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Quality Digest

FDA Compliance

Laser Scanning Helps Validate Design of New Venous Filter

Building on reliability

Published: Tuesday, March 21, 2006 - 23:00

Avoiding blood trauma is a concern when pumping cardiac patients’ blood during surgery. With this in mind, a medical-device manufacturer decided to reuse the flow-path geometry of an existing arterial filter that had been proven safe and effective in many patients. The problem was ensuring that the as-built arterial filter geometry truly matched the complex computer-aided design (CAD) model that the company planned to use as the starting point for the new design.Re-using geometry for a new filter
Veins carry blood from all organs of the body back to the heart. A number of conditions can occur as a result of localized inflammation and clotting of veins, particularly in the legs. Deep vein thrombosis is the term often used for a blood clot that develops in the deep veins of the legs. There’s some risk that a part of the clot will break loose and travel in the bloodstream to lodge in the lung, which is a condition known as a pulmonary embolus. The mainstay of treatment is anticoagulant therapy using prescription drugs, but in some circumstances a venous filter is inserted to prevent blood clots from reaching the lung.

The geometry of these venous filters is based on the design of an arterial filter that has been on the market for some time. A critical concern in the design of flow paths in any bypass-circuit device is that they minimize shear stress to avoid the disintegration of red blood cells (hemolysis) and blood clotting. The arterial filter was designed using computational fluid dynamics (CFD) to predict the blood flow through it, and some design adjustments were made to reduce shear stress. The resulting product has been on the market for a number of years and has proven safe and effective.

Wanting to take advantage of this experience, medical-device engineers duplicated much of the flow-path geometry from this earlier arterial filter design in the new venous filters. This could be done easily by scanning the existing flow-path geometry and extracting initial graphics exchange-specification curves from the triangulated point cloud in the form of a standard template library file for use in creating the new filter geometry.

Laser scanning helped validate the design of a new venous filter by ensuring that the CAD geometry used as a starting point matched a molded part that had been proven safe in the market.

Validating the CAD model
The engineers had complete confidence in the as-built product because it worked perfectly well in the market, but the flow-path geometry was too complex to fully characterize on a coordinate measuring machine (CMM) while still matching the CAD model.

When they thought about inspecting the part with a CMM, the engineers realized that the points required to fully validate the 3-D surfaces were far too numerous to capture using one-point-at-a-time contact methods. They also considered cutting the arterial filter into cross-sections and measuring them in two dimensions, but realized that they could never cut the parts to the required level of accuracy.

Laser scanners project a line of laser light onto surfaces while cameras continuously triangulate the changing distance and profile of the laser as it sweeps along, enabling the object to be accurately replicated. They collect thousands of points every second at a high level of accuracy, so they’re able to accurately digitize complicated parts. There’s no more need to maintain contact with the workpiece, and so the results are independent of the skill of the operator.

Because a small number of parts needed scanning every year, engineers thought it didn’t make sense to purchase a laser-scanning machine just yet. The machine could become obsolete before it had paid for itself, and the challenge of training operators and maintenance staff could be overwhelming. To generate millions of points of data from the as-built product, align the point cloud to a CAD model, and generate a color deviation between the surface model and the CAD geometry report, the medical-device manufacturer contracted with a laser-scanning service bureau that could provide the high accuracy and fast turnaround it needed on nearly every project.

Laser scanning provides a fast, economical solution
The engineers e-mailed the CAD geometry and shipped the physical part of the model to the laser-scanning service provider, which in turn scanned it, generating a point cloud with the millions of points needed to accurately define the complex surface geometry of the part. The laser-scanning company used software that generated a graphical comparison of the manufactured part vs. the CAD model, while color-coding differences between the two, which made comparing the parts easy and fast. The medical-device engineers were able to instantly visualize the differences between the CAD model and the as-built part.

The laser-scanning provider redid the process until the map showed minimal differences between the two parts. The remaining issues were outside features that had nothing to do with blood flow. Based on these scans, engineers at the medical-device company modified their CAD model to match it more accurately to the as-built part. The entire process of scanning the part and developing the difference map took little more than a day. The cost of the scanning and comparing was minimal, and the turnaround was much faster than normally expected for conventional CMM touch-probe–based measuring, which yields uncertain results.

When you use existing design geometry, make sure that the geometry you’re re-using actually matches that of the successful product, particularly when dealing with medical devices. Laser scanning provides a fast, accurate and economical method of validating even the most complex designs. In the case of the new venous filter, engineers quickly determined that the differences between the CAD model and the as-built product had no significance to the function of the product, but the engineers played it safe by updating the CAD geometry to match the product perfectly. The new venous filter has since gone into production and has proven just as safe and effective as its predecessors.


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