Definition
By: Gregory R. Waryasz, MD
The extensor digitorum muscle of the musculoskeletal system is characterized by being the principle extensor for the medial four fingers.
It is part of the forearm. It consists of skeletal muscle fibers.
Its unique structural features include its being a wide, flat, and fusiform shape. It covers much of the posterior forearm. The muscle belly splits into four tendons. The four tendons join with the extensor indicis tendon to pass deep to the extensor retinaculum through the tendinous sheath of the extensor digitorum and extensor indicis. On the dorsum of the hand, the tendons are tethered together by the junctureae tendinum. The tendons then spread out to their destinations of the dorsum of the fingers.
The origin is the lateral epicondyle of the humerus at the common extensor origin.
The insertion is the extensor expansions of the medial four fingers.
The blood supply is from the posterior interosseous artery and venous drainage is from the accompanying veins.
The innervation is from the posterior interosseous nerve.
The extensor digitorum muscle as well as all other bones, muscles, and ligaments of the body are derived of mesodermal origin in the embryo.
The function of the extensor digitorum is to extend the medial four fingers primarily at the metacarpophalangeal joints and also at the interphalangeal joints. The synergists are the extensor indicis, extensor digiti minimi, and the lumbricals.
Common diseases include tendinitis, tendon rupture, and injury to any part of the extensor mechanism.
Tendinitis is an overuse injury.
Tendon rupture can occur due to trauma.
A disruption of the sagittal band causes the extensor tendon to sublux between metacarpal heads. This can occur in rheumatoid arthritis or from activities such as throwing a baseball requiring significant torque. The result is loss of full active extension of the metacarpophalangeal joint.
The Boutonniere deformity is an injury to the central slip where there is flexion of the proximal interphalangeal joint and hyperextension of the distal interphalangeal joint. The injury occurs as a result of acute rupture or chronic synovitis at the proximal interphalangeal joint.
A Mallet finger is a loss of full active extension at the distal interphalangeal joint due to injury to the extensor digitorum tendon insertion. There can be an avulsion injury where a piece of bone is avulsed.
Commonly used diagnostic procedures include clinical history, physical exam, x-ray, and MRI.
It is usually treated with physical therapy, steroid injections, and NSAIDs for tendinitis. Surgery may be required for tendinitis that does not respond to conservative measures. Surgery may be required to repair tendon ruptures. Sagittal band disruption is treated initially with splinting, but chronic injuries may require releasing the ulnar sagittal band and reconstructing the extensor tendon. A Boutonniere deformity when a closed injury can be initially treated with splinting in full extension. Laceration injuries require surgical repair and pinning the joint in full extension. A Mallet finger’s treatment is typically splinting in extension for 8 weeks, but surgery may be required to repair the injured digit.
References
Lieberman J (ed), AAOS Comprehensive Orthopaedic Review, American Academy of Orthopaedic Surgeons, 2008.
Moore K, Dalley A (eds), Clinically Oriented Anatomy (5th edition), Lippincott Williams & Wilkins, Philadelphia, PA, 2006.
Wheeless’ Textbook of Orthopaedics: Extensor Digitorum (http://www.wheelessonline.com/ortho/extensor_digitorum_communis)