## Most Common Brachial Plexus Injury in Traction **Key Point:** The upper trunk of the brachial plexus (C5–C6) is the most commonly injured component in traction injuries, accounting for approximately 50–60% of all brachial plexus injuries. ### Mechanism of Upper Trunk Injury Traction injuries occur when: 1. The shoulder is forcibly depressed (downward) 2. The head is laterally flexed away from the injured side 3. This stretches the upper trunk maximally across the transverse cervical artery and anterior scalene muscle ### Clinical Presentation of Upper Trunk Injury (Erb's Palsy) | Finding | Explanation | |---------|-------------| | **Loss of shoulder abduction** | Suprascapular nerve (C5–C6) → supraspinatus paralysis | | **Loss of external rotation** | Suprascapular nerve → infraspinatus paralysis | | **Scapular winging** | Dorsal scapular nerve (C5) → serratus anterior paralysis | | **Arm hangs in adduction and internal rotation** | Classic "waiter's tip" position | | **Loss of elbow flexion** | Musculocutaneous nerve (C5–C6) | **High-Yield:** The upper trunk is vulnerable because it is the most superficial and mobile component of the plexus, making it susceptible to stretch injuries. ### Why Other Trunks Are Less Commonly Injured - **Middle trunk (C7):** Protected by its central location; injured in only ~10% of cases - **Lower trunk (C8–T1):** Injured in ~25% of cases, usually in severe hyperabduction injuries (Klumpke's palsy) - **Posterior cord:** Rarely injured in isolation; usually part of complete plexus injury **Clinical Pearl:** The "waiter's tip" deformity (arm adducted and internally rotated) is pathognomonic for upper trunk injury and is the classic teaching sign in NEET PG. [cite:Standring Anatomy 41e Ch 57]
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