Acute Compression of the Anterior Interosseous Nerve After a Proximal Both-Bone Forearm Fracture: A Case Report.

Anterior interosseous nerve (AIN) syndrome is a rare condition characterized by isolated weakness in the flexor pollicis longus (FPL) muscle, sometimes accompanied by weakness in the index flexor digitorum profundus (FDP) muscle. In this clinical case report, an 18-year-old male presented with a right proximal both-bone forearm fracture that was sustained while playing soccer, with subsequent development of AIN palsy, without sensory deficits or progressive pain. Preoperative imaging was performed, showing a proximal third radius and mid-shaft ulna fracture. Given the progressive presentation of an acute AIN palsy, the patient was indicated for urgent operative intervention. During exploration and decompression of the AIN within the pronator tunnel, the nerve was found to be in continuity but was compressed by a large hematoma and the distal radial shaft. The patient recovered full median nerve function by his six-week postoperative examination and by his final follow-up recovered full range of motion with painless return to full activities. In proximal or mid-shaft both-bone forearm fractures, a careful neurovascular exam is essential, as uncommon conditions like anterior interosseous syndrome (AIS) can present without obvious sensory deficits or pain. Potential etiology for traumatic AIN compression includes significant fracture displacement, soft tissue injury, active extravasation on advanced imaging, and/or clinical concern for compressive hematoma. Patients presenting with FPL and/or index FDP weakness in the absence of sensory deficits or pain on passive stretch may benefit from dedicated surgical exploration and decompression of the AIN to prevent irreversible nerve damage.