Shape memory resin for direct printed aligners: A scoping review

Introduction

In the past few years, the increased demands of esthetics and the innovation of aligners have caused an upheaval in the field of orthodontics and helped revolutionize this field. With the advantage of being invisible, their increasing popularity has led researchers to contribute more to aid in the development and further improve upon their shortcomings to ensure a seamless workflow. The history of aligners can be traced back to Kesling 1, who, in 1945, described a tooth positioner for the final artistic positioning of the teeth as well as an effective retaining device. Nahoum 2, in 1964 developed vacuum-formed “dental contour” thermoplastic appliances for dental use. He developed the concept of using successive appliances, with small incremental changes, to obtain major corrections that have formed the basis of the construction of the popular Essix appliance and Invisalign.

Direct-printed models have been in use for a long time in dentistry. Now, with the advancements in 3D printing technology, the focus has shifted to the direct printing of aligners, to reduce costs and improve accuracy. This technology has stemmed from the need to overcome the inherent errors that can occur during the various steps involved in the thermoforming process of aligner fabrication.

Figure 1

Flow diagram of the included studies according to the PRISMA

The whole fabrication process is prone to many errors apart from other disadvantages like wastage of material, excessive costs, and time consumption. In addition, geometric inaccuracies are also induced during the thermoforming process.19, 20, 21

Other advantages include the ability for the orthodontist to have a completely digital workflow, reduce dependency on labs, reduce production costs, and enhance practice efficiency, overall patient experience, and satisfaction with better results: the ultimate aim of every orthodontist. 3D printing the aligner not only reduces time and effort, but also results in fewer geometric inaccuracies The accuracy, fit, and clinical feasibility of direct 3D printed clear aligners have been evaluated in several studies. 22, 23

Tera Harz is a 3D printable biocompatible material produced by the company Graphy, Korea, that was approved by the Korea Food and Drug Administration (KFDA) and the European Commission (EC). The mechanical properties and behavior of this cross-linked polymer are expected to be different compared with conventional thermoplastic materials composed of non-cross-linked polymers used for thermoforming. 24

The unique property of this resin is its ability to display shape memory which enables it to perform better compared to others since lighter forces are applied to the dentition, owing to its flexibility and viscoelastic properties. 25

Materials and Methods

Results

The initial search identified 272 records for PUBMED, 7 for Cochrane Library, 300 for Google Scholar, and 31 for Web of Science.

620 records. After removing duplicates, 220 studies were screened. Based on the title and abstract, 196 full texts were screened. 90 Full-text articles were assessed for eligibility. Finally, 16 studies were identified according to the eligibility criteria to be included in the review.

The characteristics of the studies are presented in Table 1. Of the 17 included studies, 4 articles analyzed the in-vivo properties of 3D printed aligners with SMPs, while the rest were in vitro studies.

Discussion

New direct printed aligner technology has been created to address the issues with traditional thermoformed aligners in response to the growing acceptance and use of 3D printing technology in the manufacturing of models. These days, a variety of polymers are utilized to make aligners. Among them is TC-85 Tera Harz resin, which possesses a special shape memory characteristic.

Several studies have been carried out to assess various features after using an intraoral aligner for a week. After using the in-house 3D-printed aligners for a week, Can et al.3 observed no changes in their mechanical properties. Nevertheless, a single kind of printer was used to create the aligners. This is insufficient to completely describe the entire spectrum of 3D printing techniques, including material jetting, extrusion, digital light processing, and stereolithography. Many other factors, such the type of printer, printing conditions, and methods to ensure print quality control, also need to be further investigated.

Using optical profilometry, Koletsi et al. 9 evaluated the surface roughness following a week of Invisalign use. The observed increase in roughness metrics following the testing of 3D-printed aligners while they were "in-service" suggests that the appliances' material integrity and inbred traits have been exposed to intraoral and real-world clinical settings.

Surface roughness characteristics of "in-house" manufactured aligners saw significant and severe modifications at all levels due to intraoral exposure and function. After one week of intraoral use, surface roughness is inevitably changed, which may facilitate plaque buildup and biofilm adhesion. 26 It is yet unknown how using these appliances during an orthodontic treatment course may affect clinical effectiveness and safety. Furthermore, it has been questioned if the material qualities of printed aligners made with various 3D printers are uniform; this could affect how the materials' surfaces are characterized.

Using confocal laser scanning microscopy, Eslami et al. 18 examined the impact of a week-long intraoral usage on the surface roughness of Invisalign and directly printed aligners (DPAs) in comparison to their unused control counterparts.

According to this research, after one week of intraoral use, DPAs' surface roughness and porosity increased, which may have increased bacterial adherence and biofilm formation in these aligners. One important aspect influencing the development of plaque, the adhesion of biofilms, the loss of transparency, and the discoloration of aligners is the surface roughness of the aligners. 26

The study by Koenig et al. 5 showed that direct-printed aligners were more accurate than thermoformed aligners. The small sample size and the use of scanning spray, which significantly increased inaccuracy in the aligner's intaglio dimensions, were among the study's few shortcomings.

Se Yeon Park 10 used Micro CT to compare the thickness, gap width, and translucency of 3D-printed and thermoformed clear aligners. For 3D-printed CAs, the thickness increased during the fabrication process, and different thickness values were observed for each tooth type and location. However, this was an in vitro study, and only the thickness and gap width of passive-state CAs in a normal occlusion model were evaluated, not in various degrees of malocclusion.

Many studies have been conducted to assess the mechanical properties of these aligners. 4, 8, 15, 16

After comparing the mechanical characteristics of orthodontic aligners made with various commercially available 3D printing devices, Zinelis et al. 4 concluded that the mechanical characteristics of 3D-printed orthodontic aligners vary depending on the 3D printer used, which could lead to variations in their clinical efficacy. While both DLP and LCD cure the entire resin layer at once, these variations in mechanical qualities could be attributed to the various technologies utilized to flash light on the resin layer as a whole.

To assess their mechanical and viscoelastic properties in response to temperature, Se Yeon Lee et al. 8 used dynamic mechanical analysis (DMA) and static mechanical testing. Furthermore, an investigation was conducted into the shape memory feature of the photocurable resin (TC-85).

Lukas Camenisch et al 15 assessed the differences in the fundamental mechanical properties of resin-made three-dimensional (3D) printed orthodontic aligners according to the printing orientation. There were no discernible variations between the aligners or specimens produced in a vertical or horizontal orientation. The clinical implications of these results are that clinicians might consider 3D-printed aligners to be an isotropic material and, thus, a similar mechanical reaction might be anticipated intraorally under multidimensional activation.

Islam Atta et al. 16 compared the direct printed and thermoformed aligners and found that TC-85 demonstrates exceptional shape memory at oral temperature, improving adaptation, reducing force decay, and enabling, together with its higher flexibility, extensive tooth movement per step. Additionally, it maintains microhardness similar to thermoformed sheets, ensuring the durability and effectiveness of dental aligners. However, further studies are necessary to explore additional characteristics using full anatomical aligners and across diverse testing conditions. For a thorough evaluation of the effectiveness and long-term consequences related to clear aligners made with shape memory polymers and 3D printing resin, more experimental research and clinical trials are also necessary.

Mattle et al. 14 examined the mechanical characteristics of 3D-printed aligners made of resin and investigated the impact of different post-curing scenarios. They concluded that neither heat treatment nor post-curing in an N2 environment significantly changed the mechanical properties of 3D-printed resin aligners.

These aligners' cytotoxicity has also been evaluated. If any substances were released during the 14-day aging of 3D-printed aligners in water, Prastnis et al.'s 7 investigation found that they had no effect on the intracellular reactive oxygen species levels of human gingival fibroblasts and were not cytotoxic. Furthermore, an E-screen assay revealed that these potential eluates had no estrogenic effects. However because this was an in vitro study, it could not account for things like salivary enzymatic reactions, chewing pressures, dietary chemical impacts, temperature variations, or the effects of oral microbiota. Instead, it focused only on chemicals produced by the aligners' passive hydrolysis.

Since the continual insertion of new aligners in the oral cavity restores the source of elution in the oral cavity, the frequent (7–10 day) renewal of aligners introduces a variable that cannot be represented in biocompatibility studies.

A second set of research has been carried out to evaluate the force profile and force delivery quantity when utilizing these aligners. To compare the force profile of direct-printed aligners (DPA) with standard thermoformed aligners (TFA), Evan Hertan et al. 6 assessed the forces supplied by DPA in the vertical dimension. They concluded that the force profile provided by DPA was noticeably less than those shown by TFA.

James Grant et al. 11 examined the pressures and moments applied by direct printed aligners (DPAs) with varying lingual and facial aligner surface thicknesses during the lingual movement of the maxillary central incisor in all three spatial planes. Abraham McKay et al. 12 measured the stresses and moments felt by the teeth using thermoformed and 3D-printed transparent aligners. They also looked at the likelihood of a maxillary central incisor extruding in the absence of attachments using buccal and lingual pressure columns. They found that the force levels created during extrusion with transparent aligners with those 3D printed using TC-85 were significantly lower than those thermoformed using ATMOS or Zendura FLX.

However, being in vitro studies, they did not account for the natural space and elastic behavior of periodontal ligament. The study also did not simulate the effects of masticatory occlusal forces or the impact of saliva on the performance of clear aligners.

Conclusion

Advancement in orthodontic materials is influencing clinical practice. The search for efficient polymers and cost-effective techniques to reduce treatment time and patient compliance is making significant progress. It is expected that 3D printing technology will experience widespread use in everyday clinical practice in the near future

Several investigations have been performed to evaluate the various properties of this aligner material. However, the collected evidence remains quantitatively scarce and of limited reliability. The review of the literature reveals that the new Tera Harz resin has optimum properties that can enable it to be a promising alternative for thermoformed aligners, with better performance in terms of force delivery and more accuracy. However, most of the studies that have been conducted so far are in vitro and do not accurately simulate the intraoral environment.

There is also some clarity needed regarding the mechanical properties being dependent on the type of printer used and whether this has mechanical implications on the biomechanics of tooth movement in vivo as well.

The need of the hour is to further investigate the properties such as accuracy, cytotoxicity, and mechanical properties of this resin to assess the efficiency of these polymers compared to conventional orthodontic materials.

Source of Funding

None.

Conflict of Interest

None.