Factors influencing functional internal rotation after reverse total shoulder arthroplasty

BACKGROUND: Functional internal rotation (fIR) of the shoulder is frequently limited after reverse shoulder arthroplasty (RTSA). The objective of this study was to study a cohort of satisfied patients after RTSA who had comparable active mobility except for fIR and to identify factors associated with selective loss of fIR. METHODS: A retrospective cohort study was conducted to compare 2 patient groups with either poor (≤ 2 points in the Constant-Murley score [CS]) or excellent (≥8 points in CS) fIR after RTSA at a minimum follow-up of 2 years. Influencing factors (demographic, surgical or implant related, radiographic parameters) and clinical outcome were analyzed. RESULTS: Fifty-two patients with a mean age of 72.8 (±9.3) and a mean follow-up of 41 months were included in the IR≤2 group and 63 patients with a mean age of 72.1 (±8.0) and a mean follow-up of 59 months in the IR≥8 group. All patients had undergone RTSA with the same implant type and only 2 different glenosphere sizes (36 and 40) for comparable indications. A multivariate analysis identified the following significant risk factors for poor postoperative fIR: poor preoperative fIR (pts in CS: 3 [range: 2-6] vs. 6 [range: 4-8], P<.0001), smoking (17.3% vs. 6.5%, P = .004), male gender (59.6% vs. 31.7%, P = .002), less preoperative to postoperative distalization of the greater tuberosity (Δ 19.4 mm vs. 22.2 mm, P = .026), a thin humeral insert (≤3 mm: 23.1% vs. 54.8%, P = .039), and a high American Society of Anesthesiologists score (≤ III: 30.8% vs. 14.3%, P = .043). Subscapularis repair status and glenosphere size had no influence on fIR. Clinical outcome scores improved in both groups from preoperatively to last follow-up. The IR≥8 group had overall significantly better outcome scores compared to the IR≤2 group (Δ 9.3% SSV and Δ 9.5% relative CS, P < .0001). There was no difference in CS between the cohorts when the score for fIR was discarded. CONCLUSION: Independent risk factors for poor postoperative fIR after RTSA are poor preoperative fIR, smoking, male gender, less preoperative to postoperative distalization of the greater tuberosity, a thin humeral insert height, and a high American Society of Anesthesiologists score. Except for male gender, these factors are modifiable. These findings may be a valuable addition to patient counselling as well as preoperative planning and preoperative and intraoperative decision-making. The relevance of fIR for overall satisfaction is substantiated by this study.