The patient was a 13-year-old boy who complained of pain in both buttocks. Plain and reconstructive computed tomography (CT) images showed an ossified lesion within the dura mater at the L5-S2 levels, and arachnoiditis ossificans in the lumbosacral area was suspected. In the operative findings obtained after cutting the dura, a bone fragment 4.5 × 0.5 × 0.5 cm in size was observed in the center of the strongly adhesive nerve bundle of the cauda equina, which was removed en bloc. The postoperative clinical course of the patient was excellent. The case, along with a review of literature is presented.