Anatomy and Common Injuries of the Equine Forelimb

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Original Editor - Jess Bell Top Contributors - Jess Bell, Chelsea Mclene, Kim Jackson, Tarina van der Stockt, Olajumoke Ogunleye and Stacy Schiurring

Anatomy of the Forelimb

An understanding of equine anatomy is essential in veterinary practice to ensure that an accurate diagnosis is made and, therefore, that the correct management option is selected.

Normal Anatomy

The forelimb (also known as the thoracic limb) in the horse is adapted for extension and ground covering. This limb carries 55 to 60 percent of the horse’s body weight, and a large proportion of the rider’s weight as well.

The shoulder girdle (scapula, coracoid, clavicle) is greatly reduced in the equine limb. The clavicle is only present as a fibrous strip in the brachiocephalicus muscle.

Scapula:

• Wide, semilunar scapular cartilage
• Palpable tuber on the scapular spine
• No acromion

Humerus:

• Short and thickened
• Has a deep bicipital groove

Radius and Ulna:

• Only the radius supports the humerus at the elbow joint
• The ulna is fused to the radius
• Proximally, the ulna reaches to the fifth rib

Carpal Bones:

• The proximal row of carpal bones consists of the radial, intermediate, ulnar and accessory carpal bones (medial to lateral)
• The distal row consists of the first, second, third and fourth carpal bones. The first bone is small and not consistent

Metacarpal (MC) Bones:

• Only MC2, MC3, MC4 are present
• MC2 and MC4 are reduced and are also known as splint bones. They are connected to MC3 by fibrous tissue
• MC3 (also called the “cannon bone”) is well developed and carries all of the horse’s weight

Phalanges (P):

• The proximal phalanx (P1) is the longest of the phalanges
• The middle phalanx (P2) is half the length of PI. It has a large flexor tuberosity to which the superficial digital flexor tendon attaches
• The distal phalanx (P3 or the “coffin bone”) is a spongy bone to which the deep digital flexor tendon attaches

Sesamoid Bones:

• The proximal and distal sesamoid bones are clinically important. The proximal bones articulate with M3 while the distal bone (i.e. the navicular) lies within the hoof and contacts the middle and distal phalanges.

There are a number of normal anatomical variations between horses that are visible on radiography. If these features are misinterpreted, inappropriate management may be prescribed, which can have a significant impact on a patient’s outcome. Some examples of normal variations in the equine front limb are:

• Variable conformation in the dorsodistal aspect of the middle phalanx (P2) and extensor process of the distal phalanx (P3). Some horses have smooth-rounded surfaces, whereas others have pointed margins with sharp contours, which can be misdiagnosed as degenerative changes
• Some horses may have a crescent-shaped lucent region in their navicular, which needs to be distinguished from flexor cortical lysis
• Some skeletally mature horses may have an incomplete closure of the ossification centre between the ulna styloid process and distal radius. This can be mistaken for a fracture of the distal radius
• The concave margin on the glenoid notch of the scapula appears as an ovoid lucent region on x-ray, which can be misdiagnosed as a subchondral bone defect or cyst
• Some horses have an incidental variation on the mid dorsal aspect of the medial trochlear ridges of their talus - it can be flattened or focally concave. This needs to be distinguished from osteochondrosis
• The trochlear ridges of the femur of juvenile horses have irregular articular margins, which can be challenging to interpret when there is a suspicion of joint sepsis