A 6-year-old boy is brought to the emergency department after falling off gym equiplment and landing on his outstretched right hand. He reports severe pain in his right elbow. On physical examination, there is significant swelling and a puckering of the skin over the anterior aspect of the distal humerus. The right hand appears pale, and the radial pulse is difficult to palpate. He is unable to make a thumbs up sign or flex the distal interphalangeal joint of the index finger. Radiography of the elbow reveals a displaced fracture of the distal humerus just above the epicondyles.
Which of the following structures is most likely injured or entrapped in this patient?
The correct answer is:
A) Anterior interosseus nerve
This patient has a supracondylar humerus fracture, the most common pediatric elbow fracture. The puckering" of the skin, also known as the brachialis sign, indicates that the proximal bone fragment has pierced the brachialis muscle, suggesting significant displacement. In the most common type, the extension-type, the proximal fragment is displaced anteriorly. This poses a high risk to the anterior interosseous nerve (a branch of the median nerve) and the brachial artery. Injury to the anterior interosseous nerve leads to the inability to flex the distal phalanges of the thumb and index finger (the "OK sign" or "thumbs up" test). The pale hand and weak pulse further suggest brachial artery compromise, which is a surgical emergency.
Answer choice B: Axillary nerve, is incorrect. The axillary nerve wraps around the surgical neck of the humerus. It is commonly injured in anterior shoulder dislocations or proximal humerus fractures, not elbow fractures.
Answer choice C: Long thoracic nerve, is incorrect. This nerve innervates the serratus anterior. Injury, often during axillary surgery or chest trauma, results in winged scapula, not distal limb deficits.
Answer choice D: Radial nerve, is incorrect. While the radial nerve can be injured in supracondylar fractures, particularly if the fracture is displaced posteromedially, it is most classically associated with midshaft humerus fractures. Radial nerve injury would result in wrist drop.
Answer choice E: Ulnar nerve, is incorrect. The ulnar nerve is most commonly injured in flexion-type supracondylar fractures, which are rare, or fractures of the medial epicondyle. It would present with an inability to abduct/adduct the fingers and a claw hand deformity.
Key Learning Point
Supracondylar humerus fractures are pediatric emergencies due to the risk of neurovascular compromise. The anterior interosseous nerve (median nerve branch) is the most commonly injured nerve, while the brachial artery is the most commonly injured vessel. Failure to recognize and treat these injuries can lead to Volkmann ischemic contracture.
