What motor function is lost with an axillary nerve injury?

An axillary nerve injury primarily impacts the deltoid muscle, which is responsible for shoulder abduction, particularly from 15 to 90 degrees. The axillary nerve innervates the deltoid as well as the teres minor muscle, and its injury would lead to a significant loss of deltoid function. This results in weakness of shoulder abduction and an inability to lift the arm away from the body.

The other muscle functions mentioned are not directly affected by an injury to the axillary nerve. Triceps extension is primarily controlled by the radial nerve, which affects the triceps muscle in the posterior compartment of the arm. Biceps function, which includes elbow flexion and shoulder flexion, is primarily mediated by the musculocutaneous nerve. Wrist flexion involves the median and ulnar nerves, as they innervate the flexor muscles in the forearm. Therefore, it is clear that the primary motor function lost with an axillary nerve injury is that of the deltoid.