Nonossifying fibroma and fibrous cortical defects share the same tissue appearance and differ only in size, the fibrous cortical defects being smaller. They are sometimes called jibroxanthomas because they contain spindle-shaped fibroblasts and xanthoma (foam) cells.
They are the most common benign lesion of bone and have been reported in 30% of normal children. Most of these non-neoplastic developmental aberrations are asymptomatic and disappear spontaneously.
The male-to-female ratio is 1.5:1, and the peak incidence occurs around 10 years of age.
Nonossifying fibromas and fibrous cortical defects are usually localized to the metaphysis of a long bone and are most commonly found about the knee (distal femur and proximal tibia). They are uncommon in the upper extremities. Their radiographic appearance is often pathognomonic, and no additional studies are usually necessary. They are frequently seen as an incidental finding on MRI examinations of the knee and can have a varied MRI appearance.
In the early stages, when the lesion is completely lytic on plain radiographs, it may have high signal intensity on TI-weighted images as a result of an abundance of xanthoma cells containing fat. Linear structures of low signal intensity are often present and represent fibrous septa or osseous pseudosepta. When the lesion starts healing, it has low signal intensity on both T1-weighted and T2-weighted images because of increased fibrous collagen, mineralization, and, possibly, hemosiderin deposit. The lesions are always well defined and are never surrounded by bone marrow edema.