WEAR RATES FOLLOWING PRIMARY TOTAL HIP ARTHROPLASTY USING A SECOND GENERATION HIGHLY CROSS-LINKED POLYETHYLENE

Introduction Technological advances in the processing of polyethylene have led to improved survivorship of total hip arthroplasty. The purpose of this study was to determine if a second generation highly cross-linked polyethylene could improve upon wear rates compared to conventional and first generation cross-linked polyethylene in patients undergoing primary total hip arthroplasty. Methods Linear and volumetric wear rates of a second generation highly cross-linked polyethylene were evaluated following primary total hip arthroplasty. There were 44 patients with an average age of 68.6 years and mean follow-up of 5.3 years. Patients were evaluated at six weeks, one, two and five years. Wear rates were determined from digitized AP Pelvis radiograph by an independent observer using Martell9s software. Acetabular inclination and femoral head size were also evaluated to determine variability in wear rates. Results The mean linear wear rate for the entire group was 0.015mm/year (±0.055). There was a 64% decrease in linear wear rate when compared to a first generation highly cross-linked polyethylene from the same institution. There was a 90% decrease in wear rate compared to conventional polyethylene. There were no differences in the linear wear rate between the 32mm vs. larger head sizes (36mm and 40mm). However, there was a 30% increase in volumetric wear rate with larger head sizes. The mean wear rate in patients with cup inclination less than 45 degrees was 0.006mm/year compared with 0.024mm/year for those with an inclination greater than 45 degrees. The amount of linear wear was increased by 4 times in patients with a cup inclination of greater than 45 degrees. There was no evidence of any osteolysis in this group of patients. Conclusion Our data with a mean follow-up of 5.3 years, using a second generation highly cross-linked polyethylene, demonstrates a dramatic decrease in incidents of linear wear compared to conventional polyethylene and first generation highly cross-linked polyethylene (Figure 1). Of concern is the higher volumetric wear rate noted with larger head sizes and increased linear wear rates with cup inclination angles of greater than 45 degrees. Despite improvements in wear rates using a second generation highly cross-linked polyethylene, cup orientation and choice of head size play significant roles in implant survivorship.