Fractures of the humerus are classified as proximal, midshaft, and distal. In this chapter, we discuss concerns related to midshaft fractures and disorders of the muscles of the upper arm. Proximal humerus fractures are discussed in Chapter 16, and distal fractures are discussed in Chapter 14. Humeral shaft fractures are relatively uncommon, representing only 3% of all fractures.1 Humerus fractures are responsible for 370,000 emergency visits yearly, of which humeral shaft fractures represent about 13%.2,3 The incidence of humeral shaft fractures is bimodal with peaks around age 10 and age 80.4 Most humeral shaft fractures occur in children at a rate of ~290 per 100,000 population as compared to 63 to 69 per 100,000 population in the population as a whole.3–5 Among the elderly, humeral shaft fracture disproportionately affects women at a rate of ~2:1. In developing nations, limited data suggest a peak incidence during middle age with motor vehicle accidents as a primary cause.6
The humeral shaft extends from the insertion of the pectoralis major to the supracondylar ridges. Humeral shaft fractures can be described based on the number of fracture lines and relative position of fracture fragments using the AO fracture classification system. This information is useful to the orthopedic consultant as the classification of the fracture correlates with the need for surgical repair. Simple fractures (AO Type A) have one fracture line and may be spiral, oblique, or transverse. Wedge fractures (AO Type B) have multiple fracture lines and a wedge-shaped defect from the humeral shaft. The proximal and distal parts of the humerus remain in close contact. Complex fractures (AO Type C) usually include multiple fracture lines, and the proximal and distal portions of the humerus are no longer in close contact (Fig. 15–1).7
A. Type A fractures are simple and do not have an associated fracture fragment. They can be transverse, oblique, or spiral. B. Type B fractures have a free wedge-shaped fragment. The humeral shaft ends remain in close proximity. C. Type C fractures are complex (comminuted) fractures, where there is increased distance between the fractured ends of the humerus with interposed fracture fragments.
The extensive musculature surrounding the humeral shaft may result in distraction and displacement of the bony fragments after a fracture. The deltoid inserts along the anterolateral humeral shaft, whereas the pectoralis major inserts on the medial intertubercular groove (Fig. 15–2). The supraspinatus inserts into the greater tuberosity of the humeral head, resulting in abduction and external rotation. The biceps and the triceps insert distally and tend to displace the distal fragment proximally.
In humeral shaft fractures, the muscles of the proximal humerus cause displacement ...