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BMSc - Applied Ortho Semester 2 > Implant technology - unit 4 deck 1 > Flashcards

Implant technology - unit 4 deck 1 Flashcards

1
Q

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

A
  • The ankle is not frequently involved in primary OA so less attention has perhaps been paid to the design of a replacement joint.
  • In secondary OA and RA there is a tendency for the ankle to be affected in association with other joints - particularly the subtalar joint. In these cases, replacing the ankle joint alone would not be adequate.
  • The ankle functions in association with the subtalar joint and the motion of this joint has to be taken into account in the design of the replacement joint.
  • An alternative procedure, arthrodesis (fusion) is available, which is a successful operation provided the subtalar joint remains healthy
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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 ankle 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 which can damage these joints.

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4
Q

How will a patient with an ankle joint arthrodesis 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 tolerated within the human body with no short term and little long term risk of adverse toxic effects such as carcinogenesis (inducing cancer).
  • Relieve pain and restore the 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.
  • Be cost-effective
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6
Q

what are the main materials used for ankle joint replacement

A

Cobalt chrome and HDP

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7
Q

What ADL’s can be restored by an ankle replacement but not a fusion?

A

Walking and raising from a chair

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8
Q

Ankle joint replacement designs of the last 20 years has failed to meet a design which lasts a reasonably long length of time, ideally longer than the lifespan of the patient.

What are the main reasons ankle joint replacements fail?

A
  • The loads across the ankle are larger than was anticipated for early designs ==> overloading causes subsidence
  • The bony areas where ankle replacement components can be fixed are barely adequate to provide sufficient support using current cementing techniques ==> loosening due to poor fixation
  • The presence of any restriction in movement of the subtalar joint, such as stiffness induced by associated arthritis - common in rheumatoid arthritis - will subject the ankle to large axially generated torques, which will add to the loosening process in already weak anchorages
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9
Q

what is the ankle joint also known as

A

tibio-talar or talocrural joint - it is the joint between the tibia and the talus

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10
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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11
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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12
Q

what is the subtalar joint also known as

A

talo-calcaneal joint - it is the joint between calcaneus and the talus

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13
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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14
Q

The combined motion of what two joints is important for activities such as walking ?

A

The ankle and subtalar joints

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15
Q

Why must a replacement ankle joint must therefore allow sufficient dorsiflexion, what activity is this particularly important for ?

A

This is particularly important for getting up out of a chair

as this requires 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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16
Q

What is the magnitude of forces on the ankle joint during normal load bearing activities e.g. walking and like other lower limb joints what are these forces due to?

A

4/5 times BW - due to to a combination of gravitational and muscular forces.

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17
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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18
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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19
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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20
Q

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

A

500% (5x’s) BW and 70% (0.7x’s) BW

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21
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
22
Q

what do congruent ankle replacements allow for

A
  • Allow for rotation only.
  • The number of axes of rotation can be limited
23
Q

what are the 4 shapes of congruent ankle replacement joints

A

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

Think - SSC(C)

24
Q

what does the spherical design allow and therefore what can it help compensate for

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]

25
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

Medio-lateral space in the joint is limited so it has been argued that the cylindrical type is preferable

26
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

27
Q

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

A

false - there is no particular adv

28
Q

what does the conical shape implant providea and what does it require

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
29
Q

what shape of congruent implant have most ankle replacements been

A

cylindrical design

30
Q

what does the cylindrical design provide and what can it not compensate for

A

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

31
Q

what is a disadvantage of the cylindrical design

A

can cause high medio-lateral forces being transmitted causing shear stresses at the bone-implant interfaces

32
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

33
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

34
Q

what are the 2 shapes of incongruent ankle replacements

A

1 - trochlear

2 - convex-concave

Think CT

35
Q

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

A

cylindrical shape

spherical shape

36
Q

what are the problems with incongruent shapes

A
  1. higher rate of depth of wear than congruent types
  2. higher contact stresses due to a lower contact area than the congruent types
  3. less stability than congruent types due to their greater freedom of movement
37
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
38
Q

In ankle joint replacements what are used for resisting torques or sideways loads ?

A

One or two fins or other type of protrusion - they also provide a larger surface area if cement is used

39
Q

What is the difference between a congruent and an incongruent bearing surface?

A

Congruent designs have matching surfaces, incongruent ones do not and therefore tend to have more freedom of movement

40
Q

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

A

They dont need to be circular so require less mediolateral space

41
Q

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

A
  • Advantage = economical on space
  • Disadvantage = has no axial rotation or inversion-eversion motion ==> cant compensate for subtalar dysfunction
42
Q

what is the current clinical result of ankle replacements

A

65% failure at 5 years

43
Q

what are the causes of failure in ankle joint replacements?

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
44
Q

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

A
  • high contact forces
  • inversion-eversion instability
  • subluxation
  • excessive bone resection
45
Q

Which type of ankle replacements - congruent or non-congruent give the best wear resistance ?

A

Congruent

46
Q

Why has failure due to wear NOT been the main problem in congruent ankle joint replacements?

A

The bearing surface tends to be quite large so high contact stresses in a congruent design is not a problem

47
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
  • They are both uncemented with porous coated surfaces
  • Both have have hollow tibial bearing surface
  • They also both have a meniscal bearing, which 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.
48
Q

designs with or without cement seem to be better for ankle joint replacements?

A

cementless designs seem to be superior

49
Q

Give an example of a cementless ankle joint replacement

A

Koefod STAR 1985

50
Q

Describe the design of the Kofoed STAR prosthesis

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.
51
Q

List two trends in modern ankle joint prostheses design

A
  1. move towards meniscal bearing
  2. move towards cementless types
52
Q

one advantage of meniscal bearing ankle prostheses over cylindrical bearing design

A

It 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.

==> congruence (low wear) associated with congruent designs yet it provides freedom of motion (as explained in sentence above)

BMSc - Applied Ortho Semester 2 (25 decks)

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