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Software helps achieve scaffolding geometry
To overcome stress and strength issues, the group turned to design software from UK-based Within Technologies Ltd (www.withinlab.com). The software generates scaffolded lattices that can be customized to follow the contours of any desired part shape. Within’s Enhance tool controls not only the design of the internal lattice (resolution, strut thickness and topology), but also the width of the part’s walls or skin in a fluid and continuous fashion. Once an object is defined, in order to predict its real-world behavior, the Within model is integrated with FEA in a feedback loop that allows rapid iteration between optimizing the design and seeing how it will perform under stress.

Screenshot showing analysis of generic complex component by Within's software to ensure light weight and 'buildability.' Note the internal lattice structure and yellow lines at right emanating from circular portion of structure and representing the pressure exerted on the part.

The complex CAD geometries that are produced by this evaluation process are excellent candidates for laser-sintering, according to Within’s Mahdavi. "We’ve been working with additive manufacturing in metal spine, hip and other implant geometries for a few years now," he says. "This Custom IMD cranial implant research was our first PEEK design, but I was already fully convinced of the value of laser-sintering for producing patient-specific implants with optimized characteristics designed right in."

Employing the new software, the group came up with scaffold geometry that satisfied everyone’s requirements for the cranial implant (mechanical properties were targeted to meet all required ISO values for PEEK). A rim was added around the edge of the part to give it a solid border for optimum fit to existing skull bone. "With this version we saw no stress peaks in the structure at all," says Lenz. "FEA simulations, and then mechanical testing to confirm that our boundary conditions and other assumptions were correct, showed that we had come up with a strong, functional implant design."

Manufacturing technology speeds part turnaround time
The implant prototype could withstand greater than 100 megapascals (MPa) of pressure with minimal deflection, and any stress at impact dissipated quickly without being transferring to the skull. Further input from one surgeon led the group to add additional holes inside the rim border for even greater potential for osseointegration. “This kind of last-minute design change was easy to modify and virtually optimize with the software and could be promptly manufactured with our machine, then reverified with physical testing,” notes Lenz.

With their idealized design validated for strength, the research team still wanted to prove out their concept with the kinds of additional ISO-standards testing that future FDA applications would require (FDA approves materials in finished devices, not materials alone, so every new device configuration requires complete recertification). They subsequently performed:

• Polymer infiltration trials (by TNO) demonstrating even distribution of the polymer. These also revealed the benefits of the roughened lattice surface that laser-sintering could produce to enhance adherence by the polymer.
• Sterilization (electron beam, by LasMed, Poland, www.LasaMed.net validation with both high-temperature gel permeation chromatography (GPC) and infrared spectroscopy (ATR IR). These showed that the molecular weight of the PEEK samples was the same before and after electron-beam treatment, demonstrating no detectable degradation.
• Biocompatibility testing, which proved the implant to be non-cytotoxic, non-haemolytic, non-pyrogenic, non-irritant and causing no sensitization response. Contributors to these latter tests were LasMed, Tecnalia (Spain, www.technalia.com) and Rapra (UK, www.rapra.net).
  

  FEA results (left) were compared with values measured during mechanical testing (right) to prove out the strength and measure displacement of different implant designs.

  Technology benefits patients and physicians
  The socioeconomic benefits of future laser-sintered PEEK implants could be considerable, predicts EOS’s Lenz.

  "With precise customization and better fit, such implants would require less time in surgery and provide greater comfort to the patient. There are labor and material savings as well as potential improved health gain."