Evaluation and robotic simulation of the glenohumeral joint

The shoulder is the joint with the largest range of motion in the human body. It is composed of three articulations, the glenohumeral articulation providing about two thirds of its mobility which is, also the most complex and fragile of the three. For this reason, it possesses a particularly complex stabilization mechanism involving a large number of different structures, making it particularly difficult to model, evaluate and treat. Since many structures are involved in the stabilization of this articulation, pathologies affecting any of these elements may compromise the whole stabilization mechanism. In that case, a good understanding of the mechanisms involved, combined with reliable evaluation tools are necessary to provide an effective treatment. At present, different modelling approaches tried to model the phenomenon of glenohumeral instability, but none was able to replicate the whole mechanism reliably. These limitations are particularly problematic during total shoulder replacement surgeries, where the inability to reproduce healthy glenohumeral motions is the main cause of implant failure. The development of a model able to predict the onset of GH instability based on the surgical planning, would greatly improve the general survivor-ship of the implants, and is one of the main goals of this thesis. Additionally, the clinical tools available for the evaluation of the functional outcome are limited. Clinical questionnaires, static apprehension and laxity scores and two-dimensional evaluations of the articular ranges being the instruments used in most situations. Although fast and simple, these instruments are either prone to biases based on patients' interpretation and psychological state or provide only a partial evaluation of the functional limitations. Considering that the main function of the upper limb (composed of the shoulder, arm, elbow and forearm) is to place and orient the hand in space, for the latter to perform a particular action; the reachable space defined as the total volume where the upper limb is able to place the hand, is a feature of great interest. In biomechanical research laboratories, measurements with standard motion capture system enables a more comprehensive and accurate evaluation of the whole upper limb, including the evaluation of its reachable space. Unfortunately, the use of such platforms is generally too complex, expensive and time-consuming for clinical examinations. To this end, the development of new tools able to bring some of the most interesting features recorded in biomechanical laboratories to the clinical context would offer promising new solutions for the functional evaluation of patients and represent the second goal of this thesis. This thesis proposes two original instruments for the evaluation and robotic simulation of the glenohumeral joint. It offers a tool for reproducing physiological forces and motions within the glenohumeral joint, opening new opportunities for the study of glenohumeral instability. At the same time, it also proposes a second instrument allowing to evaluate the functional impairment of a patient's upper limb in an objective and reliable way, extracting information that in the past required the deployment of unrealistic amounts of resources for a clinical examination.