ARTICLE FOCUS:

• The future of cranial implants
• Custom IMD project
• Materials, software and additive manufacturing

"We envision a future where any surgeon in the world can simply send the MRI scan of their specific cranial implant requirements to a single computer that creates the CAD design, prepares an FEA validation report, then sends the data to an additive manufacturing machine where the part is quickly built and then returned to the doctor. From a technical point of view, everything is there."

That bold assessment belongs to Siavash Mahdavi, managing director of Within (more on the UK-based software company later). Whether Mahdavi’s vision will become reality isn’t the question. Rather, the question is when.

Now that knee and hip replacements have become increasingly routine, the next challenge in artificial human bone is the skull. Whether through birth defect, accident or disease, any breach of the cranium jeopardizes the fragile brain and needs fixing as quickly as possible. Yet, since every injury is unique, replacing lost bone must be done on a case-by-case basis.

Titanium-plate cranial implants, customized with CAD/CAM-processed CT data, have been the most-used solution to date. But the material and the machining methods approved to produce them can be expensive and time-consuming. Furthermore, although biocompatible, titanium is relatively stiff compared to bone and in some forms can be temperature-conductive after implantation. This can lead to long-term issues with fit and comfort in the skull.

Titanium alternative
More recently, the focus has turned to cranial implants made from PEEK (polyetheretherketone) plastic. PEEK’s lighter weight, strength, biocompatibility and other characteristics make it an attractive alternative to titanium. A line of custom-milled PEEK-based skull implants made by a US company received FDA approval this spring. And the first ever laser-sintered PEEK cranial implant prototype has now been created by a team of doctors, design engineers, and materials specialists working together in a European Union-funded research project called Custom IMD (www.CustomIMD.com).

The laser-sintered implant geometry has a unique, bone-growth-promoting lattice structure that can only be created with additive manufacturing. Extensive laboratory evaluation has been performed and animal testing is underway.

The goal of the Custom IMD project—which was partially funded by the European Commission in its 6th Framework Program and involved 22 partners from seven European countries—was to achieve supply chain integration of fully customizable medical implants, with cranial, dental, and spinal applications, in a variety of metals and plastics. The ultimate aim was a 48-hour turnaround and lower medical expenses for every patient needing an implant.

EOSINT P 800 laser-sintering system used to manufacture high-temperature PEEK cranianl implant prototypes for Custom IMD research project.

Additive manufacturing for complex geometry
After deciding on PEEK for the cranial portion of the project, Custom IMD asked laser-sintering machine manufacturer EOS to join them. EOS’s high-temperature EOSINT P 800 system is the first to operate at up to 385°C, which is needed for processing high-performance polymers such as PEEK. The company’s additive manufacturing technology also offered design freedom and rapid production turnaround times that are ideal for exploring innovative solutions to the cranial implant challenge. While EOS can supply technical grade PEEK HP3 (with an even higher melting point than standard PEEK), in this case it was decided to use the established PEEK formula.