The metacarpophalangeal joints are the joints between the metacarpal bones and the proximal phalanges. It is called the fetlock joint in ungulates.
The four main metacarpophalangeal joints include the two palmar sesamoid bones and the dorsal sesamoid bone.
Each joint has an articular capsule (joint capsule) that runs between the four bones (metacarpal bone, proximal phalanx and two palmar sesamoid bones) that form the joint with the two collateral ligaments that unite the osseous parts.
Each pair of palmar sesamoid bones of the four main joints are joined together by a palmar ligament, (formerly called the intersesmoidean ligament).
The lateral and medial collateral sesamoidean ligaments are short, flat bands on each side of the metacarpophalangeal joint, each subdivided in two parts, one connecting the lateral (or medial) surface of the sesamoid to the distal surface of the metacarpal bones palmar to the proximal attachments of the collateral ligaments of the metacarpophalangeal joint, and the second part, connecting the lateral (or medial) surface of the sesamoid to the lateral (or medial) tubercle of the base of the proximal phalanx.
The short sesamoidean ligaments are thin, flat band that extend from the distal ends of each pair of palmar sesamoid bones adjacent to the synovial membrane of the joint capsule and are attached to the palmar side of the proximal phalanx.
The cruciate sesamoidean ligaments are located on the palmar side of the short sesamoidean ligament, and extend form the bases of the sesamoid bones to the diagonally opposite tubercles on the bases of the proximal phalanx. In the first digit, there is usually only one sesamoid bone and therefore only one ligament
The dorsal sesamoid bones of the metacarpophalangeal joints are secured by delicate fibers from the tendons of the extensor digiotrum communis and the interossi musculi proximally, and by a ligament to the dorsal surface of the middle phalanx distally.
The palmar annular ligaments are developments of the palmar fascia and brace the superficial and deep digital flexor tendons at the level of the palmar sesamoid bones do the second to fifth digits. These palmar ligaments furnish insertion to the deep interdigital ligaments, which hold the digits together and support the carpal and digital pads. The digital annular ligaments are developments of the digital fascia. The deep transverse metacarpal ligament is superficial to the digital annular ligaments. It is a thickening of the fascia of the palmar metacarpophalangeal area, that forms a V-Shaped ligamentous structure that connects the palmar annular ligaments of adjacent digits and acts as a fastening mechanism for the metacarpal pad. Note that in the NAV the deep transverse ligament is clearly described in the arthrology chapter, whereas palmar annular ligaments and digital annular ligaments are considered only as part of of the palmar fascia and digital fascia, and note described as individual structures in the arthrology section, but listed with fascia digiti.
Text by Antoine Micheau, MD - Copyright IMAIOS Miller's Anatomy of the Dog, 4th Edition - Evans & de Lahunta- Elsevier