Limb Bones and Cartilages - Horse Anatomy

Introduction

For general skeletal anatomy, follow links in blue. The following section will concentrate on anatomy specific to the horse.

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 lesser and greater tubercles on the lateral and medial sides of the proximal humerus, respectively, are nearly equally well developed. Both tubercles divided into two parts (cranial and caudal). The tubercles are separated by an intertubercular groove, which is wide and has a central saggital ridge (intermediate tubercle). This ridge fits into a depression on the deep surface of the biceps tendon and restricts movement of the tendon relative to the humerus in the standing horse. The body of the bone bears the teres major tuberosity on its medial surface and the deltoid tuberosity, which is opposite, on the lateral surface. The distal end forms a cylindrical condyle that transfers weight to the radius. On the lateral aspect of the condyle there is a small saggital ridge with grooves to allow articulation with the radius and ulna. The medial and lateral epicondlyes and lateral supracondylar crest are palpable landmarks. A shallow radial fossa is just proximal to the condyle on the cranial surface. The deeper olecranon fossa, between the two epicondyles, lies opposite the radial fossa on the caudal surface. There is no supratrochlear foramen between the two fossae in horses.

Radius and Ulna

The two bones are fused, so all movement is in a sagittal plane, with very little pronation and supination. The fusion of the two bones is interrupted by interosseous space in the proximal third of antebrachium, which serves as a passage for blood vessels.

Radius

On the proximocranial aspect of the radius is the radial tuberosity, the point of insertion of the biceps tendon. At the distal end of the radius are the medial and lateral 'styloid processes. The lateral styloid process of the radius is in fact, developmentally, the distal end of the ulna.

Ulna

The ulna tapers out distally at the mid antebrachium. For the proximal quarter of the radius, the bones are joined by ligaments. It appears as a separate ossification centre in a foal until it fuses at about a year old. The olecranon tuberosity is located on its proximal extremity.

Carpal Bones

Carpal bones comprise two rows:

• Proximal row (medial to lateral): radial, intermediate, ulnar and accessory carpal bones.
• Distal row: numbered the first to fourth carpal bones, of which the first is small or absent in horses.

Metacarpals and Metatarsals

Horses are perissodactyl unguligrade animals. Unguligrade means that horses bear their weight on a horny covering (hoof) that surrounds their distal phalanx, perissodactyl means that they have only one weightbearing digit. They are virtually identical in their structure in the hindlimb and forelimb. The main differences are that in the forelimb they 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 are referring to the fore or hindlimb.

Only the third metacarpal (cannon) bone remains complete whilst the second and fourth metacarpals are greatly reduced and are known as the splint bones; metacarpal II is the medial splint and metacarpal IV the lateral splint. Metacarpal I and V are completely absent in the horse. The splint bones are approximately a third shorter than the metacarpal III. Proximally, the metacarpals articulate with carpal bones. Metacarpal II and III articulate with the second carpal bone and metacarpal IV with the fourth carpal bone.

Metacarpal III is robust and well developed, as it has to bear the entire weight transfered through the limb. The distal end of the bone has an articular surface comprised of two condylar areas seperated by a saggital ridge, which engages with a groove in the proximal phalanx. This ensures only sagittal movement of the fetlock. There is also a transverse ridge leading to two radii of curvature. The proximal aspect of metacarpal III 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 bones curve away distally from the main shaft and end in rounded nodules, which are important palpable landmarks (buttons of the splint), approximately a 1/3 to 3/4 of the way down metacarpal III.

Phalanges

Limb Bones and Cartilages - Horse Anatomy Learning Resources
Drag and Drop (Dragster) Test your knowledge using drag and drop boxes	Equine Skeleton Equine Forelimb Resources

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.

Phalanges

Limb Bones and Cartilages - Horse Anatomy Learning Resources
Drag and Drop (Dragster) Test your knowledge using drag and drop boxes	Equine Skeleton Equine Hindlimb Resources