Local Strain Measurement within Tendon

Abstract

Tendon is a dense connective tissue, responsible for transmitting the forces generated by muscles to the skeleton. It is composed of a hierarchical arrangement of crimped collagen fibres, interspersed with proteoglycan matrix and cells, known as tenocytes. During physiological loading, tendons are subjected to strains in the region of 5–6%, which result in the straightening and realignment of the collagen fibres, generating variable local strain fields within the tendon. This study demonstrates a technique for analysing local strains within viable tendon explants, during both loading and unloading of the tissue. Samples were strained in a custom‐designed rig, allowing real‐time visualisation of cell nuclei, used as local discrete markers, on a confocal microscope. Results indicated that local strains within the fascicle are smaller than the applied strains, never exceeding 1.2%, even at 8% gross applied strain. By contrast, the sliding of adjacent collagen units was recorded at each strain increment in this study, reaching a mean maximum of 3.9% of the applied displacement. Loading–unloading studies indicated that sliding behaviour is reversible up to strains of 5%, and provides the major extension mechanism within the rat‐tail tendon. This technique can be extended to further analyse shearing behaviour within the matrix.

Tendon Fascicles Exhibit a Linear Correlation Between Poisson's Ratio and Force During Uniaxial Stress Relaxation

“…Samples were attached to a mini materials test machine using custom clamps and tested in a PBS bath at room temperature. During pilot testing it was observed that the fascicles appeared to rotate a small amount (less than a quarter turn) during constant strain rate testing, which is consistent with previous observations [17]. Since both clamps were fixed, this was not true rotation.…”

“…Apparent rotation of the fascicles was observed during the ramping phase of tensile elongation below the transition strain, as previously observed [17]. This suggests the presence of a helical organization within the fascicle.…”

“…In addition to their use in many previous studies of tendon mechanics [14][15][16][17][18], tendon fascicles are easy to obtain, have a large aspect ratio that is conducive to obtaining homogenous strains and have nearly cylindrical cross sections that aid in measuring transverse strain. Rat tail tendons were obtained from a single, freshly sacrificed Sprague Dawley rat, placed in gauze, moistened using phosphate buffered saline, and frozen for future testing.…”

Poisson's ratio of collagen fibrils measured by small angle X-ray scattering of strained bovine pericardium

“…For collagen in bovine pericardium, at low strain, the Poisson's ratio appears to have a very high value (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27), but for strain above 0.09, the Poisson's ratio is in the range 2.1-2.8. For the total strain (from 0 to 0.25), the change in d-spacing and diameter gives 0 ¼ 2.1 6 0.7 (these values of 0 can be calculated from Fig.…”

“…17 It has also been suggested that in tendon, the strain may be taken up by sliding of fibrils within the tendon rather than by extension of the collagen fibrils. 18 In leather, where there is very little crimp, the reorientation of fibrils may be an important mechanism for absorbing strain. 19,20 Here, the behavior of individual fibrils of collagen I as strain is applied is studied using small angle X-ray scattering (SAXS) to simultaneously measure the fibril length extension and fibril diameter contraction.…”