Horses have about 175 bones in their body. Horses leg joints are not fully fused (grown) until the age of 3.5yrs.
A horse’s splint bones are thought to be remnants of toes from prehistoric horses. The splint bones are small bones (about the size of a pencil at the top and tapering down to be much smaller) found on each side of the cannon bone.
Sometimes a small bulge will appear on the horses lower leg, this is usually a splint bone that has popped. This happens when the splint bone becomes detached from the cannon bone. A splint might become detached due to a nutritional imbalance or trauma. It is usually not a cause for concern. In most cases a popped splint will cause mild pain to the horse but when the splint has “set” or healed it is completely pain free and is not a health or soundness concern.