In his mid-40s, 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 Carroll, describing the sound of bone grating on bone.
So last year, at the age of 54, 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 Carroll, who was 6-foot-3 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 have come from Medicare, which largely covers older patients.
“Unfortunately, it's not surprising that people getting knee replacement are overweight,” said Dr. Patricia 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 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.
“The average BMI in those patients under 65 is 33, which makes them obese, and that means that half of them are even heavier than that,” said Franklin.
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 chairman 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 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.