EMG and Delta Function in Standard RTSA vs Lateralization

Reverse total shoulder arthroplasty (RTSA) is a well-established method to treat patients with irreparable rotator cuff tears and glenohumeral osteoarthritis. The biomechanical principle implies a medialization and distalization of the center of rotation (COR). Deficiencies in internal and external rotation constitute frequently encountered functional problems. Some studies showed reduced activation of the posterior deltoid in EMG measurements, which may explain the inability to compensate these movements. Lateralized prosthetic designs demonstrated increased external rotation through an alteration of the deltoid's lever arm. The aim of the study is to investigate the impact of lateralization on functional outcome and deltoid EMG activity in comparison to a standard implantation technique.

Introduction Reverse total shoulder arthroplasty (RTSA) is a well-established method to treat patients with irreparable rotator cuff tears and glenohumeral osteoarthritis. The biomechanical principle is a medialization and distalization of the center of rotation (COR) to provide a compensatory role for the insufficient rotator cuff. Deficiencies in internal and external shoulder rotation constitute frequently seen functional problems. Some studies showed reduced activation of the posterior deltoid in EMG measurements, which may explain the inability to compensate these movements. Lateralized prosthetic designs implants demonstrated increased external rotation force through an alteration of the deltoid's lever arm. Material and Methods This study will be conducted as a monocentric randomized, prospective trial. Approximately 130 patients, scheduled for reversed total shoulder arthroplasty (RSA), will be enrolled. The patients will be divided into two groups depending on receiving RSA or a lateralized implant via randomization. Preoperatively and at 4 follow-up visits, a surface EMG (sEMG) of the deltoid muscle will be conducted and clinical scores (Constant-Murley Score and Quick DASH questionnaire) will be assessed. As patient related outcome measures, p-ASES-S and SVV will be evaluated. Pre- and postoperative x-rays and CT scans will be conducted for preoperative planning and measurement of achieved lateralization. Clinical and radiological complications will be routinely documented. Group comparisons and correlations will be performed to compare both groups pre- and postoperatively. The study's duration is scheduled for 2 years. Aim of the Study The aim of the study is to evaluate a possible difference in function and EMG activity between the two patient groups. Especially a different increase in external rotation range of motion and strength shall be investigated. Furthermore a possible connection between preoperative deltoid muscle activity and postoperative outcome shall be pointed out. Lastly, the impact of lateralization on functional outcome, shall be evaluated compared to the standard implantation technique.