A New Twist On An Age-Old Problem: Making Knee Replacements Last

"More engineering analysis goes into the washing machine in your home than into the artificial knee joints implanted in people," said B.J. Fregly, an assistant professor of mechanical and aerospace engineering at the University of Florida.

Fregly is applying the latest techniques from the world of mechanical engineering to the human body. By linking video, CT scans, computer models, specially developed computer software and other technologies, he and his colleagues expect to enhance understanding of the causes for failure of artificial and natural knee joints -- as well as improve surgical procedures and create longer-lasting artificial knees. They also are working to develop tools that will shed light on the causes of arthritis in the knee and improve the success of a surgical approach to correct damage from the disease. Seventy million adults, or about one in three Americans, suffer from some form of arthritis, according to the Centers for Disease Control and Prevention in Atlanta.

Fregly, who has published several papers on his work in the Journal of Biomechanics and the Journal of Biomechanical Engineering, said knee implants may last 20 years or longer in sedentary patients, but those who are active tend to wear out the implants more quickly – many after just 13 years. As younger and younger patients are requiring knee replacements and people are living longer, the durability and functionality of the artificial knees currently in use is becoming a cause for concern.

"We want knee implant recipients to be able to resume their favorite activities - playing tennis or going on walks - without limitations or fear of wearing out their new knees prematurely," Fregly said.

One problem is the current technology to test new implant designs is ineffective, Fregly said. Simulators designed to highlight the places artificial knees will develop wear require up to three months and cost as much as $40,000 for an assessment, yet the results don't consistently equate to what occurs in patients, Fregly said. As a result, his team – which includes Greg Sawyer, a UF assistant professor of mechanical engineering, and Scott Banks, technical director of the nonprofit Biomotion Foundation in West Palm Beach -- is developing a better way to test new implant designs. The researchers' key innovation: Combining motion data recorded from artificial knees with computer simulations of walking using physiological loads and speeds to create the first-ever computational wear models for knees, or models that make quantifiable predictions of deterioration of artificial knees.

An early version of system came within a few tenths of a millimeter of predicting the wear in an artificial knee that was recovered after a patient died – and accurately predicted the locations of the worst wear, Fregly said. "That's pretty good for a first crack," he said. "Once this model is more complete, it could allow surgeons to rapidly select knee designs on a more individual patient basis." Scott Delp, an associate professor of biomechanical engineering at Stanford University, said Fregly's research offers a unique solution to a complex problem.

"It is not possible to simulate the motions and functions of knee implants without the advanced software tools like Professor Fregly has developed," he said. "His technology offers an entirely new paradigm for simulation-based design and evaluation of knee surgery." Fregly and the others hope the research team and orthopedic surgeons eventually will work together to perform virtual surgery on a computer model, a feat that would enable them to predict the surgical parameters or modifications to an implant that will produce the best outcome. They would then track patients' long-term progress to assess the model's predictions. "Just as suits can be fitted to business executives, orthopedic surgeries will be custom tailored to the patient," Fregly said.

Fregly's group also is seeking to improve the success of a surgical procedure called tibial osteotomy, which involves realigning one of the lower leg bones to redistribute loads on the knees of patients suffering from arthritis. The problem is the procedure is successful only in some cases, Fregly said. "Many doctors today feel that the variability is so large that the operation is not worth it," he said. As part of the procedure, surgeons essentially have to make an educated guess about how much to rotate the lower leg bone, Fregly said. By recording how patients walk normally and with heel wedges that approximate the effects of the surgery, Fregly and fellow researchers, including Rafi Haftka, a professor of mechanical and aerospace engineering, are creating a computer model that could predict more accurately the correct rotation, he said. Now in the second of a three-year project funded with a $240,000 grant from the Whitaker Foundation, the research eventually could make the surgery more effective, he said.

Fregly's third project probes the causes of knee arthritis itself – an area that has long proven mysterious because it is so difficult to determine what is happening inside the knee as a person walks. The team is seeking to predict inner forces on the knee using computer models, CT scans and reverse-engineering software produced by Research Triangle Park-based Raindrop Geomagic. One result of the three-year project, funded with a $210,000 grant from the National Institutes of Health, may be a better understanding of how to guard against developing arthritis.

"It may be that we can identify load thresholds that are bad, or types of loading that are bad, so we can train people to avoid doing these things," Fregly said.