In his mid-forties, Charles Carroll, a letter carrier in Shrewsbury, Mass., started taking anti-inflammatory medicine for his achy knees. Despite his young age, osteoarthritis had begun to settle in.

Over the next decade, the pain became so bad that at times he had difficulty walking. Eventually he began skipping lunch because standing up afterward was too painful.

“My knees would snap like dry wood when I got up in the morning,” said Mr. Carroll, describing the sound of bone grating on bone.

So last year, at the age of 54, Mr. Carroll underwent his first knee replacement. He will receive his second replacement in December.

It is becoming a familiar story. While the number of annual knee replacements has doubled in the last decade, the number in people under age 65 has nearly tripled. Experts have puzzled over this surgical trend. Some attributed it to aging baby boomers trying too hard to maintain active, weekend-warrior lifestyles. Other researchers blamed it on aggressive marketing of the procedures.

Now figures from a new national database of knee replacement patients strongly suggest that obesity is the most likely reason for these surgeries in younger patients. For Mr. Carroll, who was six feet three inches and 284 pounds before his procedure, obesity was likely a factor in his knee problems.

Experts generally agree that obesity increases the risk of osteoarthritis, the leading cause of knee replacement surgery. Yet most of the data in support of this association has come from Medicare, which largely covers older patients.

“Unfortunately, it’s not surprising that people getting knee replacement are overweight,” said Dr. Patricia D. Franklin, principal investigator of the new database and director of clinical outcomes research at the University of Massachusetts Medical School. “But we were surprised that the younger population wasn’t healthier and that they didn’t have the profile of healthy active adults.”

The new database, called Function and Outcomes Research for Comparative Effectiveness in Total Joint Replacement (FORCE-TJR), collects data on total joint replacement surgeries from more than 125 orthopedic surgeons in 22 states.

A recent analysis of the data found that younger patients were more likely to be obese than older patients. In the first 9,000 patients whose outcomes were tracked in FORCE-TJR, 55 percent of patients under age 65 were obese compared with 43 percent of those 65 and older. About 11 percent of younger patients were in the morbidly obese category, with a body mass index, or B.M.I., greater than 40, compared with 5 percent in the older group.

“The average B.M.I. in those patients under 65 is 33, which makes them obese, and that means that half of them are even heavier than that,” said Dr. Franklin, who presented the findings at the annual meeting of the American College of Rheumatology in San Diego in October.

Experts had wondered whether younger patients were opting for surgery prematurely to preserve an active lifestyle. “Many people assume that younger patients have less pain and better function than older patients, that they’re doing it earlier in their disease process,” said Dr. David Ayers, a co-author of the study and chair of orthopedic surgery at the University of Massachusetts Medical School.

The data also showed that younger patients had the same levels of disability from their knee pain before the surgery as did older patients. Younger patients were equally likely to have other serious medical conditions such as diabetes and pulmonary disease as their older counterparts, and were more likely to smoke.

“It’s not a fit weekend warrior who has had a sports injury,” said Dr. Ayers. “These younger patients have significant pain and very severe functional limitations at the time of their total knee replacement surgery. They’re heavier and they’re sicker.”

The number of total knee replacements more than doubled in the past decade, from 313,618 in 2001 to 644,243 in 2011, according to national data. In patients between the ages of 45 and 64, the number jumped to about 274,000 from 102,000 in that decade. Younger people now make up 43 percent of all knee replacement patients, about an 11 percent increase in the last 10 years.

Prior research has found a strong association between obesity and osteoarthritis. Excess weight puts stress on joints, causing them to break down, and some research suggests that inflammation associated with obesity adds to the damage. Weight loss reduces knee pain in patients with osteoarthritis.

“I do believe patients are getting more osteoarthritis because of the obesity, and getting more severe arthritis at a younger age, and needing the surgery sooner,” said Dr. M. Elaine Husni, director of the arthritis and musculoskeletal center at the Cleveland Clinic. Dr. Husni was not involved with the study.

“Obesity has a tremendous impact on total joint replacement, and it’s something that every orthopedist is facing now,” said Dr. Michael Parks, an orthopedic surgeon at the Hospital for Special Surgery in New York.

Often patients with obesity have medical problems that raise the risk of complications during surgery, such as diabetes, pulmonary artery disease and poor circulation. Knee replacements are often more complicated in heavier individuals, requiring special larger implants and tools. Some studies have also reported lower implant survival time in obese patients.

The new analysis found that at six months post-surgery, those who were severely obese had similar if not greater improvement in pain and function after knee replacement therapy as those who were not. But because the implants last about 20 years, many younger patients may need surgery again in the future.

Obesity “is a modifiable risk factor for knee surgery,” said Dr. Husni. “We can change this.”

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