The last time Alicia Kline walked unassisted was in January 2021. She is in constant pain from hip arthritis and uses a walker and a wheelchair.

The problem is her obesity — her body mass index, or B.M.I., is over 50. And for someone like Ms. Kline, 54, of Reading, Pa., finding a doctor who is willing to give her a joint replacement that may ease the pain can be a near-impossible task.

Dilemmas like hers are roiling orthopedic medicine as obesity levels soar, and with them arthritis. What should orthopedists do when confronted with patients whose B.M.I. is very high?

While the science of body mass index is frequently criticized, doctors say the risks of operating on patients with B.M.I.s that fall in the upper ranges of obesity can be dire, including infections deep in the prosthetic joint that can lead to amputations and even death. And the risks escalate as a patient’s B.M.I. increases.

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Professional organizations — the American Academy of Orthopedic Surgeons and the American Association of Hip and Knee Surgeons — do not have firm B.M.I. cutoffs for operations, but orthopedists have their own.

Less than half of orthopedic surgeons in a survey published last year said they would operate on a patient with a B.M.I. over 40 — more than 22 million American adults fell into that category in 2017-18. Only 11 percent would operate on one of the 3.9 million Americans with a B.M.I. over 50. And patients with a B.M.I. over 55 would be rejected almost everywhere. Just 3 percent of U.S. orthopedists would operate.

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