Implant technology Unit 3

Question 1

The ligaments and surrounding muscles are imperative in maintaining knee joint stability. What is the main stabilising role of each of the 4 ligaments?

Answer 1

ACL - resists posterior subluxation of the femur
PCL - resists anterior subluxation of the femur
LCL - resists adduction of the joint
MCL - resists abduction of the joint
All the ligaments act together to limit distraction of the knee and to limit long axis rotation of the joint

Question 2

What is the posterior capsule and what does it do?

Answer 2

It is a band of tendonous material that resists hyper-extension

Question 3

What is subluxation?

Answer 3

Partial or complete dislocation

Question 4

What is the instantaneous centre of rotation?

Answer 4

As the knee flexes, its axis of rotation changes. This can be seen by the movement of the vertical line passing through the centre of rotation at each knee position, this being the point at which the links cross. This point is known as the instantaneous centre of rotation, because it changes at every instant of motion

Question 5

What impact does the four-bar cruciate mechanism of the knee joint have on motion?

Answer 5

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

Question 6

Why does the femur not roll of the tibia as the knee flexes?

Answer 6

Because the cruciate ligament and joint capsule prevents it from doing so

Question 7

How does the position of the instantaneous centre of rotation change as the knee moves from extension to flexion?

Answer 7

It move posteriorly and slightly distally

Question 8

The external forces acting on the knee joint are mainly compressive. Why is it that the forces on the joint are 2-6 times that of the body weight of the individual?

Answer 8

The combined effect of the gravitational (weight) forces, the contracting forces of the muscles across the joint and the balancing loads of the ligaments

Question 9

Ground reaction forces during walking also have a horizontal component directed medially which generates a turning moment on the knee. This is balanced by muscles and ligaments. For low magnitude sideways medial reaction forces, such as those that occur during normal gait, what helps to pull the joint together?

Answer 9

The quadriceps muscle, acting via the patellar tendon ligament, can pull the joint together hard enough to keep both condylar surfaces in contact with the tibial plateau

Question 10

As activities become more strenuous, what also helps keep the joint pulled together?

Answer 10

The hamstrings are used as well which leads to an increased total joint reaction force. Eventually, as the load increases, the muscles don’t have the strength to maintain contact at both condylar surfaces and so the lateral side loses contact and all the load is taken by the medial condyle. The stability of the joint then relies on the LCL to balance the turning moment

Question 11

What are the general criteria for knee joint replacements?

Answer 11

It must be tolerated within the human body with no short term and little long term risk of adverse toxic effects such as carcinogenesis.

Achieve its aim of relieving pain and restoring the activities of daily living.

Last a reasonable length of time which ideally should extend beyond the lifespan of the patient without the need for revision.

Be insertable by a competent surgeon of average ability such that a predictable outcome can be guaranteed.

Be of acceptable cost

Question 12

What are the components of a knee replacement normally made of?

Answer 12

Femoral component - cobalt chrome

Tibial component - HDP

Question 13

What are the minimal functional requirements of a knee replacement?

Answer 13

It should fully extend to 180 degrees at which point the patient should be able to stand without the need for muscular effort by the quadriceps.

It should flex to 90 degrees so that the patient can walk up and down stairs.

It should permit slight axial rotation as the knee extends to maintain natural ligament tension throughout the flexion and extension process

Question 14

Why must the posterior capsule be dissected off the back of the femur during a knee replacement surgery?

Answer 14

It must be dissected off the back of the femur to ensure that the replacement knee can fully extend

Question 15

What is done to the collateral ligaments during a knee replacement surgery?

Answer 15

They should be balanced in tension so that the bony cuts are parallel when the bones are stretched apart

Question 16

If ligaments are lost during surgery, the prosthesis design must compensate fro the functional loss. If there are no ligaments, what type of prosthesis is used?

Answer 16

A hinged prosthesis - the hinge mechanism constrains the motion of the knee into a single axis of rotation with total stability

Question 17

What are the issues related to hinge prostheses?

Answer 17

It has no “give” under lateral and long axis rotational loading and transmits the sometimes high shear forces associated with these loadings to the implant-cement and cement-bone interfaces

Question 18

What normally happens to the ACL in osteoarthritis?

Answer 18

In most cases of OA, the ACL is either destroyed or is so attenuated as to be of no mechanical value. The PCL is normally still intact

Question 19

What does the PCL do?

Answer 19

It controls the rolling motion of the tibia. If the PCL cannot be retained for a replacement knee, a necessary substitute mechanism is required as part of the prostheses. This enables the femur to rotate on the tibial plateau without sliding too far posteriorly. This means a good range of knee flexion is achieved without restrictions of movement due to soft tissues

Question 20

What are the advantages and disadvantages of retaining the PCL in a knee replacement surgery?

Answer 20

Advantages - it provides some degree of anterior-posterior stability and preserves some proprioception which helps feedback to the brain if the joint is being overloaded. Walking on stairs is also more stable with a PCL intact, although normal gait is unaffected either way.

Disadvantages - It constricts surgical dissection of the posterior capsule, which may limit full extension and it encourages the femoral component to slide over the tibial plateau which may have detrimental wear effects

Question 21

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

Answer 21

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

Question 22

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

Answer 22

The effect of constraint on load transmission and the generation of high shear stresses.

The effect of surface contact on the wear of the HDP tibial component.

The effect of the surface contact area on the stresses in the HDP tibial component

Question 23

What negative effect can occur due to the cement and prosthetic materials not being viscoelastic in nature?

Answer 23

The energy due to sudden loads is not absorbed gradually and can give rise to large instantaneous stresses at the interfaces which can cause failure in these regions. It is important therefore to have a sufficiently large area of contact. Stems and pegs on the femoral and tibial components help in this regard

Question 24

How can the rate of production of wear particles be minimised?

Answer 24

The sliding distance of the bearing should be minimised, while in order to reduce the rate of depth of wear the contact area should be increased, which can be achieved by having a wide bearing

Question 25

Other than the HDP surface shape, what 4 other design features influence prosthesis contact stress and the load transfer which affect the interfaces?

Answer 25

The thickness of the HDP component.

Whether or not the HDP has a metal backing plate.

Whether the tibial component has a stem.

The stiffness of the HDP material

Question 26

What is the minimum recommended HDP thickness?

Answer 26

8mm

Question 27

What is the purpose of a metal backing plate below the HDP?

Answer 27

The natural tibial plateau takes most of the load on the medial side. The metal backing plate is intended to distribute the high contact stresses under the condyles in order to provide an even loading on the bone beneath it

Question 28

Most tibial components have a small peg. What effect does this have?

Answer 28

It helps to reduce the incidence of loosening. This may be due to the peg assisting in reducing the high contact stresses due to uneven loading. The lateral reaction force acts to resist the rotating influence of the vertical load by providing a lateral reaction force. A central peg is therefore a useful design feature

Question 29

What does a higher Young’s Modulus of HDP mean in terms of contact stress?

Answer 29

The higher the Young’s Modulus of the HDP, the greater the contact stress. This is to be expected as the HDP deforms less. Doubling the Young’s Modulus increases the contact stresses by around 40%