Simple Bone Cyst
The simple bone cyst is a benign cystic lesion of bone of unknown etiology. It arises in the metaphysis of the bone, but with growth it tends to move away from the growth plate toward the diaphysis. It is a common bone lesion, following only nonossifying fibromas and exostoses in prevalence.
The male-to-female ratio is 2.5:l. It can be seen in any age group but is very rare before 5 years and after 20 years of age.
The most common site is the proximal humerus, followed by the proximal femur. All other locations are rare.
Simple cysts are central, lytic, expansile lesions. Their radiographic appearance is usually diagnostic, and neither MRI nor CT is usually necessary. They are well defined and have low signal intensity on TI-weighted images and high signal intensity on T2-weighted images as a result of their fluid content. Tipically, they are homogeneous on both T1-weighted and T2-weighted images unless hemorrhage has occurred. Peripheral low-signal-intensity borders represent reactive sclerosis. No edema is present in the surrounding bone marrow or muscle. CT is superior to MRI in detecting subtle infractions of the lesion.