The splint bones, or second and third metacarpal/tarsal bones, are small, thin, vestigial bones located on either side of the cannon bone. Each splint bone is attached to the cannon bone by an interosseous ligament; effectively “splinting” the cannon bone. This ligament ossifies, or turns to bone, as the horse enters adulthood by age 3-4 years. The splint bones start at the carpus or hock and extend approximately three-fourths of the length of the cannon bone, getting thinner as it extends down the leg. These bones, although small in size, can cause lameness issues for the horse.
Have you ever run your hand down your horses leg and noticed a new hard bump near the cannon bone? Chances are, you’ve felt a “splint.” This term is given to a phenomenon called interosseous desmitis…inflammation of the interosseous ligament, to put it simply. This inflammation is most commonly caused by a tearing of the interosseous ligament or external trauma, even fracture, to the splint bone. As a result of the inflammation, the affected portion of the ligament ossifies to a bony callus during the healing process. Splints commonly occur in young horses aged 2-3 years old undergoing strenuous work. Medial (inside of leg) splints are more common in forelimbs, as there is more weight-bearing load on the inner aspect of the distal limb and more chance for interference from the opposite limb. Poor conformation or shoeing that places more weight on the inside of the limb can also predispose the horse to splints. In the hind limbs, splints are more common on the outside of the limb (laterally) and are most commonly due to trauma, such as a kick from another horse.
Veterinarians can diagnose splints by palpation of the limb and radiography. Palpation may yield a warm, firm swelling on the side of the cannon bone. The horse may be sensitive to palpation of the area, but may or may not be lame! If a fracture is detected on radiographs, surgery may be necessary. Using these diagnostics, along with the location, size, and severity of the splint, as well as degree of associated lameness, your veterinarian will prescribe an appropriate treatment plan to get your horse back in action quickly. Treatments are often aimed at athletic rest, cryotherapy, anti-inflammatory medications, and bandaging. Chronic splints may require injection of corticosteroids to the area surrounding the splint in order to quiet the inflammation. If you suspect your horse has a new splint, please consult your veterinarian.