Is SDFT tendinopathy a “professional condition” in the jumping racehorse?
/Words - Jean Baptiste Pavard
Tendon and ligament disorders are one major cause of poor performance and wastage in equine athletes. The most common structures involved are the superficial digital flexor tendon (SDFT), the suspensory ligament (SL), the deep digital flexor tendon (DDFT) and the accessory ligament of the deep digital flexor tendon (ALDDFT), also called the inferior check ligament.
Thoroughbred racehorses are particularly predisposed to tendon and ligament injuries accounting for approximately 50% of all musculoskeletal injuries to competing racehorses. However, some structures are much more exposed to injuries than others in this population of equine athletes.
Most tendon injuries in racehorses occur to the forelimb tendons, with overstrain injury of the SDFT at the very top of the list. This is particularly true in jump racing, where the prevalence of superficial digital flexor tendinopathy has been found to involve up to 24% of horses in training over 2 seasons (Avella et al. 2009) and could be considered as a “professional condition”.
The higher exposure of tendon injury in jumping horses compared to flat racehorses might be explained by the fact they compete over longer distances, for more seasons and are generally older than horses that race on the flat. Another reason is very likely that the SDFT of jumping horses support bigger strains, and repetitively, when landing over fences.
The main issue for this type of injuries is that tendon healing is slow and requires a long recovery between 10 to 18 months depending on the severity of cases. Although the scar tissue of tendon injuries can be optimised with an effective rehabilitation program, its functionality remains inferior with relatively high re-injury rates in the years following the original lesion. Thus, a complete understanding of SDF tendinopathy and its major risk factors in jump racing are very important to improve prevention and early management of the condition which is a potentially career-ending condition. In the racing community, it has become crucial given big issues it involves in sporting and economic terms, as well as for the health and the welfare of racehorses.
SDF tendonitis - characteristics in jumping racehorses
SDF tendinopathy is one of the most common injuries in jump horses with a prevalence from 10 to 45% depending on epidemiological studies with some variations among trainers. Most of the cases involve the forelimbs, but hindlimb injuries also occur. Typically, lesions are found at the mid-cannon level in a central core lesion.
The disruption of the tendon fibres might generally occur in this area because it appears to be preferentially loaded and degenerates more over the time. However, injuries of the SDFT can be seen at all the levels of the tendon. They are most commonly unilateral, but bilateral SDFT injuries can also occur.
Tendinopathy is a result of mechanical overload, varying from single fibril disruption to complete rupture of the whole tendon. The most common cause of SDFT overstrain injuries in NH horses is an accumulation of damages from repetitive overloading.
The structure of the tendons in horses is matured around 2 years old, and after maturity there is very limited or no adaptation possible. It means that if tendons accumulate an excess of micro-damage over the time (tendon cells have a capacity to repair defects, but it is limited and need time), they become weaker with a loss of elasticity and strength leading to a point where higher SDFT loads / strains result in disruption of fibres with a clinical tendon injury.
Moreover, it is important to be precise that forelimb flexor tendons in racehorses function close to their maximal load / strain-bearing capacity with a narrow safety margin. While failure of the SDFT has been shown occurring for tensile strain* from 12 to 20% in vitro, peak strains within SDFT at the gallop are by around 16%. Since racehorses operate close to the functional limit of the SDFT during fast work, any risk factors that lead to higher loads on tendons during training or racing can result in clinical injury with significant disruption of tendon fibres. Some of these in NH horses are discussed below.
* % increase in length from original length / tensile strain
Causes – Risk factors
Epidemiological studies have identified risk factors for SDF tendinopathy in racehorses. As discussed previously, jump horses are at greater risk than flat racehorses and it could be partially explained by horses being older in jump racing.
Indeed, risk of SDFT injuries increases considerably with age and it appears that the prevalence in jump horses is more important in horses older than 5 years old, with the maximum injury rate seen in horses 12 to 14 years of age.
Other major risk factors identified for SDF tendinopathy are frequent high-speed work, longer race distance, harder racetrack surface, heavier bodyweight and longer training career. Although they were not clearly identified as such, fatigue in relation with exercise duration or lack of fitness and conformation / shoeing (long toe, low heel) might increase the risk of SDF tendon injuries.
In jump racing, SDF tendonitis appeared more common in steeplechasers than in hurdlers, but the reason may be the older age of the first ones rather than the type of racing.
Diagnostic
Assessment of suspected tendon injuries should be based on history and clinical signs associated with diagnostic imaging. In many racing stables, people assess forelimb flexor tendons daily which can help to detect the early lesions of SDF tendinopathy.
However, first signs may be very subtle and variable depending on history, severity and location of injury. They are usually noted within 24 hours of fast work or racing but can also develop at slower work. It is often subclinical and resolves quickly for non-severe injuries with acute lesions characterised by heat, soft tissue swelling and pain on palpation, whilst chronic ones appear with fibrosed thickening.
Overstrain SDFT injuries are classically in the mid-cannon area and present a more or less severe change in profile of the back of the limb leading to the well-known qualification of “bowed tendon”.
However, the obvious signs of inflammation (thickening and heat) are not always present even for some significant injuries and lameness doesn’t appear to be a very consistent feature associated with SDFT injuries. It is typically mild (1 to 2 grades out of 5 at the trot) and improves rapidly over the first week after the injury, however the tendon remains weakened. Consequently, the level of lameness and pain on palpation don’t have a good correlation with the severity of the lesion, except in the most severe cases.
In cases of apparent “bowed” injury with pain response on palpation, it is sufficient to consider there is likely an active tendonitis. In more subtle configuration, the need for ultrasound is indicated to confirm and assess the extent of the lesion.
It may be best to perform or repeat tendon scans at 1 to 3 weeks after clinical injury first noted. Indeed, it allows us to assess lesion severity more accurately because of ultrasonographic underestimation of lesion extent at the beginning of tendon injuries. It is also very important in cases of suspected lesions but initially not well defined.
Moreover, both tendons should be systematically examined on ultrasound for 2 major reasons. Firstly, SDFT tendinopathy are bilateral in up to 67% of cases (Webbon), and secondly it helps to differentiate active lesions versus subclinical changes on ultrasound (ex. “juvenile tendinitis”). A careful ultrasound assessment is also keen to exclude the presence of potential concomitant lesions (ex. SL desmitis).
When SDFT lesions are suspected, the horse should be put at stall rest with only short hand walking until the injury is confirmed or not by ultrasound a few weeks later.
Ultrasound is routinely used by equine veterinarians and is elected to diagnose SDFT injuries as first-line diagnostic imaging. Whilst it is particularly relevant to document tendon lesions, it has been beneficial to develop a scoring system using specific measurements in order to categorise the severity of SDFT tendinopathy.
It is also very useful to establish prognosis and monitor the healing process in line with an adapted rehabilitation program.
Prognosis and return for racing
The prognosis of SDF tendinopathy can be very variable depending on the severity of injury, the convalescence program and the type of racing. Overall, sport prognosis in the Thoroughbred is guarded with a reported return to racing from 20 to 60 % of cases. The major issue of tendon injuries in racehorses is the need for a long recovery and the high rate of re-injury due to poor regenerative capacity of tendon tissue, which is considered as a limiting factor for racing. However, return to training / racing activity is common for most mild / moderate SDFT injuries.
A study with jump racehorses affected by SDFT injuries classifying lesions severity by ultrasound established that all horses with mild lesions returned to training, and 63% raced. 50% of moderately affected horses returned to training, and 23% raced.
In severe lesions, only 30% of horses resumed training, and 23% raced. In the study, the mean of reinjury rate for horses resuming work was 40% over a period of follow-up from 9 to 30 months, but some studies with longer follow-up reported up to 80% of horses sustaining a re-injury. Also, it is remarkable to note that a significant number of re-injuries affect the opposite normal limb.
Definitely, long-term prognosis is influenced by the severity of the lesions. The more severe SDFT lesions are, the lower chance of return to racing, shorter racing career and drop in racing class of those resuming there are. Complete ruptures of SDFT are hopeless for sport prognosis, but paddock life remains possible.
The other factors established to influence the sport prognosis in racehorses affected by SDFT lesions are concomitant lesions, and more particularly bilateral tendinitis which have very poor prognosis. The less classical SDFT lesions like those at the level of carpal or proximal cannon have poorer prognosis for racing and ongoing lameness is frequently present. While it is difficult to study the influence of rehabilitation programs due to the need for a long period of follow-up, controlled exercise showed to provide better prognosis than only uncontrolled pasture rest.
Treatment & Management: How to optimise the healing of tendon lesions?
Contrary to bone, healing of tendon lesions doesn’t allow you to get back pre-injury tissue due to its poor regenerative capacity. It means the structure and function of healed tendons are modified with different mechanical properties. Thus, the aim of SDF tendinopathies’ treatment is to optimise the healing process in order to get a strong and functional repaired tendon as much as possible.
Although there are different options available in the management of SDFT lesions in racehorses, all of them should respect a long recovery with progressive return to work. As said previously, tendon healing is slow, and it is common to consider at least 12 months for return to racing in horses affected by SDFT injuries.
To understand how to manage SDFT tendinopathy, it is important to consider the different phases in the tendon healing process.
In the initial days following the injury, the acute phase is characterised by inflammatory reaction. For a long time, it was advised to control quickly and aggressively the inflammatory response to limit damage to the tendon. However, it is now more and more controversial because the initial inflammatory phase would be beneficial for the repair process of tendons.
The best management of this phase is to treat only in case of excessive pain and acute swelling through the use of anti-inflammatory drugs and cold therapy locally for a period of 3 to 5 days. During this phase, it is important to minimise exercise with stable confinement for the initial weeks. As we discussed previously, the ultrasound assessment of tendon injuries is generally best performed 1 to 3 weeks after the initiation of the injury because it allows to determine the full extent of the lesion. Thus, it is recommended to scan flexor tendons at the end of the acute phase to grade the severity of the lesion and establish a rehabilitation program and prognosis for return to racing activity.
The other crucial period in the management of SDFT tendinopathy is the rehabilitation phase which can begin soon after the inflammation subsides. The cornerstones of healing tendon are the need for time and progressive graded and controlled exercise program. Protocols are quite empirical due to the difficulty to compare long-term outcome with homogenous groups.
Indeed, the program should be determined in relation to the severity of the injury, but classically at least 6 months are necessary for return to cantering. A typical program is to introduce walking once the acute phase has passed with gradual increasing duration until 12 weeks. Ideally, monitoring of healing with ultrasound assessment every 3 months allows to control the evolution of the repair through an assessment of fiber pattern alignment and tendon/lesion size. In normal evolution, trotting can be introduced after 12 weeks and cantering after 32 weeks. Generally, the re-introduction to normal race training is resumed not before 8 to 12 months. Prognosis of SDFT lesions reported for horses rested for less than 6 months is poorer with higher risk of re-injury.
Additional therapies can be used in the aim of optimising the healing of tendon tissue after injuries. Some of them are more and more popular and promising, but it is still difficult to evaluate and compare their efficacy. These modalities have to be considered as an additional intervention to graded exercise programs.
The main interest of these therapies is not to reduce rehabilitation, but to optimise the healing process reducing the chance of re-injury after return to training. These additional therapies range from firing to intralesional therapies with PRP (Platele-rich plasma), PSGAGs (Polysulfated glycoaminoglycans), growth factors (IGF-1) or stem cells. To optimise the efficiency of these therapies, the treatment should be generally realised during the acute phase (more or less 2 weeks after the initiation of the injury).
How SDF tendinopathy can be prevented in racehorses
Prevention is very important due to long recovery and guarded prognosis linked to high re-injury rate. 23–67% of horses with tendon injury treated using conservative methods will re-injure their tendons within 2 years of the original injury.
Strategies with success in preventing/reducing the incidence of tendon injury have not been validated; however, awareness of risk factors associated with SDFT tendinitis provides some useful guidance.
Avoid excessive training to fatigue and permit sufficient recovery time after racing or high-speed training.
Avoid use of poorly prepared or inappropriate track surfaces.
Long-term use of exercise boots/bandages may also contribute to increased risk; magnitude of this risk is unknown but should be balanced against rationale for routine use of bandages in horses that are not prone to interference injuries.
Strategies to reduce risk of reinjury of a rehabilitating/ rehabilitated tendon have also not been validated; however, it is rational to limit excessive loading of tendon.
Possible aspects to assist with above: incorporate treadmill use in training programme; attention to rider weight; minimise horse accruing excessive body condition; ensure maintenance of good dorsopalmar foot balance.
Possible benefit to be derived from regular post-exercise cryotherapy (such as cold water immersion): cooling the lower limb effectively can reduce enzymatic activity in tendon and potentially inhibit cell attrition resulting from high-intensity exercise.
Tracks that are very hard result in higher speeds and increased peak impact loads. These fast tracks are therefore more likely to produce overstrain injuries of tendons.
However, tracks where the surface is uneven, slippery, or shifty seem also to contribute to damaging loading patterns on tendons. Numerous factors influence the mechanical behaviour of a track surface; the weather and track maintenance have a major influence. Moisture content affects all tracks’ mechanical properties, and extreme temperatures appear to affect some synthetic tracks’ mechanical characteristics dramatically.
Experience over years with a particular track type will allow identification of track conditions that may predispose to tendon injuries.
Fatigue is influenced primarily by the horse’s work schedule, level of fitness, and intensity of competition. Fatigue should be considered as a contributor to tendon injuries. With the onset of muscle fatigue, a horse’s stride characteristics change,13 altering the forces on the tendons. Fatigue in any sport results in an inevitable loss of form and coordination in each stride, which is likely to result in an increased risk of injury.
At high speed, lameness may result in excessive loading of the tendons in the contralateral limb.
Horses who are overweight or carrying excess weight will produce greater forces on their tendons compared with lower weight individuals.
Conclusion
In conclusion, tendon and ligament disorders prove to be a major cause of poor performance and lameness within the racing industry. With SDF tendinopathy being at the forefront of these lameness’, there are many strategies that can be adopted to prevent / reduce the incidences of tendon injuries within the thoroughbred.
