Variations during ageing in the three-dimensional anatomical arrangement of fascicles within the equine superficial digital flexor tendon.

BiTendons are constructed from collagenous fascicles separated by endotenon/interfascicular matrix (IFM). Tendons may be specialised for precision movement or to store energy during locomotion and for the latter the elasticity of the endotenon/IFM is particularly important. The equine superficial digital flexor tendon (SDFT) is a dedicated energy-storing tendon with a similar function to the human Achilles tendon. Classical anatomical descriptions portray fascicles as longitudinally arranged distinct anatomical structures. In the present study, using three-dimensional reconstruction from whole tissue slices and histological sections, the fascicles of the equine SDFT were found to adopt a complex interweaved arrangement. Fascicles were found to fully and partially converge and diverge within the tendon and fascicle bundles were observed. Fascicle morphology was not homogenous with narrowing, broadening and twisted fascicles observed in addition to relatively straight fascicles. The number of fascicle bundles observed in cross-section increased from the proximal to the distal end of the tendon, whilst the number of fascicles decreased with age in the proximal region. Fascicular patterns were not similar between the left and right limbs, across different regions or at different ages. A decrease in thickness of the endotenon/IFM between fascicles with age was found in the distal tendon region. The results provide a rationale for considering fascicular organisation when diagnosing and treating tendon injuries, for bioengineering tendon and when modelling tendon function.