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P&F Grant Awards

Year 7

Grant # 14

Region Specific Mechanics and Multiscale Strain of Human Supraspinatus Tendon.

PI: Spencer Lake, PhD


Specific Aims

Rotator cuff tendon tears are a significant source of pain and dysfunction [1-3], and surgical repair of these tendons fail at a high rate (between 20-94%) [4, 5]. The supraspinatus tendon (SST) is the most commonly injured tendon of the cuff, in part because of its highly complex in vivo loading environment [6]. As evidence of this, our work has shown very unique and highly inhomogeneous mechanical, structural, and compositional properties for the human SST [6-9]. For example, the medial SST exhibits properties typical of tendon while the lateral region (i.e., area of likely multiaxial loading) shows planar mechanical isotropy, disorganized collagen fibers and ECM composition similar to fibrocartilage [7, 10]. These unique properties likely help sustain multiaxial loading, but may limit the ability to support tensile loading and may predispose the SST to high rates of damage seen clinically. Unfortunately, much remains unknown regarding how the SST functions under non-tensile loads. In addition, multiscale force transfer in such complex loading environments has important implications in modulating mechanotransduction, tissue adaptation, and disease pathogenesis. For example, the degree to which macroscale deformations are transferred to the microscale will directly affect tissue remodeling, reorganization and repair. Understanding such multiscale relationships is critical towards elucidating structure-function-composition properties in healthy tissues and understanding how these relationships are impaired in injury and disease. This is particularly relevant for the oft-injured, poor healing SST in order to inform techniques to better prevent and treat rotator cuff injuries, as well as replace the SST with engineered constructs.

Our laboratory recently used a custom mechanical test system integrated with multi-photon microscopy to track multiscale deformations of tendon samples subjected to compression and shear [11]. We found strain attenuated at smaller length scales and quantified fiber sliding and realignment under load. Distinct regions of bovine flexor tendons exhibited differences in deformation behavior at the microscale, likely as a result of differences in compositional/organizational properties. Using similar techniques, the objective of this study is to quantify SST mechanical properties under compression and shear and correlate region-specific function with microstructural organization and composition. Further, using selective degradation, we will quantify - and elucidate mechanisms responsible for - contributions of specific constituents to multiscale mechanical behavior of human SSTs. The innovation of this study is multiaxial mechanical testing coupled with image-based multiscale strain analysis to identify microscale mechanisms in an oft-injured tendon.

Aim 1: Quantify the macroscale mechanics and macro-to-microscale strains of region-specific samples of human SST under compression and shear, and correlate properties with site-matched distributions of key ECM components. Samples from different SST regions will be loaded in compression or shear while measuring local matrix, cellular, and nuclear strains using two-photon microscopy. Parameters describing the elastic/viscoelastic mechanics and thickness-dependent multiscale strain response will be compared across SST regions, and relative quantities and distributions of ECM components (e.g., collagens, proteoglycans (PGs), elastin) will be evaluated in region-matched samples via immunohistochemistry. Hypothesis: Lateral SST will exhibit more fibrocartilage-like ECM (e.g., increased PGs, COL2) that correlates with stiffer mechanics under compression/shear, and decreased microscale deformations (i.e., strains, fiber rotations) compared to medial SST.

Aim 2: Identify mechanisms governing region-specific mechanics and multiscale strain transfer of SST under non-tensile loading. Although the role of PGs in tensile loading appears to be minimal [12-14], PGs likely contribute in compression/shear. Recent studies have also suggested a significant role for elastin [15, 16]. To elucidate the contribution of these tissue components, samples from specific SST regions will be enzyme-treated to disrupt either PGs or elastin [13, 14], and then subjected to compression or shear testing with two-photon microscopy as in Aim 1. Biochemical assays will assess the efficiency of GAG and elastin removal. Hypothesis: Selective ECM depletion will demonstrate that large PGs and elastin govern the microscale response of SST in compression and shear, respectively; ECM-disruption will alter tendon properties (i.e., mechanics, multiscale strains) more severely in lateral SST than in medial SST due to region-specific differences in ECM composition and structural organization.

The significance of this study is increased understanding of (1) multiscale strain transfer, (2) structurefunction relationships in non-tensile loading regimes, and (3) the presence/role of key constituents and mechanisms at the microscale in tendon. Further, region-specific and thickness-dependent mechanics and deformation behavior of human SST will provide significant insight into how unique properties enable this tendon to function in a complex physiologic loading environment, and how disruptions to tissue constituents may lead to degeneration and damage. Finally, this study will yield fundamental understanding that can better inform approaches to treat SST tears and/or improve the design of engineered tendon replacements.