WEARABLE SYSTEMS FOR MONITORING SHOULDER JOINT: FROM A CLINICAL PERSPECTIVE TO DESIGN, DEVELOPMENT, AND APPLICATIONS

Rotator cuff (RC) disease is a common shoulder musculoskeletal disorder entailing pain, reduced functionality, and quality of life. Management of RC disease is a hot topic embracing several fields ranging from medicine to biomechanics, sports, and engineering. To date, there is a high incidence of RC retear after surgery. Moreover, there is currently no unanimous consensus on the superiority of surgical or conservative treatment in managing patients with RC injuries. The main clinical question underlying this thesis work was developing and evaluating techniques and methods for monitoring shoulder kinematics. In this context, my thesis experimentally evaluated four technologies with an increasing level of wearability: optical motion capture (OMC) system, magneto-inertial measurement units (M-IMUs), textile strain sensors, and virtual reality (VR) devices. Scapular dyskinesis is a proven clinical condition in several shoulder pathologies, such as RC diseases. However, debates are still open in the scientific community on defining the best non-invasive method for the objective measurement of scapular kinematics. Several methods have been adopted in the literature for tracking scapular kinematics. In the present study, the acromion method has been selected as it overcomes certain limitations, such as the invasiveness of experimental procedures and the measurement of scapular orientation only in static conditions. In particular, a custom polylactic acid-based Acromion Marker Cluster (AMC) was developed to measure scapular kinematics dynamically. The L-shaped structure was realized to avoid excessive vibration during upper limb motions, resulting in a lightweight cluster positioned in correspondence with the meeting point between the acromion and the scapular spine. Moreover, a custom kinematic model and an ad-hoc pipeline were developed in Visual 3D software to extrapolate the shoulder joint kinematic variables of interest. Firstly, the intra-rater reliability of the AMC method for scapula kinematics monitoring was evaluated in a population of five healthy volunteers. Results showed that the AMC method is reliable for tracking scapular kinematics during both the elevation and lowering phases of movements performed bilaterally, although caution is needed in the interpretation of results at high humerothoracic elevation angles. In particular, the results of this investigation have shown that the AMC method had good to excellent intra-rater reliability for scapular upward-downward rotations, internal-external rotations, and posterior-anterior tilting for upper limb movements in the scapular and sagittal planes. However, poor to moderate intra-rater reliability was found for scapular internal-external rotations during upper limb movement in the frontal plane. Using the AMC method, the 3D scapular kinematics was evaluated in nine patients with RC tears and scheduled for future arthroscopic repair. In particular, differences between pathological and contralateral healthy shoulders were assessed. Results showed comparable or increased scapular motions in the affected shoulders with respect to the contralateral healthy side before surgery. This condition may result from adaptive movements of the pathological side to maintain humeral elevation. Patients with RC tear exhibit scapular dyskinesis with decreased humeral elevation and increased scapular internal rotation. Wearable motion tracking systems, based on M-IMUs, being low-cost, transportable, and unobtrusive, are increasingly emerging as systems for movement analysis outside the clinical context. In this work, a custom M-IMU-based wearable system was equipped with three commercially available Xsens DOTs. As this system is intended to be used by patients with shoulder pathologies, the choice of the number of sensors and their placement depended on the need to provide consistent measurements of shoulder kinematics and, at the same time, ease of use and modularity. In this study, a static sensor-to-segment calibration procedure and a custom algorithm for shoulder angles’ extraction were developed. The validity of the proposed methodological approach was compared against an OMC system during shoulder movements in the scapular, sagittal, frontal, and transverse planes. Ten healthy volunteers were asked to perform an experimental protocol involving bilateral clinically relevant movements. Results are promising for a potential application of the proposed M-IMU-based wearable system for shoulder motion monitoring also in clinical settings. An emerging trend in shoulder motion analysis is the use of smart textiles. This study proposed a new method for skin strain analysis of the scapular region by placing a 6x5 grid of photo-reflective markers. This investigation provided new understandings about the deformation occurring during upper limb movements that can be a factor in the optimal designing and positioning of strain sensors-based wearables. The motion tracking data were processed with an ad hoc algorithm providing the distribution of length variations in the posterior region surrounding scapulae, computed in terms of Euclidean distances between all possible combinations of markers’ pairs without prioritizing predetermined directions. Results showed that the region near the axillary fold underwent greater deformation during upper limbs elevation in the frontal, sagittal, and scapular planes. After the metrological characterization of the developed strain sensors in static and dynamic conditions, a preliminary study was conducted to track scapular sliding during bilateral upper limb elevation and to define a relationship between scapular upward-downward rotation and sensors’ resistance variations. The strain sensors were placed horizontally from the axillary fold toward the medial edge of the scapula on both scapulae of a healthy volunteer. Although the accuracy of the polymer-encapsulated strain sensors in estimating scapular upward-downward rotations showed some critical issues, flexible strain sensors represent a promising alternative for monitoring scapular motions because of their lightweight, flexibility and easy integration into substrates. To date, interest in using virtual reality (VR) devices in the orthopedic field is growing. Among the VR devices on the market, the Oculus Quest 2 was selected. As this device is intended to be used in the future during the rehabilitation of patients with shoulder musculoskeletal disorders, a preliminary evaluation of its performance in terms of translational and rotational accuracy was necessary. Results showed that Oculus Quest 2, which employs inside-out tracking, is accurate enough to measure position and orientation, so it represents a good candidate for future applications of immersive VR systems during rehabilitation sessions of patients with shoulder diseases. In conclusion, the techniques and methods developed and investigated in this thesis show promising expectations in the application of wearable systems based on different sensing technologies for monitoring the shoulder joint, although further efforts are needed for a comprehensive description of such a complex joint as the shoulder. The proposed monitoring systems and techniques in the context of patients with shoulder disorders are valuable from both a technological and clinical viewpoint, as they would influence clinicians' decision-making and the definition of therapies tailored to patients' characteristics.