Implant Technology Unit 4 Flashcards
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
what operation can be done if the subtalar joint remains healthy
arthrodesis - fusion
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
how will patient w/ subtalar joint fusion 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 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
what are the main materials used for ankle joint replacement
Cobalt chrome and high density polyethylene
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.
what is the ankle joint also known as
tibio-talar or talocrural joint
[ankle joint is 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 [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
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]
what is the force at the ankle during walking
4 to 5 x BW
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% body weight and 70% body weight
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 and 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
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]
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
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 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
what shape of congruent implant have most ankle replacements been
cylindrical design
what does the cylindrical design provide
a basic single axis replication of the ankle joint but cannot compensate for subtalar dysfunction
what is a disadvantage of the cylindrical design
creates an area of concentrated stress under asymmetrical medio-lateral loading
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
incongruent ankle replacements that are a trochlear (saddle) shape, allow for what
plantarflexion-dorsiflexion
some inversion-eversion and axial rotation
what are the 2 shapes of incongruent ankle replacements
1 - trochlear
2 - convex-concave
what are the 2 shapes of convex-concave incongruent ankle replacements
cylindrical shape
spherical shape
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
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
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
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.
what is the current clinical result of ankle replacements
65% failure at 5 years
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
what are some problems identified in ankle joint replacements that lead to issuse
high contact forces
inversion-eversion instability
subluxation
excessive bone resection
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
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
designs w/ or w/out cement seem to be better
cementless designs seem to be superior
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.
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
Congruence (low wear) yet freedom of joint motion.
example of cementless design
Koefod STAR 1985
