The radius and ulna are joined together along their entire length by a fibrous interosseous membrane and touch only at their ends to form the complex proximal and distal radioulnar joints. The ulna is a comparatively straight bone, whereas the radius has an important outward bowing. During the motions of supination and pronation, the ulna holds a relatively fixed position, while the radius rotates around it. Because these bones have such a close relationship to one another, injury to either will have a direct impact on the other. A displaced or angulated fracture of one bone typically disrupts the other or causes a dislocation at the proximal or distal radioulnar joint, such as in the Monteggia and Galeazzi fracture-dislocations.
The radius and ulna are also under the influence of numerous muscle groups, such as those that supinate and pronate. The biceps brachii and the supinator insert on the proximal radius and are the powerful supinators of the forearm. The pronator teres inserts just distal to them and onto the midsection of the radius. As its name suggests, it is responsible for pronation. Radius fractures that are located between these muscle groups will result in marked displacement of the bone, with supination of the proximal segment and pronation of the distal portion. However, if the fracture is distal to the insertion of the pronator teres, these forces tend to neutralize one another and result in less rotational deformity.
The neuroanatomy is most easily understood by appreciating the neural control of the most basic components of wrist and finger movement ( Fig, 2.61-7). The radial nerve travels over the lateral epicondyle and supplies the muscles involved in wrist extension before it gives off a branch, the posterior interosseous nerve. This branch travels around the proximal radius and controls the muscles that extend the fingers and thumb. The remainder of the radial nerve is purely sensory and innervates the posterior aspect of the hand from the thumb to radial half of the ring finger. Quite simply, the proximal portion of the radial nerve controls the more proximal function of wrist extension, while the distal branch (posterior interosseus nerve) controls the more distal function of finger extension and another branch that is purely sensory. So an isolated injury (e.g., to the posterior interosseous branch) would affect finger extension but spare wrist extension and sensation to the dorsum of the hand. The single best test of radial nerve function is to have the patient extend both the wrist and fingers against resistance, and check the sensation over the dorsum of the hand.
FIG. 261-7. A. The radial nerve controls wrist extension before branching into the posterior interosseous nerve. B. The median nerve controls wrist flexion and the flexor digitorum superficialis before branching into the anterior interosseous nerve (controls the deep finger flexors in the forearm) and a branch that innervates the thenar muscles and provides sensation to most of the palm. C. The ulnar nerve controls the intrinsic muscles and sensation to the ulnar side of the hand. [From Chin HW, Propp DA, Orban DJ: Forearm and wrist, in Rosen P, Barkin RM, et al (eds): Emergency Medicine Concepts and Clinical Practice, 3d ed, vol 1. St. Louis, Mosby Year Book, 1992.]
The median nerve controls the basic movements of wrist and finger flexion and sensation on the volar surface of the hand from the thumb to the radial half of the ring finger. The proximal portion of the median nerve innervates the muscles that control wrist flexion and the flexor digitorum superficialis before it gives off the anterior interosseous nerve. This branch controls portions of the remaining deep finger flexors: flexor digitorum profundus, flexor pollicus longus, and pronator quadratus. The remaining portion of the median nerve provides sensation to most of the volar surface of the hand plus a motor branch to the thenar muscles of the thumb (recurrent branch of the median nerve). The median nerve is evaluated by assessing each of these distal branches. A simple test of the anterior interosseous nerve is the ability to make a circle, or "OK" sign, with the thumb and index finger; if so, this nerve is likewise "OK." Abduction of the thumb (recurrent branch of the median nerve) and intact sensation on the radial side of the palm complete the evaluation of the median nerve.
The ulnar nerve provides innervation to a few forearm muscles but, more important, controls the intrinsic muscles of the hand and provides sensation to the little finger and the ulnar half of the ring finger. The ability to abduct the index finger against resistance and normal sensation on the ulnar side of the hand is an easy test of ulnar function.
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