A “blown” or damaged tendon has signalled the end of many horses’ successful racing or competition careers, although many horses do successfully return to riding and sometimes competition after a significant recovery period.

One of the most startling true facts about tendon injury is that the degree of lameness is not related to the degree of inflammation in the tendon itself. Confusingly, some horses can have quite extensive damage without being very lame, or the affected limb being very swollen. Equally, subtle thickening in the cannon region may be related to substantial tendon damage.

“Essentially, a damaged or ‘bowed’ tendon occurs when one of the tendons or ligaments responsible for supporting the leg has been torn by being overloaded. Although this is a sudden event, the harbingers would have been laid down many months beforehand,” says Roger Smith, professor of equine orthopaedics at the Royal Veterinary College.

“A useful metaphor is to think of the tendon or ligament as an old rope fraying. At any point in time, the tendon can tear and as soon as it does there’s a large inflammatory response with bags of swelling,” continues Prof Smith.

Vets generally organise tendon or ligament injuries into three levels of severity — mild, moderate and severe — and generally the more severe the damage, the poorer the prognosis.

When a vet sees a horse with swollen soft tissues at the back of the cannon bone, they have to decide which structures are damaged and how severe the damage is.

The tendons and ligaments in the horse’s lower leg include:

  • the extensor tendon at the front of the leg
  • the superficial and deep digital flexor tendons at the back of the leg
  • the suspensory ligament which runs between the bones and flexor tendons
  • the check ligament, just below the knee

The superficial flexor tendon and suspensory ligament support the fetlock. Damage to other structures such as the check ligament can occur in conjunction with injury of the superficial flexor tendon.

Injury to the check ligament tends to result in swelling centred closer to the cannon bone than with an injury to the superficial flexor tendon, which displays the classic “bow” at the back of the leg.

Damage to the suspensory ligament can occur in many sites and the clinical signs may vary. Damage that is limited to the proximal (top) part of the ligament may show only subtle lameness with minimal swelling and is common in the sport horse. This condition can be hard to detect, whereas damage lower down the suspensory usually shows more obvious swelling.
Injury to the deep digital flexor tendon is unusual but when it does occur it is normally at the level of the fetlock or below, within the tendon sheath. There is a much poorer prognosis with injury to this tendon and, frequently, persistent lameness.

The first step of diagnosis will be palpation, which will give the vet a good idea of which structure is involved, but it will be hard to tell how bad the damage is.

Next, the vet will examine how the fetlock on the injured side behaves in comparison to its pair. If there has been significant damage to the flexor tendons, the fetlock will drop towards the ground. If the suspensory ligament is severely damaged, the fetlock will be nearly on the floor.

Examination by ultrasound is the final stage of diagnosis and should take place around one-week after the injury occurred to be of most use.

Prognosis following injury depends primarily on the level of damage and the horse’s job, according to Prof Smith.

“The problem is that tendon heals well with fibrous tissue but this is not as good as the original tendon tissue and the horse will always be at risk of re-injury. It is very difficult to give an accurate prediction of the outcome. Quoted success rates range from 2%-90%.”

Horse & Hound (18 December 03)