Abstract
Introduction:
Highly cross-linked polyethylene (XLPE) was introduced into clinical practice to decrease acetabular cup wear in total hip arthroplasty (THA) based upon extremely low wear rates in vitro. Numerous short-term clinical studies using XLPE cups have shown promising improvements in wear performance. In this study we evaluated in vivo wear performance of XLPE cups compared to conventional PE (CPE) cups in primary THA at a minimum five years follow-up.
Materials and Methods:
Between October 2003 and March 2005, 114 hips were randomized to receive either a CPE cup or a XLPE cup with primary THA. A single experienced surgeon performed all cases at the same hospital. All hips received a CoCr femoral head and a cementless acetabular shell (Trilogy, Zimmer, Warsaw). One-hundred three cases received cementless femoral stems and eleven stems were cemented. Forty-six hips received a 26 mm head size CPE cup (gamma sterilized at 25 kGy), and 68 hips received XLPE cups (10 Mrad e-beam irradiation, EtO sterilized) in 26 mm (47 hips), 28 mm (11 hips) and 32 mm (10 hips) head sizes. The mean age in the XLPE and CPE groups was 57 ± 8 and 62 ± 8, respectively (p < 0.05). Mean follow up was 70 months (range, 60 to 91 months).
Supine AP radiographs were obtained postoperatively at 2, 6, 9, 12, 18, 24, 36, 48, and 60 months, and at final follow-up. Each radiograph was assessed for femoral head penetration using Roman v1.70 software (http://www.cookedbits.co.uk/roman/) and the wear magnitude was calculated relative to the 2 month postoperative radiograph.
Results:
Mean total head penetration rate was 0.008 ± 0.003 mm/yr in the XLPE group and 0.048 ± 0.005 mm/yr in the CPE group (Figure 1), an 80% reduction. The mean bedding-in penetration for the initial two years was 0.026 ± 0.015 mm/yr in the XLPE group and 0.052 ± 0.021 mm/yr in the CPE group. The long-term penetration rate averaged 0.003 ± 0.007 mm/yr in the XLPE group and 0.068 ± 0.012 mm/yr in the CPE group, a 95% reduction.
Six cases in the CPE group showed small focal osteolysis at Gruen zone 1 (one case showed osteolysis also in Gruen zone 7) around 6 years after the surgery.
Discussion and Conclusions:
Several previous studies have shown 30% to 90% reductions in femoral head penetration rates using XLPE cups compared to CPE cups. We observed similar significant improvements in wear performance with XLPE cups at medium term follow-up, especially after bedding-in penetration. We could not detect a significant difference in femoral head penetration between head sizes in the XLPE group. Femoral head penetration was significantly greater in the CPE group, especially in cases with observed osteolysis (Figure 2), where head penetration rates were greater for both early and late time periods (p < 0.05).
Mid-term follow-up of THA patients shows improved wear performance of XLPE cups and no cases of osteolysis. Although these improved bearing materials may enhance the longevity of primary THA, it is wise still to carefully position and orient the acetabular cup to provide the best hip mechanics and contact conditions compatible with enhanced wear performance.
