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The abbreviation FOP stands for Fibrodysplasia Ossificans Progressiva. Loosely translated, this means that connective tissue is gradually replaced by bone.

FOP is a rare genetic disorder where bone forms in muscles, tendons, ligaments and other connective tissue. The disease was formerly known as MOP (Myositis Ossificans Progressiva). The name was changed in 1970, because other soft connective tissues such as tendons are affected also in addition to muscles. In FOP, the body forms a second skeleton, as it were. Bone also forms around joints, immobilizing them and locking them in place. The end result is that people with FOP undergo extreme ossification, fixing them in a sitting or standing position.


Children with FOP are virtually always born without visible defects. Only deformed large toes may foreshadow their future lot.

The large toe is slightly smaller than the other toes, with the two phalanges fused, preventing mobility. During the first or second phase of their childhood, they develop connective tissue swellings in their neck, shoulders and back with bone growth – a process called “heterotopic ossification”. Other limbs follow. This disorder slowly replaces the muscles with bone that otherwise appears normal. Additional trauma, such as falling, bumping, injections into muscles, increase the risk of inflammation in the muscles, which may trigger spontaneous bone formation. Each attempt to remove extraskeletal bone results in additional bone formation.
There is significant variation in progression among people with this disease. The process is fast in some, slow in others.

How many people have FOP?

FOP occurs in at least 1 in 2 million people. 10 people have been diagnosed with FOP in the Netherlands (FOP Expertise Center, Amsterdam VUmc FOP was diagnosed in 922 people worldwide in 2017.

A US team of 15 scientists, led by Prof. Frederick S. Kaplan, MD, and Dr. Eileen M. Shore, PhD, are researching this complex disease in a laboratory. In the Netherlands, Prof.Dr. J. Coen Netelenbos and Dr. Marelise Eekhoff of Amsterdam Free University Hospital VUmc are the leading experts in this field. As yet, no adequate treatment or medicine has been discovered that controls the continuous bone formation or removes extraskeletal bone.

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