Comparing daily shoulder motion and frequency after anatomic and reverse shoulder arthroplasty

BACKGROUND: Both anatomic (TSA) and reverse total shoulder arthroplasty (RTSA) are common interventions for glenohumeral arthrosis, with the goal of relieving pain and restoring mobility. Understanding shoulder arthroplasty motion and frequency is of interest in evaluating effectiveness and in predicting bearing wear for implant development and optimization. The purpose of this study was to measure and compare the total daily shoulder motion of patients after TSA and RTSA. METHODS: Thirty-six human subjects who had undergone shoulder arthroplasty wore a custom instrumented garment that tracked upper extremity motion for the waking hours of 1 day. The 3-dimensional orientation of each humeral sensor was transformed with respect to the torso to calculate total joint motion and frequency, with comparison of TSA to RTSA. In addition, the yearly motion of the shoulder was extrapolated. RESULTS: The majority of shoulder motion occurred below 80° of elevation (P < .001), totaling on average 821 ± 45 and 783 ± 27 motions per hour for TSA and RTSA, respectively. Conversely, elevations >80° were significantly less frequent, totaling only 52 ± 44 (P < .001) and 38 ± 27 (P < .001) motions per hour for TSA and RTSA, respectively. No significant differences were detected between TSA and RTSA shoulders (P = .22) or their respective contralateral asymptomatic sides (P = .64, P = .62). When extrapolated, it was estimated that each TSA and RTSA shoulder elevated above 60° approximately 1 million and 0.75 million cycles per year, respectively. DISCUSSION: Mean shoulder motions after TSA or RTSA were not significantly different from the contralateral asymptomatic side. In addition, no significant differences were detected in shoulder motion or frequency between TSA and RTSA.