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In their retrospective review, Li et al1 have described the medium-term clinical and radiological results of Zweymüller-Plus total hip arthroplasty (THA) in 185 Chinese patients (207 hips). The articulation size was 28 mm, with conventional ultra-highmolecular-weight polyethylene (UHMWPE) over the acetabular side and third-generation alumina ceramics on the femoral side. Due to the number of patients lost to follow-up and other reasons, the final numbers of patients and hips available for analysis were 156 and 175, respectively. The mean duration of follow-up was 14.1 years. The series comprised both primary and revision THA cases. One should be aware of the complexity of the latter and that their results may not be directly comparable with the former. All the surgeries were performed before the year 2000, when UHMWPE was still widely used around the world. The authors reported satisfactory clinical outcomes.1 Two out of 175 hips were revised, making the true revision rate of 1.1%, excluding those who were lost to follow-up. Two hips were found to have excessive polyethylene wear, although the authors did not define ‘excessive’. Osteolysis or radiolucent lines were detected in 4% of the femora and 1.7% of the acetabula. There are three major limitations in this study in terms of measuring polyethylene wear and survival analysis. First, the method used to measure UHMWPE liner wear in the study was imprecise, as admitted by the authors. The modern standard of measuring wear requires the use of computer software based on the concentricity of the hip ball and liner.2 Second, the authors mentioned that their desired acetabular component position was 45° ± 10° of abduction and 15° ± 10° of anteversion. However, they did not reveal the mean postoperative angles, although malpositioning of the acetabular component is a known risk factor of excessive polyethylene wear. Third, the high number of patients lost to follow-up (13.5%) might have underestimated the revision rate and overestimated the overall survival of the prostheses. Chiu et al3 reported the results of THA in patients below 40 years of age and found UHMWPE wear and osteolysis in 56% of the hips with 28-mm Hong Kong Med J 2017;23:417–8 DOI: 10.12809/hkmj175070

Commentary on the medium-term results of ceramic-on-polyethylene Zweymüller-Plus total hip arthroplasty