Introduction Epidemiological studies have revealed that the incidence of Achilles tendon rupture is increasing and is especially high in middle age. Similarly, in horses, the superficial digital flexor tendon (SDFT) is often injured with older horses being most at risk. Tendons which play a role in elastic energy storage, such as the human Achilles tendon and equine SDFT, are much more susceptible to degenerative change and subsequent rupture than non-energy storing positional tendons, such as the human anterior tibialis tendon and the equine common digital extensor tendon (CDET). These energy storing tendons are required to operate with small safety margins and are likely therefore to incur high levels of micro-damage. The ability to repair micro-damage depends on the capacity for matrix turnover which requires both the capability to synthesise and degrade matrix components. In a previous study we have shown that the levels of matrix degrading enzymes (matrix metalloproteinases) differ significantly between the SDFT and CDET (Faram et al., 2004, Proc. BORS, Bristol) and that some matrix metalloproteinases (MMP-3) increase significantly with increasing age (Eissa et al., 2004, Proc. BSMB, Bristol). The aim of this study was to test the hypothesis that MMP derived fragments of collagen resulting from collagen breakdown are present at higher levels in the energy storing SDFT than the CDET and increase significantly with increasing age.

Methods The SDFT and CDET were harvested from the left forelimb of horses (n=20) ranging in age from skeletal maturity to senescence (5 – 30 years) and tissue from the mid-metacarpal level of each tendon analysed. A commercially available radioimmunoassay kit (Oxford Biosystems) was used to measure levels of the C-terminal telopeptide of type I collagen (ICTP). In addition, DNA levels were measured by a fluorometric assay using Hoechst 33258 dye to give an indication of tissue cellularity and collagen-linked fluorescence was measured to give an indication of the age of the collagen in the matrix. Statistical significance (p = 0.05) was evaluated using a general linear model (SPSS software) to compare tendons (SDFT and CDET) and to determine changes with age.

Results The levels of ICTP were approximately four times higher (p=0.001) in the CDET compared to the SDFT and in both tendons appeared to decrease with increasing age. DNA levels were significantly (p< 0.001) higher in the SDFT than the CDET and these levels did not change significantly with age. The collagen-linked fluorescence was significantly (p< 0.001) higher in the SDFT than the CDET and decreased significantly (p=0.006) with age in both tendons.

Discussion The results demonstrate that the SDFT is more cellular than the CDET and may therefore be expected to be more metabolically active. Contrary to this, collagen-linked fluorescence is higher in the SDFT suggesting that the matrix is older and furthermore the levels of collagen fragments are much lower in the SDFT suggesting that the collagen within the matrix is turned over more rapidly in the CDET than the SDFT. The changes in collagen-linked fluorescence and ICTP levels suggest than collagen turnover decreases with ageing and low turnover may be responsible for SDFT degneration.