Tendon injuries

Tendon injuries are unfortunately a common problem, particularly in racehorses and eventers but also occur in other sports and riding horses. They need not bring about floods of tears at their diagnosis, as most injuries are treatable. They do however need to be picked up as early to prevent mild injuries becoming more serious.
The most commonly affected tendon is the Superficial Digital Flexor Tendon (SDFT), which sits just under the skin running down the back of the cannon and into the pastern. The other common injury is to the Check Ligament, and less frequently the Deep Digital Flexor Tendon (DDFT), which both lie deep to the SDFT. Suspensory ligament injuries deserve their own discussion.

Why do tendon injuries occur?

Tendons are bundles of elastic fibres, which connect muscles to bones. These fibres can stretch a lot but have a limit to how many times and how far they can be stretched. Over time repeated bouts of exercise (especially at high speed) will wear these fibres out, particularly in the middle of the tendon where they take more of the strain and get hotter than those on the outside. This wear and tear will eventually lead to a point where the fibres can no longer cope and a tendon strain or tear occurs. Poor conformation, working when fatigued and excess high-speed training can also make injury more likely. Tendon injuries get more common as horses get older and aged sedentary horses can also suffer tendon strains or tears.

Traumatic tendon injuries can occur when horses overreach or strike themselves, stumble on rough ground or have a tumble.

After tendon injury occurs the scar tissue that forms in the initial healing is aligned poorly and is not very strong. Our treatment is aimed at improving the quality of repair by encouraging as much normal tendon fibre to develop and for the new fibres to align vertically to provide the most strength and elasticity.

What are the signs?

These vary according to the location and severity of the injury and will be subtle for mild injuries.

• Heat and swelling in the affected area. This is on the middle of the back of the cannon for SDFT injuries. If this swelling is severe it gives rise to the ‘bowed tendon’ appearance.
• Mild lameness for a few days. Some horses stay sound despite injury and injury severity does not necessarily correlate with the degree of lameness.
• Pain when the tendon is squeezed with the foot held up.
• Fetlock drop. In severe injuries. The tendon will feel very soft in this situation.
• Swelling of the tendon sheath or carpal sheath. If the injury is around the back of the fetlock or back of the knee there may not be obvious swelling but the sheath the injured tendon is in will contain more fluid than normal. These horses are usually lame.

This horse has swelling of the SDFT in the mid to low cannon giving the bowed tendon appearance

Ultrasound Scanning

They mainstay of diagnosis is an ultrasound examination. This should be carried out after 5-7 days, as the full extent of the injury may not be apparent if scanned too early. A good quality scanner is essential for examining mild injuries.

The ultrasound lets your vet:

• Assess what (if any) injury has occurred and how severe it is.
• Give you a more accurate prognosis.
• Recommend the best treatment plan for the injury.
• Establish a baseline of the injury so scans can be used to monitor healing over the coming months.

Some injuries are really subtle with only mild enlargement of the tendon and no visible disruption to the tendon fibres. Core lesions are a common injury where the centre of the tendon has a visible hole. Sometimes large sections of the tendon can be affected or occasionally injuries just to the edge of the tendon occur.

Ultrasound scan of a tendon injury. The black area is a core lesion in the SDFT

What do we do?

If you have any concern over heat or swelling in a leg it is best to have your vet examine the leg as a mild tendon injury must be rested or else more severe damage can result if exercise is continued. Picking injuries up early can save a lot of time out of work.

Treatment in the initial 10-14 days after an injury involves;

• Box rest.
• Ice application or cold hosing 2-3 times daily.
• Bandaging.
• Anti-inflammatories. Your vet may give an injection at the initial assessment followed by oral medication.

The above is aimed at reducing the initial inflammation and pain and preventing any further injury. Once the initial inflammation has stabilised a controlled exercise programme can be started.

Controlled Exercise

This is the most important aspect of treatment. Your vet will advise you what is best for a particular injury but will generally start with hand walking while still on box rest with gradual increases every 1-2 weeks.  Expect to be out of full work for 6-12 months, sometimes longer. Tendons heal very poorly and re-injury is common. Controlled exercise helps the new tendon fibres align up and down the tendon ultimately resulting in increased strength and flexibility. The effort put in with the controlled exercise early on will reap rewards later. Repeat ultrasound scans should be carried out every 3 months to monitor treatment and adjust the exercise programme accordingly.

Other treatments

Over the years there have been multitudes of treatments recommended for tendon injuries, reflecting the fact that many aren’t terribly helpful in the long run!

Current treatments include:

• Tendon injections.  A group of drugs called PSGAG’s can be used successfully to inject tendon injuries and assists in short term healing. The most exciting recent development in tendon treatment has been the advent of Stem Cells and of Platelet Rich Plasma, both of which are injected directly into the tendon soon after injury and are discussed further below.
• Tendon splitting or fenestration. A scalpel incision or lots of needle holes can be made to release the initial blood clot from a core lesion and this may also help new blood vessels to grow into the injured area. This needs to be done within 1-2 weeks of the initial injury to be helpful.
• Surgery to cut the accessory ligament of the SDFT. This allows the muscle and tendon to stretch further and reduces the incidence of SDFT re-injury. This needs to be performed under general anaesthesia. It has fallen out of favour due to other alternatives and an increased chance of a suspensory ligament injury after surgery.
• Tendon Firing. This has been carried out since the Middle Ages, and still is, despite evidence to suggest it does not really improve outcome.  Blistering is also performed occasionally. These counter-irritants are used to increase the blood supply to the injured area. Rest is essential after firing or blistering and is generally enforced by the inflammation caused.

Platelet Rich Plasma

Platelets are little blood cells responsible for clotting blood. They contain many growth factors. When injected into a tendon the growth factors encourage new blood vessels to grow into the injury site and more normal tendon fibres to develop. The horses’ blood can be taken and the platelet fraction harvested and injected immediately into the tendon. This is done under light sedation with local anaesthetic.

Stem Cells

Stem cells live in the bone marrow and have the ability to grow into many different types of tissue, according to where they find themselves. Bone marrow also contains lots of growth factors, much like platelets. Bone marrow can be taken from the horses pelvis or sternum and sent to a specialist lab to grow millions more stem cells. These are then be injected back into the injured tendon (also under sedation and local anaesthetic) suspended in liquid from the bone marrow that contains the growth factors. The stem cells then turn into new tendon cells (a process that happens very poorly normally) providing a better quality of tendon repair.

Bone marrow being harvested from the sternum for stem cell culture

While we await long-term results of PRP and Stem Cell treatments, the studies carried out on the quality of tendon repair with their use is very encouraging and they have provided the first real advance in tendon treatment for a long time.

None of the treatments available reduce the horse’s lay off period. The real aim is to reduce the risk of re-injury on return to exercise.

Will my horse come back from this?

Usually yes. 80% of racehorses with tendon injuries will return to racing and while half of those will eventually re-injure other disciplines carry a much better success rate given appropriate rest and controlled exercise.

Our goal is always to prevent injury, but when it does occur to diagnose and treat it early to provide the best possible outcome.

M Georgetti, BVSc, MACVSc, MRCVS
(This article appeared in the July 2010 issue of Central Horse News)