Surgical implications of an anteriorly positioned ulnar nerve of unknown etiology at the elbow: illustrative case.

backgroundAlthough the ulnar nerve typically follows a consistent course at the elbow, running within the condylar groove between the olecranon and the medial epicondyle, variations have been reported. The authors present the case of an anomalous ulnar nerve during cubital tunnel decompression surgery, where the nerve was found to be on an anterior course outside the ulnar groove medial to the epicondyle, entrapped by the fascia of the pronator mass. OBSERVATIONS: A 74-year-old male presented with advanced ulnar neuropathy resulting in progressive weakness and claw hand deformity of the left hand. Intraoperatively, the ulnar nerve was not found within the condylar groove between the olecranon and medial epicondyle, but was instead located medially to the epicondyle on top of the pronator retinaculum in a "transposed" position. After release, the ulnar nerve remained stable, or without subluxation into the ulnar groove on flexion and extension of the elbow. To the authors' knowledge, only similar case reports of a stable, anteriorly positioned ulnar nerve have been cited in the literature. LESSONS: When the ulnar nerve deviates from its expected position in the condylar groove, surgeons must be prepared to extend the surgical incision as needed and consider preoperative imaging to delineate aberrant nerve trajectories. https://thejns.org/doi/10.3171/CASE25788.