A horse’s legs combine form and function, with a stunning visual appeal to match their power and strength. The 20 bones in a horse leg make up an intricate system that transfers weight, controls movement, and protects soft tissue and tendons from trauma. The navicular bone, which runs along the bottom of the leg and connects to the sole of the hoof, is especially important for flexing and extending the fetlock and pastern joints.
It is only about half as thick as the wrist bone in humans, yet it supports a horse’s entire body weight while galloping. Its amazing strength stems from its complex structure, which is reminiscent of an interlocking puzzle. The main portion of the bone is divided into two articulating segments connected by an elliptical hole, called a foramen, that lets blood vessels reach the lower leg. Its shape also helps prevent stresses from causing fractures by directing stress away from the foramen to other parts of the bone.
In addition, the periosteum, a sheath that covers the bones, is covered with a layer of fat that helps bind the bones together and provide flexibility. The bones are also covered with a layer of collagen, which provides strength and resilience. Bone is a dynamic, living tissue that strengthens with use. In the horse, moderate exercise (whether a structured training regimen or simply rough play in pasture) stimulates osteoblasts to produce healthy new bone tissue. In the case of a fracture or other injury, osteoid cells can quickly fill the resulting cavities and increase the stiffness of a bone.
The distal limb bones support the knee, which is another area of great complexity in a horse’s anatomy. The knee is a hinge joint with a small, rounded bone protruding behind it, known as the accessory carpal bone. This little bone acts as a brace point and fulcrum for supportive sheets of ligament that run behind the leg from the forearm to the cannon bone.
A pea-sized hole, called a foramen, cuts through the splint bone just below the front of the knee. Its complex geometry allows for the passage of blood vessels while still allowing the splint bone to flex and extend without fracturing.
When a horse’s leg is injured, the periosteum may become damaged and circulation to the area is limited. This leads to a condition known as splints, painful enlargements of the splint bone. The splints are usually mild and resolve in time, but severe injuries can cause abnormal bone growth and lead to secondary complications. Poultices help prevent and heal leg injuries by keeping the splints tight and cooling them. They also reduce inflammation and relax muscles, cool hot spots, and decrease swelling. A good horse poultice keeps a horse’s legs working at their best.