Atraumatic intraneural heterotopic ossification of the common peroneal nerve: illustrative case.

backgroundIntraneural heterotopic ossification, or neuritis ossificans (NO), is characterized by nonneoplastic calcifications of extra-skeletal soft tissue within a peripheral nerve. This report presents an atypical case of atraumatic NO with a chronic, progressive course that led to a unilateral foot drop. In our review of the published data, there are limited reports describing NO affecting the common peroneal nerve with such profound clinical manifestation. OBSERVATIONS: A 70-year-old female presented with an 8-year history of progressive right lower extremity weakness and foot drop. Initial electrodiagnostic studies localized a peroneal neuropathy at the fibular head, with transient symptom improvement. Years later, worsening symptoms led to further evaluation. Imaging revealed an extensive, calcified lesion encasing the distal sciatic and common peroneal nerves. Intraoperatively, a rigid, 9-cm segment of ossified nerve was identified, decompressed, and biopsied. Histopathological analysis confirmed NO. Postoperative outcomes included improved motor function with ongoing rehabilitation. LESSONS: This case highlights the atypical presentation of NO, diagnostic workup, and surgical interventions. It emphasizes the need for a high index of suspicion in patients with unexplained, progressive mononeuropathies. When conventional imaging and electrodiagnostic studies are inconclusive, early referral for advanced imaging and specialist evaluation may be critical. Delayed recognition can result in significant neurological impairment. https://thejns.org/doi/10.3171/CASE25623.