Mandibular simple bone cyst in a patient with first and second branchial arch syndrome

Simple bone cysts are known as pseudocysts and are thought to arise due to growth or developmental abnormalities, but few reports have been published on this phenomenon. We here report a rare case of a left-sided simple bone cyst in a patient with first and second branchial arch syndrome. This 13-year-old girl had regularly visited the orthodontic department for follow up of a jaw deformity and malocclusion since the age of 4 years. She reported to our department for examination of a radiolucent area in the computed tomography images of the middle to left side of the mandible, which had been found incidentally during orthodontic follow-up. Her facial features were asymmetric due to hypoplasia of the jaw. There was a bone-like bulge, without pain, at the labial gingiva around the left lower canine. She had not experienced trauma. We diagnosed this as a simple bone cyst after biopsy. Under general anesthesia, we performed curettage and promoted organizing. At 6 months after the operation, the radiolucent area had been partially replaced with new bone. This is a rare case indicating that a simple bone cyst can arise from a hypoplastic area of the jaw.