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:

Question

Accepted Answer

A. Weakness of elbow extension occurs because the radial nerve supplies the triceps muscle. ## 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).

Answer

B. Sensory loss occurs in the distribution of the superficial radial nerve (dorsum of thumb, index, and middle fingers)

Answer

C. Weakness of wrist extension occurs due to paralysis of extensor carpi radialis longus and brevis

Answer

D. Loss of thumb abduction results from paralysis of the abductor pollicis longus muscle

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

Motor Deficits in Radial Nerve Injury

Sensory Deficits

Analysis of Each Option

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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

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