American Board of Surgical Assistants (ABSA) Orthopedic Practice Test

In a humeral shaft fracture, displacement and angulation are primarily influenced by the muscles acting on the humerus. When a fracture occurs, the surrounding muscles exert their tension, which can pull the fragments of the bone out of alignment.

The pectoralis major muscle, which is responsible for adduction of the arm, will contract and pull the proximal fragment medially and anteriorly. This contraction contributes to a specific angulation at the fracture site. Meanwhile, the deltoid muscle, which is responsible for abduction, can pull the distal fragment laterally or superiorly, depending on the direction and severity of muscle contraction.

As a result, the combined effect of the pectoralis muscle adducting and the deltoid muscle abducting leads to displacement and angulation of the fractured humeral shaft. The concurrent action of these two muscle groups results in an imbalance of forces, causing the fracture fragments not just to separate, but also to align in a way that creates an angle between them, leading to the characteristic deformity seen in such fractures. Thus, the correct choice highlights the involvement of both muscle actions in the process of displacement and angulation in a humeral shaft fracture.