Describe the two types of fibrous dysplasia.

Monostotic fibrous dysplasia occurs in 85% of patients and has a predilection for the ribs, femur, and skull.
Polyostotic fibrous dysplasia occurs in 15% of patients, typically the younger ones (mean age, 8 years). This form is most likely to occur in the pelvis, and when it does is nearly always also present in the ipsilateral proximal femur. It is also common to the proximal femur without pelvic involvement and in the tibia.