The relationships between cyclic fatigue loading, changes in initial mechanical properties, and the in vivo temporal mechanical response of the rat patellar tendon

doi:10.1016/j.jbiomech.2011.10.008

Journal of Biomechanics

Volume 45, Issue 1, 3 January 2012, Pages 59-65

https://doi.org/10.1016/j.jbiomech.2011.10.008 Get rights and content

Abstract

Damage accumulation underlies tendinopathy. Animal models of overuse injuries do not typically control loads applied to the tendon. Our in vivo model in the rat patellar tendon allows direct control of the loading applied to the tendon. Despite this advantage, natural variation among tendons results in different amounts of damage induced by the same loading protocol. Our objectives were to (1) assess changes in the initial mechanical parameters (hysteresis, stiffness of the loading and unloading load-displacement curves, and elongation) after fatigue loading to identify parameters that are indicative of the induced damage, and (2) evaluate the relationships between these identified initial damage indices with the stiffness 7 day after loading. Left patellar tendons of adult, female retired breeder, Sprague-Dawley rats (n=68) were fatigue loaded per our previously published in vivo fatigue loading protocol. To induce a range of damage, fatigue loading consisted of either 5, 100, 500 or 7200 cycles that ranged from 1 N to 40 N. Diagnostic tests were applied before and immediately after fatigue loading, and after 45 min of recovery to deduce recoverable and non-recoverable changes in initial damage indices. Relationships between these initial damage indices and the 7-day stiffness (at sacrifice) were determined. Day-0 hysteresis, loading and unloading stiffness exhibited cycle-dependent changes. Initial hysteresis loss correlated with the 7-day stiffness. k-means cluster analysis demonstrated a relationship between 7-day stiffness and day-0 hysteresis and unloading stiffness. This analysis also separated samples that exhibited low from high damage in response to both high or low number of cycles; a key delineation for interpretation of the biological response in future studies. Identifying initial parameters that reflect the induced damage is critical since the ability of the tendon to repair depends on the damage induced and the number of applied loading cycles.

Introduction

Tendinopathies leading to tendon rupture are common and debilitating clinical problems. Degeneration in ruptured tendons and matrix disorganization in macroscopically ‘healthy’ tendons suggest pre-rupture damage accumulation (Kannus and Jozsa, 1991, Tallon et al., 2001). Consequently, animal models have been used to investigate the response of the tendon to overuse or cyclic loading (Wang et al., 1995, Ker et al., 2000, Schechtman and Bader, 2002). Chronic injury from treadmill running, muscle stimulation, and repeated reaching tasks have shown histological and mechanical evidence of degeneration (Backman et al., 1990, Soslowsky et al., 2000, Nakama et al.,). Despite insight gained from these models, their inability to control the loads applied directly to the tendon may introduce confounding variability.

We have previously developed and utilized an in vivo model of fatigue damage accumulation in the rat patellar tendon (Fung et al., 2010). Briefly, the rat patellar tendon was chosen because: (1) the rat is a small animal that has been used to investigate tendon biomechanics; (2) the patellar tendon can be directly loaded without direct instrumentation of the tendon; and (3) the patellar tendon exhibits clinical tendinopathy. Our model allows us to accurately control the loading applied to the tendon and titrate input parameters such as number of cycles, loading magnitude, and loading frequency to investigate specific clinical conditions. Despite improvement gained from controlling the load directly applied to the tendon, animal-to-animal variation in tendon size and strength results in varying amounts of damage induced by the same fatigue loading protocol. We expect that the response of the tendon to fatigue loading reflects the additive effect of the number of applied cycles to induce damage and the resulting amount of induced damage, suggesting that due to natural variation between animals, both of these aspects should be considered in interpreting the response of the tendon to fatigue loading. However, little is known about the relationships between applied number of fatigue loading cycles, initial recoverable, and non-recoverable changes in mechanical properties and the mechanical properties of the tendon at a later time point. Therefore, the objectives of this study were to (1) assess the relationship between the applied number of fatigue loading cycles and the recoverable (transient effect of loading) and non-recoverable (damaging to the matrix) changes in initial mechanical parameters (damage indices) after fatigue loading to identify parameters that are indicative of the amount of damage induced within the tendon, and (2) evaluate the relationship between the number of applied fatigue loading cycles and initial damage indices with the mechanical response 7 days after fatigue loading. We hypothesized that (H1): Non-recoverable changes in hysteresis, tendon length, stiffness of the loading and unloading load-displacement curves and measures characterizing the toe-region (initial mechanical parameters) will depend on the number of applied fatigue cycles; (H2): The stiffness 7 day after fatigue loading will correlate with the amount of initial induced damage, as indicated by the damage indices.

Section snippets

Methods

Following IACUC approval, adult, female retired breeder, Sprague-Dawley rats (n=68) (Charles River Laboratories, Ltd., Wilmington, MA) were anesthetized with isoflurane (2–3% by volume, 0.4 L/min) and their left patellar tendons (PT) were exposed (Fung et al., 2010). Under aseptic conditions, a clamp fixed the tibia at ∼30° knee flexion. A clamp gripped and connected the patella to a 50-lb load cell and actuator of a servo-hydraulic loading system, allowing loading of the PT without contact with

Initial mechanical parameters assessed after fatigue loading

For all groups, no significant differences were found between diag₃ and diag_2, (recoverable) in any day-0 parameter evaluated (except disp_toe for the 5 cycle group). However, a significant difference between diag₃ and diag₁ (non-recoverable) was found in hysteresis, elongation, loading, and unloading stiffness, disp_toe (for the 5 cycle group only) and D (non-recoverable D will be denoted by ΔD) (Fig. 4). No significant non-recoverable changes were found in load_toe for any cycle number group.

Discussion

We have identified initial damage parameters that serve as indices of the damage induced in the tendon and assessed their effectiveness to cluster the 7-day stiffness. Initial non-recoverable changes were found in hysteresis, loading and unloading stiffness, elongation, and ΔD, suggesting that these parameters could serve as indices of the damage induced in the tendon. For the damage levels induced in this study, the lack of change in the load_toe and disp_toe suggests that these measurements

Conflict of interest statement

We have no conflicts of interest to disclose.

Acknowledgments

This study was supported by the National Institutes of Health (AR052743 and AR058123).

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The relationships between cyclic fatigue loading, changes in initial mechanical properties, and the in vivo temporal mechanical response of the rat patellar tendon

Abstract

Introduction

Section snippets

Methods

Initial mechanical parameters assessed after fatigue loading

Discussion

Conflict of interest statement

Acknowledgments

Journal of Biomechanics

Journal of Biomechanics

Journal of Biomechanics

Journal of Shoulder and Elbow Surgery

Journal of Biomechanics

Journal of Shoulder and Elbow Surgery

Journal of Prosthetic Dentistry

Chronic achilles paratenonitis with tendinosis: an experimental model in the rabbit

Journal of Orthopaedic Research