Splints and bucked shins
The second and fourth metacarpal and metatarsal bones are located on either side of the cannon bones and are commonly referred to as splint bones.
If these bones become inflamed or fractured, either through chronic stress (i.e. poor conformation) or a traumatic injury, the horse is said to have "popped a splint." The area over the affected splint bone is initially painful when touched, and the horse may or may not become lame. A permanent hard swelling may remain in the location of the injury, but is usually not painful after the initial inflammation has subsided.
Treatment for this condition generally consists of rest and anti-inflammatory drugs. Horses with splints generally have a good prognosis to return to work, however, occasionally they can develop a callus around a splint bone fracture that damages the suspensory ligament, which runs along the back of the cannon bone. If this occurs, surgery may be needed to remove part of the affected splint bone, and there is a greater risk of long-term lameness.
Bucked shins is the common name for stress microfractures of the dorsal cortex (front aspect) of the front cannon bones. This condition is most commonly seen in 2-year-old racehorses just entering race training. In fact, the incidence of this disease among this population is estimated at 70%.
Signs of this condition include pain when touching the front of the cannon bone, swelling (first soft, then hard), and sometimes lameness. Bucked shins occur when the stresses put on the legs by training at high speeds exceed the bone's ability to adapt to that stress.
Treatment for this condition generally includes rest and anti-inflammatory drugs. Other treatments, such as blistering and pinfiring are common, but have not been shown to be more effective than rest. Once the pain and lameness have resolved, training can resume at a slower pace. A hard bony swelling may remain on the front of the affected bone(s). This condition does not seem to affect future race performance.
Lately, shockwave treatment has been advocated to provide pain relief and allow continued training during bone remodeling.