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FOP stands for Fibrodysplasia Ossificans Progressiva. This means that connective tissue is gradually replaced by bone. The first cases were described in the 17th and 18th century.

An estimated 2,500 people worldwide suffer from FOP. This comes down to about one in two million people. By way of comparison: if a soccer stadium holds 100,000 people, you would need to fill almost 20 stadiums to find one person with FOP. FOP was diagnosed in 922 people worldwide in 2017.

Technically and surprisingly, the answer is “yes”. But there is more to it. Surgical interventions frequently aggravate the disease. New bone will grow back and further limit mobility.

The extraskeletal bone formed in FOP grows by gradual conversion of connective tissue into cartilage and bone. This happens in the same way that bone forms (and heals) after a fracture. The process is also quite identical to the way bone forms in the embryo. The abnormality in FOP bone formation is therefore not in the way it forms, but the time and place it forms.

FOP is characterized by the occurrence of flare-ups in muscles or around joints, which often starts off a process of local bone formation, frequently locking joints into place. Flare-up is a localized, sudden aggravation of the disease accompanied by swelling, redness and pain. The flare-ups may develop spontaneously or be triggered by trauma, an injection or surgery.

Unfortunately, people with FOP cannot be cured. The “P” in FOP stands for “Progressiva”. This means that FOP is progressive, or will get worse over time. Because FOP is a hereditary disease, which means the cause is genetic, people with FOP are born with the disease. However, the extraskeletal bone is not necessarily present yet at birth. Also, patients will not outgrow their disease and the extraskeletal bone will never disappear. The body of an FOP patient does not form extraskeletal bone all the time. A person with FOP could live for months or years without experiencing a flare-up. Nevertheless, there is always a risk of extraskeletal bone forming. This can happen after a (mild) injury, such as a fall, or even without any warning at all. But conversely, an injury does not always trigger an FOP flare-up. So it is unclear why the disease is at times active, and dormant the rest of the time.

FOP affects the neck, spine, chest, shoulders, elbows, wrists, hips, knees, ankles and many other areas. Progressive ossification (conversion into bone) follows a characteristic pattern. The extraskeletal bone usually forms in the neck, spine and shoulders first before it develops in the elbows, hips and knees. The muscles of the diaphragm, tongue, eyes, face and heart are usually unaffected. FOP impacts mobility because joints, such as the knees and elbows, connect bones that are essential to movement. In FOP, the extraskeletal bone replaces the ligaments covering the joints, and the muscles and tendons that enable the joints to move. As a result, movement of the affected areas becomes very difficult or even impossible.

No, there is significant variation of FOP among patients. The greatest variation is seen in the times that bone is formed (the flare-ups) and the rate at which this happens. Another variation is the degree of mobility limitation. For example: an elbow may become immovable, or still allow some movement.

As yet, no effective treatment is known. But there is hope. Clinical studies are now underway with two potential drug candidates, and other compounds are currently being tested in the laboratory for effectiveness.

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