Ankle joint replacement

Question 1

what joint does the ankle joint function in association with?

Answer 1

the subtalar joint

Question 2

what is the purpose of joint fusion

Answer 2

relives pain in a stiff joint without the need to provide compensation for the resulting loss of movement

Question 3

following fusion, what do` the resulting biomechanical changes in motion and load transmission lead to

Answer 3

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

Question 4

why would a person walk out toed following ankle fusion?

Answer 4

so that the subtalar joint in effect acts as a dorsiflexor of the foot

Question 5

what activities of daily living can be restored by an ankle replacement but not provided by fusion

Answer 5

walking and rising from a chair

Question 6

joint reaction force might reach ? times body weight

Answer 6

5

Question 7

what will restricted movement of the subtalar joint, such as stiffness, subject the ankle to?

Answer 7

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

Question 8

what are the 3 main reasons for failure of a prosthesis

Answer 8

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)

Question 9

normal ROM of ankle

Answer 9

about 25 - 30 degrees in both dorsiflexion and plantar flexion

Question 10

what is the subtalar joint?

Answer 10

talus - calcaneus

Question 11

what motion does the subtalar joint allow?

Answer 11

inversion-eversion

Question 12

axis of the subtalar joint is nearly at ? to the axis of the ankle joint

Answer 12

right angles

Question 13

what does getting up from a chair involve at the ankle in order for the trunk to move forwards?

Answer 13

dorsiflexion of the ankle

Question 14

whats the difference between a congruent and an incongruent ankle replacement?

Answer 14

congruent - matching bearing surfaces

incongruent do not

Question 15

congruent types allow ? only

Answer 15

rotation

Question 16

why is a spherical shape better than a cylindrical shape for ankle replacement

Answer 16

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

Question 17

what is an inherent disadvantage of a cylindrical shaped ankle replacement

Answer 17

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

Question 18

despite the known disadvantages which shape of ankle replacement is the most common so far

Answer 18

cylindrical

Question 19

what can cylindrical replacements not compensate for

Answer 19

subtalar dysfunction

Question 20

what is the main feature of incongruent ankle replacement types and why could this be advantageous?

Answer 20

there is less constraint in the movement&raquo_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

Question 21

examples of incongruent ankle prosthesis shapes

Answer 21

trochlear (saddle)

convex-concave

Question 22

what movements does a trochlear replacement allow

Answer 22

plantar-dorsiflexion
inversion-eversion
axial rotation

Question 23

give 3 problems with incongruent prostheses

Answer 23

smaller contact area&raquo_space; higher contact stresses
higher rate of depth of wear
less stability due to greater freedom of movement

Question 24

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

Answer 24

dont need to be circular so less medio-lateral space is needed

Question 25

state an advantage and a disadvantage pf cylindrical designs compared to spherical ones

Answer 25

adv - economical on space

disadv - no inversion/eversion or axial rotation allowed

Question 26

name some common causes of ankle prosthesis failure

Answer 26

aseptic loosening 
infection
wound healing problems
medial/lateral subluxation
subsidence of talar component

Question 27

name 2 trends in modern ankle prosthesis design

Answer 27

moving towards cement less and moving towards meniscal bearing

Question 28

whats the advantage of meniscal bearings

Answer 28

it allows congruence (reducing contact stresses) without limiting joint motion