Are there other diseases like FOP? Not quite, but there are many rare bone diseases we don't completely understand yet. Osteogenesis imperfecta (OI) is also known as "brittle bone disease," and it's just like it sounds -- someone with OI could have literally hundreds of fractures in his lifetime. Unlike with FOP, the actual structure of the bones is faulty in people with OI. Remember that osteogenesis is another word for bone formation -- here, that very bone formation is imperfect. Collagen fibers in bones help keep them strong and structurally sound. In people with osteogenesis imperfecta, one of two things happens -- either the collagen isn't formed right or there isn't enough of it. Without that internal support, the bones can't withstand stress. You could break a bone just by coughing. Samuel L. Jackson's character in "Unbreakable" suffers from a form of brittle bone disease.
Melorheostosis is another rare disease affecting the bones and soft tissues. In melorheostosis patients, the compact bone thickens in a way it shouldn't. In X-rays, observers say that the bones look like they are covered in dripping candle wax [source: Melorheostosis Association]. As with FOP, melorheostosis often limits motion -- the muscles contract because connective tissues, such as ligaments and tendons, are shortened. Sometimes the joints turn to bone and freeze, and connective tissues calcify. One limb is often longer than the other, and limbs and hands or feet can be deformed. Like FOP and the other diseases on this page, melorheostosis is a painful disease.
Paget's disease enlarges and deforms bones. Osteoblasts, the bone-building cells, make bone that is too weak. Osteoclasts, the cells that break bones down, destroy too much bone. Paget's is painful and sometimes results in scoliosis and bowed limbs. When Paget's affects the skull, sometimes the head looks too big and the person loses his hearing. And surprisingly, even though many of the bones are dense and some are too large in Paget's patients, the bones can also break very easily.
Like with Paget's, people who suffer from osteopetrosis (not to be confused with osteoporosis), have lots of bone mass, but the bone is fragile. In osteopetrosis, the osteoclasts don't do their job. Osteoclasts are supposed to reabsorb bone. They work together with osteoblasts to make sure that the right amount of bone is in the body -- osteoblasts make the bone matrix, and osteoclasts eat it up. With osteopetrosis, there's too much new bone and the old bone isn't disappearing fast enough, which throws the whole system out of whack. Osteopetrosis is sometimes called marble bone disease -- in some forms of the disorder, the appearance of the bones is changed.
To learn more about FOP, bone disorders, and related topics, visit the links on the next page.