A Kirigami-inspired Shoulder Patch to Identify Shoulder Humeral Rotation

Abstract

Background: The shoulder's complexity, with six degrees of freedom, makes measuring glenohumeral joint movement challenging. Monitoring shoulder kinematics is crucial for injury prevention, rehabilitation, and optimizing performance, especially in restoring internal and external rotation for athletes and post-surgery patients. However, quantifying shoulder rotation in clinical settings is difficult due to the large, expensive setups required for motion capture and the precise calibration needed for inertial measurement units. Objectives: Evaluate a novel, engineered, wearable sensor system for improved monitoring of shoulder rotation, using a sensor platform that conforms to the shape of the shoulder with on-board strain gauges to measure movement; examine how well this patch could identify differences in shoulder kinematics between internal and external rotation as individuals moved their arm through specified movements. Methods: A custom kirigami-inspired shoulder patch measured local strain changes during shoulder movement. Seventeen participants performed internal and external rotations of the right arm, following Codman's paradox. A trigger identified three movement stages: forearm from above to bent over head, internal/external rotation, and return to neutral. Strain gauge data was low-pass filtered, and time normalized to movement percentage. Differences in strain voltage between internal and external rotations were analyzed using one-dimensional statistical parametric mapping (SPM). Results: The movement patterns for each strain gauge on the kirigami shoulder patch were consistent within participants, indicating uniform performance without hardware discrepancies across trials. Intra-subject SPM models showed that 14/17 participants had significant differences between internal and external rotation in at least one movement phase, with the most common phase being 33-66% of the movement, when in internal/external rotation. This suggests that conformal sensors can detect skin deformation as the humerus rotates. Three of four strain gauges detected significant temporal differences between internal and external rotation (all p < 0.047). The most effective strain gauges were distal (over the humeral head) and posterior (over shoulder muscle fascia). The posterior gauge showed more significant voltage changes during internal rotation, likely due to the posterior capsule tightening. The proximal gauge, farthest from the humeral head, was less effective in detecting rotation differences. While the patch could differentiate between internal and external rotation, further research is needed to evaluate its ability to detect other shoulder movements and to design patches for a broader range of users. Conclusion: A new class of kirigami-inspired wearable sensors conforming to the shoulder can detect differences in skin deformation corresponding to the underlying humeral rotation. Given their lower cost and enhanced portability, expanded use of this sensor platform may better inform clinical decision making for shoulder pathologies than traditional approaches for measuring shoulder kinematics (e.g., marker-based motion capture).