The equine carpus is the region located in between the radius and the metacarpals. It is composed of 8 bones, organized in 2 rows, and 3 horizontal joints.
The most distal row contains the bones C1, C2, C3 and C4 (medial to lateral) and makes up the carpometacarpal joint with the metacarpals below. The carpometacarpal joint has very little ginglymi movement and some authors state that it does not open. The first and second carpal bones (C1 and C2) are usually fused and sometimes listed as a single bone.
The most proximal joint is the radiocarpal (antebrachial) joint. It is formed by the radius and the first row of carpal bones (Radial, Ulnar, Intermediate and Accessory).
The midcarpal joint is located in between the proximal and distal rows of carpal bones.
Intercarpal joints are located between adjacent carpal bones, and intercarpal ligaments ensure the bones are held in place. Also, medial and lateral collateral carpal ligaments provide stability to the carpus.
The accessory carpal bone does not bear weight directly. Its main function is to act as a sesamoid bone for the attachment of the tendons of the Flexor Carpi Ulnaris and Extensor Carpi Ulnaris (Ulnaris Lateralis) muscles. Four ligaments, located palmar to the lateral collateral carpal ligament, connect the accessory carpal bone to the carpus and surrounding regions: Accesorioulnar, Accessoriocarpoulnar, Accessorioquartal, Accessoriometacarpalry carpal bone does not bear weight directly. Its main function is to act as a sesamoid bone for the attachment of the tendons of the Flexor Carpi Ulnaris and Extensor Carpi Ulnaris (Ulnaris Lateralis) muscles. Four ligaments, located palmar to the lateral collateral carpal ligament, connect the accessory carpal bone to the carpus and surrounding regions: Accesorioulnar, Accessoriocarpoulnar, Accessorioquartal, Accessoriometacarpal
Dorsally, the tendons of the Extensor Carpi Radialis, Extensor Carpi Obliquus (Abductor Digiti I Longus) and Common Digital Extensor muscles are responsible for extending the carpus. These muscles are innervated by the Radial Nerve.
The Extensor Carpi Radialis tendon originates from the Extensor Carpi Radialis muscle located in the lateral epicondyloid crest of the humerus and inserts in the metacarpal tuberosity of the third metacarpal bone (cannon bone). Besides extending the carpal joint and flexing the elbow, this tendon stabilizes the carpus.
The Extensor Carpi Obliquus muscle is located on the lateral and adjacent border of the cranial surface of the radius. Its tendon of insertion passes over the Extensor Carpi Radialis tendon crossing the medial face of the carpus to insert in the head of the second metacarpal bone (medial splint bone).
The tendon of the Common Digital Extensor muscle originates from the cranial aspect of the distal humerus and lateral aspect of proximal radius and courses distad to insert into the proximal extremities of the proximal and middle phalanges and the extensor process of the distal phalanx.
Branches from the Radial, Transverse Cubital and Cranial Interosseous arteries form a vascular network in the dorsal aspect of the carpus called the Rete Carpi Dorsale.
The tendons of the Lateral Extensor, Ulnaris Lateralis, and Flexor Carpi Ulnaris muscles are found on the lateral aspect of the carpus.
The tendon of the Lateral Extensor muscle aids to extend the carpus and the digit. It originates on the lateral tuberosity of the radius and lateral collateral ligament of the elbow joint and the body of the ulna. It courses distad to insert in the proximal extremity of the proximal phalanx. The Radial nerve provides innervation to this muscle.
The Ulnaris Lateralis (Extensor Carpi Ulnaris) muscle is located on the lateral epycondyle of the humerus, caudal and distal to the lateral collateral ligament of the elbow joint. Its tendon makes its insertion on the lateral surface and proximal border of the accessory carpal bone and fourth metacarpal bone (lateral splint bone). This tendon helps flexing the carpal joint and extending the elbow. It is innervated by the Radial nerve.
The Flexor Carpi Ulnaris muscle originates from the medial epicondyle of the humerus and medial surface and caudal border of the olecranon. Its tendon of insertion blends with the flexor retinaculum, crossing over the caudal aspect of the carpus to make its insertion on the proximal edge of the accessory carpal bone. It is innervated by the Ulnar nerve.
Caudally, the tendons of the Deep Flexor and Superficial flexor muscles provide flexion to the carpal joint.
The Superficial Flexor muscle originates from the medial epicondyle of the humerus and caudal surface of the radius. Its tendon inserts bilaterally in proximal interphalangeal joint (distal extremity of the proximal phalanx and proximal extremity of the middle phalanx). It aids in flexing the digit and carpus and extending the elbow. It is innervated by the Ulnar nerve.
The tendon of the Deep Flexor muscle originates from the medial epicondyle of the humerus, medial surface of olecranon and the middle and caudal surface of the radius. It is the largest muscle of the flexor group. It runs deep to the superficial flexor tendon through the carpal canal and inserts in the semi-lunar crest of the distal phalanx. It aids in flexing the digit and the carpus and extending the elbow joint. It is innervated by the Medial and Ulnar nerves.
Check (Accessory) ligaments provide tendoligamentous support to the superficial and deep flexor tendons.
The Radial Check Ligament (Accessory Ligament of the Superficial Digital Flexor Tendon) is basically the radial head of the Superficial Flexor muscle. It joins the tendon of the humeral head under the proximal part of the flexor retinaculum and contributes to the medial wall of the carpal canal.
The Subcarpal Check Ligament (Accessory Ligament of the Deep Digital Flexor Tendon) originates from the palmar carpal ligament and joins the Deep Digital Flexor tendon near the middle of the metacarpus.