The superficial extensors of the forearm
Anatomy and supply
The superficial extensors of the forearm are group of three muscles located at the posterior side of the forearm. Due to their superficial course, they form the surface of the lateral side of the posterior forearm where they can be easily palpated, especially during hand extension. They all share a common extensor tendon arising from the lateral epicondyle of the humerus together with the extensor carpi radialis brevis from the radial musculature. The three superficial extensors are:
- Extensor digitorum muscle: originates from the lateral epicondyle of the humerus. Distally it divides into four smaller tendons which insert at the posterior surface of the middle and distal phalanges of the second to fifth fingers. Hereby the insertion tendons build a sheet-like tendinous expansion, the dorsal aponeurosis, covering the medial, posterior and lateral surfaces of the distal phalanges. The dorsal aponeurosis serves as a place of insertion for various ligaments and muscles.
- Extensor digiti minimi muscle: arises from the lateral epicondyle of the humerus and inserts at the dorsal aponeurosis of the little finger. Its' muscle belly often blends with the extensor digitorum.
- Extensor carpi ulnaris muscle: has its' origin at both the lateral epicondyle of the humerus (humeral head) and the posterior side of the ulna (ulnar head). This muscle's insertion is located at the base of the fifth metacarpal bone.
Like all extensors of the forearm, these three muscles are innervated by the radial nerve. The radial nerve divides into a superficial and deep branch at the height of the radial head. While the superficial branch continues along the brachioradialis the deep branch penetrates the supinator muscle after which it branches off the posterior interosseus nerve (C6-C8). There upon the posterior interosseus nerve supplies the superficial extensors with motor branches.
As the name suggests, all superficial extensors of the forearm fulfill a dorsal extension in the wrist joint. Furthermore, the extensor digitorum extends the metacarpophalangeal (MCP), proximal interphalangeal (PIP) and distal interphalangeal joints (DIP) of the second to fifth fingers. Due to the fact that its' insertion tendons are partly attached to each other through the dorsal aponeurosis, an extension of one finger simultaneously leads to a slight extension of the neighboring fingers. The extensor digiti minimi extends the MCP, DIP and PIP of the little finger. The contraction of the extensor carpi ulnaris pulls the hand medially (ulnar abduction).
Approximately 2-3 % of all people have a so called 'small brother' of the extensor digitorum, the extensor digitorum brevis manus. It originates around the area of the distal radius and inserts at the dorsal aponeurosis of one or more fingers. This accessory muscle is normally harmless but can compress the posterior interosseus nerve during hypertrophy causing pain in the wrist joint. During clinical examination, it appears like a bulge at the dorsum of the hand, which is why it is often mistaken as a tendon ganglion, cyst or even a soft-tissue tumor. In order to prevent misdiagnoses and unnecessary surgeries one should always consider an accessory muscle as a differential diagnosis when facing pain in the wrist joint.
The cause for the development lies in the incomplete migration of the forearm muscles during embryogenesis. It is assumed that this muscle anomaly represents an atavism, a “throwback” to our phylogenetic ancestors. It reminds us that our distant ancestors controlled their hands and fingers solely by short, intrinsic hand muscles. The takeover of this function by forearm muscles with long tendons has only developed through the course of evolution.