Mainly because of the advancement of digital technology, there has been a trend toward in-office design and fabrication of orthodontic brackets and appliances, including 3-dimensional (3D)–printed resin brackets. This technology enables the orthodontist to create brackets with patient-specific designs and tailored biomechanical properties, which can potentially optimize tooth movements and reduce treatment time. In addition to this individualized treatment, the printed brackets may also offer a more esthetic appeal for some patients. During the process of 3D printing, postprocessing occurs once the material is printed and before its use. The postprocessing steps include postrinsing and postpolymerization, also referred to as postcuring. This postpolymerization process is required to promote cross-linking of the remaining polymers. However, the conversion from monomers to polymers may not be fully completed, resulting in residual monomers. The degree of residual monomers is influenced by the method of polymerization chosen. Because of the large variety of 3D printers, postprocessing units, and resins, there are various procedures that can be followed, involving the rinsing time and UV exposure time in these postprocessing steps. With these diverse parameters to consider, it may be difficult for clinicians to decide what the optimum postprocessing route is. In addition, it can be a problem for clinicians who acquire 3D printers from different manufacturers while using resins manufactured by another company.

An important consideration when dealing with dental resins is the biocompatibility of the products to human oral tissues. Biocompatibility is the ability of a material to perform its desired function without eliciting an undesirable systemic or local response. This concept is important for clinicians when using materials that will be placed and remain in close contact with the periodontium of the mouth. Traditional brackets are typically made from stainless steel or ceramics and have been used with proven biocompatibility. However, the introduction of 3D-printed brackets, which are made of resins or polymers, is a novel area in need of more investigation. These residual monomers can be released into the oral cavity, inducing cytotoxic effects within the dental pulp and adjacent gingival soft tissue in contact with the 3D-printed materials. They have also been found to cause continuous inflammatory response when applied to pulp tissue and can promote the growth of numerous dental caries bacteria. One study evaluated the cytotoxic effects on both human gingival and periodontal ligament (PDL) cells of 3 dental resin liquids that are used for temporary crowns, relining dentures, and denture border extensions. It was found that all leached monomers from the resins reduced cell viability in a dose-dependent manner. An additional study investigated the cytotoxic effects of orthodontic resins by examining the monomer leaching and resulting impact on mouse fibroblast cells. Both cured and uncured forms of the resins were tested, finding that all materials were cytotoxic, with uncured resins being significantly more toxic. Both Lai et al and Uomo et al raised the concern of the release of residual monomers and emphasized the potential for inadequate polymerization to impair cellular functions and induce inflammatory responses, leading to irritation of the oral tissues. In contrast, one study evaluated 4 resin materials used for temporary restorations that are fabricated through additive manufacturing. While evaluating the cell viability and inflammatory cytokine expression over time, they found that direct contact with these resins reduced cell viability; however, the remaining leached monomers had minimal impact on cell viability. Understanding the 3D-printing process, compositions of the materials, and potential immediate and long-term biologic responses within the oral cavity are important to ensure the biocompatibility and efficacy of 3D-printed brackets in clinical practice settings. With the potential of the remaining uncured monomers to reduce cell viability within the oral tissues, it is important to find the most appropriate protocols for postprocessing to minimize these deleterious effects on the oral environment.

Adequate rinsing is a key step for postprocessing, but there is no universally defined optimal time, and results have varied across past studies and resins. The importance of rinsing before light curing is continually stressed, as trapped monomers are harder to remove once cured. One study suggests that increasing the rinsing time significantly improved the biocompatibility of 3D-printed resins used for crowns and bridges, especially when using isopropyl alcohol (IPA) for up to 90 minutes. Additional research supported this in a different context, examining root-end filling materials and the effect of prolonged rinsing for 2 weeks on both gingival and PDL fibroblasts. They concluded that rinsing facilitated the removal of toxic by-products released during the setting of materials, suggesting that leachable components are major contributors to cytotoxicity. It has also been suggested that oxygen-shielding methods, such as glycerin immersion during postpolymerization, can significantly increase the degree of conversion of polymer resins. Acting as an oxygen barrier, the glycerin prevents oxygen inhibition at the resin surface, leading to improved mechanical properties and reduced cytotoxic effects. With postcuring in the air, the curing zone is physically limited between the cured region and the oxygen inhibition layer.

It was hoped that this study would extend these concepts to orthodontic 3D-printed resin brackets, supporting these minor modifications in the postprocessing protocol that can potentially lead to a safer, more biocompatible material to be used in the clinical setting. Although 3D-printed resin has gained popularity in restorative dentistry for fabricating provisional and permanent restorations, orthodontics has recently seen a surge of interest in its application for appliances and brackets. These resins have been previously approved for restorative dentistry and are potential resins useful for orthodontic applications, specifically 3D-printed brackets. This study sought to investigate if two 3D-printed resin materials, fabricated with altered rinsing and postpolymerization parameters, exhibit different levels of cytotoxicity.

Material and methods

This study was reviewed and approved by the institutional review boards of Louisiana State University Health Science Center, New Orleans (institutional review board number: 004826). Written informed consent was obtained for the use of these cells in research experiments involving the soft tissues of the oral cavity.

This study evaluated the biocompatibility of two 3D-printed dental resins intended for orthodontic bracket fabrication, SprintRay ceramic crown (SprintRay Inc, Los Angeles, Calif) and SprintRay crown (SprintRay Inc). Using the SprintRay Pro55S printer and SprintRay ProCure 2 unit (SprintRay Inc), standardized 10 mm × 2 mm resin discs were fabricated and subjected to various postprocessing protocols. The disc model was designed using Meshmixer (Autodesk Inc, San Francisco, Calif) computer-aided design software and exported in a Standard Tessellation Language format for printing with a layer height of 50 microns. The appropriate resin material was selected within the SprintRay Pro55S printer software (SprintRay Inc) settings to ensure compatibility with the resin. The printer automatically adjusted the parameters, such as exposure time, based on the selected resin. The manufacturer-recommended initial rinsing protocol was completed on all discs. Discs were brushed with a soft-bristled brush and 99% IPA for 5 seconds to remove all residual resin. Discs were air-dried for 10 minutes. Postprint curing was accomplished by placing the discs in the SprintRay Procure 2 unit (SprintRay Inc) for further polymerization. The unit was preset to the manufacturer’s recommended curing time and temperature for each resin. Detailed information on the preparation of each resin can be found in Table I , including the manufacturer’s recommended postprocessing protocols. Three postprint processes—no postprint cure vs postprint cure, postprint cured with prolonged rinsing, and air cured vs glycerin cured—were examined ( Table II ).

Table I

Manufacturer’s recommended postprint processing and intended use

Resin	Washing + drying	Postcuring	Intended use
SprintRay ceramic crown, A1 (resin 1)	Use ≥91% IPA to wash the device using one of the following methods: Submerge in a small bowl of IPA and scrub with a brush.	ProCure 2: select the preprogrammed material profile: • rc405 nm wavelength • 50 μm print layer thickness • Light energy of 28.8 mW/cm 2	Definitive full single crowns. Definitive partial crowns in the anterior and posterior areas. Individual and fixed single veneers. Artificial teeth for dental prostheses. Individual and removable monolithic full and partial dentures.
SprintRay crown (resin 2)	Use ≥91% IPA to wash the device using one of the following methods: Submerge in a small bowl of IPA and scrub with a brush.	ProCure 2: select the preprogrammed material profile. • 405 nm wavelength • 50 μm print layer thickness • Light energy of 28.8 mW/cm 2	Definitive full single crowns. Definitive partial crowns in the anterior and posterior areas. Individual and fixed single veneers.

Table II

Print processes evaluation in the respective groups

Process 1: no postprint cure vs postprint cure
Group 1 IPA rinse + no postcure	Group 2 IPA rinse + postcure per manufacturer’s instructions
Process 2: postprint cure with prolonged rinsing
Group 3 Fresh → 1 d → 1 wk → 2 wk IPA rinse + postcure per manufacturer’s instructions + 100 mL distilled water serial rinse
Process 3: air-cured vs glycerin-cured
Group 4 IPA rinse + postcure per manufacturer’s instructions	Group 5 IPA rinse + postcure in glycerin

Postprint process #1: For each resin, no postprint curing (group 1) was compared with standard postprint curing (group 2) using the SprintRay ProCure 2 (SprintRay Inc), as described above. Eight samples were examined for each group.

Postprint process #2: For each resin, 8 samples per resin were subjected to standard postprint curing (group 3) using the SprintRay ProCure 2 (SprintRay Inc), as described above, followed by serial washes with 100 mL distilled water for 1 day, 1 week, and 2 weeks. For these experiments, fresh denotes cured but unrinsed materials.

Postprint process #3: For each resin, standard postprint curing in air (group 4) was compared with standard postprint curing submerged in glycerin (group 5) using the SprintRay ProCure 2 (SprintRay Inc), as described above. Eight samples were examined for each group.

Cytotoxicity was assessed using PDL fibroblast cells. A 23-year-old male requiring premolar extraction during orthodontic treatment, with no systemic diseases, provided samples for PDL fibroblasts. PDL fibroblasts were isolated from PDL remnants attached to the central region of the tooth. Cells were maintained in minimal essential media–alpha (MEM-α; GIBCO, Life Technologies, Grand Island, NY) containing 10% fetal calf serum, 200 units/ml of penicillin, and 200 μg/ml streptomycin and grown at 37°C with 5% CO ₂ and 100% humidity. Cells were passaged via incubation with 0.25% trypsin with ethylenediaminetetraacetic acid for 3 minutes at 37°C. PDL fibroblasts between the 5th and 12th passages were used in this study. PDL fibroblasts were chosen for this study, as they have been determined to be slightly more sensitive to some soluble cytotoxic molecules than gingival fibroblasts, and these cells would be most important for the effectiveness of any orthodontic tooth movement that would require these types of 3D-printed brackets. ^,

For each group, cells were plated into a single 24-well polystyrene plate at 10,000 cells per well in 400 μL MEM-α. Cells were allowed to adhere to the substrates for 24 hours before exposure to test solutions. Cytotoxicity was determined by exposing cells to a single disc of test material for 24 hours ( Fig 1 ). After the 24-hour incubation period, the test material was removed, and the cells were rinsed twice in 400 μL MEM-α. To examine cell survival, cells were treated with Calcein AM (Invitrogen,Thermo Scientific, Carlsbad, Calif) (1-2 nM) for 1 hour (400 μL per well). Cells were then rinsed twice with 400 μL MEM-α and immediately quantified using a BioTek Synergy2 fluorescent multiwell plate reader (BioTek, Winooski, Vt). Data were exported to MS-Excel (Microsoft, Redmond, Wash) for subsequent analysis.

Only gold members can continue reading. Log In or Register to continue

Share this:

• Click to share on Twitter (Opens in new window)
• Click to share on Facebook (Opens in new window)

Related

Related posts:

1. The periodontal ligament–periosteum axis: An underexplored pathway for alveolar bone adaptation
2. Orthognathic surgery need following BAMP therapy in unilateral complete cleft lip and palate with moderate to severe maxillary deficiency: a prospective study
3. Effect of premolar extractions on occlusal contacts and chewing efficiency: A case-control study
4. Evaluation of pharyngeal airway volume, soft-tissue changes, and risk of obstructive sleep apnea after bimaxillary orthognathic surgery in patients with skeletal Class III malocclusion using cone-beam computed tomography and the STOP-BANG questionnaire: A long-term study

Stay updated, free dental videos. Join our Telegram channel

Join