Ankle replacement designs

Question 1

What are the 2 classifications of ankle replacements?

Answer 1

Congruent or incongruent

Question 2

What is the difference between a congruent and incongruent ankle replacement?

Answer 2

Congruent = matching bearing surfaces 
Incongruent = not matching bearing surfaces

Question 3

Name 4 types of congruent ankle replacement

Answer 3

Spherical
Spheroidal
Conical
Cylindrical

Question 4

Why is a spherical design good?

Answer 4

Allows freedom of rotation and therefore provides compensation for a degenerate subtalar joint

Question 5

Why does the spherical design need careful positioning during insertion?

Answer 5

It has a specific centre of rotation

Question 6

Why is it argued that the cylindrical type is better than the spherical when compared?

Answer 6

Cylindrical gives greater angle of plantarflexion/dorsiflexion for the same medio-lateral width of bearing surface

Question 7

What movements are enabled by the spheroidal shape?

Answer 7

Plantarflexion-dorsiflexion
Inversion-eversion
(no axial rotation because curvature is different in the sagittal and frontal planes)

Question 8

How does the spheroidal shape compare to the spherical shape?

Answer 8

Spheroidal has no particular advantages over the spherical type

Question 9

What are the disadvantages of the cylindrical design?

Answer 9

Cannot compensate for subtalar dysfunction

Creates an area of concentrated stress under symmetrical medio-lateral loading

Question 10

What is the main feature of incongruent replacements?

Answer 10

Less constraint in the movement - so some horizontal motion is possible

Question 11

Why is the horizontal motion allowed in incongruent replacements advantageous?

Answer 11

Can reduce load transmission to the bone-cement-prosthesis interfaces by transferring some of the load to the soft tissues

Question 12

Name 2 examples of incongruent ankle joint replacement deigns

Answer 12

Trochlear

Convex-concave

Question 13

What motion does a trochlear (saddle) shaped replacement allow?

Answer 13

Plantarflexion-dorsiflexion
Inversion-eversion
Axial rotation

Question 14

What are the 3 main problems with incongruent shapes?

Answer 14

Higher rate of depth of wear than congruent
Higher contact stresses (due to lower contact area)
Less stability (due to freedom of movement)

Question 15

What shape have most ankle replacements been?

Answer 15

Cylindrical

Question 16

What is the Oregon prosthesis made from?

Answer 16

Carbon reinforced polyethylene tibial component

Vitallium talar component

Question 17

What materials have most ankle replacements been made from?

Answer 17

CoCr or SS for one component

HDP for the other

Question 18

Which 2 prostheses used a ceramic material instead of a metal?

Answer 18

Oonshi

Takakura

Question 19

Why are one or two fins or other type of protrusion often used?

Answer 19

Resisting torque and sideways loads
helps to maintain fixation and reduce subsidence
provides larger surface area for keying the cement

Question 20

List 6 possible causes of failure of an ankle replacement

Answer 20

Aseptic loosening of a component 
Lateral or medial subluxation 
Subsidence of the talar component 
Impingement of the joint 
Wound healing problems 
Infection

Question 21

Which design has been found to give better wear resistance?

Answer 21

Congruent (but failure due to wear has not been the main problem)

Question 22

Which 2 ankle prostheses have had the best results so far?

Answer 22

New Jersey LCS prosthesis

Beuchel-Pappas

Question 23

What are the basic design features of the New Jersey LCS and Becuel-Pappas prostheses?

Answer 23

uncemented with porous coating
Hollow in the tibial bearing surface (BP deeper)
2 talar fins
Meniscal bearing

Question 24

What are the advantages of the meniscal bearing (found in New Jersey LCS and BP)?

Answer 24

Provides congruent bearing surfaces at the tibial and talar interfaces - but without the usual disadvantage of rigid transmission of medio-lateral and rotational shear forces associated with constrained designs which has been linked to loosening problems

Question 25

Which design came first New Jersey LCS or Beuchel-Pappas?

Answer 25

Beuchel-Pappas us a later version of the LCS

Question 26

What deficiences were found in the New Jersey LCS that the Beuchel-Pappas intended to correct?

Answer 26

Subluxation (due to lack of lateral stability in the bearing) Talar subsidence

Question 27

Do cemented or cementless designs seem n the whole to give better results?

Answer 27

Cementless designs

Question 28

What are the main features of the Kofoed STAR prosthesis?

Answer 28

Cylindrical bar on tibial component slide into holes drilled in the tibia Stabilising fin is used for fixation of the talar component Front and back designed to rest on cortical bone Interfaces coated with hydroxyapatite HDp floating meniscus is partially contrained for plantar/dorsiflexion by a ridb on the top of the talar component

Question 29

What have clinical results of the Kofoed STAR prosthesis shown?

Answer 29

Over 4 year period very good with no signs of radiological loosening (if good results continue will add to evidence that meniscal bearing prostheses are the best design solution)

Question 30

Which 2 ankle prostheses have been derived from finite element analysis?

Answer 30

Calderdale (Pipino) | Oonshi

Question 31

Why does the Calderdale prosthesis have a tibial stem?

Answer 31

Analysis found there was a real risk of the cancellous bone in the distal tibia not being strong enough to bear the tibial component directly and it would be better to transfer some of the load to the more proximal cortical bone

Question 32

Why does the Calderdale prosthesis have the talar component cemented on top of the talar dome?

Answer 32

Studies found that resection of the talar dome would result in considerable loss of strength in the underlying bone

Question 33

What issues were noted in Oonshi's finite element analysis and how were these addressed?

Answer 33

Tibial component sinking into the tibia - so their component partly rested on the anterior and posterior cortical bone of the distal tibia