SPLINTS

Anatomy

In horses, the cannon bone of each leg is flanked on either side by a splint bone. The uppermost portion of each splint bone begins at the back of the knee/hock, extending downwards approximately three-quarters the length of the cannon bone (Fig 1A). Each splint bone is firmly attached to the cannon bone by the strong interosseous ligament.

The splint bones have erroneously been described as ‘vestigial’ structures, that are remnants from a time when horses walked on three toes that with evolution are no longer needed. If an entire splint bone is removed it will in fact result in knee/hock arthritis and long-term lameness. Additionally, the splint bones are important sites of attachment for numerous ligaments and support the small bones within the knee and hock. They are also thought to support or ‘splint’ the cannon bone, hence their name.

How Splints Develop

‘Splints’ are a common complaint in horses and occur due to either excessive strain or direct trauma to either the splint bone itself or to the interosseous ligament.

‘True’ splints occur when the interosseous ligament develops a small tear or sprain. This results in inflammation and swelling of the ligament, evidenced by a hot, painful bump on the lower limb, most commonly in the middle one third of the splint bone (Fig 1B). The inside splint on the forelimb is most commonly affected. This is the most commonly recognised form of a splint and is usually seen in younger horses whose musculoskeletal systems have not yet completely matured, although any horse can be affected. With this type of splint, lameness is often the first sign noted. There may be a history of exercise on hard ground, particularly of high-speed exercise.

Trauma to the splint bone itself may result in bone inflammation, resulting in swelling, heat, pain and ultimately lameness. Horses that do not track-up straight but wind/wing their forelimbs are particularly prone to striking their inside splint with the opposite forelimb. This type of splint may result in larger bumps, as the injured splint bone produces many layers of new bone in response to continued inflammation. Again, this form of splint is also most commonly found on the inside of the forelimb but any splint bone can be affected. If the degree of trauma is more severe, for example a kick from another horse, the splint bone may fracture (Fig 2A). In response, the body begins to form a bony callus to bridge the fracture gap. In this case it is usually the outermost splint bone that is affected.

Diagnosis and Treatment

Apart from the classic signs of heat, pain, swelling and lameness, diagnosis may be aided by the use of x-rays, ultrasound, and occasionally MRI. X-rays are particularly important to rule out the presence of a fracture, as slightly different treatment may be recommended for these cases. Ultrasound is also important, as the majority of splints have concurrent damage to the branches of the suspensory ligament where it attaches to the back of the fetlock.

Once a splint has been recognised, it is imperative that it is treated promptly and appropriately to safeguard future soundness. The mainstays of treatment include rest, cold therapy and anti-inflammatories in the early period to prevent any further damage and decrease the inflammatory response. Fortunately, the majority of cases respond well to conservative therapy, and a full recovery can be expected in three weeks to three months. One of the most common mistakes is returning the horse back to exercise prematurely, before the splint has fully settled, resulting in a secondary flare-up. In cases that do not respond to conservative management, adjunctive treatments such as shockwave therapy or direct infusion of the area with corticosteroids may be recommended. Ultimately recurring injuries or those with an obvious radiographic fracture may benefit from surgical removal of the fractured portion of the splint bone (Fig 1C & 2B). This results in a definitive cure and more expedient return to exercise.

Prevention

Unfortunately it is impossible to completely prevent the occurrence of splints but there are a few strategies, which may help avoid them:

  • Avoid fast exercise on hard ground
  • Minimise the intensity and frequency of exercise in young horses
  • Select horses with good conformation, i.e. avoid base narrow, toed-out horses and those with off-set knees
  • Use protective splint boots in horses at risk of striking themselves
  • Maintain correct hoof balance

    • Overall, splints carry an excellent prognosis for a return to a full athletic career and, if treated correctly from the onset, result in nothing more than a cosmetic blemish in the long-term.

    Turlough McNally MVB Dip ACVS Dip ECVS MRCVS is a member of the Veterinary Ireland Equine Group and is a specialist in equine surgery at Anglesey Lodge Equine Hospital, The Curragh, Co Kildare

    Email: hq@vetireland.ie

    Telephone: 01-4577976