Subscapularis function after total shoulder arthroplasty: electromyography, ultrasound, and clinical correlation

Background The literature lacks electromyographic (EMG) examination of subscapularis function in the postoperative period after total shoulder arthroplasty (TSA). The primary purpose of this study was to document EMG activity of the subscapularis after TSA and to correlate it with clinical and ultrasound findings. Methods The study included 30 patients who were at least 1 year (average, 2.1 years) from surgery, status post TSA approached through a standard subscapularis tenotomy. Patients returned for a physical examination, ultrasound evaluation, and EMG evaluation. Patients also completed postoperative surveys: the American Shoulder and Elbow Surgeons questionnaire, the Simple Shoulder Test, and the 12-Item Short Form Health Survey. Results The American Shoulder and Elbow Surgeons, Simple Shoulder Test, and physical 12-Item Short Form Health Survey scores improved from preoperatively to postoperatively, respectively, 45.3 to 76.8 (P = .0002), 3.9 to 9.0 (P < .0001), and 33.9 to 42.8 (P = .017). Six patients had a positive lift-off test result, and the belly-press test result was negative in all patients. Two patients had a subscapularis rupture on ultrasound. The postoperative EMG finding was normal in 15 patients; in the other 15 patients, there was evidence of chronic denervation with reinnervation changes: 30% subscapularis, 27% infraspinatus, 20% supraspinatus, 20% teres minor, and 13% rhomboids. Conclusions This is the first study using a comparison EMG evaluation to document subscapularis function after TSA. EMG evaluation showed that active denervation of the subscapularis was not evident in any patient at least 1 year after TSA. However, in half of the patients, there was evidence of chronicdenervation and reinnervation changes across 5 muscle groups. We theorize that surgical exposure, traction, and the use of interscalene regional anesthesia may contribute to these unexpected EMG results.