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.
There seems to be but little doubt that in many cases the character of displacement of the fragments in fracture of the shaft of the humerus is due to the mode of injury and not to muscular action. This being so accounts for there being less uniformity in these fractures than in those higher up, which have already been considered. There are some cases, however, in which muscular action does play a part and the possible influence of the muscles should be understood.
The line of fracture is usually more or less oblique, in rare cases nearly transverse, but the displacement is often not marked. Notwithstanding this latter fact, non-union of fracture of the shaft of the humerus is one of the most frequent of any in the body.
Muscular action shows its influence most markedly in producing displacements in three directions, viz., in towards the body, out away from the body, and directly anteriorly.
There are two main points where fracture occurs; immediately above the insertion of the deltoid and below it.
The bone may be fractured immediately above the deltoid insertion. In this case the powerful axillary fold muscles, pectoralis major, teres major, and latissimus dorsi, being attached to the upper fragment, tend to draw it toward the body, while the deltoid tends to draw the lower fragment out. The influence of the other muscles, biceps, coracobrachialis, and triceps, would be to increase the overlapping (Fig. 289).
This is the more common site of fracture. The line of fracture is most apt to be from above downward and outward. The upper fragment is displaced anteriorly by the coracobrachialis and anterior portion of the deltoid and is drawn outward by the deltoid aided by the supraspinatus. To relax the deltoid the arm is sometimes dressed in an abducted position (Fig. 290). Non-Union. - The humerus has muscles attached to it almost throughout its entire length, and when the sharp ends of the fragments are displaced they probably become fixed in the surrounding muscle, and proper apposition of the fragments is prevented, hence non-union. Hamilton believed that lack of proper fixation was also a prominent cause. The Radial (Musculospiral) Nerve. - In fracture of the shaft of the humerus, paralysis of the extensors due to injury of the radial nerve is comparatively common. It also occurs from pressure due to the use of crutches, to sleeping on the arm, etc. The other nerves are too far removed from the bone to be injured, but the radial (musculospiral) lies on the bone in the radial (musculo-spiral) groove in approximately the middle third of the bone. It comes into contact with the bone posteriorly above the insertion of the deltoid and leaves the bone on its outer anterior surface to pass between the brachialis anticus and brachioradialis (supinator longus)muscles. Paralysis may be caused (1) by direct injury to the nerve at the time the fracture is received. (2) By subsequent changes in the nerve due to its being stretched over the sharp edge of a fragment. (3) By being included in callus. The last is probably much more rarely the case than the two former (Fig. 291). Paralysis should be examined for early in the course of treatment. Too often it is detected only after the splints have been removed, and then it is apt to be ascribed to improper treatment or to misapplied pressure. The symptoms of involvement of the musculospiral nerve are wrist-drop and diminution of the power of supination, also some sensory changes in the dorsum of the hand and forearm.
Fig. 289. - Fracture of the shaft of the humerus just above the insertion of the deltoid and below the insertion of the axillary fold muscles. The lower fragment is seen to be drawn outward by the deltoid; the upper fragment is seen to be drawn inward by the pectoralis major, latissimus dorsi, and teres major.
Fig. 290. - Posterior view of a fracture of the shaft of the humerus just below the insertion of the deltoid, showing the influence of that muscle in producing abduction of the upper fragment.
This nerve is frequently paralyzed from pressure in cases in which there is no fracture, as from sleeping on the arm, the use of crutches, and also in certain systemic affections, such as lead poisoning. It supplies the triceps, part of the brachialis anticus, brachioradialis (supinator longus), and extensor carpi radialis longior muscles in the arm, and then proceeds to the forearm. The branch to the triceps is given off before the nerve enters the musculospiral groove, hence is not often injured, and loss of extension of the forearm is not often present; even paralysis of the other muscles mentioned is not common, the forearm muscles being mostly affected. The branch to the inner head of the triceps also supplies the anconeus.