Greater Deltoid Coverage on Pre-Operative MRI Correlates with Improved Early Functional Outcomes after Reverse Shoulder Arthroplasty.

Introduction: Reverse shoulder arthroplasty (RSA) translates deltoid contraction into abduction and rotational forces, allowing the deltoid to replace the function of the deficient rotator cuff. While RSA is classically contraindicated in patients with deltoid deficiency, there is no consensus in the literature as to the importance of the deltoid muscle in predicting clinical outcomes. Therefore, the extent or arc of the deltoid wrapped around the humeral head, and the thickness of the deltoid muscle at specific intervals are investigated as possible predictors of post-RSA outcomes in this study.

Methods: A retrospective review of prospectively collected data was performed in a tertiary institution. Patients who had undergone a reverse shoulder arthroplasty from 2011 to 2021 were included. Pre- and post-operative radiographic measurements of deltoid wrap arc anteriorly and posteriorly, as well as thickness at 45-degree intervals were taken. These were correlated with forward flexion and abduction range of motion (ROM) and clinical scores pre-operatively and at 3, 6 months, 1 and 2-year post-operatively. A statistical significance of p<0.05 was taken.

Results: Forty-five cases from forty-one patients were included. From the plane of the native glenoid, the deltoid arc ranges at an average of 21.5˚ anteriorly to 168˚ posteriorly. The thickness of the deltoid at 45˚ was correlated with greater flexion and abduction ROM at the 6-month mark (p=0.037 and p=0.014 respectively). Greater deltoid thickness at 45^o was also correlated with better early functional outcomes (Constant, University of California-Los Angeles and Oxford scores) at 6-months (p=0.007, p=0.004 and p=0.031 respectively). No correlation was found with late functional outcomes at 2-years.

Conclusion: Patients who undergo RSA with a greater pre-operative deltoid thickness, especially at 45˚ from the anterior, are associated with better forward flexion and abduction ROM, as well as improved functional outcomes 6 months post-operatively. There may be value in prehabilitation to improve early post-operation outcomes.

Level of evidence: 3: