Degenerative changes of the deltoid muscle have impact on clinical outcome after reversed total shoulder arthroplasty

Introduction Despite a high complication rate, subjective and objective results of reversed shoulder arthroplasty for severe rotator cuff lesions associated with osteoarthritis, fracture sequelae, or revision of hemiarthroplasty are favorable. However, whether the changes in biomechanics of the joint may lead to structural changes in the remaining rotator cuff and the deltoid muscle, and may thereby alter the clinical result, has rarely been described. This study investigates the context between postoperative fatty infiltration of the remaining rotator cuff and the deltoid muscle, and the clinical outcome after reversed shoulder arthroplasty. Methods Twenty-three patients with an average age of 73.1 ± 5.6 years were evaluated at an average of 26 ± 16.8 months. Evaluation included assessment of Constant Score, disabilities of the arm, shoulder and hand score, radiographic evaluation with measurement of the humeral lengthening and medialization of the center of rotation, and magnetic resonance imaging or computed tomography to classify changes in the rotator cuff and the deltoid muscle. Results There was a significant correlation between fatty infiltration of the deltoid muscle and the clinical score results. Patients with a longer follow-up showed a higher amount of degenerative changes of the deltoid than patients with a shorter follow-up. Degenerative changes of the rotator cuff showed no significant correlation to clinical results. The amount of arm lengthening, medialization of the center of rotation, and scapula notching showed no correlation to clinical results or degenerative changes of the deltoid and the rotator cuff. Conclusions These data show that degeneration of the deltoid muscle is of impact for clinical outcome. Although other factors like disuse or pain as a reason for muscular changes cannot be fully excluded, this degeneration might occur after time due to biomechanical changes after reversed arthroplasty. However, prospective studies with longer follow-up periods and repetitive assessment of muscle quality are necessary to further support these results.