340 THE JOURNAL OF BONE AND JOINT SURGERY A. A. Besong, B Eng, Research Student J. Fisher, D Eng, Professor Department of Mechanical Engineering J. L. Tipper, PhD, Research Fellow E. Ingham, PhD, Senior Lecturer Department of Microbiology University of Leeds, Leeds LS2 9JT, UK. M. H. Stone, FRCS, FRCS Ed, Consultant Orthopaedic Surgeon Leeds General Infirmary, Great George Street, Leeds LS1 3EX, UK. B. M. Wroblewski, FRCS, Consultant Orthopaedic Surgeon, Professor of Orthopaedic Biomechanics The John Charnley Research Institute, Wrightington Hospital for Joint Disease, Hall Lane, Appley Bridge, Wigan, Lancashire WN6 9EP, UK. Correspondence should be sent to Professor J. Fisher. ©1998 British Editorial Society of Bone and Joint Surgery 0301-620X/98/28234 $2.00 Quantitative comparison of wear debris from UHMWPE that has and has not been sterilised by gamma irradiation A. A. Besong, J. L. Tipper, E. Ingham, M. H. Stone, B. M. Wroblewski, J. Fisher From the University of Leeds, England U ltra-high-molecular-weight polyethylene (UHMWPE) components for total joint replacement generate wear particles which cause adverse biological tissue reactions leading to osteolysis and loosening. Sterilisation of UHMWPE components by gamma irradiation in air causes chain scissions which initiate a long-term oxidative process that degrades the chemical and mechanical properties of the polyethylene. Using a tri-pin-on-disc tribometer we studied the effect of ageing for ten years after gamma irradiation in air on the volumetric wear, particle size distribution and the number of particles produced by UHMWPE when sliding against a stainless-steel counterface. The aged and irradiated material produced six times more volumetric wear and 34 times more wear particles per unit load per unit sliding distance than non-sterilised UHMWPE. Our findings indicate that oxidative degradation of polyethylene after gamma irradiation in air with ageing produces more wear. J Bone Joint Surg [Br] 1998;80-B:340-4. Received 8 January 1997; Accepted after revision 22 December 1997 Osteolysis and subsequent loosening as a result of the generation of wear particles are now recognised as a major cause of long-term failure in total joint arthroplasty. Liver- more, Duane and Murray 1 have shown a positive correla- tion between the amount of resorption of the proximal part of the femur and wear, and tests in vitro 2-4 have demon- strated that the macrophage response to particulate wear debris is dependent on the number and size of particles. These studies have shown that cells are more reactive to small phagocytosible particles and irregularly shaped par- ticles than to larger and more regularly shaped particles. 3,4 It is therefore important to reduce the wear volume of ultra- high-molecular-weight polyethylene (UHMWPE) and the number of wear particles to improve the long-term clinical performance. The volume, size, morphology and concentration of the wear particles produced are likely to depend on tribological factors such as the properties of the material and the loads and movements experienced at the contact surfaces. The smooth hard counterface (typically Ra = 0.01 to 0.05 m) slides against a concave surface producing UHMWPE wear particles by adhesive and fatigue wear. Many particles of various sizes are produced in each cycle. The wear rates and volumes increase dramatically as the femoral counter- face becomes roughened due to third-body damage; the counterface asperities remove particles by abrasive actions or by low cycle fatigue. 5 A second wear process that may produce larger polyethylene particles 5 is called ‘macro- scopic polymer asperity wear’ and is associated with cyclic deformation and strain accumulation of much larger asper- ities (up to 10 m high) on the polymer surfaces. Evidence from explanted acetabular cups 6-8 has shown that fatigue failure of the polymer surface just below the asperity (=5 m deep) can produce long fibres or cigar-shaped particles of up to 100 m in length. Most polyethylene wear particles are submicron in size. 9,10 Until recently, sterilisation of UHMWPE was by gamma irradiation in the presence of air causing the generation of free radicals which then initiated a series of long-term oxidation reactions. The degree of oxidation in polyethyl- ene has been shown to depend on the dose of radiation, 11 the post-irradiation age 12 and the environmental condi- tions. 11,13,14 Studies have indicated that it causes deteriora- tion of important mechanical properties such as tensile strength, 15 impact strength, 16 toughness, fatigue strength 16,17 and Young’s modulus. These time-dependent changes have been shown to affect the volumetric wear. 18,19