Accessory flexor carpi ulnaris muscle

Last revised by Dr Joachim Feger on 10 Sep 2021

The accessory flexor carpi ulnaris (AFCU) is a rare accessory muscle of the forearm and wrist and a normal anatomical variant that can be found in addition to a normal flexor carpi ulnaris muscle.

The accessory flexor carpi ulnaris muscle is considered very rare 1,2.

It has been associated with other muscular variants of the forearm such as 2-4:

The accessory flexor carpi ulnaris muscle is an extra muscle of the medial volar forearm with features of the normal flexor carpi ulnaris muscle and variably of the palmaris longus muscle. It originates from the medial surface of the ulna and courses radial or deep to the normal flexor carpi ulnaris muscle inserting at the pisiform radial and separate to the insertion of the latter and variably to the flexor retinaculum 1,2.

Variant insertions of the accessory flexor carpi ulnaris include the hook of the hamate, the abductor digiti minimi or the base of the fifth metacarpal bone 5.

On ultrasound, the accessory flexor carpi ulnaris muscle is characterized by a muscle like well-defined hypoechoic echotexture 2.

The accessory flexor carpi ulnaris muscle can be visualized on MRI of the wrist and forearm on the ulnar side deep to flexor carpi ulnaris muscle 1.

According to Le Double, the accessory flexor carpi ulnaris muscle was described first by the French anatomist Jean-Francois Jarjavay in 1857 6.

It might be confused with a pathologic condition on imaging such as a soft tissue tumor 4.

The accessory flexor carpi ulnaris muscle has been implicated in the following clinical conditions 1:

  • volar ulnar swelling
  • ulnar nerve entrapment

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