Multiscale strain analysis of tendon subjected to shear and compression demonstrates strain attenuation, fiber sliding, and reorganization

Abstract

ABSTRACT:The manner in which strains are passed down the hierarchical length scales of tendons dictates how cells within the collagen network regulate the tissue response to loading. How tendons deform in different hierarchical levels under shear and compression is unknown. The aims of this study were: (i) to evaluate whether specific regions of bovine deep digital flexor tendons exhibited different strain attenuation from macro to micro length scales, and (ii) to elucidate mechanisms responsible for tendon deformation under shear and compression. Samples from distal and proximal regions of flexor tendons were subjected to three-step incremental stress-relaxation tests. Images of tissue markers, photobleached lines on collagen fibers, and nuclei locations were collected before and after loading. Results showed that strain transfer was attenuated from tissue to local matrix under both shear and compression. Nuclear aspect ratios exhibited smaller changes for distal samples, suggesting that cells are more shielded from deformation in the distal region. Collagen fiber sliding was observed to contribute significantly in response to shear, while uncrimping and fiber reorganization were the predominant mechanisms under compression. This study provides insight into microscale mechanisms responsible for multiscale strain attenuation of tendons under non-tensile macroscale loading. Keywords: tendon; multiscale strain transfer; shear; compression; two-photon microscopy In addition to experiencing tensile loading, tendons often function in complicated in vivo loading environments. In such cases, cell-driven alterations to the compositional and structural properties of tendons lead to mechanical properties that can vary with anatomical locations, regions within the tendon, species, and age. For example, the supraspinatus tendon (SST), one of four musculotendinous units in the rotator cuff of the shoulder, experiences shear and compressive forces as a result of the wide range of motion of the shoulder joint and complex interactions with neighboring anatomy. 1-4 As a result, specific locations of the human SST exhibit significantly different fiber alignment and tensile modulus during tensile loading, 5,6 as well as compositional differences (i.e., collagens, proteoglycans) by location. 3,7 Similarly, the bovine deep digital flexor tendon (DDFT) functions in a multiaxial physiological loading environment with forces that vary at different anatomical locations: the proximal region is mostly loaded in tension, while the distal region is also compressed against the navicular bone during normal joint function. 8-13 Our previous study, 14 which evaluated the DDFT as an easily accessible example of a complexly loaded tendon with tissue-level properties that vary by location, 8 demonstrated that the proximal and distal regions exhibited different elastic mechanical properties, yet similar viscoelastic properties, and inhomogeneous proteoglycan distribution and collagen organization.Several previous studies have explored how strains t...

Experimental evaluation of multiscale tendon mechanics

“…Our work showed that sliding between collagen fibers primarily regulated the mechanical response of distal and proximal regions from deep digital flexor tendon (DDFT) subjected to shear loading . Different from the response to shear, DDFT showed both fiber realignment and sliding as physical mechanisms governing the mechanical response in compression …”

“…(A) D‐period change of mouse SST during static tensile loading and AFM‐acquired images measuring fibril D‐period with line highlighted (adapted, with permission, from Connizzo et al). (B) A measure of microscale fiber sliding, termed between rows rotation, increased with increasing applied shear strain, while images taken via multi‐photon microscopy showed fiber sliding during shear stress relaxation testing (white arrows denote photobleached lines broken due to fiber sliding; adapted, with permission, from Fang et al). (C) Crimp frequency of tendon in tensile loading decreased from the toe to linear regions of the stress‐strain curve and representative images of polarized light showing the change of crimp pattern (adapted, with permission, from Miller et al).…”

“…On the fiber level, tendon structural characteristics, such as helical twist and crimp, have been visualized using imaging techniques, including polarized light, interference, and phase contrast microscopy (Table ) . The proposed biomechanical implications of these specific structural features have been examined in a number of studies, utilizing confocal/multi‐photon/second harmonic generation (SHG) microscopy and quantitative polarized light imaging (QPLI, Table ) …”

“…) . Both experimental and computational approaches have been employed to determine and predict tendon anisotropic, inhomogeneous, and viscoelastic properties under different loading scenarios at a single scale . This review focuses on studies that have evaluated tendon structural and compositional features at different length scales via experimental techniques, and conclusions regarding the contributions of these features to tendon mechanics.…”

Collagenous Extracellular Matrix Biomaterials for Tissue Engineering: Lessons from the Common Sea Urchin Tissue

“…Of note, the sliding mechanism prevails in the connective tissues of human and other animals [114]—sliding of collagen fibre bundles, i.e., fascicles, has been observed [115,116] during tissue deformation. For a MCT to change from a compliant to a stiffened state and vice versa, other mechanisms would be involved in regulating this transition process.…”