Additive ManufacturingProduces Polymeric Cranial Implants

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W ith regard to modern medical applica-tions, additive manufacturing is makingheadway—literally—when it comes to

building cranial implants. One company finding suc-cess in this area is Oxford Performance Materials(OPM), South Windsor, Conn., who received the firstFDA 510(k) clearance for its polymer laser-sinteredOsteoFab Patient-Specific Cranial Device (OPSCD).The customizable implant restores voids in the skullcaused by trauma or disease. Manufactured in a mat-ter of hours with additive manufacturing (AM) tech-nology from EOS, Munich, Germany, the device wassuccessfully implanted in a patient missing a signifi-cant portion of cranial bone a few days later. “It wasvery large, measuring nearly six inches across,” saysOPM president Scott DeFelice. “It fit perfectly.” OsteoFab technology, OPM’s brand for additively

manufactured medical and implant parts producedfrom PEKK material contributed to the success.PEKK (Poly-Ether-Ketone-Ketone) is a high-per-formance thermoplastic with many exceptional prop-erties (see sidebar). It has mechanical and thermalqualities that make it highly suitable for cranial re-construction. For example, PEKK has a density andstiffness similar to bone, is lighter than traditionalimplant materials such as titanium and stainless steel,

and is chemically inert and radiolucent so it will notinterfere with diagnostic imaging equipment. Mostimportantly, PEKK is also osteoconductive, meaningbone cells will grow onto it.In some implants, the surrounding bone pulls

away over time. And with most implants, fastenersare very important because if bone does not growinto the implant, they hold everything in place. Be-cause PEKK has osteoconductive properties, long-term implant stability may be easier to achieve.

Laser sinteringBecause low-volume parts with complex shapes

were needed, AM was the logical choice. PEKK wasalready being sold as a raw material in pellet, film,and extruded bar stock for orthopedic and spine ap-plications, but occasionally OPM had requests fora one-off product with a complex organic shape. Itwas already being molded and machined, but be-cause these processes have substantial limitationsin terms of tolerance and geometry, other optionswere explored. Laser sintering lifts manufacturability restrictions

that traditional processes impose—for instance, draftangles in molding and corner design for CNC tool-ing. It also does not require upfront costs such astooling and molding, so it is well suited for creatingone-off, patient-specific parts (Figs. 1 and 2). Lasersintering does not generate the level of waste thatsubtractive cutting and milling do. PEKK, unlike its cousin PEEK, has a high melt-

ing point relative to other polymers, so all currentcommercial AM processes were immediately re-moved from consideration until OPM discoveredEOS, makers of the high-temperature EOSINT P 800laser-sintering system (Fig. 3). “EOS is a clear leaderfor high-temperature industrial 3D printers,” says De-Felice. “We found their technology to be the onlylaser-sintering system in the world that can run high-temperature materials such as PEKK.”

Path to FDA approvalThe path to commercialization of patient-spe-

cific implants was arduous. Aside from the not-so-trivial prerequisite of the right molecule andthe right process, DeFelice says climbing themountain of regulatory requirements was a daunt-ing task. “For starters, you need an ISO 13485compliant facility that has design controls and anappropriate clean manufacturing environment.You also need to be compliant with CFR 21 cGMP(current Good Manufacturing Practices). Add tothat a completely validated process and ISO 10993

TECHNICAL SPOTLIGHT

A Peek at PEKK: Thermoplastic Shape Shifting

While PEKK shares some of the qualities of its better-known cousinPEEK—high mechanical strength, heat resistance, and biocompatibil-ity, to name a few—it is no ordinary thermoplastic. The difference isstructural: PEEK is a homopolymer, made up of identical monomerunits. By contrast, PEKK is polymorphous, which means it has lots ofmolecular “knobs.” By adjusting the polymer’s manufacturing process,or incorporating different additives, melting points, crystallization lev-els, and mechanical properties, PEKK is capable of supporting manydifferent applications and a variety of customized materials from thesame base molecule.OPM is a participating member of America Makes, the National

Additive Manufacturing Innovation Institute (NAMII), a White Houseinitiative to advance additive manufacturing as a critical process in theU.S. “In the context of NAMII, we’re establishing what are called B-Basis allowables, which are structural design parameters for aerospaceuse, for PEKK,” says DeFelice. “We are very active in that market.” In the broader market, in addition to its recently FDA-cleared Os-

teoFab cranial implant, OPM is providing OXFAB materials and criti-cal parts for other demanding sectors such as the nuclear, aerospace,chemical processing, and semiconductor industries. As DeFelice says,“After fourteen years of mixing, blending, and modifying this molecule,we’re just getting started.”

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biocompatibility data on your finished parts.” Once all that “stuff” is out of the way, the lifecy-

cle of a patient-specific cranial implant starts withthe patient. If someone comes to the hospital with askull injury, a CT or MRI scan is performed and pro-duces a slice file similar to data used to build partsvia laser sintering.That slice file is then sent to OPM where 3D de-

sign software is used to create an implant that pre-cisely fits the patient’s anatomy. The implant is thenprinted or “grown.” This “growing” phase is entirelyautomatic—the laser-sintering system lays down athin layer of powder on its build platform. Guided bythe lowest slice of the implant design file, a high-tem-perature laser melts a cross section of the implant.When that layer is done, the build platform lowers,and a new powder layer is distributed on top of theold one, and the laser melts the next cross section.The process repeats until the entire implant is built.Laser sintering is capable of producing practicallyany shape geometry to match the precise needs of in-dividual patients.When the implant is removed from the leftover

powder (Fig. 4), it must be inspected for quality. Inaddition to mechanical and analytical testing, a struc-tured light scanner is used to do 100% line-of-sightmetrology inspection to certify its dimensional accu-racy. Finally, the implant is shipped to the hospital.“The total process from receiving the data to ship-ping the implant takes less than two weeks,” accord-ing to DeFelice. (Figs. 5 and 6).

More implant types possibleHaving successfully created and obtained clear-

ance for their cranial product, OPM is making plansto move throughout the body to use the technology.OPM will only operate in highly regulated, high-riskmarkets such as medical. “We are after critical parts,and biomedical is the pinnacle of that,” DeFelice says.“That means you need the right material, the rightprocess, the right quality system, and the rightmetrology. When the patient is on the operating tableand the part shows up and doesn’t fit, you are puttingsomeone’s life at risk. With the first implant case,

where the implant was very large, extensive areas ofcritical tissues were exposed during surgery. Everysecond is critical in that situation.”

For more information: Scott DeFelice is president andCEO of Oxford Performance Materials (OPM), 30 SouthSatellite Rd., South Windsor, CT 06074, 860/698-9300,www.oxfordpm.com.

Fig. 1 — A batch of implants are set up forproduction. The EOSINT P 800 system canrun multiple, different designs in a singlebuild. All images are courtesy of Fred Smith Associates.

Fig. 2 — 3D digital model of a cranial implant.

Fig. 3 — Using patient-specific 3D digitaldata, the cranial implant is additively manufactured with an EOSINT P 800 high-temperature plastic laser-sintering system.

Fig. 4 — The completed partis cleaned of anyresidue powder.

Fig. 5 —Cleaned implants.

Fig. 6 —Skull model demonstrates howan implant is customized to fit thecranial hole.

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