While more than 900,000 total knee replacement surgeries were performed in the U.S. in 2011 to treat debilitating knee osteoarthritis, the success rate of post-operative functional gains vary widely, according to new research findings presented this week at the American College of Rheumatology Annual Meeting in Washington, D.C.

Knee osteoarthritis is caused by cartilage breakdown in the knee joint. Factors that increase the risk of knee osteoarthritis include obesity, age, prior injury to the knee, extreme stress to the joints, and family history. In 2005, 27 million Americans suffered from osteoarthritis, and one in two people will have symptomatic knee arthritis by age 85.

Starting in 2008, researchers at Arcadia University and the University of Massachusetts Medical School reached out to 179 people with knee OA patients who were undergoing total knee replacement surgery, 68 percent of these people were female -- with a mean age of 65.1. All the participants wore an accelerometer ankle device to measure walking before surgery, with a mean use of 3.3 days worn. The accelerometer was worn by 174, 163 and 168 participants at least one valid day at baseline, eight weeks and six months after surgery. In addition to patient self-reports and accelerometer information on those patients, the researchers requested and examined physical therapy records of 90 individuals who completed outpatient rehabilitation and 27 who completed their rehabilitation in home-care.

The goal was to analyze how physical therapy might be related to levels of post-operative walking function, says Carol Oatis, PhD, PT, lead investigator in the study and a professor of physical therapy at Arcadia University.

"Our findings demonstrated wide variability in the utilization of physical therapy in these subjects, in the amount of physical therapy, the number of days in physical therapy, and also, wide variability in the kind of physical therapy after surgery," says Oatis.

Examination of the accelerometer data showed that participants' average and median daily step counts were approximately 1,000 steps fewer at eight weeks after surgery than they were prior to surgery. At six months after surgery, the mean and median increase in steps from pre-operative levels was only 738 and 354 steps, respectively. However, 30 percent of the participants ended physical therapy after eight weeks and were left to continue their rehabilitation alone, says Oatis. Approximately 40 percent completed their rehabilitation after nine weeks.

"What struck me was that a large percentage of people had been discharged from physical therapy while their physical activity level was still greatly below their preoperative levels," she says. "I thought that was a pretty stunning picture of the relationship between the timing of rehabilitation services and functional activity."

It is unclear what communication or direction physical therapists are giving to patients after total knee replacement, and there is no standardization of postoperative rehabilitation, Oatis notes. "We don't know what physical therapists are saying to patients about what they need to work on after the rehabilitation ends. Many patients have a higher expectation of functional ability following surgery than they actually attain. By recognizing the disconnect, we can be more overt in conversations with patients, and help them focus on physical activity and behavioral goals after surgery."

Funding for this study was provided in part by the National Institute of Arthritis and Musculoskeletal and Skin Diseases.