Deltoid Fatigue Part 2: A Longitudinal Assessment of Anatomic Total Shoulder Arthroplasty Over Time.

Abstract Background Gradual loss of overhead range of motion (ROM) has been observed following reverse shoulder arthroplasty (RSA). It remains unclear if this is caused by the effect of RSA design on muscle fiber lengthening or is part of the natural aging process of the shoulder musculature. Although studies have attempted to evaluate deltoid fatigue after RSA, there is a paucity of literature evaluating this effect after anatomic shoulder arthroplasty (aTSA), which would be expected to occur due to aging alone. The purpose of this study is to evaluate the effect of time on overhead ROM after aTSA and compare this to previous data on a similar cohort of RSAs. We hypothesized that overhead ROM would decrease gradually over time in both groups without differences between prosthesis types. Methods A retrospective review of 384 aTSAs without complications was performed over a 10-year period. All shoulders were treated for primary osteoarthritis (OA) using a single implant system. Patients were evaluated longitudinally at multiple postoperative time points. At least one follow-up visit was between 1-2 years postoperatively and another at least 5 years after surgery. ROM and patient reported outcome measures (PROM) were evaluated using linear-mixed models for repeated measures. These results were compared to a previously-evaluated cohort of 165 well-functioning RSAs analyzed using the same methodology. Results Primary aTSA shoulders were observed to lose 0.7° of abduction per year starting 1 year postoperatively (p=0.001). Smaller losses were observed in ER (-0.3°/year, p=0.06) and IR (-0.04/year, P Discussion In the well-functioning aTSA, gradual loss of ROM occurs in all planes of motion except forward elevation. However, these losses are small and have little meaningful impact relative to MCID thresholds on PROMs. Progressive loss of abduction seen in both aTSA and RSA is likely secondary to aging of the periscapular and rotator cuff musculature. When compared to RSA, loss of motion following aTSA was statistically similar, calling in to question the belief that RSA-induced deltoid fatigue leads to loss of overhead motion over time.