Ankle joint replacement Flashcards
what joint does the ankle joint function in association with?
the subtalar joint
what is the purpose of joint fusion
relives pain in a stiff joint without the need to provide compensation for the resulting loss of movement
following fusion, what do` the resulting biomechanical changes in motion and load transmission lead to
leads to abnormal loadings on the knee and subtalar joint on the same leg, and a shortening of stride which has consequences for the opposite limb joints
why would a person walk out toed following ankle fusion?
so that the subtalar joint in effect acts as a dorsiflexor of the foot
what activities of daily living can be restored by an ankle replacement but not provided by fusion
walking and rising from a chair
joint reaction force might reach ? times body weight
5
what will restricted movement of the subtalar joint, such as stiffness, subject the ankle to?
large axially generated torques which will add to the loosening process because axially generated forces will have no modulation by thigh muscle controlled subtalar motion
what are the 3 main reasons for failure of a prosthesis
overloading causing subsidence (up to 5 x body weight generated at ankle)
loosening due to poor fixation (current techniques make the bony areas around the ankle barely inadequate for sufficient support)
high torques which can cause interface loosening (axially generated torques cant be prevented if there is restriction of movement at the subtalar joint e.g. OA of this joint)
normal ROM of ankle
about 25 - 30 degrees in both dorsiflexion and plantar flexion
what is the subtalar joint?
talus - calcaneus
what motion does the subtalar joint allow?
inversion-eversion
axis of the subtalar joint is nearly at ? to the axis of the ankle joint
right angles
what does getting up from a chair involve at the ankle in order for the trunk to move forwards?
dorsiflexion of the ankle
whats the difference between a congruent and an incongruent ankle replacement?
congruent - matching bearing surfaces
incongruent do not
congruent types allow ? only
rotation
why is a spherical shape better than a cylindrical shape for ankle replacement
spherical allows a greater angle of plantar/dorsiflexion rotation for the same media-lateral width of bearing surface, and medio-lat space is limited in the joint
what is an inherent disadvantage of a cylindrical shaped ankle replacement
creates an area of concentrated stress under asymmetrical medio-lateral loading
despite the known disadvantages which shape of ankle replacement is the most common so far
cylindrical
what can cylindrical replacements not compensate for
subtalar dysfunction
what is the main feature of incongruent ankle replacement types and why could this be advantageous?
there is less constraint in the movement»_space; some horizontal motion is possible which could be advantageous as it can reduce the load transmission to the bone-cement-prosthesis interface by transferring some load to the soft tissues
examples of incongruent ankle prosthesis shapes
trochlear (saddle)
convex-concave
what movements does a trochlear replacement allow
plantar-dorsiflexion
inversion-eversion
axial rotation
give 3 problems with incongruent prostheses
smaller contact area»_space; higher contact stresses
higher rate of depth of wear
less stability due to greater freedom of movement
why are cylindrical bearing surfaces more economical in the use of space than non-cylindrical ones?
dont need to be circular so less medio-lateral space is needed
state an advantage and a disadvantage pf cylindrical designs compared to spherical ones
adv - economical on space
disadv - no inversion/eversion or axial rotation allowed
name some common causes of ankle prosthesis failure
aseptic loosening infection wound healing problems medial/lateral subluxation subsidence of talar component
name 2 trends in modern ankle prosthesis design
moving towards cement less and moving towards meniscal bearing
whats the advantage of meniscal bearings
it allows congruence (reducing contact stresses) without limiting joint motion
