Superficial digital flexor tendinopathy

What it is

Superficial digital flexor tendinopathy (tendonitis, "bowed tendon") is a partial rupture of the fibres of the superficial digital flexor tendon. This tendon works as a spring: it stores elastic energy as the fetlock sinks during stance and returns it at push-off, and it works very close to its failure limit at the gallop. With age and exercise it accumulates microdamage and degenerative changes in the matrix without clinical signs; clinical injury appears when a load exceeds the strength of an already weakened tendon. It is therefore considered an overstrain injury on top of prior degeneration rather than an isolated trauma.

The typical lesion is a core lesion in the mid-metacarpal region, where the cross-section of the tendon is smallest. The repaired tissue is a scar that is less elastic than normal tendon, which explains the frequent reinjuries, in the same tendon or the contralateral one.

Who it affects

Mainly racehorses, jump racers more than flat racers, and high-level event horses and showjumpers. Risk increases with age and years of training, and is higher in entire males. The thoracic limbs are almost always affected; in the pelvic limb the injury is rare and usually affects the hock region, where the tendon can luxate off the calcaneus.

Anatomy involved

• Superficial digital flexor muscle of the thoracic limb: its muscle belly, on the caudal aspect of the forearm, continues as a tendon that runs down the palmar aspect of the metacarpus, the most superficial one in the region, and inserts on the first and second phalanges. The accessory ligament (proximal check ligament) joins it to the radius and is part of the stay apparatus: it lets the horse bear weight standing with very little muscular effort. The lesion lies mainly in the middle third of the metacarpus, although it can extend into the fetlock and pastern.
• Superficial digital flexor muscle of the pelvic limb: in the horse it is almost entirely tendinous; it forms a cap over the calcaneal tuberosity and runs down the metatarsus. Its injuries are much less common than those of the thoracic limb.
• Metacarpus: the tendon lies on the palmar aspect of the third metacarpal bone, superficial to the deep digital flexor tendon and the suspensory ligament, which lies against the bone between the splint bones. Ultrasonography is described by zones along the metacarpus.

Clinical signs

Heat, pain on palpation and swelling of the palmar aspect of the metacarpus, which in severe injuries gives it a curved ("bowed") profile seen from the side. Lameness may be marked in the acute phase but is often mild or resolves within a few days even when the lesion is significant. Small lesions may show only as thickening or pain on palpation of the tendon with the limb lifted, especially after training.

Diagnosis

• Careful palpation of the tendon with the limb weight-bearing and lifted, comparing both limbs.
• Ultrasonography: confirms the lesion and measures its extent (cross-sectional area, percentage of the tendon affected, echogenicity and fibre alignment). It is repeated every few weeks or months to guide the return to work.
• Ultrasonography is best done a few days after injury, when the core lesion is well defined.
• Differential diagnosis: lesions of the suspensory ligament, the deep digital flexor tendon and its accessory ligament, digital sheath tenosynovitis, and trauma to the skin and subcutaneous tissue.

Treatment and prevention

• Acute phase: cold, compression bandaging, box rest and non-steroidal anti-inflammatory drugs for the first days.
• Controlled exercise programme: walking in hand and a very gradual return to work over 9–12 months or more, guided by ultrasonographic checks. This is the basis of treatment and cannot be shortened.
• Intralesional treatments: bone marrow- or adipose-derived mesenchymal stem cells, platelet-rich plasma and other biological products. There are few controlled trials and the quality of the evidence is low (see Key studies).
• Surgery: desmotomy of the accessory ligament of the superficial digital flexor tendon and tendon splitting for acute core lesions have variable results.
• Prevention: progressive training, avoiding firm ground, monitoring the tendons of horses in training by palpation and ultrasonography, and withdrawing from intense work those that show changes.
• The horse is a food-producing animal in many countries and drug use is subject to national regulations and withdrawal periods. In the EU, the equine identification document (passport) states whether the horse is excluded from slaughter for human consumption, and this decides which medicines may be given.

Key studies

• Kasashima et al. (2004), retrospective study of 10,262 flat-racing Thoroughbreds registered with the Japan Racing Association in 1999. Forelimb superficial digital flexor tendonitis affected 11.1% (1,130 cases) and suspensory desmitis 3.61%; in the hindlimb there were only 6 and 14 cases. The risk of tendonitis was higher from 3 years of age and in entire males. It is based on the veterinary records of a single organisation.
• Reardon et al. (2012), retrospective cohort of hurdle race starts in the UK between 2001 and 2009, with 1,031 starts with injury and 168,637 without. Firmer going, older age at first race, a previous tendon injury and racing in summer were associated with higher risk; racing more often in the previous 9–12 months and a more successful trainer, with lower risk. It only includes injuries during races, not in training.
• Dyson (2004), comparative study of 219 horses with superficial digital flexor tendonitis followed for at least 2 years after returning to work. The reinjury rate was 42.5–44.4% with controlled exercise alone and with hyaluronan or polysulphated glycosaminoglycans; with beta-aminopropionitrile fumarate it was 16% in the treated limb, but the overall injury rate, including the contralateral limb, did not change. Reinjury risk increased from showjumpers to event horses to racehorses. Groups were not randomised.
• Godwin et al. (2012), cohort of 141 racehorses treated with bone marrow-derived mesenchymal stem cells and followed for at least 2 years. There were no adverse effects, and reinjury was 27.4% in the 113 with follow-up, lower than published for jump racehorses with other treatments. The comparison was with historical controls from other studies, which limits the conclusion.
• Geburek et al. (2016), randomised, placebo-controlled clinical trial in 20 horses: a single intralesional injection of platelet-rich plasma (n = 10) or saline (n = 10). Lameness decreased significantly sooner with plasma (week 8 versus 12) and the repair tissue was better organised, with no difference in tendon cross-sectional area. At 12 months, 80% of treated horses versus 50% of controls had regained their level of performance, and at 24 months, 60% versus 50%. It is a small trial.
• M'Cloud et al. (2024), systematic review and meta-analysis of 21 studies with a comparison group on stem cells and platelet-rich plasma for tendon and ligament injuries (17 in the meta-analysis). No biological treatment increased the likelihood of returning to performance, but stem cells, alone or with platelet-rich plasma, were associated with fewer reinjuries. Most studies were of low quality, heterogeneous and at high risk of bias.