Scientists think people might be better off if our knees worked more like elbows, and a pair of University of Chicago researchers has designed an artificial knee joint that does just that.
The new knee joint has impressed several European surgeons and this autumn will begin studies required by the Food and Drug Administration so that it may be marketed in this country, perhaps within two years.
The Chicago scientists didn’t set out with an elbow obsession to rearrange anatomy willy-nilly, but they did begin with the notion that nature’s design of the human knee is flawed.
Because natural evolution depends upon incremental changes in the structures of existing animals, it is limited in the materials and designs available to accommodate new functions. Working up from crawling, hopping reptiles to four-legged mammals to two-legged primates that walk around upright proved a major challenge for the natural evolution of the knee.
Using an assortment of ropelike ligaments, muscles and tendons to tie bones together at the knee while allowing great freedom of movement, nature has given us a joint prone to failure caused by playing at sports, climbing stairs or just growing older.
Replacing 150,000 to 200,000 knees a year is a billion-dollar business in the United States, and many people in that business think that artificial knees ought to overcome nature’s shortcomings.
“The function of the normal knee is optimal, but the design is not,” said Dr. Lawrence A. Pottenger, an orthopedic surgeon at the U. of C. “It’s becoming more common for people to try to design something better to improve on nature.”
Pottenger and his co-inventor, Dr. Louis Draganich, director of orthopedic biomechanics at the university, began their quest for a better knee nearly a decade ago following a conversation Pottenger had with an executive of a firm that makes artificial knee joints.
“I told him that I hated all prosthetic knees, but that I hated his the least,” Pottenger recalls of his backhanded compliment to the executive. “He said if I was so dissatisfied, I ought to design something better myself. I took that as a personal challenge.”
Most artificial knee joints are made with cobalt chrome installed at the end of a patient’s upper leg bone, or femur, that rests against a component made of heavy-duty polyethylene plastic attached to the lower bone, or tibia.
The problem is that as metal rests against the hard plastic at a very small point of contact, it wears away the polyethylene and can cause joint failure. Also, people with artificial knees may experience difficulty rising from a sitting position or climbing stairs.
The Chicago researchers’ goal was a joint that would maximize contact between metal and plastic in all positions, spreading the weight load and reducing wear. Another goal was to allow full movement with a minimum of wires or springs to stabilize the bones.
The product of these labors is a prosthetic knee joint called TRAC (Two Radii Area Contact) that appears stunningly simple when compared with a model of a natural human knee joint.
Key to TRAC is a metal component that has an inner track and outer track with a crossover segment joining each so that no matter what position the knee assumes there will be contact with a large portion of the underlying plastic surface, minimizing wear.
The knee’s wide range of motion is enabled by a rotational device incorporated in the plastic embedded in the tibia.
“The principle behind it is much more like the way an elbow works than a knee,” said Pottenger.
For some years now, Pottenger has given lectures at the university with Paul Sereno, the dinosaur hunter and anatomy expert. Exploring such things as how a dinosaur’s jaw evolved into an ear has given Pottenger a new appreciation of evolutionary limitations that can result in something as functionally important yet so fragile as the human knee.
But these thoughts of dinosaurs and elbows came after development of the new knee joint. TRAC’s design stems from theories about optimal joint function that Draganich and Pottenger conceived over the years using computerized patient information.
They tested these theories by putting patients who had various kinds of artificial knees on a computerized gait-analysis apparatus to observe how they functioned and to capture the information in a computer.
Many orthopedists over the last 20 years have sought to design a better artificial knee. Some try to stick to nature’s model, but more and more of them are striking out in new directions as Pottenger and Draganich have.
So there are a lot of variations, however slight, in prosthetic knees.
“We’d compare the walks of patients with different prosthetic designs to people with normal knees,” said Draganich. “We studied effects on different muscle groups and checked this against our hypotheses.”
Working with ARCH Development Corp., the agency that helps commercialize university research, the scientists have a relationship with Biomet Inc., of Warsaw, Ind., which makes replacement joints and other medical devices.
“We talk to Biomet engineers frequently to discuss details of design, testing or manufacturing,” said Draganich.
The arrangement works well for Biomet, said its chief executive, Dane Miller, because the firm doesn’t have basic research expertise or access to patients as the University of Chicago scientists do.
Over the years, about 90 patients at the U. of C. have received TRAC knee joints under a research protocol, and Pottenger has traveled to several foreign countries to install the joints in patients as local surgeons watched to learn the techniques involved.
TRAC knees have generated considerable excitement in Europe, said Chuck Niemier, Biomet’s senior vice president for international operations.
“This is the knee of the future,” he said. “Its design is unique. We see this as a tremendous opportunity with FDA testing starting in the United States this year. With luck, we could get approval to market the knees here in two years.”
Biomet is just gearing up to make surgical instruments needed to install the TRAC knee joints and may have trouble keeping up with demand initially. Niemier said that one set of instruments sent to Australia for a demonstration was supposed to be returned for use in Europe, but the Australian surgeons liked TRAC so much they haven’t sent back the instruments.
The quest of Pottenger and Draganich for fame and fortune in the crowded field of artificial knees has an ironic twist because Pottenger’s wife, Bobbie, now needs an artificial knee but won’t get a TRAC even though the researchers believe it is best.
Pottenger would replace his wife’s knee with a TRAC, but it’s against university policy for a surgeon to operate on a close family member.
Enrolling Pottenger’s wife in the coming FDA trials could be a conflict of interest because she might not complain even if she found the implant less than ideal.
“She’s put up with our work inventing and developing this from the first, but she won’t be able to benefit from that work, and that’s a shame,” said Pottenger.
