A 2-year-old girl is brought to the emergency department by her father after she suddenly started crying and refused to use her right arm. The father reports that the injury occurred while he was pulling her upward by her hand to prevent her from tripping on the sidewalk. On physical examination, the child is holding her right arm in a slightly flexed and pronated position against her body. There is no visible swelling, bruising, or deformity of the elbow, wrist, or shoulder. The child cries when the examiner attempts to supinate the forearm but allows palpation of the entire limb.
Which of the following is the most likely mechanism of this injury?
The correct answer is:
B) Slippage of the annular ligament over the radial head
This patient is presenting with radial head subluxation, commonly known as nursemaid's elbow. This is a frequent injury in children aged 1–4 years. The mechanism involves sudden axial traction on a pronated and extended forearm (e.g., pulling a child up a curb or swinging them by the arms). Because the annular ligament is relatively lax in toddlers, the radial head can slip partially out of the ligament, which then becomes trapped in the radiohumeral joint. The classic clinical presentation is a child who holds the arm pronated and flexed, which is(the position of least pain, and refuses to use it. Diagnosis is clinical. X-rays are usually normal and not required unless there is a history of direct trauma or focal swelling.
Answer choice A: Rupture of the ulnar collateral ligament, is incorrect. This is typically an overuse injury seen in throwing athletes like baseball pitchers due to repetitive valgus stress. It would present with medial elbow pain, not the acute refusal to move the arm seen in a toddler.
Answer choice C: Tearing of the interosseous membrane, is incorrect. This membrane connects the radius and ulna along their shafts. Injuries here typically occur with high-energy forearm fractures, which would involve significant swelling and pain along the forearm.
Answer choice D: Transverse fracture of the distal humerus, is incorrect. This refers to a supracondylar humerus fracture, the most common pediatric elbow fracture. However, it usually results from a fall on an outstretched hand (FOOSH) and presents with significant swelling, deformity, and S-shaped bruising.
Answer choice E: Upward displacement of the coronoid process, is incorrect. The coronoid process is part of the ulna. While it can be fractured during an elbow dislocation, this typically requires significant force and would present with gross deformity and swelling, unlike the subtle presentation of nursemaid's elbow.
Key Learning Point
Nursemaid's elbow (radial head subluxation) is caused by axial traction on a pronated forearm, leading to the annular ligament becoming displaced and trapped between the radius and humerus. It is treated by reduction, either by hyperpronation or by supination followed by flexion, which typically results in immediate relief and return of function.
