Accessory flexor carpi ulnaris: case report and review of the literature

Abstract

Most of the accessory muscles of the forearm described in the radiology literature are located either in the radial aspect of the forearm or towards the hypothenar eminence. We present an unusual case of an ulnar-sided distal forearm accessory flexor carpi ulnaris muscle presenting as a "pseudotumour" demonstrated with both ultrasound and MRI, rarely reported in the current surgical and anatomical literature. Given the location and relation to the ulnar nerve towards Guyon's canal, the accessory muscle may also predispose to distal ulnar nerve entrapment.

Figure 1.

Medical photograph of the “pseudotumour“ towards the ulna aspect right distal forearm.

Figure 2.

Transverse ultrasound images for “mirror“ comparison of bilateral distal forearms at the level of pronator quadratus muscle demonstrating the accessory muscle bulk deep and separate to normal FCU (white arrows). Note the ulna neurovascular bundle deep to the accessory muscle (asterisk). FCU, flexor carpiulnaris; FDSI, flexor digitorum superficialis.

Figure 3.

Axial PD weighted images (TR 3314; TE 30) at (a) level just proximal to Guyon’s canal, (b) approximately 3 cm proximal to the radiocarpal joint, (c) at level of pisiform and (d) coronal T₁ weighted (TR 590.52; TE 20) images demonstrating the accessory muscle (white arrows), larger FCU proper (white block arrows), and ulna nerve (asterisks) including their separate tendon insertions to pisiform. Note the normal appearing PL tendon. FCU, flexor carpiulnaris; PD, proton density; PL, palmaris longus; TE, echo time; TR, repetitiontime.