Deltoid muscle activity in patients with reverse shoulder prosthesis at 2-year follow-up

Purpose: The reverse shoulder prosthesis (RSP) was developed to relieve pain and improve functional outcomes in patients with glenohumeral arthritis and deficiency of the rotator cuff. Even if clinical and functional outcomes regarding the use of the RSP were reported by literature, data concerning progressive deltoid adaptation to this non-anatomic implant are still missing. The purpose of our study was to correlate clinical and functional outcomes with deltoid fibers activity and muscle fatigability in patients with reverse shoulder prosthesis at 2 years follow-up. Methods: Twenty patients with reverse shoulder prosthesis due to symptomatic deficient or nonfunctional rotator cuff associated with osteoarthritis were referred by Cervesi Hospital Shoulder and Elbow Surgery Unit. Exclusion criteria were: axillary nerve palsy, a nonfunctioning deltoid muscle, diabetes, previous trauma, malignancy. Furthermore patients who received the RSP for revision arthroplasty, proximal humerus fractures were excluded. All the patients underwent clinical and functional evaluation with the support of electromyography measurement focused on deltoid activity. Results: RSP surgical treatment in shoulder osteoarthritis confirms his good outcome in terms of pain relief. At 2 years anterior and lateral deltoid electromyographic activity was significantly lower compared with contralateral side (p < 0.001). Posterior deltoid activity was no detectable. Range of motion at 2 years of follow-up decreased in terms of forward flexion (p = 0.045), abduction (p = 0.03) and external rotation (p < 0.001). Conclusions: Our study demonstrates that even if the patients remain pain-free, progressive deterioration of the deltoid activity is unavoidable and may lead to poor functional outcomes overtime.