Rat model of Achilles tendon disorder - A pilot study

Three-month-old male rats were subjected 3 times weekly for 1 h to eccentri c exercise of one triceps surae muscle (30 stimulations/min) under general anesthesia in order to induce Achilles tendon disorder corresponding to par atenonitis and tendinosis in man. Net muscle work during the sessions range d between 0.67 and 4.37 mJ (mean 1.72, SD 0.77). After 9 and 13 sessions, r espectively, 2 rats started to show gait alterations during the functional test which was performed 2-3 times weekly. These rats were killed after add itional sessions which showed a worsening of the limp. The other trained ra ts and controls did not limp and were killed after 7-11 weeks. Histologic e valuation of the Achilles tendons from the exercised limb showed in the maj ority of the cases hypervascularization, increased number of nerve filament s and increased immunoreactivity for substance P and calcitonin gene-relate d peptide. The tendons from the nonstimulated limb looked normal, The distr ibution of collagen types I and II appeared normal in the tendon and its in sertion to the calcaneus. Inflammation of the epi- and paratenon could be p rovoked in the rat, but tendon changes corresponding to chronic tendinosis did not develop within 11 weeks with the used training regime. The clinical relevance of this model for chronic tendon disease needs to be evaluated f urther.