Hip Implants

Question 1

What is a labrum?

Answer 1

A cartilaginous extension which deepens the acetabulum socket and provides more mobility by not acting as a geometric stop.

Question 2

Name the 5 reasons that someone might get a hip replacement.

Answer 2

Osteoarthritis.
Rheumatoid arthritis.
Traumatic arthritis.
Hip fractures.
Avascular necrosis - head of femur loses blood supply.

Question 3

What is the KTH complex?

Answer 3

Knee thigh hip.

Question 4

What are the three components of the modern hip implant?

Answer 4

1. Femoral stem and head (metal)
2. Acetabular component
3. Bone cement (PMMA)

Question 5

Explain the importance of head size.

Answer 5

Larger heads result in less dislocation but greater acetabular wear.

Question 6

Why would a femoral head be made porous?

Answer 6

For greater surface area which leads to better bone growth/integration.

Question 7

What is the Austin Moore device?

Answer 7

Small hole in stem containing bone growth. Anchorage depended on bone growth into hole. This was unreliable.

Question 8

What is HA?

Answer 8

Hydroxyapatite which is coated onto some implants to promote bone growth.

Question 9

What are some design considerations for hip implants?

Answer 9

Hip must support the body weight and range of motion.

Question 10

What is the range of motion for the human hip?

Answer 10

140 degrees flexion/extension

75 degrees rotation

Question 11

What are the forces acting on the hip?

Answer 11

Muscles
Body weight
Normal forces

Question 12

What is the angle of inclination?

Answer 12

Angle between the shaft of the femur and the neck of the femur. Normally around 125 degrees. It affects loading pattern and limb length.

Question 13

What does a greater angle of inclination mean?

Answer 13

Valgus deformation (coxavalga). Distal end of joint moves laterally. Greater axial load on head of femur.

Question 14

What does a smaller angle of inclination mean?

Answer 14

Varus deformation (coxavara). Knee points inward. Increased bending moment on neck of femur.

Question 15

What is the angle of torsion?

Answer 15

The bottom end of the femur naturally twists inwards by about 15 degrees.

Question 16

What does an increased angle of torsion mean?

Answer 16

Feet point inward (pigeon toed)

Question 17

What does a decreased angle of torsion mean?

Answer 17

Feet point outward (duck feet)

Question 18

What is the current design for hip implants?

Answer 18

Femoral stem with separate head piece which allows for length adjustment.
2 part acetabular component - metallic shell anchors to bone, separate liner (made of plastic) attaches to metal shell.

Question 19

What are the three types of hip replacement?

Answer 19

Resurfacing
Mini hip
Total hip replacement

Question 20

What are the criteria for selecting which type of hip replacement to use?

Answer 20

If the patient has really good bone quality with a fractured acetabular shell - just a femoral ball and acetabular cup is implanted.

Decent bone quality = mini hip. Halfway between resurfacing and total hip replacement.

Question 21

What is the articular interface?

Answer 21

Space between the acetabular cup and femoral head.

Question 22

Describe the surgical procedure for a hip replacement.

Answer 22

Remove top of femur.
Cup implant fitted into prepared hip socket.
Tapered hip prosthesis inserted into femur.
Femoral head fitted into prosthetic stem.
Femoral head inserted into cup implant.

Question 23

Rank the most common materials in terms of the amount of wear.

Answer 23

1. Metal on plastic
2. Metal on metal
3. Ceramic on plastic
4. Ceramic on ceramic

Question 24

What is the most common combination of materials?

Answer 24

Metal on polyethylene

Question 25

Describe the issue with device wearing.

Answer 25

Devices wears, implant loosens slightly until there is a small gap, greater bone resorption, then osteolysis.

Question 26

Describe some advantages (3) and disadvantages (1) for ceramic on ceramic hip implants.

Answer 26

Pros: excellent wear resistance, no toxic side effects, no evidence of osteolysis. Cons: Can cause squeaky hip.

Question 27

Describe some advantages (2) and disadvantages (1) for ceramic on polyethylene hip implants.

Answer 27

Pros: new ceramics have low fracture rates, less expensive than ceramic on ceramic Cons: more expensive than metal on polyethylene

Question 28

Describe some advantages (2) and disadvantages (2) for metal on metal hip implants.

Answer 28

Pros: allow for larger ball heads, greatly reduce wear Cons: metal poisoning, not good for women or people with reduced kidney function

Question 29

Name the 5 surgical techniques used to implant a hip replacement.

Answer 29

``` Posterior Lateral Antero-lateral Anterior Minimally invasive ```

Question 30

Describe some advantages (2) and disadvantages (2) to the posterior approach.

Answer 30

Pros: provides good access to femur and acetabulum, preserves hip abductors. Cons: most invasive, high dislocation risk

Question 31

Describe some advantages (1) and disadvantages (1) to the lateral approach.

Answer 31

Pros: lower dislocation risk Cons: requires elevation of hip abductors which occasionally results in improper healing.

Question 32

Describe some advantages (5) to the anterio-lateral approach.

Answer 32

Pros: minimally invasive (most common), allows access without cutting any muscles or tendons, implant more likely to stay in place, less pain, quicker recovery Cons:

Question 33

Describe some advantages (1) and disadvantages (1) to the anterior approach.

Answer 33

Pros: reduces dislocation rates in smaller diameter heads Cons: 10% rate of numbness due to nerve injury

Question 34

Describe some advantages (1) and disadvantages (1) to the minimally invasive approach.

Answer 34

Pros: reduces soft tissue damage Cons: visualization and accuracy are impaired - unintended fractures or soft tissue injury.

Question 35

Name 8 risks and complications involved in hip replacement surgery.

Answer 35

``` Dislocation Osteolysis Metal sensitivity Metal toxicity (metallosis) Metal on metal implant failure Nerve palsy Chronic pain Unequal leg lengths ```

Question 36

Describe dislocation.

Answer 36

Most common complication. | Liable to occur until soft tissue heals.

Question 37

Describe osteolysis

Answer 37

Body reacts to wear debris. | Inflammatory response causes bone resorption then loosening of implant.

Question 38

Describe metal sensitivity

Answer 38

Causes pseudotumors. | Can trigger immune reactions.

Question 39

Describe metal toxicity.

Answer 39

Implants release ions into the blood stream.

Question 40

Describe metal on metal implant failure.

Answer 40

Release particles due to wear. | Cobalt and chromium ions enter bloodstream.

Question 41

Describe nerve palsy

Answer 41

Low incidence. | Resolved over time.

Question 42

Describe chronic pain

Answer 42

Groin pain of muscles rub against acetabular cup. | Pain in cold or damp weather.

Question 43

Describe unequal leg lengths.

Answer 43

Leg can be lengthened or shortened. Most common complaint. Can be mental or caused by improper implant selection.