Splint bones denote the second and fourth metacarpal (front limb) and metatarsal (hind limb) bones in the horse. Each horse owns a total of eight splint bones: one along the inside and one along the outside of each of the four limbs. Each splint bone is attached to the adjacent cannon bone (the third metacarpal bone or MC3 in the front limb and the third metatarsal bone or MT3 in the hind limb) by an interosseous ligament.
Each splint bone is comprised of three parts:
The second and fourth metacarpal bones (known as MC2 and MC4, respectively) communicate with the lowest joint of the carpus (known as the carpometacarpal joint) in the horse's fore limb. The second and fourth metatarsal bones (known as MT2 and MT4, respectively) communicate with the lowest joint of the tarsus (known as the tarsometatarsal joint) in the horse's hind limb. Since splint bones communicate with the joints above, each has one or more articular surfaces covered with a layer of cartilage. The articular surface(s) reside at the proximal margin (i.e. along the head) of the splint bone.
Some degree of weightbearing load is experienced by each splint bone as it provides columnar support to the structures residing above. Splint bones are stabilized by their length (which allows them to function as a lever-arm) as well as their ligamentous attachment to their parent cannon bone.
Since the center of gravity falls towards the medial side (inside) of the equine limb, the medial splint bones (i.e. the second metacarpus/MC2 and second metatarsus/MT2) generally experience more weightbearing load than their lateral (outside) counterparts. Therefore, the medial (inside) splint bones are considered more important (from a functional standpoint) than the lateral (outside) splint bones.
Furthermore, since the horse carries approximately 65% of the weight on the thoracic (front) limbs, the thoracic splint bones experience more weightbearing load than do the pelvic (hind) splint bones.
The splint bones can therefore be listed in the following order of functional importance:
- Thoracic medial (inside) splint bone (the second metacarpal bone or MC2)
- Pelvic medial (inside) splint bone (MT2)
- Thoracic lateral (outside) splint bone (MC4)
- Pelvic lateral (lateral) splint bone (MT4)