Science in brief: recent advances into understanding tendon function and injury risk.

Tendon injury is one of the most common musculoskeletal injuries sustained by horses competing in all disciplines, with a particularly high prevalence reported in racing Thoroughbreds [1–4]. The forelimb superficial digital flexor tendon (SDFT) is far more likely to be injured than any other tendinous structures in the equine limb, with ∼90% of tendon injuries in racehorses occurring to this tendon [1,5]. Other tendinous structures at risk of injury are the suspensory ligament (SL), which accounts for ∼7% of tendon injuries [1,5]. While tendon injuries occurring to horses competing in other disciplines are less well reported, a small number of studies have shown a significant risk of injury to the SDFT, SL and deep digital flexor tendon (DDFT), depending on the discipline undertaken ([4,6]; for a more detailed review of tendon injury risk, please refer to Thorpe et al. [7] and references within). Tendon injury is thought to occur due to an accumulation of microdamage over a number of loading cycles rather than an acute injury [8]. It is well established that the risk of sustaining an SDFT injury increases significantly with ageing [1,5,9,10], which may be because aged tendons have undergone a larger number of loading cycles and so are more likely to have accumulated microdamage. While tendon injury risk has been well characterised, the efficacy of many treatments is still to be established, and recurrence of tendon injuries is common [11]. The intrinsic healing property of tendon is low, and few horses return to previous activity levels [12,13]. While some treatments, including stem-cell therapy, show promising results in preliminary studies [14,15], more research is required to establish the true efficacy of these treatments. In order to develop effective treatment regimens, the alterations that occur during tendon injury and healing need to be understood better; however, this is difficult when there is still much that is not known about basic structure–function relationships within tendon and how these are altered with ageing and injury. All tendons have the same basic structure, in which collagen type I molecules are aligned in a hierarchical manner, aggregating to form fibrils, fibres and fascicles and tendons [16] (Fig 1). At the larger hierarchical levels, the collagen is interspersed with a predominantly noncollagenous matrix, rich in proteoglycans, glycoproteins and other fibrous proteins, including elastin [16,17] (Fig 1). This organisation results in a structure with high uniaxial strength, in which extension is facilitated by sliding between collagenous units at each hierarchical level [18–21]. Recent research has shown that there are numerous differences in tendon structure, composition and organisation between tendons with different functions [22–24]; these differences confer the distinct mechanical properties required by functionally different tendons. Recent work has focused on characterising structure–function relationships in the equine SDFT, due to both its extreme energy-storing role and its high risk of injury. Along with the SL, this energy-storing tendon stretches and recoils up to 16% with each stride [25] and experiences stresses of up to ∼90 MPa [26]. These values approach the failure properties of the SDFT measured in vitro [27,28]. It is therefore important to understand how the SDFT is specialised to be able to resist the continual application of large stresses and strains in vivo and to characterise changes that occur with ageing that predispose aged tendons to injury. Comparison of the SDFT with the positional common digital extensor tendon (CDET), which experiences much lower stresses and strains in vivo [22], has provided important insights into specialisation of tendon mechanical properties.