Can a shoulder continuous passive motion device be considered functionally safe after rotator cuff repair? An in-vivo study of evasive motion, muscle activation, and patient feedback

Background: Early mobilization without muscle activation is important after arthroscopic rotator cuff repair (RCR). Motor-driven continuous passive motion (CPM) devices may support rehabilitation, but it is unclear whether motion remains passive or setup errors could push joint motion beyond ROM limits. Purpose: To assess whether patients can relax shoulder musculature and avoid evasive movements during CPM, and to quantify how altered device setup affects joint motion and muscle activity. Methods: Shoulder kinematics and bilateral surface EMG of four shoulder muscles were recorded during CPM-guided abduction/adduction and internal/external rotation using 3D motion capture. Patients completed 10 min in the “optimal” setup, followed by four suboptimal positioning conditions (2 min each). Patients then used the CPM device at home for 4 weeks and reported pain, comfort, compliance, and satisfaction. Results: Thirteen patients (56.9±6.6 years) were assessed 12.9±2.8 days after arthroscopic RCR. Actual ROM differed from device-guided ROM (p<0.001), but discrepancies were similar between sides (p>0.05) and remained below approved limits (50-90° abduction and 0-30° external rotation). Suboptimal positioning showed a main effect on ROM (p<0.015); post-hoc differences occurred only for abduction (p=0.002) and remained within safe limits. Muscle activity stayed low and below activation thresholds; deltoid and upper trapezius activation remained within acceptable limits. Pain and discomfort were low (≤0.72±0.96 and ≤0.87±0.91; 0–10 scales), abduction ROM improved from 37±16° to 46±19°, and satisfaction was high (reuse willingness 9.1±1.58/10). Conclusion: Early after RCR, the CPM device provides safe passive shoulder mobilization with very low muscle activation, limited sensitivity to setup errors, low pain/discomfort, and high acceptance.