Fibrous Dysplasia (FD) is an uncommon bone disease that can affect any bone in the body. The severity of the disease covers a wide spectrum. It can affect a single bone and go unnoticed for years, or it can affect virtually every bone, start very early in life, and result in significant physical impairment. It is caused by a defective gene in the cells that form bone. The cause of the gene defect is unknown, but the defect is neither inherited from the person's parents, nor passed on to the person's children. When the long bones (the bones of the legs and arms) or flat bones (ribs and pelvis) are affected, the bones weaken, may bow, are often painful, and will frequently fracture. Affected bones in the skull often expand, may cause disfigurement and, again, are often painful.

The areas of the skeleton that are affected are probably established early in life (often long before the disease is even detected). Once the affected areas are established, it is uncommon for new areas to be affected. Affected areas may increase in size or severity, but in general most of the problems as far as fractures and expansion have occurred by the age of 15. Once a bone is affected, it never returns to normal. Pain is commonly associated with FD, often is not present in childhood, usually starts later in the course of the disease, and if present, usually continues.

FD in the craniofacial bones tends to have a 'sclerotic' appearance on plain radiographs. CT scanning is the best technique for imaging FD in the skull, and with this technique lesions have a 'ground glass' appearance. In children and young adults, the lesions appear homogeneous on CT, but in older patients the appearance is mixed, with the development of 'cystic' lesions in some areas, although true fluid-filled cysts are rare. The density of these areas is that of soft tissue, so while they may have a cystic appearance they are not true cysts. That said, it is possible for true cysts to develop in FD - both in the long bones, and (more often) in the craniofacial bones. This occurs in about 5% of the patients with FD. The cysts tend to have a more aggressive course. They can expand rapidly and produce symptoms which vary, depending on the location. These usually require surgical intervention. Biopsy of FD lesions can confirm the diagnosis if doubt remains after review of the radiographs.

FD can be associated with birth marks (café-au-lait spots), and a number of endocrine problems such as precocious puberty, hyperthyroidism, low blood phosphorus, excess growth hormone, and others. When this occurs, this is known as McCune-Albright syndrome. The birth marks can often be the first sign of the disease. However, almost any combination can occur. For example, severe bone disease (with or without endocrine problems) can be associated with minimal skin disease, and vice versa.

There is no cure for FD, but there are treatments for the various problems associated with it. Surgery remains the mainstay of treatment for the long bone disease. Surgical treatment is complicated and is best performed by surgeons with experience with FD. Medications known as bisphosphonates (pamidronate (Aredia), zoledronate (Zometa), etc.) have been shown to be very effective in relieving pain. In general, surgery in the bones of the skull usually is not medically required. Although the nerves to the eyes and ears are often surrounded by FD, blindness and significant hearing loss can occur although this is very uncommon.