A 35-year-old man presents with wrist drop and loss of sensation over the dorsum of the thumb and index finger after a mid-shaft humeral fracture sustained in a motor vehicle accident. Regarding the clinical consequences of radial nerve injury at this level, all of the following are true EXCEPT:

Explanation

## Radial Nerve Injury — Motor and Sensory Deficits ### Clinical Presentation of Radial Nerve Palsy **Key Point:** Mid-shaft humeral fractures are the most common cause of radial nerve injury (Saturday night palsy, crutch palsy). The nerve is vulnerable in the radial groove. ### Motor Deficits in Radial Nerve Injury **High-Yield:** The radial nerve innervates the **posterior compartment of the arm and forearm**: | Muscle | Action | Innervation | Deficit | |--------|--------|-------------|----------| | **Triceps** | Elbow extension | Radial nerve (proximal) | **Preserved** in mid-shaft fracture | | **Extensor carpi radialis longus (ECRL)** | Wrist extension | Radial nerve (distal) | **Paralyzed** → wrist drop | | **Extensor carpi radialis brevis (ECRB)** | Wrist extension | Posterior interosseous nerve (PIN) | **Paralyzed** → wrist drop | | **Abductor pollicis longus (APL)** | Thumb abduction | Posterior interosseous nerve (PIN) | **Paralyzed** → loss of thumb abduction | | **Extensor pollicis longus (EPL)** | Thumb IP extension | Posterior interosseous nerve (PIN) | **Paralyzed** | | **Extensor digitorum** | Finger extension | Posterior interosseous nerve (PIN) | **Paralyzed** → finger drop | **Clinical Pearl:** The **triceps muscle is innervated by the radial nerve proximal to the spiral groove**. In mid-shaft humeral fractures, the nerve is injured **distal to the triceps branch**, so elbow extension remains intact. This is a key differentiating feature. ### Sensory Deficits **Mnemonic:** **SRN-DAM** = **S**uperficial **R**adial **N**erve supplies **D**orsum of thumb, **A**djacent fingers, **M**iddle finger (lateral half). The superficial radial nerve (terminal sensory branch) provides sensation to: - Dorsum of thumb - Dorsum of index finger - Dorsum of middle finger (lateral half) - Dorsum of hand (lateral half) ### Analysis of Each Option | Option | Accuracy | Comment | |--------|----------|----------| | Wrist extension weakness (ECRL/ECRB) | ✓ Correct | Classic finding; causes wrist drop | | Thumb abduction loss (APL) | ✓ Correct | APL is innervated by PIN (distal radial nerve) | | Sensory loss (dorsum thumb/index/middle) | ✓ Correct | Superficial radial nerve distribution | | Elbow extension weakness (triceps) | ✗ **INCORRECT** | Triceps is innervated **proximal** to the injury site | **Warning:** Do not confuse radial nerve injury (wrist drop, finger drop) with **axillary nerve injury** (shoulder abduction loss) or **musculocutaneous nerve injury** (elbow flexion loss).