Definition
Catastrophic polyethylene wear is macroscopic premature failure of the polyethylene bearing caused by excessive mechanical loading, adverse implant mechanics or progressive loosening.
It is classically described in total knee arthroplasty, although severe polyethylene failure can occur in other arthroplasties.
Mechanisms
Important contributors include:
- inadequate polyethylene thickness
- low-conformity or unfavourable articular geometry
- high contact stress
- abnormal kinematics
- malalignment
- instability
- excessive rollback or edge loading
- manufacturing or sterilisation-related material degradation
- progressive loosening
Polyethylene thickness
Thin polyethylene increases contact stress. Historical designs with very thin inserts were particularly vulnerable to fatigue failure, delamination and fracture.
Articular surface geometry
A flatter, less congruent bearing has a smaller contact area and therefore higher contact stress. Increasing congruity increases contact area and reduces peak stress.
Kinematics and alignment
Varus or valgus malalignment, instability and abnormal femoral rollback can concentrate load on a small part of the insert and accelerate wear.
Material factors
Polyethylene properties are influenced by:
- resin and manufacturing method
- machining
- sterilisation technique
- oxidation
- cross-linking
Older polyethylene sterilised with gamma irradiation in air was particularly susceptible to oxidation and fatigue failure.
Clinical and radiographic features
- pain
- recurrent effusion
- instability
- progressive asymmetric joint-space narrowing
- osteolysis
- component migration or loosening
- metal-on-metal contact in advanced failure
Management
Treatment depends on implant stability and bone loss.
- isolated polyethylene exchange may be considered when components are well fixed, well aligned and otherwise satisfactory
- revision arthroplasty is required when there is component loosening, malposition, instability or major wear-related damage
References
- Orthobullets. TKA Polyethylene Wear & Manufacturing.
- Orthobullets. Wear & Osteolysis Basic Science.