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.
These are the most common fractures of the humerus. The surgical neck of the humerus is usually defined as the portion between the lower part of the tuberosities and the upper edge of the tendons of the pectoralis major and latissimus dorsi muscles. Often, however, the tendons of these two muscles continue almost or quite up to the tuberosities, hence there is little or no interval here and the line of fracture then passes through the upper part of these tendons.
The fractures occur both from direct and indirect violence and the direction of the force has probably something to do with the displacement of the fragments.
It can readily be seen that if a blow is received on the humerus below the tuberosities while the arm is in a somewhat abducted position the head will be supported by the glenoid process (head) of the scapula and the bone will be fractured through the surgical neck and driven in towards the body, and, as the scapula is supported posteriorly, the movable lower fragment is displaced anteriorly. After the fracture has occurred, and possibly in some cases aided by the peculiar direction of the fracturing force, the lower fragment is drawn upward by the muscles running from one side of the fracture to the other. These are the deltoid, biceps, coracobrachialis, and the long head of the triceps. The typical displacement is for the upper fragment to be abducted and some say rotated out - this latter is not without doubt. The lower fragment is certainly in front and to the inside of its normal position.
The abduction of the upper fragment is due to the unresisted action of the supraspinatus muscle. The subscapulars in front and the teres minor and infraspinatus behind nearly or quite balance each other, thus causing little or no lateral displacement. The displacement inward and anteriorly of the lower fragment, is due to the action of the violence as already detailed and is aided by the action of the pectoralis major, the teres major, and latissimus dorsi muscles, all of which pass from the lower fragment just below the seat of fracture inward to the trunk.
Fig. 260. - Fracture through the anatomical neck of the humerus.
The longitudinal displacement is peculiar. As the lower fragment is drawn up its upper end may be felt through the deltoid muscle below and toward the inner side of the acromion. While the displacement in most cases is not marked, in some the lower fragment can readily be felt in the axilla (Fig. 261).
Sometimes instead of the lower fragment being displaced inward it goes outward. In this case as it rises it pushes the head and tilts it inward while it passes farther outward.
Fig. 261. - Fracture of the surgical neck of the humerus. The upper fragment is held out by the supra" spinatus, while the lower fragment is drawn in by the pectoralis major, latissimus dorsi, and teres major muscles and the arm abducted by the deltoid.crepitus is felt, the upper end of the lower fragment can often be palpated, and on rotating the arm the head of the bone is found to lie stationary.
The ideal treatment is extension with the patient in bed and the arm abducted. As the upper fragment cannot be brought in, an effort may be made to bring the lower one out. As these are usually treated as walking cases a common dressing employed is a shoulder-cap with the arm bound to the side; sometimes an axillary pad is used to keep the arm away from the body. In cases of fracture associated with luxation of the head of the bone, replacement can sometimes be effected by traction in the abducted position and pressure on the head, general anaesthesia being used (see description of direct method of reduction under dislocation of the shoulder, page 236).
To aid in the reduction McBurney devised a hook which he inserts into the upper fragment, pulling it toward the glenoid cavity.