Polyethylene wear debris can cause osteolysis
and the failure of total hip arthroplasty. We present the five-year
wear rates of a highly cross-linked polyethylene (X3) bearing surface
when used in conjunction with a 36 mm ceramic femoral head. This was a prospective study of a cohort of 100 THAs in 93 patients.
Pain and activity scores were measured pre- and post-operatively.
Femoral head penetration was measured at two months, one year, two
years and at five years using validated edge-detecting software
(PolyWare Auto). At a mean of 5.08 years (3.93 to 6.01), 85 hips in 78 patients
were available for study. The mean age of these patients was 59.08
years (42 to 73, the mean age of males (n = 34) was 59.15 years,
and females (n = 44) was 59.02 years). All patients had significant
improvement in their functional scores (p <
0.001). The steady
state two-dimensional linear wear rate was 0.109 mm/year. The steady
state volumetric wear rate was 29.61 mm3/year. No significant
correlation was found between rate of wear and age (p = 0.34), acetabular
component size (p = 0.12) or clinical score (p = 0.74). Our study shows low steady state wear rates at five years in
X3 highly cross-linked polyethylene in conjunction with a 36 mm
ceramic femoral head. The linear wear rate was almost identical
to the osteolysis threshold of 0.1 mm/year recommended in the literature. Cite this article:
The aim was to assess the wear rate of highly Cross Linked (X3) polyethylene with the use of 36mm femoral heads in total hip arthroplasty (THA). We have previously reported our early results and raised some concern regarding the potential excessive femoral head penetration rates. These results give the 2 year wear rates following this initial bedding-in phase. There were 100 consecutive patients who had a THA with the same femoral and acetabular components using a 36mm femoral head and X3 polyethylene that were assessed prospectively. Validated computer software (Polyware) was used to assess linear and 3 dimensional wear using standardised x-rays. Examinations were performed at 2,12,18 and 24 months. There were 40 hips that had completed the 2 year x-ray examination (average 2.4 years). The mean 2-dimensional linear wear rate was 0.17 mm/yr and the mean volumetric wear rate was 113.73 mm3/yr. Steady state wear was achieved after the 2 month and before the 1 year examination. The steady state wear rate was 0.001mm/yr. There was no difference in wear rate with the different sized cups used and wear rate was independent of liner thickness. The early high wear rates reported have now settled into a more expected pattern of steady state wear similar to other results presented in literature with the use of smaller femoral heads. Using a 36 mm femoral head has not adversely increased the wear rates compared to smaller head sizes when used in conjunction with X3 polyethylene in the short term for THA. These results suggest that the wear rate of X3 is not compromised even with thinner liners and raise the possibility of safely using even larger head sizes with this polyethylene.
The early results with highly cross-linked polyethylene have been encouraging and have increased the ability to use larger head diameters to improve the range of motion and decrease the dislocation rate, the commonest cause of early complications following total hip arthroplasty (THA). Wear rates with 32 mls heads have been satisfactory however there have been very few independent studies looking at early polyethylene wear in 36 mm heads. This study assessed the rate of polyethylene wear of a 36mm ceramic femoral head and a highly cross-linked polyethylene (X3 Stryker) liner in THA. This prospective study reviewed 100 consecutive THAs in young patients (mean age 58 years) who had undergone THA with the same 36mm ceramic femoral head and highly cross linked polyethylene liner. All patients received the same femoral stem (ABG, Stryker) and acetabular cup (Trident, Stryker). Two surgeons performed all procedures. Patients were assessed radiologically immediately postoperatively, at 10 weeks and at one year. Validated computer software (Polyware) was used to assess both volumetric and linear wear. At one year the mean two-dimensional linear wear rate was 0.51 mm/yr. Mean three-dimensional linear vector wear rate was 0.59 mls per year with a mean volumetric wear rate of 322.6 mms per three years. Cup size ranged from 52–62 mms and the correlation coefficient between cup size and three-dimensional linear wear rate was −0.100. The correlation coefficient between cup size and volumetric wear rate was −0.009 confirming no significant correlation between cup size and wear. Larger size femoral heads are associated with a higher volumetric wear compared to linear wear rate when using conventional polyethylene. This study demonstrated much higher early linear wear rates compared to other studies using 28 and 32 mms heads. This higher rate may be associated with the creep phenomenon and early bedding-in in the early stages after a THA and although this is of concern these results should be interpreted with caution.
