Abstract
Traditional hip prostheses, which involve metal on poly-ethylene articulations, have shown good survivorship at ten years, but in the long term, wear debris induced osteolysis has been found to cause loosening and failure. Specifically, micron and submicron size polyethylene wear particles generated at the articulating surfaces enter the periprosthetic tissues, activate the macrophages causing adverse cellular reactions and bone resorption. Recent laboratory, retrieval and clinical studies have shown that oxidation of the traditional polyethylene irradiated in air, causes wear to increase by a factor of three following either storage on the shelf for five years or following implantation in vivo for 15 years. Furthermore, damage or scratching of metallic femoral heads has been shown to increase wear by a factor of two. In vitro cell culture studies with real polyethylene wear particles, have shown that the intensity of the adverse cellular reactions is critically dependent on the size of the polyethylene wear particle with the smallest particles 0.1 to 1 mm being the most active. A novel model has been developed to predict functional biological activity and osteolytic potential, by integrating wear rates, particle analysis and cell culture studies.
Stabilised and crosslinked polyethylenes have been investigated and been found to reduce wear rates by a factor of three compared to oxidised and aged materials. A moderate level of crosslinking reduced wear from 50 to 35 mm3 per million cycles compared to non crosslinked materials. However, against scratched femoral heads, the wear rate of both stabilised and cross-linked polyethylene was elevated to levels where the functional biological activity remains a concern in the long term. Alternative bearing surfaces, metal on metal, and alumina ceramic on ceramic provide potential to substantially reduce wear. Metal on metal bearings have shown mean wear rates of 1.5 mm3/year in the hip joint simulator, with very small, 30 nm size particles. Alumina ceramic ceramic have also shown very low wear rates of approximately 1 mm3/year, even in the presence of microseparation and rim contact, with small 10 nm size wear particles and larger particles up to 1 mm in size caused by grain boundary fracture. The functional biological activity and osteolytic potential of the alumina ceramic couple is predicted to be at least ten times lower than crosslinked polyethylene.
New ceramic materials (zirconia toughened alumina) have been shown to further reduce ceramic ceramic wear. Furthermore, novel differential hardness ceramic on metal bearings have shown even lower wear rates. The currently available hard on hard bearings and the recent further improvements of these bearing couples, indicate that osteolysis free lifetimes well beyond 20 years are now possible.
The abstracts were prepared by Professor Jegan Krishnan. Correspondence should be addressed to him at the Flinders Medical Centre, Bedford Park 5047, Australia.
