CAD/CAM Has Use Beyond Prosthesis Design and Fabrication

Computer-assisted design and manufacturing systems may be efficient tools for recording residual limb anthropometrics and 3-D shape information.

“The CAD systems I studied are able to be used for clinically relevant measurements with accuracy and repeatability,” Mark D. Geil, PhD, director of the Biomechanics Laboratory in the department of kinesiology and health at Georgia State University in Atlanta, told O&P Business News.

Geil examined the consistency and accuracy of optical and electromagnetic shape-capturing systems of two computer-assisted design (CAD) systems – the Tracer Omega and the T-Ring II (Ohio Willow Wood, Mount Serling, Ohio). The Tracer Omega is a contact scanner and the T-Ring II is a noncontact optical scanner. He compared the results achieved with these systems with published anthropometrics from standard measurement tools: standard tape measure, force gauge tape measure, anthropometer, VAPC and Ritz Stick.

The results were published in the Journal of Rehabilitation Research and Development.

His study included four board-certified prosthetists and orthotists who had completed the Ohio Willow Wood training course for, and had clinical experience using, the two systems. Seven first- or second-year students enrolled in the Georgia Institute of Technology Master of Science program in Prosthetics and Orthotics also participated. These students had completed a semester-long course in CAD/computer-assisted manufacturing (CAM) that included the Ohio Willow Wood training course; however, the students had no clinical experience with the systems.

Consistent measurements

The digitally captured shape of one of the models used in the study with landmarks represented
Pictured: The digitally captured shape of one of the models used in the study with landmarks represented.
Image reprinted with permission of Mark D. Geil, PhD.

Geil averaged the measurement results across all participants and both CAD systems. All measurement sites produced consistent measurements, with a mean range of 0.96 cm, he wrote.

Students were slightly more consistent than practitioners. Practitioners had a larger average error in length measurement; other sites showed similar errors, according to the study.

“Differences were present between groups, but no result suggested clinically significant differences in consistency or measurement error between the students and practitioners,” Geil wrote. “This result implies that a baseline level of training is all that is required to produce consistent and accurate anthropometric measurements with digital shape capture.”

He stressed, however, that experience is still important in using CAD/CAM systems correctly.

“Clinical experience almost certainly plays a larger role in model modification than in proper digital shape capture,” he wrote.

The two systems also produced consistent results for the same measurement.

When he compared the digital results to published anthropometrics collected with traditional tools, again, Geil found that the results were similar. The average difference between digital and hand measurements was about 5 mm.

“This result suggests that, at least for linear and circumferential shape measures at distinct landmarks, measures obtained by either digital system in comparison with analog measurement with hand tools have no clinically significant differences,” he wrote.

Typically, practitioners assume that CAD/CAM systems are strictly for designing and fabricating a prosthetic socket, according to Geil.

“I think the results indicate a hidden use for CAD. For practitioners who have access to CAD shape capture systems, the systems can be used instead of hand tools for the measurement of residual limb dimensions,” Geil said in an interview. “An optical system can capture in just a few seconds the same diameters and circumferences that take a few minutes to record with calipers and tape measures, and it also provides permanent access to thousands more measurements. So, even if computer aided manufacturing or C-Fab is never intended, this research suggests the utility of CAD systems for routine digital shape capture.”

CAD/CAM benefits

CAD/CAM offer several benefits over traditional prosthesis manufacturing techniques.

“If we ignore the up-front cost of the equipment, the benefits are numerous,” Geil said. “Residual limb shape can be captured and modified in less time using fewer materials. Both are a cost savings,” he said.

CAD is also a time saver, he said. Digitization time has decreased from minutes to seconds. Socket fabrication using the automated carving system takes only minutes and still produces accurate results, according to the study. The total time for prosthesis fitting is significantly shorter.

In addition, CAD/CAM is easily integrated with central fabrication sites, which makes it usable in developing countries.

“For me, though, the biggest benefit provided by CAD is the ability to store the shape of countless residual limbs digitally,” Geil said. “A repository of digital images allows practitioners to quickly and accurately compare changes in an individual’s limb volume over time, find out where those changes are occurring, and monitor the effects of certain treatments. I’m a believer in quantification, which is why I’m conducting these studies in clinimetrics. I also think that routine quantification in clinical practice is important to the future of post-hoc research in prosthetics and orthotics.”

For more information:

  • Geil M. Consistency, precision and accuracy of optical and electromagnetic shape-capturing systems for digital measurement of residual-limb anthropometrics of persons with transtibial amputation. J Rehabil R D. 2007;44:515-524.

Colleen Owens is a correspondent for O&P Business News.

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