Types
1. Fixed bearing
Insall-Burstein II knee
- cannot be fully conforming
- otherwise would be very constrained to axial rotation
- would transfer large rotational stresses to the prosthesis bone interface
2. Mobile bearing
LCS (meniscal bearing)
- allow fully conforming articulations
- because they allow unconstrained axial rotation at the poly / tibial interface
- reduced axial stress to the prosthesis bone interface
PE wear in TKR
2 types
1. Articular Wear
Kinematic conflict
- increasing contact area reduces contact stress
- but it reduces ROM
Highest
- low conformity
- round on flat designs (PCL retaining)
- increased ROM but high contact stresses in the poly
- sliding and skidding
- delamination and particle production
Lowest
- highly conforming designs
- PCL retaining
- this produces less ROM
This is the kinematic conflict
- want to increase conformity to decrease wear
- but want increased ROM
2. Under-surface Wear
- between poly and tibial baseplate
- no locking mechanism is 100% reliable
- some movement occurs
- resulting in particle production
- one way to avoid this is to use an all poly tibia
Mobile bearing
Goal
1. Maximise conformity by allowing mobility of the bearing surface
2. Increase contact area & decrease long term wear
- reduce stresses at implant - implant and implant bone interface
3. Recreate normal knee kinematics
Advantage
Decreased wear due to decreased contact stresses (unproven)
- may compensate for any malrotation
- do they solve the kinematic conflict by allowing highly congruent surfaces whilst maintaining good ROM? (unproven)
Disadvantage
1. Bearing Dislocation
- soft tissue and ligamentous balancing crucial
- severe deformity is a contra-indication
2. Anterior soft tissue impingement with AP translation
Types
A. IR and ER
- cone in cone constraint mechanism
- backward motion of one condyle / forward motion of the other
- i.e. LCS RP (low contact stress rotating platform, PS)
B. IR and ER on medial axis
- better stimulation of anatomic motion
C. IR and ER + AP
- meniscal bearing
- relies on ligamentous structures for stability
- CR or PS
- i.e. LCS meniscal bearing CR
D. Guided motion IR and ER + AP
- controlled by intercondylar saddle shaped cam
- attempt to reproduce normal knee kinematics
- rollback with flexion / roll forward with extension
Biomechanical Studies
LCS Mensical Bearing
LCS Meniscal Bearing wear simulator
- 1% loss over 10X106 cycles
Contact area
- 200 mm x mm in fixed bearing
- >1000 mm x mm in mobile bearing
Contact stresses
- reduced from 25mPa to <5mPa
Results
ROM / Function
Rahman et al J Arthoplasty
- RCT of mobile v fixed bearing
- no difference in ROM or functional scores
Ladermann Knee 2008
- RCT of mobile v fixed bearing followed up for 7 years
- no difference in ROM or outcome
2 x meta-analysis show similar findings
Survival / Wear
Australian Joint Registry 2010
9 year revision rate with OA as primary diagnosis
Fixed bearing 4.7%
Mobile Bearing Rotation 5.7%
Mobile Bearing Sliding 6.7%