SUNDAY, Nov. 11 (HealthDay News) -- People who earn $35,000 or less a year may have better outcomes after knee replacement surgery than those who make more money, U.S. researchers say.

The study finding is based on data from patients who had knee replacements at the Mayo Clinic over the past few decades. The researchers found that lower-income patients reported less pain and better knee function at their two-year checkups than higher-income patients.

The finding "runs counter to what many might have expected to see. We need to work to understand it further," study author Dr. David Lewallen, an orthopedic surgeon at the Mayo Clinic in Rochester, Minn., said in a Mayo news release.

However, this finding can begin to help doctors figure out why some patients do better than others after knee surgery, Lewallen explained.

One possible explanation is that many lower-income patients delay the surgery as long as possible. This means that their knees tend to be in worse condition when they receive a knee replacement and their feeling of improvement after the procedure is more dramatic, Lewallen suggested.

"This is one small piece of a very large puzzle in understanding patient outcomes following a well-defined surgery that we know is very effective for most," he said.

The study was scheduled for presentation Saturday at the annual meeting of the American College of Rheumatology in Washington, D.C.

The data and conclusions of research presented at medical meetings should be viewed as preliminary until published in a peer-reviewed journal.

More information

The American Academy of Orthopaedic Surgeons has more about knee replacement.