ENGINEER AND SURGEON TEAM UP TO SOLVE ARTIFICIAL JOINT PROBLEMS
The heroine in the "Princess and the Pea" fairy tale showed her noble lineage by suffering a sleepless night because of one pea under a layer of 20 mattresses. But you don't have to be exquisitely sensitive to appreciate that physically small imperfections, like a shoe that's just a shade too narrow or a dental filling that barely overflows the cavity, can be a royal pain.
Researchers at the University of Illinois at Chicago are convinced that small imperfections in the fit of artificial joints - slight misalignments between the femoral and tibial components of a prosthetic knee joint, for example - are at fault in rare but bothersome cases of prosthetic joint failure, and they have developed tools and a technique to address the problem.
"We've come up with a way for the surgeon to do what we call dynamic fitting," said Farid Amirouche, professor of mechanical engineering. "We developed a clinical procedure that takes advantage of a new type of pressure sensor to detect an imbalance in the prosthetic joint while the patient is still in the operating room."
Amirouche and his student, Luke Aram, collaborated with Mark Gonzalez, M.D., a professor of clinical orthopaedics in UIC's College of Medicine, to adapt new technology to an engineering problem in medicine. Prosthetic knee, hip or other joints, which are made of plastic-like synthetic material, may not function smoothly if the components are geometrically misaligned or if there is some imbalance in the patient's ligaments supporting the joint. If the fit is not corrected, one moving part improperly grinds against another as the patient moves about and flexes the joint. This dramatically shortens the lifetime of the joint and can cause the patient pain, or even infection as the plastic is ground into a powder and enters the bloodstream. Currently, the surgeon may test the fit of the joint with his or her hands or, at best, with a single measurement of the force acting at one point inside the joint. That may not be good enough to detect a slight error that could lead to excessive wear, notes Amirouche. Some surgeons have used pressure-sensitive film to measure the pressure distribution across the joint, but the time to process the film limits its usefulness in the operating room and the film records the pressure distribution at only one time. At the heart of Amirouche's team's innovation is an array of sensors about 6 square centimeters in area that they use to measure the contact pressure inside the joint while it flexes through its range of motion. "New pressure sensing technology recently made it possible to build paper thin load cells that measure contact pressure instantaneously," said Amirouche. "These sensors enable surgeons to make the necessary adjustments, based on instantaneous contact pressure measurements, right in the operating room."
The surgeon slides the sensor array into the newly installed joint at the place where one component makes contact with the other; in the case of the knee joint, it's where the femoral and tibial components make contact. The sensor continuously records the pressure at more than 80 discrete points across the contact area. The pressure signals travel along wires to a computer next to the surgeon, where they appear on a screen so that the surgeon can monitor how the pressure changes as he or she rotates the patient's bone through 90 degrees -- the normal angle of motion. The readings can be compared to standards developed through clinical testing. If the sensor picks up points where the pressure is outside the ranger permitted by the standards, it's probably an indication that the fit is not good, and the surgeon can modify the fit accordingly and try again. Manufacturer of prosthetic joints DePuy, a division of Johnston and Johnston, is supporting the development of these tools so it can establish rigorous guidelines on the proper installation of its product.
Amirouche said the surgeon's ability to measure the pressure at different points in the contact area inside the joint, and to do it as the joint flexes, is a significant boon to the patient.
"Older techniques to determine the pressure only gave a reading at one point, and in engineering terms that gives you forces and moments in three directions," he said. "Here we have measurements over a whole contact area between the moving parts of a joint, while the joint is in motion. This will be key in reducing pain and other problems for patients."
As an engineer, Amirouche is also looking ahead to other potential benefits of the pressure sensor.
"Now we can have a better model or representation of the knee," he said, obviously eager to explore the analytical vistas opening up before him. "We'll be able to understand, for example, the effect of carrying different loads of weight. I think this will open up for research the whole issue of joint stability, especially in athletes."
- UIC -
RSS