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Fit to Print (Part 2) by Dr. Rooz Khosravi

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Categories: Orthodontics;
Fit to Print (Part 2) 

Examining the options for 3D printers, thermoforming resins, postprocessing protocols and branding for aligners created in-office

Part 2 of 2

by Dr. Rooz Khosravi

In Part 1 of this article, which ran in November’s issue, I defined an in-office aligner (IOA) system and discussed why to implement it at your practice. I also reviewed components of an IOA system. This article dives deeper into various 3D-printing technology, the postprinting processes, thermoplastic foils and branding your IOA system.

3D printers
Advances in desktop 3D printing, introduction of various digital software packages to move teeth, and availability of various thermoforming plastic films all contributed to adoption of in-house protocols to fabricate aligners. This trend aligns with 3D-printing most dental appliances in-office as part of opting for digital dentistry.

Most desktop 3D printers use one of seven printing technologies: vat photopolymerization (also known as stereolithography, or SLA); material extrusion; material jetting; binder jetting; powder-bed fusion; directed- energy deposition; or sheet lamination. In dentistry, the most commonly used is vat photopolymerization, followed by a metal-selective laser-sintering process.

This article focuses on vat photopolymerization 3D printers, which use ultraviolet-spectrum light to cure photosensitive liquid resin. The vat photopolymerization process is divided into three subgroups: laser beam or classic SLA; direct light processing (DLP); and LCD-masked SLA (Fig. 1).

Independent of the printing technology, all 3D-printed objects at this time are fabricated in an incremental process of layer-by-layer deposition. Attempts on volumetric 3D printing are promising.1

Fit to Print (Part 2)
Fig. 1: Schematic representation of three common vat photopolymerization 3D-printing technologies. (Courtesy of Mary Farahzadi)

Laser beam (classic SLA): The stereolithography 3D-printing concept—also earlier referred to as rapid prototyping—was introduced by Charles Hull in the 1980s. SLA printers follow the same structure: A beam of laser is guided through two mirrors and projected on the membrane, the transparent base of a tank. The polymerization process occurs close to the beam of light. Basically, each layer of 3D-printed parts is fabricated through an incremental brush-like process.

Form 3 SLA printers by Formlabs are now the dominant SLA 3D printers in dentistry. Form 3 models are equipped with low-force stereolithography (LFS), a redesigned light-processing unit (LPU), and software improvements over previous versions. These collective changes put Form 3 printers in good position to partially fulfill the 3D-printing needs of an IOA system. The main drawback of laser-beam SLA printers such as Form 3s is their relatively slow printing speed compared with alternatives on the market, such as DLP and LCD printers. Printing speed might not be a critical factor if the practice production volume (number of parts printed per week) is low. The extremely user-friendly dental 3D-printing solution offered through the Form 3 desktop printer drew dental providers to this 3D printer at the expense of reduced printing speed.

Direct light processing: Direct light processing—also known as digital light processing—printers share similar parts as laser-beam SLA printers, except the layer-by- layer projection protocol of UV light on the membrane. Specifically, the projected UV-light layer on the tank polymerizes the resin layer by layer according to the data encoded by a slicer program. Next, the build plate moves vertically upward, pauses, and repositions downward close to the membrane, determined by the print layer thickness (50–200 microns).

The print time of DLP printers is independent of the number of parts on a build plate per print run, because all the data in a layer project at once. Instead, the print time is dictated by the height of the object and the per-layer build plate oscillation time. DLP printers share common parts like the DMD unit because of a limited supply of these parts; nonetheless, DLP printer manufacturers customize other parts of a printer.

The manufacturers of DLP printers use the following factors to reduce the print time or improve the quality of details in parts printed by these printers:
  • Continuous DLP (build plate optimization) printing.
  • Force feedback sensors.
  • Resin tank design.
  • DMD resolution.
  • Dual projectors.
In sum, current attributes associated with DLP printers favorably position these printers for an IOA. It has been speculated that volumetric 3D printing might change this dynamic in the future.

LCD masked SLA: Masked-LCD or LCD printers use a liquid crystal display to block the UV light projected on a resin tank. LCD printers have been majorly improved in the past five years. The initial improvements for LCD printers were in the homogeneity and intensity of the UV light. The next round of improvements were on the resolution and structure of the display: 4K, 5K and 8K LCDs have been slowly integrated into 3D printers with the promise to improve the details of 3D-printed parts.

It is important to note that higher-resolution displays bring a new slew of challenges in these printers, such as reduction of light intensity passing through a high-resolution display. Monochrome displays were another introduced solution to improve the UV light quality polymerizing the resin. The conventional RGB (red/green/blue) was replaced with a monochrome display made of black and white pixels aiming to achieve a consistent high-quality UV light and ideally to increase the lifespan of the display.

The inexpensive entry point to 3D printing is the appealing factor to use masked-LCD printers for an IOA system. LCD printers are approximately 10 times cheaper than counterpart SLA or DLP printers. Nonetheless, the lack of training and specifically designed dental resins are the biggest hurdles to jump adding LCD printers in a dental clinic.

Additionally, most dental providers are using liquid dental resins optimized for SLA and DLP printers with LCD printers. The challenge of this approach is often poor calibration of printing setup, resulting in inaccuracy in printed parts and potential lack of complete polymerization. Suboptimal polymerization is a critical issue when dental resins will be utilized for more than 24 hours in a patient’s mouth.

Postprinting processes

Dental models printed using vat polymerization protocol require postprinting processes to remove unpolymerized liquid resin from the parts after they’re removed from the printer. Next, the cleaned dental model (or a 3D-printed part) undergoes a UV light polymerization process in a cure box. This step finalizes the polymerization of remaining unpolymerized liquid resin—often the outer layers.

Two approaches are available to wash the liquid resin from 3D-printed models: a DIY magnetic stirrer unit or an automated wash unit. During the wash process, 90%–99% isopropyl alcohol (IPA) is often used.

Factors to consider in building a DIY magnetic stirrer unit are: compatibility of the 3D printer build plate to the IPA container; cleanability of the IPA container; and resistance of the parts to IPA.

To build a magnetic stirrer unit, one would need the stirrer, a container for the IPA, a lid adjusted to hold the build plate (ideally a custom design) and a cross magnet. When possible, use a shallow glass container (Fig. 2), and to minimize alcohol evaporation, cover it overnight.
Fit to Print (Part 2)
Fig. 2: A do-it-yourself magnetic stirrer unit. The lid was 3D-printed with the SprintRay Pro printer and was designed to fit the build plate.

The ideal wash step requires a dirty and a clean station, with two DIY magnetic stirrer units. To clean the printed dental models, do not remove the models from the build plate; submerge the invertedly build plate into the IPA when the magnetic stirrer is on for three to five minutes. Repeat this process at the clean station.

An automated wash unit works on principles similar to a DIY magnetic stirrer. SprintRay, Formlabs and Anycubic are examples of manufacturers that offer a spectrum of automated to semiautomated solutions (Fig. 3).
Fit to Print (Part 2)
Fig. 3:An automated wash and dry unit by SprintRay.

Formlab and Anycubic offer magnetic stirrer units, but the main challenge with both are their nondental-specific design. One can remove the models from a build plate (a messy process) and use the metal basket to clean the models. In this approach, the IPA container is often half or less full with IPA. Alternatively, one can fi ll the container of the wash to reach the models when a build plate with attached models is placed on top of the container. These two approaches are protocols to work around the design.

The SprintRay wash and dry unit was specifically designed to clean and partially dry dental appliances. With two IPA reservoirs and a propeller to splash (microjetting) IPA on the surfaces of dental models, this automated wash unit eliminates the need for two stations (saving lab space), reduces the consumption levels of IPA and simplifies the process.

Avoid discarding dirty IPA in the drain; it should be discarded through professional local or city chemical waste venues. Distilling dirty IPA is appealing to reduce the consumption of IPA, but I encourage avoiding this step because of the potential biological hazard associated with this process.

 Cure box

The postprinting curing process is a critical step to maintain safety while 3D-printing dental appliances, including dental models of aligner fabrication. The quality (magnitude and consistency) of UV light during the postprinting curing step determines the success of polymerizing uncured resins. Use commercial 3D-printing cureboxes that pass the quality control for this step. Most DIY and nail LED cure boxes generate inconsistent light and lack the heating component designed in most dental cure boxes. The next generation of dental cure boxes, such as ProCure 2 by SprintRay (Fig. 4), uses powerful concentrated UV lights to reduce the curing time approximately by 25x.

Fit to Print (Part 2)
Fig. 4: ProCure 2 by SprintRay combines high-power LED technology with motion to significantly reduce the cure time.

Thermoforming plastics

Expansion of the clear aligner market drove the introduction of numerous thermoplastic films to the dental community. Limited studies on some of these films have made comparing them rather challenging.

Collectively, the thermoplastic films can be categorized based on their structure (mono versus multiple layers); material, such as co-polyester (PEG), polycarbonate (PU) or a blend of various materials; and thickness (0.5–1 mm). All of these characteristics affect the handling of the thermoforming during fabrication, patient comfort and, most importantly, the levels of force-inducing tooth movement. Table 1 goes into more detail.
Fit to Print (Part 2)
Table 1: Highlighted thermoplastic foils and their properties. The recommended applications are based on limited internal studies and clinical experience; further rigorous studies on thermoplastic foils are required to confirm those applications.

Packaging

Setting up an IOA system comes with the challenges associated with establishing the components of this system, but an IOA system can also be a differentiating factor in a crowded market. Branding your IOA system is an opportunity to project added value to what your practice offers patients. You can utilize marketing companies that provide comprehensive solutions to brand and deliver your aligners, while companies like uLab Systems have also established services to help brand your services (Fig. 5). Alternatively, you can piecemeal the process using design and marketing freelancers,
Fit to Print (Part 2)
Fig. 5: Customized packaging developed for my practice’s PORTH aligners, designed by uLab Systems.

It is of importance that selling aligner manufactured in-house under a specific brand is subjected to all regulatory policies, including a 510(k) clearance from the FDA.

 What are the steps to establish an in-office aligner system?

Like all systems in a practice, one should start small, gauge the progress and dive deeper accordingly. The most common trend in adaption of an IOA system initiates with setting up a digital lab.

A lean design is more appealing in high-rent metropolitan areas, and a well-designed small digital lab can carry the needs of most orthodontic clinics. It is a myth that one needs multiple rooms with a complex HVAC system to establish a digital lab! Most 3D printers are not sensitive to the dust from polishing aligners, and a good aligner trimming protocol requires minimal polishing. These are examples of inaccurate assumptions preventing providers from adapting integration of an IOA in their practices.

After building a lean digital lab, a well-developed retainer program brings value to your patients and provides a good return of investment on purchasing the equipment. This could be combined with a nightguard program for your patients.

The progression next includes taking on correcting minor relapses with fewer than 10 aligner sets, or orthodontic treatments that require less than six months of care. One can always push the limits of fabricating appliances in-house or build a protocol combining internal and external aligner fabrication. Companies like uLab systems or Align Technology now offer on-demand aligners assisting you with adjusting your needs to render clear aligner therapy. Implementing an IOA system is the dip—a temporary setback that will get better if you keep pushing, as described by Seth Godin.

Reference
1. Science 363 (6431): 1075–79.

Author Bio
Dr. Rooz Khosravi
Dr. Rooz Khosravi is a clinical assistant professor at the University of Washington and speaks on implementation of in-office aligner systems and 3D printing. Khosravi also established the Digital Orthodontics Hub, a study club that offers training courses on digital orthodontics. In addition to private practice and academic life, he is an orthodontist-scientist consultant at uLab Systems, SprintRay and Bay Materials. In these capacities, he assists with accelerating the development of advanced software and materials for digital orthodontics.

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