Extensor digitorum

Musculus extensor digitorum

Also known as: extensor digitorum communis, EDC, extensor expansion, dorsal hood

The main extensor of the medial four fingers. It arises from the lateral epicondyle and forms the extensor expansions (dorsal hoods) of digits 2–5.

Overview

Extensor digitorum arises from the lateral epicondyle and divides into four tendons that pass through the 4th extensor compartment with extensor indicis. On the back of the hand the tendons are linked by oblique intertendinous connections (juncturae tendinum), which limit independent extension of the ring finger. Over each MCP joint the tendon widens into a triangular extensor expansion (dorsal digital expansion, "hood"). The lumbricals and interossei insert into this expansion. It then divides into a central slip to the base of the middle phalanx and two lateral bands that reunite to insert on the base of the distal phalanx.

It mainly extends the MCP joints. Extension of the interphalangeal joints needs the lumbricals and interossei acting through the expansion. It also helps extend the wrist.

Parts & subdivisions

Extensor expansion (dorsal hood)

Aponeurotic hood over the MCP joint and proximal phalanx. Receives the lumbricals and interossei.

Central slip

Inserts on the base of the middle phalanx and extends the PIP joint.

Lateral bands

Reunite as the terminal tendon on the base of the distal phalanx and extend the DIP joint.

Juncturae tendinum

Oblique bands linking the tendons on the dorsum of the hand.

Anatomy

Origin

Lateral epicondyle of the humerus (common extensor origin).

Insertion

Extensor expansions of digits 2–5 (central slip → middle phalanx; lateral bands → distal phalanx).

Action

Extension of the MCP joints (and, with the intrinsic muscles, the IP joints) of the medial four digits; assists wrist extension.

Innervation

Posterior interosseous nerve (deep branch of the radial nerve).

Nerve roots

C7–C8

Arterial supply

Posterior interosseous artery.

Clinical significance

Mallet finger

Avulsion of the terminal extensor tendon (lateral bands) from the distal phalanx, e.g. when a ball strikes the fingertip. The DIP joint droops and cannot be actively extended. Treated with a splint holding the DIP joint in extension for 6–8 weeks.

Boutonnière deformity

Rupture of the central slip lets the lateral bands slip below the axis of the PIP joint. The result is PIP flexion with DIP hyperextension. Seen after trauma and in rheumatoid arthritis.