A solitary bone cyst is a common lesion which is seen in tubular bones in patients that are less than 30 years old. Specifically, these most commonly occur within the metaphysis. These are a centrally located lytic lesion with a thin sclerotic margin. Pathologic fracture through the simple bone cyst is common. When a piece of cortical bone lies within the cyst in its dependent portion, this is know as the fallen fragment sign.
Treatment includes surgical intervention or steroid injection.
Occasionally, a simple bone cyst may undergo spontaneous regression. Bone and Joint Imaging, 2nd Edition .