3D printing has many applications in medicine and surgery, ranging from patient specific tools to biofidelic, patient-specific models, to facilitate complex surgical planning. Of course, 3D printing is also expected to revolutionize medical device manufacturing by enabling more geometrically complex and patient-specific implants.1-5 In orthopedics, these patient-specific implants have, thus far, seen broadest applications in complex devices needed for revision surgeries or oncology cases. In addition to patient-specific implants, 3D printing has also been successfully commercialized for mass manufacturing of complex titanium alloy implants with porous bone ingrowth surface for the hip, knee, and spine. 3D printing of implants is here, and it is here to stay. 

So what, exactly, is 3D printing? Unlike conventional subtractive manufacturing, in which material is machined from a stock cast or forged shape to arrive at a final part, in 3D printing, also known as additive manufacturing 

(AM), components are fabricated by depositing successive, often microscopic layers, adding layer upon layer to 

a device from the bottom up. Today there are many additive manufacturing methods under development, but the most widely used techniques reported in the literature for biomedical applications are either based on sintering microscopic powder particles together, which are collectively referred to as powder-based fusion, for example using laser energy (referred to as Selective Laser Sintering, SLS);6-9 or based on extrusion of filaments, known as Fused Filament Fabrication (FFF).10 In the literature, FFF is synonymous with Fused Deposition Modeling (FDM). Because FDM is trademarked by a single manufacturer, FFF was introduced as a noncommercial alternative nomenclature for the same AM technology. 

 Although AM has been used in nonmedical industries for over a decade, it is still a new and growing technology for implant applications, especially so for implants fabricated from PEEK. This article reviews the current state-of-the-art for 3D printing of PEEK implants. The article considers the motivation and key drivers for 3D printing, some noteworthy papers describing AM of PEEK implants, and concludes with some considerations regarding the regulatory aspects of 3D printing. 

Why 3D Print PEEK?

AM is not a panacea, and from the perspective of a device manufacturer, certainly not all products warrant commercialization using a 3D printing approach. Indeed, a key aspect of professional certification in AM technologies is the economics of production relative to traditional subtractive manufacturing techniques. From the manufacturer’s perspective, with production of implants having standard sizes, important considerations include start-up manufacturing costs, raw material cost, the cost of any secondary finishing processes, as well as the time necessary to complete all the required steps for producing a finished part. These factors all contribute to the cost 

of a final implant component. Consider, for example, an orthopedic or spinal device with a porous surface for bone ingrowth. At a conceptual level, the device itself needs to be manufactured, and the porous coating needs to be produced and applied to the outside of the device. There are time delays in all the steps to make the device, as well as time delays before the device can be coated. The longer the process takes, the further in advance a manufacturer needs to plan their production or create a backlog of inventory to satisfy short-term demand. 

AM of PEEK implants has the potential to address many of these limitations. Medical grade PEEK is expensive, and machining results in expensive waste when turning down a rod or extruded plate to create an implant. Injection molding produces less waste, but has a high start-up expense to produce the necessary molds. Finally, not all implants are suitable for machining or injection molding, and specific requirements in the molding equipment are necessary to process a high temperature polymer such as PEEK. For example, porous implants with complex parts are challenging to produce with conventional techniques. If, however, it were possible to simultaneously 3D-print a device and its porous coating at the same time, even with post-processing, the economics may favor 3D printing over a staged traditional production process. After a decade of investment in metal AM, these obstacles have already been overcome, as evidenced by selected orthopedic and spine devices already clinically available by major manufacturers and third-party service providers, such as Materialise.