Measurement Systems in Medical Manufacturing
There's little margin for error in the production of medical devices
By Brian J. Hogan
In the US and in other industrial countries with aging populations, it has become commonplace to observe that the manufacture of medical devices and systems is a growth industry. But the production of medical devices poses special problems for manufacturing engineers, because of the materials employed and the unforgiving tolerances demanded by medical designs.
As the saying goes, "everyone knows" that today's measurement systems are critical to success in medical manufacturing. But what are they actually being used for? We asked companies working in the fields of medical manufacturing and precision measurement to tell us what they see happening in their areas. Here's what they said.
Dynamic scanning technology can help manufacturing engineers solve certain high-volume metrology problems. Last year, Uwe Kemmer, production manager at CeramTec (Plochingen, Germany)--an international OEM supplier of high-performance ceramics--experienced a bottleneck caused by an increasing number of orders. The company produced such a large number of parts that it experienced capacity problems with existing CMMs.
CeramTec already owned five Prismo CMMs from Carl Zeiss (Minneapolis, MN) that were dedicated to providing 100% inspection for their high-tech products. "We don't perform spot checks, we inspect each part," says Kemmer. A decision had to be made. The easiest solution would have been to increase machine capacity and purchase another CMM, but the space in the measuring lab was not designed for an additional machine.
Carl Zeiss recommended upgrading three of the PRISMO CMMs with VAST Navigator technology. This dynamic-scanning technology permits faster data acquisition without compromising machine accuracy. It offers two major benefits: time savings and faster scanning speeds. For a typical task such as the inspection of a cylinder bore, the user can save as much as 30% of measuring time. The decision to upgrade instead of buying another CMM saved CeramTec time as well as money.
A little history: Until 1995, Ceramtec measured all ball heads and sockets for prosthetic hip joints using custom-made gages. Back then, because artificial hip joints were a standardized product, this technology was acceptable. It's very cumbersome, however, to measure an inner ball with a custom-made gage. In addition, the gage can be used only for this specific task. When several versions must be produced, there comes a time when costs outweigh benefits.
To deal with this situation, CeramTec decided to purchase its first CMM. From that moment on, the company was able to measure any type of inner or outer forms. With this flexible measuring technology, 100% inspection of artificial hip joint components was no longer a problem.
On today's prosthesis market, there are numerous versions of hip joints that CeramTec manufactures for its customers. "For custom products you need flexible measuring technology," Kemmer explains. This observation specifically applies to cap inserts. In the past, the part either "fit" or "didn't fit." Today, by using CMMs, Kemmer can define a preset measuring uncertainty. In addition, inspection of very small cones finally makes sense. Also, with VAST Navigator Kemmer was able to reduce measuring time on the Prismo by 23% without sacrificing accuracy.
Three of the five CMMs in use at CeramTec can now be loaded automatically. The company uses pallets that are loaded with parts, which are then automatically placed on the first available CMM. For Kemmer, the advantage is obvious: "The operator loads three or four pallets and the loading device takes care of the rest."
Today, CeramTec performs up to 3000 measuring runs per day in a multishift operation. Form measurement accuracy is at a maximum of 5 µm. "This is the smallest tolerance that we need here," says Kemmer. Probing accuracy of the machine is at 0.8 µm. The high accuracy required for the form measurements is not a matter of aesthetics. It extends the lifetime of the artificial joint. Substantial form deviations would lead, in the long run, to higher wear and tear. From the patient's point of view, it's this fact which confirms that the metrological investment paid off.
Growth in demand for medical devices creates special problems for manufacturers. Quality must be maintained, that's a given, but measurement and inspection can't be allowed to become a bottleneck.
It's estimated that more than 700,000 people in the US will undergo either hip or knee replacements this year. And as baby boomers age, industry experts say that number could skyrocket.
The surge in demand for orthopedic implants--while a boon to manufacturers of these devices--is accompanied by the challenge of accelerating product development and increasing production volume while maintaining a critically high level of accuracy.
Exactech (Gainesville, FL), a manufacturer of orthopedic implant devices, sought to boost production by streamlining its inspection methods when it realized that the capacity of current equipment was over-extended. Its search for new systems led the company to CMM manufacturer LK Metrology Systems (Brighton, MI). Exactech sent a complex implant along with a CAD model to LK, and asked the company to demonstrate its capability to inspect the part.
Pete Forest, senior quality engineer at Exactech, found that LK's PT Scan CMM had the stability, stiffness, and smoothness of motion to offer both scanning and traditional touch probe measuring technology. Plus, LK's Camio Studio software was compatible with his existing Unigraphics CAD system.
Today, an LK CMM is up and running at Exactech, where the company uses LK's Camio Studio software for both in-process inspection and new-product development. The 3-D DMIS-based Camio Studio generates inspection routines from CAD files, while its reverse-engineering tools are used to capture 3-D models of new parts. Parts are measured by the LK CMM and a 3-D CAD math data is created. After manufacturing, the new part is scanned and a series of points taken, then compared to the solid model to determine how well the actual part meets the design. "The reverse engineering process is fast and accurate--which is just what we were looking for," explains Forest.
When manufacturing products such as Exactech's femoral components, the ability to hold tight tolerances is crucial. One such part is a replacement for the top portion of the knee that's attached to the end of the thighbone that articulates on the bottom portion of the knee. The geometry here is very complex as there are two radiused surfaces--each having compound radii--that must fit right. Verifying that the parts meet stringent requirements is done with CAD models and surface scanning. This helps Exactech ensure a perfect match for the patient.
According to Forest, parts with short arc lengths pose a particular challenge in the measurement process, which is why the ability to compare point clouds to a solid model is useful. When a 3-D model is created using the LK CMM, it's done with a data structure that has thousands of points. Using scanning technology, Exactech can go back and create that same data structure off a part, take a series of those points, and compare theoretical to actual points. Scanning is significantly faster than manual inspection. In fact, Exactech personnel have seen a 30-min manual inspection reduced to 10 min. High-speed analog scanning allows data collection rates of up to 1000 points per second at scanning speeds over 6.5 ips (165 mm/sec).
Offline programming is also helping in the streamlining efforts. Previously, users had to teach the CMM an inspection routine online using an actual part. With Camio Studio, new inspection routines are generated from CAD-file data in an off-line mode to optimize machine utilization and improve throughput. Forest estimates a decrease in inspection programming time of 50%.
Intravenous (IV) catheters and hypodermic syringes (both referred to as sharps in the medical game) are designed with points to puncture a patient's skin. It's not unusual, however, for nurses and doctors to accidentally puncture themselves (suffer a needlestick) when handling or removing these devices. In fact, one out of every seven US healthcare workers is accidentally stuck by a contaminated sharp every year. It has been estimated that over 600,000 needlesticks occur annually, leading to 1000 infections and more than 100 deaths.
Given today's heightened concerns about blood-borne disease transmission, manufacturers of sharps are designing protective mechanisms to prevent inadvertent punctures of their handlers. Since 1984, manufacturers have filed more than 1000 patents, the FDA has approved 150 safer medical devices, and more than 100 such products are currently available.
One method of preventing sharps from inflicting inadvertent needlesticks causes the point to retract into the plastic body of the device when it's removed from the patient. Another causes metal fingers to surround the sharp's point, effectively shielding it from the user. Successful implementation of these ideas requires robust mechanical designs.
Keep in mind that sharps are typically in the 0.5-mm-diam range. With design tolerances typically in the ±0.0005" (0.013-mm) range, or better, video-measuring machines often are used to verify these critical dimensions.
Video measurement offers a number of advantages:
- Video measurement is noncontact. Because it is images that are measured, there is no chance for deformation of the part that might be caused by a contacting probe (this is especially important for pliable and thin-walled plastic components).
- Different areas of small parts can be measured with appropriate magnifications and illumination. For example, overall outer dimensions can be measured at low magnifications with backlight illumination for highlighting edges. Smaller metal components can then be measured at higher magnifications with appropriate surface illumination for these typically cylindrical parts. Zoom lenses provide a wide range of magnifications, which can be programmed to occur automatically.
- Shrinkage of plastic parts can be quantified by measuring parts while still warm from the mold and again as they cool.
- Transparent, translucent, and opaque parts of any color can be measured without any surface preparation requirements.
- Video measurement systems are accurate and repeatable. A typical SmartScope Flash system made by Optical Gaging Products (OGP; Rochester, NY) provides gage repeatability and reproducibility (GR&R) of <10% at typical tolerances for this type of part.
- And most important to manufacturing engineers, an automated video-measurement system performs identical measurements for every part, allowing lots of tens or hundreds of parts to be measured at a time.
While sharps sell for a few dollars each, manufacturing efficiency is necessary to minimize total costs. Measurements must be done to ensure quality but not add costs to the products. This is where video-measurement technology provides an advantage. Once a measurement routine for a single part is created, it's repeated for a series of parts. Depending on stage travel of the measurement machine, fixtured sets of 50 parts or more can be measured at one time with no user interaction.
Because video measurement is based on magnified images of the part, there is an additional benefit when measuring plastic parts--flaws in the injection mold flow are clearly visible.
The advantages of video-measurement systems make them appropriate for many other manufactured medical products. Prostheses (such as replacement hips and knees) are measured for critical dimensions including profiles of their highly polished surfaces. Even if these parts are stainless steel, stylus probes can mar their polished surfaces. Video measurement systems with lasers (multisensor video measurement systems) can profile these surfaces without contact as part of a complete measurement routine. In the system metrology software, point clouds of laser-scan data are combined with video data to more thoroughly analyze each part. More measurements are performed, and the part is handled less than when using separate, stand-alone measurement systems.
Medical device components have critical quality requirements in terms of function, tolerance, and appearance. The huge variety of sizes and tones of color and texture schemes characteristic of these products makes accurate viewing, inspection, and gaging problematic.
Lavezzi Precision Inc. (Glendale Heights, IL) specializes in the manufacture of all sizes of medical devices and assemblies, ranging from subminiature occluding balls to cardiovascular blood-pump housings. A variety of equipment is used in the manufacturing processes, and each machine has been chosen for its ability to maintain tight tolerances. The company uses the Hawk noncontact measurement system from Vision Engineering Inc. (New Milford, CT), on its shop floor as well as for final inspection.
"The Hawk not only allows in-process checks to be automated, such as measuring diameters, hole location, and providing geometric-tolerance information," says Lavezzi's David Gartner, "but also has the ability to gather data that enables us to analyze the manufacturing process statistically."
The system provides a true optical image display combined with fully automatic video-edge detection. According to Vision Engineering, both of these technologies have been available individually, but not as a combined package.
Hawk can be run manually, motorized, or in fully automatic mode. For rapid product changeover, manual operation allows quick setup in any measurement task. For high-volume throughput, automation allows repeated, objective inspection much faster than manual measurement. Between these extremes, there are a whole host of application areas that benefit from partial automation--where components cannot be automatically checked, or where product variety makes multiple measurement routines impractical. Hawk permits both techniques to be used in one system, which opens up many opportunities where previously either technology alone would be insufficient.
At Specialized Medical Devices Inc. (SMD; Lancaster, PA), a new project from a long-standing customer spurred an upgrade of inspection capabilities.
The new job included stainless steel parts with partial ID and OD spheres with size and relational tolerances of ±0.001" (0.025 mm). Volumes would reach several hundred parts per day, and a review of existing hand tool, optical comparator, and dial indicator inspection methods revealed that SMD had no practical means for ensuring the dimensional quality of these features.
Rather than lose business or risk jeopardizing a good account, QC manager Marc Liberatore undertook an evaluation of CMMs. He selected the Phoenix DCC CMM from Helmel Engineering Products Inc. (Niagara Falls, NY). An automatic system equipped with a Renishaw touch-trigger probe, the CMM had the required accuracy in the measuring envelope needed and accurate 3-D measurement of internal features via light contact with the ruby stylus.
A focused, four-day off-site training session at Helmel's facility for two SMD associates helped the company get up to speed with DCC CMM technology. Technical support from Helmel and part holding fixtures produced in-house allowed the CMM to be up and running in a couple weeks. The machine is in a centrally located open room on SMD's air-conditioned production floor without any special environmental controls.
The company has trained a number of inspection personnel and machine operators to use the CMM, and it has become a fundamental inspection tool. Each machine operator typically checks a part in process about every two hours to ensure conformance. "Machine operators have been accustomed to inspecting their own work," says Quality Technician Craig Hartman, who participated in the implementation of the CMM. "Any initial skepticism quickly passed, and operators have developed faith in the CMM by seeing the results."
The CMM has allowed SMD to manufacture more difficult parts with challenging geometric forms, and to produce more consistent results. Since acquiring the machine, SMD has added more five-axis machining centers and 10-axis screw machines to its manufacturing facility. The complex parts produced by the sophisticated multiaxis equipment can only be inspected using a CMM, and SMD recently purchased a second Phoenix machine to give operators greater access.
According to VP Jack Fulton, the CMMs also help SMD keep up with documentation required by customers in the medical device industry. "They tend to require FDA and ISO certifications, and these come with higher inspection criteria," he explains. "Customers are more sensitive to first-article and inspection documentation, packaging, and so on. They are more demanding in that they need to know more about our processes."
Fulton says that the medical market also typically features accelerated production schedules with rapid development changes. "You never reach maturity in a product, because it is quickly replaced by a newer design, and new programs make demands on inspection. By definition a flexible 3-D measuring device, the CMM adapts readily to almost any dimensional task thrown at it," he says.
The CMM has also proven to be a time saver, and has made SMD's inspection process more efficient. Parts that formerly would have been inspected using a surface plate with indicators and height gages can now be quickly measured using a CMM that will also provide more detailed information.
This article was first published in the May 2005 edition of Manufacturing Engineering magazine.