Bearings

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%