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Fibrodysplasia ossificans progressiva (FOP)



Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disorder that causes soft tissues to transform permanently into bone. These bones grow abnormally in the muscles, tendons, ligaments and other connective tissues, forming bridges of extra bone across the joints. As a result, movement in the areas affected by FOP is greatly restricted and sometimes impossible. The condition affects many areas of the body including, but not limited to, the neck, spine, chest, shoulders, elbows, wrists, hips, knees, ankles and jaw.

FOP is an autosomal dominant condition, which means that a person needs only to get the gene for FOP from one parent to inherit the disease. In most cases, however, FOP is a new mutation or an accident of nature. In fact, the parents of FOP patients typically don't have the disease. A person with FOP, however, has a 50 percent chance of passing it on to his or her child.

Worldwide, FOP affects only about 2,500 people, or one in 2 million, of all ethnicities, ages and races. Because FOP is such a rare disease, experts believe that 80 percent or more of the cases are misdiagnosed and its prevalence may be much higher than known.

Experts in both pediatric and adult orthopedics, orthopedic surgery and rheumatology at UCSF Benioff Children's Hospital are among the few in the country who readily diagnose and treat FOP. In addition, researchers at UCSF are investigating the rate of misdiagnosis of FOP and the most common causes for misdiagnosis. For information about FOP treatment at UCSF Benioff Children's Hospital, please call (415) 476-7242.

Signs & symptoms

The hallmark symptom of fibrodysplasia ossificans progressiva (FOP) is a malformation of a newborn's big toe. This malformation, which is apparent at birth, consists of a short big toe with an abnormal turning of the toe called a valgus deviation.

During early childhood, most of those with FOP form painful fibrous nodules, or tumor-like swellings, over the neck, back and shoulders. These nodules often develop after a child experiences some sort of trauma to the body, such as a bump or fall. Episodes also can occur without any warning or may not occur at all. In most cases, the nodules transform into bone during a process known as heterotopic ossification.

When the body starts to generate new bone, the patient usually experiences a painful flare-up. Tissue swelling, joint stiffness and serious discomfort can occur. Some may have a low-grade fever. Flare-ups can last as long as six to eight weeks. The disease then progresses along the trunk and limbs of the body. These lesions slowly replace the body's muscles with normal-looking bone.


In most cases, an accurate diagnosis of fibrodysplasia ossificans progressiva (FOP) can be made based on a patient's characteristic malformation of the big toe, in addition to rapidly changing swellings on the head, neck or back.

Due to a lack of knowledge of FOP among doctors, the rate of misdiagnosis of the disease is estimated at 80 percent or higher. These errors in diagnosing FOP have caused pain and suffering for FOP patients and their families worldwide. For instance, misdiagnosis has led to unnecessary invasive procedures, such as biopsies, as well as permanent complications from medical interventions, including loss of mobility.

Three of the most common misdiagnoses for FOP are cancer, aggressive juvenile fibromatosis, also called desmoid tumors, and progressive osseous heteroplasia, another rare disease characterized by the abnormal growth of bone.


Unfortunately, there is no effective treatment for fibrodysplasia ossificans progressiva (FOP). Surgery is not an option for removing the excess bones because surgery often results in more bone formation. And these new bones don't disappear on their own.

The good news is that researchers are investigating FOP and new treatments. For example, a drug is being developed that may help to control bone growth. Medications also are available to help relieve symptoms of FOP, such as pain and inflammation.

Since FOP is a progressive disease, it typically gets worse over time as the patient ages, but the rate of new bone formation differs for each person and the disease's progression is generally unpredictable.

Although a person is born with FOP, extra bone may not appear at birth and he or she may go months or years without experiencing a flare-up, which signals the development of new bone.

For information about FOP treatment at UCSF Benioff Children's Hospital, please call (415) 476-7242.

UCSF Benioff Children's Hospitals medical specialists have reviewed this information. It is for educational purposes only and is not intended to replace the advice of your child's doctor or other health care provider. We encourage you to discuss any questions or concerns you may have with your child's provider.

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