Musculoskeletal Shoulder Simulation

Correct diagnosis and deep understanding of a condition is the general prerequisite for providing an optimal treatment. Injuries and diseases affecting the musculoskeletal system and their impact are difficult to reproduce in controlled and repeatable settings. Impairments greatly restrict the patient’s ability to carry out common daily tasks vital for remaining independent. In particular, many activities rely on the extensive reachability offered by the upper limb.

The shoulder offers the greatest range-of-motion in the whole human body and plays a key role in enabling the execution of common daily tasks vital for remaining independent, which is only possible thanks to its unique structure. Indeed, the shoulder consists of loosely connected bones maintained in their desired positions by many muscles, which play a vital role in moving the arm and transferring the load from the upper limb to the trunk. However, it means that the various observable properties are the result of a complex set of neurological processes, which coordinates the many muscles for actuating the upper limb. Active motion, as the result of conscious decision, is a key aspect of understanding pathology and their potential solution. Unfortunately, it is this particular aspect that is difficult to study for practical and ethical reasons. Alternative to clinical and ex-vivo studies falling under the aforementioned concerns, numerical models of the musculoskeletal system offer a great opportunity for an ethical, reproducible, and flexible means to analyze in-depth the human movement and its intrinsic properties. However, several outstanding challenges in modeling have long delayed the application of such models in concrete clinical and biomechanical scenarios.

Throughout my thesis, musculoskeletal models of the shoulder were developed with the aim of deepening the understanding of orthopedic conditions and their surgical treatment options. The first attempt is a fully volumetric and continuum-based musculoskeletal model of the shoulder joint and girdle. Its simulation is faster than usual volumetric models owing to a simpler geometrical representation of the muscles, while retaining a similar degree of fidelity to the anatomical knowledge. Muscles of the shoulder girdle are all integrated with a free-hanging scapula, whose muscles are controlled by a position-tracking controller. Such a comprehensive model allows for analyzing scapular dysfunction as well as glenohumeral pathologies and their impact on the arm movement.¹

The main musculoskeletal model developed in the thesis presents an alternative to earlier muscle modeling paradigms, where muscle parts are represented as thin-shell surface elements instead of the more common lines or volumetric elements. The proposed representation combines the advantages of both alternative representations. Surface-based muscle segments are easy to generate and fast to simulate, allows for spatial variation of biomechanical parameters thanks to the texture-like visualization of such model, and do not require manual intervention to enforce a muscle to pass along anatomically known positions. Via a tracking controller, which computes the muscles activation required for the model to follow a given trajectory, the model was able to reproduce various activities of daily living.²

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The model was applied to two clinical scenarios. In the first, rotator cuff tears are simulated along with the presence of a reverse implant used in Reverse Shoulder Arthroplasty. The goal was to quantify the changes of reaction forces occurring within the glenohumeral joint when a reverse prosthesis is implanted with an intact or near-intact rotator cuff, where an anatomical implant would have been usually indicated.³

The second scenario concerns the use of musculoskeletal models for evaluating muscle transfers. If the rotator cuff is nearly intact, consisting of only one or two irreparable torn tendons, a surgical option of muscle transfer might be preferred by the surgeon. It involves moving the attachment of an intact muscle onto the area of a ruptured muscle in order for the transferred muscle to replace some of its lost function. The pectoralis major is the main muscle considered for replacing an irreparable subscapularis. Muscle activation patterns were compared between several configurations during activities of daily living to better understand the effectiveness of the procedure.⁴