A 32-year-old man presents with a fracture of the mid-shaft of the humerus following a motor vehicle accident. Which is the most common site of radial nerve injury in humeral shaft fractures?

Explanation

## Radial Nerve Injury in Humeral Shaft Fractures **Key Point:** The radial nerve is most vulnerable at the **radial groove (spiral groove)** in the middle and distal thirds of the humerus, where it lies in close proximity to the bone. ### Anatomical Basis The radial nerve follows a predictable course along the humerus: 1. Enters the arm posterior to the axillary artery 2. Passes through the radial groove (spiral groove) on the posterior surface of the humerus 3. Pierces the lateral intermuscular septum in the distal third 4. Emerges anterior to the lateral epicondyle **High-Yield:** The radial groove is the **narrowest and most fixed point** where the nerve lies directly against bone, making it vulnerable to compression and stretching during fracture displacement. ### Incidence of Radial Nerve Injury by Site | Fracture Location | Incidence of RN Injury | Most Common Site | | --- | --- | --- | | Surgical neck | 2–3% | Proximal traction | | Mid-shaft (radial groove) | 10–18% | **Radial groove (most common)** | | Distal third | 5–10% | Distal traction | **Clinical Pearl:** Mid-shaft humeral fractures account for approximately 60% of all radial nerve injuries associated with humeral fractures, and the radial groove is the injury site in the vast majority of these cases. ### Why the Radial Groove? - The nerve is **tethered** by branches to triceps and the lateral intermuscular septum - Fracture displacement causes **direct compression** or **stretching** of the nerve against the bone edges - The groove provides **minimal soft tissue protection** compared to proximal or distal sites **Mnemonic:** **"GROOVE is the GROOVE"** — The radial groove in the middle-distal humerus is the groove where radial nerve injury grooves (occurs most commonly). ### Clinical Presentation Radial nerve injury at this level results in: - **Wrist drop** (loss of extensor carpi radialis longus and brevis, extensor carpi ulnaris) - Loss of thumb extension (extensor pollicis longus) - Loss of finger extension (extensor digitorum) - Sensory loss over the dorsal first web space **Warning:** Do not confuse with posterior interosseous nerve (PIN) injury, which occurs distal to the radial groove and spares sensation.