Implant technology - unit 4 deck 1 Flashcards
what are some suggested reasons for the lack of success in ankle joint replacements
- 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
what operation can be done if the subtalar joint remains healthy
arthrodesis - fusion
what is adv and disadv of fusion of the ankle 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 which can damage these joints.
How will a patient with an ankle joint arthrodesis walk?
will walk out toed so that the subtalar joint acts as a dorsiflexor of the foot
what is the general criteria for ankle joint replacement
- 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
what are the main materials used for ankle joint replacement
Cobalt chrome and HDP
What ADL’s can be restored by an ankle replacement but not a fusion?
Walking and raising from a chair
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?
- 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
what is the ankle joint also known as
tibio-talar or talocrural joint - it is the joint between the tibia and the talus
what is the normal range of motion of the ankle
25 to 30 degrees in both dorsiflexion and plantarflexion
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
what is the subtalar joint also known as
talo-calcaneal joint - it is the joint between calcaneus and the talus
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
The combined motion of what two joints is important for activities such as walking ?
The ankle and subtalar joints
Why must a replacement ankle joint must therefore allow sufficient dorsiflexion, what activity is this particularly important for ?
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]
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?
4/5 times BW - due to to a combination of gravitational and muscular forces.
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
What is the range of motion of the ankle joint during walking?
around 15 degrees in both plantar flexion and dorsiflexion
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.
What are the approximate maximum vertical and fore-aft loads on the ankle joint
500% (5x’s) BW and 70% (0.7x’s) BW
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
what do congruent ankle replacements allow for
- Allow for rotation only.
- The number of axes of rotation can be limited
what are the 4 shapes of congruent ankle replacement joints
1 - spherical
2 - spheroidal
3 - conical
4 - cylindrical
Think - SSC(C)
what does the spherical design allow and therefore what can it help compensate for
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]
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
Medio-lateral space in the joint is limited so it has been argued that the cylindrical type is preferable
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
true or false - the spheroidal shape replacement has a massive advantage over the spherical type
false - there is no particular adv
what does the conical shape implant providea and what does it require
- provides a single axis of plantarflexion/dorsiflexion rotation and some medio-lateral resistance
- requires a greater amount of bone resection than a cylindrical shape
what shape of congruent implant have most ankle replacements been
cylindrical design
what does the cylindrical design provide and what can it not compensate for
a basic single axis replication of the ankle joint but cannot compensate for subtalar dysfunction
what is a disadvantage of the cylindrical design
can cause high medio-lateral forces being transmitted causing shear stresses at the bone-implant interfaces
what is the main feature of incongruent shapes of ankle replacement joints
less constraint in the movement so that some horizontal motion is possible
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
what are the 2 shapes of incongruent ankle replacements
1 - trochlear
2 - convex-concave
Think CT
what are the 2 shapes of convex-concave incongruent ankle replacements
cylindrical shape
spherical shape
what are the problems with 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
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
In ankle joint replacements what are used for resisting torques or sideways loads ?
One or two fins or other type of protrusion - they also provide a larger surface area if cement is used
What is the difference between a congruent and an incongruent bearing surface?
Congruent designs have matching surfaces, incongruent ones do not and therefore tend to have more freedom of movement
Why are cylindrical bearing surfaces more economical in the use of space than non-cylindrical ones
They dont need to be circular so require less mediolateral space
List one advantage and one disadvantage of cylindrical designs compared with spherical designs.
- Advantage = economical on space
- Disadvantage = has no axial rotation or inversion-eversion motion ==> cant compensate for subtalar dysfunction
what is the current clinical result of ankle replacements
65% failure at 5 years
what are the causes of failure in ankle joint replacements?
- 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
what are some problems identified in specific ankle joint replacements that lead to issuse
- high contact forces
- inversion-eversion instability
- subluxation
- excessive bone resection
Which type of ankle replacements - congruent or non-congruent give the best wear resistance ?
Congruent
Why has failure due to wear NOT been the main problem in congruent ankle joint replacements?
The bearing surface tends to be quite large so high contact stresses in a congruent design is not a problem
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
- 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.
designs with or without cement seem to be better for ankle joint replacements?
cementless designs seem to be superior
Give an example of a cementless ankle joint replacement
Koefod STAR 1985
Describe the design of the Kofoed STAR prosthesis
- 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.
List two trends in modern ankle joint prostheses design
- move towards meniscal bearing
- move towards cementless types
one advantage of meniscal bearing ankle prostheses over cylindrical bearing design
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)
