Image Interpretation: Dynamic ultrasound demonstrates anterior dislocation of the ulnar nerve with elbow flexion, followed by subluxation of the medial head of the triceps muscle over the medial epicondyle. With elbow extension, the medial triceps and subsequently the ulnar nerve snap posteriorly back into the retrocondylar groove. The ulnar nerve is normal in caliber and echotexture. No anconeus epitrochlearis muscle is identified.

Teaching Pearl: Snapping triceps syndrome is a rare cause of medial elbow pain and results from dynamic dislocation of the medial head of the triceps muscle over the medial epicondyle during elbow flexion, often occurring in conjunction with ulnar nerve instability or dislocation. Patients typically report palpable or audible snapping or popping over the medial epicondyle with elbow flexion and extension. Symptoms of ulnar neuropathy are common but may be absent. The condition is most often seen in the setting of anatomic variants that predispose the medial head of the triceps to translate over the medial epicondyle, most commonly varus angulation of the distal humerus, though other contributing factors may include shallow trochlear anatomy or post‑traumatic changes. Management is typically surgical and may involve ulnar nerve transposition and/or lateral transposition or excision of the dislocating medial head of the triceps. Accurate pre‑operative dynamic ultrasound evaluation is essential to determine whether snapping involves the ulnar nerve alone or occurs in combination with medial triceps subluxation, as unrecognized triceps involvement is a well‑described cause of persistent or recurrent post‑surgical symptoms. For dynamic assessment, the transducer should be placed over the retrocondylar groove in short axis to the ulnar nerve, and the patient should be instructed to slowly flex and extend the elbow while observing for sequential instability of the ulnar nerve and medial triceps. If symptoms are not readily elicited, resisted elbow extension from a fully flexed position may also reproduce snapping and improve visualization of the subluxation. References: Spinner et al. Medial or lateral dislocation (snapping) of a portion of the distal triceps: a biomechanical, anatomic explanation. J Shoulder Elbow Surg, 2001 Nov-Dec; 10(6): 561-7. Disclaimer: No financial disclosures. The views expressed herein are those of the authors and do not reflect the official policy of the Department of the Army, Department of the Air Force, Defense Health Agency, Uniformed Services University, Department of War, or the U.S. Government.