Ch. 9 - Total Hip Replacements

Question 1

What is the aim of a THR?

Answer 1

The aim is to restore normal articulation and motion btw the femoral head and acetabulum.

Question 2

What does a THR usually consist of?

Answer 2

Generally the THR consists of a femoral stem inserted into the intramedullary canal of the proximal femur, a femoral head and plastic liner (which are fairly congruent), and an acetabular cup component which is fixed to the pelvis (within which the plastic line sits in).

Question 3

What is the goal of hip resurfacing?

Answer 3

The goal is to replace only the degenerated joint surfaces to restore smooth articulation btw femur and pelvis. It is more common in younger patients because more of the original bone anatomy can be preserved.

Question 4

What is the Swedish Arthroplasty Register?

Answer 4

The Swedish Arthoplasty Register includes all operations related to hip implant in Sweden starting from 1979. The aggregate data of this database is publicly available.

Question 5

What does the graphs of the Swedish Arthroplasty Register (slide 6) show regarding the trends of THR primary and revision procedures over the past 25 years?

Answer 5

• THR increase over the past 25 years
• Steady increase in primary procedures
• Slower increase in revision procedures (this could indicate device design has improved)

Question 6

What does this graph (slide 7) show regarding the trend in primary THR operations over the past 50 years in Sweden?

Answer 6

• Increase in operations appears roughly linear over past 50 years
• Step change in the early 90s

Question 7

What can be highlighted regarding the causes for THR?

Answer 7

• Main cause is primary OA
• With increasing age, hip fracture becomes a larger factor (probs due to increase in fragility fractures due to osteoporosis)
• For patients under 50, childhood disease is another primary diagnosis

Question 8

What are the different components of the THR usually made of?

Answer 8

• Cup: metal or PE
• Liner: ceramic or PE
• Head: ceramic or metal
• Femoral stem: metal

Question 9

How is the implant procedure different for cemented vs uncemented designs?

Answer 9

• For the cemented design, the shaft of the femur is reamed and then a plug inserted into the shaft to prevent the spread of cement through the medullary canal. The cement surrounds the implant and fills the medullary cavity. For the acetabular component, the acetabulum is reamed and then the component placed using cement.
• For the uncemented design, the femoral component is covered with a porous coating which fits tightly into the medullary canal to enhance bony ingrowth. The uncemented acetabular cup design also has a porous coating but is also attached with screws to ensure it remains in place.

Question 10

What is wrong with these THR images?

Answer 10

Image 1 - The image is labelled “cemented hip prosthesis” and yet it shows both the femoral stem and the acetabular cup with a porous coating. A porous coating is only present in uncemented designs.
Image 2 - The acetabular component has a smooth surface and is fixed with cement, which is accurate. However, the femoral stem has a porous coating and is also fixed with cement, which is incorrect.

Question 11

What are the advantages of a short femoral stem THR design?

Answer 11

• Normal anatomy
• Reduced stress-shielding
• Preserve bone and soft-tissue
• Less invasive surgery

Question 12

How do preferred THR fixation methods differ between young and older populations? Why?

Answer 12

The most common method for younger patients is the fully uncemented THA because they tend to have good bone quality and are thus expected to have good bony ingrowth. Furthermore, it is more likely a younger patient will need a revision surgery within their lifetime and in this case the revision surgery is simplified with a short stem design as the surgeon has a) a lager bony footprint and b) does not have to remove the cement from the previous THA.
For older patients, the use of fully cemented THA is most popular because they may have diminished bone quality.

Question 13

What anatomical considerations must be taken into account in the design of a THR?

Answer 13

1. Femoral neck shaft angle - affects the relationship btw the femoral shaft and the articulation of the femoral head within the joint
2. Femoral version - angle of the femoral neck relative to the axis of the condyles of the distal femur
3. Acetabular version - angle measured relative to the medial-lateral axis of the pelvis
4. Femoral neck length
5. Femoral offset
6. Patient natural ROM
7. Patient muscle attachments
8. Size of the implant

Question 14

Describe the complications that could arise from using a shorter or longer than ideal femoral neck length.

Answer 14

• Shorter neck: muscles become slack and are unable to provide proper moment arms and function. Overall leg length may become shorter in which case risk for degeneration of the contralateral hip is high due to imbalanced loading.
• Longer neck: muscles are tight and may become torn or damaged.

Question 15

Describe the problem with using a too short or too long neck offset.

Answer 15

• Muscle function will not be optimal and may result in loss of function
• Although leg length will probably not change, the kinematics, as well as the magnitude and direction of the transmitted joint forces will change, given as the moment arms of the muscle and joint forces may change dramatically

Question 16

What are the implications of having a smaller vs. larger diameter joint?

Answer 16

Smaller diameter joint:
- smaller contact area
- greater contact stress, which may increase wear rate of surfaces
- may lead to impingement over ROM
- increased risk of dislocation (smaller jump distance)
Larger diameter joint
- lower wear rate, BUT larger contact area
- ROM may be submaximal

Question 17

How is wear rate affected by component materials?

Answer 17

• Highest wear rate observed with metal-PE implants
• Wear rate reduced by 3x with ceramic-PE
• Both metal-metal and ceramic-ceramic have low wear rates
• Metal-PE releases large amount of particulates
• Metal-metal have small amount of particulate (but these particles trigger a more severe reaction)

Question 18

What are possible long-term failure modes for a THR?

Answer 18

1. Cement fracture
2. Bone-cement interface fracture
3. Bone loss due to osteolysis from wear debris
4. Bone loss due to stress-shielding

Question 19

For failure in the proximal zone of a THR, what analysis method would you use to analyse a) cement fracture, b) bone-cement interface failure?

Answer 19

a) BOEF

b) FEA

Question 20

For failure in the central zone of a THR, what analysis method would you use to analyse a stem fracture?

Answer 20

CBT, BOEF, FEA

Question 21

For failure in the distal zone of a THR, what analysis method would you use to analysis a) cement fracture, b) bone fracture?

Answer 21

a) BOEF

b) FEA

Question 22

As the ratio of the stiffness of the prosthetic to that of the bone increases, what happens to the load carried by the bone?

Answer 22

It decreases.

Question 23

Imagine we are interested in analysing the stresses in the central zone of a THR. Under which circumstances would we use BOEF over CBT?

Answer 23

• If we are interested in the load transfer between components or in the stresses in transition zones.

Question 24

In FEA, what does the level of detail of the geometry depend on?

Answer 24

It is largely dependent on the size of the mesh elements.

Question 25

Consider the FEA model shown in slide 38. Give examples of 3 different aspects that could be investigated using this model.

Answer 25

1. The effect of the flat or round stem design for the distal portion of the femoral stem
2. The effect of neck length
3. The role of cemented or uncemented designs by considering differing material properties of the interface elements between the bone and the implant

Question 26

Consider a musculoskeletal model software such as OpenSim. What are the system inputs and outputs?

Answer 26

Inputs:
- GRFs
- Kinematics
- Geometry
Outputs:
- Muscle forces
- Joint contact forces

Question 27

Why may it be useful to implant patients with instrumented hip prostheses?

Answer 27

They allow us to measure the direction and magnitude of the forces transmitted over the hip joint in vivo. This provides accurate data that can be used, for example, to generate boundary conditions for FEA of THA patients.

Question 28

To run an FEA we may need to input material properties. How can these values be obtained for local bone?

Answer 28

Using CT imaging, the inherent relationship between bone density and image voxel intensity can be used to capture local variations in bone strength and density.

Question 29

Suggest a possible approach to combat hip joint loading pattern that could result in stress shielding.

Answer 29

One method would be to incorporate a variable stiffness stem, such that the implant is stiffer proximally than distally. This would provide a balance in the loading transmitted across the cortical bone of the femur, this minimising the effects of stress shielding.

Question 30

What are the complications of higher stiffness THR implants?

Answer 30

• Decrease physiological loading
• Proximal cortical atrophy
• Distal cortical and medullary bone hypertrophy