An Update on 3D-printed Orthodontic Aligners

1. Professor, Department of Orthodontics and Dentofacial Orthopaedics, Saveetha Dental College and Hospital/Saveetha Institute of Medical and Technical Sciences, Chennai, Tamil Nadu, India. 2. Professor, Department of Orthodontics and Dentofacial Orthopaedics, Saveetha Dental College and Hospital/Saveetha Institute of Medical and Technical Sciences, Chennai, Tamil Nadu, India.

Abstract

Clear Aligner Treatment (CAT) is an orthodontic technique used to align teeth with removable and scarcely visible appliances. Conventionally, these are produced through the process of thermoforming. The inherent disadvantages of thermoforming include increased surface roughness leading to plaque accumulation, compromised biomechanics due to a reduction in force delivered and flexure of the aligner, and altered mechanical properties, such as increased opacity, water sorption, and hardness. Direct Three-dimensional (3D)-printed aligners, or Direct Printed Aligners (DPA), introduce a new frontier to aligner technology and are a recent addition to the ever-improving field of orthodontics. Through 3D printing, the various disadvantages of thermoformed aligners like surface roughness, extent and definition of aligner borders, undercuts, and differential thickness of the aligner can be controlled to enhance the accuracy of aligner fit with lesser reliance on attachments. 3D printing of aligners is more environmentally friendly since there is no subtractive process for thermoforming or post-processing of the TA. Various methods of 3D printing, such as selective laser melting, selective laser sintering, Stereolithography (SLA), and Digital Light Processing (DLP), can be applied to the printing of clear aligners. Challenges in printing primarily involve maintaining transparency and designing support during the printing process. The present review aimed to include a detailed description of all aspects of direct 3D-printed aligners.

Clear aligner appliances, Orthodontic appliances, Printing, Removable, Three-dimensional

The Clear Aligner Treatment (CAT) is an orthodontic technique that aims to align teeth through the use of removable and barely noticeable appliances (1). Although conceptualised by Harold D. Kesling in 1945, the pivotal moment in the history of aligners occurred in 1998 with the introduction of Computer-aided Design (CAD)/Computer-aided Manufacturing (CAM) technology by Zia Chishti and Kelsey Wirth (2). Consumer awareness, and consequently the demand for aligner treatment, has surged in the last decade, particularly among adult patients, those with aesthetic concerns, and individuals with periodontal compromise (3). The increased comfort of removable appliances during activities such as eating, brushing, and flossing provides patients with a more pleasant experience, potentially contributing to a higher preference for aligners over fixed appliances (4).

Drawbacks of Thermoformed Aligners (TA)

Thermoforming of the aligner sheet reduces both the delivered force and the flexure of the appliance (5). Additionally, aligners permit the deposition of plaque on their surfaces, which is comparable to fixed appliances, partly due to surface roughness formed during the thermoforming process (6). Ryu JH et al., reported increased opacity, water sorption, and hardness after thermoforming the tested material (7). The irregularities in thickness also affect the fitting accuracy of the aligner (8). This process might help bypass thermoforming errors and potentially exceed its quality (9). Aligner setups typically include 0.25 mm of movement in each set. Studies indicate a discrepancy of 0.3 mm in some regions between the clear aligner and the model after thermoforming (10),(11). This discrepancy could imply that the planned tooth movement may not accurately translate to the treatment outcome.

Direct 3D-printed Aligners

Direct 3D printing of the aligner refers to an aligner that has been printed without the intermediate thermoforming process, thus negating the requirement of a physical model for aligner fabrication. Direct 3D printing offers the potential for improved precision, shorter supply chains and lead time, and lower costs (12),(13). Direct printing potentially might enable control of differential thickness and increase the versatility of aligner biomechanics and application (14). Direct 3D printing of aligners has an edge over conventional methods since it allows digital design of the appliance borders, smooth edges, and digitally defined undercuts leading to a better fit. Since errors associated with making a cast and thermoforming process would be negated, direct printing would result in higher precision of fabricated aligners. The thickness of the aligner at varying regions of the aligner can also be customised, reducing the need for attachments (15). DPA produces substantially fewer carbon emissions and less waste since there is no subtractive process of 3D printing a model for the thermoforming process nor post-processing of the TA (16).

3D Printing Technologies

Additive printing or 3D printing was first invented by Wilfried Vancraen in 1990 (17). It has revolutionised many industries, from prosthodontics, restorative dentistry, and implantology to instrument manufacturing (18). Among the various types of additive manufacturing or 3D printing, Vat photopolymerisation is most suited to 3D aligner printing.

During the process of photopolymerisation, a light-curable resin, i.e., a photopolymer, is stored in a Vat and treated with visible or Ultraviolet (UV) light from different types of sources depending on the type of Vat polymerisation, which initiates polymerisation to form a solid resin. Operating on this principle, multiple layers of resin are sequentially fabricated from a sliced Standard Tessellation Language (STL) file (19).

Vat photopolymerisation is of three types: SLA, DLP, and continuous DLP/continuous liquid interface production. The Liquid Crystal Display technique (LCD) is a subtype of DLP. The challenge of 3D printing an aligner lies within its design—an intricate shell structure—with the added demand for transparency. For instance, producing small patent features in clear materials using 3D printing might be difficult and may necessitate the use of biocompatible photoquenchers (20). However, as seen in studies by Zinelis S and Panayi N and Venezia P et al., accuracy and the mechanical properties of a DPA rely not only on the type of printer but also on differences between different companies (21),(22). The salient features of the different types of 3D printing technologies for aligners are mentioned in (Table/Fig 1) (23),(24),(25),(26),(27),(28).

Resins used for Manufacture of Direct Print Aligners

A direct print aligner material must be compatible with 3D printing, aesthetic, durable, stable, biocompatible, cost-effective, and possess appropriate mechanical properties (14). The resins currently used to print DPA have been described below in (Table/Fig 2) (27),(28),(29),(30),(31),(32),(33),(34),(35),(36).

Designing Software

For designing aligners for direct printing, software options have been on the rise in recent years. Available software includes OnyxCeph™ (Image Instruments, Chemnitz, Germany), Maestro 3D (Ortho Studio v.5.2, AGE Solutions S.r.l., Pontedera, Italy), Deltaface (Coruo, Limoges, France), Lux Align by LuxCreo (USA), Blue Sky Plan by Blue Sky Bio (USA), and uLab Systems, Inc. (California, USA). Deltaface software permits location-specific differential thickening of the aligner to either facilitate or restrict tooth movement (37),(38). Workflow for Fabrication of Direct Print Aligners (Table/Fig 3).

Designing the Aligner

The aligner is designed virtually using drawing tools on the pre-treatment model. The software automates the subsequent sets of aligners depending on the desired tooth movements. While the aligner thickness generally ranges from 0.25 mm to 1.2 mm, the thickness of the aligner can be customised locally to favour or restrain tooth movement (37),(39). The trim line and border of the aligner may be customised to a high trim line or a low trim line depending on the amount of force required during the stage of treatment (39). Another important factor to note is the management of undercuts in the aligner design. Black triangles or generalised spacing need to be blocked out, or aligner material in these spaces may act as a wedge and unintentionally open up spaces. Blocking out undercuts may also lead to loss of retention, and care needs to be exercised during this process (37). Once designed, the aligners are exported to the printing system in Standard Tessellation Language (STL) format. Each printer has its software for printing with different tools and ways of support positioning. Supports should be designed and positioned where needed for accurate 3D printing.

Designing of Supports

The supports can be designed so that the aligner sets are printed vertically or horizontally. Horizontal positioning allows for faster printing; however, fewer sets can be printed in a cycle because the aligners would occupy more space and require more support. When positioned vertically, the aligner would require fewer supports and allow for more sets to be printed in each cycle, but printing would take more time and have a higher risk of errors due to an increase in the number of layers. The z-axis resolution for printing used is 100 μm, which ensures adequate printing accuracy (40).

Preparing the Resin and Printing

To reduce the risk of failure, the resin must be homogeneous and stirred while maintaining its temperature around 30oC (40).

Removal of Excess Resin

All resins before printing and UV curing are toxic and allergenic. Once printing is finished, the aligner is removed from the printer’s platform and placed in a centrifugation machine with its internal parts facing outward to remove the excess uncured resin. Centrifugation should take approximately 5-6 minutes at 500-600 rpm. Manual resin removal can be done after centrifugation. Failure to eliminate resin from the aligner might lead to excessive curing of the resin and an ill-fit of the aligner due to the increased internal thickness of the aligner (40).

Curing

The supports can be retained after curing to prevent distortion of the shape of the aligner. The next step is to remove the supports and cure the aligner. In the Graphy system, direct print aligners are cured in a UV curing unit called Tera Harz (Graphy, Korea, Seoul). This curing unit is designated for printed aligners with high-intensity LEDs and is equipped with a nitrogen generator to ensure curing in the absence of oxygen, as oxygen inhibits complete polymerisation which could affect the mechanical properties of the aligner. Complete polymerisation enhances the transparency of the aligner while also producing a fully biocompatible aligner (40). Although the same wavelength (405 nm) was used by all printers, other important parameters that determine the extent and depth of cure remain unknown (41).

Polishing

Following curing, the aligner is polished using rotating handpiece brushes, and a thin layer of resin may be applied to achieve a smoother surface, followed by 2-3 minutes of additional curing. Polishing is primarily done at the junctions of the supports and the aligner. Finally, the aligner is submerged in hot water for a few seconds to remove the remaining resin or other particles (40),(42).

Properties of Direct Printed Aligners (DPA)

Aligners in clinical use are subjected to forces that are both short-term and long-term in nature. The properties of different DPAs as reported in the literature are described in (Table/Fig 4) (29),(34),(35),(43),(44),(45).

Cytotoxicity

The 3D-printed materials are initially very toxic, and after polymerisation, the toxicity gradually reduces. Therefore, post-curing and processing, as advised by the manufacturers of the resins, are essential for reducing the levels of toxicity (46). DPA materials exhibited higher levels of cytotoxicity within the first 24 hours, which then slowly and progressively decreased. These results suggest that further investigation is required to evaluate the therapeutic efficacy of DPA and determine their qualities in an intra-oral environment (46). Dental LT® resin and Accura 60 SLA have not yet received clearance for use in DPA. However, based on the E-screen assay, neither Dental LT nor Accura 60 demonstrated any oestrogenic effects. The study found Dental LT clear resin to be less cytotoxic than Accura 60 SLA (47). According to Rogers HB et al., exposure to Dental LT® caused a severe phenotype that led to rapid gamete degeneration before meiosis resumed and may have a negative effect on reproductive health. The polycarbonate-based material Accura 60® demonstrated the highest level of cytotoxicity on day 1, and variations in intragroup cell viability for Accura 60® were statistically significant. This is due to the increased BPA leaching associated with polycarbonate. Animal studies and in vivo studies are required to confirm the effect of DLT on reproductive health (48).

Conclusion

Direct Printed Aligners (DPAs) are the future in the field of orthodontics. With the right setup and a digital workflow in place, a DPA can quickly replace its conventional counterpart. The mechanical properties of DPAs are, to a large extent, dependent on the 3D printer used, and thus, differences in their clinical efficacy are anticipated. Forces delivered by DPAs in the vertical dimension are more consistent and of lower magnitude. However, in order to safely apply the use of 3D aligners to everyday clinical practice, to widen the scope of its application, and to draw decisive conclusions on the effectiveness of direct-printed aligners, further studies, possibly Randomised Control Trails (RCTs), should be conducted.

DOI and Others

DOI: 10.7860/JCDR/2024/67084.19279

Date of Submission: Sep 03, 2023
Date of Peer Review: Oct 25, 2023
Date of Acceptance: Feb 02, 2024
Date of Publishing: Apr 01, 2024

AUTHOR DECLARATION:
• Financial or Other Competing Interests: None
• Was informed consent obtained from the subjects involved in the study? NA
• For any images presented appropriate consent has been obtained from the subjects. NA

PLAGIARISM CHECKING METHODS:
• Plagiarism X-checker: Sep 05, 023
• Manual Googling: Oct 29, 2023
• iThenticate Software: Jan 31, 2024 (15%)

ETYMOLOGY: Author Origin

EMENDATIONS: 8