Fibrous dysplasia is considered as a rare disease (prevalence below 1 in 5,000). However, the exact prevalence of the disease is difficult to determine, due to the frequency of asymptomatic cases. Even though it is not a tumour (it is in fact a disease of the osteoblast cells), FD is often classified as a bone tumour, for diagnostic imagery reasons and due to its differential anatomopathological diagnosis. It represents approximately 7% of bone lesions with a benign tumour appearance. Fibrous dysplasia also affects both sexes. Diagnosis is most often made between the age of 5 and 30. Bone lesions are not necessarily visible at birth, but their presence is programmed.  They generally appear during childhood and can develop with skeletal growth. More often bone lesions develop less after puberty. It is uncommon for new lesions to appear after bone maturity is complete. All bones of the body can be affected. The effect can be monostotic or polyostotic. In monostotic fibrous dysplasia (FD) (60 to 80 % of cases), the most frequently affected areas are the ribs (45 % of all sites, the most common cause of benign rib lesions), the neck of the femur and the skull (the maxilla, the cranial vault and the mandible). The pelvis, vertebrae and other long bones (the humerus, tibia, etc.) are also described as being affected. The metaphyseal diaphyseal areas of long bones are typically affected. In polyostotic FD, a unilateral distribution or a unilateral predominance of affected bone sites is often observed (hemimelic distribution). In cases of polyostotic forms, 2/3 are diagnosed before the age of 10.

Created 09 june 2010