Project Summary/Abstract Anterior shoulder instability is a common problem in physically active individuals and has serious long-term implications due to the high rate of injury recurrence and the resulting extended time out of work. The injury leading to anterior instability often includes damage to both the humeral head (Hill-Sachs lesion) and glenoid bone (bony Bankart lesion). The current surgical guidelines for treating anterior instability in the presence of bony lesions are based upon biomechanical studies that measured glenohumeral contact regions (the “glenoid track”) in cadaver specimens during static simulated loading. Cadaver-based biomechanical studies cannot replicate in vivo loads, kinematics, and muscle activity, and so the current guidelines lack the accuracy and generalizability needed to guide surgical decision-making in patients with anterior shoulder instability and subcritical (10% to 20%) glenoid bone loss. A critical need exists to improve our understanding of how anatomy and kinematics affect glenohumeral articular contact during in vivo movement so that the optimal surgical treatment can be identified in this challenging patient cohort. The objectives of this study are to define the dynamic glenoid track and to develop and validate a patient-specific evaluation process, based upon the dynamic glenoid track, to predict shoulder function after three common surgeries to treat anterior shoulder instability. In this mechanistic ancillary study, in vivo kinematics from dynamic biplane radiography (DBR) will be combined with cartilage morphology from MRI to measure glenohumeral contact regions during loaded and unloaded circumduction and during external rotation at 30°, 60°, 90° and 120° of abduction (the dynamic glenoid track). The dynamic glenoid track will be measured in healthy controls and compared to the previously reported static glenoid track. Data from healthy shoulders will then be used to develop a model to predict cartilage contact and instability in patients based upon bone lesion size and location. The validity and robustness of the model will be evaluated by testing patients from the parent study prior to surgery. Finally, the ability of the patient-specific models to predict cartilage contact regions after surgery will be evaluated using pre and post-surgery data from patients in the parent study. The Specific aims are: 1) to compare the in vivo dynamic glenoid track in healthy shoulders to the previously reported static glenoid track, 2) to develop and validate a patient-specific evaluation process for predicting glenohumeral cartilage contact during dynamic in vivo movement, and 3) to determine the accuracy of the patient-specific model in predicting dynamic glenohumeral cartilage contact after surgery. Achieving these objectives will provide insight into the anatomic and kinematic mechanisms that lead to the loss and restoration of anterior shoulder stability, and will clarify and improve the surgical guideli...