May_AMP_Digital

Additive Manufacturing

Produces Polymeric Cranial Implants

ADVANCED MATERIALS & PROCESSES •

MAY 2014

ith regard to modern medical applica-

tions, additive manufacturing is making

headway—literally—when it comes to

building cranial implants. One company finding suc-

cess in this area is Oxford Performance Materials

(OPM), SouthWindsor, Conn., who received the first

FDA 510(k) clearance for its polymer laser-sintered

OsteoFab Patient-Specific Cranial Device (OPSCD).

The customizable implant restores voids in the skull

caused by trauma or disease. Manufactured in a mat-

ter of hours with additive manufacturing (AM) tech-

nology from EOS, Munich, Germany, the device was

successfully implanted in a patient missing a signifi-

cant portion of cranial bone a few days later. “It was

very large, measuring nearly six inches across,” says

OPM president Scott DeFelice. “It fit perfectly.”

OsteoFab technology, OPM’s brand for additively

manufactured medical and implant parts produced

from 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 thermal

qualities that make it highly suitable for cranial re-

construction. For example, PEKK has a density and

stiffness similar to bone, is lighter than traditional

implant materials such as titanium and stainless steel,

and is chemically inert and radiolucent so it will not

interfere with diagnostic imaging equipment. Most

importantly, PEKK is also osteoconductive, meaning

bone cells will grow onto it.

In some implants, the surrounding bone pulls

away over time. And with most implants, fasteners

are very important because if bone does not grow

into the implant, they hold everything in place. Be-

cause PEKK has osteoconductive properties, long-

term implant stability may be easier to achieve.

Laser sintering

Because low-volume parts with complex shapes

were needed, AM was the logical choice. PEKK was

already being sold as a raw material in pellet, film,

and extruded bar stock for orthopedic and spine ap-

plications, but occasionally OPM had requests for

a one-off product with a complex organic shape. It

was already being molded and machined, but be-

cause these processes have substantial limitations

in terms of tolerance and geometry, other options

were explored.

Laser sintering lifts manufacturability restrictions

that traditional processes impose—for instance, draft

angles in molding and corner design for CNC tool-

ing. It also does not require upfront costs such as

tooling and molding, so it is well suited for creating

one-off, patient-specific parts (Figs. 1 and 2). Laser

sintering does not generate the level of waste that

subtractive cutting and milling do.

PEKK, unlike its cousin PEEK, has a high melt-

ing point relative to other polymers, so all current

commercial AM processes were immediately re-

moved from consideration until OPM discovered

EOS, makers of the high-temperature EOSINT P 800

laser-sintering system (Fig. 3). “EOS is a clear leader

for high-temperature industrial 3D printers,” says De-

Felice. “We found their technology to be the only

laser-sintering system in the world that can run high-

temperature materials such as PEKK.”

Path to FDA approval

The path to commercialization of patient-spe-

cific implants was arduous. Aside from the not-

so-trivial prerequisite of the right molecule and

the right process, DeFelice says climbing the

mountain of regulatory requirements was a daunt-

ing task. “For starters, you need an ISO 13485

compliant facility that has design controls and an

appropriate clean manufacturing environment.

You also need to be compliant with CFR 21 cGMP

(current Good Manufacturing Practices). Add to

that 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 cousin

PEEK—high mechanical strength, heat resistance, and biocompatibil-

ity, to name a few—it is no ordinary thermoplastic. The difference is

structural: PEEK is a homopolymer, made up of identical monomer

units. By contrast, PEKK is polymorphous, which means it has lots of

molecular “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 many

different applications and a variety of customized materials from the

same base molecule.

OPM is a participating member of America Makes, the National

Additive Manufacturing Innovation Institute (NAMII), a White House

initiative to advance additive manufacturing as a critical process in the

U.S. “In the context of NAMII, we’re establishing what are called B-

Basis allowables, which are structural design parameters for aerospace

use, 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.”