Researchers work to develop effective implants
DesJardins (right) and doctoral students Roy Rusly (left) and Eric Lucas work to help design better knee implants.
Even if just a tiny part of the knee hurts, it still hurts and may severely impact a patient’s ability to walk. Yet surgical implants that replace just the knee’s damaged portion are far from ideal and may even negatively impact the remaining knee structure. This is particularly true for patients who suffer with osteoarthritis.
That’s where John D. DesJardins, PhD, steps in. DesJardins is director of the Frank H. Selling and C. Dayton Riddle Orthopaedic Education and Research Laboratory at Clemson University in South Carolina. In DesJardins’ bioengineering laboratory, he and doctoral students apply mechanical engineering principles – stresses, strains, forces and loading – to figure out how best to treat joint problems.
Specifically, DesJardins’ research today focuses on unicompartmental knee arthroplasty, a surgical procedure to treat osteoarthritis when the damage is limited to one knee compartment. When compared to total knee replacement, the unicompartmental procedure theoretically should offer many advantages: bone preservation, more joint movement, improved ability to perceive joint position and motion, increased range of motion and faster recovery time.
Sounds great, right? Yet the reality is that the unicompartmental procedure is more complex and has a slightly lower success rate compared to a total knee replacement. Some of the problems that develop after the unicompartmental operation include the progression of arthritis in the rest of the knee, polyethylene wear of the implant and aseptic loosening that causes the bond between the implant and the bone to fail.
“Believe it or not, it’s sometimes harder to replace a tiny piece than it is to just get rid of the whole thing. The knee is very complex. It has a lot of tissues that guide the motion,” DesJardins explained. He likened the situation to replacing just one tire on your car – it throws off the alignment.
“In this particular study, we’re looking at issues like alignment,” DesJardins said. “If you replace just one half, what’s with the other half? Are you making the situation worse by taking the loading that’s on one side and sticking it over on the other? Remember, you’re not replacing it with cartilage; you’re replacing it with plastic and metal. The forces that go through the knee are definitely going to be changed. That distribution could affect what’s left in your knee.”
Thus, DesJardins and his students are investigating the effects of different unicompartmental replacements and ligament configurations on the movement and pressures on the knee. The idea is to prevent detrimental weight loading (particularly when someone stands up or sits down) and bad motion that increase stress on the joint.
To that end, DesJardins’ lab now features a knee simulator that he and his students have constructed from scratch over the last two years. The unicompartmental replacement study, which is funded in large measure by the Orthopaedic Research and Education Foundation, is the first one using the new contraption, designed to analyze the impact of different implant devices and surgical procedures on both mechanically equivalent plastic bones and cadaver bones. “As a tool set in our laboratory, this was needed,” DesJardins said. “You can’t buy these; you have to make them.”
DesJardins surmised that there are probably about 15 similar knee rigs, each custom-built, in laboratories worldwide. Generically, they are called “Oxford” rigs, because the developer of the first such device was with the University of Oxford. A motor at the top of the device creates the loading pressure that quadriceps demand when someone stands up – about 1,000 pounds of force. Also, the device features an array of instrumentation to measure forces and stresses.
Cadaver specimens may be mounted on the rig, offering the ability to simulate lower-extremity pressures on human bone and tissue. Clemson works with the Greenville Hospital System and its institutional review board to obtain cadaver knees from tissue banks nationwide. DesJardins’ laboratory partners with regional surgeons, who conduct a surgical procedure on a cadaver knee just as they would on a live patient. Then the altered knee is loaded into the Oxford device. “Thus we see how different variations in the surgeons’ technique might affect the results, and we can quantify that for them,” he said.
DesJardins said another study is now using the rig to study the patella.
The Clemson lab offers surgeons a variety of opportunities. In fact, DesJardins described his lab as being “on the front lines” when it comes to orthopaedic-surgery improvements. “We work on devices that are actively being used, so we can offer an opinion on their use directly,” he said. “When we tell a surgeon to use four screws instead of three screws, the very next day, they go in and do it.”
His laboratory also works in the area of gait analysis, measuring how stride and accidents that damage the legs, such as a tear of the anterior cruciate ligament, can affect a person’s motion.
DesJardins lab is also one of the nation’s largest repositories of post-use total joint replacements. “We’re very interested to look at that old implant, because we can maybe figure out what went wrong and how the implant performed in the body,” he said. The research involves both physicians and medical-device manufacturers.