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BMSc SB > Implant Technology Unit 4 > Flashcards

Implant Technology Unit 4 Flashcards

1
Q

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

A
  • 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
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2
Q

what operation can be done if the subtalar joint remains healthy

A

arthrodesis - fusion

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3
Q

what is adv and disadv of fusion of the subtalar joint

A

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

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4
Q

how will patient w/ subtalar joint fusion walk

A

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

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5
Q

what is the general criteria for ankle joint replacement

A
  • 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
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6
Q

what are the main materials used for ankle joint replacement

A

Cobalt chrome and high density polyethylene

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7
Q

what are the 3 main reasons ankle prostheses fail

A

1 - Overloading causing subsidence,

2 - loosening due to poor fixation,

3 - high torques, which can cause interface loosening.

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8
Q

what is the ankle joint also known as

A

tibio-talar or talocrural joint

[ankle joint is between the tibia and the talus]

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9
Q

what is the normal range of motion of the ankle

A

25 to 30 degrees in both dorsiflexion and plantarflexion

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10
Q

how many axis rotation is there is the ankle

A

1 single axis of rotation

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

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11
Q

what is the subtalar joint also known as

A

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

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12
Q

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

A

inversion-eversion

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

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13
Q

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

A

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]

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14
Q

what is the force at the ankle during walking

A

4 to 5 x BW

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15
Q

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

A

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

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16
Q

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

A

around 15 degrees in both plantar flexion and dorsiflexion

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17
Q

what kinematic function does the subtalar joint perform

A

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.

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18
Q

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

A

500% body weight and 70% body weight

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19
Q

what are the 2 classifications of ankle joint replacements

A

congruent - i.e. have matching bearing surface

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

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20
Q

what do congruent ankle replacements allow for

A

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

21
Q

what are the 4 shapes of congruent ankle replacement joints

A

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

22
Q

what does the spherical design allow

A

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
Q

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

A

cylindrical

24
Q

what does the spheroidal shape provide

A

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

25
Q

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

A

false

- there is no particular adv

26
Q

what does the conical shape implant provide

A

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
Q

what shape of congruent implant have most ankle replacements been

A

cylindrical design

28
Q

what does the cylindrical design provide

A

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

29
Q

what is a disadvantage of the cylindrical design

A

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

30
Q

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

A

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

31
Q

what is an adv of incongruent shapes of ankle replacement joints

A

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

32
Q

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

A

plantarflexion-dorsiflexion

some inversion-eversion and axial rotation

33
Q

what are the 2 shapes of incongruent ankle replacements

A

1 - trochlear

2 - convex-concave

34
Q

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

A

cylindrical shape

spherical shape

35
Q

what are the problems w/ incongruent shapes

A

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
Q

how have most ankle replacement joints been made

A

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
Q

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

A

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

38
Q

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

A

Adv - economical on space

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

39
Q

what is the current clinical result of ankle replacements

A

65% failure at 5 years

40
Q

what are causes of failure

A

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
Q

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

A

high contact forces

inversion-eversion instability

subluxation

excessive bone resection

42
Q

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

A

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
Q

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

A

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
Q

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

A

cementless designs seem to be superior

45
Q

what are features of a cementless design

A

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
Q

List two trends in modern ankle joint prostheses design.

A

move towards meniscal bearing

move towards cementless types

47
Q

one advantage of meniscal bearing ankle prostheses over cylindrical bearing design

A

Congruence (low wear) yet freedom of joint motion.

48
Q

example of cementless design

A

Koefod STAR 1985

BMSc SB (18 decks)

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