Fibrodysplasia ossificans progressiva

Fibrodysplasia ossificans progressiva (/ˌfaɪbroʊdɪˈspleɪʒ(i)ə ɒˈsɪfɪkænz prəˈɡrɛsɪvə/; abbr. FOP), also called Münchmeyer disease or formerly myositis ossificans progressiva, is an extremely rare connective tissue disease in which fibrous connective tissue such as muscle, tendons, and ligaments turn into bone tissue (ossification). It is the only known medical condition in which tissue of one organ system changes into that of another. It is a severe, disabling disorder with no cure.

FOP is caused by a mutation of the gene ACVR1. The mutation affects the body's repair mechanism, causing fibrous tissue including muscle, tendons, and ligaments to become ossified, either spontaneously or when damaged as the result of trauma. In many cases, otherwise minor injuries can cause joints to become permanently fused as new bone forms, replacing the damaged muscle tissue. This new bone formation (known as "heterotopic ossification") eventually forms a secondary skeleton and progressively restricts the patient's ability to move. Bone formed as a result of this process is identical to "normal" bone, simply in improper locations. Circumstantial evidence suggests that the disease can cause joint degradation separate from its characteristic bone growth.

Surgical removal of the extra bone growth has been shown to cause the body to "repair" the affected area with additional bone. Although the rate of bone growth may differ depending on the patient, the condition ultimately leaves sufferers immobilized as new bone replaces musculature and fuses with the existing skeleton. This has earned FOP the nickname "stone man disease".