Bones - Horse Anatomy

Thoracic Limb

Scapula

The scapular is roughly triangular in shape and flat. It lies against the thoracic wall in a cranioventral direction. It does not form a true articulation with the trunk, instead forming a synsarcosis; meaning it is attached by only muscle. The dorsal border points towards the spinal column and joins the scapular cartilage. The scapular cartilage is crescent shaped, enlarging the area of attachment for muscles of the scapula as well as acting as a shock absorber. This cartilage ossifies, thus becomes more brittle, with age. In the horse, the cartilage extends caudally to the level of the withers; which is extensive compared with other species.

The spine of the scapular ends in a bony prominence ventrally, known as the acromion. In the horse, this structure becomes smaller distally. A second bony prominence, the tuberosity, exists dorsal to the middle of the scapular spine.

Humerus

The intertubercular groove has a central sagittal ridge.

Radius and Ulna

The two bones are fused, so all movement is in a sagittal plane, with very little pronation and supination. The ulna tapers out distally at the middle of the radius shaft. For the proximal quarter of the radius, the bones are joined by ligaments. There is an interosseous space for the passage of blood vessels and distal to this the bones are fused. The lateral styloid process of the radius is in fact, developmentally, the distal end of the ulna. It appears as a separate ossification centre in a foal until it fuses at about a year old.

Carpal bones

Carpal bones comprise two rows:

1. Proximally (mediolaterally):radial, intermediate, ulnar and accessory bones.
2. Distally: 1st carpal bone is small or absent, II-IV are all present.

Metacarpals and Metatarsals

Horses are perissodactyl unguligrade animals. Unguligrade means that horse's bear their weight on a horny covering (hoof) that surrounds their distal phalanx, perissodactyl means that they have only one weight-bearing digit. They are virtually identical in their structure in the hindlimb and forelimb. The main differences are in the forelimb we have metacarpals and the metacarpophalangeal joint, the hindlimb equivalents are the metatarsals and the metatarsophalangeal joint. Also in anatomical planes we use the term palmar for forelimb and plantar for hindlimb. These terms are all interchangable in the below article depending on whether you want to talk about the fore or hindlimb.

Only Metacarpal III (cannon bone) remains complete whilst metacarpals II and IV are greatly reduced and are known as the splint bones. Metacarpal III has a distal articular surface with two condylar areas separated by a sagittal ridge. This separates the two sesamoids on the palmar aspect of the joint and ensures only sagittal movement of the fetlock. There is also a transverse ridge leading to two radii of curvature. The proximal part has articulating facets on its palmar side where the splint bones articulate. It has a metacarpal tuberosity on the dorsomedial surface of the proximal end. The splint bones are held to metacarpal III by strong ligaments which are usually ossified and can lead to lameness if there is excessive bone production. They curve away distally from the main shaft and end in palpable nodules 2/3rds to 3/4s the way down the cannon bone.

Phalanges

Proximal Phalanx

The proximal phalanx proximally has two shallow articular surfaces separated by a small sagittal groove. Distally there are two convex areas separated by a sagittal groove.

Middle Phalanx

The middle phalanx is half the length of the proximal phalanx, its proximal articular surface is ridged so it can articulate with proximal phalanx and the distal end resembles that of the proximal phalanx.

Distal Phalanx

The distal phalanx is rounded to a point distally. The proximal articular surface has two concavities separated by a ridge. The palmar border meets with the distal sesamoid bone. The dorsal aspect has a marked extensor process, where the common digital extensor attaches. The solar surface has two distinct areas, the more palmar area is the roughened flexor surface and the remainder is smooth, concave and crescent-shaped. The parietal surface of the distal phalanx is that which conforms to the hoof wall. It is rough, porous and has processes on each side heading in a palmar direction. The ungual (collateral) cartilages attach to these processes, they are squarish plates which are palpable above the level of the hoof. They become more fibrous in life and may ossify.

Distal Sesamoid (Navicular) Bone

The distal sesamoid in horses is known as the navicular bone. It is elongated transversely and articulates with both the distal and middle phalanx. The navicular is separated from the deep digital flexor tendon by a bursa; the flexor surface of the navicular is covered by cartilage. The proximal and distal ends are grooved and possess large foramina. The bone also has a sagittal ridge.

Pelvic Limb

Pelvic Girdle and Hip

The pelvic girdle is formed by two hip bones which are joined ventrally at the cartilagenous pelvic symphysis and articulate dorsally with the sacrum. The three components of each hip bone are the ilium, pubis and ischium. The bone that articulates with the hip bones to form the hip joint is the femur.

Equine Bone Specifics

Within the equine ilium, the tuber coxae is visible and an important palpable landmark. The iliac crest is thin and concave whilst the ileal wing is orientated in an vertical manner. The pubis has a pubic groove that crosses the ventral surface of the iliopubic eminence towards the acetabulum. In the groove lies the accessory ligament of the head of the femur. Specifically in the stallion, a dorsal pubic tubercle exists. The ischium has an ischial tuberosity that is linear in shape whilst the ischial arch is shallow and irregular. The femur has a notched femoral head that is wedge-shaped and opens towards the medial side of the head. The greater trochanter extends beyond the the dorsal limit of the femoral head. The horse has an extra process called the third trochanter that is situated on the lateral aspect of the proximal third of the shaft and allows the superficial gluteal muscle to insert here. The caudal distal aspect of the femoral shaft is hollowed out forming the supracondylar fossa providing a larger area of origin for the superficial digital flexor muscle. The medial trochlear ridge is larger than lateral trochlear ridge and has a protuberance projecting proximally.