Proximal Sesamoid Bones in the Forelimbs of Domestic Animals

Proximal sesamoids of the forelimbs are small, nodular bones located at the back (palmar side) of the metacarpophalangeal joints (commonly known as the fetlock joint in large animals).

These bones are embedded within the tendons of the flexor muscles, primarily the deep digital flexor tendon, and play a crucial biomechanical role in limb movement.

Proximal Sesamoids of Ox

The proximal sesamoids are a pair for each digit in the ox. They are situated behind the distal extremity of the large metacarpal and afford increased leverage to the flexors of the digits.

Each bone has the form of a three-sided pyramid with the base below and the apex above. The base presents a facet for the first phalanx. The dorsal surface articulates with the distal extremity of the large metacarpal on its volar aspect.

Each sesamoid also presents a facet for articulation with a similar facet of its fellow. The abaxial and axial sesamoids of each digit articulate with each other. The opposing faces of the axial sesamoids are covered in life by fibrocartilage and form a groove for the gliding of the deep flexor tendon of the digit.

Comparative Anatomy of Proximal Sesamoids

Comparative anatomy of proximal sesamoids refers to the study and comparison of the structure, number, position, and function of proximal sesamoid bones across various animal species — particularly domestic animals like the horse, sheep, goat, pig, dog, rabbit, and fowl — in comparison with the ox, which serves as the standard reference.

Sheep and Goats

The number of proximal sesamoids in sheep and goats is similar to that of an ox. The abaxial ones are compressed from side to side, and the axial ones are compressed from front to back.

Horse

There is only one digit (the third), and hence one pair of proximal sesamoids in the horse. Each is much larger and distinctly pyramidal in shape.

It articulates only with the large metacarpal, but neither with the first phalanx nor with its fellow. The abaxial surface is concave.

Pig

The proximal sesamoids of the forelimbs of the pig are similar to those of an ox.

Dog

Nine proximal sesamoids are present in dogs: one for the first digit and two for each of the other digits present in the forelimb. Besides, there is a dorsal sesamoid in the capsule of each metacarpo-phalangeal joint.

Rabbit

Many proximal sesamoid bones are present on the metacarpo-phalangeal joints in the forelimb of the rabbit.

Fowl

The sesamoids are absent in the fowl.