3D Printing In Dental Technology

The more vigorous innovations are focused in the realm of 3D printing and related materials for restorative dentistry applications. With each successful execution of digital technology, the benefits are quickly realized for the laboratory, dentists, and ultimately patients. This article will explore the different additive manufacturing methods, also known as 3D printing technologies, with their solutions as they relate to dentistry.

The more commonly adopted current manufacturing technology is the subtractive or milling method. This digital manufacturing method produces complex geometries by taking a block or puck of material and making strategic cuts under a Computer Numeric Controls (CNC) apparatus (milling machine) to remove all that is not necessary until the final object is attained.² Often this method of manufacturing tends to be less expensive, with more affordable apparatus technology and materials; however, counter indications exist that significantly affect final output. Potential inefficiencies in subtractive manufacturing directly relate to geometry and tool paths obstacles. The more complex the geometry, the more complex the tool path strategy needs to be to attain the appropriate part. Often, with increasingly complex geometries and undercuts, the tools simply cannot gain access to mill properly. This results in one of two scenarios: Either the part cannot be milled, or over-milling will occur. Over-milling happens when the desired product is thinned out or more material is removed than desired because the tools were unable to gain access properly. An example of this occurrence is when the intimate internal detail of a crown is physically smaller than the smallest tool a milling machine deploys. The result is over-milling that detail and therefore compromising the strength, fit, and viability of the product and/or material.

Commonly known as 3D printing, additive manufacturing is a process that starts with an empty platform, tray, or build plate and successively builds a 3-dimensional object. Appropriate CAM software slices the 3D electronic design file, called Standard Tessellation Language (STL), which is an open-formatted CAD file of a complex geometry, into layers of fixed thickness. The printer then processes each layer, usually from the bottom up, creating an extremely accurate final object.^3,4 Currently, in dentistry, five additive technologies are utilized, and they fall into two categories: The first group is Digital Light Projection (DLP), Jet, and Stereo-Lithography Apparatus (SLA); the second is Selective Laser Sintering (SLS) and Selective Laser Melting (SLM). The term additive manufacturing encompasses technologies such as Rapid Prototyping (RP), Direct Digital Manufacturing (DDM), Layered Manufacturing (LM), and 3D Printing.⁵ Both polymer and metallic applications provide solutions for the dental professional — everything from surgical guides and models, to castings, and actual dental restorative components for both temporary and permanent solutions (Figure 1).⁵ Each technology has its own distinctions and each system varies in terms of materials available. How those materials are solidified and how they can be used often differentiates among the technologies.⁶ The use of 3D printing has made it possible to build objects with complex geometries and cavities that otherwise would have been difficult to attain with subtractive manufacturing.⁵

Polymer Resin Additive Manufacturing

The first and most utilized 3D printing technology to emerge into the dental market is the Stereo-Lithography Apparatus (SLA). This method was patented by Charles Hull, co-founder of 3D Systems, in 1986.⁵ The process converts liquid plastic into solid 3D geometric objects, through sequential printing, layer by layer, of liquid plastics (photopolymer resin)3 that are hardened with an ultraviolet light source or laser.⁶ There could be up to 10 layers per millimeter and depending on the indication and precision desired, these layers can be manipulated. Different printers offer a different level of precision, typically within a range of 100 to 35 microns. The platform or build plate holding the 3D objects, or the 3D objects themselves, are lowered after each layer is printed and the resin is cured, and a wiper blade spreads more build material uniformly across the workspace and repeats the process until the object is fully printed. The smaller the layer (precision) increments, the longer the print output will take. The time required to print an object depends on the amount of times the print head has to pass over the object to fulfill the print. SLA typically has a slower build time and requires post-print processing, cutting the final part from the support material, removal of excess material, and placement of the part in a UV oven for final curing.^4,6 In the dental setting, this technology is used in fabricating dental models as well as patterns for fixed and removable dental prostheses (Figure 2).

Digital Light Processing (DLP) was created in 1987 by Larry Hornbeck of Texas Instruments and is very similar to SLA. The two additive technologies are differentiated by the light source. The resin hardens quickly when affected by a large amount of light. The result of DLP is a significantly faster print process with excellent resolution, and significantly less materials need to be used for detailed production.⁵ EnvisionTEC deploys this technology in its dental 3D printers to provide fast, accurate, and proper 3D complex geometries in the dental space (Figure 3). Similar post-processing to that of SLA is required to obtain the final product.

Fused deposition modeling (FDM) technology was developed in the 1980s by Scott Crump, from Stratasys.⁵ This technology affords a new type of high performance and engineering-grade thermoplastics, acrylonitrile butadiene styrene (ABS) or polycarbonate (PC) filaments, which allows the geometric objects to be printed in excellent mechanical, thermal, and chemical quality plastics. This technology utilizes heat, as well as light, to build objects layer by layer from the bottom up, while heating and extruding thermoplastic filament and simultaneously curing.^5,6 The level of precision and surface finish is enhanced, due to the nature of these materials and technology; a typical layer thickness ranges from 16 to 50 microns. Furthermore, some printers provide the ability to print multiple materials as part of the same print job.^7,8 This provides the ability to 3D print models for crowns, bridges, diagnostic wax-ups, and veneer try-ins using proper color and/or texture/surface detail (Figure 4).⁹

Nearly all forms of additive manufacturing require some form of post-processing. This generally means the removal of support materials, unused build materials, and jetted resin. It is accomplished by soaking the finished printed parts in a liquid that dissolves or softens the support materials.³

Metallic Additive Manufacturing

Selective Laser Sintering (SLS) is an additive technology that uses a laser to form a solid 3D object out of powdered materials in a build plate. This technique was developed by Carl Deckard, a student at Texas University, and Professor Joe Beaman in 1980.⁵ Unlike the previously mentioned polymer additive technologies, SLS does not need any support structures; the object is being printed because it is surrounded by unsintered powder. This technology uses a strategic selectively high-power laser, which is controlled through the use of CAM software, to fuse small particles of metal into a mass that has the desired 3-dimensional shape, continuously scanning cross-sections of the surface of the powder bed. After each cross-section pass, strategically sintering the powder, the powder bed is lowered by one-layer thickness until the part is complete.⁶ The materials available to print in a large variety of industries can range from nylon, ceramics, and glass to some metals such as aluminum, steel, and silver.⁵ In dentistry, this additive technology is being used to produce a variety of metal substructures and frames in non-precious, semi-precious, and gold alloys.

Selective Laser Melting (SLM) utilizes a similar manufacturing technology to that of SLS; however, the high-power laser is set to melt the fine metal particles as it passes; the energy of the laser is so intense that the metal powder melts fully and therefore creates a solid, 3-dimensional, printed part — rather than sintered layers (Figure 5). This technology was initiated as part of a German research project by a group from Fraunhofer Institute for Laser Technology ILT in 1995.⁵

These technologies and dental restorative solution offerings will only become more robust. Staying abreast and knowledgeable with new technologies can help ensure the dental professional stays competitive and relevant in the current dental environment. Although these solutions are readily available, most come at a significant price point with hardware and materials. If the volume justifies the costs, and calculations are performed for ROI, laboratories have several venues to attain these technologies and fulfill the manufacturing in house. The entry costs can range from $4,900 for a small 3D system pattern printer, to others that could cost well over $100,000 for those laboratories that differentiate themselves by exhibiting more versatility and robust 3D printers. Furthermore, large corporate providers can fulfill the need for these technologies in the form of outsourcing. Argen Corporation and Bego, among other centers, are well equipped and versed in additive technology, which can take a laboratory’s digital design to additive manufacturing for a fast and considerably inexpensive return. These benefits can be immediately realized by laboratories and dentists alike.

Advances in this arena and space are steadily developing. Companies such as GE have invested heavily in utilizing metal 3D printing for industrial applications, creating prototypes, and even end parts for jet engines.¹⁰ Bio-printing and other medical uses have surfaced with 3D printing technology, allowing doctors, residents, and patients to see in real time what their organs and tumors look like. Australian reconstructive plastic surgeon David Hunter-Smith states, “If you can print them, you can hold them and turn them around in your hand. It’s an exact model for patient education and pre-operative planning; it’s really good.”¹¹ Advances are constantly emerging that would increase the benefits while reducing the entry-level expense to incorporate additive technologies in daily operations. In dentistry and dental technology alike, these technologies are creating opportunities to enhance the level of service and increasing the range of offerings to achieve optimal results.

Daniel Alter, MSc, MDT, CDT, is a consultant and Professor at New York City College of Technology.