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Flashcards in Implant Technology Unit 4 Deck (48):
1

what are some suggested reasons for the lack of success in ankle joint replacements

- ankle is not freq involved in primary OA so less attention has been paid

- in secondary arthritis and RA the ankle is affected in association w/ other joints, particularly the subtalar joint, therefore replacing ankle joint alone would not help

- ankle functions in association with the subtalar joint and the motion of this joint has to be taken into account in designs of replacement joints

2

what operation can be done if the subtalar joint remains healthy

arthrodesis - fusion

3

what is adv and disadv of fusion of the subtalar joint

adv - fusion relieves pain in a stiff joint w/out need to provide any compensation for resulting loss of movement

disadv - resulting bio-mechanical changes in motion and load transmission leads to abnormal loadings on the knee and subtalar joint on the same leg, and a shortening of stride

4

how will patient w/ subtalar joint fusion walk

will walk out toed so that the subtalar joint acts as a dorsiflexor of the foot

5

what is the general criteria for ankle joint replacement

- be tolerable in human body w/ no short term risk and little long term risk of adverse toxic effects

- relieve pain and restore activities of daily living

- Last a reasonable length of time which ideally should exceed the expected life span of the patient

- Be insertable by a competent surgeon of average ability such that a predictable outcome can be reasonably guaranteed.

- cost effective

6

what are the main materials used for ankle joint replacement

Cobalt chrome and high density polyethylene

7

what are the 3 main reasons ankle prostheses fail

1 - Overloading causing subsidence,

2 - loosening due to poor fixation,

3 - high torques, which can cause interface loosening.

8

what is the ankle joint also known as

tibio-talar or talocrural joint

[ankle joint is between the tibia and the talus]

9

what is the normal range of motion of the ankle

25 to 30 degrees in both dorsiflexion and plantarflexion

10

how many axis rotation is there is the ankle

1 single axis of rotation

axis is not perpendicular to the sagittal plane but is inclined downwards and posteriorly on the lateral side

11

what is the subtalar joint also known as

talo-calcaneal joint [joint between calcaneus and the talus]

12

what motion does the subtalar joint allow and what does this movement allow

inversion-eversion

helps to allow the foot to stand flat on level and uneven surfaces, which the ankle joint alone cannot achieve

13

what activity of daily living, apart from walking is the subtalar joint important for

getting up from a chair

involved dorsiflexion of the ankle for the trunk to move forward

if both ankles are affected then a supreme upper limb effort is required to stand [difficult for frail or severe RA patients]

14

what is the force at the ankle during walking

4 to 5 x BW

15

what forces must be resisted by an ankle replacement to prevent subluxation of the joint

fore-aft forces that occur at foot-ground contact during walking that produce a shear force at the ankle joint

16

What is the range of motion of the ankle joint during walking?

around 15 degrees in both plantar flexion and dorsiflexion

17

what kinematic function does the subtalar joint perform

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.

18

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

500% body weight and 70% body weight

19

what are the 2 classifications of ankle joint replacements

congruent - i.e. have matching bearing surface

incongruent - i.e. do not have matching bearing surface

20

what do congruent ankle replacements allow for

allow for rotation only and the number of axes of rotation can be limited

21

what are the 4 shapes of congruent ankle replacement joints

1 - spherical
2 - spheroidal
3 - conical
4 - cylindrical

22

what does the spherical design allow

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

[has a specific centre of rotation and therefore required careful position during insertion]

23

if the spherical and cylindrical types have the same medio-lateral width of bearing surface, what style gives a greater angle of plantarflexion/dorsiflexion rotation

cylindrical

24

what does the spheroidal shape provide

plantarflexion-dorsiflexion and inversion-eversion motion, but no axial rotation because its curvature is different in the sagittal an frontal planes

25

true or false - the spheroidal shape replacement has a massive advantage over the spherical type

false
- there is no particular adv

26

what does the conical shape implant provide

provides a single axis of plantarflexion/dorsiflexion rotation and some medio-lateral resistance

requires a greater amount of bone resection than a cylindrical shape

27

what shape of congruent implant have most ankle replacements been

cylindrical design

28

what does the cylindrical design provide

a basic single axis replication of the ankle joint but cannot compensate for subtalar dysfunction

29

what is a disadvantage of the cylindrical design

creates an area of concentrated stress under asymmetrical medio-lateral loading

30

what is the main feature of incongruent shapes of ankle replacement joints

less constraint in the movement so that some horizontal motion is possible

31

what is an adv of incongruent shapes of ankle replacement joints

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

32

incongruent ankle replacements that are a trochlear (saddle) shape, allow for what

plantarflexion-dorsiflexion

some inversion-eversion and axial rotation

33

what are the 2 shapes of incongruent ankle replacements

1 - trochlear

2 - convex-concave

34

what are the 2 shapes of convex-concave incongruent ankle replacements

cylindrical shape

spherical shape

35

what are the problems w/ incongruent shapes

higher rate of depth of wear than congruent types

higher contact stresses due to a lower contact area than the congruent types

less stability than congruent types due to their greater freedom of movement

36

how have most ankle replacement joints been made

as cylindrical type

and most have been made of combo of cobalt chrome or stainless steel for one component and HDP for the other

37

Why are cylindrical bearing surfaces more economical in the use of space than non-cylindrical ones

They don’t need to be circular as seen from the top so require less mediolateral space

38

one advantage and one disadvantage of cylindrical designs compared with spherical designs.

Adv - economical on space

Disadv - has no axial rotation or inversion-eversion motion.

39

what is the current clinical result of ankle replacements

65% failure at 5 years

40

what are causes of failure

aseptic loosening of a component

lateral or medial subluxation of the joint

subsidence of the talar component

impingement of the joint

would healing problems

infection

41

what are some problems identified in ankle joint replacements that lead to issuse

high contact forces

inversion-eversion instability

subluxation

excessive bone resection

42

what is the difference seen in congruent and incongruent designs and why is the thought to be the case

congruent designs
- have been found to give better wear resistance than non-congruent designs
- as the bearing surface tends to be quite large so high contact stress are not a problem

43

the best results so far for ankle replacements have been obtained from the Jersey LCS prosthesis and from the Beuchel-Pappas prosthesis - what design features have thought to be the cause of its success

both have hollow tibial bearing surface

both have a meniscal bearing
- provides congruent bearing surfaces at the tibial and talar interfaces
- but w/out disadv of rigid transmission of medio-lateral and rotational shear forces associated w/ constrained designs, which has been linked to loosening problems

44

designs w/ or w/out cement seem to be better

cementless designs seem to be superior

45

what are features of a cementless design

Cylindrical bars on the tibial component slide into holes drilled in the tibia,

Stabilising pin used for fixation of the talar component

Both the front and the back of the component are designed to rest on cortical bone. Interfaces to the bone on both components are coated with hydroxyapatite

HDP floating meniscus is partially constrained for plantarflexion-dorsiflexion movement by a rib on the top of the talar component.

46

List two trends in modern ankle joint prostheses design.

move towards meniscal bearing

move towards cementless types

47

one advantage of meniscal bearing ankle prostheses over cylindrical bearing design

Congruence (low wear) yet freedom of joint motion.

48

example of cementless design

Koefod STAR 1985