Different regions of bovine deep digital flexor tendon exhibit distinct elastic, but not viscous, mechanical properties under both compression and shear loading Fei Fang, Amrita S. Sawhney, Spencer P. Lake n Department of Mechanical Engineering & Materials Science, Washington University in St. Louis, USA article info Article history: Accepted 22 July 2014 Keywords: Tendon Compression Shear Stress relaxation Multiaxial loading abstract Tendons in different locations function in unique, and at times complex, in vivo loading environments. Specifically, some tendons are subjected to compression, shear and/or torsion in addition to tensile loading, which play an important role in regulating tendon properties. To date, there have been few studies evaluating tendon mechanics when loaded in compression and shear, which are particularly relevant for understanding tendon regions that experience such non-tensile loading during normal physiologic function. The objective of this study was to evaluate mechanical responses of different regions of bovine deep digital flexor tendons (DDFT) under compressive and shear loading, and correlate structural characteristics to functional mechanical properties. Distal and proximal regions of DDFT were evaluated in a custom-made loading system via three-step incremental stress-relaxation tests. A two- relaxation-time solid linear model was used to describe the viscoelastic response. Results showed large differences in the elastic behavior between regions: distal region stresses were 4–5 times larger than proximal region stresses during compression and 2–3 times larger during shear. Surprisingly, the viscous (i.e., relaxation) behavior was not different between regions for either compression or shear. Histological analysis showed that collagen and proteoglycan in the distal region distributed differently from the proximal region. Results demonstrate mechanical differences between two regions of DDFT under compression and shear loading, which are attributed to variations of composition and microstructural organization. These findings deepen our understanding of structure–function relationships of tendon, particularly for tissues adapted to supporting combinations of tension, compression, and shear in physiological loading environments. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction Tendons are soft connective tissues that transfer forces from muscle to bone in order to mobilize and/or stabilize joints. Elaborately organized as a complicated hierarchy spanning Ang- strom to centimeter scales, tendons are composed of cells (typi- cally fibroblasts) and a highly hydrated extracellular matrix (ECM) of collagens, proteoglycans, elastin, and other minor proteins (Tresoldi et al., 2013; Vogel, 2003). Tendons predominantly sup- port uniaxial forces along the principal direction of collagen fiber alignment and exhibit mechanical nonlinearity, viscoelasticity, and anisotropy (Vogel, 2003; Zhu et al., 2014). In addition, some tendons (e.g., human supraspinatus tendon) also experience vary- ing levels of compressive, shear, and/or frictional forces as they wrap around bones or interact with neighboring anatomical structures (Koob and Vogel, 1987a, 1987b; Lake et al., 2009). Even different locations within the same tendon can function in unique, and at times complex, in vivo loading environments. Another tendon subjected to complex physiological loading is the bovine deep digital flexor tendon (DDFT); while the proximal region of this tendon is loaded predominantly in tension, the anterior side of the distal region contacts the sesamoid bone of the lower leg and experiences compression and (likely) shear (Koob and Vogel, 1987a). This unique loading environment greatly influences tissue composition and organization as cells remodel the ECM in response to a multiaxial, non-uniform mechanical stimulus. Thus, while the proximal region exhibits properties typical for tendon (i.e., highly organized and aligned collagen fibers, relatively low levels of large proteoglycans), the distal region contains a fibrocartilage-rich zone with characteristics common to multiaxially-loaded tissues (e.g., less aligned collagen, increased glycosaminoglycans). Many studies have investigated how tendon behaves under tensile loading, namely the mechanical response and the way in Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jbiomech www.JBiomech.com Journal of Biomechanics http://dx.doi.org/10.1016/j.jbiomech.2014.07.026 0021-9290/& 2014 Elsevier Ltd. All rights reserved. n Correspondence to: 1 Brookings Drive, Campus Box 1185, St. Louis, MO 63130, USA. Tel.: þ1 314 935 3161; fax: þ1 314 935 4014. Journal of Biomechanics 47 (2014) 2869–2877