A 32-year-old man presents with wrist drop and inability to extend the thumb after a fracture of the mid-shaft of the humerus. Which is the most common site of radial nerve injury in such fractures?

Explanation

## Most Common Site of Radial Nerve Injury in Humeral Fractures ### Anatomical Basis **Key Point:** The radial nerve runs in the radial groove (spiral groove) on the posterior surface of the mid-shaft of the humerus, making this the most vulnerable site during mid-shaft humeral fractures. The radial nerve follows a predictable course: 1. Exits the axilla posteriorly 2. Enters the radial groove at the junction of proximal and middle thirds 3. Travels in the groove between the medial and lateral heads of triceps 4. Pierces the lateral intermuscular septum in the distal third 5. Emerges anterior to the lateral epicondyle ### Incidence of Radial Nerve Injury by Fracture Location | Fracture Site | Incidence of RN Injury | Mechanism | |---|---|---| | **Mid-shaft (radial groove)** | **10–18%** | **Direct trauma, angulation, traction** | | Proximal third | 2–3% | Rare, nerve already exited groove | | Distal third | <5% | Nerve already anteriorly displaced | | Supracondylar | Uncommon | Nerve protected by anterior displacement | ### Clinical Presentation of Radial Nerve Palsy **High-Yield:** The classic triad of radial nerve injury: 1. **Wrist drop** — loss of extensor carpi radialis longus and brevis 2. **Thumb extension loss** — extensor pollicis longus paralysis 3. **Finger extension loss** — extensor digitorum paralysis **Clinical Pearl:** Sensation loss over the dorsal first web space (between thumb and index finger) is a key sensory finding. ### Why the Radial Groove is Most Vulnerable **Mnemonic:** **GROOVE** = **G**reat vulnerability, **R**adial nerve tethered, **O**scillating fracture edges, **O**sseous trauma, **V**ascular compromise, **E**xcoriation - The nerve is tethered within the groove with limited mobility - Fracture fragments can directly lacerate or compress the nerve - Angulation and displacement cause traction injury - The nerve's intimate relationship with bone makes it susceptible to ischemia from vascular injury ### Prognosis **Key Point:** Most radial nerve injuries from humeral fractures are neurapraxia or axonotmesis (not complete transection), with 90% recovering spontaneously within 3–4 months with conservative management. [cite:Standring Anatomy 42e Ch 48]