Compressive creep characteristics of extruded ultrahigh-molecular-weight polyethylene.

The static compressive creep behavior of ultrahigh-molecular-weight polyethylene (UHMWPE) was studied under physiologic conditions. Specimens were machined from the center and periphery of extruded GUR 4150HP rod stock and were subjected to constant pressures of 2, 4, or 8 MPa for intervals as long as 10(4) min. The creep strain (creep divided by initial thickness) was compared to the pressure and duration of loading by using analysis of variance and linear regression analysis. The amount of creep strain increased rapidly in the early period of testing and was followed by a reduced rate of creep, which reached a steady state after approximately 4000 min. The amount and rate of creep strain increased linearly with pressure. Surprisingly, the rate of creep strain varied with the radial position in the rod stock: specimens obtained from the periphery had 8-19% larger creep strain rates than did specimens obtained from the center (p = 0.1 to p < 0.001). These results advance the characterization of creep's contribution to the in vivo penetration of the metallic component into the UHMWPE component, thereby facilitating the measurement of true in vivo wear. These data also help explain the azimuthally nonuniform deformation observed in retrieved acetabular cups.