"Fibrous dysplasia is a developmental dysplastic disorder of bone in which
immature woven bone is formed directly from abnormal fibrous connective tissue.
It is characterized by expanding
tissue within affected bones and
predominantly is a lesion of the growing skeleton. It is termed a
dysplasia because of the inability of
involved tissue to form mature lamellar bone from the immature, woven precursor.
Monostotic fibrous dysplasia affects only one bone, most commonly the ribs, proximal femur, and craniofacial bones. Polyostotic affects many bones, up to 75% of the skeleton. Monostotic fibrous dysplasia is 7-10 times more common than polyostotic fibrous dysplasia. It also can be associated with systemic conditions, including precocious puberty and skin pigmentation (as in McCune Albright syndrome)."
http://www.emedicine.com/orthoped/topic487.htm ,updated last 6/1/2005