Epiphyseal extension of a unicameral bone cyst.

Epiphyseal extension of a unicameral bone cyst is rare. We report a case of a 13-year-old boy with three pathological fractures through a unicameral bone cyst with epiphyseal involvement in the proximal humerus. These lesions initially tends to expand the humeral epiphysis laterally and progress medially. They also commonly cause a slip of the epiphysis in a medical direction. They also have a greater association with growth retardation and lesser degree of recurrence than their metaphyseal counterpart.