The clavicle has three functions:
1 to transmit forces from the upper limb to the axial skeleton;
2 to act as a strut holding the arm free from the trunk, to hang supported principally by trapezius;
The weakest point along the clavicle is the junction of the middle and outer third. Transmission of forces to the axial skeleton in falls on the shoulder or hand may prove greater than the strength of the bone at this site and this indirect force is the usual cause of fracture.
When fracture occurs, the trapezius is unable to support the weight of the arm so that the characteristic picture of the patient with a fractured clavicle is that of a man supporting his sagging upper limb with his opposite hand. The lateral fragment is not only depressed but also drawn medially by the shoulder adductors, principally the teres major, latissimus dorsi and pectoralis major (Fig. 121).
The humerus (Fig. 122)
The upper end of the humerus consists of a head (one-third of a sphere) facing medially, upwards and backwards, separated from the greater and lesser tubercles by the anatomical neck. The tubercles, in turn, are separated by
Where the upper end and the shaft of the humerus meet there is the narrow surgical neck against which lie the axillary nerve and circumflex humeral vessels. The shaft itself is circular in section above and flattened in its lower part. The posterior aspect of the shaft bears the faint spiral groove, demarcating the origins of the medial and lateral heads of the triceps between which wind the radial nerve and the profunda vessels.
The lower end of the humerus bears the rounded capitulum laterally, for articulation with the radial head, and the spool-shaped trochlea medially, articulating with the trochlear notch of the ulna.
The medial and lateral epicondyles, on either side, are extra-capsular; the medial is the larger of the two, extends more distally and bears a groove on its posterior aspect for the ulnar nerve.
Three important nerves thus come into close contact with the humerus —the axillary, the radial and the ulnar; they may be damaged, respectively, in fractures of the humeral neck, midshaft and lower end (Fig. 122).
It is an important practical point to note that the lower end of the humerus is angulated forward 45° on the shaft. This is easily confirmed by examining a lateral radiograph of the elbow, when it will be seen that a vertical line continued downwards along the front of the shaft bisects the
capitulum. Any decrease of this angulation indicates backward displacement of the distal end of the humerus and is good radiographic evidence of a supracondylar fracture.
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