Making one-of-a-kind knee implants: Medical Manufacturing

When Dan Doiron took the job of lead CNC programmer at Micron Products Inc., it didn’t take him long to conclude that the company’s sophisticated template-based manufacturing processes could be improved with a more flexible CAM system. However, being a newcomer, he was reluctant to argue against what was already working. Instead, he bided his time.

All images courtesy Micron Products

In the past 5 years, the amount of advanced CNC equipment in Micron Product’s orthopedic machining work cell has doubled while the number of programmer/machinists has remained about the same due to the use of more efficient advanced CAM technology.

His opportunity came a little over a year later when Micron, a Fitchburg, Mass.-based contract manufacturer that specializes in sensors, plastic injection molding and machining medical devices, landed a major new contract. It would require the company to manufacture highly sculptured knee-replacement prostheses made of cobalt chrome, what Doiron calls “the most unforgiving material I’ve ever come across.” Creating CNC programs to make these products with the resident CAM software presented numerous issues, including:

• Devising adequate workholding to hold parts securely without marring surface finish while allowing tools to reach more areas on the part in a single setup.
• Dealing with exceptional levels of tool breakage encountered because of the hardness of materials combined with the long tool reach that is sometimes required makes tools more vulnerable to breakage.
• The ever-present need to maintain quality levels when machining an exceptionally difficult material while still improving cycle times.

With the sales process for the cobalt-chrome knee-replacement prostheses underway, Doiron had prepared a machining strategy for these new parts using Dynamic Machining Technology, a feature in the latest versions of Mastercam CAD/CAM software, developed by CNC Software Inc., Tolland, Conn.

For many months, he had been experimenting with a Mastercam machining strategies using an evaluation copy provided by MACDAC Engineering, Somers, Conn., the local Mastercam reseller. With support from the reseller via GoToMeeting, Doiron was able to present his management with a CAM solution that not only addressed the challenges of machining cobalt chrome, but improved the template-based manufacturing processes used to make all of Micron’s products.

The Replacements

Micron makes one-of-a-kind knee replacements—femorals, tibial trays and ultrahigh-molecular-weight-polyethylene inserts—for major medical device suppliers. Each part is customized to produce an ideal fit for each patient. For example, the company produces more than 80 tray designs and each has an infinite amount of dimensional and geometric variability based on measurements taken from individual patients.

Lead times on the implants are often as short as 7 days. However, when emergency surgery is required, Micron can achieve even shorter turnarounds. One factor enabling fast turnaround is that all of the designs reside in CAM templates that can be transformed into CNC programs.

Programmer/machinist Dan Doiron uses Mastercam’s backplot simulation on every implant to ascertain that CNC toolpaths will not encounter interference.

“With the previous CAM software, it took about 45 minutes to produce a CNC program from the template,” Doiron said. “Now it takes less than a third of the time using Mastercam.” The CAD model is imported from SolidWorks and regenerates the toolpaths based on the template.

Workpieces for the tibial trays and femorals are usually castings. However, if one is unavailable, Mastercam can set new boundaries so the knee part can be machined from a monolithic workpiece.

Getting a Grip