Mechanical Overload Followed by Consecutive Collagenase Injections: Developing a Multifactorial and Long-Lasting Animal Model of Induced Achilles Tendinopathy

Category: Basic Sciences/Biologics; Hindfoot; Sports Introduction/Purpose: Different animal models of Achilles tendinopathy have been proposed in the literature. They usually involve the induction of tendinopathic findings by either chemical stress (most commonly with one or more injections of collagenase, mimicking intrinsic factors) or mechanical stress (by repetitive exercise-induced stress with treadmill running exercises, simulating extrinsic risk factors). To date, no study has evaluated the combination of a mechanical trigger followed by collagenase injections, replicating the logical and sequential steps involved in the development the human pathology. Our goal was to develop this novel animal model of Achilles tendinopathy and to compare histological and functional findings with animals subjected to isolated mechanical or chemical stress, as well as to controls. Methods: Sixty-four Sprague-Dawley rats were divided into four groups (n=16): isolated treadmill running protocol (15o uphill running, 20meters/minute, 1hour/day, 3 weeks duration, weeks 2-4); isolated injections of collagenase (0.1mg each, 3 injections total, weeks 5-7); treadmill protocol (weeks 2-4) followed by three consecutive collagenase injections (weeks 5-7); and controls, no running and three injections of normal saline (weeks 5-7). Five animals from each group were sacrificed at weeks 8 and 10. Six animals by group were sacrificed at week 12. Gait analysis was performed at weeks one (after acclimation), five (following running protocol), eight (following injection protocol) and twelve (just before latest sacrifice time-point). Histological findings were assessed by the Movin Tendinopathy Score (eight parameters, scored from 0-3, total score 0-24), assessing collagen arrangement, structure, and stainability, cellularity, vascularity, nuclear rounding, hyalinization and presence of glycosaminoglycans. Gait parameters included stand and swing times, stride length, duty cycle and swing length. Results: After 8 weeks, significantly increased tendinopathic scores (p