Fibrous dysplasia represents a disturbance of normal bone development specifically a defect in osteoblastic differentiation and maturation that originates in the mesenchymal precursor of the bone. In fibrous dysplasia, the lesion expands, which leads to a distortion and weakening of bone. Although the lesion is not encapsulated, it tends to remain enclosed within a shell of cortical bone. This shell can be thinned as a result of the pressure exerted on it. As it slowly progresses, fibrous dysplasia can cause skeletal destruction and deformity.