Slipped hock is the term used to describe an injury where the superficial digital flexor tendon becomes dislocated from its normal position at the point of the hock, to one side.
The tendon itself is a continuation of the superficial digital flexor muscle (see diagram, left), which is attached in the upper part of the limb at the stifle. The muscle turns into tendon about halfway between the stifle and hock, and the tendon then runs in a straight line over the calcaneous (the point of the hock) where it is held in place by strong, thick bands of tissue attaching to the bone on either side. To lubricate the passage of the tendon, there is also a fluid-filled bursa which sits between the tendon and the cartilage-covered bone surface.
A slipped tendon is an uncommon injury, which results from the tearing of one of the attachments of the tendon to the bone at the point of the hock. Once the attachment is torn, there is no longer an anchor to keep the tendon from slipping off to the side.
Although the tendon can slip off either side, most commonly this happens to the outside after the inside attachment has torn. The exact cause of tearing is often unknown, but it usually occurs during fast exercise – whether the horse is galloping in a paddock or being ridden. Mis-jumps or falls are the cause of tearing in some horses.
All types can be affected by the condition, from leisure horses to racehorses. There is no specific conformation which seems to predispose to this problem. Unlike the acute tears and injuries described so far, occasionally a horse with straight hocks and hyper-extending fetlocks has been observed to suffer from the condition, sometimes in both hind limbs. In the cases I’ve seen, however, this is a less common cause of the condition.
Immediately after suffering the injury, the horse will be obviously lame and will quickly develop severe swelling at the point of the hock.
Quite commonly, the attachment does not tear completely, meaning that the tendon slips on and off the point of the hock depending on the position of the leg. This can cause the horse anxiety and often leads to him panicking due to the instability of the tendon. One of the most useful treatments at this stage is sedation, particularly ACP which can relax the horse. He should also be treated with pain relief and anti-inflammatory treatments to improve comfort.
In cases where the attachment has torn completely, the tendon will luxate (slip) to the side and stay in that position. This is called a stable luxation. Diagnosis can usually be made by careful clinical examination, although this may be more difficult shortly after injury due to swelling.
In unstable luxations, the horse will need to be seen in action to diagnose the condition, where the tendon can be seen moving abnormally at the point of the hock. Ultrasound examination enables identification of the torn attachment, along with the extent and location of the damage.
Treatment can be either conservative or surgical. In horses with stable luxations, conservative treatment is appropriate and consists of rest and anti-inflammatory medications.
This approach can also be used in unstable luxations. In recently reported cases from the University of Zurich, five out of eight horses were able to return to some level of work eventually. Typically, this takes between six and nine months, although a mild mechanical lameness should be expected long term. In those horses unable to return to work, most will become pasture sound.
It is generally accepted that surgery gives the best chance of returning the horse to athletic work long term, by offering a definitive treatment for unstable luxations — when the tendon is continually slipping on and off the point of the hock.
Two types of surgery can be performed. The first and more traditional treatment is to repair the torn attachment and re-position the tendon back into its normal position. When successful, this returns a horse back to work with a normal gait.
The surgery requires quite major dissection, however, and carries significant risks — including infection and failure of the repair. Cases treated surgically in the recently reported series from Zurich University documented a third of cases (three out of nine) not surviving until discharge from hospital. Of the six that did survive, however, four returned successfully to work.
A novel surgical approach, pioneered by Newmarket Equine Hospital surgeon Ian Wright, involves using keyhole surgery to cut across (transect) and remove the damaged attachment. This creates a stable luxation of the tendon that is positioned permanently off to the side of the point of the hock.
Good success has been achieved with this approach, with most horses (six out of seven) returning to work and being used for varying activities. A persistent alteration to the horse’s action may be expected due to the altered position of the tendon and it may be more significant in certain disciplines.
Treatment options must be considered carefully, with decisions made only once the owner is aware of the chances of success — and the risks of complications — with different options.
Ref Horse & Hound; 17 August 2017