A splint is a soft swelling or a bony enlargement that occurs between the splint bone and cannon bone. The size of a splint can vary from a tiny bump to a large, ugly swelling, and its effects range from an unsightly blemish to a severe lameness. A new, acute, or "hot," splint is usually accompanied by heat, swelling, and pain; the horse may or may not be lame. An old "cold" splint is a hard bump or blemish that usually does not cause pain.
Splints can have several causes. The syndrome is common in young horses in which the ligament attachment of the splint bone to the cannon bone has not yet hardened into bone. Stretching or tearing of the ligament causes new bone to be laid down, and a firm swelling results. Sometimes, too, the splint bone fractures. Splints in young horses usually occur on the inside of the front legs, since this area bears the most weight. Certain conformation faults predispose horse to developing splints. Horses with "bench knees" (a condition in which the knee is offset to the inside relative to the cannon bone) place more stress on the splint bone. The horse with a base-narrow, toed-out stance is prone to hit the inside of one leg with the opposite foot and a cause of a splint.
Splints can occur in horses of any age because of trauma or a blow to the leg. Traumatic splints occur in either the outside or inside of the leg, and are just as common on the hind legs as on the front.
Always have your veterinarian radiograph (X-ray) the splint to be sure there is no fracture. Young horses can sometimes continue to work if training is gradual and no fracture is found. However, to reduce the size of the lump, allow the horse to rest and wrap the leg until the splint "cools out" (no heat or pain remains). Alternate cold therapy with sweat wraps to minimize tissue reaction and reduce the size of the bump, since the size of the splint can only diminish while the splint is hot. A permanent blemish often remains after the splint has cooled, but the horse is usually not lame.
Large splints can cause lameness even after they are cold if there is enough bone growth to interfere with movement of the ligaments and tendons at the back of the leg. This is less likely to happen if you spot the problem early and take extra precautions to prevent it from progressing.