Fibrous Dysplasia

What happens in fibrous dysplasia

Bone is constantly being formed by cells called osteoblasts. In fibrous dysplasia, a gene change makes some of these bone-forming cells permanently switched on. Instead of maturing, they produce fibrous tissue and thin, irregular bone. On X-ray this looks like ground glass.

The gene involved is GNAS. The change happens by chance in one cell during early development, so only cells descended from it carry it. This is called mosaicism. It explains why the condition is patchy, why it varies so much between people, and why it is not passed on to children.

Lesions are usually established by about age 10 to 15. They may grow during childhood and adolescence and tend to become less active in adulthood.

Types of fibrous dysplasia

Symptoms

Many people have no symptoms. When present, they include:

• Bone pain, often a dull ache, more common in adults
• Fractures after little or no injury, often in the thigh bone (see pathological fractures)
• Bowing of the upper thigh bone, called a "shepherd's crook" deformity
• Limb length difference and a limp
• Facial or skull asymmetry, a bony lump, or teeth that shift
• Rarely, compression of the optic nerve (vision loss) or hearing nerve
• Scoliosis when the spine is involved

Associated features to check for

• Light-brown birthmarks with jagged borders ("coast of Maine" café-au-lait macules)
• Early puberty, especially vaginal bleeding in young girls (precocious puberty)
• Low phosphate in the blood from a hormone (FGF23) made by the abnormal bone, which can worsen bone softening (hypophosphatemia)
• Overactive thyroid and excess growth hormone

How it is diagnosed

• X-ray: typically a ground-glass area with a thin, expanded outer shell of bone.
• CT scan: best for the skull and face.
• Bone scan: shows all affected bones, important to tell monostotic from polyostotic disease.
• Biopsy: when the imaging is not typical, to exclude other conditions. Genetic testing of the tissue can find the GNAS change.
• Blood tests: phosphate, vitamin D, and hormone tests if McCune-Albright syndrome is possible.

Conditions that can look similar include a simple bone cyst, ossifying fibroma, non-ossifying fibroma and Paget's disease of bone in older adults.

Treatment

There is no medicine that reverses fibrous dysplasia. Treatment targets symptoms and complications.

Observation

Small, painless lesions that are not at risk of fracture need only monitoring, often with periodic X-rays.

Pain relief

• Over-the-counter pain relievers such as acetaminophen (paracetamol) or ibuprofen, at the label dose.
• Bisphosphonates (IV or oral) can reduce bone pain in some adults, though they do not heal the lesions.

Surgery

• Internal fixation with intramedullary rods or plates to treat or prevent fractures and correct bowing, especially in the femur. Rods are preferred over bone grafting alone, because grafts tend to be reabsorbed into the dysplastic bone.
• Osteotomy to correct deformity.
• Craniofacial surgery to reshape bone for appearance or to relieve pressure on nerves. Routine "preventive" decompression of the optic nerve is generally not recommended without vision loss.

Treating associated problems

Correcting low phosphate and vitamin D, and treating hormone excess, improves bone strength and reduces fractures.

Outlook

Most people with monostotic fibrous dysplasia lead normal lives. Polyostotic disease is more likely to cause repeated fractures and deformity, but lesions usually become quieter after puberty. Change to bone cancer such as osteosarcoma is rare, well under 1%. The risk is higher with excess growth hormone and after radiation therapy, which is why radiation is avoided.

Frequently asked questions

Is fibrous dysplasia cancer?

No. Fibrous dysplasia is a benign bone condition. In very rare cases, well under 1%, a lesion can turn into a bone cancer. New or rapidly increasing pain, swelling or a growing mass should be checked promptly.

Is fibrous dysplasia hereditary?

No. It is caused by a GNAS gene change that occurs by chance in some cells after conception. Because it is not present in egg or sperm cells, it is not passed on to children, and siblings are not at increased risk.

Does fibrous dysplasia get worse with age?

Lesions usually grow during childhood and early adolescence, then become less active in adulthood. Pain may still occur in adults. New growth after adulthood is uncommon and should be checked for complications such as cysts or, rarely, malignant change.

What is the difference between monostotic and polyostotic fibrous dysplasia?

Monostotic fibrous dysplasia affects one bone and is often found by chance. Polyostotic affects several bones, usually appears earlier, causes more fractures and deformity, and is more often linked to McCune-Albright syndrome. A bone scan shows which type you have.

Can fibrous dysplasia be cured?

There is no cure, and the abnormal bone does not return to normal. However, most people need little or no treatment. Surgery can correct deformity and prevent fractures, and medicines can reduce pain and correct hormone or phosphate problems.