During a difficult shoulder dislocation reduction, a 35-year-old man develops acute loss of sensation over the lateral aspect of the shoulder and upper arm, along with weakness of shoulder abduction. The axillary artery remains palpable and intact. Which anatomical relationship explains this nerve injury?

Explanation

## Axillary Nerve Anatomy and Injury Mechanism **Anatomical Course of the Axillary Nerve:** - **Origin:** Terminal branch of the posterior cord (C5–C6) - **Path:** Exits the axilla posteriorly through the **quadrangular space** (also called the axillary nerve space) - **Boundaries of quadrangular space:** - **Superior:** Teres minor - **Inferior:** Teres major - **Medial:** Long head of triceps - **Lateral:** Surgical neck of humerus **Clinical Significance:** - The nerve is vulnerable during shoulder dislocation reduction because the humeral head displacement stretches the nerve as it passes through the quadrangular space - The nerve can also be injured by fractures of the surgical neck of the humerus - Posterior circumflex humeral artery accompanies the nerve but is separate from the axillary artery proper **Key Point:** The axillary nerve's posterior course through the quadrangular space makes it vulnerable to traction injuries during shoulder manipulation, while the axillary artery (which runs anterior and medial) remains intact. **Mnemonic:** "TTLH" = Teres minor (superior), Teres major (inferior), Long head of triceps (medial), Humerus (lateral)