COLD-EXTRUSION DEFORMATION OF UHMWPE IN TOTAL KNEE REPLACEMENT PROSTHESES

The use of metal-backed tibial plates in total knee replacement prosth eses can result in the flow of ultra-high molecular weight polyethylen e (UHMWPE) from the tibial insert into a cavity on the metal tray surf ace. A study of the relationship between the thickness of UHMWPE inser ts and the amount of cold extrusion is reported here. An attempt was m ade to correlate the occurrence of cold extrusion with computer-aided analysis. UHMWPE samples of varying thickness, from 3 mm to 10mm, were placed over cobalt-chrome (Co-Cr) discs. The Co-Gr discs had a 5 mm d iameter hole placed central ly to simulate a tibial tray cavity. A cyc lic load was applied at 20 Hz through a Co-Cr spherical indentor for a million cycles. The application of cyclic loading on UHMWPE samples r esulted in cold-extrusion values comparable to those reported for retr ieval analysis studies. Results after fatigue loading show that the sa mples do not suffer any gross surface damage. A shiny depression was v isible at the load application site and the surface roughness value wa s decreased. The amount of cold extrusion increased with decreasing UH MWPE sample thickness. From the results, a minimum UHMWPE thickness of 12 mm is required if cold extrusion of UHMWPE is to be eliminated.