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Background: The subscapularis is an important mover and stabilizer of the glenohumeral joint. Since the advent of shoulder arthroscopy, partial tears are found in 43% of rotator cuff patients. While partial tears to the upper band occur more commonly, little is known about the structure and mechanical behavior of the individual bands. Therefore, the objective of this study was to measure tensile mechanical properties, corresponding collagen fiber alignment, and histology in the upper and lower bands of the rat subscapularis tendon., Materials and Methods: Thirty adult Sprague-Dawley rats were euthanized and subscapularis tendons dissected out for mechanical organization (n = 24) and histologic assessment (n = 6). Collagen organization was measured with a custom device during mechanical testing., Results: Linear-region modulus at the insertion site was significantly lower in the upper band compared to the lower band, while no differences were found at the midsubstance location. The upper band was found to be significantly less aligned and demonstrated a more rounded cell shape than the lower band at the insertion site., Discussion: This study demonstrated that the 2 bands of the subscapularis tendon have differential mechanical, organizational, and histological properties, which suggests a functional deficit exists to the upper band of the subscapularis and may be contributing to the prevalence of partial subscapularis tears., Conclusions: Clinicians should be aware that the upper band of the subscapularis tendon may be at higher risk of developing tears, based on decreased mechanical properties and a more disorganized collagen fiber distribution., (Copyright © 2012 Journal of Shoulder and Elbow Surgery Board of Trustees. Published by Mosby, Inc. All rights reserved.)

A recent study (Lake et al., 2009); reported the properties of human supraspinatus tendon (SST) tested along the predominant fiber direction. The SST was found to have a relatively disperse distribution of collagen fibers, which may represent an adaptation to multiaxial loads imposed by the complex loading environment of the rotator cuff. However, the multiaxial mechanical properties of human SST remain unknown. The objective of this study, therefore, was to evaluate the mechanical properties, fiber alignment, change in alignment with applied load, and structure-function relationships of SST in transverse testing. Samples from six SST locations were tested in uniaxial tension with samples oriented transverse to the tendon long-axis. Polarized light imaging was used to quantify collagen fiber alignment and change in alignment under applied load. The mechanical properties of samples taken near the tendon-bone insertion were much greater on the bursal surface compared to the joint surface (e.g., bursal moduli 15-30 times greater than joint; p<0.001). In fact, the transverse moduli values of the bursal samples were very similar to values obtained from samples tested along the tendon long-axis (Lake et al., 2009). This key and unexpected finding suggests planar mechanical isotropy for bursal surface samples near the insertion, which may be due to complex in vivo loading. Organizationally, fiber distributions became less aligned along the tendon long-axis in the toe-region of the stress-strain response. Alignment changes occurred to a slightly lesser degree in the linear-region, suggesting that movement of collagen fibers may play a role in mechanical nonlinearity. Transverse mechanical properties were significantly correlated with fiber alignment (e.g., for linear-region modulus rs=0.74, p<0.0001), demonstrating strong structure-function relationships. These results greatly enhance current understanding of the properties of human SST and provide clinicians and scientists with vital information in attempting to treat or replace this complex tissue., (Copyright 2009 Elsevier Ltd. All rights reserved.)

Tendon exhibits nonlinear stress-strain behavior that may be partly due to movement of collagen fibers through the extracellular matrix. While a few techniques have been developed to evaluate the fiber architecture of other soft tissues, the organizational behavior of tendon under load has not been determined. The supraspinatus tendon (SST) of the rotator cuff is of particular interest for investigation due to its complex mechanical environment and corresponding inhomogeneity. In addition, SST injury occurs frequently with limited success in treatment strategies, illustrating the need for a better understanding of SST properties. Therefore, the objective of this study was to quantitatively evaluate the inhomogeneous tensile mechanical properties, fiber organization, and fiber realignment under load of human SST utilizing a novel polarized light technique. Fiber distributions were found to become more aligned under load, particularly during the low stiffness toe-region, suggesting that fiber realignment may be partly responsible for observed nonlinear behavior. Fiber alignment was found to correlate significantly with mechanical parameters, providing evidence for strong structure-function relationships in tendon. Human SST exhibits complex, inhomogeneous mechanical properties and fiber distributions, perhaps due to its complex loading environment. Surprisingly, histological grade of degeneration did not correlate with mechanical properties.