Tumor-like growths that consist of replacement of the medullary bone with fibrous tissue, causing the expansion and weakening of the areas of bone involved. Especially when involving the skull or facial bones, the lesions can cause externally visible deformities. The skull is often, but not necessarily, affected, and any other bone or bones may be involved. Fibrous dysplasia can either effect isolated bones (Monostotic fibrous dysplasia) or also generalized all bones of the body (Polyostotic fibrous dysplasia).