Introduction: Energy storing tendons, such as the human Achilles tendon, suffer a much higher incidence of rupture than non- energy storing positional tendons, such as the anterior tibialis tendon. Similarly, in the horse partial rupture of the energy storing superficial digital flexor tendon (SDFT) and suspensory ligament (SL) occurs much more frequently than to the deep digital flexor tendon (DDFT) and common digital extensor tendon (CDET) which are not involved in energy storage. In order to function effectively, energy storing tendons experience strains during high speed locomotion which are much closer to failure strain than non-energy storing tendons. Therefore, these tendons are likely to sustain high levels of microdamage, hence cell metabolism may also be higher in order to repair damage and maintain matrix integrity. Maintenance of the matrix requires not only synthesis of new matrix components but also degradation of matrix macromolecules which is achieved, in part, by a family of matrix metalloproteinase enzymes (MMPs). In this study we test the hypothesis that the energy storing equine SDFT and SL which are prone to degenerative changes have higher levels of MMP2 and MMP9 than the positional DDFT and CDET that are rarely injured.

Methods: Tendons (SDFT, DDFT, SL, CDET) were harvested from the distal part of the forelimbs of 18 month old Thoroughbred horses (n = 12). Tissue from the mid-metacarpal region of each tendon was snap frozen, lyophilised, powdered and MMPs extracted. Gelatin zymography was used to determine levels of the pro and active forms of the gelatinase enzymes, MMP2 and MMP 9. Proteolytic activity (units per mg dry weight tissue) was quantified based on densitometry measurements and standardised between gels using an equine neutrophil MMP extract. Statistical significance was evaluated using a general linear model (SPSS software).

Results: The main activity observed in all tendon samples was that of proMMP2. Quantification showed that the energy storing SDFT (23.4 ± 10.95) and SL (18.9 ± 5.3) had significantly higher levels than the non-energy storing DDFT (2.90 ± 0.99) and CDET (4.06 ± 2.06). Active MMP2 levels were lower than the pro form and were not sufficient to quantify. However, there appeared to be more in the energy storing structures compared with the non energy storing structures. MMP9 activity was detected in some samples. A higher number of the CDET extracts contained MMP9 activity compared to extracts from the other structures.

Discussion: The results of this study show higher levels of MMPs in energy storing structures than in non-energy storing structures. This suggests that there may be an increased demand for repair of micro-damage in these tendons and hence an increased capacity for matrix degradation. Previous studies on energy storing structures in the horse have shown that they do not undergo adaptive hypertrophy or a change in structural architecture in response to mechanical demand, unlike non-energy storing structures. The results of this study indicate that this lack of adaptation in energy storing structures is not due to a general deficiency in cell activity but may be a means of preventing increase in tendon stiffness and a subsequent decrease in efficiency. In order to maintain tendon integrity MMP activity must be matched by mechanisms to inhibit activity and/or to synthesize new matrix components. Degeneration may therefore occur when there is an imbalance between these processes.