Unit 4: Ankle Joint Replacement

Question 1

Why are ankle joint replacements unsuccessful?

Answer 1

• joint isn’t involved in primary OA so it gets less attention
• if secondary OA or RA then other joints tend to be involved eg. subtalar
• ankle joint moves in assoc. with the subtalar joint so a replacement must accomodate this
• arthrodesis is successful at removing pain

Question 2

What complications does arthrodesis cause?

Answer 2

abnormal loadings on the knee and subtalar joint on the same leg
shortening of stride which has consequences for the opposite limb joints.

The patient will walk out toed so that the subtalar joint, in effect, acts as a dorsiflexor of the foot. These effects can cause long term damage to these joints.

Question 3

What bones does the ankle joint lie between?

Answer 3

tib/fib and talus

Question 4

What bones does the subtalar joint lie between?

Answer 4

talus and calcaneus

Question 5

What are ankle joint replacements commonly made out of?

Answer 5

metal (cobalt chrome or stainless steel) and HDP

Question 6

What activities of daily living can an ankle replacement allow for that arthrodesis does not?

Answer 6

walking

rising from seated

Question 7

Why do ankle replacements not last a reasonable length of time?

Answer 7

large loads
small area for bone fixation
restriction of movement at subtalar joint causes large axially generated torques that result in loosening

Question 8

3 main reasons an ankle replacement fails

Answer 8

1. Overloading causing subsidence,
2. loosening due to poor fixation,
3. high torques which can cause interface loosening.

Question 9

What is the normal ROM of the ankle joint?

Answer 9

25-30 degrees of dorsi and plantarflexion

Question 10

What is the ROM of the ankle joint during walking

Answer 10

around 15 degrees of dorsi and plantarflexion

Question 11

What kinematic function does the subtalar joint perform?

Answer 11

Provides eversion-inversion of foot which, in combination with the ankle joint motion, helps to provide axial rotation and ease of planting the foot on uneven surfaces.

Question 12

What are the approximate maximum vertical and fore-aft loads on the ankle joint?

Answer 12

500% of body weight for vertical

70% of body weight for fore-aft

Question 13

What do fore-aft forces during ground contact produce? What might this cause in a replacement?

Answer 13

shearing force

subluxation

Question 14

What is the difference between a congruent and an incongruent bearing surface?

Answer 14

a congruent design has a matching bearing surface

Question 15

Which type of congruent design can compensate for subtalar dysfunction?

Answer 15

spherical

Question 16

Does a spherical or cylidrical design have a greater angle of plantar/dorsiflexion for the same medio-lateral width?

Answer 16

cylindrical

Question 17

Which congruent design requires more bone resection?

Answer 17

conical

Question 18

What type of congruent design is most commonly used?

Answer 18

cylindrical

Question 19

What is a disadvantage of the cylindrical design?

Answer 19

creates area of concentrated stress under asymmetrical medio-lateral loading

cannot compensate for subtalar dysfunction

Question 20

What is the main feature of incongruent designs?

Answer 20

they have less constraint of movement so there is some horizontal motion which reduces load transmission at the interface

Question 21

What are the problems with incongruent designs compared to congruent designs?

Answer 21

higher rate of depth of wear
lower contact area –> higher contact stresses
inc freedom of movement –> less stability

Question 22

List two trends in modern ankle joint prostheses design.

Answer 22

• meniscal bearings

- cementless types.

Question 23

List one advantage of meniscal bearing ankle prostheses over cylindrical bearing design.

Answer 23

Congruence (low wear) yet freedom of joint motion.

Question 24

Why do ankle replacements fail?

Answer 24

• aseptic loosening
• lateral or medial subuxation
• subsidence of talar component
• impingement of joint
• wound healing problems
• infection

Question 25

Are congruent or non-congruent designs more wear reistant?

Answer 25

congruent

Question 26

What may help future ankle replacement designs from failing?

Answer 26

fundamental biomechanical research and the testing of designs prior to clinical use

especially finite element analysis studies