Superficial Digital Flexor Tendon
Injuries to the SDF tendon of the hind limb are something we see quite often but their nature differs from that of the forelimb. In the fore limb, rupture through repetitive strain is by far and away the most likely origin of injuries. In the hind limb however, injuries tend to fall into two groups.
1. Physical trauma
A common injury in racing is when a fore foot of a chasing horse catches the hind limb of a horse in front. The resultant wounds can go straight through the skin and damage the underlying SDF tendon which lies just under the skin. There are no overlying muscles and little fat beneath the skin to protect the tendon. Kick injuries are also a frequent cause of injury to the hind limb SDF tendon. This is most worrying when the wound is on the point of the hock and damages the SDF tendon directly underneath where it runs over the bone. The tendon is under much pressure at this point so any injury is under a lot of strain. This affects its ability to heal. There is also a bursa at this site. This is the structure that, when inflamed, gives rise to “capped hocks”. A wound into this bursa means infection can become established. Unless this is treated aggressively and urgently, then recovery is unlikely.
Most SDF tendon injuries of the hind limb are treated with supportive management to allow the wound to heal. We then begin a gently ascending walking out programme to stimulate tendon fibre alignment and ensure the fibres heal in plane of workload. This minimises chances of re-injury.
One piece of good news is that an SDF injury as a result of a wound means that the tendon either side of the injury is healthy. This means that if the correct environment can be established (free of infection, skin healing, gentle exercise), healing can be very successful and complete.
2. Subluxation of the SDF tendon off point of the hock
As mentioned earlier, the SDF tendon runs down the back of the limb and passes directly over the point of the hock. It is held firmly in position by connective tissue at this site but it is the point of greatest pressure in the horse’s limb. At strenuous exercise, the SDF tendon can “break free” of its moorings. Needless to say this causes a sudden and profound lameness. After several days’ rest and pain relief, it will be noted that the architecture of the hock has changed with the tendon now running down the side of the hock, sometimes the inside but usually the outside. Although surgery can be performed to replace the tendon to its original site, this is considered quite heroic and unfortunately is rarely successful. Much more usually, the horse is gently recuperated with an ascending programme of exercise. While active competition will no longer be possible, low grade ridden exercise should be achievable.
The hind limb suspensory ligament is a frequent source of lameness, particularly in dressage and event horses. Injury is most usually the result of repetitive strain. It often presents as a mild lameness affecting both hind limbs because both suspensory ligaments have been injured.
Sometimes it presents not as a lameness at all but rather with owners noticing minor changes in gait e.g. transitions from trot to canter are not as smooth as before or that the horse moves better on one rein than another. While the lameness is usually low grade, recovery from hind limb suspensory injuries can be slow. Shock wave treatment can be helpful but is not as successful as when used in the corresponding fore limb injuries. As a result, many Vets now encourage owners to consider a surgical procedure where the nerve supplying the ligament is cut. Although a relatively minor procedure, it must be performed under general anaesthetic. While it is often very successful, it contravenes FEI rules so such horses are not eligible to compete at higher levels of competition.
When treated by conservative means, recuperation involves a gently ascending programme of walking exercise from the stable, gradually increasing to straight line trot work. If the surgical route is taken however, recovery is generally much faster and patients can sometimes be back in full work within 8 weeks.
Annular Ligament Desmitis
The annular ligament is a structure that surrounds the tendons where they pass down the back of the fetlock. The structure is present in both fore and hind limbs and is most commonly injured in ponies. It is damaged when the fetlock is overly extended, putting pressure on the ligament. Treatment involves the injection of steroid into the sheath which lies within the ligament. The steroid injection is allied to a gently ascending exercise programme from stable rest. Recovery is usually complete. In those ponies where recovery is not as good, then an operation on the injured ligament can be performed which carries a high degree of success.