In anatomical parlance, the forearm is the part of the body which extends from the elbow to the wrist and is not to be confused with the arm which extends from the shoulder to the elbow. The extensor muscles in the forearm are extensor carpi ulnaris, extensor digiti minimi, extensor digitorum, extensor indicis, extensor pollicis longus, extensor pollicis brevis, extensor carpi radialis brevis, extensor carpi radialis longus. These extensor muscles are supplied by the radial nerve. Other muscles in the forearm also innervated by the radial nerve are supinator and abductor pollicis longus. Note that all these muscles are situated in the posterior half of the forearm (posterior when in the anatomical position). Also,Brachioradialis, Anconeus, Triceps brachii, and Extensor carpi radialis longus innervated muscular branches of the radial nerve in the arm.
Wrist extension is achieved by muscles in the forearm contracting, pulling on tendons that attach distal to (beyond) the wrist. If the tendons, the muscles, or the nerves supplying these muscles, are not working as they should be, wrist drop may occur. The following situations may result in wrist drop:
Stab wounds to the chest at or below the clavicle may result in wrist drop. The radial nerve is the terminal branch of the posterior cord of the brachial plexus. A stab wound may damage the posterior cord and result in neurological deficits including an inability to abduct the shoulder beyond 15 degrees, an inability to extend the forearm, reduced ability to supinate the hand, reduced ability to abduct the thumb and sensory loss to the posterior surface of the arm and hand.
The radial nerve can be damaged if the humerus (the bone of the arm) is broken, because it runs through the radial groove on the lateral border of this bone.
Persistent injury to the nerve is also a common cause through either repetitive motion or by applying pressure externally along the route of the radial nerve as in the prolonged use of crutches or extended leaning on the elbows.