The influence of preoperative teres minor muscle fatty degeneration on active external rotation following reverse total shoulder arthroplasty

Background: Reverse total shoulder arthroplasty (rTSA) improves shoulder elevation in patients treated for cuff tear arthropathy (CTA) or irreparable massive cuff tears. Patient satisfaction can be limited by reduced active external rotation (AER). Rotator cuff muscles that externally rotate the shoulder are infraspinatus and teres minor (TM). Aim: The purpose of this study was to assess the correlation between preoperative TM fatty degeneration and postoperative AER after rTSA performed for CTA or irreparable cuff tears. Methods: Constant scores and active range of motion were consecutively collected for 109 shoulders in 97 patients (mean: 75.73 ± 8.94 years; 31 male, 66 female) over a 10-year period. AER was evaluated with the humerus in adduction (AER1) and in abduction. TM muscle atrophy was scored according to Goutallier's classification, assessed on preoperative computed tomography scans. Results: Multivariate analysis showed that TM fatty infiltration was a predictor of AER1. AER1 decreased by 4.9 degrees preoperatively and by 6.4 degrees at the final follow-up, for each increment in Goutallier grade (P = .02). Postoperatively, AER evaluated with humerus in abduction improved significantly (P < .001), but did not correlate with TM Goutallier grade. At a mean follow-up of 38 months (range: 24 to 96), mean Constant score improved from 20.5 ± 11.1 to 68.4 ± 14.9 (P < .001), as did shoulder active range of motion in all planes including AER1 (P < .001). Conclusion: This is the first study to quantify the inverse correlation between AER and TM Goutallier grade, both preoperatively and after rTSA; this information guides prognosis for patients with TM degeneration undergoing rTSA. Further studies are necessary to have a better understanding and find reliable solutions. Level of evidence: Level III; Case Series