Deltoid Fatigue: A Longitudinal Assessment of Reverse Shoulder Arthroplasty Over Time

Background: Studies evaluating the mid-term performance of reverse shoulder arthroplasty (RSA) have identified a drop in the Constant-Murley score between 6 and 8 years after surgery, which is most affected by a loss of forward elevation and strength. Alterations of the deltoid length and moment arm after RSA leads to non-physiologic stress on the deltoid muscle. Concern has arisen that the long-term implications of increased deltoid work may be causing “deltoid fatigue.” The purpose of this study is to evaluate the long-term effects of RSA on overhead range of motion (ROM) and to validate the hypothesis of “deltoid fatigue.”

Methods: A retrospective review of 165 RSA over a 5 year period was performed. Diagnoses were limited to cuff tear arthropathy (CTA), osteoarthritis with rotator cuff deficiency (OA), and irreparable rotator cuff tear (RCT). All procedures were performed using a single implant system. Patients were evaluated longitudinally at multiple time points. Patients were required to have a minimum of three follow-up visits, with at least one exceeding 5 years. ROM and patient reported outcome measures (PROM) were evaluated using linear-mixed models for repeated measures to evaluate changes in outcome measures over time. A secondary analysis was performed to assess the influence of patient demographic factors on observed changes in ROM and PROMs.

Results: Primary RSA shoulders were observed to lose 0.8° of forward elevation and abduction per year starting at 1 year postoperatively (p=0.006), without a significant drop at mid-term follow-up. No significant change in external rotation or internal rotation was observed. Males and patients with a diagnosis of OA showed greater baseline overhead ROM at one year postoperatively, but the subsequent rate of functional decline occurred at similar rates regardless of age, gender or indication.

Discussion: This study challenges the previous theory of “deltoid fatigue” resulting in a significant loss of overhead ROM beginning 6 to 8 years after index arthroplasty. However, a slower progressive decline in overhead ROM in well-functioning RSA shoulders was observed, averaging 0.8 degrees of overhead ROM per year. This progressive deterioration occurs at a slightly greater rate than the that observed in the natural shoulder. The observed rate of functional decline was found to be independent of age, gender and preoperative diagnosis.

Level of evidence: Level IV; Case Series; Treatment Study.

Keywords: decline; elevation; long-term; loss of motion; overhead; progressive; reverse shoulder arthroplasty.