Fibrous dysplasia is a skeletal disorder where normal bone is gradually replaced by fibrous (scar-like) tissue mixed with immature bone. This weakens the affected areas, sometimes leading to pain, fractures, or visible deformity. It accounts for about 5% of all benign bone lesions, with an estimated incidence of 1 in 5,000 to 10,000 people. The condition is not inherited and is not a form of cancer, though it develops from a genetic mutation that occurs randomly during early embryonic development.
What Happens Inside the Bone
Fibrous dysplasia begins with a spontaneous mutation in a gene called GNAS. This mutation happens very early in fetal development and affects only some cells in the body, not all of them. The mutated gene causes a signaling pathway inside bone-forming cells to stay permanently switched on, flooding those cells with a chemical messenger called cAMP. Under normal circumstances, this signal would turn on and off as needed. In fibrous dysplasia, it never shuts down.
The result is that bone-forming cells in the affected area never fully mature. Instead of producing strong, organized bone, they create a disorganized mix of fibrous tissue and fragile, poorly structured bone. The bone’s outer shell (cortex) thins out, and the normal marrow inside is replaced with this weaker material. Over time, the affected bone loses mechanical strength and can become prone to fractures or bending under the body’s own weight.
Monostotic vs. Polyostotic Forms
Fibrous dysplasia comes in two main forms based on how many bones are involved. The monostotic form affects a single bone and is by far the more common type. Many people with monostotic disease have no symptoms at all. Their condition is often discovered incidentally when an X-ray or scan is done for an unrelated reason.
The polyostotic form involves multiple bones, sometimes on one side of the body, sometimes on both. It tends to cause more noticeable problems: greater pain, more fractures, and more visible skeletal changes. In some cases, polyostotic fibrous dysplasia is part of a broader condition called McCune-Albright syndrome, where the same underlying gene mutation also affects the skin and hormone-producing glands. People with this syndrome may develop characteristic light-brown (café-au-lait) skin patches with irregular borders, along with early puberty or overactive thyroid, adrenal, or pituitary glands.
Common Symptoms
The most frequently reported symptom is bone pain, which can come from the fibrous changes themselves or from small stress fractures developing in weakened bone. Swelling over the affected area is also common and may be the first thing someone notices.
When fibrous dysplasia affects the thigh bone (femur), the bone can bow outward into a shape sometimes called a “shepherd’s crook” deformity. This happens because the weakened bone gradually bends under normal body weight. Leg bone involvement can also cause one leg to become shorter than the other, leading to a limp or changes in how you walk. Fractures in these weight-bearing bones are a significant concern.
In the skull and face, expanding fibrous tissue can compress nearby nerves, potentially affecting vision or hearing. Facial asymmetry is possible when craniofacial bones are involved, with one side appearing more prominent than the other.
How It’s Diagnosed
Fibrous dysplasia has a distinctive appearance on imaging. On standard X-rays, affected bone often shows a hazy, frosted look sometimes described as “ground glass.” A ring of denser bone around the lesion, called the rind sign, is particularly common in the upper thigh bone. Over time, lesions can develop cystic areas and become more varied in appearance.
CT scans are considered the best imaging tool for fully evaluating fibrous dysplasia. They show the exact boundaries of each lesion, the degree of cortical thinning, and whether any soft-tissue changes raise concern. For long bones, a plain X-ray is usually sufficient, but craniofacial lesions almost always warrant a CT scan because of the complex anatomy involved.
Blood tests play a supporting role. A bone-specific enzyme called alkaline phosphatase (ALP) tends to be elevated in people with active fibrous dysplasia, and its levels can reflect how active the disease is. Tracking ALP over time helps gauge whether lesions are growing or stabilizing, and higher levels may predict disease progression.
Treatment for Pain and Bone Weakness
There is no cure for fibrous dysplasia, so treatment focuses on managing symptoms and preventing complications. For bone pain that doesn’t respond to standard painkillers, intravenous bisphosphonates (a class of drugs that slow bone breakdown) can help. Studies show that IV bisphosphonate therapy significantly decreases bone pain and reduces markers of bone turnover. It may also improve the appearance of lesions on imaging, with some filling in of weakened areas or thickening of the outer bone shell.
Oral versions of these medications have not shown meaningful pain relief over placebo in controlled trials, so they are generally not recommended for this purpose. A newer approach uses a targeted medication that blocks a specific protein (RANKL) involved in bone breakdown. Early reports show reductions in bone turnover and improvements in bone formation, though this treatment is not yet standard.
Neither bisphosphonates nor other current medications have been shown to reduce the risk of fractures in fibrous dysplasia. Their role is primarily to manage pain and slow disease activity.
When Surgery Is Needed
Surgery becomes an option when fibrous dysplasia causes persistent pain that doesn’t respond to medication, significant skeletal deformity, or a fracture. The goals are straightforward: relieve pain, correct the deformity, and stabilize the bone to prevent further breaks.
For large lesions with a high fracture risk, the typical approach involves scraping out the abnormal tissue (curettage), packing the space with bone graft material, and securing the bone with metal hardware. When the upper thigh bone bows or fractures, a metal rod placed inside the bone provides reliable long-term support. Corrective procedures to realign bent bones are especially important in children, where early intervention can prevent worsening deformity during growth.
Fibrous dysplasia lesions can recur after surgery, particularly in younger patients whose skeletons are still growing. Long-term follow-up with periodic imaging is a standard part of care.
Risk of Malignant Transformation
One of the most common concerns people have after diagnosis is whether fibrous dysplasia can turn into bone cancer. The risk exists but is very low, estimated at less than 1% of cases. When transformation does occur, the most common result is osteosarcoma. A sudden increase in pain, rapid swelling, or a change in the appearance of a known lesion on imaging can signal this rare complication. Any new or worsening symptoms in a previously stable lesion warrants prompt evaluation.
Living With Fibrous Dysplasia
For many people, particularly those with a single affected bone, fibrous dysplasia is a manageable condition that may never require treatment beyond monitoring. Lesions tend to be most active during childhood and adolescence, when the skeleton is growing, and often stabilize after skeletal maturity. Periodic imaging and blood tests to track alkaline phosphatase levels help determine whether a lesion is stable or progressing.
People with polyostotic disease or McCune-Albright syndrome typically need more comprehensive, ongoing care that addresses both the skeletal and endocrine components. Weight-bearing exercise is generally encouraged to maintain bone strength, but high-impact activities may need to be modified depending on which bones are involved and how weakened they are. A coordinated care plan with specialists familiar with the condition makes the biggest difference in long-term outcomes.