Comparative analysis of mechanical properties of orthodontic aligners produced by different contemporary 3D printers

Abstract Objective The aim of this study was to compare the mechanical properties of orthodontic aligners among different commercially available 3D printing devices. Materials and Methods Five 3D printers (Ka:rv LP 550, Swinwon; “KAR”), (L120, Dazz 3D; “L12”), (MiiCraft 125, Miicraft Jena; “MIC”), (Slash 2, Uniz; “SLS”) and (Pro 95, SprintRay; “PRO”) were used to prepare orthodontic aligners with dental resin (Tera Harz TC‐85DAW, Graphy). The central incisors of each aligner were cut, prepared and evaluated in terms of Martens‐Hardness (HM), indentation‐modulus (EIT) and elastic‐index (ηIT) as per ISO14577‐1:2002. Force‐indentation curves were recorded and differences among printers were checked with generalized linear regressions (alpha=5%). Results Statistically significant differences were seen for all mechanical properties (P < .05), which were in descending order: HM (N/mm2) as median (Interquartile Range [IQR]): SLS 108.5 (106.0‐112.0), L12 103.0 (102.0‐107.0), KAR 101.5 (97.5‐103.0), MIC 100.0 (97.5‐101.5) and PRO 94.0 (93.0‐96.0); EIT (MPa) as mean (Standard Deviation [SD]): SLS 2696.3 (124.7), L12 2627.8 (73.5), MIC 2566.2 (125.1), KAR 2565.0 (130.2) and PRO 2491.2 (53.3); and ηIT (%) as median (IQR): SLS 32.8 (32.3‐33.1), L12 31.6 (30.8‐32.3), KAR 31.3 (30.9‐31.9), MIC 30.5 (29.9‐31.2) and PRO 29.5 (29.1‐30.0). Additionally, significant differences existed between liquid crystal display (LCD) and digital light processing (DLP) printers for HM (P < .001), EIT (P = .002) and ηIT (P < .001), with aligners from the former having higher values than aligners from the latter printer. Conclusion Under the limitations of this study, it may be concluded that the mechanical properties of 3D‐printed orthodontic aligners are dependent on the 3D printer used, and thus, differences in their clinical efficacy are anticipated.


| INTRODUC TI ON
Orthodontic aligners present a highly aesthetic treatment alternative to fixed appliances which make them exceedingly desirable, especially among the adults. 1,2 The old-fashioned indirect fabrication technique involves the production of a series of dental models where the thermoplastic material is shaped accordingly either with applied air pressure or under vacuum. 3 These translucent sequential positioners mainly made of polyethylene terephthalate glycol (PETG) or polyurethane (PU) are able to displace teeth in an incremental fashion 4,5 to various degrees of clinical effectiveness. 6 Despite the increased demand for this treatment option, orthodontic aligners remain till today an expensive, tedious and time-consuming option, which may discourage patients and clinicians alike.
The introduction of direct three-dimensional (3D) printing technology presents as a breakthrough that solves this problem by providing a low-cost treatment that can be provided at the same day within the dental office and bypassing the dental lab. In contrast to the conventional indirect technique, the manufacturing process of the direct 3D-printed technique circumvents the step of the physical construction of the dental model and the aligner is directly constructed based on electronically stored 3D dental data. 7,8 The materials used are also quite different with epoxy resins and photopolymers being predominant. 7 Multiple 3D fabrication systems and processes have been developed and employed for that purpose including stereolithography, fused deposition modelling, direct pellet-fused deposition, selective laser sintering, multi-jet photocured polymer process or continuous liquid interface production technology. 7,8 Direct light processing (DLP) and liquid crystal display (LCD) are fast 3D printing processes which utilize a conventional light source applied to the entire photopolymer resin and gains acceptance and preference by the majority of aligner 3D-printer manufacturers. 7 However, this "do it yourself" trend, as tempting as it appears to be, lacks scientific data regarding fundamental parameters like the material's mechanical properties as well as biocompatibility. 9 In the literature, there is vast information about the conventionally used thermoplastic aligners like Invisalign and other clear aligner systems. This is, however, not the case for the directly 3D-printed appliances, which not only are fabricated by different materials, but the end product might also be affected by the manufacturing process itself as well as the printer's specifications. 10 Thus, the clinician is found in a strenuous position where one can quickly produce an aligner, but is unaware of its fundamental properties that impact its clinical performance. These include important mechanical and surface properties like stiffness, elastic relaxation, hardness and roughness that not only can influence treatment efficiency, but may be also associated with iatrogenic implications on the patient's health. The first two characteristics are decisive on light continuous tooth force implementation which is important for proper periodontal biological response. In addition, hardness is associated with resistance to abrasive stimuli deriving from the opposing arch while roughness is related to plaque accumulation and discoloration. 11,12 Thus, it becomes evident that the arbitrary clinical application of the direct 3D-printed aligners due to lack of knowledge makes imperative the need of material quality control at a wide spectrum.
Therefore, the aim of this research is to investigate 3D-printed aligners deriving from five different 3D printing devices as far as their mechanical behaviour concerned. The null hypothesis set was that no statistical differences exist in the mechanical properties of 3D-printed aligners printed with different printers.

| Sample preparation
Five different 3D printers were included in this study (Table 1)

TA B L E 1
Brand names of 3D printers tested along with their corresponding manufacturer, printing technology, XY resolution, minimum layer thickness and codes (group name) used in this study. All printers are equipped with a source that emits at 405 nm

| Instrumented Indentation Testing (IIT)
The four central incisors of each aligner were cut off and embed-

| Statistical analysis
The numerical data of HM, E IT and η IT were initially checked for normality visually and with the Shapiro-Wilk test. Descriptive statistics included means with standard deviations (SD) for normal and medians with interquartile ranges (IQR) for skewed data. and an openly provided dataset. 15  The measured results for all three outcomes are given in

| D ISCUSS I ON
The present study evaluated the mechanical properties of orthodontic aligners manufactured via direct process with different properties of orthodontic applications is currently available. 8,9 Therefore, at the best of our knowledge, there are not similar studies in dental literature and thus no comparison with literature data is feasible.
Interestingly, significant differences were identified for all mechanical properties among the printers tested. Given that all groups share the same resin and identical post-curing process at the same device, the only source for these differences might be attributed to the 3D printing process of each printer itself. It must be noted here that as per the ISO 14 577 standard, the specimen thickness should be higher than 10 times the maximum indentation depth, which, for this study, should be around 0.40 mm.
Specimen thickness in the current study was however 0.35 mm, as this is dictated by the preparation instructions for orthodontic aligners and varying the thickness of the tested specimens would not correspond to the clinical reality, reducing the applicability of this study's results. Likewise, no standard specimens of fixed shape or orientation were printed in this study and 3D printing aligners with irregular morphology might introduce variation in printing precision. 21 However, as mentioned before, this would result in the specimens not corresponding to aligners used in reality and would have impaired the applicability of this study's findings.

| CON CLUS IONS
Within the limitations of this study, the mechanical properties are dependent on 3D printing devices used for the manufacturing of orthodontic aligners.

ACK N OWLED G EM ENTS
Open Access Funding provided by Universitat Zurich.

CO N FLI C T O F I NTE R E S T
The authors declare no conflict of interest or financial interest.

AUTH O R CO NTR I B UTI O N
All authors have contributed to conceptualizing of the work, writing and reviewing the manuscript. In addition, SZ and SNP acquired and analysed data; TE overviewed the coordination of the work.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are freely available (10.5281/zenodo.4701075).