Osteoid Osteoma of the Femoral Neck Mistaken As a Synovial Herniation Pit

A 36-year-old man presented with 1 year of atraumatic left lateral thigh, groin, and hip pain, and imaging consistent with the diagnosis of femoroacetabular impingement and a labral tear. Imaging concurrently demonstrated a synovial herniation pit. The patient underwent hip arthroscopy, which included femoroplasty, acetabuloplasty, labral debridement, and synovectomy. His pain persisted and further workup confirmed an osteoid osteoma that was mimicking a synovial herniation pit. The osteoid osteoma was treated with radiofrequency ablation. At 18 months follow-up, the patient reported complete resolution of his symptoms. We present the case to highlight distinguishing imaging and clinical findings of these similar-appearing lesions. While neither condition is particularly rare individually, the misidentification of osteoid osteoma as a synovial herniation pit is a unique feature of this case that lead to the patient's protracted clinical course.