A 32-year-old male labourer presents to the orthopaedic clinic with a 3-week history of weakness in wrist and finger extension following a fracture of the mid-shaft humerus sustained during a fall. On examination, he has wrist drop, inability to extend the thumb at the interphalangeal joint, and sensory loss over the dorsal first web space. The fracture has been managed conservatively. Which nerve is injured and at what anatomical location is the lesion most likely to occur?

Explanation

## Anatomical Course of the Radial Nerve The radial nerve is the largest branch of the posterior cord of the brachial plexus (C5–T1). It has a characteristic spiral course around the humerus. ### Course Through the Arm 1. **Axilla to spiral groove**: The radial nerve descends posteriorly in the axilla, lying medial to the axillary artery. 2. **Spiral groove (radial groove)**: The nerve enters the spiral groove (also called the radial groove) on the posterior surface of the mid-shaft humerus, where it lies in close contact with the bone. 3. **Exit point**: It emerges from the spiral groove at the junction of the middle and distal thirds of the humerus, piercing the lateral intermuscular septum. 4. **Anterior compartment**: It then descends between the brachialis and brachioradialis muscles. 5. **Elbow region**: Just proximal to the elbow, it divides into its terminal branches: the posterior interosseous nerve (PIN) and the superficial radial nerve. ### Vulnerability in Mid-Shaft Humeral Fractures **Key Point:** Mid-shaft humeral fractures carry the highest risk of radial nerve injury (10–18% of cases) because the nerve is tethered within the spiral groove and cannot escape lateral displacement of fracture fragments. **Clinical Pearl:** Wrist drop (inability to extend the wrist) is the hallmark sign of radial nerve injury at the level of the spiral groove, as it affects the extensor carpi radialis longus (ECRL) and extensor carpi radialis brevis (ECRB). ### Clinical Findings in This Case | Finding | Nerve Segment Affected | |---------|------------------------| | Wrist drop | Radial nerve proximal to spiral groove | | Loss of thumb IP extension | Posterior interosseous nerve (extensor pollicis longus) | | Sensory loss, dorsal first web space | Superficial radial nerve (sensory branch) | **High-Yield:** The combination of wrist drop + thumb IP extension loss + dorsal first web space sensory loss indicates a radial nerve injury at or just distal to the spiral groove. The sensory loss confirms involvement of the superficial radial nerve, which branches off distal to the spiral groove. ### Why the Spiral Groove? - The spiral groove is the narrowest, most vulnerable segment of the radial nerve's course. - Fracture fragments, haematoma, and callus formation can compress or stretch the nerve. - The nerve cannot retract laterally because it is bound by the intermuscular septum. **Mnemonic: WRIST DROP = Radial nerve at Spiral groove** — the classic triad of wrist drop + thumb extension loss + dorsal web sensory loss localizes the lesion to the distal spiral groove or just beyond. ## Differential Localization ```mermaid flowchart TD A["Radial Nerve Injury"]:::outcome --> B{"Which segment?"}:::decision B -->|"Axilla (proximal)"|C["Loss of triceps, wrist/finger extension, dorsal arm sensation"]:::outcome B -->|"Spiral groove (mid-shaft humerus)"|D["Wrist drop + thumb IP extension loss + dorsal web sensory loss"]:::outcome B -->|"Distal to spiral groove"|E["Wrist/finger extension loss, but triceps intact"]:::outcome B -->|"At/distal to elbow"|F["PIN: finger extension loss (thumb IP, fingers 2-5 at PIP/DIP)"]:::outcome B -->|"Wrist/hand"|G["Superficial radial: sensory only (dorsal hand)"]:::outcome D -->|"This case"|H["Mid-shaft humeral fracture"]:::action ``` [cite:Clinically Oriented Anatomy by Moore & Dalley, 8e; Robbins & Cotran Pathologic Basis of Disease 10e Ch 27]