Ch. 10 - Total Knee Replacement

Question 1

How many DOF does the knee joint have?

Answer 1

6 DOF (3 rotation + 3 translation)

Question 2

Which muscle is responsible for knee extension?

Answer 2

Quadriceps

Question 3

Which muscle is responsible for knee flexion?

Answer 3

Hamstrings

Question 4

When is a joint in stable equilibrium?

Answer 4

A joint is stable when small force changes result in only small changes in motion.

Question 5

What is the goal of a knee replacement?

Answer 5

To create a bone-implant system which will provide a patient with renewed normal joint function.

Question 6

What is 2 tasks are required to achieve the goals of a total knee replacement?

Answer 6

1. Design the prosthetic component

2. Design the surgical procedures necessary to implant the device

Question 7

What design specifications must we consider when designing a total knee replacement?

Answer 7

1. Greater functional complexity than the hip due to kinematic restraint by soft tissues
2. New components must work with existing soft tissue structures and replace lost structures
3. Must provide normal ROM
4. Must transmit normal joint forces
5. If muscle travelling distances and resting lengths are maintained, the forces across the joint should stay the same
6. Simpler fixation requirements bc we are replacing surface components
7. Must deal with structural damage to articulating surface components wrt wear
8. Surfaces are non conforming, so these high stresses can lead to loosening

Question 8

Describe the different types of knee replacements that exist.

Answer 8

While both femoral and tibial components are always replaced, TKRs may be tricompartmental, bicondylar or unicondylar.

Question 9

When would you implant a unicondylar knee replacement?

Answer 9

When the patient has normal cruciate and collateral ligaments.

Question 10

How can you classify bicondylar knee replacements according to the level of constraint required?

Answer 10

Consider whether both cruciate AND collateral ligaments are missing or insufficient, or only the cruciate ligaments are

Question 11

Describe a total condylar knee replacement design.

Answer 11

• Sagittal femoral profile matches the native condyle
• Tibial plateaus are concave to give AP support and thus substitute ACL function
• Cam on femoral component can additionally be used to also provide adequate PCL function

Question 12

What is the challenge of designing a PCL-retaining total condylar design?

Answer 12

The design must accomodate the ligaments both structurally and kinematically.

Question 13

Consider a single-axis hinge total condylar knee replacement design. What are the advantages and disadvantages of this design?

Answer 13

+ Very simple, so leads to consistent and reproducible results among surgeons

• Axis of rotation of the knee in reality moves wrt tibia during flexion and cannot be simply replaced by a fixed axis if surrounding soft tissue are to function normally
• Fixed axis causes all joint loads to be transmitted through the prosthesis. This eliminates any load sharing with the surrounding soft tissue, thus resulting in an increased risk of interface failure and loosening due to high stresses.

Question 14

In a knee joint, how can one control the relative motion of the components?

Answer 14

1. Control loads across the joint
2. Control the geometry of articulating surfaces
3. Control the soft tissues

Question 15

What is the disadvantages of an asymmetric condylar design vs. a symmetric one? Why would an asymmetric design still be used?

Answer 15

• Asymmetric may not be used for left and right knee interchangeably
• Asymmetric designs mean the hospitals need to carry a larger inventory
• Asymmetric designs have increased component complexity
• Increased cost associated with the use of an asymmetric design
  + Asymmetric designs can better approximate normal articular geometry

Question 16

What do toroidal (2 radii) and conforming (1 radius) femoral and tibial surfaces have in common?

Answer 16

• Both are PCL retaining or substituting
• Both provide nearly normal knee flexion
• Both approximate shape of condyle in sagittal plane
• Both require ACL substitution
• Both deal with varus-valgus moments

Question 17

How do toroidal and single-radius knee replacement designs deal with varus-valgus moments?

Answer 17

• Toroidal: Since the load goes through the centre, edge loading is limited
• Single-radius: Design allows ML angulation before lift-off, an increasing moment is required for lift-off to occur

Question 18

What are the 3 main structural considerations we must keep in mind ensure the success of a total knee replacement?

Answer 18

1. Component failure
2. Component fixation
3. Surface damage

Question 19

Although component failure in a total knee replacement is rare, what factors may contribute to its occurrence?

Answer 19

• Reduced fatigue strength from porous coating procedure
• Stress concentrations near the PCL notch or fixation pegs
• Crushing of trabecular bone under the tray (cortical bone is removed to place the prosthesis)

Question 20

What factors may contribute to the failure of component fixation in a total knee replacement?

Answer 20

• Geometry of the plate
• Fixation of the peg configuration
• Metal backed or not
• Varying trabecular bone properties

Question 21

How does varying load position affect bone loading and how can this be modelled?

Answer 21

This can be modelled using the principle of Beam on an Elastic Foundation. This tells us that deformation is determined by load P, bending rigidity of the beam EI, and stiffness of the elastic foundation k.
The greatest deformation will be near where the load is applied but not necessarily directly underneath it. The greatest lift-off occurs away from the load.

Question 22

What important lessons can we learn from analysing bone loading in a knee replacement using the Beam on Elastic Foundation theory?

Answer 22

1. A component loaded at the edge puts the underlying trabecular bone at risk.
2. A component loaded at the edge increases the chance of lift-off away from the load
3. If the bone-implant interface fails in tension, compressive forces on the bone increase
4. The smallest maximum compressive stress occurs for rigid beam behaviour with a centred load
5. A rigid beam subject to off-centre forces will tilt, and compressive and tensile stresses will increase at the interface

Question 23

Describe a potential simple FE model of a PCL substituting knee replacement tibial component.

Answer 23

• Cortical shell elements that increase in thickness in proximal-distal direction
• Trabecular bone modelled with computationally more expensive brick elements

Question 24

Based on the results of a 3D FEA of the tibial component of a bicondylar knee prosthesis, discuss the advantages and disadvantages of metal backing from a stress perspective, considering a range of possible loading conditions.

Answer 24

• Compression within bone tissue is decreased
• Unwanted tension when load is eccentric w.r.t center (cases 4,5,6,8), solved by mechanically interlocking the metal backing to the tray.

Question 25

Explain what the main clinical advantage of using metal backing would be.

Answer 25

By using metal backing, surgeon can implant different types of PE plateaus and thus try out different configurations to meet the optimal one.

Question 26

Explain the potential advantages of including a peg vs no peg in a bicondylar knee prosthesis design.

Answer 26

• Although not very significant, the peg can function as a moderate stress reducer
• Provide resistance to tilting
• Provide resistance to torsion

Question 27

To compute loading in the tibial component of a knee replacement using FEA, we assume bonding between the component and the underlying bone. What does BOEF theory tells us about the effect of these surfaces not being bonded?

Answer 27

The maximum compressive stress will be even higher. Bonding minimises the stress on the bone.

Question 28

Why is it important for a tibial component design to include a mechanical interlock mechanism between on cement and the device, or a porous surface to encourage bone ingrowth?

Answer 28

Because by bonding the component to the bone, we minimise stresses on the bone.

Question 29

What important lessons can we learn from analysing bone loading in a knee replacement using FEA?

Answer 29

1. It is important to balance the knee so that single plateau loading is avoided
2. Use techniques and devices which maximise interlock between the device and the bone
3. Choose devices and techniques which avoid edge loading

Question 30

How is the clinical success of knee replacements evaluated in the long run?

Answer 30

• Survival analysis: how long does the implant last before there is a need for revision surgery
• Retrieval analysis: examine the revised component to determine failure modes