Tendon injuries are a common occurrence in the athletic horse. Although injury can occur in any tendon in any type of horse and age, the superficial digital flexor tendon of the lower limb is most frequently affected, particularly in the racehorse and event horses. It is characterised by heat, pain and swelling of the limb with variable lameness. Management of the injury in the early stages is imperative to achieving a favourable outcome.
The role of tendons in the musculoskeletal system is to attach muscle to bone to assist movement of the skeleton. In the horse, as these tendons pass over bony prominences such as joints they are often enclosed in a synovial structure called a tendon sheath (or bursa) to enable frictionless movement of tendons over joints.
They are composed of numerous longitudinal elastic fibres, which can become injured if excessive force is placed on the limb, e.g. during fast speeds. Disruption to these tendon fibres casues inflammation in the immediate post-injury phase. Over time as the tendon heals, the injured elastic tendon fibres are filled with fibrous scar tissue resulting in a tendon with reduced elasticity and loss of normal function.
Injury to the flexor tendon most commonly occurs in the mid cannon region. The severity of injury can vary from mild strains, where little or no lameness can be seen to compete rupture of the tendon where the horse exhibits a severe lameness. The first sign of a tendon injury is frequently an increase in skin temperature over the affected area. This is often accompanied by swelling and pain on digital palpation in the acute stage. These are all signs of the presence of inflammation, as the tendon heals and the affected area is filled with scar tissue the profile of the limb may appear bowed.
The aim of therapy is to maximise the chance for the tendon to function normally, by reducing inflammation in the acute phase and encouraging improved fibre alignment in the long-term. The following steps are aimed at reducing inflammation:
- Rest – depending on the severity of injury, the horse will require a period of box rest to reduce the strain placed on the healing tendon.
- Ice for 1-2 hours per day – remember never to place ice directly onto the skin, always ensure there is a barrier such as a bandage to prevent skin burns.
- Support – a light compression bandage, will help to reduce swelling of the limb. In cases of complete tendon rupture a cast may be applied.
- Non-steroidal anti-inflammatory medication, eg bute (phenylbutazone) – Your vet will be able to advise you if anti-inflammatory medication is indicated.
- Corticosteroids – These are potent anti-inflammatories which may be given by your vet as a one-off treatment in the immediate post-injury phase.
Ultrasound is an essential tool in the diagnosis and management of tendon injuries. In some cases, the tendon lesion may not be visible on ultrasonography for up to 7 days post-injury. Lesions are seen as dark (hypoechoic) areas within the tendon on ultrasound, which represent areas of loss of normal fibre pattern. The cross-sectional area of the tendon damaged will help to determine an appropriate treatment plan and prognosis. Ultrasound examination will be repeated at 3-4 week intervals to assess healing, enabling appropriate adjustments in the post-injury exercise programme to be made.
There are many management and treatment options available for tendon injuries, which indicates that no single one is perfect! Your vet may advocate any or some of the following depending on the type of injury.
Polysulphated glycosaminolglycans (PSGAGs)
Anti-inflammatories which are administered by weekly injection for 4 weeks. Their clinical benefits are not proven at present.
Injected into the affected area of tendon, aimed at reducing adhesion formation. Often combined with corticosteroids to reduce inflammation.
Stem cell therapy
In stem cell therapy the cells are taken from the horses own bone marrow (sternum), and injected into the lesion in the aim of filling the lesion with the horses own cells which can transform into new tendon cells. This therapy is still in the early stages and results so far have been variable.
This is produced from the bladder of pigs and when injected into the lesion, acts as a scaffold for cells to migrate across to fill in the lesion.
Therapeutic ultrasound, extracorporeal shockwave therapy (ESWT) and electromagnetic field therapy are often used but no clinical studies have proved their efficacy over a controlled exercise regime alone.
Depending on the severity of injury healing can take anything between 3 and 12 months. In some cases the horse may never return to its previous level of activity. A favourable outcome will depend on a number of factors and a combination of treatment methods most importantly in association with a controlled exercise regime.