Tendinitis

Tendinitis refers to inflammation within a tendon, which in severe cases can be a career ending injury. Any tendon in the horse’s limbs can be affected but by far the most commonly involved is the superficial digital flexor tendon (SDFT) which runs down the back of the cannon bone. This article will focus on SDFT tendinitis. 

 The SDFT, together with the suspensory ligament prevents the fetlock from hitting the ground when the leg is in the weight bearing phase. At speed the pressure placed on these two structures is enormous and at the gallop, the SDFT is loaded close to its mechanical limits. With the rare exception of traumatic tendon injuries (ie. a kick or blow to the tendon directly) most cases of tendinitis are the result of repetitive strain on the tendon. Muscle fatigue, poor conformation or foot balance and lameness in another limb place the tendon under increased strain and predispose to injury. 

Tendinitis is typically associated with fast exercise and so the condition is more common in eventers and racehorses than dressage horses, but injuries can also occur in the paddock so any horse could be affected. 

When tendons become injured, a complicated series of events occurs at a cellular level resulting in breakdown of tendon fibres, cell death, blood vessel damage, haemorrhage into the injury site and inflammation. The most obvious sign of tendinitis is heat, pain and swelling of the area with the classic appearance of a ‘bowed tendon’, as seen below, often being the tell tale sign! If the affected area of the tendon is within a tendon sheath, fluid filling of the sheath may also occur. 

While lameness may be present, it can be intermittent and does not always match with the severity of the injury. Diagnosis of tendinitis can often be presumptively made based on clinical examination – the localised swelling and response to palpation of the tendon. 

However, ultrasound evaluation is recommended in the vast majority of cases to establish, the severity and type of injury and to better guide treatment and rehabilitation programs. ‘Bandage bows’ result from too much or uneven bandage pressure and can look quite similar to tendinitis with heat, pain and swelling. However, the inflammation is confined to the tissues around the tendon, not the tendon itself. They’re usually of little significance, so distinguishing a bow from a case with actual tendon involvement is very important in terms of long term management and prognosis. Depending on the nature of the injury, ultrasonographic measurements are usually recorded and might include the change in cross-sectional area of the tendon compared to the normal one (how enlarged it is), the length of the lesion (how much of the length of the tendon is affected) and the cross-sectional area of the lesion as a proportion of the tendon. Other important information includes the type of tear – core lesions (those in the centre of the tendon) have different treatment options to tears affecting the edge of a tendon. 

Ultrasound images showing the left (injured) and right (normal) SDFT. The yellow dotted line outlines the edge of the SDFT which is significantly larger in the injured limb. Additionally the regular speckled pattern which represents normal fibre alignment has been disrupted and replaced by fluid which looks like a black hole (marked by white arrows).