Discussion

Thorpe, C.T., Birch, H.L., Clegg, P.D., Screen, H.R.C.
Aims: To investigate the microstructural strain response and cell strain environment in the injury prone equine superficial digital flexor tendon (SDFT) and identify age-related alterations. Methods: Fascicles were dissected from the SDFT of 4 horses aged 4 - 6 years, and 4 horses aged 18 - 20 years. Fascicles were incubated in the collagen stain 5-dicholorotriazynl fluorescein, viewed under a confocal microscope and a grid photobleached onto each fascicle. Strain was applied in 2% increments up to 10% and images taken at each increment. Grid deformation was quantified to measure longitudinal strain, perpendicular strain, fibre sliding and fascicle rotation. Results: Longitudinal strains were heterogeneous, smaller than applied strain, and showed no age-related changes. Large compressive strains were measured perpendicular to the direction of applied strain; the magnitude of these strains decreased with ageing. A small amount of fibre sliding occurred; levels did not change with ageing. However, the amount of rotation, which increased with each strain increment, decreased significantly with age. Conclusions and practical significance: Strain distribution throughout fascicles was complex and heterogeneous, with small longitudinal strains, but large compressive strains perpendicular to the loading axis. Strain is dissipated throughout the matrix by a combination of fibre extension and sliding. Fascicle rotation suggests a helical organisation to fascicle structure; this may enable more efficient extension and recoil. These data suggest that, while cells within fascicles experience tensile strains much smaller than the applied strain, they are also subjected to shear and compressive strains. The decreased compressive strain and rotation with ageing may be due to changes in matrix organisation, which may alter the local cell strain environment. Decreased rotation also indicates some loss of efficient recoil. These changes may cause altered cell and mechanical responses to loading, potentially contributing to the increased risk of SDFT injury with ageing. Acknowledgements: Funding: Horserace Betting Levy Board.