Impact of Fatty Infiltration of the Teres Minor Muscle on the Outcome of Reverse Total Shoulder Arthroplasty

Abstract

Reports have demonstrated that reverse shoulder arthroplasty restores overhead elevation but fails to restore active external rotation. The teres minor muscle-tendon unit contributes to active external rotation, and its deficiency may impair the clinical outcome. It was therefore the purpose of this study to evaluate the influence of fatty infiltration of the teres minor muscle on the clinical outcome after reverse total shoulder replacement. Forty-two shoulders in forty-two patients (average age, seventy-one years) with painful cuff tear arthropathy or an irreparable rotator cuff deficiency with pseudoparesis were treated with a reverse Delta-III shoulder arthroplasty and followed clinically for a minimum of twenty-four months. Preoperatively, fatty infiltration of the teres minor was assessed, according to the grading system of Goutallier et al., with use of magnetic resonance imaging. The effect of teres minor fatty infiltration on the subjective and objective outcomes of the reverse shoulder arthroplasty was evaluated. The thirty shoulders with stage-0, 1, or 2 fatty infiltration of the teres minor muscle (group 1) had a significantly better ultimate Constant score, a significantly better subjective shoulder value, and significantly greater preoperative-to-postoperative improvement than the twelve shoulders with stage-3 or 4 fatty infiltration (group 2). In group 1 the relative Constant score increased by an average of 41% and the subjective shoulder value increased by an average of 44%, whereas in group 2 the respective increases were 32% (p = 0.033) and 25% (p = 0.018). Group 1 had an average increase of 6.2 points in the score for extremity positioning, whereas group 2 gained only 5.3 points (p = 0.033). Group 1 had a net gain of 9 degrees of external rotation with the arm at the side compared with an average net loss of 7 degrees in group 2 (p < 0.001). Stage-3 or 4 fatty infiltration of the teres minor compromises the clinical outcome of reverse shoulder arthroplasty for the treatment of irreparable rotator cuff tears.