Ultra high molecular weight polyethylene (UHMWPE), the bearing surface used in toal joint arthroplasty (TJA) has been associated with premature failure due to wear. However, stress analysis, finite element analysis and kinematic analysis have been unable to accurately predict clinical failures. It was postulated that these shortcomings were due, in part, to the lack of consideration of UHMWPE microstructure which arises due to incomplete consolidation of polymer powder when bulk stock is fabricated.

It was hypothesized that UHMWPE possessed heterogeneous mechanical and chemical properties related to the microstructure. The approach was to examine the two and three dimensional microstructure by staining thin sections with SO2 gas to identify localized regions of oxidation in unimplanted medical grade UHMWPE which had been oxidized in a controlled fashion by heating in an air circulating oven. Next, a series of samples were tested mechanically using a custom-designed impact test apparatus in order to examine the relationship between extent of oxidation, microstructure and mechanical performance. Finally, a series of failed knee arthroplasties were examined in order to identify whether the heterogeneous spatial distribution of oxidation was also present in the clinical environment.

Two and three dimensional microstructural characterization indicated that grain boundaries were preferentially oxidized at low levels of oxidation and with increasing levels of oxidation gradually more of the interiors of the grains were oxidized. Frontal plane thin sections from a series of failed knee arthroplasties demonstrated heterogeneous oxidation similar to in vitro specimens and an oxidation distribution as a function of depth from the articular surface. The impact testing of in vitro aged specimens demonstrated a continuum of failure modes which changed from tough to ductile to brittle behavior as the level of oxidation increased.

The studies indicated that heterogeneous chemical and mechanical properties are central to the true performance of the current UHMWPE material and must be considered when modeling its behavior. Furthermore, steps to control oxidation and optimize microstructure should be implemented to improve long term performance of polymer bearing surfaces in TJA.