Musculus flexor carpi ulnaris
The flexor carpi ulnaris has two heads: a humeral head from the medial epicondyle of the humerus and an ulnar head from the olecranon. These converge to insert on the accessory carpal bone. Its primary action is to flex the carpus. It is innervated by the ulnar nerve (roughly C8–T2). The muscle contributes to carpal stability and flexion during the swing phase.
Origin: Medial epicondyle of the humerus (humeral head) and olecranon (ulnar head)
Insertion: Accessory carpal bone
Action: Flexes the carpus
Innervation: Ulnar nerve (C8-T2)
Isolated pathology is uncommon. The muscle or its insertion can be involved in trauma to the caudal forearm or accessory carpal region. Ulnar-nerve lesions affect it.
## Overview Fracture of the accessory carpal bone is a relatively uncommon but performance-limiting injury of the equine carpus. The accessory carpal bone sits on the palmar aspect of the carpus and
Swelling or pain behind the knee or on the outside of the forearm after trauma should be checked. Most soft-tissue injuries improve with rest.
## Overview Fracture of the accessory carpal bone is a relatively uncommon but performance-limiting injury of the equine carpus. The accessory carpal bone sits on the palmar aspect of the carpus and serves as an insertion point for the flexor carpi ulnaris and ulnaris lateralis muscles. Most fractures occur in the frontal (vertical) plane. Because of the constant pull of the attached muscles, many fractures heal by fibrocartilaginous nonunion rather than bony union. Effects include acute lameness, carpal canal swelling, and long-term mechanical limitation or osteoarthritis if the fracture involves the joint or carpal sheath. ## Causes Proposed mechanisms include external trauma, asynchronous contraction of the flexor muscles, or a “nutcracker” effect when the horse lands on a partially flexed carpus. Racehorses and horses that jump or fall onto the carpus are most commonly affected. Horizontal fractures and avulsion fractures also occur but are less frequent. ## Diagnosis Horses typically present with acute, often severe lameness and may stand with the carpus flexed. Palpation reveals pain over the accessory carpal bone; crepitus or instability may be felt. Radiographs (lateromedial and flexed lateromedial views) confirm the fracture. Ultrasound evaluates soft-tissue involvement and carpal sheath effusion. Computed tomography can further characterize complex fractures. ## Treatment Conservative management with 3 to 6 months of stall rest and controlled exercise is often chosen for non-displaced or minimally displaced fractures. Surgical options include lag-screw fixation, fragment removal when the fragment is intra-articular or within the carpal sheath, or, in select cases, partial resection. Recent biomechanical studies support both dual lag-screw and single lag-screw plus plate constructs. Carpal canal involvement increases the risk of adhesions and warrants earlier surgical intervention in many cases. ## Recovery and Prognosis Prognosis for return to athletic use is fair to good with appropriate treatment, but many horses have residual mechanical limitation or require a longer convalescence than other carpal fractures. Internal fixation generally offers the best chance of full athletic recovery (often 9 to 12 months). Conservative management can succeed, especially in non-athletes, but nonunion is common. ## Prevention Avoidance of falls onto a flexed carpus and careful management of high-speed or jumping work reduce risk. No specific preventive program has been validated. Signs include: acute lameness of the affected limb; swelling of the carpus or carpal canal; pain on palpation of the accessory carpal bone; reluctance to flex the carpus; occasional crepitus
Isolated pathology is uncommon. The muscle or its insertion can be involved in trauma to the caudal forearm or accessory carpal region. Ulnar-nerve lesions affect it.
Swelling or pain behind the knee or on the outside of the forearm after trauma should be checked. Most soft-tissue injuries improve with rest.
See the Flexor Carpi Ulnaris on our interactive 3D horse model. Open 3D Model