EVALUATING THE ANTERIOR STABILITY PROVIDED BY THE GLENOHUMERAL CAPSULE: A FINITE ELEMENT APPROACH

The shoulder is the most dislocated joint in the body, with over 80% of dislocations occurring in the anterior direction. During anterior dislocation, the glenohumeral capsule within the shoulder is the primary joint stabilizer. Physical diagnostic examinations of the capsule following dislocation are crucial for clinicians to determine the location and extent of pathology in the capsule, and diagnoses from the examinations are used to decide the type, location, and extent of surgical repair to restore capsule function. These examinations, however, are not standardized for joint position among patients. Since capsule function is dependent upon joint position and can vary among patients, the lack of standardized joint positions for the examinations leads to misdiagnoses of the location and extent of capsule pathology that subsequently result in improper surgical procedures to restore capsule function. In fact, over 50% of re-dislocations and over 80% of pain and loss of motion following surgery have been shown to result from misdiagnoses of pathology to the capsule. Therefore, the objective of the current work was to suggest joint positions where the stability provided by the capsule is consistent among patients in response to application of an anterior load. Two subject-specific finite element models of the glenohumeral joint were developed based on experimental data collected from two cadavers, and that used an isotropic hyperelastic constitutive model to represent the capsule. The models were validated by comparing predicted capsule strains in the models to experimental capsule strains in clinically relevant joint positions. The models were used to identify clinically relevant joint positions where the distributions of predicted capsule strain were correlated with an r2 value greater than 0.7, and to further identify if strain was higher in specific regions of the capsule than others at these positions. The clinically relevant joint positions resulting from application of a 25 N anterior load at 60° of glenohumeral abduction and 10° - 40° of external rotation resulted in distributions of strain that were correlated with an r2 value greater than 0.7. Of these positions, those with 20° - 40° of external rotation resulted in capsule strains that were significantly higher in the glenoid side of the anterior band of the inferior glenohumeral ligament than in other regions of the capsule. Therefore, the current work suggests that standardizing physical diagnostic examinations for anterior instability at joint positions with abduction and a mid-range of external rotation may allow clinicians to more effectively diagnose the location and extent of capsule pathology resulting from anterior dislocation, and may ultimately lead to an improvement in surgical outcomes. The current work further provides an excellent foundation to evaluate the stability provided by the capsules of multiple subject-specific finite element models of the glenohumeral joint, to identify multiple joint positions for physical examinations that can be used to diagnose pathology throughout the glenohumeral capsule