Development of a solitary bone cyst--a report of a case suggesting its pathogenesis.

The origin and natural course of solitary bone cysts (SBC) still remain controversial. Knowledge of the process of cyst formation and enlargement would be helpful for investigating its pathogenesis. Only two papers have described a radiodense nidus of the diaphysis as a precursor. Their cases were unique in that the initial lesions were in the diaphysis and that the cysts did not abut onto the epiphyseal line. This study reports a case in a patient with a tiny erosive lesion in the endosteal surface of the humeral metaphysis, which developed expansively into a typical large cyst over 6 years. Serial roentgenograms taken every year demonstrated the process of cyst enlargement. This is the first longitudinal study of a case with SBC documented from its onset.