The objective of this study was to determine the strain distribution in the inferior glenohumeral ligament at 08, 308, and 608 of external rotation with an anterior load applied to the joint. Five cadaver shoulders were dissected free of all soft tissue except the glenohumeral capsule and a 7 Â 11 grid of strain markers were affixed to the inferior glenohumeral ligament. The location of these strain markers was then determined for a reference strain configuration and while a 25 N anterior load was applied to simulate a clinical exam for instability. The magnitude and direction of the maximum principal strains were then determined at each joint position. For all specimens, the magnitude of the maximum principal strains were significantly greater for 308 and 608 of external rotation when compared to 08 of external rotation. Furthermore, when comparing 308 to 608 of external rotation, three of the five specimens were significantly different. Additionally, the previously described regions of the inferior glenohumeral ligament could not be identified with a qualitative evaluation of the strain distribution pattern for each specimen at all external rotation angles. This indicates that our current description of the three regions of the inferior glenohumeral ligament does not correspond to its functional role. Additionally, the directions of the maximum principal strains across the inferior glenohumeral ligament became more aligned with one another as external rotation was increased. The complex strain distributions observed indicates that future studies should treat the inferior glenohumeral capsule as a continuous sheet of fibrous tissue. ß

Inconclusive findings regarding the collagen fiber architecture and the material properties of the glenohumeral capsule make it unclear whether the material symmetry of the glenohumeral capsule is isotropic or anisotropic. The overall objective of this work was to use a combined experimental and computational protocol to characterize the mechanical properties of the axillary pouch of the glenohumeral capsule and to determine the appropriate material symmetry. Two perpendicular tensile and finite simple shear deformations were applied to a series of tissue samples from the axillary pouch of the glenohumeral capsule. An inverse finite element optimization routine was then used to determine the material coefficients for an isotropic hyperelastic constitutive model by simulating the experimental conditions. There were no significant differences between the material coefficients obtained from the two perpendicular tensile deformations or finite simple shear deformations. Furthermore, stress-stretch relationships predicted by utilizing the material coefficients from one direction were able to predict the responses of the same tissue sample in the perpendicular direction. These similarities between the longitudinal and transverse material behaviors of the tissue imply that the capsule may be considered an isotropic material. However, differences did exist between the material coefficients obtained from the tensile and shear loading conditions. Therefore, a more advanced constitutive model is needed to predict both the tensile and shear responses of the material.

Surgical repair of the glenohumeral capsule after dislocation ignores regional boundaries of the capsule and is not sex specific. However, each region of the capsule functions to stabilize the joint in different positions, and differences in joint laxity between men and women have been found. The objectives of this research were to determine the effects of region (axillary pouch and posterior capsule) and sex on the material properties of the glenohumeral capsule. Boundary conditions derived from experiments were used to create finite-element models that applied tensile deformations to tissue samples from the capsule. The material coefficients of a hyperelastic constitutive model were determined via inverse finite-element optimization, which minimized the difference between the experimental and finite-element model-predicted load-elongation curve. These coefficients were then used to create stress-stretch curves representing the material properties of the capsule regions for each sex in response to uniaxial extension. For the axillary pouch, the C1(men: 0.28 ± 0.39 MPa and women: 0.23 ± 0.12 MPa) and C2(men: 8.2 ± 4.1 and women: 7.7 ± 3.0) material coefficients differed between men and women by only 0.05 MPa and 0.5, respectively. Similarly, the posterior capsule coefficients differed by 0.15 MPa (male: 0.49 ± 0.26 MPa and female: 0.34 ± 0.20 MPa) and 0.6 (male: 7.8 ± 2.9 and female: 7.2 ± 3.0), respectively. No differences could be detected in the material coefficients between regions or sexes. As a result, surgeons may not need to consider region- and sex-specific surgical repair techniques. Furthermore, finite-element models of the glenohumeral joint may not need region- or sex-specific material coefficients when using this constitutive model.

The glenohumeral joint is the most dislocated major joint in the body and the axillary pouch of the glenohumeral capsule is the primary stabilizer at the extreme ranges of external rotation. [1] Procedures to repair the capsule following dislocation result in 12–25% of patients still experiencing pain and instability. [2] Studies performing clinical exams have found inconsistent data on differences between males and females. Increased laxity in the glenohumeral joint of females has been found as well as overall hypermobility when compared to males. [3,4] However, others have found no differences in overall joint stiffness between genders. [5] These findings suggest that a difference in the mechanical properties might exist between genders. Therefore, the objective of this study was to determine the effects of gender on the mechanical properties of the axillary pouch during tensile loading. A combined experimental and computational approach was used to evaluate the properties of the tissue. This data could potentially be utilized to improve surgical procedures and necessitate gender-specific repair techniques.

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