Introduction Overuse conditions of the Achilles tendon likely develop due to a combination of extrinsic and intrinsic risk factors. The purpose of this study was to investigate the individual and combined effects of uphill treadmill running (extrinsic risk factor) and collagenase injection (intrinsic risk factor) on the histological appearance of rodent Achilles tendons. Previous studies have shown that collagenase injection leads to inflammation of the tendon,1 while exercise can lead to adaptation and tendon hypertrophy.2 Our hypothesis was that collagenase injection would result in initial tendon inflammation which would become degenerative when combined with mechanical loading.
Methods Twenty-four mature high-capacity running rats were studied. All rats received a collagenase injection into one Achilles tendon at baseline. After 1 week, animals were randomly divided into either a run or cage-control group. The run group ran on a treadmill at a 15° incline, 5 days/week at increasing duration and speed. Tendons were harvested at either 4 or 10 weeks after the start of the running regimen. Specimens were fixed in formalin and embedded in paraffin. H&E stained sections were evaluated using a modified Bonar scale by two blinded and independent assessors, with a third to resolve disputes.
Results At 4 weeks, higher levels of pathology were seen in the injected tendons than in the non-injected tendons (ANOVA: p=0.007). No side to side difference was seen at 10 weeks (ANOVA: p=0.309). There was no difference between running and non-running groups at 4 weeks (ANOVA: p=0.579) or 10 weeks (ANOVA: p=0.429). No interaction effects were seen between injection and running at 4 weeks (ANOVA: p=0.934) or 10 weeks (ANOVA: p=0.847).
Discussion In high capacity running rats, uphill treadmill running using these experimental parameters did not independently create histopathology of the Achilles tendon. Furthermore, the effect of collagenase did not have a long-term effect and was not influenced by running. Therefore, the combination of collagenase injection and uphill treadmill running does not seem to create a histological presentation similar to the degeneration seen in human Achilles tendinopathy.
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