Abstract
Summary Statement
Fatigue loading has an age-specific effect on tendon fascicle micro-mechanics, with greater fibre sliding in aged samples indicating a decreased mechanical integrity, and a reduced ability to withstand cyclic loading, which may partially explain the age-related risk of tendon injury.
Introduction
The human Achilles and equine superficial digital flexor (SDFT) tendons function as energy stores, experiencing large, repetitive stresses and strains1 and are therefore highly susceptible to injury, particularly in aged individuals. We have previously observed rotation within SDFT fascicles in response to applied strain, which indicates the presence of helical sub-structures within this tendon. Further, we have shown that this rotation decreases with ageing, suggesting alterations to the helix sub-structure and a difference in the extension mechanisms in aged tendons. We therefore hypothesise that cyclic fatigue loading (FL) will result in alterations in fascicle extension mechanisms which are age specific.
Methods
Fascicles (n= 6–8/tendon) were dissected from the forelimb SDFTs of 6 young (aged 3–6 years) and 5 old horses (aged 18–20 years). Half the fascicles underwent 1800 cycles of FL to 60% of their predicted failure stress, while the remaining fascicles acted as unloaded controls. Following FL, fascicles were stained with 5-dicholorotriazynl fluorescein, secured in a straining rig and viewed under a confocal microscope. A grid was photobleached onto each fascicle and images were taken at 2% strain increments up to 10%. Deformation of the grid was quantified by measuring changes in longitudinal strain, deviation from the vertical gridline and rotation of the horizontal gridline. Statistical significance was set at p<0.05.
Results
In agreement with previous studies, local longitudinal strains were smaller than the overall applied strain, and did not differ between FL and control groups or with ageing. Deviation of the vertical gridline, representing fibre sliding, did not differ between FL and control samples from young horses, but there was a significant increase in fibre sliding in FL samples from old horses (p<0.01). By contrast, grid rotation decreased significantly in FL samples from young horses, and with ageing (p<0.01) but showed no alteration with FL in aged samples.
Discussion
The results support the hypothesis, showing that FL causes age-specific alterations in fascicle extension mechanisms. Our previous findings suggest that, in samples from young horses, extension is facilitated by the unwinding of helical sub-structures, which is indicated by the grid rotation observed. The results of the current study show that FL causes a significant decrease in this rotation, suggesting the helix sub-structures are altered by FL, which may reduce the ability of fascicles to extend and recoil. By contrast, in aged samples this rotation decreased, indicating ageing also causes alterations to the helical sub-structures. FL of aged samples resulted in increased fibre sliding, indicating that damage may be occurring between the collagen fibres, which is likely to decrease the mechanical integrity of the fascicles. The observed age-specific alterations in extension mechanisms after FL suggest that fascicles from aged tendons, where structure of the helix is already compromised, suffer increased fibre sliding. Fibre sliding may initiate a cell response predisposing aged tendons to degenerative changes and subsequent injury.
