| Summary: | Abstract Background Although shoulder arthroplasty is less common than knee or hip arthroplasty, the number of procedures being performed is increasing rapidly. The treatment effect is a simple method to measure outcome of joint replacement. The method was applied to measure results of total hip/knee arthroplasty but not yet for shoulder arthroplasty. Methods Included were patients with unilateral cuff arthropathy (Hamada grades > = 2) treated with reversed total shoulder arthroplasty (RSA) in this prospective multicenter study. The patients were assessed with the ASES questionnaire. The treatment effects (TE) was calculated for each patient. TE = score reduction/baseline score. A positive TE means amelioration, TE = 0 unchanged, and a negative TE means worse. The primary aim was to calculate the TE’s for RSA at 6, 12, 24, and 60 months postoperatively. The secondary aim was to analyze the influence of confounders (preoperative Hamada grade, age, gender, dominance, side of the affected shoulder, general co-morbidities measured using ASA grade). Results Two hundred three patients were included for this analysis of whom 183 patients had a complete 2 year follow up. Two years postoperatively the mean ASES score augmented significant from 20.5 to 78.7 (p < 0.001). The 2 year TE’s ranged from 1 to 0.09. We had no patient with a negative TE. A higher Hamada grade was associated with better TE’s (Hamada grade 4+ vs. 2, p-value 0.042). For age and dominant side there were weak associations where those aged 80+ and dominant side had better TE’s. The patients with higher ASA grade had lower TE’s (ASA grade 4+ vs. 1, p-value 0.013). The mean TE’s were 0.77 at 6-months, 0.81 at 1 year, 0.76 at 2 years and 0.73 at 5 years. Conclusions The outcome for reverse shoulder arthroplasty can be measured with the treatment effect method; the 2 years TE’s vary from 1 to 0.09. The mean treatment effects change little in the first five postoperative years (from 0.73 to 0.81). The confounders for better TE’s were: higher severity of cuff arthropathy (Hamada grade 3, 4 and 5), less co-morbidities (ASA Grade 1), higher age (80+) and dominant side. Gender did not influence the 2-year TE’s. Trial registration Comité intercantonal d’éthique (Jura, Fribourg, Neuchâtel), number 01/2008, 24.09.2008.
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