This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The forearm may require to be operated on for disease or injuries of the bones, tumors, foreign bodies, wounds, etc. In operating on this region of the body it is to be constantly borne in mind that it contains a multitude of structures each of which is essential to the proper performance of some special function. Injury to these structures is followed by a corresponding functional disability. Attempts at brilliant operating are out of place and the surgeon should be exact, careful, and even tender in his handling of the various structures.
The nerves that supply the forearm are given off high up near the elbow, hence the are not usually in danger of injury. The median, ulnar, and superficial branch of the radial nerve pass to the hand and they, if possible, are to be avoided.
It is therefore evident that as far as the arteries and nerves are concerned operations in the lower part of the forearm are less dangerous than those in the upper. With the muscles it is just the opposite. In the lower half the muscles become tendinous and soon form groups or masses of tendons. These tendons are separated by thin connective-tissue sheaths or synovial membrane which allow them to move freely as the muscles contract. Any interference with these sheaths or their contents causes an outpouring of inflammatory material that binds them together and fetters their action. As healing takes place contraction sets in and the patient is left with a useless claw-like hand. For these reasons large incisions and displacements and interference with tendons are to be avoided whenever possible.
As the muscles mostly run longitudinally the incisions should also be longitudinal. Division of the superficial veins is not liable to cause trouble, but the large radial, median, or ulnar veins on the anterior surface may be plainly visible and then the incision should be made so as to avoid wounding them.
The only superficial nerve to be so avoided is the superficial branch of the radial. It is alongside of the radial artery in its middle third, but about 7 or 8 cm. (3 in.) above the wrist it leaves the artery and winds under the brachioradialis to go down the outer and posterior surface of the radius. It is here to be looked for and avoided, as it furnishes sensation to the thumb, index, middle, and half of the ring fingers.
If it is desired to penetrate the muscles their direction is to be remembered. The superficial flexor muscles arise from the internal condyle, hence the incision should point upward toward it. The direction of the pronator radii teres is from the internal condyle to the middle of the radius. The deep flexors are parallel with the bones.
Posteriorly the extensor group of muscles tends toward the external condyle. A third group on the radial side comprises the brachioradialis and the extensor carpi radialis longior and brevior. The tendon of the first lies on the outer surface of the radius with the other two immediately posterior to it. Crossing the posterior and outer surface of the radius in its lower third are the extensor ossis metacarpi pollicis and extensor brevis pollicis tendons.
If it is desired to reach the bones the ulna can be exposed posteriorly where it is subcutaneous in its entire length by an incision between the flexor carpi ulnaris and extensor carpi ulnaris. The deep fascia is attached to the bone at this point.
If it is desired to expose the radius, H. Morris (Clin. Soc. Trans., vol. x, p. 138) has advised going in between the brachioradialis and the extensor carpi radialis longior. He used the superficial branch of the radial nerve as a guide to the desired interspace.
If an incision were made upward from the outer surface of the styloid process of the radius one would first encounter the tendons of the extensor brevis pollicis and extensor ossis metacarpi pollicis muscles. These being displaced posteriorly would reveal the brachioradialis tendon crossing from beneath the posterior border of the radius; 5 to 7 cm. (2 to 3 in.) above the styloid process would be the superficial branch of the radial nerve. Following the nerve and edge of the brachioradialis tendon would lead to the interspace between it and the extensor carpi radialis muscle posteriorly. When the middle of the forearm was reached the insertion of the pronator teres would be encountered, and from that point up the bone would be covered by the supinator (brevis).
Operations on the median nerve (page 319) and the ulnar nerve (page 320) have already been alluded to.
In operations involving the upper third of the radius the deep branch of the radial (posterior interosseous) nerve is liable to be wounded as it passes through the supinator (brevis) muscle. It is best avoided by elevating the muscle from the bone and raising the nerve along with it, for it does not rest immediately on the bone but has some muscular fibres intervening.
The arteries have already been sufficiently described.