A crippling disease called FOP is so rare, it usually goes undiagnosed. A documentary may change that.

As a baby in Sao Paulo, Brazil, Jessica Beltramin had surgery to straighten her contorted legs, then spent a year in hip-high casts.

That was the beginning of biopsies, operations, and injections that made her torment immeasurably worse. At age 9, she was correctly diagnosed with one of the world’s rarest diseases: fibrodysplasia ossificans progressiva, FOP. People with the genetic disorder are progressively crippled and immobilized as their muscles, tendons and ligaments turn into bone. Any injury or attempt to remove the misplaced bone triggers explosive, painful new growth.

“I don’t suffer anything like I did before with all the treatments and all the medications," Beltramin, frail and bent but still walking at age 28, says in a new documentary. “I suffered things I didn’t need to at all.”

The documentary, Tin Soldiers, is taglined “More than a film, it’s a call to action.”

The 79-minute movie is the centerpiece of a worldwide education and outreach campaign that aims to find the estimated 4,000 undiagnosed FOP sufferers and connect them to FOP families and experts — the small but mighty group that funded the film.

“I am so proud to be associated with the Tin Soldiers documentary," said University of Pennsylvania physician Frederick Kaplan, who provides commentary as the world’s leading FOP researcher. “We need to find everyone with FOP wherever they are to end isolation, to prevent harm, to lessen misery, to afford participation in clinical trials, and to build a truly global FOP community.”

Amanda Cali of Mountain Lakes, N.J., executive producer of the film and a patient advocacy dynamo, said, “I remember how isolated I felt 25 years ago when my son was diagnosed. There were only 83 known cases in the world. Now we have about 900 confirmed cases. So we don’t have even a quarter of" the estimated total.

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Ironically, recognizing a potential case is easy — if doctors know what to look for — because patients are born with bent big toes. Kaplan points out the bunion-like deformities while examining a boy in the movie.

Another irony: The obscure disease is a research hotbed thanks to the 2006 discovery by Kaplan’s lab of the gene mutation that causes it. Now, dozens of universities and drug companies are studying the molecular mechanics of FOP because it has implications for many bone diseases. As the documentary explains, seven potential therapies are in clinical trials for FOP, offering the first hope for more than just symptom relief.

But as Cali is acutely aware, trials need patients — even if their rare disease afflicts just 1 in 1.5 million.

About a year ago, Cali pitched the idea of a documentary to Odette Schwegler, a South African journalist and filmmaker who had done a segment on FOP for a TV news magazine.

“The story [of FOP] just touched me on another level,” said Schwegler, who has a history of supporting charities, especially in South Africa’s desperately poor communities.

What started with a plan to film at an FOP meeting grew into a sprawling, four-continent chronicle of resilience and courage. Schwegler’s crew trekked to big cities, small towns, affluent suburbs, and dangerous slums to follow FOP patients, their families, and doctors.

In Allentown, Stacey and David Scoble are shown holding the seventh annual “Bingo for a Cure,” inspired by their 13-year-old son, Joshua, who must use a wheelchair. The event has raised $1 million for research.

In Gugulethu, a township near Cape Town, South Africa, 52-year-old Thozamile Mciki talks about being shunned by neighbors who think he is “bewitched,” about hoping FOP becomes treatable, and about wishing the outhouse weren’t so far from his living quarters.

“Anyway, I would say I’m lucky to be this age, having FOP,” he says, smiling.

In Manhattan, Whitney Weldon, 28, zips along in her motorized wheelchair, visiting restaurants to review their food — and accessibility — on social media. “FOP is all about adaptation,” she says.

In family videotapes, Cali’s son Ian is shown through the years, laughing in his high chair, playing softball, and roughhousing with his brother. Now 30 and a computer application developer, he discusses how anxiety about FOP’s unpredictable “flare-ups” can be as disabling as the ossification.

“I look back on all my major flare-ups. In my head, I had already jumped down the rabbit hole,” he says. “Now, I try to maintain my mental independence more than my physical independence.”

The film premiered in Johannesburg in October and is slated to be screened at film festivals around the world. The co-producers are also getting invitations to showcase it at medical and educational gatherings. The University of Pennsylvania showed it last month; sites in Australia, Prague, Holland, Kenya and Namibia are upcoming.

Posters, public service announcements, and other pieces of the outreach campaign are now rolling out. A nonprofit, Tinsoldiers.org, has been created to raise funding and recruit volunteers.

“The more we get, the more we can do,” Schwegler said.