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AY BMSc Ankle Replacement > Ankle replacement designs > Flashcards

Ankle replacement designs Flashcards

1
Q

What are the 2 classifications of ankle replacements?

A

Congruent or incongruent

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2
Q

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

A 
Congruent = matching bearing surfaces 
Incongruent = not matching bearing surfaces
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3
Q

Name 4 types of congruent ankle replacement

A

Spherical
Spheroidal
Conical
Cylindrical

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4
Q

Why is a spherical design good?

A

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

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5
Q

Why does the spherical design need careful positioning during insertion?

A

It has a specific centre of rotation

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6
Q

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

A

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

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7
Q

What movements are enabled by the spheroidal shape?

A

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

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8
Q

How does the spheroidal shape compare to the spherical shape?

A

Spheroidal has no particular advantages over the spherical type

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9
Q

What are the disadvantages of the cylindrical design?

A

Cannot compensate for subtalar dysfunction

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

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10
Q

What is the main feature of incongruent replacements?

A

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

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11
Q

Why is the horizontal motion allowed in incongruent replacements advantageous?

A

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

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12
Q

Name 2 examples of incongruent ankle joint replacement deigns

A

Trochlear

Convex-concave

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13
Q

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

A

Plantarflexion-dorsiflexion
Inversion-eversion
Axial rotation

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14
Q

What are the 3 main problems with incongruent shapes?

A

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

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15
Q

What shape have most ankle replacements been?

A

Cylindrical

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16
Q

What is the Oregon prosthesis made from?

A

Carbon reinforced polyethylene tibial component

Vitallium talar component

17
Q

What materials have most ankle replacements been made from?

A

CoCr or SS for one component

HDP for the other

18
Q

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

A

Oonshi

Takakura

19
Q

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

A

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

20
Q

List 6 possible causes of failure of an ankle replacement

A 
Aseptic loosening of a component 
Lateral or medial subluxation 
Subsidence of the talar component 
Impingement of the joint 
Wound healing problems 
Infection
21
Q

Which design has been found to give better wear resistance?

A

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

22
Q

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

A

New Jersey LCS prosthesis

Beuchel-Pappas

23
Q

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

A

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

24
Q

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

A

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

25
Q

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

A

Beuchel-Pappas us a later version of the LCS

26
Q

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

A

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

27
Q

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

A

Cementless designs

28
Q

What are the main features of the Kofoed STAR prosthesis?

A

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

29
Q

What have clinical results of the Kofoed STAR prosthesis shown?

A

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)

30
Q

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

A

Calderdale (Pipino)

Oonshi

31
Q

Why does the Calderdale prosthesis have a tibial stem?

A

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

32
Q

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

A

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

33
Q

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

A

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

AY BMSc Ankle Replacement (3 decks)

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