Histopathological, biomechanical, and behavioral pain findings of Achilles tendinopathy using an animal model of overuse injury

Abstract

Animal models of forced running are used to study overuse tendinopathy, a common health problem for which clear evidence for effective and accessible treatments is still lacking. In these models, pain evaluation is necessary to better understand the disease, help design and evaluate therapies, and ensure humane treatment of the animals. Therefore, the main objective of this study was to evaluate pain and pathologic findings in an animal model of moderate Achilles tendinopathy induced by treadmill running. Air puffs, instead of electrical shocks, were used to stimulate running so that pain associated with stimulation would be avoided. Pressure pain sensitivity was evaluated in vivo using a new instrumented plier, whereas spinal cord peptides were analyzed ex vivo with high‐performance liquid chromatography tandem mass spectrometry. Tendon histologic slides were semiquantitatively evaluated, using the Bonar score technique and biomechanical properties, using the traction test. After 8 weeks of treadmill running (2 weeks for adaptation and 6 weeks for the lesion protocol), the protocol was stopped because the air puffs became ineffective to stimulate running. We, nevertheless, observed some histologic changes characteristic of overuse tendinopathy as well as decreased mechanical properties, increased Substance P and dynorphin A peptides but without pressure pain sensitivity. These results suggest that air‐puffs stimulation is sufficient to induce an early stage tendinopathy to study new therapeutic drugs without inducing unnecessary pain. They also indicate that pain‐associated peptides could be related with movement evoked pain and with the sharp breakdown of the running performance.

Sustained Exposure of Substance P Causes Tendinopathy

“…To the best of our knowledge, there are two popular animal models to investigate tendinopathy. First, collagenase could be injected into the tendon to create tendinopathy, but it was reported that this model is closer to the acute injury model, not chronic tendinopathy [ 32 , 33 ]. Furthermore, past studies have used animals running on treadmills as their models in the generation and observation of tendinopathy.…”

Mechanobiology of young and aging tendons: In vivo studies with treadmill running

“…Another recent study in a rat model of overuse injury to assess Achilles tendinopathy shows some histological changes characteristic of overuse tendinopathy (higher cell density, more cell clusters, and disorganized collagen) as well as decreased mechanical properties, increased substance P, and dynorphin A peptides but without pressure pain sensitivity. 33 While the above-described studies demonstrated changes characteristic of overuse tendinopathy, some recent studies show contrasting results. For example, using the same model as that of Glazebrook, another study observed that uphill running improved rat Achilles tendon tissue mechanical properties, and altered gene expression without inducing pathological changes even after the running speed was progressively increased.…”

“…Although MGF and tenocyte‐related gene upregulation are important for tendon repair and/or remodeling as observed with MTR, ITR may also cause degenerative changes in tendons by inducing aberrant differentiation into non‐tenocyte phenotypes such as adipocytes, chondrocytes, and osteocytes. Another recent study in a rat model of overuse injury to assess Achilles tendinopathy shows some histological changes characteristic of overuse tendinopathy (higher cell density, more cell clusters, and disorganized collagen) as well as decreased mechanical properties, increased substance P, and dynorphin A peptides but without pressure pain sensitivity . While the above‐described studies demonstrated changes characteristic of overuse tendinopathy, some recent studies show contrasting results.…”

Development of overuse tendinopathy: A new descriptive model for the initiation of tendon damage during cyclic loading

“…Because disruption to collagen and excessive mechanical loading are both common features of tendinopathy, gaining a detailed understanding of how repeated mechanical loading affects tendon collagen at its various levels of structural hierarchy is of significant importance. At the microscale, in vivo animal models have demonstrated that abnormally high levels of physical activity can cause fragmentation and disorganization of collagen fibres within tendon . In vitro experiments have provided a better description of the progression of structural disruption with cyclic loading.…”