Characterizing the Effects of Simulated Injury on the Structure and Function of the Glenohumeral Capsule.

Anterior glenohumeral dislocation is a significant clinical problem, which can result in the glenohumeral capsule being loaded beyond its elastic limit (permanently deformed). Diagnosing and treating this pathology is difficult as currently no reliable method for determining the location and extent of capsular damage exists. Consequently, following repair ~20% of patients experience recurrent dislocations and ~50% are at risk for developing osteoarthritis. Existing knowledge of the structure and function of the capsule following permanent deformation is minimal and may be the root cause behind these issues. A greater awareness of how injury affects this structure will enable specific at-risk locations to be targeted during repair. Additionally, validated finite element models of the glenohumeral joint may be able to improve patient treatments; but require adequate constitutive models to describe normal and injured capsule behavior. Therefore, the goal of this work was to evaluate the effect of anterior dislocation on the structure and function of the capsule from three levels: microstructure, tissue, and joint; and to suggest improvements to a constitutive model for the capsule. At the microstructural level, collagen fiber alignment increased with uniaxial extension and was able to predict the location of tissue failure. Following simulated injury of tissue samples from the capsule, the stiffness and modulus of stress-stretch curves increased by 47% and 128%, respectively, but changes were not detectable in the parameters of a phenomenological constitutive model. Anterior dislocation permanently deformed the capsule and resulted in increased anterior translations and glenohumeral contact forces of 18-48% and 41-148% at three joint positions. Finally, a structural constitutive model was found to better predict the complex capsule behavior than the phenomenological model; but accounting for non-affine fiber kinematics may further improve the accuracy of computational models. This work suggests that surgical repair procedures targeting the anterior capsule based on increased anterior translation during pre-operative physical exams are not addressing the appropriate region of the capsule; rather the posterior axillary pouch suffers the most damage following anterior dislocation. Therefore, current physical examinations may not be capable of identifying specific locations of tissue damage and future research to standardize physical exams is warranted