Achilles Tendon Injuries in Elite Athletes: Lessons in Pathophysiology from Their Equine Counterparts

Abstract

Superficial digital flexor tendon (SDFT) injury in equine athletes is one of the most well-accepted, scientifically supported companion animal models of human disease (i.e., exerciseinduced Achilles tendon [AT] injury). The SDFT and AT are functionally and clinically equivalent (and important) energy-storing structures for which no equally appropriate rodent, rabbit, or other analogues exist. Access to equine tissues has facilitated significant advances in knowledge of tendon maturation and aging, determination of specific exercise effects (including early life), and definition of some of the earliest stages of subclinical pathology. Access to human surgical biopsies has provided complementary information on more advanced phases of disease. Importantly, equine SDFT injuries are only a model for acute ruptures in athletes, not the entire spectrum of human tendonopathy (including chronic tendon pain). In both, pathology begins with a potentially prolonged phase of accumulation of (subclinical) microdamage. Recent work has revealed remarkably similar genetic risk factors, including further evidence that tenocyte dysfunction plays an active role. Mice are convenient but not necessarily accurate models for multiple diseases, particularly at the cellular level. Mechanistic studies, including tendon cell responses to combinations of exercise-associated stresses, require a more thorough investigation of cross-species conservation of key stress pathway auditors. Molecular evidence has provided some context for the poor performance of mouse models; equines may provide better systems at this level. The use of horses may be additionally justifiable based on comparable species longevity, lifestyle factors, and selection pressure by similar infectious agents (e.g., herpesviruses) on general cell stress pathway evolution.

Pathological tendons maintain sufficient aligned fibrillar structure on ultrasound tissue characterization (UTC)

“…Similar data are not available in humans. However, structural similarities between horses and human have been described in the literature (Kristoffersen et al., ; Patterson‐Kane et al., ; Patterson‐Kane & Rich, ). It is also important to note that the UTC method used in this study differs from the original articles (e.g., transducer moving automatically compared with moving manually, transducer captures over 12 cm compared with 9.6 cm, the region of tendon selected for quantification) (Rosengarten et al., ).…”

Tendon Functional Extracellular Matrix

“…The SDFT provides a good model for the human Achilles tendon, prone to aetiologically similar tendinopathies 47 , but is also a particularly highly loaded energy storing tendon 40 , providing an excellent structure-function model to study the structural mechanisms enabling the tendon to perform its mechanical function.…”

Tendinopathy: Is Imaging Telling Us the Entire Story?

“…69 van Schie et al 95 scanned a cross-section of individuals with and without Achilles tendon pain and reported that the percentage of intact and aligned tendon bundles (echo type I) was reduced in the symptomatic tendon, with increases in echo types that represent disorganized structure (echo types III and IV). The ability to semi-quantify the structural integrity of the tendon has allowed for classification of the tendon pathology based on a continuum (FIGURE 7), treatment efficacy to be investigated, 21,23,24 detection of subtle changes in response to load, 29,79 and demonstration that the asymptomatic Achilles tendon is structurally compromised in individuals with unilateral tendon pain.…”