The flexor carpi ulnaris is one of the superficial flexor muscles of the forearm along with the pronator teres, flexor carpi radialis, palmaris longus, and flexor digitorum superficialis (FDS) muscles. These superficial forearm flexor muscles attach through a common flexor tendon onto the medial epicondyle of the humerus.

The accessory flexor carpi ulnaris is a physiologic variant that is located in an anteromedial plane to flexor digitorium superficialis and between flexor carpi ulnaris and flexor digitorum superficialis, extending to the radial aspect of flexor carpi ulnaris. In one case report, this variant originates at the medial epicondyle 1-cm posterolateral to the flexor carpi ulnaris origin and forms a separate tendon 5 cm above the proximal border of the flexor retinaculum.[4] This variant inserts to the flexor retinaculum and wrist bones, including the triquetral and hamate.

There are various insertions and origins described for the accessory flexor carpi ulnaris. The brevis variant of the accessory flexor carpi ulnaris may insert into the pisiform, the hook of the hamate, abductor digiti minimi, or fifth metacarpal. Other variants of the accessory flexor carpi ulnaris described in the literature have been found to insert on the flexor pollicus longus tendon, flexor digitorum profundus tendon for the index finger, and triquetral bone.

These physiologic variants are important because abnormal anatomical structures may compress the ulnar nerve within the Guyon canal.[5] Previous studies have found ulnar artery thrombosis to be associated with abnormal flexor carpi ulnaris variant in the Guyon canal. Knowledge of these anatomic variants is important during preoperative, surgical planning so providers can prepare for modified surgical exposures.

The flexor carpi ulnaris is important for many surgical exposures, including exposure of the shaft of the ulna. When treating for open reduction and internal fixation of ulnar fractures, osteotomy of the ulna, ulnar lengthening, ulnar shortening, or treatment of the fibrous anlage of the ulna in cases of ulnar clubhand, the primary surgical exposure aims to exposure the ulnar shaft through the internervous plane between the extensor carpi ulnaris and flexor carpi ulnaris muscles. The ulnar nerve travels down the forearm under the flexor carpi ulnaris, and the surgeon must be careful to strip the flexor carpi ulnaris from the ulna epiperiosteally. The ulnar nerve may be damaged if dissection into the Flexor carpi ulnaris is achieved, and thus the surgeon must identify the ulnar nerve between the two heads of the Flexor carpi ulnaris before flexor carpi ulnaris stripping off the ulnar shaft.

The flexor carpi ulnaris is surgically important for the volar approach to the ulnar nerve at the wrist. This volar approach is used to decompress the canal of Guyon in cases of ulnar nerve compression. The volar carpal ligament is seen as a continuation of the deep palmar fascia and flexor carpi ulnaris fibers, and the flexor carpi ulnaris fascia must be incised to allow muscle retraction to reveal the ulnar nerve and artery. The ulnar nerve and artery lie just beneath the flexor carpi ulnaris and volar carpal ligament, and care must be taken during superficial and deep surgical dissection to protect the ulnar nerve.

During subcutaneous transposition of the ulnar nerve for cubital tunnel syndrome, the deep and superficial aponeurosis covering the two flexor carpi ulnaris heads are released. A transverse arc of fascia is found distal to the flexor carpi ulnaris ulnar hiatus and released to avoid iatrogenic compression. The surgeon must be careful to avoid ligating the flexor carpi ulnaris motor branches during Flexor carpi ulnaris release.

The neurovascular bundle of the forearm, including the ulnar artery, vein, and venae comitantes, is formed by the flexor carpi ulnaris, FDS, and flexor digitorum profundus. The flexor carpi ulnaris is an important anatomical guide when the wrist is flexed and ulnarly deviated.

Cubital tunnel syndrome is the second most common compression neuropathy of the upper extremity and involves the ulnar nerve.[6] Compression and entrapment of the ulnar nerve most commonly occur between the two heads of the Flexor carpi ulnaris aponeurosis. The cubital tunnel is a passage between the 2 heads of the Flexor carpi ulnaris. They connect via the continuation of the fibroaponeurotic covering of the epicondylar groove, also known as the Osborne ligament. Flexor carpi ulnaris compression of the ulnar nerve may manifest as paresthesias of the small finger, ulnar half of the ring finger, and ulnar dorsal hand with night symptoms exacerbated by sleeping with the arm in flexion. During arm flexion, the cubital tunnel flattens as the ligament stretches, causing ulnar nerve compression.

Prolonged compression of the ulnar nerve by the Flexor carpi ulnaris may lead to not only decreased sensation in the ulnar 1-1/2 digits, but patients may complain of weak grasp and thumb pinch (Froment’s sign) from loss of metacarpal phalangeal joint flexion power and thumb adduction.[7] Muscle wasting and clawing of the ring and small fingers indicate chronic compressive syndrome.

Flexor carpi ulnaris tendinopathy is an overuse injury that presents with focal volar and ulnar-sided wrist pain that is reproduced with wrist flexion and ulnar deviation. The pisiform is a sesamoid within the Flexor carpi ulnaris tendon that is thought to contribute to the tendonitis pain at the wrist.