Racing on the edge - The new research perspectives on SDFT and SL injuries in thoroughbreds
Article by Laura Steley
Injuries to the superficial digital flexor tendon (SDFT) and suspensory ligament (SL) are sadly a common occurrence in racehorses, with jump racing in particular placing these structures under intense mechanical stress. In a large UK study of 1223 UK National Hunt racehorses over two seasons, the rate of tendon and suspensory ligament injuries (TLIs) in training was 1.9 per 100 horse-months at risk. Of these, around 89% were SDFT injuries, with the remainder being SL injuries. The study also found that risk of TLIs varied by trainer and increased with the horse’s age. (Ely et al., 2009). Another noteworthy study performed regular ultrasonography on 263 National Hunt horses over two seasons, concluding that 24% of them showed structural changes of the SDFT, even before an injury was clinically evident. (Avella et al., 2009).
Focussing primarily on the SL, a study found SL injuries in racehorses to have an incidence of 2.7 per 1000 starts, with a prevalence range of 3.6% to 10% depending on type of race and population studied. (Davies et al., 2021). In flat Thoroughbreds, rates of SDFT tendinitis in training/populations range between roughly 3.4% and 11.1%, and SDFT injuries are responsible for a significant portion of horses retiring from their racing careers. (Kasashima et al., 1999).
These two structures lie at the heart of the suspensory and passive stay apparatus, functioning like elastic springs which absorb energy and aim to prevent excessive extension of the fetlock. When the strain placed upon them is too great, too frequent, or inadequately managed, micro-damage builds up over time, leaving the limb vulnerable to more serious breakdown. This makes SDFT and SL injuries among the most significant welfare, performance, and economic concerns in the horse racing industry.
It is for this reason veterinary scientists and researchers alike will not be deterred from trying to find answers to the critical questions of; why the prevalence of this type of injuries is so high, what we can do to help prevent them occurring and what is the best rehabilitation strategy when the worst happens.
Anatomy & Function
The SDFT is the continuation of the superficial digital flexor muscle, the top of the tendon attaches to the humerus, radius and ulna. The tendon proper begins on the back of the lower radius and extends down the surface of the limb, lying superficially to the deep digital flexor tendon (DDFT) and making it readily palpable along the length of the metacarpus or commonly known, cannon bone. Enclosed within the flexor tendon sheath, the SDFT glides over the DDFT and suspensory ligament, maintaining close anatomical relationships that are critical to their function. Just above the fetlock, the superficial flexor tendon forms a small “tunnel” of tissue that the deep flexor tendon passes through before continuing down the leg. At the level of the pastern, the SDFT divides into two branches that insert on both the long pastern (P1) and the short pastern (P2). Functionally, the SDFT acts not only as a flexor of the pedal bone/foot (P3) but also as a vital part of the passive stay apparatus, resisting over-extension of the fetlock and pastern joints. Its elastic properties allow it to store and release energy during locomotion, making it a key contributor to the horse's stride efficiency.
The SL is a highly modified ligament that originates from the upper, back area of the cannon bone, with fibres also arising from the bottom row of carpal bones. From its broad origin, the SL runs downwards along the cannon, positioned deep (or below) to both the superficial and deep digital flexor tendons. In the mid-region of the cannon its fibres become more discrete and near the bottom third of the cannon the ligament divides into medial and lateral branches. Each branch curves around the outside of the upper sesamoid bones at the back of the fetlock, before continuing forward as the extensor branches, which attach into the main extensor tendon running down the front of the leg.
Functionally, the SL is the cornerstone of the suspensory apparatus, supporting the fetlock and preventing its overextension under the heavy loading of movement. Together with the sesamoid bones and lower sesamoidean ligaments, it acts as a dynamic support structure, storing elastic energy and releasing it to aid forward propulsion. Its critical role in both stability and energy efficiency explains why the SL is so frequently impacted by injury and disease.
In conclusion, the SDFT and SL are very interconnected, both anatomically and functionally. The SDFT primarily provides flexion of the foot and helps support the fetlock and pastern, while the SL suspends the fetlock and forms the foundation of the suspensory apparatus. Both are integral to the horse’s remarkable efficiency of movement and endure significant forces during training and on the racecourse.
Recent findings
A study at the Royal Veterinary College (Hertfordshire) by Hanousek and colleagues (2024) investigated how injury to the palmar supporting structures of the fetlock, the SDFT in the forelimb and the SL in the hindlimb, affects limb biomechanics. Using a retrospective cohort of clinical cases, the authors measured limb stiffness and fetlock conformation in injured and uninjured horses with a validated, non-invasive technique combining floor scales and electrogoniometry (electronic sensors to measure joint angles during movement). In uninjured horses, forelimb stiffness was found to be significantly greater than hindlimb stiffness, reflecting their different roles in locomotion, while fetlock conformation did not differ between limbs. Age did not influence stiffness in this mature population (aged between 3yo and 35yo). This corresponds with the knowledge that tendon maturity is thought to be reached between 2 and 3 years old.
In horses with forelimb SDFT injuries, there was no long-term difference in stiffness or conformation between the injured and opposite uninjured limbs, even with follow-up examinations extending beyond three years. This suggests that SDFT injuries, despite fibrotic healing, may recover mechanical function sufficiently to restore overall limb stiffness. By contrast, hindlimb SL injuries were associated with both increased limb stiffness and greater fetlock extension, consistent with elongation of the ligament following injury. These changes persisted regardless of injury duration, indicating a lasting alteration in the biomechanics of the hindlimb fetlock after SL damage.
The findings highlight important distinctions between the biomechanical consequences of SDFT and SL injuries. While SDFT lesions appear to permit functional compensation within the limb, SL injuries lead to measurable and persistent changes in stiffness and fetlock angle. Clinically, these differences may explain the poor prognosis often associated with chronic suspensory disease and underline the importance of fetlock support during rehabilitation.
A review paper published last year, by Guest and colleagues provides a comprehensive analysis of the SL, emphasising its central role in athletic performance and its high susceptibility to injury. The review highlights that, despite the clinical significance of SL disease across a range of equestrian disciplines, research into its anatomy, biomechanics, and pathology falls short of that of the superficial and deep digital flexor tendons. The SL is described as a unique structure; an evolutionary adaptation from a muscle to a predominantly fibrous ligament that functions both as a key component of the suspensory apparatus and as an elastic energy store during movement.
The paper explores how the specialised functions of the SL open it up to being particularly vulnerable, especially in sports horses where the SL operates close to its mechanical limits. Large-scale studies on the prevalence and risk factors for SL injury are still relatively scarce; however, SL injuries consistently represent a large proportion of musculoskeletal conditions, with particularly high prevalence in racehorses. Prognosis is often guarded, as mentioned in the previously discussed study, especially in chronic hindlimb disease, where persistent biomechanical changes and reduced performance are common. The take home message of the review is the need for greater research attention to this structure, suggesting that improved understanding of its biology, loading environment, and healing response is critical to advancing prevention and treatment strategies.
In summary, Hanousek et al., 2025, provides experimental evidence of the long-term biomechanical consequences of SL injury, showing persistent increases in limb stiffness and fetlock extension in affected hindlimbs. In contrast, Guest et al., 2024 shines the spotlight on how the SL remains comparatively understudied despite its clinical importance, noting limited epidemiological data and gaps in our understanding of its biology, biomechanics and healing.
Advice from the frontline . . .
Tendon injuries remain one of the most frustrating problems in racing, hence the subject being debated and discussed in great depth at the Beaufort Cottage Educational Trust lectures, which recently took place in Newmarket, UK.
Mark Johnston’s perspective was a reminder of the hard reality: in flat horses especially, very few make a full return to their peak once a tendon has gone. Many may race again but shortened careers and recurrent issues are common. Johnston feels ultrasound is useful, but nothing replaces the trainer’s eye and feel, looking at tendon profile, monitoring for bowing, and using hands to assess changes. The advice is to act fast: throw every anti-inflammatory option at the injury in its early stages, manage workload with care and accept that rest alone rarely works. Prevention, early detection, and realistic expectations are the trainer’s best tools. He also raised the question of whether tighter rules around racecourse medications might now limit the ability to prevent some injuries that, in his experience, were reduced when prophylactic treatments were allowed.
Professor Roger Smith discussed the science behind why these injuries happen. He explained that the SDFT is like an elastic spring storing and releasing energy every stride, but in Thoroughbreds it is working close to its limits. Damage often builds up gradually, causing small changes in the tendon matrix. Age and repeated training reduce the tendon’s ability to repair itself and enzymes released after hard work can make things worse. His strongest advice was about prevention: avoid hard ground, spread out intense sessions, and ideally allow 72 hours between big gallops to let the tendon recover. He also highlighted the value of early detection using imaging like ultrasound, Doppler, or UTC to spot problems before they become injuries. When things do go wrong, rehabilitation has to be long, structured, and monitored closely, with new biologic treatments like stem cells and platelet-rich plasma showing promise for improving repair and reducing re-injury.
From a purely rehabilitation viewpoint, Amelia MacArthur discussed how she manages tendon injuries within her rehabilitation yard. Her message was clear: one size does not fit all. Every horse’s recovery should be tailored to its injury, temperament, and long-term goals. She uses tools like a water treadmill, deep sand gallop, and hacking, but stresses the basics first: controlled hand-walking, safe handling, and even simple in-stable physiotherapy exercises. Ultrasound checks and gradual increases in loading are essential, and work on surfaces must be varied and adjusted depending on the case. She also drew attention to rider weight and balance, something which is often overlooked and showed how even body condition can make or break a horses recovery.
The rehabilitation advice following a SL injury is much the same as with tendons, with an emphasis on early controlled exercise, progressive loading and close monitoring. As we discussed earlier, recovery can be more challenging due to the SL’s role in supporting the fetlock and its tendency to heal with elongation and prognosis is generally more favourable for forelimb injuries than hindlimb cases. Ultimately, the best approach is prevention through thoughtful training management, early detection, and timely intervention, rather than reliance on rehabilitation alone once major damage has occurred.
Our industry strives to not only continue to provide the best possible care of our equine partners in this wonderful sport but also further improve welfare and decrease injury rates on and off the racecourse. Across Europe standards are high, with close monitoring and consistent improvements made by bodies such as the IHRB and the Equine Injury in Irish Racing Risk Reduction Project, France Galop and the Equine Welfare Charter, the BHA and Horse Welfare Board. It is safe to say with all the dedicated research, fast developing technology and the support of our governing bodies we are on the right track!
