Implant technology - unit 3 deck 2

Question 1

what is meant by the word “constraint” in context of modern knee replacement?

Answer 1

• The relationship between tibial and femoral bearing surface geometries.
• The more constrained they are the less freedom of movement they have to slide and rotate in different directions.

Question 2

what are the important functional design features of knee replacements

Answer 2

• To provide an acceptable ROM of joint combined with good stability under loading
• Screw home mechanism or some equilavent that allows standing up straight w/out the need to apply the quad muscle

Question 3

Anatomically shaped replacements can only meet these important design features if what?

Answer 3

The ligaments are retained

Question 4

If one or more ligaments cannot be used in a knee replacement what must be used ?

Answer 4

A prosthesis designed to compensate for the functional loss

Question 5

what knee implant design is used if there is no ligaments intact and what movement does it allow?

Answer 5

hinged prosthesis - constrains the motion of the knee to a single axis of rotation with total stability

Question 6

what is the problem with hinged joint prosthesis

Answer 6

1. has no give under lateral and long axis rotational loading
2. and transmits the high shear forces associated with these loadings to the implant-cement and cement-bone interfaces which can lead to loosening

Question 7

Are the collateral ligaments usually preserved in people undergoing a knee replacement ?

Answer 7

Yes they are usually intact or or can be made to function properly by corrective adjustment to their length so that they are balanced in tension with the prosthesis in place

Question 8

Are te cruciate ligaments usually preserved in those undergoing a knee replacement ?

Answer 8

• The ACL is usually destroyed
• PCL usually preserved

Question 9

what condition often causes the destruction or degradation of the ACL

Answer 9

OA

Question 10

how does the preservation of the PCL influence the knee replacement design used

Answer 10

PCL controls rolling motion of the tibia
implant choice depends of whether PCL is retained or removed
if not retained it is necessary to substitute a mechanism within the prosthesis - this may be a simple stop or a more sophisticated cam-like device

Question 11

why is it important to use a mechanism to replace PCL in knee replacements if necessary?

Answer 11

This will enable the femur to rotate on the tibial plateau without sliding too far posteriorly thus allowing a good range of knee flexion w/out restriction of movement due to soft tissue

Question 12

what are the theoretical advantages of retaining the PCL in a knee replacement ?

Answer 12

• It provides some degree of anterior-posterior knee stability and that it may preserve some proprioceptive activity, i.e. it may offer some sensory feedback to the brain to protect against overloading the joint.
• Walking on stairs is more stable

Question 13

what are disadv of retaining PCL in a knee replacement ?

Answer 13

It constricts a free surgical dissection of posterior capsule which:

• may limit full exntesion
• and enourages femoral component to slide over tibial bearing which may have deterimental surface wear effects

Removal of PCL allows the use of more congruent joint surfaces which reduces HDP wear

Removal may also correct deformity

Question 14

what is the design of prostheses that retain the PCL and what needs to be determined carefully?

Answer 14

• Have fairly flat tibial plateau like that on natural tibia
• It is normal to slope the tibial component posteriorly by about 10 degrees to encourage femoral component to roll back on the tibial
• The height, position and anterior- posterior angulation of the tibial plateau need to be carefully determined at surgery if the PCL is to work properly.

Question 15

In PCL retaining knee replacements what happens if the PCL is too tight or too loose?

Answer 15

• if PCL is too loose - allows forward movement of the femur on the tibia so that normal rolling back motion no longer works
• if PCL is too tight - there will be restricted degree of flexion, excessive rolling back of the femur on the tibia. Also compression of the 2 prothetic joint surfaces together posteriorly, causing high contact stresses

Question 16

what problems are associated with PCL retaining and other flat tibial plataeu prosthesis designs?

Answer 16

HDP wear problems
Fatigue problems

Question 17

Why does a replacement knee need to have a fairly flat tibial plateau when the PCL is retained?

Answer 17

Because the PCL could otherwise become lax or too tight during flexion- extension movement.

Question 18

What are the what are 3 important mechanical factors relating to the surface shape of a knee prosthesis?

Answer 18

1. the effect of constraint on load transmission and the generation of high shear stresses
2. the effect of surface contact on wear of the HDP tibial component
3. the effect of surface contact area on the stresses in the HDP tibial component.

Question 19

Almost all knee replacements are made from a HDP (tibial component) and CoCr (femoral component) bearing surfaces - how do they wear related to each other?

Answer 19

• HDP being softer material wears out first while CoCr is hardly affected by wear
• HDP component is also prone to fatigue failure under loading >> eventually results in failure of the joint

Question 20

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

Answer 20

In the knee it has a smaller bearing surface contact area, so stresses in the material are higher

Question 21

HDP is known for its adverse affects of its wear debris on bone tissue, leading to bone resorption (discussed in unit 2 for the hip) - what is another of its side effects?

Answer 21

1. Its surface becomes stiffer due to increase in density after sterilisation with gamma radiation and over time after implantation due to oxidisation.
2. The greater stiffness increases the joint contact stress under loading and therefore makes the HDP tibial component more prone to wear.

Question 22

effect of degree of constraint on load transmission

Answer 22

Unit 3 page 15 - prob just pie learning this

Question 23

Why is it important to have a sufficiently large area of contact in knee prostheses?

Answer 23

Cement and prosthesis materials are much stiffer than the soft tissues and not viscoelastic, so the energy due to sudden loads is not absorbed gradually and can give rise to large instantaneous stresses at the interfaces which can cause loosening and failure in these regions

Question 24

What are used in the design of knee prostheses to help ensure they have a sufficiently large area of contact?

Answer 24

Stems and pegs on the femoral and tibial component

Question 25

Which models in Fig. are most representative of a natural knee joint and why?

Answer 25

D and E

The femoral component should be fairly circular and the joint should only partially constrained.

Question 26

if the prosthesis does not loosen or the components break, and there are no medical problems i.e. infection - then what determines the useful life of a knee or prothesis in general?

(hint its the same as in unit 2 when discussing hip prosthesis)

Answer 26

rate of wear of the HDP component

Question 27

what is the equation for volume of wear (same as abrasive wear eq.)

Answer 27

v = volume of wear 
c = coefficient of wear/constant 
N = applied load across bearing surface (normal load) 
s = distance that the bearing slides 
p = hardness of the surface being worn

Question 28

For a given activity N is defined, so how can the formula for wear volume be simplified for the purpose of comparing the wear rate of different surface geometries of knee prostheses using the same CoCr femoral component and HDP tibial component (==> c and p are constants)?

Answer 28

v = K × s

where K is some constant.

==> This formula says that the volume of wear material produced is proportional to the sliding distance moved

Question 29

Referring to the simplified equation for volume of wear (v = K x s) what is the sliding distance (s) moved to achieve a rotation of θ degrees proportion to and therefore what equation can be created for volume of wear ?

Answer 29

s = rθ

since r = d/2

then the equation v = k (d/2 x θ)

Question 30

Referring to the equation used to calculate volume of wear how does the diameter of the bearing surface affect volume of wear?

Answer 30

Referring to the equation v = k (d/2 x θ)

A smaller diameter of bearing will therefore reduce the volume of wear material

Question 31

what value determines the life of the HDP bearing?

Answer 31

the rate of DEPTH of wear

[not the rate of volume of wear]

Question 32

what is the equation which relates volume of wear to depth of wear ?

Answer 32

v = A x t

v = volume of wear 
A = area of contact 
t = depth of wear 

thus depth of wear can be reduced by having a large surface area of contact

Question 33

How can for a particular volume of wear the life of a HDP bearing be increased state the equations involved?

Answer 33

Considering the equation

v = A x t

For a particular volume of wear e.g. 100 then you know the depth of wear (t) is the thing you want to decrease so you can increase the area of contact (A) which will then decrease the depth of wear as the volume of wear is remaining constant

the area of contact (the grey area A) is equal to the length of the arc = (s = d × θ) × W

where W is the width of the bearing. The area of contact can therefore be increased by increasing d and W, but increasing d increases the volume rate of wear (as we have just seen) so it is preferable to increase W rather than the length of arc (d x θ)

Question 34

To summarise how is volume of rate of wear reduced and how is depth of wear reduced?

Answer 34

to minimise rate of production of wear particles i.e. volume rate of wear, the SLIDING distance of the bearing surfaces is minimised

to reduce rate of depth of wear the contact area should be increased, by having a wide bearing

Question 35

How does the diameter of a bearing affect volume wear?

Answer 35

The greater the diameter, the greater the volume of wear

Question 36

How does the area of contact of a bearing affect depth of wear?

Answer 36

The greater the contact area, the lower the depth of wear

Question 37

In order to reduce wear, why is it preferable to increase bearing width than to increase bearing diameter?

Answer 37

Increasing the diameter also increases volume wear, wereas increasing the bearing width does not increase volume wear

Question 38

what can HDP fatigue cause

Answer 38

premature wear and subsurface lamination cracking at the joint

Question 39

if a model knee prosthesis had a lower contact area than another, what would it be more at risk too

Answer 39

contact stresses much higher and therefore will be more prone to fatigue failure under the cyclical loading that occurs during flexion and extension of a loaded knee

[thus larger contact area superior]

Question 40

the moment due to lateral GRF can cause one of the condyles to lift off and place all the load on the other one - what prostheses design is suited to deal with this

Answer 40

one with a curved edge to spread out the load

the curved surface maintains a higher contact area when horiztonal and tilted

Question 41

why is the HDP tibial component prone to fatigue failure?

Answer 41

• Because the contact points on the tibial plateau change between compression and tension and loading and unloading,
• These repetitive cyclical loading conditions make it susceptible to fatigue failure if the loads are high.

Question 42

Why is Model J in Fig. better than Model I in reducing contact stresses?

Answer 42

Because it provides a larger contact area during uneven loading of the knee

Question 43

what are the important design features that influence prosthesis contact stress and load transfer [affecting the interface stresses between the tibial component and the underlying bone]?

Answer 43

• • HDP surface shape
• • thickness of HDP component
• • whether or not HDP has a metal backing plate
• • whether the tibial component has a stem
• • stiffness of the HDP material

Question 44

how does the thickness of the HDP component and stress relate

Answer 44

• the thinner the HDP component, the greater it is stressed
• due to the stress being unable to be distribute evenly in the material

Question 45

what is the minimum thickness of HDP tibial component without metal backing plate?

Answer 45

8mm thickness

if a metal tray is used, HDP tibial component can be thinner to limit total thickness for surgical reasons

Question 46

when is metal backing tray of value

Answer 46

when the soft tissue tightness forces the choice of a prosthesis of minimum thickness upon the surgeon

Question 47

why is the ability to vary the tibial height in a prosthesis important and how can this be done ?

Answer 47

essential to ligament balancing - If the joint is to be compressed its ligaments must be tight, or at least maintained at a length that does not allow the joint to open up medially or laterally when loaded with a lateral turning moment]

Tibial height can be altered through interchangable HDP inserts

Question 48

how does the natural tibial plateau distribute load and how does this compare to the replacement tibial plateau

Answer 48

• natural tibial plateau takes most of the load on the medial side (at least 60% but sometimes more)
• replacement tibial plateau will be similarly loaded
• • medial edge of the plateau will be more highly stressed thus transferring a higher load and stress to underlying bone than on the lateral side

load is taken proximally by cancellous bone only and is transferred to cortical bone more distally

Question 49

what is the tibial backing tray under the HDP tibial component function

Answer 49

To distribute high contact stresses under the condyles in order to provide an even loading on the bone beneath it, this is provided due to the high stiffness of the metal plate

Fig. shows how the concentration of load on the top surface of the HDP is spread out over the contact area with bone

Question 50

what is the main disadvantage seen in tibial backing metal trays?

Answer 50

Disadvantage occurs if the knee if loaded unevenly:

Normally it is the medial side that takes the largest proportion of the load

The stress concentration on the underlying medial bone is greater if a metal tray is present than if the component is only HDP because the metal is so much stiffer.

There will also be a tensile stress between the plate and the bone laterally. The bone cement between them will not tolerate this well

Question 51

most tibial components have a small peg between 30-50mm long - what is the function of this

Answer 51

• found that tibial components loosen mainly from sinkage in the bone, probably due to gradual done failure from high localised stresses
• providing central peg reduces the incidence of loosening

[peg helps reduce the high contact stresses due to uneven loading that could cause the bone to fail and subside]