Unit 4: Ankle Joint Replacement

Question 1

Suggest 3 reasons why there has been a lack of success to dae regarding ankle replacements

Answer 1

Ankle not usually involved in primary OA - less attention paid to it

In sec OA & rheumatoid ankle is often affected alongside other joints (e.g. subtalar) so just replacing ankle wouldn’t b adequate

Ankle function in assoc with subtalar joint

Arthrodesis is also available

Question 2

What are the advantages of arthrodesis?

Answer 2

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

Question 3

What are the disadvantages of arthrodesis?

Answer 3

Leads to abnormal loading on the knee and subtalar joint on the same leg

Shortens stride causing problems in opposite limb joints

Question 4

How does a patient walk after arthrodesis?

Answer 4

Out toed - so the subtalar joint acts as a dorsiflexor of the foot

Question 5

What issues exist with ankle replacement fixation?

Answer 5

Bony areas where components can be fixed are barely adequate to provide support using cementing techniques

Tibial bone is soft cancellous which rapidly widens and then narrows

Talus is cut across where the neck meets the body leaving a very small area for fixation

Question 6

What effect does restriction in movement of the subtalar joint (e.g. stiffness from arthritis) have on joint loading?

Answer 6

Will subject the ankle to large axially generated torques (with no modulation by the thigh muscle controlled subtalar motion)

Question 7

What istructures is the ankle joint between?

Answer 7

Tibia and talus

Question 8

What is the ankle joint also known as?

Answer 8

Tibio-talar or talocrural joint

Question 9

What is the normal range of motion of the ankle joint?

Answer 9

25-30 degreee in both plantar and dorsiflexion

Question 10

Where does the axis of rotation of the ankle joint lie?

Answer 10

Not perpendicular to the sagittal plane but is iclined downwards ad posteriorly on the lateral side

Question 11

What is the subtalar joint also known as?

Answer 11

Talo-calcaneal joint

Question 12

What movement does the subtalar joint allow?

Answer 12

inversion/eversion

Question 13

What movement is particularly important in the ankle for getting up out a chair/

Answer 13

Dorsiflexion

Question 14

What is the difference between kinetics and kinematics?

Answer 14

Kinematics = the study of motion 
Kinetics = the study of motion and forces

Question 15

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

Answer 15

Vertical = 500% BW

Fore-aft = 70% BW

Question 16

What are the 2 types of ankle replacement?

Answer 16

Congruent (matching bearing surfaces)

Incongruent (not matching surfaces)

Question 17

Why do spherical designs of replacement require careful positioning during insertion?

Answer 17

They have a specific centre of rotation

Question 18

Why is a spherical design advantageous?

Answer 18

Allows freedom of rotation and therefore provides compensation for a degenerate subalar joint

Question 19

What are the disadvantages of a cylindrical design over a spherical design?

Answer 19

Reduced range of plantar/dorsiflexion

Reduced medio-lateral shear resistance

Question 20

What movements does a spheroidal shape provide?

Answer 20

Plantar/dorsiflexion
Invesrion/eversion
NO axial rotation

Question 21

What advantages does spheroidal design have ove the spherical type?

Answer 21

Doesn’t have any particular advantages over the spherical type

Question 22

What is the disadvantage of the conical type?

Answer 22

Requires a greater amount of bone resection than a cylindrical shape

Question 23

What are the movements provided by the conical type?

Answer 23

Single axis of plantar/dorsiflexion rotation

Some medio-lateral resistanceresistance

Question 24

What are the disadvantages of cylindrical design?

Answer 24

Provides basic single axis replication that cannot compensate for subtalar dysfunction

Some designs have a mortice to substitute for mediolateral support but if tight fit can cause shear stresses at bone-implant interface

Creates area of concentrated stress under symmetrical medio-lateral loading

Question 25

Most ankle replacements have been of which type?

Answer 25

Cylindrical

Question 26

What is the main feature of incongruent shapes?

Answer 26

Less constraint in the movement so some horizontal motion is possible

Question 27

Why might less constraint in movement be advantageous in ankle replacement?

Answer 27

Reduce load transmission to the bone-cement-prosthesis interfaces by transferring some of the load to the soft tissues

Question 28

What are 3 problems with incongruent shapes?

Answer 28

Higher rate of depth of wear than congruent types

Higher contact stresses due to a lower contact area

Less stability than congruent types due to their greater freedom of movement

Question 29

What movement does a trochlear (saddle) shape allow?

Answer 29

Plantar/dorsiflexion

Some inversion/eversion and axial rotation

Question 30

What materials have most ankle replacements been made of?

Answer 30

Combination of cobalt chrome or stainless steel for one component and HDP for the other

Question 31

What have been common causes of failure of ankle replacements?

Answer 31

Aseptic loosening of a component 
Lateral or medial subluxation of the joint 
Subsidence of the talar component 
Impingement of the joint 
Wound healing problems 
Infection

Question 32

Which design provides better wear resistance? Congruent or Incongruent?

Answer 32

Congruent - bearing surface tends to be large so high contact stresses less of a problem

Question 33

Which 2 designs of prosthesis have had the most success?

Answer 33

New Jersey LCS prosthesis

Beuchel-Pappas

Question 34

What are the similarities between the New Jersey LCS prosthesis and the Beuchel-Pappas?

Answer 34

Uncemented
Porous coated surfaces
Hollow in the tibial bearing surface
Meniscal bearing

Question 35

Why are meniscal bearings good in ankle replacement?

Answer 35

Provides congruent bearing surfaces at the tibial and talar interfaces - without the usual disadvantage of rigid transmission of medio-lateral and rotational shear forces

Question 36

What are the known deficiences in the LCS?

Answer 36

Subluxation (due to lack of lateral stability in the bearing)
Talar subsidence

Question 37

How are cementless designs fixated?

Answer 37

Clyindrical bars on the tibial components slide into holes drilled in the tibia

Stabilising fin used for fixation of the talar component

Both front and back are designed to rest on cortical bone

Interfaces on both components coated with hydroxyapatite

HDP floating meniscus is partially constrained by a rib on the top of the talar component

Question 38

Why does the Calderdale prosthesis have a tibial stem?

Answer 38

Designers found from finite element analysis that cncellous bone in the distal tibia was not strong enough to bear the tibial component directly