Horses have a total of eight splint bones. The splint bones are located, one laterally and one medially, against the cannon bones of both the fore and hind limbs. These thin bones are believed to be the vestigial remains of toes possessed by the three-toed ancestors of today's equus. As you may recall from your studies, the anatomical name for the lateral splint in the front limb is the "fourth metacarpal bone." The medial splint of the forelimb is the "second metacarpal bone." In the hind limb the lateral splint is called the "fourth metatarsal bone" and the medial splint is called the "second metatarsal bone."
The splint bones are not attached to the cannon by bone, rather they are held in place by an interosseous ligament. The lack of bony support can leave this area open to injury. When a horse is referred to as having a "splint," an injury to either the splint bone, or the interosseus ligament has occurred. The most common form of injury is what we call a "popped splint" in which the interosseous ligament has been damaged. When a young horse is worked too much, or on a hard surface, the stress put on the immature bones of the legs can cause the interosseous ligament to become bruised and strained. The ligament may tear and localized heat and swelling will occur.
"Splints" are most often seen on one, or both, forelimbs and present themselves as painful swollen lumps. They occur most often on the medial side of the cannon; although a lateral "splint" may be seen on some horses. A "splint" may also be caused by the horse hitting itself with a hoof, either due to poor leg conformation, or due to overactive play. The splint bone itself can be fractured and this type of injury can occur to either the fore, or hind limbs.
While the injury is fresh, the horse will be sore and lame. The body will heal the interosseous ligament in a few weeks, by first laying down fibrous tissue. Once the fibrous tissue is in place, the area will ossify so that the splint bone becomes fused to the cannon bone. The ossification leaves a small to large lump on the side of the cannon. As long as there is no interference with the carpal joint, the bump is merely considered to be a cosmetic flaw. Despite the residual bump, ossification of the injured area actually results in a stronger splint region that is not easily re-injured.
Splint injuries may need to be x-rayed to determine if the splint bone itself has been fractured. A "splint" will heal with time and rest, although anti-inflammatory creams and counter-irritants may be applied. Pin firing, in which a hot needle is inserted into the interosseous space to further irritate the area, is used sometimes to speed healing. Pin firing has also been used to cause a healthy interosseous ligament to swell and ossify to prevent a splint injury at an inconvenient time, such as a week before a big race. Small rows of white dots along the cannon are telltale signs of pin firing, as the process kills the hair follicle, causing the hair to grow in white.
As with any injury likely to occur, it is best to look towards prevention of splint injuries. Conditioning young horses, especially those with less than perfect leg conformation, slowly and on good footing will help prevent work-related splint injuries. The application of "splint boots," or wrapping the legs for work and for turnout, or exercise may also help with prevention.
© 2002 Krishna JM Abel & EquiTouch® Systems, Inc.