A 28-year-old construction worker sustains a fracture-dislocation of the shoulder with posterior displacement. Which is the most common cause of radial nerve injury in proximal humeral fractures?

Explanation

## Mechanism of Radial Nerve Injury in Proximal Humeral Fractures **Key Point:** **Traction injury** is the most common mechanism of radial nerve injury in proximal humeral fractures, occurring when the proximal fragment is displaced and stretches the nerve. ### Anatomical Relationship at the Surgical Neck The radial nerve: - Arises from the posterior cord of the brachial plexus (C5–C8) - Enters the arm posterior to the axillary artery - Lies in close proximity to the surgical neck of the humerus - Has limited mobility due to muscular attachments (triceps branches) **High-Yield:** Unlike the radial groove (where compression dominates), the proximal humerus injury mechanism is predominantly **traction** because the nerve is stretched across the displaced fracture site. ### Mechanisms of RN Injury by Fracture Location | Fracture Site | Primary Mechanism | Secondary Mechanism | Incidence | | --- | --- | --- | --- | | Surgical neck (proximal) | **Traction** | Compression by swelling | 2–3% | | Radial groove (mid-shaft) | **Compression** | Traction | 10–18% | | Distal third | **Traction** | Compression | 5–10% | **Clinical Pearl:** In proximal humeral fractures with posterior displacement (as in this case), the proximal fragment is pulled posteriorly, causing the radial nerve to be **stretched over the anterior aspect of the fracture site**, resulting in traction injury. ### Why Traction, Not Compression? 1. **Limited soft tissue buffer** — The nerve is tethered by muscular branches and has minimal slack 2. **Displacement vector** — Posterior displacement of the proximal fragment pulls the nerve backward and stretches it 3. **Nerve mobility** — Unlike at the radial groove (fixed in bone), the nerve at the surgical neck has some mobility but is constrained by attachments 4. **Timing** — Traction injury occurs immediately at the time of fracture, whereas compression develops over hours/days **Mnemonic:** **"PROX = TRACTION"** — Proximal humeral fractures cause traction injury to the radial nerve. ### Clinical Presentation Radial nerve injury at the surgical neck presents with: - Weakness of **all radial nerve-innervated muscles** (triceps, brachioradialis, wrist/finger extensors) - **Sensory loss** over the dorsal first web space - **Wrist drop** and inability to extend fingers **Warning:** Do not confuse with axillary nerve injury (which occurs with anterior shoulder dislocation and causes deltoid paralysis and loss of sensation over lateral shoulder).