Applied Anatomy of Deltoid

Axillary Nerve Injury

The axillary nerve, which lies on the deep surface of the deltoid as it winds around the surgical neck of the humerus, may be damaged by:

The rounded contour of the shoulder is lost, and the greater tubercle of the humerus becomes prominent, since the deltoid (which normally caps the shoulder) is paralysed and wasted.
Deltoid is paralysed, resulting in loss of the power of abduction of the arm from 0° to 90° at the shoulder joint.
Sensory loss occurs over the lower half of the deltoid, in a characteristic patch of skin known as the “regimental badge” area — corresponding to the cutaneous distribution of the upper lateral cutaneous nerve of the arm (a branch of the axillary nerve).

Intramuscular injections are commonly given into the deltoid.
To avoid injury to the axillary nerve (which winds around the surgical neck of the humerus deep to the muscle), injections should be given into the upper and outer quadrant of the deltoid region (lower half of deltoid, away from the nerve’s course).

In subacromial bursitis, pressure applied over the deltoid just below the acromion, with the arm hanging by the side, causes pain.
When the arm is then abducted, pressure over the same point causes no pain, because the inflamed bursa disappears under cover of the acromion during abduction.
This change in tenderness with arm position is known as Dawbarn’s sign, and is suggestive of subacromial/subdeltoid bursitis — usually secondary to supraspinatus tendon inflammation.

Clinical Scenario	Key Finding(s)
Axillary nerve injury (shoulder dislocation / surgical neck fracture)	Loss of rounded shoulder contour, prominent greater tubercle, loss of abduction (0-90°), regimental badge sensory loss
Intramuscular injection	Given in upper-outer quadrant of deltoid to avoid axillary nerve
Subacromial bursitis	Positive Dawbarn’s sign (pain on pressure with arm by side, relieved on abduction)