Editor’s note: This article is the first in a two-part series on three-dimensional printing techniques, including the design and implantation of custom implants. The next installment will appear in the February issue of AAOS Now.
Three-dimensional (3D) printing is an exciting technology that has been adopted by almost every major industry over the past two decades. This rapidly advancing field has enabled fabrication of previously impossible geometries, including almost limitless 3D structures (Fig. 1) that can be created from an expanding variety of materials, including metals, plastics, and even living cells (Fig. 2). The benefits of 3D printing include extreme flexibility to customize shapes, increased intricacy/complexity of manufactured products, elimination of assembly steps, reduction in material waste, and the promise of “just in time” manufacturing capabilities.
3D printing has given us the ability to create anatomic models; patient-specific instrumentation; and implants ranging from complex, noncustom, “off-the-shelf” devices to truly custom implants. All of these aspects of 3D printing are important to the patients we treat, and orthopaedic surgery is uniquely situated within medicine in its ability to improve patients’ lives with custom implants. In fact, the FDA recently reported that of all 3D-printed implants cleared for commercial use, most are for orthopaedic surgery, including several subspecialties, such as arthroplasty, spinal fusion, lower extremity, and upper extremity. This article describes the basics of 3D printing of custom implants.
Generally, 3D printing (or additive manufacturing) refers to the process of creating a 3D object layer by layer. This is a broad definition and encompasses many different 3D printing technologies that range in material selection, part size, accuracy, and costs. This process is different than that of traditional or subtractive manufacturing techniques where orthopaedic implants are machined from a block of metal or cast into molds. Because 3D-printed implants are built layer by layer, much more complex shapes can be created because each layer can have a drastically different shape than the one prior and the one after. This creates underhangs, overhangs, interconnected pores, spaces within the implant, and many more features that are not possible with machining or molding. For example, many of the custom implants being used to fill critical-sized bone defects from cancer or trauma require osseointegration from the surrounding bone. This requires bone ongrowth and, more importantly, bone ingrowth through the implant. As opposed to traditional manufacturing, 3D printing techniques allow for fully interconnected pores or channels for bone ingrowth (Fig. 3).
Today, custom metallic implants are 3D printed with powder bed fusion technology. In this technique, a thin layer of metal powder is deposited on the build platform of the printer, and then a source of thermal energy, which is either a laser or electron beam, selectively fuses the appropriate region as indicated by the original design. After the energy sources fuse the powder to the prior layer, the building platform is lowered by a predetermined layer thickness, another layer of powder is deposited, and the appropriate regions of the powder are fused once again. This process continues until each layer has been fused properly, resulting in the desired implant shape.
After printing, an implant is far from ready, as it must undergo additional post-processing steps to be removed from the build platform and prepared for surgery (Fig. 4). During powder bed fusion, the first layer of metal powder is fused to the building platform. The custom implant must be removed via a process called electric discharge machining. After removal, the implant may undergo high-temperature annealing or stress-relieving treatments to optimize strength and fatigue resistance. Surface treatment of 3D-printed implants is commonly used to alter the surface roughness or chemistry. Physical treatments including microblasting or mixed-media mechanical polishing processes are often employed. For example, a custom implant that is to articulate with cartilage or a polyethylene component must be polished until smooth (Fig. 5, available in the online version). Some custom implants may require machining to achieve features that are not achieved by 3D printed. Milling processes may be used for threaded features, through holes or interlocking component interfaces. Finally, the implant must be inspected by the manufacturer’s quality-control division to ensure the parts have been manufactured to engineering specifications. The implant is now ready to be shipped.
The most common indications for custom implants are for fusion to surrounding bone, articulation with adjacent cartilage, or both. The most commonly used metals for 3D-printed custom implants are titanium and cobalt-chromium alloys. It is important to consider metal properties and select the right material based on the function of the implant to be designed. Porous titanium implants can be designed to have a Young’s modulus similar to cortical bone, have excellent biocompatibility for osseointegration, and therefore are typically chosen for custom implants designed for fusion. However, a limitation of titanium is low wear resistance, which may lead to implant failure and makes it a poor material choice as a bearing surface. Some coating technologies have been proposed for improving the wear resistance of titanium, including nitride coatings. Cobalt-chromium alloys possess high strength and have excellent wear properties, making them a logical choice for custom implants requiring articulation.
Orthopaedic surgeons can use 3D printing technology to improve upon surgical technique and preoperatively plan for difficult surgeries. However, the true power of 3D printing in our specialty is the ability to treat patients using 3D-printed implants that have superior anatomic fit. Today, we have the capabilities to treat complex pathologies involving deformity, segmental bone loss, and joint articulations. As this technology continues to improve and becomes more widely available, its clinical applications will also continue to expand. However, like all new technologies, 3D printing cannot be considered a panacea and should be approached with caution. Although there are no definitive long-term data on the success of these implants, some early research has shown promise from small case series. Moreover, we must adhere to FDA regulation on truly custom implants.
Implant cost is also a concern. Although the cost is variable across the country, the costs have decreased over the past six years. Moreover, specifically for limb salvage, the upfront cost of implants may be less of a burden to our healthcare system than multiple external prostheses. Surgeons should weigh the benefits of 3D-printed custom implants versus off-the-shelf implants in improving patients’ lives.
The next article in the series will discuss multiple facets of custom implant creation, including steps in the design process, timeline, and FDA guidance and regulations for custom devices.
Samuel Adams, MD, FAAOS, is an orthopaedic surgeon clinician-scientist at Duke University specializing in reconstructive foot and ankle surgery. He is an early adopter of 3D printing technology to improve patient care. He is a member of the AAOS Devices, Biologics, & Technology Committee.
- Ricles LM, Coburn JC, Di Prima M, et al: Regulating 3D-printed medical products. Sci Transl Med 2018;10:eaan6521.