the proximal bone-stem interface or if cement is used at the bone-cement-stem interfaces and in the cement itself

47
Q

What is the balance between when considering rigidity of a stem?

A

Proximal stress shielding and keeping proximal interface shear stresses low

48
Q

How is the load taken by the bone in the central load sharing region calculated?

A

Rigidity of bone divided by total rigidity

49
Q

How is the load taken by the stem calculated in the load sharing region?

A

Rigidity of stem divided by total rigidity

50
Q

Why is cement not involved in calculating the loads taken in the load sharing region?

A

Rigidity of cement is so low that we can assume that it takes a very small proportion of the load

51
Q

How is load transferred from the stem to the bone?

A

As a shear force

52
Q

What are isoelastic stems and why are they not used?

A

Stems with the same stiffness as bone

Improve stress shielding but generate high shear stresses at the interface causing failure

53
Q

How does cement affect load transfer?

A

Allows good contact avoiding stress concentrations

Stresses in cement depends on its thickness

54
Q

What problems can be caused by too thin a cement layer?

A

Very high cement stresses and can cause bone resorption at the proximal end of the femur

55
Q

What problems can be caused by too thick a cement layer?

A

High cement stresses

56
Q

Why is a more flexible cement preferred to a stiffer cement?

A

Stiffer cement results in higher proximal and distal cement and interface stresses

57
Q

Why might a collar be used in a replacement?

A

Allows compressive load transfer from stem to bone (reduces stress shielding and lowers stresses in the cement in the proximal medial region)

58
Q

What is the calcar?

A

The thick cortical part of the femoral neck

59
Q

Why is it argued that a collar shouldn’t be used in hip replacement?

A

Collar-collar contact area acts as a pivot about which the stem rotates (distal end of stem then prone to high stress concentrations)

60
Q

What is the theory behind the Exeter hip?

A

Cement-metal interface is weak and the stem should be free to slide and form an interference fit

61
Q

How does press fit reduce stress shielding?

A

By promoting hoop stresses in the bone

62
Q

Why are cementless stems coated with hydroxyapatite?

A

Helps bone ingrowth and potenitally eliminates metal debris from bone-metal abrasion

63
Q

What is the downside of stems fully coated in hydroxyapatite/

A

They promote stress shielding of the bone

64
Q

What are the downsides of cementless stems?

A 
Thigh pain (from lack of distal contact)
Fibrous tissue growth 
Breakdown of bone-hydroxyapatite bond (after a few years)
65
Q

Why do many stems have a proximal wedge shape/

A

So the stem can rest on the bone allowing transmission of compressive forces as well as shear forces

66
Q

How can the offset distance from the head to the neutral axis of the stem be reduced?

A

Reducing the length of the neck of the stem or increasing the angle between the long axis and the axis of the neck

67
Q

What is the downside of reducing the offset distance of the head from the neutral axis of the stem?

A

Increases joint rection force (giving rise o greater wear and acetabular bone-implant stresses)

68
Q

Why is there much less need to reduce bending moments in the stem?

A

Modern stem materials are much stronger

69
Q

How can bending moments in the stem be reduced?

A

Reducing head offset distance

70
Q

What two factors affect the frictional forces between two materials?

A

The surface properties of the two materials in contact

The magnitude of the load pressing them together

71
Q

What equation is used to calculate frictional force?

A

F = µN

72
Q

Why is HDP used when other polymers have a much lower coefficient of friction?

A

Alternative materials are either toxic or have poorer wear properties

73
Q

What is used most frequently as the other bearing surface to HDP and why?

A

Ceramics - excellent corrosion resistance, low friction and cause less wear on HDP

74
Q

What is the consequence of a replacement joinnt having a higher contact friction than the normal joint?

A

Much higher shear force transmitted to the acetabulum

75
Q

Why do most hip replacemenets nowadays use small heads?

A

Smaller interface force

76
Q

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

A

Adhesive wear

Abrasive wear

77
Q

What is adhesive wear?

A

Occurs because two bearing surfaces stick to each other when they are pressed together and one (usually the softer) is torn off by the harder one

78
Q

How can adhesive wear be reduced?

A

Bearing surfaces should be made of matrials that have a low level of adhesion
Lubricants can be used

79
Q

What is abrasive wear?

A

Occurs because surfaces are not perfectly smooth

80
Q

How can abrasive wear be minimised?

A

Highly polished surfaces

Good circulation of lubricant ( to remove wear particles)

81
Q

How can the volume of wear be calculated?

A

V = c.N.s / p

c = coefficient of wear 
N = applied load 
s = distance the bearing slides 
p = hardness of the surface being worn
82
Q

Why are HDP wear particles a problem?

A

Cause adverse tissue reactions - local inflammation - bone resorption - prosthesis loosening

83
Q

List 3 ways of reducing wear based on the wear equation?

A

Reduce loading on the joint (N)
Minimise sliding distance (s)
Find alternative materials for the head that reduce wear in HDP (c)

84
Q

How can the sliding distance be reduced?

A

Use a femoral head with a small radius because the bearing surface moves less as slides

85
Q

What are the pros and cons of alumina?

A

More scratch resistant than CoCr so cause less abrasive wear of HDP

But more brittle and prone to fracture than metals

86
Q

Why is Zirconia good?

A

Tougher and more fracture resistant than alumina also harder and more scratch resistant

87
Q

What are the two disadvantages of a small diameter head?

A

Rate of depth of wear is greater (as contact area is less)

Increased likelihood of dislocation in the post-op period (increased likelihood of neck impingement on the edge of the cup)

88
Q

Why is the accelerated wear caused by smaller head diameter a problem/

A

Although prostheses usualy fail for another reason first joint loses range of motion as the material wears because the neck of the ste of the femoral component contact the cup

89
Q

Why is it important to reduce the diameter of the neck as the dimeter of the head is reduced?

A

In order to maintain a required range of motion

90
Q

How is the normal acetabulum orientated?

A

At 45⁰ relative to the coronal and sagittal planes of the body and faes slightly backwards

91
Q

How does the diameter of the head affect the contact pressure between the head and the cup?

A

Contact pressure increases as diameter of the head decreases

92
Q

What surface contact angle is recommended to minimise contact pressure on the cup?

A

At least 120⁰

93
Q

What is the radial clearance?

A

The difference in the radius between the cup and the head (the cup always being slightly bigger)

94
Q

If the thickness of the HDP cup is reduced or a stiffer material used how does this affect the radial clearance and why?

A

Radial clearance must be reduced - to spread point contact load to avoid excessive cntact stress on the HDP

95
Q

What methods can be used to fix cups into place?

A

Cement
Fixed with screws
Push fitted
Incorporate a threaded stem which is screwed into a female threa cut into the bone

96
Q

Why are threaded cups not generally used?

A

Result in bone resorption from the high stress concs in the region of sharp threads

97
Q

What are the advantages of a metal backing to the cup?

A

Helps to hold the plastic in place (reduces tendency to creep and sitort) - thereby avoiding high contact stresses and focal wear on the HDP

98
Q

What are the disadvantages of a metal backing to the cup?

A

Increases head-cup contact pressure - depends on the thickness of HDP (thinner = higher contact pressure)

99
Q

What can cause loosening of the cup?

A

Mechanical overstressing or ingress of HDP wear particles which ultimately leads to resorption of the trabecular bone at the bone-cement interface

100
Q

How can the cup be made a stiffer structure to reduce areas of high contact stress?

A

Metal backing
Thick layer of HDP
Thick layer of cement

101
Q

Describe the process of cup loosening

A

HDP wear particles migrate from the rim of the cup along the bone interface - causing progressive absorption over a number of years - integrity of bone-cement interface is lost - fibrous membrane forms between the materials

102
Q

Moving the cup nearer the mid-line reduces load at the hip joint. Why do modern joint replacements not utilise this?

A

Have to breach subchondral bone plate (softer, weaker bone)

Deepened cup leads to earlier impingement of the femoral neck on the rim of th acetabulum

103
Q

What is the failure rate in hip replacement?

A

10% before 10 years

1% of the remainder per year

104
Q

If there is no failure or infection and the implant does not loosen why do most prostheses fail?

A

Acetabular wear

105
Q

Do prostheses tend to loosen or wear out first?

A

Loosen (due to wear particles and the role of bacteria and infection)

106
Q

What is a revision/exchange arthroplasty?

A

Removal of the failed prostheses and all surrounding inflammatory tissue, exclude infection and then add a new prosthesis

AY BMSc Implant Technology (13 decks)

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