Digital workflow for 3D-printed complete dentures

Digital dental workflows are no longer a promise of the future, but the reality we live in today. The entire work protocol from diagnosis to treatment has digital elements, even though there are still gaps when it comes to the complete digitalisation of the dental office.

Do you consider yourself a digitised professional because you have an intra-oral scanner? That is only a small but very signicant part of the framework, especially when it comes to a full digital workflow for a complete or partial denture.

Phases of the CAD/CAM process in digital dentistry and 3D dental prostheses:

Methodology

The methodology used for CAD/CAM involves optical scanning of the jaw without the need for alginate and employing those scans in design software to design and 3D-print dental appliances with maximum accuracy. By using the digital STL le from the design process, we can realise the denture or bridge we see on screen. To do this, we need a high-speed scanner (Fig. 1) for complete marginal denition of soft and hard tissue. We use the Medit i700. The same efficiency is also provided by the Medit i500. We also need an adequate bite registration that allows us to determine the vertical dimension and correct intermaxillary relation for the positioning of the prosthesis, whether complete or partial, based on the centric relation and the peripheral tissue (mucosa and lips). We need an automatic means of sending all the scan data to the associated laboratory or a design program to interpret the scanned data and design the prosthesis ourselves. Finally, once the prosthesis has been printed, we must follow a protocol that guarantees a material with the required characteristics of resistance, polishing and nal nishing for better adaptation and durability in the patient’s mouth. Naturally, the digital method also requires time and training that in the medium to long term will allow us optimal results.

Steps for a complete scan of an edentulous patient

1) We start with a complete scan of the upper jaw from the retromolar area to the palate and palatal rugae. It is important to have some particular anatomy that differentiates one side from the other so that the software does not confuse the scanned sides and cross-reference the information. The tissue of the vestibular part must be retracted and separated for better t of the future printed prosthesis. Although it is complex, it is essential, and in fact there are techniques that result in a better-quality scan to achieve this peripheral sealing.

2) We then continue with the scanning of the lower jaw, from the posterior region, as well as the upper jaw, passing along the plateau or gingiva-covered alveolar ridge to the opposite side. It is important to be able to obtain a good extension of the lower jaw in the same way as in the upper jaw so that the denture can be fully extended for a better fit.

3) Next, we obtain a bite registration to establish the relationship between the upper and lower jaws, whether dentate or edentulous.

Starting the 3D-printing process
At this point, after taking scans of both arches, we send the STL le to and transform it into a printable model using the Model Builder software (Medit), which provides us with a suitable structure for classic models with a base and articulators. Afterwards, it can be sent to the SprintRay printer in two different ways, depending on the user’s preferences and experience:

1) Sending to RayWare Cloud: Printing of the lower and upper jaw models is a user-friendly process thanks to an articial intelligence (AI) program which arranges the designs in the best way possible automatically on the build platform for fast and optimal printing. In this case, the scan les must be saved on the PC and uploaded to the SprintRay Cloud program.

2) Using RayWare Desktop: From the same Medit app, the scan les are uploaded to the printing program directly. In this case, the user must arrange the designs on the build platform for good retention and print quality. This program provides the possibility of adding supports and deciding on the print position, based on the user’s training.

Obtaining the jaw models
With both methods, we will have the scanned jaws ready for the fabrication of the bite registration to record the intermaxillary relations. Having models of the jaws enables creation of a base plate for a classic occlusal rim (Fig. 2) that provides:
– the bite registration;
– the occlusal dimension;
– the relationship between the jaws; and
– the future size of the teeth and prosthesis in relation to the patient’s lips (facial prole).

Once we have both jaw models printed and the base plate with adequate wax registration, we mark reference points on the wax that we are going to scan with a marker or coloured composite (Fig. 3) and scan both registrations in the bite sequence.

Once we have obtained all this data according to the scanning techniques of the sequence protocol guided by the Medit program, we will be able to send it to the laboratory for final design.

Viewing the prosthetic design and understanding the STL files
Our laboratory designs the prosthesis digitally, and we receive the prosthetic design for our inspection and control. The design can be viewed on a mobile device (Fig. 4) or on our computer to check and make any necessary changes.

Once we have received these images and approved the design, we receive an STL le that can be sent to the SprintRay printing system to print via RayWare Cloud or via RayWare Desktop.

Once we receive the final STL file, we download it to our computer. The le contains the design of the dental pieces that can be grouped in a single block (Fig. 5) or separated by segments. These pieces will be the ones that we will cement to the printed denture base. The le also contains the design of the denture base with detailed anatomy, as traditionally done in wax denture design systems. In this case, the details will be digitised, imitating the gingiva and roots. Later, we will be able to provide more detail with stains and colours that mimic the gingiva in a more natural way (Fig. 6).

Printing the prosthesis
The STL le that we have on our computer is sent to RayWare Cloud for adaptation and placement, through an AI process, on the virtual platform that appears on the program’s screen (Fig. 7). We must classify the parameters (Fig. 8) for printing and follow the intuitive steps that the program prompts, such as:
1) the name of the patient or the print job;
2) the type of material that will be used for the printing of the base or the teeth;
3) the quality of the surface nish, smoother for structures with a more detailed nish and less detailed for a faster print;
4) the type of support that will x the structure to the platform; and
5) other details that we can customise or leave as the program default.

When the printing process is started, the time that it will take is shown, as is the amount of resin to be used and other data that will later be saved in the RayWare Cloud file.

For the manufacturing of the dental pieces, SprintRay offers material that has similar wear and tear properties to those of a natural tooth. The material is called SprintRay Crown (Fig. 9). For the base of the prosthesis, there are different colour options that provide a similar chroma depending on the gingival colour we choose.

After printing
When the printing is nished, the platform will be lifted out of the resin tank and the piece will be nished. We will only have to remove the supports that held the prothesis to the print platform (Fig. 10). If RayWare Cloud was used, the supports will be a mesh or net that can be removed easily by hand, greatly improving the process of nishing the prosthesis, unlike other systems that require support removal with pliers and cutters because they are very thick and leave a residue on the prosthetic structures, requiring polishing. That is why we use RayWare Cloud, as it provides us with supports that are very easy to remove without leaving any residue.

Post-processing: Washing and drying
The liquid resin that remains on the printed pieces must be washed off thoroughly. For this, SprintRay has an automatic washing and drying system that ensures that the printed piece is free of resin residue, ready for polymerisation and nishing. SprintRay Pro Wash/Dry (Fig. 11) is the rst multistage automated wash and dry system designed for dental 3D printing, offering the speed, cleanliness and ease of use that the digital workflow demands. Thanks to its high-powered motor, high-precision propeller and innovative alcohol delivery method, this system can complete a full two-stage wash and dry cycle in less than 9 minutes, meaning that the dentist or assistant does not need to be manually involved in the process.

Post-processing: Polymerising in ProCure 2
The pieces must be polymerised with an ultraviolet light that closes micropores and gives the prosthesis the texture and uniformity necessary to obtain a surface of adequate rigidity and/or exibility according to the characteristics of the material used. That is where ProCure 2 (Fig. 12) ts into the digital workflow, offering an easy and automated program requiring less than 5 minutes with preset proles for each approved material. With ProCure 2, no manual manipulation of light intensity or duration of the polymerising process or heat is necessary.

Assembly and nishing of the prosthesis
Now we assemble both the base of the prosthesis and the teeth in order to use the same resin that we have in the tray to cement the two together. The constructed piece is then placed in ProCure 2 for final fixation. Subsequently, we polish the nal product using classic polishing pastes and various polishing brushes, nishing with the fabric brush that gives it a special nal shine (Figs. 13 & 14). The work can be nished by glazing it with a GC glaze and polymerising it again in ProCure 2 or with a common composite polymerising lamp that gives a brighter nish. It should be noted that there are different stains or owable composites that can be used to further customise the prosthetic finish.

Conclusion
Although the digital workow of 3D-printing complete prostheses may seem complex and lengthy at first, process automation with the latest technology means that dental clinics that embark on this new and wonderful adventure have only benets to look forward to in the long run.

Dr Fernando Gérman, Spain