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Implant technology > hip joint replacement > Flashcards

hip joint replacement Flashcards

1
Q

what are the 2 methods for holding the femoral and acetabular components in place

A

bone cement or cementless contact - the latter involves cutting a reciprocal shape in the bone and hammering the prosthesis (or occasionally screwing it) in

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2
Q

what does ‘press fit’ rely on

A

close surface contact between the stem and the bone

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3
Q

what does bone cement help to do

A

it helps to distribute loads between bone and prosthesis

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4
Q

give a pro and a con to using ceramics

A

good frictional and wear properties

brittle dan subject to sudden failure

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5
Q

what characteristic of metals gives rise to stress shielding

A

very stiff relative to bone

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6
Q

most commonly used implant materials

A 
cobalt chrome alloy
stainless steel
titanium and titanium alloy
high density polyethylene (HDP)
polymethylmethacrylate (PMMA) bone cement
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7
Q

HDP provides a good _____?

A

bearing surface

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8
Q

disadvantage of HDP

A

undesirable tissue reaction when fragmented

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9
Q

For daily living, what must the hip be able to do

A

extend slightly
flex to at least 30 degrees
abduct when weight bearing
rotate when in full extension

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10
Q

what part of the pelvis is the normal weight bearing part when sitting

A

ischial part of the pelvis

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11
Q

how many groups of muscles and ligaments act across the hip joint?

A

7

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12
Q

why is the hip joint termed indeterminate?

A

more than one muscle being active at one time means that forces acting on the femur and the pelvis and across the joint cannot be calculated precisely and must be approximated

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13
Q

why is standing on one leg commonly used for analysis of stresses in hip prosthesis

A

muscle forces can be estimated to some degree of confidence because some muscles aren’t active at all
also its believed to generate high bending stress in the femur

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14
Q

highest hip moments occur in which plane?

A

coronal

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15
Q

how can we calculate compressive stress at a section of the bone

A

compressive force component divided by cross sectional area of the bone

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16
Q

describe 4 ways of preventing a stem from sinking into the medullary canal

A

tapering the stem
proximal collar
fixing the bone to the stem (bone ingrowth / adhesion)
using cement strong enough to withstand the shear stress

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17
Q

2 ways of reducing stem implant interface shear stresses

A

proximal collar
tapering
- both allow some of the load to be taken in compression

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18
Q

why does the hip produce a bending stress on the femur

A

the joint force vector is not along the neutral axis

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19
Q

how to calculate bending stress

A

My / I

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

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20
Q

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

A

the stem takes some of the bending load, reducing the stresses in the proximal end of the femur

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21
Q

what is a radial stress and where are they greatest

A

stresses directed radially outward from a central point

greatest at the points of bone stem contact

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22
Q

what do radial stresses cause in turn

A

hoop stresses

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23
Q

what type of stress are hoop stresses (primarily)

A

tensile

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24
Q

under what circumstances does the femur experience high hoop stresses due to the presence of a femoral stem

A
  • when a loose prosthesis experiences a bending load

- when a tapered stem is loaded axially and presses against the femur

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25
Q

how to avoid excessive hoop stresses

A

ensuring the stem is long enough

providing a good fit of the stem in the medullary cavity

26
Q

why is it beneficial to use non-circular section for a femoral stem

A

helps resist rotational shear forces

27
Q

why does it cause a problem if the subchondral cortical bone is broken during a joint replacement

A

below the cortical bone is cancellous bone which is normally not highly stressed and if the cortical bone is broken, the load will have to be taken by the underlying cancellous bone

28
Q

why does there tend to be higher stress concentrations in contact regions between replacement head and cup, compared to natural hip joint

A

the replacement head and cup usually have a smaller diameter&raquo_space; contact area smaller&raquo_space; higher stress concentrations

29
Q

what is more toxic, cement monomer or polymer?

A

monomer

30
Q

what happens as the cement polymer sets that can damage body tissues?

A

exothermic reaction and a lot of heat is generated

31
Q

can PMMA bone cement bond chemically to bone and the prosthesis?

A

no

32
Q

why is their a lot of interest in hydroxyapatite coated cementless prostheses

A

they can bond chemically to bone (hydroxyapatite is a natural constituent of bone)

33
Q

what is there a tendency for developing at the metal or cement interface with the bone and what can be done to minimise it

A

fibrous tissue forming creating a layer with no shear or tensile strength
to minimise: micro motion should be kept to a minimum after surgery (» accurate fit at surgery followed by controlled weight bearing)

34
Q

when is it particularly important to prevent the production of a fibrous layer

A

when hydroxyapatite coating is being used as the layer prevents proper ingrowth of bone into hydroxyapatite coated prostheses

35
Q

advantages of a cemented prosthesis

A

stem doesn’t need to be an exact fit

cement will fill all the gaps so there will be an even stress distribution

36
Q

what are the problems with PMMA cement

A

PMMA fragments cause adverse tissue reactions which can lead to bone resorption

the cement does not form an adhesive bone so it cant resist tensile forces

37
Q

what is the main disadvantage of isoelastic stems

A

proximal shear stresses too high&raquo_space; interface bonds fail

38
Q

what is the downside of more load being transferred proximally from the stem to the bone

A

higher shear forces and interface shear stresses generated

39
Q

why is thickness of the cement layer important and what is the ideal thickness

A
  • too thin a cement layer (< 2mm) can lead to bone resorption due to cement debris from fractured cement layer causing adverse tissue reaction
  • too thick and too thin can both cause high cement stresses
  • ideally between 3 and 7 mm proximally and minimum of 2 mm distally
40
Q

effect of a proximal collar on load transfer from the stem to the femur

A

the collar transfers load from the stem to the bone proximally, helping to reduce stress shielding and proximal interface shear stresses

41
Q

whats the main argument against using a collar

A

it acts as a pivot which causes fretting wear at the pivot and high stem stresses distally

42
Q

advantage of coating the whole stem with hydroxyapatite

A

helps bone ingrowth

43
Q

what is the disadvantage of fully coating a stem with hydroxyapatite

A

promotes stress shielding of the bone

44
Q

how does a tapered wedge help proximal load transfer

A

by transferring much of the load to the bone in compression rather than in shear

45
Q
  • name 2 ways that the neck of a femoral stem can be altered to reduce the bending moment of the stem
  • how do these modifications affect hip joint force
A
  • shortening the neck
  • increasing the angle of the neck (making it more vertical)
  • both of these modifications increase the joint reaction force&raquo_space; increases wear
46
Q

how does femoral canal geometry change with age

A

cortical bone thins&raquo_space; the canal becomes wider

47
Q

why is cobalt chrome preferred to stainless steel or titanium

A

cobalt chrome and titanium have better corrosion resistance than stainless steel
titanium has poor wear properties
» cobalt chrome is the best option

48
Q

have ceramics got good corrosion resistance?

A

yes, excellent

49
Q

what are the 2 main types of wear that occur at bearing surfaces

A

abrasive and adhesive

50
Q

why does adhesive wear occur

A

the 2 bearing surfaces stick together when they’re pressed together and one (usually the softer one) is torn off by the harder one

51
Q

why is HDP used as a bearing surface in replacement joints

A

it has low coefficient of friction with metals

52
Q

disadvantage of using HDP as a bearing surface

A

fragments of HDP produce adverse tissue reaction that leads to bone resporption

53
Q

main disadvantages of using a smaller diameter head

A
  • rate of depth of wear is greater

- increased chance of dislocation in post-op period due to higher chance of neck impingement on the edge of the cup

54
Q

important features in the design of the acetabular cup

A

thickness of HDP layer
lining with metal backing plate
outer dimension of the acetabulum

55
Q

what is radial clearance

A

difference in radius between the cup and the head

56
Q

when must radial clearance reduce

A

stiffer material used or thickness of the HDP cup is reduced

allowing contact load to be spread over a greater area

57
Q

why is there a minimum recommended thickness of HDP cup

A

to avoid excessive bearing contact stresses

58
Q

advantage of using a metal backing plate on the cup

A

helps hold the plastic in place and prevents creep and distortion

59
Q

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

A
  • ingress of HDP at bone interface
  • bone resorption
  • migration of HDP along the interface until loosening occurs
60
Q

methods of reducing adhesive wear

A
  • reduce joint loading
  • keep sliding distance as small as possible i.e. femoral head with small radius
  • find alternative materials for the head that reduce wear in the HDP i.e. ceramics such as alumni or zirconia
61
Q

what are alumni and zirconia

A

ceramic metals

62
Q

why is centralisation the cup not common practice despite it lowering the load at the hip joint?

A

the effect is probably marginal and there are two major disadvantages

  • to move the cup more centrally the strong cortical bone would have to be breached and the prosthesis would therefore be mounted on much weaker bone
  • would result in earlier impingement of the neck on the rim of the acetabulum which limits motion and increases the risk of dislocation

Implant technology (10 decks)

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