Fibrous dysplasia is a skeletal disorder that is characterized by the replacement of normal bone with fibrous bone tissue. It may involve one bone (monostotic) or multiple bones (polyostotic). Fibrous dysplasia can affect any bone in the body. The most common sites are the bones in the skull and face, the long bones in the arms and legs, the pelvis, and the ribs. Though many people with this disorder do not have any symptoms, others may have bone pain, abnormally shaped bones (deformities), or an increased risk of fractures (broken bones). The problems a person experiences depend on which bones are affected, and may arise from compression and displacement of adjacent structures to the lesions. For example, the legs can be of different lengths, leading to a limp, the bones of the sinuses can be affected, leading to chronic sinus congestion or headache. This condition can occur alone or as part of a genetic disorder, such as McCune-Albright syndrome.