3D Printing Technologies and Protocols to Enhance the Dental Workflow

Andrew T. Moshman, DMD

Three-dimensional (3D) printing, also referred to as additive manufacturing or rapid prototyping, is quickly gaining popularity within dentistry.^1,2 The emergence of in-office 3D printing has provided dental professionals with a low-cost option to expedite and streamline access to a variety of dental procedures. Clinical applications of 3D printing in dentistry include occlusal splints, clear aligner orthodontic therapy, study models for diagnosis and treatment planning, temporary crown-and-bridge restorations, dentures, and surgical guides for implant, periodontal, endodontic, and oral surgery procedures.^2,3

A major advantage of 3D printing is that it allows components that have traditionally required a dental laboratory for fabrication to be produced in the office; this can save considerable time and expense. As with all technologies, 3D printing requires a learning curve, as well as a plan for incorporating its use into clinical practice.

Additive manufacturing (ie, 3D printing) and subtractive manufacturing (ie, milling) are considered CAD/CAM technologies. Whereas 3D printing involves adding material layer by layer until completion of the object being fabricated, milling entails the controlled removal of material from a solid block until the desired object is achieved. Milling is used extensively in the fabrication of fixed prosthodontic restorations, because 3D printing materials are still not capable of creating restorations as strong, durable, or esthetic as necessary.^4,5

Types of 3D Printing Technologies

The most common types of 3D printers for use in the dental office include fused deposition modeling (FDM) printers and resin printers.

FDM printing uses a motorized extrusion head to deposit thermoplastic material on top of a print bed. After each horizontal layer has been completed, the printer's extrusion head is raised and begins to deposit the next layer of thermoplastic material on top of the previous one. A commonly used FDM printer is the Prusa i3 MK2 (Prusa Research, prusa3d.com).

Resin printers operate primarily via laser stereolithography (SLA) or digital light processing (DLP) methods. SLA and DLP 3D printing processes use an ultraviolet (UV) light source to photopolymerize a light-activated resin material. The resin is stored in a tank with a translucent base. Below the tank, UV light is emitted through the base to cure a single layer of resin that is in contact with a suspended build plate. After one layer has printed, the build plate is raised to allow photopolymerization of the next layer.

SLA printers achieve light-curing via a UV laser that traces through each layer of the preprogramed design before moving onto the next horizontal layer. DLP printers achieve light-curing in a similar fashion, but instead of using a laser that must trace through each layer, a DLP printer projects a UV light image that can cure the entire layer at once. An example of a popular SLA printer is the Form 3 (Formlabs, formlabs.com), and a commonly used DLP printer is the SprintRay Pro (SprintRay, sprintray.com).

Steps for 3D Printing

Application of 3D printing largely requires the same diagnostic information as does traditional lab work. Dawood et al outlined the following steps for 3D printing in dentistry¹: (1) acquiring the 3D patient model, (2) creating the STL file, (3) preparing the model for printing, (4) 3D printing, and (5) post-processing.

For instance, the fabrication of a nightguard or temporary crown with 3D printing starts with a patient model that will be imported into a CAD software. The model can be obtained through either a desktop scan of a dental stone model after an analog impression with polyvinyl siloxane or polyether, or a digital impression from intraoral optical scanners. Both methods will be saved in STL file format.

STL stands for "stereolithography" or "Standard Triangle Language," and the smallest single unit of an STL is a triangle. An STL file represents the outer surface architecture of a 3-dimensional object. In dentistry, the STL file consists of the outer surfaces of the teeth and the periodontium.

Once a digital model has been obtained, the work that was traditionally done on a laboratory bench with a physical model is now designed using CAD computer software. The 3D object will be digitally designed and saved as a separate STL file. The preparation of the 3D object depends on the desired application (eg, occlusal guards/splints, dentures, temporary crowns, etc).

Although the designed 3D object is saved in STL format, 3D printers require slicer software to convert the STL image to a printer-compatible format. The slicer software "slices" the 3D object into cross-sectional layers that will be printed one at a time. Slicer software also includes custom parameters such as layer thickness, exposure time, and printing orientation.^6,7 The settings can be adjusted depending on the particular type of 3D printing material being used.

Slicing software also renders customizable support structures, including rafts and supports. A raft is a flat horizontal structure of cured material that improves retention to the build plate, prevents warping, and improves overall accuracy.⁸ Supports prevent distortion or deformation of the printed object; they support it while printing and are designed for easy removal once printing has finished.⁸ Once the proper settings are selected, the slicer processes the 3D object and exports it for printing.

The final step is post-processing of the completed 3D print. Post-processing of FDM prints is relatively simple: It involves removing the support structures and polishing any rough areas after the supports are removed. Post-processing for resin printers (SLA and DLP) is more involved. After a successful resin print, uncured resin remains on the object. Additionally, the cured resin has not yet achieved complete polymerization. Removing unreacted resin helps the object achieve optimum mechanical properties,⁹ and this is usually accomplished via an ethyl or isopropyl alcohol wash.^9,10 Following treatment with solvent to remove uncured resin, UV light-cure accomplishes final polymerization.⁹ Support structures are then removed, and the resulting rough areas are polished. Completed 3D prints must be properly sterilized prior to surgical use; however, care must be taken so that the sterilization does not result in deformation of the printed object.^11,12

Use of Cone-Beam Computed Tomography Imaging for 3D Printing

Many applications of 3D printing require only the 3D external surface geometry available from a digital impression. These include nightguards, fixed provisional restorations, dentures, and dental models for clear orthodontic aligners. Other applications of 3D printing require additional diagnostic information, such as that which is obtained through cone-beam computed tomography (CBCT). Examples include surgical applications for implantology, oral surgery, periodontology, and endodontics. CBCTs provide exact 3-dimensional analysis of hard tissue and its proximity to vital anatomic structures. The smallest data unit of a CBCT is called a voxel. CBCTs are saved as a digital imaging and communications in medicine (DICOM) file.

While STL models obtained from a digital impression are already in the native format for 3D printing, DICOM files must first be converted into STL file format. Converting a DICOM file into an STL file introduces some degree of error into the 3D model.¹³ Error occurs while filtering out radiographic artifacts, such as beam hardening and noise, and during the process of segmenting anatomical structures.^14,15 Therefore, the design of a toothborne surgical guide is often more accurate when based on an intraoral optical scan rather than a CBCT. Fortunately, most CAD software will allow users to work with a CBCT and intraoral optical scan simultaneously. For instance, in the case of an implant surgical guide, the CBCT can be used to plan the implant position within the available bone, while the optical scan can be used to design the toothborne surgical guide.

Clinical Example

The following is an example of the production of a 3D-printed toothborne surgical guide for implant fabrication. An intraoral optical scan was taken using an intraoral scanner (TRIOS 3, 3Shape, 3shape.com) (Figure 1), and a CBCT scan was taken (Carestream CS 9600, Carestream Dental, carestreamdental.com). CAD software (Blue Sky Plan, BlueSkyBio, blueskybio.com) was used to analyze the CBCT for available bone in the maxillary left first bicuspid (No. 12) position, and, using the patient model from the intraoral optical impression, to design a surgical guide (Figure 2).

The No. 12 crown was digitally waxed-up against the opposing arch for prosthetically driven implant planning. The surgical guide was saved and exported as an STL file (Figure 3). Slicer software for Anycubic^® (anycubic.com) was used to set the 3D object build orientation, as well as the input custom settings for NextDent SG resin (NextDent, nextdent.com). Support structures were generated in the software, and the object was exported for 3D printing (Figure 4).

A DLP 3D printer (Anycubic Photon DLP 3D Printer, Anycubic) was used to print the surgical guide for the No. 12 implant. After printing, uncured resin was removed with 91% isopropyl alcohol, and final UV light-curing was performed. Support structures were removed and rough areas were polished with a slow-speed handpiece (Figure 5). The surgical guide and metal sleeve used for guiding the implant pilot drill were sterilized prior to intraoral use. The surgical guide was used to place a Straumann^® Bone Level Tapered 3.3 mm x10 mm implant (Straumann, straumann.com) (Figure 6). The implant was restored with a screw-retained zirconia crown (Figure 7 and Figure 8).

Conclusion

As 3D printing increases in popularity, its applications in dentistry can be expected to expand. Beyond the various types of 3D printing technologies, there are also many types of 3D printing materials, each with different properties that can be selected for the appropriate desired application. For instance, many photosensitive resins are available; some are used to print diagnostic models, while others are used to print surgical guides. An eventual evolution of additive manufacturing will encompass the ability to print fixed prosthetic restorations that are strong, esthetic, and suitable for long-term use. New 3D printing technologies and materials, such as bioprinting cells for tissue engineering, will likely lead to new treatment modalities and improved patient care.¹⁶

About the Author

Andrew T. Moshman, DMD
Adjunct Assistant Professor, New York City College of Technology, Brooklyn, New York; Private Practice, Brooklyn, New York

References

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