What nerve is likely damaged if a patient cannot abduct the arm beyond 25 degrees?

When a patient is unable to abduct the arm beyond 25 degrees, this commonly indicates a problem with the deltoid muscle, which is primarily responsible for arm abduction. The deltoid is innervated by the axillary nerve, which arises from the brachial plexus (specifically from the C5 and C6 nerve roots).

In normal anatomical function, a patient can execute the initial phase of arm abduction involving the supraspinatus muscle—this is effective from 0 to about 15 degrees of abduction. However, further abduction from 15 to 90 degrees primarily engages the deltoid muscle. If the axillary nerve is damaged, the patient will experience weakness or inability to effectively use the deltoid, limiting their ability to abduct the arm beyond approximately 25 degrees.

In contrast, the radial nerve, musculocutaneous nerve, and median nerve innervate different muscles and functions, primarily affecting extension of the wrist and fingers, elbow flexion, and hand movements. These nerves do not have a direct effect on the deltoid's ability to abduct the arm, which is why they are not associated with the described impairment in shoulder abduction.