A Passive Ankle Dorsiflexion Testing System for an In Vivo Model of Overuse-induced Tendinopathy

Summary

This protocol presents a testing system used to induce quantifiable and controlled fatigue injuries in a rat Achilles tendon for an in-vivo model of overuse-induced tendinopathy. The procedure consists of securing the rat's ankle to a joint actuator that performs passive ankle dorsiflexion with a custom-written MATLAB script.

Abstract

Tendinopathy is a chronic tendon condition that results in pain and loss of function and is caused by repeated overload of the tendon and limited recovery time. This protocol describes a testing system that cyclically applies mechanical loads via passive dorsiflexion to the rat Achilles tendon. The custom-written code consists of pre- and post-cyclic loading measurements to assess the effects of the loading protocol along with the feedback control-based cyclic fatigue loading regimen.

We used 25 Sprague-Dawley rats for this study, with 5 rats per group receiving either 500, 1,000, 2,000, 3,600, or 7,200 cycles of fatigue loads. The percentage differences between the pre- and post-cyclic loading measurements of the hysteresis, peak stress, and loading and unloading moduli were calculated. The results demonstrate that the system can induce varying degrees of damage to the Achilles tendon based on the number of loads applied. This system offers an innovative approach to apply quantified and physiological varying degrees of cyclic loads to the Achilles tendon for an in vivo model of fatigue-induced overuse tendon injury.

Introduction

As tendons connect muscle to bone and experience daily repetitive motions throughout their lifetime, they are highly prone to overuse injuries that are painful and limiting and result in impaired mechanical function, affecting 30-50% of the population¹. Tendinopathies are chronic conditions considered overuse injuries due to repetitive fatigue motions and inadequate healing to pre-injury levels. Both upper and lower extremities are commonly affected, including the rotator cuff, elbow, Achilles tendon, and patellar tendon^2,3,4,

Protocol

This study was conducted per Institutional Animal Care and Use Committee (IACUC) approval at Beth Israel Deaconess Medical Center. Animals were anesthetized using 5% isoflurane for induction and 2.5% for maintenance, and care was taken to avoid hypothermia.

1. Setting up the testing system

1. Control passive ankle rotation by a stepper motor to apply consistent rotation and torque. Control the stepper motor with a microcontroller. Use the inputs from the 3D position a.......

Discussion

This study presents a method to cyclically load the rat Achilles tendon with a passive ankle dorsiflexion system for an in-vivo overuse-induced tendinopathy model. The importance of the system lies in its ability to isolate the Achilles tendon, apply quantifiable loads without surgically accessing the tendon, and measure in-vivo tendon properties.

In 2010, Fung et al. presented a rat patellar tendon fatigue model with a custom-built testing system¹⁴. .......

Materials

Name	Company	Catalog Number	Comments
1/32'' Aluminum beads
2.5% isoflurane
3D digitizing pen	Polhemus, Vermont, NH, USA
3D electromagnetic positioning and orientation sensor	Polhemus, Vermont, NH, USA
5% isoflurane
Customized device: 1) Assembly, sensors, 3D printed animal bed and ankle mount actuator			Assembled as described in manuscript
MATLAB code	MATLAB, Natick, MA, USA
Microcontroller	Ivrea, Italy	Arduino UNO, Rev3
Nose cone
Scalpel and scalpel holder			No. 11 scalpel
Sprague-Dawley rats	Charles River Laboratories, Wilmington, MA, USA		11-13 weeks old
Stepper driver	SparkFun Electronics, Niwot, CO 80503	DM542T
Stepper motor	SparkFun Electronics, Niwot, CO 80503	23HE30-2804S
Straight forceps
Torque sensor assembly	Futek Inc., Irvine, CA, USA	FSH03985, FSH04473, FSH03927
Water heating pad