To investigate the influence of the linear wear rate of polyethylene, the osteolysis threshold of the linear wear rate, and the related factors on polyethylene wear in an uncemented metal-backed-cup of a primary total hip arthroplasty.

This study examined 87 hips from 75 patients (57 men and 18 women) who had undergone a primary total hip arthroplasty with a minimum follow-up of five years. The mean age was 47.3 years (range, 25-68 years). The mean follow up period was 90 months (range, 60-148 months). The most frequent diagnosis was avascular necrosis of the femoral head. 18 Harris-Galante I and 69 Harris-Galante II cups, were used on the acetabular side, and 66 Anatomic and 21 Harris-Galantestems were used on the femoral side. Radiographic polyethylene wear was estimated using the method reported by Livermore, and the above mentioned factors that influence polyethylene wear were investigated and analyzed statistically.

The overall mean linear wear rate of polyethylene was 0.21 mm/year. The linear wear rate of 22 hips with osteolysis and 65 hips without osteolysis was 0.29 mm/year and 0.12 mm/year respectively, showing a significant difference (p=0.001). The patients with high activity had a significantly higher mean linear wear rate (0.28 mm/year) than that those patients with low activity (0.16 mm/year) (p=0.048). Patients younger than 40 year-old had a significantly higher mean linear wear rate (0.23 mm/year) than the patients older than 40 year-old (0.19 mm/year) (p=0.049). However, there were no correlations between the mean linear wear rate and gender, weight, diagnosis, the type of cup and the thickness of the polyethylene.

The polyethylene wear rate in an uncemented metal-backed-cup was significantly affected by age and the activity of the patients, which strongly influenced the development of osteolysis. Therefore, in order to reduce the incidence of aseptic loosening due to osteolysis in young patients with high activity, a bearing surface with mean linear wear rate of 0.12 mm/year or less is recommended.