Quantitative analysis of the plain radiographic appearance of unicameral bone cysts

The authors determine the features that distinguish the radiographic appearance of unicameral bone cysts (UBC) from other solitary lesions of bone and to develop an ordered differential of similar-appearing lesions.

Seven hundred nine cases of solitary bone lesions, including 40 UBCs, were analyzed according to demographic, anatomic, and plain radiographic features. Vector analysis of both classic and new groups of features was performed to determine the sensitivity and specificity of radiographs for UBC and an ordered differential based on radiographic features.

The features of the UBCs in this study compared favorably with those in the literature. The male:female ratio was 3.3:1, with an average age of 15 +/- 13 years. Age ranges and lesion diameters were similar for both sexes. Thirty-three of the 40 (83%) UBCs were in long bones, and seven were in the pelvis or calcaneus. All 40 UBCs were geographic, medullary, and lytic. None had an associated soft-tissue mass. Fifty-five percent had pathologic fractures, and 10% had fallen fragment signs. Ninety-eight percent had no cortical break, and 88% had well-defined margins.

Classic descriptions of UBCs can be refined to improve sensitivity without sacrificing specificity. The best vector in this study had a sensitivity of 80% and a specificity of 93% and included these radiographic features: metaphyseal, diaphyseal, or flat bone location, geographic, lytic, medullary-based, no matrix, no satellite lesions, no subarticular extension, no soft-tissue mass, no cortical break, and central location in long bones. An ordered differential diagnosis, starting with the most likely lesion, includes enchondroma, aneurysmal bone cyst, fibrous dysplasia, nonossifying fibroma, Brodie abscess, and chondrosarcoma.