|The Structure of The Equine Lower Forelimb|
By Stephen Champion, BSc (Hons) BVM&S MRCVS,
I have decided in this latest article on aspects of applied anatomy of the horse, to discuss the anatomy of the lower fore limb. For the purposes of this discussion this consists of the portion of the fore leg from below the carpus (knee). In general equine practice, we find this part of the leg is most commonly damaged, both due to trauma from internal forces (such as in show-jumpers, eventers and racehorses) and from external trauma from kicks and cuts and riding injuries in the field. The lower limb is highly vulnerable due to its position close to the ground, its rapid motion and its poor coverage with protective muscle. It is also notable for its poor healing capacity, which means both internal and external injuries can take many weeks or months to heal. The anatomy of the lower fore limb can be extrapolated in simple terms to that of the hind limb.The Bony Skeleton The skeleton in this area comprises the metacarpal bones and the three phalangeal bones, the last one being encapsulated within the hoof. The third metacarpal bone, or cannon bone, is the main bone leading from the knee to the fetlock and is extremely strong, due to its cylindrical shape. On either side of this bone, lie the second and fourth metacarpal bones, otherwise known as the splint bones. These are much smaller and taper down the leg, ending at a button, about three quarters of the way down the cannon. These are prone to damage from trauma such as kicks, but the fractured end can often be removed surgically to leave the horse sound. They are attached to the cannon bone by a long interosseus ligament, which can become stressed and inflamed causing a painful lump called a “splint”. Splints are common in younger horses and often settle down with box rest and anti-inflammatory treatment, again to leave the horse sound. The end of the third metacarpal bone joins the first phalangeal bone at the fetlock joint. To the rear of this joint are two small, pyramid-shaped bones called the proximal sesamoid bones which give the fetlock joint its thickness and are fractured more often than any other bone in the forelimb. They are connected to the phalangeal bones by a complicated series of ligaments and to each other by a thick palmar ligament. The fetlock joint has great mobility due in part to its large joint capsule that surrounds the joint. This extends upwards attached to the cannon bone as two pouches. These can become distended; a condition known as “windgalls”, which is common particularly in older horses but does not necessarily cause lameness. The three phalangeal bones are the “finger” bones of the horse, leading from the fetlock down into the hoof capsule (refer to the previous article on the anatomy of the hoof). The second phalanx is half the length of the first and attached to the first at the pastern joint. It is attached to the small, triangular third phalanx at the so-called coffin joint at the level of the coronet. The small navicular bone, responsible for many a great horse’s demise, is found just behind the coffin joint, again within the hoof capsule. The Soft Tissue Structures of the Lower Leg The movement of the fetlock and pastern of the horse are controlled by muscles whose bulk is largely confined to the part of the leg above the carpus. These muscles are continued below the carpus as tendons, which are fibrous structures serving to attach muscle to bone. These tendons course down the front and back of the cannon bone to insert onto the bones of the phalanges. The tendons running down the front of the leg are the common and lateral extensor tendons, whose function is to extend the fetlock. The tendons running down the back of the leg are the flexor tendons, responsible for flexing the leg at the fetlock. These tendons, particularly the flexor group are highly prone to damage, due in part to the immense forces imposed upon them within the performing horse, and partly by their vulnerable position on the leg. The two flexor tendons on the back of the cannon bone are called the superficial and deep flexor tendons, due to their relative positions on the leg, and can be felt together as quite “fleshy” structures behind the cannon bone. These run together over the back of the fetlock and attach onto the phalanges and are held in place in this area by three annular ligaments. They are protected from friction as they glide backwards and forwards, by a number of sheaths, which wrap around the tendons. The deep flexor tendon is strengthened and supported in its position by a further short structure, the accessory check ligament that runs diagonally and upwards from the deep tendon to insert on the carpus. This serves to dissipate some of the forces that run along the deep flexor tendon. Between the deep flexor tendon and the cannon bone, runs the interosseus ligament, perhaps more commonly known as the suspensory ligament. This runs from carpus to fetlock and is continued below the fetlock as the sesamoidean ligaments. It stabilises the fetlock of the horse and forms an integral part of the horse’s stay apparatus, a collection of muscles, tendons and ligaments whose combined efforts keep the horse upright (usually!). Furthermore, the energy stored in the ligament as the horse lands and the fetlock extended, is released at the end of the stride, providing forward impetus. As we have discussed above, the lower limb of the horse is vulnerable to damage and complicated both in terms of anatomy and function. Healing is slow and sometimes incomplete and any trauma to this area needs to be assessed carefully. The majority of injuries in this area should be examined by a veterinary surgeon as soon as possible after the event. Bony injuries are usually assessed using radiography. Tendon and ligament injuries can be effectively assessed using diagnostic ultrasound, and a treatment plan and prognosis decided upon. It is worth noting that acute tendon injuries which receive the most appropriate treatment in the first few hours and days after injury have a much more favourable prognosis for full return to work than those left to chance and “hit and miss” treatment.