What Is Fibrous Dysplasia?

Fibrous dysplasia is a chronic, benign bone disorder in which normal bone and bone marrow are replaced by fibrous connective tissue and immature, abnormal woven bone. This replacement process weakens the affected bone, making it more prone to pain, deformity, and fracture. The condition is caused by somatic mutations in the GNAS gene that occur after conception, so it is not inherited from a parent.

Types of Fibrous Dysplasia

• Monostotic fibrous dysplasia: Affects a single bone, most commonly the femur, tibia, rib, or skull. This is the most common form.
• Polyostotic fibrous dysplasia: Affects multiple bones and is usually more severe, often involving the long bones, pelvis, and skull.
• McCune-Albright syndrome: A form of polyostotic fibrous dysplasia accompanied by café-au-lait skin spots and endocrine abnormalities, such as precocious puberty or thyroid disease.

Symptoms

Symptoms vary depending on the bones involved. Common features include bone pain, swelling, limping, leg length discrepancy, facial asymmetry, dental problems, headache, and vision or hearing changes when the skull is affected. Affected bones are weak and may break after minor trauma, resulting in a pathological fracture.

Diagnosis

Fibrous dysplasia is often found on imaging studies performed for other reasons. X-rays typically show a characteristic ground-glass appearance with a thin rim of dense bone. CT scans and MRI help define the extent of the lesion, while a bone scan can identify multiple sites of disease. A bone biopsy may be performed to confirm the diagnosis, and genetic testing can identify mutations in the GNAS gene.

Treatment

There is no cure for fibrous dysplasia, so treatment focuses on relieving symptoms and preventing complications. Management approaches include:

• Regular monitoring and observation of stable lesions.
• Medications such as bisphosphonates to reduce bone pain and bone turnover.
• Surgical procedures, including curettage, bone grafting, internal fixation, and corrective osteotomy. Surgery may also be needed to decompress nerves when the skull is involved, particularly the optic nerve.

Outlook

The course of fibrous dysplasia is unpredictable. Many people with monostotic disease remain stable for years, while polyostotic disease can progress and cause significant disability. Malignant transformation is extremely rare, occurring in less than one percent of cases. Regular follow-up with a specialist is recommended.

Key Terms

• Fibrous dysplasia
• GNAS gene
• Monostotic
• Polyostotic
• McCune-Albright syndrome
• Ground-glass appearance
• Bisphosphonates
• Pathological fracture
• Craniofacial fibrous dysplasia

More Information

For further reading, consider the National Institutes of Health and the Mayo Clinic.