WEAR AND DEFORMATION OF UHMWPE IN TOTAL KNEE REPLACEMENT PROSTHESES

Abstract

The use of metal-backed tibial plates in Total Knee Replacement (fKR) prostheses can result in the flow of Ultra High Molecular Weight Polyethylene (UHMWPE) from the tibial insert into a cavity on the metal tray surface. This under-surface flow of UHMWPE is especially marked in prostheses designs where the tibial tray has cavities, such as screw-holes for cement-less fixation. This under-surface flow into these cavities is described here as "cold extrusion" of UHMWPE. It is not a surface cold-flow effect and is akin to an extrusion process. A study of the relationship between the UHMWPE inserts thickness and the amount of cold extrusion is reported here. An attempt was made to correlate the occurrence of cold extrusion with articular surface damage. UHMWPE samples of varying thickness, from 3mm to 10mm, were placed over CobaltChrome (Co-Cr) discs. The Co-Cr discs had a 5mm diameter hole placed centrally to simulate a tibial tray cavity. A cyclic load was applied at 20Hz through a Co-Cr spherical indentor for a million cycles. The application of cyclic loading on UHMWPE test pieces resulted in cold-extrusion values comparable to those reported for retrieval analysis studies. Results after fatigue loading show that the samples do not suffer any gross surface damage. A shiny depression was visible at the load application site and the surface roughness value was decreased. The amount of cold extrusion increased with decreasing UHMWPE sample thickness. From extrapolation of the results, a minimum UHMWPE thickness of 12mm is required if cold extrusion of UHMWPE is to be eliminated.