Kelly Hanna’s leg was amputated on a summer day in 2020, after a Michigan doctor who called himself the “leg saver” had damaged her arteries by snaking metal wires through them to clear away plaque.

It started with a festering wound on her left foot. Her podiatrist referred Ms. Hanna to Dr. Jihad Mustapha. Over 18 months, he performed at least that many artery-opening procedures on Ms. Hanna’s legs, telling her they would improve blood flow and prevent amputations.

They didn’t — for Ms. Hanna or many of his other patients. Surgeons at nearby hospitals had seen so many of his patients with amputations and other problems that they complained to Michigan’s medical board about his conduct. An insurance company told state authorities that 45 people had lost limbs after treatment at his clinics in the past four years.

Dr. Mustapha is no back-alley operator working in the shadows of the medical establishment, an investigation by The New York Times has found. With the financial backing of medical device manufacturers, he has become a leader of a booming cottage industry that peddles risky procedures to millions of Americans — enriching doctors and device companies and sometimes costing patients their limbs.

The industry targets the roughly 12 million Americans with peripheral artery disease, in which plaque, a sticky slurry of fat, calcium and other materials, accumulates in the arteries of the legs. For a tiny portion of patients, the plaque can choke off blood flow, leading to amputations or death.

But more than a decade of medical research has shown that the vast majority of people with peripheral artery disease have mild or no symptoms and don’t require treatment, aside from getting more exercise and taking medication. Experts said even those who do have severe symptoms, like Ms. Hanna, shouldn’t undergo repeated procedures in a short period of time.