Is continuous passive motion of the shoulder safe after rotator cuff surgery? A biomechanical in-vivo study of evasive motion and muscle activation with use of ARTROMOT-S3

Research Question: Early shoulder mobilization without muscle activation is crucial for recovery after rotator cuff surgery (Altintas et al., 2019). Motor driven continuous passive motion (CPM) devices, like the ARTROMOT-S3, could support the mobilization process, but there are concerns if patients can fully relax their muscles during use (Wang et al., 2015), and concerns about the sensitivity of the optimal device positioning and the corresponding range of shoulder movement. Therefore, the goal of this study was to assess patients' ability to relax shoulder muscles and resist unintended movements during continuous passive motion (CPM), as well as to evaluate the impact of suboptimal setup positions on these factors. Methods Thirteen patients (56.9 ± 6.6 years; 4 males, 9 females) were evaluated during passive shoulder mobilization using a CPM device (ARTROMOT-S3) 12.9 ± 2.8 days post arthroscopic rotator cuff repair surgery. A 3D motion capture system (Qualisys) and surface electromyography (EMG, Noraxon Ultium System) recorded shoulder movements and muscle activity of the healthy and injured side. Patients used the CPM device for 10 minutes in the optimal setup position, followed by four suboptimal positioning conditions recorded for 2 minutes each. EMG activity of four shoulder muscles (injured and non-injured sides) was normalized to resting values. Range of motion (RoM), joint displacement, and muscle activity during passive shoulder abduction, adduction, and rotation were analysed. Subsequently, patients used the CPM device at home for four weeks, reporting pain, compliance, and comfort via a questionnaire. Statistical analyses used paired t-tests and repeated measures ANOVAs with post-hoc evaluation using Bonferroni-Holm correction (p < 0.05). Results There were significant differences noted between RoM of the device and actual shoulder movement (p < 0.001). However, the discrepancies (difference between RoM of the CPM device and the shoulder RoM) were similar between the injured and healthy sides during abduction and rotation (p > 0.05) and RoM values of the shoulder remained within the safe RoM settings specified for the device. Suboptimal positions revealed a significant main effect (p < 0.015) on shoulder RoM. However, only abduction RoM showed a significant post hoc difference (p = 0.002), which remained smaller than the device’s full range. Muscle activity was low, with values not reaching activation threshold levels, supporting passive mobilization. Minimal activation of the deltoid and upper trapezius remained within acceptable limits. Pain scores were low (all means < 0.72 ± 0.96 out of 10 on the VAS Scale), comfort ratings were high (all means < 0.87 ± 0.91 out of 10), and RoM in abduction improved over four weeks (37° ± 16° to 46° ± 19°). Patient satisfaction was high (willingness to reuse: 9.1 ± 1.58 out of 10). Conclusion The ARTROMOT-S3 CPM device ensures effective and safe passive mobilization post-surgery, with low pain and high compliance, supporting early recovery after rotator cuff repair.