Final answer:
The axillary nerve can be injured due to a strong blow to the shoulder or a hard fall onto an outstretched hand, potentially resulting from fractures to the humerus or shoulder dislocations.
Step-by-step explanation:
Injuries to the axillary nerve can occur due to mechanisms such as a strong blow to the shoulder or a hard fall onto an outstretched hand. These traumas can lead to damage either directly from the impact or indirectly through associated fractures, such as a distal humerus fracture.
Two common injuries of the pectoral girdle following such traumas include fractures of the clavicle or scapula and dislocations of the shoulder joint. These injuries may have subsequent effects on the axillary nerve, which primarily innervates the deltoid and teres minor muscles and plays a crucial role in shoulder abduction. A humeral head dislocation, often occurring in an inferior direction due to strong muscular and ligamentous support in other directions, can stretch or tear the axillary nerve.
Furthermore, a fracture of the humerus can damage the axillary nerve due to its close proximity to the bone. For instance, a fracture across the distal humerus, above both epicondyles (supracondylar fracture) or separating one or both of the epicondyles from the body of the humerus (intercondylar fracture) may lead to nerve injury. The resulting nerve damage can present as weakness in shoulder abduction and a loss of sensation over the lateral aspect of the shoulder.