Effect of maturation and aging on the histomorphometric and biochemical characteristics of equine superficial digital flexor tendon

Objective - To assess tendon morphology and non-reducible crosslink concentration, and associations of these findings with horse age and previously reported mechanical and ultrasonographic findings. Sample Population - Superficial digital flexor tendon samples were obtained from 23 horses aged 2 to 23 years. The tendons had undergone ultrasonography and were submitted to biomechanical testing in the physiologic range prior to sample acquisition. Procedure - Samples were sectioned in a transverse plane; then dorsal, palmar, central, lateral, and medial regions were evaluated for fascicle cross-sectional area (CSA) septal width, and vessel density (the product of vessel numbers and vessel CSA per field). Contiguous samples were analyzed for collagen crosslinking. Results - Central fascicles were significantly larger than fascicles in other tendon regions. Fascicle CSA decreased significantly with increasing age. Because total tendon CSA is unrelated to increasing age, fascicle numbers appeared to increase with increasing age. Regional or age effects on septal width were not found. There was no age or regional effect on vessel numbers, density, or fractional area. Fascicle CSA was positively correlated with total tendon CSA; fascicle CSA was negatively correlated with elastic modulus. Hydroxypiridinium concentration tended to increase with increasing horse age; this effect was associated with a positive correlation between hydroxypiridinium values and elastic modulus. Conclusions - Equine superficial digital flexor tendon undergoes an increase in structural organization and an increase in nonreducible crosslinks with maturation and aging. These changes are associated with an increase in elastic modulus.