Models of Tendinitis in the Horse

Introduction
Tendon injuries are common in all athletic activities in both humans and horses. Strains of the superficial digital flexor tendon (SDFT) account for up to 46% of limb injuries in racing Thoroughbreds and were reported to be the most important reason for retirement of racehorses from racing. SDFT strains are often career-threatening events because recovery is slow and return to exercise is associated with a high re-injury rate4, which is believed to be a consequence of the compromised biomechanical function of the tendon after scarring. The precise etiopathogenesis of tendon injury in humans and horses remains a topic of controversy and confusion. The most significant histopathological feature in human overuse tendon injury is the absence of inflammatory cells, which has prompted the introduction of the term ‘tendinosis’, a degenerative condition of unknown aetiology, to replace the more commonly used diagnosis of ‘tendinitis’, as used in horses. Overstrain injury mostly affects the core of the SDFT in the middle to distal third of the metacarpal region. The loss of collagen crimp angle and length in the central core of the SDFT with age and exercise6 as well as the susceptibility of the central core of the SDFT to hypoxia and its tendency to heat up with exercise may explain this site predilection for overstrain injury.

Various models of induced tendinitis have been used to study the pathology of tendon healing and the modulatory effect of different treatment methods on this process in the horse. Most studies have used a model of collagenase- induced enzymatic injury to the SDFT. However, in recent years alternative models have been proposed in an attempt to avoid some of the documented problems associated with the collagenase-induced model of injury. These newer mechanical models are mostly based on surgically-induced injury to the SDFT, although some have also used thermal injury created by laser or radiofrequency energy. Recent studies on tendinitis models in the horse have focussed on how to create a controllable core lesion in the SDFT that shows a similar ultrasonographic and histopathological appearance and progression as naturally occurring tendinitis in both equine and human athletes.  [...]