Cold extrusion deformation of UHMWPE in total knee replacement prostheses

The use of metal-backed tibial plates in total knee replacement 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. A study of the relationship between the thickness of UHMWPE inserts and the amount of cold extrusion is reported here. An attempt was made to correlate the occurrence of cold extrusion with computer-aided analysis. UHMWPE samples of varying thickness, from 3 mm to 10 mm, were placed over cobalt-chrome (Co-Cr) discs. The Co-Cr discs had a 5 mm diameter hole placed centrally to simulate a tibial tray cavity. A cyclic load was applied at 20 Hz through a Co-Cr spherical indentor for a million cycles. The application of cyclic loading on UHMWPE samples 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 the results, a minimum UHMWPE thickness of 12 mm is required if cold extrusion of UHMWPE is to be eliminated.