Anisotropic, inhomogeneous and nonlinear structure-function of human supraspinatus tendon

Tendon exhibits inhomogeneous, anisotropic and nonlinear mechanical behavior. This is likely due, in part, to the organization and behavior of the collagen fiber network. However, the quantitative collagen fiber alignment and realignment under load has not been quantified in tendon. The supraspinatus tendon (SST) is particularly interesting due to its complex loading environment and high frequency of degeneration and injury. The limited success of treatment strategies illustrates the need for a better understanding of SST properties. Therefore, the objective of this study was to evaluate the mechanical properties, fiber alignment, realignment under load, and structure-function relationships in human SST. Samples from six regions of SST were tested in both longitudinal and transverse orientations and a novel polarized light imaging technique was used to quantify fiber alignment during mechanical testing. A structurally-based fiber dispersion constitutive model was utilized to further examine structure-function relationships. Mechanical and organizational properties were found to be highly inhomogeneous and nonlinear. In both testing orientations, there were significant correlations between initial fiber alignment and mechanical parameters, demonstrating strong structure-function relationships in SST. Significant fiber realignment under load was particularly pronounced in the toe-region, suggesting that realignment may play a role in mechanical nonlinearity. A fiber dispersion model fit well to the experimental dataset and results suggest that differences in relative fiber alignment are primarily responsible for the differences in moduli values in these locations. The highly complex properties of the human SST may be due to the unique loading environment of the rotator cuff. For example, the bursal samples near the tendon-to-bone insertion exhibit planar mechanical isotropy, which may be a result of complex loading in those locations such as compression, shear and off-axis tensile forces. Very significant data correlations demonstrate that the complex mechanical behavior of SST is largely due to the organization of the underlying collagen fiber network. Taken together, these results greatly enhance the knowledge of SST properties and will be very useful for clinicians and scientists in order to properly diagnose, treat, prevent or repair SST injury, as well as design and develop synthetic or biologic tissue-engineered tendon replacements