Definition
Loss of material from solid surface due to mechanical action
Types of wear
TKR
- more complex movements compared with THR
- rolling, sliding and rotation
- delamination, pitting and fatigue failure of the poly surface
Types
- adhesive wear (most common in TKR)
- abrasive wear
- third body wear
- fatigue wear - delaminating
- wndersurface wear
Patient factors
Age
Weight
Activity
Inflammatory arthritis
UHMWPE
Site of wear
Frontside
- articular side wear
- conformity
- PS v CR
- mobile v fixed bearing
Backside
- undersurface wear
- between baseplate and poly
Manufacturing
1. Compression moulding of poly
- moulding the powder into shape required
- less wear in vitro
- 0.05mm wear/yr
2. Machined poly
- from a poly sheet or bar
- may generate subsurface cracking
- predispose to sublamination
- 0.11mm /yr wear
Sterilisation
Gamma irradiation in air
- creates oxygen free radicals
- then stored in oxygen rich environment
- created a sub surface band of highly oxidised PE
- decreased mechanical strength
- increased wear
- worsened by long storage times (8-10 years)
Modern sterilisation
- ethylene oxidation / gamma irradiation
- oxygen free environment
Conformity
Conforming prostheses i.e. LCS & IB II approx 1/2 max stress levels of other designs
1. Highly conforming surface stresses are maximal at the surface
2. Increased contact area
- increases stress distribution
Thickness of Poly
Thinner poly inserts exhibit greater wear than thicker inserts of the same design
- yield strength of poly exceeded if < 4mm poly
- if add 4mm poly reduce stresses by 27%
- definitely if < 6 mm
- no improvement > 8 mm
Backside wear
Locking mechanisms
- snap fit
- tongue in groove
These mechanisms may lose stability over time
- increase particulate wear
Lead to interest in all poly base plate
Femoral component
Decrease wear at post-cam interface
Increase surface smoothness and hardness
- oxinium
Surgical Factors
Mechanical alignment
- coronal
- sagittal
- rotation
All can contribute to abnormal loading and wear