Fibrodysplasia ossificans progressiva (FOP) is a rare genetic condition in which the body gradually turns soft tissues, including muscles, tendons, and ligaments, into bone. It affects roughly 1 in 1.5 million people worldwide, making it one of the rarest diseases known to medicine. The extra bone forms outside the normal skeleton, progressively locking joints into place and severely restricting movement over a person’s lifetime.
What Causes FOP
FOP is caused by a single mutation in the ACVR1 gene, which controls a receptor involved in bone growth. In nearly all people with classic FOP, the same point mutation (a substitution called R206H) overactivates a signaling pathway that normally tells the body when and where to build bone. With this mutation stuck in overdrive, the body misreads signals from its environment and begins forming cartilage and bone in soft tissues where they don’t belong.
The mutation is inherited in an autosomal dominant pattern, meaning only one copy of the altered gene is enough to cause the disease. Most cases, however, arise from a new spontaneous mutation rather than being passed down from a parent. FOP affects all ethnicities and occurs equally in males and females.
The Earliest Sign: Malformed Big Toes
The most recognizable clue at birth is shortened, inward-turning big toes. This malformation is present in virtually all children with classic FOP and often appears before any extra bone has formed. Because FOP is so rare, this toe abnormality is frequently overlooked or attributed to other conditions. Recognizing it early is critical, since the correct diagnosis changes everything about how the child should be managed going forward.
How the Disease Progresses
Extra bone formation in FOP follows a predictable geographic pattern. It typically begins in the upper back and neck during childhood, then spreads outward and downward: from the spine to the shoulders, then to the hips, knees, and eventually the jaw and limbs. Doctors describe this as a cranial-to-caudal (head to tail) and axial-to-appendicular (trunk to limbs) progression.
The process happens in episodes called flare-ups. During a flare-up, painful soft-tissue swelling appears, often resembling a tumor or deep bruise. Over days to weeks, this swelling transforms into solid bone. Each episode permanently locks another joint or restricts another range of motion. Between flare-ups, the disease can seem dormant, but the cumulative effect is a “second skeleton” that bridges across joints and fuses them in place.
By their 30s, many people with FOP have lost significant mobility in the spine, shoulders, and hips. The jaw often fuses partially or completely, making eating difficult. Most people with FOP eventually require a wheelchair and assistance with daily activities.
What Triggers Flare-Ups
Any trauma to the body can set off a new episode of bone formation. Common triggers include falls, surgery, intramuscular injections, and even viral illnesses like the flu. This is why people with FOP are advised to receive vaccines as subcutaneous injections (into the fat layer under the skin) rather than the standard intramuscular route. Avoiding contact sports, preventing falls, and taking precautions against common infections are all part of daily life with FOP.
Even well-intentioned medical procedures carry risk. Biopsies of suspicious swellings, a routine step when doctors suspect a tumor, can trigger explosive new bone growth at the biopsy site. Improper treatment has caused irreparable damage or permanent disability in more than half of FOP patients, largely because the condition was misdiagnosed before the correct one was identified.
Why Misdiagnosis Is So Common
FOP is frequently mistaken for bone cancer, fibromas, or bursitis, especially early on when a painful lump appears in a child’s back or neck. The swellings look alarming on imaging, and the instinct to biopsy them is strong. But in FOP, that biopsy will cause more bone to form. The safest path to diagnosis is genetic testing for the ACVR1 mutation, which confirms the condition without any tissue trauma. The characteristic malformed big toes, combined with a history of soft-tissue swellings, should raise immediate suspicion and prompt genetic testing rather than a biopsy.
Life Expectancy and Major Complications
The estimated median lifespan for people with FOP is 56 years, though the range varies widely. Among those who have died, the median age at death was 40, with a range from 3 to 77 years. The most common cause of death is cardiorespiratory failure from thoracic insufficiency syndrome, accounting for 54% of deaths at a median age of 42. This happens because extra bone gradually encases the rib cage, restricting the lungs’ ability to expand. Pneumonia is the second most common cause (15%), and complications from falls account for about 11%.
Dental care and any surgical procedure present unique challenges. Many people with FOP develop fusion of the jaw joint and cervical spine, making it impossible to open the mouth fully or tilt the head back. Standard methods for placing a breathing tube during anesthesia are often impossible. Medical teams use specialized fiberoptic techniques through the nose, position the patient with extreme care to avoid pressure on soft tissues, and keep a surgical airway kit on standby. Every touch, every repositioning, carries the risk of triggering new bone.
Treatment Options
In 2023, the FDA approved the first treatment specifically for FOP: palovarotene (brand name Sohonos). It is approved for adults and children aged 8 and older for females and 10 and older for males. The drug works by reducing the volume of new extra-skeletal bone that forms. Patients 14 and older take a low daily dose, with a higher dose for 12 weeks during flare-ups. Younger patients receive adjusted doses based on body weight.
Palovarotene does not reverse bone that has already formed. Its goal is to slow the accumulation of new bone during flare-ups. Before its approval, management was limited to corticosteroids for inflammation during flare-ups, careful pain management, and a long list of precautions to avoid triggering new episodes. Physical therapy is used cautiously, focused on maintaining the range of motion that remains rather than aggressively stretching joints, which could provoke new bone growth.
Living With FOP
Daily life with FOP revolves around prevention. Avoiding falls is paramount, which means adapting living spaces, using mobility aids early, and steering clear of activities with any risk of impact. Respiratory care becomes increasingly important as the chest wall stiffens, and many people benefit from breathing exercises and monitoring for signs of declining lung function. Nutrition can become complicated as jaw mobility decreases, sometimes requiring soft or liquid diets.
Because the disease is so rare, with only about 800 confirmed cases worldwide, finding knowledgeable specialists can be difficult. Patient organizations and a small number of academic medical centers serve as hubs for coordinating care, connecting families, and enrolling patients in clinical research.