Imp Tech U4

Question 1

Are there any good ankle replacements currently in use?

Answer 1

No (fusion is a suitable alternative)

Question 2

Why does standing up from a chair require good motion of the ankle joint?

Answer 2

Dorsiflexion for trunk to move forwards

Question 3

What are the 2 main joints in the ankle? What motions do they allow?

Answer 3

Tibio-talar (tibia-talus); dorsi/plantarflexion

Subtalar (talus-calcaneus); inversion/eversion

Question 4

What type of forces go through the ankle joint?

Answer 4

Compression (4-5xBW)

Shear (0.4-0.7xBW) from fore-aft forces due to ground contact

Question 5

What are congruent and incongruent replacement designs?

Answer 5

Congruent = matching bearing surfaces (allow rotation only)

Incongruent =not matching bearing surfaces (less constraint in movement so some horizontal motion)

Question 6

What are the 4 types of congruent ankle prosthesis?

Answer 6

Spherical
Spheroidal
Conical
Cylindrical

Question 7

What are the advantages/disadvantages of a spherical design?

Answer 7

Pros:
- freedom of rotation (good compensation for subtalar join)

Cons;

• Specific centre of rotation so needs to be carefully positioned
• Less shear resistance than a cylindrical design

Question 8

What are the pros/cons of the cylindrical design?

Answer 8

Pros:

• greater plantar/dorsiflexion for the same width (than spherical)
• useful when space is limited

Cons

• doesn’t allow any eversion/inversion (can’t compensate for subtalar joint)
• Can cause an area of concentrated stress if asymmetrically loaded (med/lat)
• can cause high shear stress at interface

Question 9

What are the pros/cons of the spheroidal design?

Answer 9

Pros’
- allows plantar/dorsiflexion and eversion/inversion

Cons;
- no advantage of spherical

Question 10

What are the pros/cons of the conical design?

Answer 10

Pros; allows plantar/dorsiflexion

Cons; requires greater bone resection than cylindrical

Question 11

What are the pros/cons of incongruent shapes of ankle prosthesis?

Answer 11

Pros;
- allow some horizontal motion (transfers load onto soft tissues and reduces load to the interface)

Cons;

• higher depth of wear than congruent
• high contact stress (as smaller contact area)
• less stability (greater freedom of movement)

Question 12

What are the causes of failure of ankle prostheses?

Answer 12

Aseptic loosening
Lateral/medial subluxation
Ta;ar subsidence
Impingement of the joint
Wound healing problems 
Infection

(not many studies into specific feature causing failure - wear is not the main problem)

Question 13

What 2 ankle prostheses have the best results?

Answer 13

Buechel Pappas and New Jersey LCS

Question 14

What is shown to be better; cemented or cementless?

Answer 14

Cementless

Question 15

How are the tibial and talar components fixed on LCS or Buechel-pappas prostheses?

Answer 15

Tibial; Using a cylindrical bar which slides into holes drilled into the tibia

Talar; stabilising fins

Question 16

What features of the LCS/Buechel-pappas prostheses are successful?

Answer 16

Floating meniscal bearing surface

• partially constrained for plantar/dorsiflexion
• no rigid transmission of shear forces like with constrained designs

Question 17

How is it advised that a prosthesis is sealed to the talus?

Answer 17

Cemented to the top (stronger when no bone resection)

Question 18

How is it advised that a prostheses is attached to the tibia? Why?

Answer 18

Tibial stem (cancellous bone in distal tibia isn't strong enough to take load of tibial component directly - transfers some load to stronger proximal bone)
Should be rested on anterior/posterior cortical bone to prevent sinking into tibia