A 32-year-old male labourer presents with inability to extend the thumb at the interphalangeal joint and wrist. On examination, he has wrist drop and sensory loss over the dorsal first web space (between thumb and index finger). He reports a fracture of the mid-shaft humerus 2 weeks ago, which was treated conservatively. Which nerve is injured and at what anatomical level?

Explanation

## Clinical Presentation Analysis **Key Point:** Mid-shaft humeral fractures are the classic cause of radial nerve injury in the spiral groove, occurring in 10–15% of cases due to the nerve's intimate relationship with the bone in this region. ### Anatomical Correlation The radial nerve travels in the **spiral groove (radial groove)** on the posterior surface of the mid-shaft humerus. A fracture at this level causes direct nerve trauma or compression from callus formation. ### Clinical Features of Radial Nerve Injury at Spiral Groove Level | Feature | Findings | |---------|----------| | **Motor deficit** | Wrist drop (loss of wrist extension via ECRB, ECRL, ECU) + thumb IP extension loss (EPL) | | **Sensory loss** | Dorsal first web space (radial-supplied area) | | **Nerve branches affected** | Both motor (posterior interosseous nerve) and sensory (superficial radial nerve) branches | | **Mechanism** | Direct trauma or compression in spiral groove | **High-Yield:** The radial nerve is most vulnerable to injury at the **mid-shaft humerus** (spiral groove), making humeral fractures the most common cause of radial nerve palsy in clinical practice. ### Why This Level, Not Others? ```mermaid flowchart TD A[Radial Nerve Course in Upper Arm]:::outcome --> B{Anatomical Level?}:::decision B -->|Axilla to mid-humerus| C[Radial nerve proper]:::action B -->|Spiral groove mid-humerus| D[Most vulnerable to fracture]:::urgent B -->|Below elbow| E[PIN & superficial branch]:::action D --> F[Wrist drop + thumb IP loss + web space sensory loss]:::outcome C --> G[Fracture here causes complete radial nerve injury]:::action ``` **Clinical Pearl:** The **2-week timeline** and **conservative treatment** are key: delayed radial nerve palsies can occur from callus compression or nerve entrapment in healing bone. ## Why Correct The combination of **wrist drop** (ECRB/ECRL loss), **thumb IP extension loss** (EPL loss), and **dorsal web space sensory loss** (superficial radial nerve) indicates injury to the **complete radial nerve at the spiral groove level**, where both motor and sensory branches are still together. [cite:Clinically Oriented Anatomy Moore 8e Ch 6]