The mechanobiological aetiopathogenesis of tendinopathy: is it the over-stimulation or the under-stimulation of tendon cells?

While there is a significant amount of information available on the clinical presentation(s) and pathological changes associated with tendinopathy, the precise aetiopathogenesis of this condition remains a topic of debate. Classically, the aetiology of tendinopathy has been linked to the performance of repetitive activities (so-called overuse injuries). This has led many investigators to suggest that it is the mechanobiologic over-stimulation of tendon cells that is the initial stimulus for the degradative processes which have been shown to accompany tendinopathy. Although several studies have been able to demonstrate that the in vitro over-stimulation of tendon cells in monolayer can result in a pattern(s) of gene expression seen in clinical cases of tendinopathy, the strain magnitudes and durations used in these in vitro studies, as well as the model systems, may not be clinically relevant. Using a rat tail tendon model, we have studied the in vitro mechanobiologic response of tendon cells in situ to various tensile loading regimes. These studies have led to the hypothesis that the aetiopathogenic stimulus for the degenerative cascade which precedes the overt pathologic development of tendinopathy is the catabolic response of tendon cells to mechanobiologic under-stimulation as a result of microscopic damage to the collagen fibres of the tendon. In this review, we examine the rationale for this hypothesis and provide evidence in support of this theory.