The demand for orthodontic appliances with a low impact on orofacial esthetics, especially in the anterior region, continues to be high due to increased societal pressure regarding self-optimization and perfection [1
]. Currently, practicing orthodontists have the choice between numerous esthetic multibracket appliances for fixed therapy, which differ in geometry, material composition, and treatment efficiency. Regarding therapy efficiency, the most relevant factors are torque efficacy, bracket wing stability, binding and notching, resistance to the intraoral environment, adhesion to the enamel, and the possibility of gentle removal from the enamel. Regarding esthetics, important factors include color, size, and color stability. Patients’ wish for almost invisible orthodontic appliances has led to the development of tooth-colored bracket materials, primarily made of ceramics, polymer materials or a combination of both [3
In contrast to lingual bracket systems, which have the best esthetic appearance and very effective torque, tooth-colored brackets can be integrated into the proven classic straight-wire concept [5
]. Ceramic brackets are considered an established alternative to conventional metal brackets and are characterized by high stability and rigidity as well as a low tendency to deformation and discoloration [8
]. However, the hardness of the ceramic material can lead to an increased risk of enamel abrasion [10
]. Further practical challenges can include problems regarding adhesive bonding to the enamel, high shear bond strength, high sliding resistance, high susceptibility to fracture and chipping during torque loading, and the time-consuming debonding procedure [11
Brackets made of polymer material provide another esthetic alternative to metallic brackets. Polycarbonates are thermoplastics, which allow easy matching to the tooth color thanks to their transparent properties [14
]. In addition, they have been shown to exhibit low sliding resistance [15
]. However, given their poor stability and stiffness, resulting in permanent plastic deformation even at low force, polymer brackets are only rarely used in practice [18
]. Testing of different material compositions of polycarbonate and polyurethane with inorganic fillers, such as glass fibers or ceramic particles, and/or a metal cover for the slot have also failed to meet clinical requirements [3
]. The effective torque of brackets made of these materials was continuously significantly lower than that of ceramic and metal brackets [8
]. In addition, their color stability was only moderate [20
]. This was also observed for novel resins used in three-dimensional (3D) printing [22
The material-specific shortcomings of both ceramic and current polymeric brackets have led to efforts in developing a more suitable material for tooth-colored brackets. With 3D printing being on the rise in dentistry, the first steps to produce in-office printed brackets have already been published [23
]. A novel high-performance resin, Permanent Crown Resin ([PCR], Formlabs Inc., Somerville, MA, USA), approved for the fabrication of permanent dental crowns has recently become available for 3D stereolithographic printing (SLA). If PCR meets the requirements for prosthetic crowns, we hypothesized that it might also be suitable as a bracket material. The objective of this study was to investigate the material-specific characteristics of an in-office manufactured PCR bracket in terms of fabrication quality, fabrication precision, and torque transmission after simulated artificial aging in a thermocycler. Our specific interest was the effective torque of the PCR brackets, the specific dimension in which all previous polymer materials have performed poorly. Such in-office manufactured PCR brackets could offer an alternative to conventional tooth-colored bracket systems, as they provide advantages in terms of personalization regarding bracket color, bracket prescription, and bracket base.
In the present study, a novel high-performance resin, which has been shown to be suitable for dental crown fabrication, was transferred to orthodontic brackets manufacturing and compared with conventional bracket materials (ceramic, metal) regarding slot precision and effective torque [27
]. Results showed comparable performance of the novel PCR bracket to the established bracket materials regarding both metrics.
To detect possible visible defects, a visual inspection of the brackets was performed under a digital microscope. This step served as an initial evaluation of the manufacturing quality. Special attention was paid to the implementation of the digital bracket design especially regarding rounded edges, excess resin residues, and polishing defects. Of course, this methodology only has limited validity regarding biomechanical properties; thus, further tests were carried out [17
In accordance with previous studies, the dimension of the used wires was measured with a digital micrometer [33
]. The manufacturing precision in the slot area was determined with specially calibrated plug gages. Based on ISO certification, the plug gages were selected with an accuracy of ± 0.0004 mm and were, thus, considered suitable for determining slot sizes [31
]. It should be noted that this procedure does not allow precise three-dimensional slot measurement. However, in the present study the slot size measurements were only relevant for the subsequent interpretation of the torque values; thus, the comparability of slot sizes between the material groups suffices. Further studies will be necessary to verify the exact morphological characteristics of the PCR slots, e.g., by using established methods such as visual measurement in a digital optical microscope, machine measurement using specially manufactured inspection systems, or with the aid of a micro-CT (computed tomography) [14
The obtained results of the slot measurements and wire dimension were all within the range specified by the DIN 13996 [32
]. The observed oversizing of the slot heights of the conventional bracket groups was expected and is consistent with previous literature [37
]. For the PCR brackets, the slot size was set at 5% above the target of 0.022″ to meet the required precision of 50 µm of the PCR material for SLA printing. Surprisingly, the print precision in the slot size area exhibited a deviation of only 10 µm, showing higher precision than anticipated. In contrast to the metal and ceramic brackets, the PCR brackets displayed a difference in the slot height with slot entrance being 3% larger than the slot bottom on average. Furthermore, the PCR brackets showed a higher variance in slot sizes than the other two material groups. A possible explanation for these differences could be the differences in manufacturing of the bracket groups. The ceramic brackets used in the present study were manufactured using the ceramic injection molding process (CIM), the metal brackets using the metal injection molding process (MIM), each procedure followed by a mechanical finishing process. Both are standardized mechanical processes that ensure a high degree of precision [42
]. The resolution of 3D printing and manual post-processing can be assumed to have been the causes for the higher variance of the measurements of the PCR brackets.
Polymers have the property of absorbing water and therefore swell due to the water absorption in the intraoral environment [4
]. To mimic real life behavior of the material, artificial aging according to ISO 10477 was performed [34
]. It should be noted that thermocycling only conditionally mirrors the real clinical situation and is limited regarding influences such as food intake and the associated pH fluctuation, chewing and abrasion of surfaces, home and professional tooth cleaning, and mineral concentration in saliva. Further research is necessary to determine the influence of these factors on material properties.
The OMSS was utilized to determine torque movements. This is in line with current literature, which deems the OMSS suitable for in vitro studies of biomechanical issues related to different tooth movements in all spatial planes [4
]. In line with previous literature, torque transmission was higher for steel arch wires than for TMA arch wires. Reasons for this may be the different E‑moduli and/or the different flexural strengths [49
The obtained mean torque values of the PCR brackets were generally comparable to, and at times slightly higher than those of the reference groups. These results differ from the results of previous studies of polymer brackets with a wide variety of polymer compositions regarding torque stability. In these studies, the polymer brackets never achieved the effective torque values of the ceramic and/or metal reference groups, due to deformation and slot expansion [4
]. Surprisingly, the small deformations of the bracket slots of the PCR brackets after torque loading, as seen during the visual inspection, had no effect on the maximum torque values in the present study. Effects on the tip and friction behavior have not been investigated. One reason for the good performance of the novel high-performance polymer for bracket fabrication could be its special composition [27
]. Another reason could be the novel manufacturing 3D-printing process.
The variance of torque values was higher for the PCR brackets than for the reference groups. This is probably due to the higher variation in slot width of the PCR brackets resulting in higher variation of slot play. Slot play describes the empty space between the bracket and the arch wire caused by the combination of oversized bracket slots and/or undersized arch wire cross sections. This causes torque to be initiated only after the arch wire has undergone a certain amount of torsion [7
]. For example, in a study by Joch et al., examinations of self-ligating and conventional brackets revealed actual high slot play due to oversized slots [33
In general, the effective torque values measured in the present study far exceeded those of clinical relevance. In daily practice, the effective applied torques are usually between 5 and 20 Nmm [13
]. Our simulation in the OMSS provided initial evidence that the in-office manufactured PCR brackets can meet these clinical requirements. Further in vivo research is necessary to validate these findings.
That the novel PCR brackets could be a promising alternative to established material groups in clinical practice is further supported by the fact that only one of 30 PCR brackets broke during torque loading in the present study. In contrast, 9 of 30 ceramic brackets shattered. One explanation could be the differences of these materials regarding their mechanical properties. In the study by Grzebieluch et al., the material properties of novel polymers, including PCR, were investigated [23
]. The reported values of the flexural modulus (4.37–4.69 GPa) were significantly lower than those of polycrystalline alumina oxide ceramics [30
]. The relatively low flexural modulus could explain the lower fracture susceptibility and higher tendency for deformation. Another reason for the high stability of the PCR brackets could be the relatively thick bracket base design. The thick base design is necessary to allow individualization to the tooth surface for future clinical use. Nevertheless, this only allows limited comparability of the ceramic and PCR brackets regarding fracture susceptibility.
The possibilities of 3D printing are continuously expanding. It is not surprising that initial trials with in-office printed brackets have already taken place [22
]. To establish its clinical use, further studies must determine whether the used PCR material really meets the requirements for a bracket material. The possibility of torque transfer to the tooth is of particular research interest, as it represents an essential movement in active orthodontics. Therefore, it was considered critical to investigate the novel high-performance resin in terms of torque stability prior to clinical application.
Limitations of this study mainly concern that in vitro investigations have only limited ability to reproduce clinical conditions. This study provides initial evidence that a self-designed polymer bracket can be a possible alternative to conventional bracket systems. The PCR bracket would give the orthodontist the possibility to program the bracket in a personalized manner and offer individualized therapy, which could improve therapy in combination with a digital set-up [54
]. The scope for programming the bracket extends from an individualized bracket base to variations in slot and stem dimensions, to color selection and positioning on the tooth surface. The possible advantages of such an individualized bracket system have yet to be explored more deeply [25
]. To reach its full potential, the programming should be based on a 3D data set of the patient’s teeth and bony conditions. In addition, the individualized bracket bases must be positioned precisely with the help of a suitable bonding tray. To enable future clinical use, further studies regarding friction properties, discoloration tendencies, shear bond strength, and positioning accuracy are necessary.
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