Bones - Horse Anatomy

Thoracic Limb

Scapula

The horse lacks an acromion, it has a tuber scapular half way along the spine. The scapular cartilage is reasonably extensive compared to other species.

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

The arrangement of the metatarsals are similar to those of the metacarpals. 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.