MEDICAL METROLOGY
a comparator version of the deviation color map one often sees with scanning systems. “We recognize it is cumbersome to match parts manually, like knee or hip joint replacements, against Mylar overlays,” he said. “There is also the management overhead of manag- ing those overlays.” Human error is always a possibility when matching parts to overlays. New overlays need to be plotted as parts change. Medical applications include replacement hips, knees, or bone screws. “All of these parts from a medical manufacturer need inspection to 100%,” said Arenal. “The video comparator improves the processing speed as well as the quality” compared to the traditional optical comparator.
While choosing the right device is a balance between accuracy requirements, size of the part, speed and cost, he also observes that increasing concerns over product liability is driving attention towards more accurate measurements. In the context of medical devices, CT scanning is becom- ing more useful. “Something like a Romer Arm is ideal for measuring engineered joints, where tolerances are actually a little more forgiving,” explained Solberg. “But, there are many instances where the CT scanner is absolutely the only way to go because of its accuracy. A Romer Arm and scanner is capable of holding ±50 microns (0.002"), while our CT Scan- ner is certifi ed to ±2 microns (0.000078").” He noted such accuracy certifi ed for metrology is especially critical to the customer. “Only recently have [some] CT scan- ners been certifi ed by the German VDI/VDE group,” he said, referring to the German standards organiza- tion that serves as certifying organization for many metrology processes.
A variety of medical implants being checked on the Renishaw Equator.
Starrett’s series of Horizontal Digital Video Comparators, or HDV series, are targeted for the shop fl oor, to be operated next to the production tool. The most recent release in the series is the HDV500, released in October 2015. It boasts an X/Y accuracy of 3.0 + 1 L (33 mm) μm for objects that are up to 250-mm tall. Control is either full CNC or motorized manual joystick. Stage travel is 500 mm in X and 200 mm in Y.
Choosing the Right Device It would seem that variety in instruments is the spice of
measurement. But, there are good reasons for this, espe- cially in medical device manufacturing. “Which device and accuracy to choose is application specifi c,” explained Dean Solberg, co-founder of Exact Metrology (Cincinnati, OH). He lists the devices his contract measurement and scanning services business uses or delivers as including metrology grade CT scanning, structured light devices, and Romer Arms equipped with laser scanners.
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CT scanning is ideal for many classes of medi- cal parts, those with small and intricate details often hidden away inside the part. This includes 3D printing, a parts manufacturing technique that seems ideal for making many medical parts, according to Solberg. “It means you can measure that part without having to cut it open,” he said. Exact Metrology has recently partnered with ProCon X-Ray GmbH, a German developer of X-ray inspection equipment for industrial applications, to sell the ProCon line of CT metrology devices in the United States. “I think the biggest impact with medical metrology right now is the ongoing development of the CT scanning,” he said. CT scanners like the ProCon allow users to swap out two different power ratings, such as 160 or 225 KV, each with its own advantage. And the smaller desktop models are ideal for smaller medical parts, such as stents or implants, that need critical inside dimensions measured. Recently, Exact Metrology has even entered the direct patient care business in a project to deliver systems that pre- cisely guide radiation treatments robotically. Another applica- tion of CT scanning somewhat directly in patient care is in root canal procedures, according to Dr. James Gutmann, DDS and professor emeritus of Texas A&M University (Dallas, TX). He recently described research that exploited the technology of the Nikon Metrology XT H 225 X-Ray CT system for clinical research. MicroCT refers to techniques for especially sharp images. “The new technology will impact research greatly, along with the alteration of clinical techniques. MicroCT
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