Capacity for sliding between tendon fascicles decreases with ageing in injury prone equine tendons: a possible mechanism for age-related tendinopathy?

Age-related tendinopathy is common in both humans and horses; the initiation and progression of which is similar between species. The majority of tendon injuries occur to high-strain energy storing tendons, such as the human Achilles tendon and equine superficial digital flexor (SDFT). By contrast, the low-strain positional human anterior tibialis tendon and equine common digital extensor (CDET) are rarely injured. It has previously been established that greater extension occurs at the fascicular interface in the SDFT than in the CDET; this may facilitate the large strains experienced during locomotion in the SDFT without damage occurring to the fascicles. This study investigated the alterations in whole tendon, fascicle and interfascicular mechanical properties in the SDFT and CDET with increasing age. It was hypothesised that the amount of sliding at the fascicular interface in the SDFT would decrease with increasing horse age, whereas the properties of the interface in the CDET would remain unchanged with ageing. Data support the hypothesis; there were no alterations in the mechanical properties of the whole SDFT or its constituent fascicles with increasing age. However, there was significantly less sliding at the fascicular interface at physiological loads in samples from aged tendons. There was no relationship between fascicle sliding and age in the CDET. The increase in stiffness of the interfascicular matrix in aged SDFT may result in the fascicles being loaded at an earlier point in the stress strain curve, increasing the risk of damage. This may predispose aged tendons to tendinopathy.