Tendon Injuries

By P.R.E.Cantor BVSc, MRCVS

One of the greatest worries to owners and trainers of point-to-pointers, apart from a fatal accident, must be that their horse will sprain a tendon which is often referred to as “breaking down“. The damage to the tendon may be slight and therefore hardly noticeable, or fairly severe which may result in a “bowed tendon”. Such injuries are nearly always athletic related and therefore occur most commonly in racehorses, hunters and eventers.

A bowed tendon occurs when a horse ruptures a large number of fibres in the superficial digital flexor tendon of a forelimb, usually just behind the cannon bone, but can extend to below the fetlock. Normal healthy tendon fibres run parallel to the tendons and are very slender but, following rupture of some of the fibres, haemorrhage occurs which causes swelling in the tendon and disruption in the alignment of the fibres. During the healing process the ensuing cross linking between the damaged fibres and the build up of scar tissue results in thickening of the tendons and hence the bowed appearance. This build up of scar tissue causes tendon shortening, restricts flexibility and therefore increases the chance of re-injury.

In the past it was thought that the majority of tendon injuries involved the deep flexor tendon, but with the advent of ultrasound scanners it has now been proved that it is nearly always the superficial digital flexor tendon that is damaged. The reason for this is that the SDF takes most of the loading early in the weight bearing phase of the stride.

With the scanner it is possible to examine the tendon fibres both longitudinally and in cross section, and therefore one can accurately see what proportion of fibres are damaged, and in which area of the tendon. In my experience the greater majority of tendon injuries are in the mid cannon region.

Injury to the SDF tendon occurs either as the result of external injury (e.g. being struck into) or in the majority of cases the tendon being stretched beyond its elastic limit. In the latter case there are innumerable factors which may be responsible, some of which can be listed as follows.

1.Fitness of the horse - Invariably horses “break down” in the last stages of a race when they are tired and become incoordinated, which puts more stress on the tendons. The fitter the horse the less likely it is to tire and theref ore break down.

2.Ground surface - obviously the heavier the going the quicker horses will tire.

3.Weight - The more weight a horse carries, either as rider, saddle or weights, again the earlier it will tire.

4.Rider - A good rider is invaluable. Knowing a horse’s capabilities will prevent “overstretching” the horse.

5.Shoeing - It is essential that the horse is well shod and balanced in order to give it the best possible action. In addition particular attention should be given to the foot pastern axis since the longer the toe of the hoof the more effort is needed f or the horse to break over at the toe and the more strain is put onto the SDF tendon.

6.Conformation - Good conformation gives better action and therefore reduces fatigue.

7.Bandaging - If bandages are not applied properly this can cause ischemia (deficiency of blood) in the tendons. It is thought that prior to injury of a tendon heat is generated in the centre of the tendon and this causes the damage. Bandaging other than for protection may therefore be considered undesirable.

If there is any doubt that a tendon has been injured it should be examined by a veterinary surgeon immediately, in order to prevent further damage. All tendons should be scanned post injury but this is best delayed for 5 to 7 days, because initially the limb is too painful to allow detailed examination with the scanner and, also, the true extent of the damage cannot be ascertained. At the time of injury the inflammatory exudate produced at the site of the sprain causes further destruction to the damaged as well as the healthy fibres and therefore the lesions enlarge during the first few days after injury. The healing of the tendon should then be monitored at intervals by scanning.

Treatment for a sprained tendon should be instigated immediately. A non steroidal anti-inflammatory drug (e.g. phenylbutazone) should be administered, cold compresses applied to the affected limb, and a support bandage applied to the opposite forelimb. The cold compresses and cold hosing should be continued for 48 hours after which cold applications should be applied during the day and warm at night. One thing I cannot emphasise enough is that you must be extremely careful with the bandaging. Owing to the swelling caused by the injury, bandages are often applied too tightly which can result in some horrific pressure sores. I have seen some cases where these sores have resulted in worse damage than the initial tendon injury. I would therefore recommend that when bandaging one should preferably use a soffban followed by two layers of gamgee under a bandage.

During this initial inflammatory phase, which lasts 1 to 2 weeks, the horse should be stabled. It is often thought that a raised heel or rest shoe should be fitted, but this is contra indicated since by raising the heel one is reducing the tension of the deep flexor tendon and increasing it of the superficial digital flexor tendon.

Following the inflammatory phase, within a few days the repair phase begins during which the formation of scar tissue occurs. Finally there is the remodelling phase when the nature of the tendon fibres change and the tendon begins to fine down and take a better shape.

During these latter two phases the horse can and should have controlled exercise and can then progress to being turned out in a confined area, before eventually going out to pasture.

In addition to conservative treatment various other procedures are often carried out in an effort to improve the healing process and possibly reduce the rest period such as tendon splitting, check ligament desmotomy, physiotherapy or pinfiring, the efficacy of which are debatable.

For many years the hunt has been on to find drugs which would improve the quality of healing and reduce the convalescent time. One such drug which has been available for some time is Adequan which can either be injected into the lesion or given by intramuscular injection. This is believed to improve the quality of the repair tissue but does not reduce the rest period. The latest drug to be tested which was heralded as a “miracle cure” is called BAPTEN (Beta-aminoproprionitrile fumarate). This drug prevents the formation of cross links and therefore improves re-modelling of the tendons, but it does not accelerate healing; it is administered intra tendinously 1 to 3 months after injury via multiple needle punctures every other day for at least 5 treatments. However this drug is still experimental and is not licensed for general use.

Tendons have a relatively poor blood supply and therefore are slow to heal and, to date, there are no medications or treatments which can greatly reduce the convalescent times, which are roughly as follows;

Mild injury 6-9 months

Moderate injury 9-12 months

Severe injury 12-18 months

Tendons which have been injured should be scanned prior to the horse being put back into work, in order to establish that healing is complete. The scans will also show the quality of healing which will be a good indication as to the future ability of the horse.