Stoneman syndrome, formally called fibrodysplasia ossificans progressiva (FOP), is a genetic condition in which the body gradually turns soft tissues like muscles, tendons, and ligaments into bone. It affects roughly 1 in every 1 to 3 million people worldwide, making it one of the rarest diseases known to medicine. The name “Stoneman syndrome” comes from the way the body slowly becomes encased in a second skeleton, progressively locking joints into place.
What Causes It
FOP is caused by a mutation in the ACVR1 gene, which controls a receptor involved in bone growth signaling. In more than 95% of cases, the mutation is identical: a single letter change in the genetic code that swaps one amino acid for another. This tiny alteration leaves the receptor stuck in an “on” position, constantly sending signals that tell the body to build bone in places it shouldn’t.
The result is heterotopic ossification, the formation of bone outside the normal skeleton. When soft tissue is damaged or inflamed, even slightly, the mutated receptor hijacks the healing process. Instead of repairing muscle with muscle, the body lays down true bone, complete with marrow. This new bone is permanent and cannot be reabsorbed.
The Earliest Sign Most Doctors Miss
Nearly all children with FOP are born with malformed big toes, typically short, curved inward, or missing a joint. This toe abnormality is present at birth, before any extra bone has formed, and it’s the single most reliable early clue. Some children also have unusually short thumbs or other subtle skeletal differences.
Despite this telltale sign, 87% of people with FOP initially receive a wrong diagnosis. The most common misdiagnosis is cancer, given to about a third of patients. That misdiagnosis often leads to a biopsy, which is one of the worst things that can happen. Two-thirds of misdiagnosed patients undergo unnecessary biopsies or surgeries, and nearly half of all patients report permanent mobility loss directly caused by those medical interventions. The trauma of a biopsy triggers explosive new bone growth at the site, doing exactly the kind of harm the procedure was meant to prevent.
How the Disease Progresses
FOP follows a predictable geographic pattern through the body. It typically begins in the upper back, neck, and shoulders during early childhood, then moves downward and outward toward the arms, hips, and legs over the following years and decades. The spine and jaw are commonly affected. Bone formation tends to appear first along the back and top of the body before reaching the front, and closer to the trunk before the hands and feet.
Progression happens through “flare-ups,” episodes of painful soft tissue swelling that eventually harden into bone over weeks to months. About half of all flare-ups appear to happen spontaneously, with no obvious trigger. The other half are linked to some form of physical trauma. Most people with FOP are confined to a wheelchair by their late twenties or early thirties and need lifelong help with daily activities. Joints don’t just stiffen; they fuse completely, locking the body into whatever position the new bone holds.
What Triggers Flare-Ups
The list of known triggers reads like a catalog of everyday risks. Intramuscular injections are particularly dangerous: 25% of people who received one reported an immediate flare-up at the injection site, and 84% of those flare-ups resulted in permanent new bone. This means routine childhood vaccinations given into muscle can cause irreversible harm if the diagnosis isn’t recognized first.
Other documented triggers include:
- Blunt trauma from bumps, falls, or contact sports
- Muscle fatigue or stretching, including physical therapy exercises
- Surgical procedures and biopsies of any kind
- Dental work involving jaw injections
- Viral illnesses, particularly flu-like infections
This is why the current approach to managing FOP centers on avoidance. Every preventable injury is a potential trigger for bone that will never go away.
Why Surgery Makes It Worse
The most counterintuitive aspect of FOP is that you cannot surgically remove the extra bone. Attempts to cut it out provoke explosive new episodes of bone growth, often producing more bone than was removed. Even emergency procedures like fasciotomy, where surgeons cut into tissue to relieve dangerous pressure, will worsen flare-ups and must be avoided. Current medical guidelines are blunt: avoid surgery except in true emergencies, and never operate to remove heterotopic bone.
Life Expectancy and Cause of Death
FOP significantly shortens lifespan. The estimated median life expectancy is 56 years, though individual outcomes vary widely, with recorded deaths ranging from age 3 to 77. The primary cause of death is cardiorespiratory failure from thoracic insufficiency syndrome, which accounts for 54% of deaths at a median age of 42. This happens when bone formation around the ribcage and spine gradually restricts the lungs’ ability to expand. Pneumonia causes another 15% of deaths, and complications from falls account for 11%, both closely tied to the progressive immobility and chest restriction the disease creates.
The First Approved Treatment
For decades, there was no approved therapy for FOP. That changed when the FDA approved Sohonos (palovarotene), the first drug specifically indicated for the condition. It’s approved for adults and children (females 8 and older, males 10 and older) and works by reducing the volume of new bone that forms.
Patients 14 and older take a low daily dose, then increase it during flare-ups for a 12-week course: four weeks at a higher dose followed by eight weeks at a moderate dose. Younger patients receive weight-adjusted amounts. The drug doesn’t reverse existing bone growth, but it can slow the accumulation of new bone, which in a disease defined by relentless progression represents a meaningful shift. Clinical trials enrolled 164 people with FOP, a substantial number given how rare the condition is.
Beyond palovarotene, a phase 3 trial has been completed for a different approach: blocking a protein called Activin A, which researchers identified as a key driver of the abnormal bone formation pathway. Early work had focused on blocking bone growth signals directly, but pharmaceutical researchers found that targeting Activin A was the more effective strategy. This represents a second potential treatment option in a disease that had none until recently.
Living With FOP
Day-to-day management revolves around protecting the body from triggers. That means avoiding contact sports, being cautious about falls, skipping intramuscular injections in favor of alternatives when vaccines are needed, and steering clear of unnecessary medical procedures. When flare-ups do occur, treatment focuses on managing pain and inflammation while the episode runs its course. Current guidelines emphasize early and accurate diagnosis, careful avoidance of injury and medical harm, symptom relief during painful flare-ups, and making the most of whatever mobility remains.
Because the disease is so rare and so easily misdiagnosed, awareness of the malformed big toe sign at birth is one of the most important factors in preventing iatrogenic harm. A correct early diagnosis doesn’t change the genetic reality, but it can spare a child years of damaging procedures that accelerate the very condition they’re meant to investigate.