Extensor digitorum brevis manus muscle

The extensor digitorum brevis manus (EDBM) muscle is an accessory muscle in the hand and is a normal anatomical variant. 

Epidemiology

It is thought to be present in ~ 3% of the population ¹. It is often painless although rarely can present as a painful mass over the dorsal aspect of the hand. It can be bilateral in up to half of cases.

Gross anatomy

The EDBM muscle lies along the ulnar side of the extensor tendon of the index finger (usually fourth wrist compartment ⁵). It commonly arises at the distal end of the radius and posterior radiocarpal ligament to insert, most commonly on the index finger. However, insertion can also be seen on the 3^rd, 4^th, or 5^th digits as well as multiple insertions on more than one digit. 

Blood supply

• arterial supply - posterior interosseous nerve

Nerve supply

• posterior interosseous nerve

Clinical presentation

The muscle is usually painless but may occasionally be associated with exercise-induced pain or tenosynovitis of the extensor tendons.

Radiographic features

Plain film

Can be normal

Ultrasound

Sonography may reveal a soft-tissue mass with a musclelike echo texture, on real time this usually undergoes morphologic changes during active finger extension.

MRI

Signal characteristics include

• T1/T2/PD - Iso dense to muscle on all sequences
• C+ (Gd) - no enhancement in uncomplicated cases (e.g. unless there is any inflammation etc.)

Summary

• origin - distal radius and posterior radiocarpal ligmanet
• insertion - extensor hood of index or middle fingers (variable)
• innervation - posterior interosseous nerve
• action - extension of the fingers