Fibrous dysplasia is a dysplastic bone disorder wherein immature woven bone forms directly from abnormal fibrous connective tissue. Fibro-osseous tissue within affected bones expands as a lesion of growing bone. The term “dysplasia” refers to the person’s inability to form mature lamellar bone.

The true incidence is not known, but there are estimates that about 10% of benign bone tumors are the result of fibrous dysplasia.

Fundamentally fibrous dysplasia appears to be a disorder of postnatal cancellous bone maintenance or an abnormality of bone-forming mesenchyme.