The term polyostotic fibrous dysplasia means "abnormal fibrous tissue growth in many bones". In the places that are affected, normal bone is replaced by irregular masses of fibroblast cells. When this occurs in weight-bearing bones, such as the upper leg bone, limping, deformity and fractures may result. Regions of fibrous dysplasia are also very common in the bones that form the skull and upper jaw. If these areas begin to expand, skull and facial asymmetry may be the result. PFD (Polyostotic Fibrous Dysplasia) can often be seen in a plain x-ray picture of the skelton. A more sensitive method of finding lesions is a bone scan. In a bone scan, a small amount of radioactivity is injected into a vein, taken up by the abnormal tissues, and detected by a scanner. 

     The severity of bone disease in MAS is quite variable. Some children may be minimally affected, with no asymmetry, deformity or fracture and lesions detected only by a bone scan. In a few children, lesions are found only in the base of the skull. By repeating bone scans at intervals of one to two years, it has been seen that in some children, the bone disease may become more extensive over time. Severe bone disease can have permanent effects on physical appearance and mobility.

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