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CAD adds long life to new knees
By Benjamin B. Ames, Contributing Editor -- Design News, December 16, 2002
Gainesville, FL — When the cartilage between the ends of your tibia (shin bone) and femur (thigh bone) begins to deteriorate, the result is called knee-joint osteoarthritis. The condition includes painful friction, and can only be treated by swapping a metal-and-plastic artificial knee for your natural joint. A quarter-million Americans get the operation every year.
Yet transplants typically last just 20 years, so patients who are young or active will often outlive their new knee. That spells trouble, because it's almost impossible to anchor second and third replacements to the same bone.
So why can't we fix the problem? That's the question asked by B.J. Fregly, an assistant professor in the Dept. of Mechanical and Aerospace Engineering (MAE) at the Univ. of Florida. "More engineering analysis goes into the washing machine in your home than into the artificial knee joints implanted in your body," Fregly says.
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Geometry measured from CT scans is used in Geomagic to create the final polygonal model of a full knee. |
With a PhD in biomechanics, he first got the idea to improve the process while working as a software developer on the Pro/MECHANICA Motion product for Parametric Technology Corp. (PTC; Needham, MA). That's also where he learned his philosophy of "design it/refine it, rather than make it/break it." Precise computer modeling now allows him to make delicate design changes, instead of just starting from scratch with each failed implant.
That's different from the industry's usual technique, which puts physical prototypes of artificial knees into a wear simulator machine, a $40,000 tool that takes three months to simulate the steps of 20 years of use.
Fregly's complaint about this method is that it ignores the patient. "There's not a one-size-fits-all knee," he says. Variables include walking gait versus stair climbing, and individual walking styles. And that 20-year lifespan drops to just 13 years in one-third the cases of young and active patients.
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The knee model is merged with a full-body computer model and the result tuned to reproduce fluoroscopic image-derived and gross-motion data simultaneously. |
So Fregly's new approach is to fatigue-test an artificial knee according to the specific motion and size of the patient it was made for. He starts by getting a CAD model of the fake knee from its manufacturer, then records a motion-capture image of the patient, and matches the two together.
The goal of this process is to predict how quickly a new transplant will wear out. "Wear is the big issue; it comes from a combination of motion and load," he explains. So he's working with the university's MAE Dept. and with the Biomotion Foundation (West Palm Beach, FL) to predict it.
He starts by getting CAD models from knee-makers in Unigraphics (EDS; Plano, TX) or PTC Pro/ENGINEER file formats. That gives him a precise shape, but he still has to resurface the models with Rhino (Rhinoceros 3D; Seattle, WA) or Geomagic (Raindrop Geomagic; Research Triangle Park, NC), because he needs tight geometry tolerance—and a patchy surface with many seams is harder to calculate than a single, big surface. Then he uses SolidWorks (Dassault Systemes; Concord, MA) to translate files between those steps.
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By automatically aligning CAD models of the knee component with its segmented outlines, Geomagic allows creating composite bone-implant polygonal models that can be used for fluoroscopic image matching. |
The next software challenge in the process is to bridge the gap between deformable and rigid models. So he uses motion capture x-rays to record an individual's walking movements. That's tough to do from the outside. For instance, it's very difficult to calculate the force of various muscles on the bones. "Muscles are linear actuators that create rotational movement," he explains. "We have to figure out the moment arm (leverage) of each muscle on the tibia and femur bones."
"Then we match it to the CAD model," he says. "That measures motion pretty accurately, but it doesn't know loads. So I put it into my computer model of a knee, which takes 10-20 minutes to run a simulation on a high-end PC."
Recently, a patient donated her artificial knee to Fregly's team after she died, so they could check the accuracy of their predictions. He was thrilled to find that his prediction of 0.7 mm of wear was fairly close to the actual 1.0 mm.
Fregly's method is awaiting federal National Institutes of Health funding for widespread use, and is being applied to other skeletal conditions in the meantime.
go to www.designnews.com/info and enter the numbers below:
Unigraphics from
EDS:
Enter 535
Pro/ENGINEER from
PTC:
Enter 536
Rhino by
Rhinoceros 3D:
Enter 537
Geomagic from
Raindrop Geomagic:
Enter 538
SolidWorks by
Dassault Systemes:
Enter 539
Talkback
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Anybody know a place where I can download a 3D model of a knee to use in SolidWorks?
- 2006-12-20 00:51:15
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