Unit 4: Ankle Joint Replacement Flashcards
Suggest 3 reasons why there has been a lack of success to dae regarding ankle replacements
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
What are the advantages of arthrodesis?
Relieves pain in a stiff joint without need to provide any compensation for the resulting loss of movement
What are the disadvantages of arthrodesis?
Leads to abnormal loading on the knee and subtalar joint on the same leg
Shortens stride causing problems in opposite limb joints
How does a patient walk after arthrodesis?
Out toed - so the subtalar joint acts as a dorsiflexor of the foot
What issues exist with ankle replacement fixation?
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
What effect does restriction in movement of the subtalar joint (e.g. stiffness from arthritis) have on joint loading?
Will subject the ankle to large axially generated torques (with no modulation by the thigh muscle controlled subtalar motion)
What istructures is the ankle joint between?
Tibia and talus
What is the ankle joint also known as?
Tibio-talar or talocrural joint
What is the normal range of motion of the ankle joint?
25-30 degreee in both plantar and dorsiflexion
Where does the axis of rotation of the ankle joint lie?
Not perpendicular to the sagittal plane but is iclined downwards ad posteriorly on the lateral side
What is the subtalar joint also known as?
Talo-calcaneal joint
What movement does the subtalar joint allow?
inversion/eversion
What movement is particularly important in the ankle for getting up out a chair/
Dorsiflexion
What is the difference between kinetics and kinematics?
Kinematics = the study of motion Kinetics = the study of motion and forces
What is the approximate maximum vertical and fore-aft loads on the ankle joint?
Vertical = 500% BW
Fore-aft = 70% BW
What are the 2 types of ankle replacement?
Congruent (matching bearing surfaces)
Incongruent (not matching surfaces)
Why do spherical designs of replacement require careful positioning during insertion?
They have a specific centre of rotation
Why is a spherical design advantageous?
Allows freedom of rotation and therefore provides compensation for a degenerate subalar joint
What are the disadvantages of a cylindrical design over a spherical design?
Reduced range of plantar/dorsiflexion
Reduced medio-lateral shear resistance
What movements does a spheroidal shape provide?
Plantar/dorsiflexion
Invesrion/eversion
NO axial rotation
What advantages does spheroidal design have ove the spherical type?
Doesn’t have any particular advantages over the spherical type
What is the disadvantage of the conical type?
Requires a greater amount of bone resection than a cylindrical shape
What are the movements provided by the conical type?
Single axis of plantar/dorsiflexion rotation
Some medio-lateral resistanceresistance
What are the disadvantages of cylindrical design?
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
Most ankle replacements have been of which type?
Cylindrical
What is the main feature of incongruent shapes?
Less constraint in the movement so some horizontal motion is possible
Why might less constraint in movement be advantageous in ankle replacement?
Reduce load transmission to the bone-cement-prosthesis interfaces by transferring some of the load to the soft tissues
What are 3 problems with incongruent shapes?
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
What movement does a trochlear (saddle) shape allow?
Plantar/dorsiflexion
Some inversion/eversion and axial rotation
What materials have most ankle replacements been made of?
Combination of cobalt chrome or stainless steel for one component and HDP for the other
What have been common causes of failure of ankle replacements?
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
Which design provides better wear resistance? Congruent or Incongruent?
Congruent - bearing surface tends to be large so high contact stresses less of a problem
Which 2 designs of prosthesis have had the most success?
New Jersey LCS prosthesis
Beuchel-Pappas
What are the similarities between the New Jersey LCS prosthesis and the Beuchel-Pappas?
Uncemented
Porous coated surfaces
Hollow in the tibial bearing surface
Meniscal bearing
Why are meniscal bearings good in ankle replacement?
Provides congruent bearing surfaces at the tibial and talar interfaces - without the usual disadvantage of rigid transmission of medio-lateral and rotational shear forces
What are the known deficiences in the LCS?
Subluxation (due to lack of lateral stability in the bearing)
Talar subsidence
How are cementless designs fixated?
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
Why does the Calderdale prosthesis have a tibial stem?
Designers found from finite element analysis that cncellous bone in the distal tibia was not strong enough to bear the tibial component directly
