An accessory flexor digitorum superficialis indicis muscle is an unusual accessory muscle of the hand and wrist and a normal anatomical variant.
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Summary
- origin: flexor digitorum superficialis tendon near the transverse carpal ligament
- insertion: metacarpal head of the index finger near the A1 pulley
Gross anatomy
The accessory flexor digitorum superficialis muscle of the index finger is characterized by several variations that have been reported in the literature. It usually arises from the flexor digitorum superficialis tendon at the level of the transverse carpal ligament and runs through the carpal tunnel and within the flexor tendon sheath to insert near the metacarpophalangeal joint of the index finger 1-5.
Variant anatomy
Variations of the accessory flexor digitorum superficialis indicis muscle include the following 1:
- replacement of the normal tendon with a muscle belly within the palm
- digastric muscle with components in the palm and the forearm
- muscular component extending into but not through the carpal tunnel
Radiographic features
MRI
The accessory flexor digitorum superficialis indicis muscle can be nicely visualized on an MRI of the wrist. It travels deep in the subcutaneous tissue through the carpal tunnel and the distal tendon or palmar muscle belly extends into the flexor tendon sheath 1.
History and etymology
The accessory flexor digitorum superficialis indicis muscle was first described by the Scottish-American anatomist and statistician Donald Mainland in 1927 as ‘an uncommon abnormality of the flexor digitorum sublimis muscle'.
Clinical importance
Depending on the variant anatomy the accessory flexor digitorum muscle of the index finger might be confused with a pathologic condition such as a tumor 1,2.
Related pathology
The accessory flexor digitorum muscle of the index finger has been implicated in the following clinical conditions 1,2:
- carpal tunnel syndrome
- chronic wrist pain
- wrist tumor
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