Muscle Activation Patterns during Active External Rotation after Reverse Total Shoulder Arthroplasty: An Electrophysiological Study of the Teres Minor and Associated Musculature.

BACKGROUND: Preoperative teres minor insufficiency has been identified as a risk for poor restoration of external rotation (ER) after reverse shoulder arthroplasty (RTSA). However, there has been little investigation regarding muscle-activation patterns generating ER. This prospective study sought to determine timing and activation levels of shoulder-girdle musculature during ER in well-functioning RTSAs with an intact teres minor utilizing a lateralized design.

METHODS: Patients at least 1-year from RTSA with functional ER, American Shoulder and Elbow Surgeons (ASES) score >70, superior rotator cuff deficiency, and intact teres minor were identified. Electrophysiological and kinematic analyses were performed during ER in modified neutral (MN; arm at side with 90 degrees of elbow flexion) and abduction (AB; shoulder abducted 90 degrees with 90 degrees elbow flexion). Dynamometer-recorded torque and position were pattern-matched to electromyography (EMG) during ER. Root mean square (RMS), integrated EMG (microvolts*msec; standard deviation [SD]) and median frequency ([MF] Hertz [Hz], SD) were calculated to determine muscle recruitment. Pairwise t-test analysis compared muscle activation (p<0.05 showed significance).

RESULTS: After an a priori power analysis, 16 patients were recruited. Average ASES and visual analog scale (VAS) scores, and ASES sub-score for ER in AB ("comb hair") were 87.7, 0.5, and 2.75/3, respectively. In AB, muscle activation began with upper trapezius, middle trapezius, and latissimus dorsi then anterior deltoid activating to neutral. With ER beyond neutral, the teres major (9.6 microvolts*msec, SD 9.2) initiated ER followed by the teres minor (14.1 microvolts*msec, SD 18.2) and posterior deltoid (11.1 microvolts*msec, SD 9.3). MF analysis indicated equal contribution of teres major (1.1 Hz, SD 0.5), teres minor (1.2 Hz, SD 0.4), and posterior deltoid (1.1 Hz, SD 0.4) in ER beyond neutral. In MN, the upper and middle trapezius were not recruited to the level as AB. For ER beyond neutral, the teres major (9.5 microvolts*msec, SD 9, MF 1.1 Hz, SD 0.5), teres minor (11.4 microvolts*msec, SD 15.1, MF 1.1 Hz, SD 0.5), and posterior deltoid (8.5 microvolts*msec, SD 8, MF 1.2 Hz, SD 0.3) were activated in similar sequence and intensity as AB. No differences in muscle-activation duration or intensity were noted between teres major, teres minor, or posterior deltoid (p>0.05).

CONCLUSION: Active ER after RTSA is complex and not governed by a single muscle-tendon unit. This study establishes a sequence, duration, and intensity of muscle activation for ER in well-functioning RTSA. In both tested positions, the teres major, teres minor, and posterior deltoid function equally and sequentially to power ER.