A 28-year-old male motorcyclist presents with a flaccid arm following a road traffic accident. Examination reveals loss of abduction and external rotation of the shoulder, along with loss of sensation over the lateral aspect of the shoulder. Which is the most common site of injury to the brachial plexus in such traumatic scenarios?

Explanation

## Most Common Site of Brachial Plexus Injury in Trauma ### Epidemiology of Traumatic Brachial Plexus Injuries **Key Point:** Upper trunk injuries account for approximately 50–60% of all traumatic brachial plexus injuries, making them the most common site of injury in traction-type mechanisms. ### Mechanism of Upper Trunk Injury The upper trunk (formed by C5 and C6 nerve roots) is most vulnerable to injury because: 1. **Traction mechanism** — Shoulder depression with head tilting away (as in motorcycle falls, forceful lateral neck flexion) stretches the upper trunk maximally. 2. **Anatomical position** — The upper trunk lies superficially at the angle between the anterior and middle scalene muscles, making it exposed to direct trauma. 3. **Supraclavicular location** — Most traumatic injuries occur above the clavicle, where the upper trunk is the most accessible structure. ### Clinical Presentation of Upper Trunk (Erb's Palsy) | Feature | Finding | |---------|----------| | **Affected nerve roots** | C5, C6 | | **Motor loss** | Shoulder abduction (supraspinatus, infraspinatus), external rotation (infraspinatus), elbow flexion (biceps, brachialis) | | **Sensory loss** | Lateral shoulder and lateral forearm | | **Classic posture** | "Waiter's tip" — arm hangs at side, internally rotated, extended at elbow | | **Reflex loss** | Biceps reflex (C5–C6) | ### Comparison with Other Trunk Injuries | Trunk | Frequency | Mechanism | Key Features | |-------|-----------|-----------|---------------| | **Upper (C5–C6)** | 50–60% | Shoulder depression + head tilt away | Waiter's tip, loss of abduction/external rotation | | **Lower (C8–T1)** | 25–30% | Arm hyperabduction or forceful upward traction | Claw hand, loss of intrinsic hand muscles | | **Middle (C7)** | 10–15% | Direct blow to shoulder | Wrist drop, loss of elbow/wrist extension | | **Complete plexus** | 5–10% | Severe avulsion injuries | Pan-plexopathy | **High-Yield:** In motorcycle accidents and birth-related injuries (shoulder dystocia), upper trunk injury is the most common pattern because the mechanism inherently involves lateral neck traction. ### Why Upper Trunk Is Most Vulnerable **Clinical Pearl:** The upper trunk is the **first structure to be stretched** when the shoulder is depressed and the head is tilted away, because it spans the widest distance between its root attachments (C5–C6 exit foramina) and its continuation as the upper trunk in the supraclavicular region. [cite:Clinically Oriented Anatomy 8e Ch 6]