The extensor digitorum brevis manus (EDBM) is a rare anomalous muscle on the dorsal wrist and hand, present in approximately 1-3% of individuals. It presents as a soft tissue mass on the dorsal wrist, typically becoming more prominent with wrist flexion, and is most commonly confused with a dorsal wrist ganglion. It may also be associated with a true dorsal ganglion or tenosynovitis.
| Origin | Variable: dorsal wrist capsule, carpus, or distal radius; typically in the fourth extensor compartment region |
|---|---|
| Insertion | Extensor mechanism of the index or middle finger, joining the EDC tendon or the extensor hood |
| Nerve Supply | Posterior interosseous nerve (radial nerve branch) |
| Blood Supply | Dorsal carpal arterial branches |
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When present it assists extension of the index or middle finger.
The EDBM is a well-recognised diagnostic pitfall on the dorsal wrist: its muscle belly presents as a firm dorsal wrist swelling that increases with wrist flexion and disappears with wrist extension, the opposite behaviour from a typical dorsal ganglion. Ultrasound confirms the muscular nature of the mass (echogenicity identical to adjacent muscle, movement with finger extension). MRI differentiates it from soft tissue tumour. Excision is curative when the mass is symptomatic from impingement against the extensor retinaculum during wrist movements.
When present it is felt as a soft, mobile swelling on the back of the hand near the bases of the second and third metacarpals, which becomes firmer when the fingers are extended against resistance.
The extensor digitorum brevis manus presents as a dorsal wrist mass that enlarges with wrist flexion and finger extension, mirroring the dynamic properties of the underlying muscle; its firm consistency and EMG activity during finger extension distinguish it from the tense fluid-filled nature of a true dorsal ganglion.
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