The foreleg forms part of the appendicular skeleton of the horse. Compared to the hindleg the foreleg is suspended in place by muscles and tendons, which gives it great mobility, and it is relatively straight with few angles, which makes it prone to concussion injuries. Thus, the foreleg conformation has a big affect on the athletic ability and soundness of the horse.
Mechanics of the front leg
The scapula is a flat triangular bone that joins onto the humerus. The spine of the scapula (a ridge) divides it into two halves, providing attachment for the shoulder muscles. How flat or bulging the shoulder is will depend on size and development of these muscles.
The humerus forms the elbow joint with the ulna and radius. Where they join, the humerus has a deep groove called the olecranon fossa. This is where the olecranon process (the free end of the ulna) will enter when the weight is placed on the leg, locking the joint and so stabilize the entire leg.
When the weight is taken off the leg the elbow joint unlocks and is capable of a wide range of forward and backward motion (about 100°). The elbow can only bend when the knee (carpus) is bent.
The knee (carpus) too can only flex in one direction. It consists of 7 bones. These bones are called the carpal bones, except for the 7th bone which is referred to as the accessory carpal bone.
Anatomy: Horse vs Human
Heather Smith Thomas made a beautiful comparison, in her book “The Horse Conformation Handbook”, between the anatomy of the horse's lower leg and that of the human hand. It is rather fascinating to think that the horse is, actually, standing on its fingers!
The knee (carpus) corresponds to the human wrist and contains the same 7 bones arranged in an identical fashion. The cannon bone corresponds to the human middle finger. The middle knuckle in the human hand is the equivalent of the horse's fetlock joint. The two splint bones corresponds to the 3rd and 4rt metacarpals, but have shrunk to become narrow. The long and short pastern bones together with the coffin bone, in that order, corresponds to the phalanges in the hand.
Tendons and ligaments of the lower foreleg
The Superficial Digital Flexor (SDF) Tendon and Superior Check Ligament
The SDF muscle runs along the back of the radius and narrows near the the carpus to form the SDF tendon. As the SDF tendon passes behind the carpus through the carpal canal it is a round shape. It becomes flattened and half moon shaped as it continues down the limb, cupping itself around the rounder digital flexor tendon. The SDF tendon branches at the fetlock joint, each attaching to a pastern bone. The SDF tendon serves to support the two pastern bones.
The superior (radial) check ligament attaches the SDF muscle to the carpus even though it is a ligament. All check ligaments serve to protect the tendons from overstretching.
The SDF tendon is commonly inured in horses, especially race horses such as Thoroughbreds and Quarter Horses. Injury to this tendon will lead to what is called a “bowed tendon”.
The Deep Digital Flexor (DDF) Tendon and Inferior Check Ligament
The DDF muscle runs along the back of the radius. Just above the carpus it narrows to form the round DDF tendon which joins the SDF tendon through the carpal canal behind the carpus. At the back of the fetlock the DDF tendon passes through a sheath that provides lubrication and nutrients to the tendons. The DDF tendon then widens and flattens, crossing the the navicular bone and attaches to the coffin bone (third phalanx). Between the DDF tendon and the navicular bone lie the navicular bursa ( a fluid filled sac) which helps to cushion the tendon as it bends.
The inferior (carpal) check ligament descends from the carpus' base and attaches to the DDF tendon at the mid-cannon bone.
Injury to the DDF tendon is uncommon.
The Suspensory Ligament
It originates behind the cannon bone below the carpus. As it descends down the cannon it runs along the DDF tendon. It divides into two branches, each of which attaches to one sesamoid bone. These branches continue towards the front of the pastern to join the common digital extensor tendon. The main function of the suspensory ligament is to stabilize the lower leg and to prevent overextension.
Injury to the suspensory ligament is common.
The Sesamoidean Ligaments
These ligaments help to stabilize the fetlock joint and is made up of the following:
1. Oblique and straight sesamoidean ligaments (located behind the pastern deep under the DDF tendon).
2. Intersesamoidean ligaments (located between the two sesamoids).
3. Collateral sesamoidean ligaments (located on both sides of the fetlock joint).
4. Cruciate sesamoidean ligaments (descends from each sesamoid bone's base and cross to attach to opposite side of pastern).
Injury is uncommon.
The Palmar Anular Ligament
The palmar anular ligament covers the back of the fetlock joint, creating a channel through which the DDF and SDF tendons pass. It holds these tendons firmly in place against the fetlock bones.
Injury to this ligament itself is uncommon, but swelling and injury to the DDF and SDF tendons can cause problems.
The Foreleg Extensor Tendons
These tendons have two very important functions. They help to move the leg forward (extend the leg) and help to stabilize the front joints. There are three extensor tendons in the foreleg:
1. The extensor carpi radialis tendon. This tendon attaches to the front of the cannon bone and helps to extend the knee joint and move the leg froward.
2. The common digital extensor tendon. This tendon attaches to the coffin bone and helps to extend the toe forward.
3. The lateral digital extensor tendon. This tendon is found behind the common digital extensor tendon and runs on the side of the leg. It attaches tot he first pastern bone (beneath the fetlock joint) and helps to extend the leg.