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AY BMSc Implant Technology > Unit 3: Knee Joint Replacement > Flashcards

Unit 3: Knee Joint Replacement Flashcards

1
Q

What is the knee dependent on for stability?

A

Ligaments
Integrity of the posterior joint capsule
Good musculature

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2
Q

What is the purpose of the ACL?

A

Resists posterior subluxation of the femur

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3
Q

What is the purpose of the PCL?

A

Resists anterior subluxation of the femur

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4
Q

What is the purpose of the LCL?

A

Resists ADDuction of the joint

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5
Q

What is the purpose of the MCL?

A

Resists ABDuction of the joint

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6
Q

What is the purpose of the posterior capsule?

A

Resists hyperextension

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7
Q

How are the ACL and PCL named?

A

According to their anterior and posterior attachments to the tibia, not the femur

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8
Q

What is meant when it is said that the ligaments in the knee move nearly isometrically?

A

They keep the same length as they move and do not lengthen or shorten

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9
Q

How does the centre of rotation and point of contact between the femur and tibia change as the knee flexes?

A

The centre of rotation and point of surface contact both move posteriorly

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10
Q

How do the medial and lateral compartments of the tibia plateau differ?

A

Medial compartment = slightly concave

Lateral compartment = slightly convex

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11
Q

What is the “screw home” mechanism?

A

Tibia externally rotates 5 degrees in the last 15 degrees of extension because the medial tibial plateau articular surface is longer than the lateral tibial plateau

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12
Q

What is the purpose of the “screw home” mechanism?

A

“locks” knee decreasing the work performed by the quadriceps while standing

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13
Q

What is the full range of knee flexion?

A

About 140 degrees

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14
Q

How does the knee move from flexion to extension?

A

Combination of rolling and sliding motion

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15
Q

What limits the rolling distance to allow the knee to flex fully without rolling up against the posterior capsule?

A

Cruciate ligaments

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16
Q

Approximately what magnitude of forces go through the knee joint during normal daily activities?

A

2-6X body weight

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17
Q

What causes the compressive joint force in the knee?

A

Action of the quadriceps acting via the patellar ligament

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18
Q

What combines to form the total joint reaction force of 4 BW in the knee joint?

A

Patellar force of 3 BW + Ground Reaction force of 1 BW

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19
Q

What are the other components other than the vertical component of ground reaction force?

A

Fore-aft GRF = 20% BW

Medial horizontal component = 5% BW

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20
Q

How does the knee combat low magnitude sideways medial reaction forces?

A

Quads acting via the patellar tendon pull the joint together hard enough to keep both condylar surfaces in contact with the tibial plateau

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21
Q

How does the knee combat high magnitude sideways medial reaction forces?

A

Lateral collateral ligament balances the turning moment

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22
Q

What implications do the high loads that act on the medial compartment of the knee have on joint replacement design?

A

Tibial component needs to be able to transfer igh medial compartment loads on its upper surface to the underlying bone without causing high compressive stresses

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23
Q

What are most commercially available knee replacements made of?

A

Femoral component = cobalt chrome

Tibial component = HDP

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24
Q

What problems were seen when trying titanium components instead of cobalt chromium?

A

Metal wear particles were taken up by the synovial membrane causing blackening

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25
Q

What are the minimum functional kinematic requirements of a knee replacement?

A

Fully extend to 180⁰
Stand without quad effort
Flex to 90⁰
Allow slight axial rotation to maintain natural ligament tension in flex/ext

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26
Q

How is it ensured that the two bearing surfces are cut parallel in a knee replacement?

A

Use a radiograph of the feur and draw a line from the centre of the femoral head to the knee centre

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27
Q

In knee replacement, why must the posterior capsule be dissected off the back of the femur?

A

To ensure that the replacement knee can fully extend

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28
Q

How are ligaments balanced in knee replacement surgery?

A

Lengthened tightened ligaments to match slack ones

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29
Q

What is meant by the term ‘constraint’?

A

Relationship between tibial and femoral bearing surface geometries - more constrained = less freedom of movement to slide and rotate in different directions

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30
Q

What type of prosthesis is used if there are no ligaments intact?

A

Hinged prosthesis

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31
Q

What are the downsides of a hinged prosthesis?

A

Only one axis of rotation - under lateral and log axis rotation transmits high shear forces to implant-cement and comeent-bone interfaces

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32
Q

Which ligament is usually either destroyed or of no mechanical value in OA?

A

ACL

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33
Q

What are the theoretical advantages of retaining the PCL?

A

Provides a degree of ant-post stability

May preserve some proprioception

34
Q

What are the disadvantages of retaining the PCL?

A

Constricts free surgical dissection of the posterior capsule which may limit full extension
Encourages the femoral component to slide over the tibial bearing which may have detrimental surface wear effects

35
Q

Why is it normal to slope the tibia posteriorly by about 10⁰?

A

Ro encourage the femoral component to roll bback on the tibial component

36
Q

What happens if the PCL is too loose or too tight?

A

Too loose - allows forard movement on the femur on the tbia so that the normal rolling back motion no longer works
Too tight - restricted flexion, excessive rolling back of the femur on the tibia and possible compression of the 2 prosthetic joint surfaces together posteriorly generating high contact stresses

37
Q

What is the main problem that have been found with CL retaining prostheses?

A

HDP wear and fatigue problems

38
Q

What are the 3 important factors to consider when looking at the surface shape of a knee prosthesis?

A

Effect of constraint on load transmission
Effect of surface contact on wear of the HDP tibial component
Effect of surface contact area on the stresses in the HDP tibial component

39
Q

How does the stiffness of HDP change and how does this affect the joint?

A

Becomes stiffer due to an increase in density after sterilisation with gamma radiation and over time after implantation due to oxidisation

Increased stiffness increases joint contact stress making the HDP more prone to wear

40
Q

Why is it the HDP not CoCr component that is first affected by wear?

A

HDP is softer than CoCr

41
Q

Why is HDP fatigue and wear more of a problem in knee replacements than hip?

A

The bearing surface contact area is smaller so the stresses in the material are higher

42
Q

Most knee replacements are designed to provide some constraint in which two directions?

A

Axial rotation

Lateral motion

43
Q

Why do sudden loads give rise to large instantaneous stresses at the interfaces?

A

Cement and prosthesis materials are much stiffer than the soft tissues and not viscoelastic - so energy due to sudden loads is not absorbed gradually

44
Q

How does the constraint of the prosthesis affect the length of stem required?

A

The more constrained the motion of the prosthesis the greater the length of stem required

45
Q

What formula can be used to calculate the volume of wear?

A

v = c.N.s / p

c = coefficient of wear 
N = applied load 
s = distance that the bearing slides 
p = hardness of the surface being worn
46
Q

What is the relationship between the rate of volume of wear and the rate of depth of wear?

A

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

47
Q

In summary, how do you minimise the rate of production of wear particles (i.e. the volume rate of wear)?

A

Minimise the sliding distance

48
Q

How do you minimise the rate of depth of wear?

A

Increase contact area (can be achieved by having a wide bearing)

49
Q

What is lamination cracking at the joint associated with?

A

A combination of both high contact stress and a cyclical sliding motion between the surfaces

50
Q

What developments in recent years have helped to reduce HDP fatigue to some extent?

A

Improved quality of HDP manufacturing process and the use of ultra high molecular weight polyethylene

51
Q

How does the stress differ from the central contact area to the periphery?

A

Central part of contact area = always in compression

periphery = always in tension

52
Q

How does the thickness of the HDP component affect stress and why?

A

The thinner the HDP component the greater it is stressed - because the stresses cannot be distributed evenly in the material

53
Q

What are the current recommendations for HDP thickness?

A

8mm thickness minimum for HDP tibial components without a metal backing tray

54
Q

On which side does the natural tibial plateau take most load?

A

On the medial side (60%)

55
Q

What is the purpose of a metal backing plate?

A

Distribute the high contact stresses under the condyles in order to provide an even loading on the bone beneath it

56
Q

What is the downside of a metal backing tray?

A

stress concentration on underlying medial bone is greater because the metal is so much stiffer

Also tensile stress between the plate and the bone laterally

57
Q

Why is there now a trend towards all HDP components rather than a metal backing?

A

Recent research has shown that a thicker HDP (>10mm) behaves similar to a metal tray

58
Q

What effect does a metal peg when used with a metal backing plate?

A

Reduces stresses by 20-40% - helps to reduce high contact stresses due to uneven loading that could cause the bone to fail and subside

59
Q

How does the stiffness of the HDP affect contact stress?

A

The higher the Young’s modulus of the HDP the greater the contact stress (as the HDP deforms less)

60
Q

How does the femoral component of a knee replacement differ from a normal knee?

A

Replacement is symmetrical

Normal knee - medial condyle is larger than the lateral condyle

61
Q

Why do replacements not copy the natural asymmetry of the knee joint?

A

V expensive

In practice no shown benefit to asymmetry

62
Q

Why are partially constrained shapes generally favoured?

A

They provide the required degree of functional movement
Do not suffer greatly from loosening due to overstressing
They limit the range of sliding motion to help reduce wear

63
Q

How does PCL affect the shape of the tibial component?

A

If PCL retained - need flattish surface profile

64
Q

What are the 2 aims of posterior stabilised designed prostheses?

A

Prevent posterior femoral subluxation of the femur over the tibia
Cause the femur to ‘roll back’ as it flexes

65
Q

How are components anchored in knee joint replacement?

A

PMMA cement
Sometimes projections

Fem component - press fit
Tibial component - screw fixation or a stem

66
Q

What is the purpose of the patella?

A

Provides a better leveage for the patellar tendon - lower force required for flexion - lower joint reaction force

67
Q

How does the shape of the surface of the patella affect HDP wear?

A

Conforming shapes contoured to match the femur wear less than convex (non-conforming) shapes

68
Q

Why is wear worse in metal backed patellae?

A

Because the HDP is insufficiently thick to distribute the loads and is therefore prone to higher contact stresses than the less rigid all-HDP component

69
Q

In which condition is it preferable to replace all joint surfaces?

A

Rheumatoid arthritis

70
Q

What comprises a meniscal bearing prosthesis?

A

Metal femoral component
Metal tibial component
HDP meniscus

71
Q

What are the disadvantages of meniscal prostheses?

A

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

72
Q

What is a knee hemi-arthroplasty?

A

Replacing only one side of the tibio-femoral joint

73
Q

In what situation is hemi-arthroplasty used?

A

Alternative to osteotomy in younger patients who have a painful and deformed joint that is not severe enough to warrant joint replacement but with too advanced a disease process to permit osteotomy

74
Q

In hemi-arthroplasty why are pegs or lugs used to limit the loss of bone stock?

A

In case of the need to perform a revision operation

75
Q

What are the main problems with revision of TKR?

A

Loss of bone sotck for anchoring the revision prosthesis

Hard to allign/stabilise due to ligament damage

76
Q

What design is used for revision of TKR?

A

Linked hinge design

77
Q

Describe the linked hinge design used in revision of TKR

A

Bearing surface with a high degree of constraint built into the tibio-femoral bearing

High peg projecting above the tibial plateau which is partially captured by a central groove in the femoral component

78
Q

What are augmentation blocks?

A

Flat or wedge shaped blocks that can be added to prosthesis in revision TKR to achieve contact between the prosthesis and bone

79
Q

If there is gross loss of bone through the loosening process how can a revision TKR be anchored?

A

Intramedullary stems

80
Q

What is reaming?

A

A process of enlarging a hole using a special tool which removes only small amounts of material

AY BMSc Implant Technology (13 decks)

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