Background: Osteolysis resulting from wear debris production from the bearing surfaces is a major factor limiting long-term survival of hip implants. Oxidized zirconium head on crosslinked polyethylene (XLPE) is a modern bearing coupling. However, midterm in vivo wear data of this coupling are not known.
Questions/purposes: The purpose of this study was to investigate in vivo whether the combination of an oxidized zirconium femoral head on XLPE produces less wear than a ceramic head on XLPE or a ceramic head on conventional polyethylene (CPE) couplings and whether any of these bearing combinations results in higher hip scores.
Methods: Between 2003 and 2007, we performed 356 total hip arthroplasties in 288 patients; of those, 199 (69.1%) patients (199 hips) were enrolled in what began as a randomized trial. Unfortunately, after the 57(th) patient, the randomization process was halted because of patients' preference for the oxidized zirconium bearing instead of the ceramic after (as they were informed by the consent form), and after that, alternate allocation to the study groups was performed. Hips were allocated into four groups: in Group A, a 28-mm ceramic head on CPE was used; in Group B, a 28-mm ceramic head on XLPE; in Group C, a 28-mm Oxinium head on XLPE; and in Group D, a 32-mm Oxinium head on XLPE. The authors prospectively collected in vivo wear data (linear wear, linear wear rate, volumetric wear, and volumetric wear rate) using PolyWare software. Preoperative and postoperative clinical data, including Harris and Oxford hip scores, were also collected at regular intervals. Of those patients enrolled, 188 (95%) were available for final followup at a minimum of 7 years (mean, 9 years; range, 7-12 years).
Results: All bearing surfaces showed a varying high bedding-in effect (plastic deformation of the liner) up to the second postoperative year. At 5 years both oxidized zirconium on XLPE groups showed lower (p < 0.01) volumetric wear (mean ± SD mm(3)) and volumetric wear rates (mean ± SD mm(3)/year) (Group C: 310 ± 55-206 ± 55 mm(3)/year, Group D: 320 ± 58-205 ± 61 mm(3)/year) when compared with ceramic on CPE (Group A: 791 ± 124-306 ± 85 mm(3)/year) and ceramic on XLPE (Group B: 1420 ± 223-366 ± 88 mm(3)/year) groups. For those patients who had completed 10 years of followup (20 patients [44.5%] of Group A, 21 [45.7%] of Group B, 23 [47.9%] of Group C, and 22 [44.9%] of Group D), at 10 years, both oxidized zirconium on XLPE groups also showed lower (p < 0.01) volumetric wear (mean ± SD mm(3)) and volumetric wear rates (mean ± SD mm(3)/year) (Group C: 356 ± 64 to 215 ± 54 mm(3)/year, Group D: 354 ± 50 to 210 ± 64 mm(3)/year) when compared with ceramic on CPE (Group A: 895 ± 131 to 380 ± 80 mm(3)/year) and ceramic on XLPE (Group B: 1625 ± 253 to 480 ± 101 mm(3)/year) groups. When wear rates of both oxidized zirconium groups were compared, no differences were found at any time interval with the numbers available. Two hips (one from Group A and one from Group B) are scheduled for revision as a result of wear and osteolysis. There were no differences in hip scores among the groups with the numbers available.
Conclusions: In this study, in vivo wear parameters were lower when the combination of an oxidized zirconium head on XLPE liner was used at an average of 9 years (range, 7-12 years) followup. Further larger-scale clinical studies should confirm these findings and evaluate osteolysis and revision rates in association with the use of this bearing coupling.
Level of evidence: Level II, therapeutic study.