Aneurysmal bone cyst originating from the surface of the bone, either within the cortex or subperiosteally, is an uncommon anatomic subtype. This article reports the clinical and radiologic evaluations and treatment outcomes of 10 patients with surface aneurysmal bone cysts that were surgically treated between 1982 and 2014. Mean age at the time of surgery was 22.4 years (range, 11-44 years). According to Capanna's radiographic evaluation criteria, 6 of the lesions were classified as type V and 4 were classified as type IV. Radiographically, periosteal shell formation was observed to be complete in 4 patients, partial in 3, and absent in 3, and 6 patients had Codman's angle or buttress formation. In 1 patient, computed tomography scan showed birdcage-like ossification attached to the surface of bone. Magnetic resonance imaging showed fluid-fluid levels in 5 patients. All of the patients had standard curettage and high-speed burr application as an adjuvant. No patient had local recurrence at the end of the follow-up period of 98.4 months (range, 13-288 months). These findings show the importance of careful radiologic evaluation and biopsy to better plan a treatment strategy when surface aneurysmal bone cyst is included in the differential diagnosis. The finding of fluid-fluid levels on magnetic resonance imaging or computed tomography is not pathognomonic for primary aneurysmal bone cyst; however, the absence of this finding does not rule out the diagnosis. The rate of local recurrence after curettage plus high-speed burr is reasonably low, and other adjuvant procedures should be used whenever needed. [Orthopedics. 2016; 39(5):e897-e903.].
