Anatomy and Location
- Origin: The muscle originates from the lateral epicondyle of the humerus (epicondylus lateralis) as part of the common extensor tendon.
- Forearm topography: Belongs to the superficial layer of the posterior forearm compartment. In a cross-section of the middle third of the forearm, it lies posteriorly alongside the extensor carpi ulnaris (m. extensor carpi ulnaris).
- Insertion: Tendons extend to all digits except the thumb. They pass to digits II–V via the dorsal digital expansions (aponeurosis dorsalis).
- Tendon slips: The central slip attaches to the base of the middle phalanx, and the lateral slips attach to the base of the distal phalanx. The tendon of the extensor digiti minimi (m. extensor digiti minimi) merges with the tendon for the little finger.
Wrist Passage
The tendons pass through the common extensor tendon compartment at the wrist.
- Synovial sheath: Contained within the common synovial sheath for the extensor digitorum and extensor indicis tendons (vagina tendinum mm. extensoris digitorum et extensoris indicis).
- Sheath borders: The base of the triangular sheath faces distally toward the digits. Proximally, the sheath extends 10 mm above the extensor retinaculum (retinaculum mm. extensorum), and distally it ends blindly at the mid-metacarpal level.
Blood Supply and Innervation
- Blood supply: Posterior interosseous artery (a. interossea posterior).
- Innervation: Radial nerve (n. radialis), C7–C8.
Function
- Extends digits II–V at the metacarpophalangeal (MCP) and interphalangeal (IP) joints.
- Assists in wrist extension.
- Assists in abduction of the fingers.
Clinical Significance
- Tendon injuries: Disruption of the central slip leads to a boutonnière deformity. Rupture of the lateral slips results in a mallet finger deformity.
- Neurological assessment: Needle electromyography (EMG) of the extensor digitorum is used to evaluate denervation and reinnervation changes in mononeuropathies. It is also tested to assess motor function and trophic status.
Differential Diagnosis: Extensor Digitorum Longus
In the lower limb, there is a homologous muscle — the extensor digitorum longus (m. extensor digitorum longus). It originates from the lateral condyle of the tibia, the anterior surface of the fibula, the interosseous membrane, and the crural fascia; it is innervated by the deep fibular nerve (n. peroneus profundus) and extends digits II–V of the foot. Its tendons divide on the dorsum of the foot: the intermediate parts attach to the bases of the middle phalanges, and the lateral parts attach to the bases of the distal phalanges.