Collagenase-Induced Achilles Tendon Injury

Forty 12-weeks-old male Sprague Dawley rats (Rattus norvegicus) (Envigo, Huntingdon, UK) (mean body weight 347 ± 9 g) were used in this study. In the absence of suitable data to perform the power analysis, the sample size was calculated according to the Mead’s resource equation (E = N-T, 10<E<20). Thus, considering two treated limbs per animal for a total of 16 treated tendons per time-point, the sample size was calculated as follows: E = (16–1)-(4–1) = 12. The rats were randomly divided into two treated groups that were injected within the Achilles tendon with collagenase type I (collagenase from Clostridium histolyticum, Worthington Lakewood, NJ, USA, 185 IU/mg): 1) collagenase 1 mg/mL defined as low dose (LD); 2) collagenase 3 mg/mL defined as high dose (HD). In each group, the contralateral tendon was treated either with phosphate buffered saline (PBS) and served as control (CTRL), or left untreated and served the sham control (S-CTRL). For each time point, 4 tendons were injected with LD, 4 tendons with HD, 4 tendons received PBS and 4 tendons were left untreated (n = 4). Tendon explants were analyzed at 3, 7, 15, 30 and 45 days.