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AN10.1-13 | Axilla, Shoulder and Scapular region — Part 5
Axillary Nerve Injury — The Deltoid Danger Zone (AN10.12)
The axillary nerve (C5, C6) is a terminal branch of the posterior cord. It wraps around the surgical neck of the humerus — the narrow region just below the humeral head. This makes it vulnerable in two situations:
- Fracture of the surgical neck of the humerus — common in elderly patients who fall on an outstretched hand
- Anterior dislocation of the shoulder — the humeral head displaces forward and stretches the nerve
- Intramuscular injection in the wrong site — injecting too low in the deltoid can hit the axillary nerve
Consequences of axillary nerve injury:
• Paralysis of deltoid — can't abduct the arm beyond 15° (supraspinatus can still initiate, but deltoid can't continue)
• Paralysis of teres minor — weakness of lateral rotation
• Loss of sensation over the regimental badge area — a patch of skin over the lower deltoid (where a military regiment badge would sit)
Safe injection zone: For intramuscular injections in the deltoid, always inject in the upper third of the deltoid, approximately 2–3 finger-breadths below the acromion. This avoids the axillary nerve, which runs at the level of the surgical neck (lower portion of the deltoid).
IMAGE PLACEHOLDER
The axillary nerve wrapping around the surgical neck of the humerus, showing the regimental badge area (sensory territory) and the safe injection zone in the upper third of the deltoid muscle
SELF-CHECK
A medical student is about to give a deltoid intramuscular injection. To avoid damaging the axillary nerve, the injection should be placed:
A. In the lower third of the deltoid, near the deltoid tuberosity
B. In the upper third of the deltoid, 2-3 finger-breadths below the acromion
C. At the exact centre of the deltoid muscle
D. At the posterior border of the deltoid
Reveal Answer
Answer: B. In the upper third of the deltoid, 2-3 finger-breadths below the acromion
The axillary nerve wraps around the surgical neck of the humerus at the level of the lower deltoid. To avoid it, inject in the upper third of the deltoid — 2–3 finger-breadths below the acromion. This is above the path of the axillary nerve. Injecting too low risks paralysing the deltoid.
CLINICAL PEARL
Rotator cuff tears are extremely common — by age 60, up to 30% of people have a partial tear (often asymptomatic). The supraspinatus tendon is most commonly torn because it has a relatively poor blood supply in its 'critical zone' (near its insertion on the greater tubercle) and is repeatedly compressed between the humeral head and the acromion during abduction. Patients present with a painful arc — pain between 60° and 120° of abduction (the range where supraspinatus is maximally compressed). Below 60° and above 120°, the pain decreases. MRI is the investigation of choice. Treatment ranges from physiotherapy (strengthening the remaining rotator cuff muscles) to arthroscopic surgical repair.