Migrated Sacral Facet Joint Cyst Mimicking a Tarlov Cyst: A Case Report.

We report the case of a 78-year-old male presenting with debilitating left-sided S1 radicular pain, associated with a positive Lasègue sign and numbness in the S1 dermatome. The patient exhibited mild motor deficits, but his pain was severe enough to impair standing and ambulation. Magnetic resonance imaging (MRI) revealed a cystic lesion at the S1 level; however, the exact etiology remained unclear-differential diagnoses included a Tarlov cyst or a facet joint cyst. No other spinal comorbidities were noticed. Initial conservative management, including intravenous analgesics and C-arm-guided S1 nerve root blocks, failed to provide relief. Due to persistent symptoms, surgical exploration was undertaken. Intraoperatively, a facet joint cyst was identified and successfully excised. Postoperative recovery was uneventful, and the patient reported significant pain relief and functional improvement. This case highlights an unusual presentation of a facet joint cyst located at the sacral level without a visible connection to the facet joint, raising diagnostic challenges on imaging.