Mechanical Overload Followed by Consecutive Collagenase Injections: Developing a Multifactorial and Long-Lasting Animal Model of Induced Achilles Tendinopathy
Authors: Netto Cesar de Cesar , Zhang Zijun , Goncalves Mario Lobao , Cychosz Chris , Li Shuyuan , Duchman Kyle R. , Goetz Jessica E. , Femino John E. , Chimenti Ruth , Schon Lew C. 
Published: November 2020

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<0.001) were found in animals subjected to collagenase injections (16, CI 13.1-18.9) and to running/collagenase (17.4, CI 14.4-20.3), when compared to controls (1.6, CI -1.3-4.50) and running (3, CI 0.1-5.9). Similarly, after 10 weeks significantly increased scores were found in the same groups, with slight severity regression: controls (1, CI -0.8-2.8), running (2.2, CI 0.4-4.0), collagenase (10, CI 8.2-11.8) and running/collagenase (17.6, CI 15.8- 19.4). After 12 weeks, collagenase group demonstrated reversion of the findings (3.3, CI 1.6-5.1), and wasn’t different than control (2.1, CI 0.4-3.9) and running groups (2.5, CI 0.3-4.7). However, significantly increased pathological findings were noted in the running/collagenase group (20.0, CI 18.2-21.8) consistent with chronic tendinopathic process. Gait analysis results presented in Figure1. Conclusion: When compared to other models of induced Achilles tendinopathy and to controls, the novel animal model induced by a mechanical trigger and sustained by chemical stress demonstrated progressively increased histological tendinopathic scores after 12 weeks. Findings observed after isolated mechanical or chemical stresses were temporary, not maintained at latest follow- up. Steps involved in tendinopathy development, as well as the observed histological results of the combined running/collagenase model, replicate better the findings of human chronic Achilles tendinopathy. Applications for this novel model are promising, potentially supporting a better understanding of early/late findings as well as treatment options for Achilles tendinopathy. [Figure: see text]