Identifying internal and external shoulder rotation using a kirigami-based shoulder patch

Abstract

Internal and external rotation of the shoulder is often challenging to quantify in the clinic. Existing technologies, such as motion capture, can be expensive or require significant time to setup, collect data, and process and analyze the data. Other methods may rely on surveys or analog tools, which are subject to interpretation. The current study evaluates a novel, engineered, wearable sensor system for improved internal and external shoulder rotation monitoring, and applies it in healthy individuals. Using the design principles of the Japanese art of kirigami (folding and cutting of paper to design 3D shapes), the sensor platform conforms to the shape of the shoulder with four on-board strain gauges to measure movement. Our objective was to examine how well this kirigami-inspired shoulder patch could identify differences in shoulder kinematics between internal and external rotation as individuals moved their humerus through movement patterns defined by Codman's paradox. Seventeen participants donned the sensor while the strain gauges measured skin deformation patterns during the participants' movement. One-dimensional statistical parametric mapping explored differences in strain voltage between the rotations. The sensor detected distinct differences between the internal and external shoulder rotation movements. Three of the four strain gauges detected significant temporal differences between internal and external rotation (all p < .047), particularly for the strain gauges placed distal or posterior to the acromion. These results are clinically significant, as they suggest a new class of wearable sensors conforming to the shoulder can measure differences in skin surface deformation corresponding to the underlying humerus rotation.

Keywords: Biomechanics; Biomechatronics; Embedded electronics; Monitors; Sensors.

Figure 1.

a) Top view of the kirigami-inspired shoulder patch lying flat with four strain gauges. b) Anterior view of the kirigami patch placed over the shoulder, with ribbon cabling from the patch to the electronics placed within an armband. c) The electronics contained within the armband, including a Wheatstone bridge board and Bluetooth module board.

Figure 2.

Illustration of the movements performed by participants while wearing the kirigami shoulder patch. a) The participant begins with their arm to the side and ends with their arm above their head. b) The participant begins with their arm above their head and ends with their forearm resting on their head. After completing movements a-b, participants were instructed to lower their arms in one of two paths. c-i) The participant begins with their forearm resting on their head and moves their arm in front of them in the sagittal plane into internal rotation, followed by d-i) the participant begins with their arm in internal rotation and moves it back into neutral position. Alternatively, c-ii) The participant begins with their forearm resting on their head and moves their arm to the side, in the coronal plane, into external rotation, followed by d-ii) the participant begins with their arm in external rotation and moves it back into a neutral position. Finally, four triggers were used to signal important time points in the movement. The timing of these triggers relative to the movements performed is shown at the bottom of the figure.

Figure 3.

Representative data from one participant performing five trials resulting in internal rotation (red traces) or external rotation (black traces). The displayed data has been filtered and normalized to 100% of the activity movement. Each column shows resultant data from each of the four strain gauges, with the visible representation of the strain gauge location above each plot.

Figure 4.

Data averaged across all 17 participants, with the top row showing the averaged time-series data for each of the four strain gauges after it has been filtered and normalized to 100% of activity movement. The mean of the five trials for external rotation is the bold black line with the standard error of the five trials shown in the gray shaded area. The mean of the five trials for internal rotation is the bold red line, with the standard error of the five trials for internal rotation in the pink-shaded area. The bottom row displays the results from a t-test using statistical parametric mapping for each of the four strain gauges for the entire group, where the gray shaded area represents the area in which there is a statistical difference between internal and external rotation.