Elsevier

Journal of Biomechanics

Volume 43, Issue 2, 19 January 2010, Pages 274-279
Journal of Biomechanics

Early response to tendon fatigue damage accumulation in a novel in vivo model

https://doi.org/10.1016/j.jbiomech.2009.08.039Get rights and content

Abstract

This study describes the development and application of a novel rat patellar tendon model of mechanical fatigue for investigating the early in vivo response to tendon subfailure injury. Patellar tendons of adult female Sprague-Dawley rats were fatigue loaded between 1–35 N using a custom-designed loading apparatus. Patellar tendons were subjected to Low-, Moderate- or High-level fatigue damage, defined by grip-to-grip strain measurement. Molecular response was compared with that of a laceration-repair injury. Histological analyses showed that progression of tendon fatigue involves formation of localized kinked fiber deformations at Low damage, which increased in density with presence of fiber delaminations at Moderate damage, and fiber angulation and discontinuities at High damage levels. RT-PCR analysis performed at 1- and 3-day post-fatigue showed variable changes in type I, III and V collagen mRNA expression at Low and Moderate damage levels, consistent with clinical findings of tendon pathology and were modest compared with those observed at High damage levels, in which expression of all collagens evaluated were increased markedly. In contrast, only type I collagen expression was elevated at the same time points post-laceration. Findings suggest that cumulative fatigue in tendon invokes a different molecular response than laceration. Further, structural repair may not be initiated until reaching end-stage fatigue life, where the repair response may unable to restore the damaged tendon to its pre-fatigue architecture.

Introduction

Tendinopathy and tendon rupture are common painful and debilitating clinical problems. The Achilles, patellar and rotator cuff tendons are particularly prone to tendinopathies (Kannus and Jozsa, 1991; Maffulli et al., 2004) due to their tensile overload, material and structural inhomogeneity, and poor healing response (Leadbetter, 1992; Maganaris et al., 2004; Mehta et al., 2003; Riley, 2004). Observed degenerative changes in ruptured tendons suggest pre-rupture damage accumulation (Kannus and Jozsa, 1991; Tallon et al., 2001). Additionally, matrix disorganization with cellular proliferation and lipid accumulation was observed in macroscopically “healthy” tendons from older subjects (Kannus and Jozsa, 1991), suggesting damage accumulation. It is thus thought that cumulative injury from overuse causes degeneration leading to tendon weakening and failure.

Overuse/cyclic loading protocols have been used to investigate the mechanical basis of degradation and matrix degeneration in tendons. Studies evaluating the relationship between applied stress and number of cycles to failure of devitalized tendons and ligaments showed progressive degradation of mechanical properties (Ker et al., 2000; Pike et al., 2000; Schechtman and Bader, 1997, Schechtman and Bader, 2002; Wang and Ker, 1995; Wang et al., 1995; Wren et al., 2003). In vivo tendon modeled overuse injuries by utilizing treadmill running, muscle stimulation and repeated reaching tasks (Backman et al., 1990; Nakama et al., 2005; Soslowsky et al., 2000) and showed tendon damage, including fiber disorganization, microtears, and reduction of modulus and maximum stress. Furthermore, inability of degenerative changes to recover with time (Soslowsky et al., 2000) indicate a poor repair response likely from disrupted collagen fibrillogenesis, which is required for the scar tissue deposition, collagen synthesis, and tissue remodeling seen in wound healing (Hiranuma et al., 1996; Oshiro et al., 2003; Sakai et al., 2001; Williams et al., 1984)). However, the process of microstructural damage initiation and accumulation from loading, and the resulting tendon weakening and cellular-matrix response should be evaluated.

Previously, we investigated fatigue-induced damage accumulation in tendons using an ex vivo model of rat flexor digitorum longus (FDL) tendons (Fung et al., 2008). Results demonstrated changes in morphology and mechanical properties representing key mechanistic events during fatigue life. In this study, we developed an in vivo animal model to evaluate tendon response to fatigue loading at various damage levels based on previous ex vivo studies. For comparison, we concurrently evaluated the response of the patellar tendon to laceration and suture-repair. We hypothesize that fatigue-induced damage in tendons does not elicit the typical healing response seen in laceration. More specifically, while the extent to which a fatigue damage tendon may undergo any structural repair is unknown, we expect that its response to fatigue damage accumulation will differ from that of a lacerated tendon in that it will not exhibit the expression spectrum of collagens that accompanies the lacerated tendon's attempt to undergo structural repair.

Section snippets

Animal model for in vivo fatigue loading

Our criteria for selecting an animal model were (1) a small animal previously used to investigate tendon biomechanics, tissue adaptation, and biochemical and molecular studies of tendon physiology and pathology; (2) a tendon that can be fatigue loaded with control; and (3) a tendon that exhibits tendinopathy in humans. Candidates evaluated included the FDL, Achilles, patellar, supraspinatus and tail tendons in mice, rats, rabbits and dogs.

Patellar tendons in adult female Sprague-Dawley rats (n

Animal model

At completion of the loading protocol, gross inspection of the knee joint and the patellar tendon showed no damage from clamping. Rats resumed normal cage activity with no evidence of lameness or inability to use the loaded limb within 2 h of recovery. At all time points, there was no gross evidence of acute inflammation at the gripping sites or in the tendon. Incision sites showed only normal granulation tissue.

Biomechanics

No tendons failed during the course of fatigue loading. Tendons were loaded for

Discussion

We developed an in vivo model in which rat patellar tendons were fatigue loaded to precisely defined levels of cumulative damage to explore mechanisms of tendon degeneration. Results showed that low level fatigue damage is represented by isolated kinked fiber patterns that transversely span across several fibers. In mid-fatigue life, tendons exhibited a greater density of deformation patterns. Surprisingly at Low- and Mid-levels, tendon damage caused increased stiffness and decreased

Conflict of interest

None of the authors have any relevant commercial relationships which may lead to a conflict of interest.

Acknowledgements

This study was supported by Aircast Foundation and NIH (AR41210, AR44927, AR49967, and AR52743).

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