CHARACTERIZING THE MECHANICAL PROPERTIES OF THE GLENOHUMERAL CAPSULE: IMPLICATIONS FOR FINITE ELEMENT MODELING

The glenohumeral joint is the most dislocated major joint in the body; however despite such a high rate of injury, the proper treatment protocol remains unclear. Rehabilitation has proved to be insufficient with an 80% chance of redislocation in teenagers and 10-15% chance after the age of 40. Following surgical repair, nearly 25% of patients still experience redislocation and complain of joint stiffness and osteoarthritis. In an attempt to improve these results, the normal function of the glenohumeral capsule has been evaluated using both experimental and computational methods. Recent data (strain and force patterns) suggests that the capsule functions multiaxially. Therefore, simple uniaxial methods may not be sufficient to fully characterize the tissue and identify the appropriate constitutive model of the tissue. Inconclusive data has been presented in the literature regarding the collagen fiber architecture and the mechanical properties of the capsule that make it unclear whether the capsule is an isotropic or a transversely isotropic material. For instance, the collagen fiber architecture has been shown by one researcher to be randomly distributed, while another researcher reported the fibers to be aligned. In addition, the axillary pouch has been shown to be the primary stabilizer of the glenohumeral joint in positions of extreme external rotation while the posterior region of the capsule has been shown to stabilize the joint in positions of extreme internal rotation. The rate of dislocations, however, is more frequent in the position of external rotation. Therefore, the overall objective of this work was to utilize a combined experimental - computational methodology to characterize the mechanical properties of the axillary pouch and posterior region of the glenohumeral capsule. Using an isotropic constitutive model, the stress-stretch relationship of the axillary pouch and posterior regions in response to two perpendicular tensile and finite simple shear elongations showed no statistical difference. Further, the constitutive coefficients of pure tensile and simple finite shear elongations in the direction parallel to the longitudinal axis of the anterior band of the inferior glenohumeral ligament (longitudinal) were able to predict the response of the same tissue sample in the direction perpendicular to the longitudinal axis of the anterior band (transverse). These similarities between the longitudinal and transverse elongations of the tissue imply that the capsule is an isotropic material and functions to resist dislocation the same in all directions, rather than just along the longitudinal axis of the anterior band of the inferior glenohumeral ligament, as previously thought. Further, the coefficients of the axillary pouch and posterior regions of the capsule showed no statistical difference, suggesting that these regions have similar mechanical properties, despite a difference in geometry. Thus, when developing finite element models of the glenohumeral capsule, an isotropic constitutive model should be utilized; and both the axillary pouch and posterior regions could be evaluated using the same coefficients. However, due to discrepancies when comparing the constitutive coefficients of tensile and shear elongations, an update to the constitutive model is required. With the proper representation of the glenohumeral capsule known, finite element models can be developed to pursue the understanding of normal joint function, including the effects of age and gender, as well as injured and surgically repaired joints.