Abstract
Most overuse tendinopathies are thought to be associated with repeated microstrain below the failure threshold, analogous to the fatigue failure that affects materials placed under repetitive loading. Investigating the progression of fatigue damage within tendons is therefore of critical importance. There are obvious challenges associated with the sourcing of human tendon samples for in vitro analysis so animal models are regularly adopted. However, data indicates that fatigue life varies significantly between tendons of different species and with different stresses in life. Positional tendons such as rat tail tendon or the bovine digital extensor are commonly applied in in vitro studies of tendon overuse, but there is no evidence to suggest their behaviour is indicative of the types of human tendon particularly prone to overuse injuries. In this study, the fatigue response of the largely positional digital extensor and the more energy storing deep digital flexor tendon of the bovine hoof were compared to the semitendinosus tendon of the human hamstring. Fascicles from each tendon type were subjected to either stress or strain controlled fatigue loading (cyclic creep or cyclic stress relaxation respectively). Gross fascicle mechanics were monitored after cyclic stress relaxation and the mean number of cycles to failure investigated with creep loading. Bovine extensor fascicles demonstrated the poorest fatigue response, while the energy storing human semitendinosus was the most fatigue resistant. Despite the superior fatigue response of the energy storing tendons, confocal imaging suggested a similar degree of damage in all three tendon types; it appears the more energy storing tendons are better able to withstand damage without detriment to mechanics.
Highlights
Many painful tendon lesions are thought to be the result of a gradual accumulation of micro-injuries.[1]
A significant difference between the response of the two bovine tendons was only apparent at 1800 cycles and at no time point did a statistically significant variation exist between the human semitendinosus and bovine flexor tendon
Despite considerable stress relaxation after 300 cycles of loading, all samples including the bovine extensor tendon fascicles retained their tensile strength during quasi-static testing at this first time point (Figure 2)
Summary
Many painful tendon lesions are thought to be the result of a gradual accumulation of micro-injuries.[1] Tendinopathy is often referred to as an overuse pathology and can, in some ways, be considered analogous to the fatigue failure of common engineering materials during cyclic loading.[2] This is an oversimplification, as tendon is not a fibrereinforced composite, but a living tissue maintained by its resident cell population, the tenocytes. Damage to tendon probably occurs every day as a result of a variety of normal activities, but this damage only accumulates when equilibrium cannot be maintained between the rate of damage and the cellular driven rate of repair.[3] The progression of this damage can be challenging to monitor in situ, and as such, in vitro fatigue testing routes are of significant importance for understanding overuse damage in tendons. Tendon mechanics are defined by the hierarchically organised collagen within the tissue, providing a uniaxially strong material, in which failure of one or a few collagen components within the hierarchy does not
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