Effect of cross-linking on the wear beha vior of ultra high molecular weight polyethylene (UHMWPE) used in total joint replacements

Abstract

INTRODUCTION The major problem in total joint replacements remains to bc the wear debris induced peri-prosthetic osteolysis (mostly from UHMWPE components) that leads to aseptic loosening. Recently, cross-linking was shown to reduce the adhesive wear of UHMWPE under bovine serum lubrication. In this study, we investigated the prop&es of radiation crosslinked IJHMWPE as a function of cross-link density using the following two methods of irradiation: (i) irradiation at room temperature with subsequent melt-annealing (Method A) and (ii) irradiation with in-situ phase transformation (Method B). METHODS Bi-Directional pin-on-disk (POD) testing up to 2 million cycles with a Paul physiologic loading (peak load = 750 lb.) curve was used to determine the wear behavior as a function of cross-link density. The mechanical properties, thermal properties, density and surface oxidation was also determined. The cross-link density and molecular weight between cross-links was determined using equilibrium volume swelling. RESULTS The physical properties of the cross-linked UHMWPE varied significantly between the two methods used. At equal dose levels. the Method B always yielded far better mechanical properties than Method A. While the yield strength, ultimate tensile strength, density, and crystallinity decreased with increasing radiation dose, the wear resistance of the cross-linked polymer increased dramatically following both methods. DISCUSSION The inserted figure shows the wear behavior of UHMWPE as a function of radiation dose for both methods. The wear rate of the cross-linked polymer followed a transition from high wear to negligibly low wear. The transition to undetectable wear occurted at around absorbed radiation dose of 150 kGy for both methods, above which dose the pins tested on the POD did not show any sign of wear. In fact the original machining marks were still present after 2 million cycles of bi-directional loading. The method B which leads to minimum changes in material properties with dramatic improvement in the wear resistance of UHMWPE promises to be a preferable method of cross-linking IJHMWPE for applications in total joint arthroplasty as bearing surfaces.