Introduction

Flexor carpi ulnaris is a superficial flexor muscle of the forearm that flexes and adducts the hand. It is the most powerful wrist flexor. The flexor carpi ulnaris originates from two separate heads connected by a tendinous arch. The humeral head arises from a flexor tendon origin on the medial epicondyle, while the ulnar head arises from the olecranon and upper three-fourths of the subcutaneous border of the ulna by an aponeurosis. Flexor carpi ulnaris inserts on the fifth metacarpal bone, the hook of hamate, and the pisiform bone of the wrist. Flexor carpi ulnaris inserts into the hook of hamate through the pisohamate ligament and inserts into the 5 metacarpal bone through the pisometacarpal ligament.[1]

Structure and Function

Flexor carpi ulnaris flexes and adducts (ulnar deviates) the hand at the wrist. Flexor carpi ulnaris is also a weak flexor of the elbow.[2]

Embryology

The upper limb musculature arises from the dorsolateral somite cells that move into the limb around the fourth week to form muscles.[3] With growth and further lengthening of the limb buds, the muscle tissue divides into separate extensor and flexor components determined by connective tissue derived from the lateral plate mesoderm. The fibroblast growth factor synchronizes the developmental schema between the anterior-posterior and the proximal-distal axis by ensuring Sonic hedgehog expression in cells contained within the zone of polarizing activity. In turn, the sonic hedgehog ensures that the fibroblast growth factor is present in the apical ectodermal ridge.[4] Once the limb buds form, the primary ventral rami pass through into the mesenchyme, and the ulnar nerve arises from the ventral branch to eventually supply the flexor carpi ulnaris.

Blood Supply and Lymphatics

Flexor carpi ulnaris is supplied by the ulnar collateral arteries and small ulnar artery branches. The ulnar artery reaches the flexor carpi ulnaris along with the middle third of its muscle belly. Flexor carpi ulnaris lymphatic drainage is part of the upper limb lymph system consisting of superficial and deep lymphatic vessels. The superficial vessels around the basilic vein go to the cubital lymph nodes, which are proximal to the medial epicondyle of the humerus. Vessels around the cephalic vein go to the axillary lymph nodes. The deep lymphatic vessels also drain lymph from the flexor carpi ulnaris and follow the major deep veins, eventually terminating in the humeral axillary lymph nodes.[5]

Nerves

Flexor carpi ulnaris is innervated by the muscular branch of the ulnar nerve, C7, and C8. Unlike the median nerve, which passes between the two heads of the pronator teres, the ulnar nerve passes between the two heads of the flexor carpi ulnaris.[6]

Muscles

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.[2]

Physiologic Variants

The accessory flexor carpi ulnaris is a physiologic variant 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.[7] 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.[8] 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.

https://teachmeanatomy.info/upper-limb/areas/ulnar-canal/

Surgical Considerations

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 inter-nervous 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.[9]

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. 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.[10]

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.

Clinical Significance

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.[11] 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 two 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.[12] Muscle wasting and clawing of the ring and small fingers indicate chronic compressive syndrome.[13]

Other Issues

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 contributes to the tendonitis pain at the wrist.[14]