Comparison of linear wear rate according to femoral head sizes in metal on conventional UHMWPE liner

Abstract

Purpose: It has been generally accepted that the amount and rate of linear wear following total hip arthroplasty are influenced mainly by femoral head size in the conventional UHMWPE acetabular liner, with linear wear rate increasing as femoral head size decreases. The aim of this study is to identify the relationship between wear rate and polyethylene cup thickness in addition to femoral head size. Materials and Methods: We conducted a retrospective review of all patients who had undergone primary cementless total hip arthroplasty using conventional UHMWPE (HGP2) acetabular liner between July 1992 and December 2002. One hundred and twenty-eight hips (34 hips of female, 94 hips of male) of 64 patients on which 28 mm femoral head was used in combination with different polyethylene acetabular linear thicknesses and 102 hips (41 hips of female, 61 hips of male) of 81 patients on which 22 mm femoral head was used were included. Clinical and radiographic postoperative assessment of patients was performed at 6 weeks, 3 months, 6 months, and annually thereafter. Clinical assessment was performed using Harris hip score (HHS), UCLA activity score, visual analogue scale (VAS), and range of motion (ROM). Radiographic analysis included measurement of acetabular component position and of polyethylene wear using a validated radiographic technique (Dorr method). The mean age of patients at the time of operation was 45.3 (24-81) years old, and the mean follow-up period was 10.8 (96-144 months) years. Results: At a mean of 10.8 years, clinical results as determined by Harris hip score did not differ among different liner thickness and head sizes. For males, the linear wear rate was 0.137 mm/year in the 28 mm femoral head group and 0.139 mm/year in the 22 mm femoral head group. For females, the linear wear rate was 0.169 mm/year in the 28 mm femoral head group and 0.136 mm/year in the 22 mm femoral head group. In the 28 mm femoral head group, the linear wear rates of 6.2, 7.2, 8.2, 9.2, 10.2, 11.2, 12.2, 13.2, 14.2, 15.2, and 16.2 mm-thick polyethylene liners were 0.223, 0.197, 0.190, 0.182, 0.130, 0.104, 0.095, 0.086, 0.070, 0.064, and 0.059 mm/year, respectively, while in the 22 mm group, the linear wear rates of 6.1, 7.1, 9.1, 10.1, 11.1, 12.1, 13.1, 14.1, and 15.1 mm-thick polyethylene liners were 0.172, 0.164, 0.148, 0.139, 0.137, 0.138, 0.123, 0.122, and 0.114 mm/year, respectively. Overall, the usage of thinner liners leads to increased linear wear rates. In addition, in cases where liners thinner than 10 mm had been used, the larger femoral heads produced higher linear wear rates, while in cases where liners thicker than 10 mm had been used, larger femoral heads produced lower linear wear rates. On the other hand, volumetric wear rate was higher in the 28 mm femoral head group than in the 22 mm group when the thickness of the liner was less than 13 mm. Conclusion: This study demonstrated that the linear wear rate of the conventional polyethylene liner was not always higher in those hips with smaller femoral heads than in those with larger femoral heads, but rather different according to the thickness of the polyethylene liner. With thin conventional polyethylene liners (less than 10 mm), larger femoral heads (28 mm) induce a higher linear wear rate than with smaller heads (22 mm), while with thicker conventional polyethylene liners (more than 13 mm), larger femoral heads induce a lower volumetric wear rate than with smaller heads.