Statement of problem
Information related to the optical and surface properties, including health compatibility, surface roughness, and esthetics, of 3D-printed dental materials is scarce.
The purpose of this in vitro study was to compare the physical and surface properties of a 3D-printed resin with those of materials used for interim restorations.
Material and methods
A 3D-printed resin (PR) (NextDent C&B MFH; 3D Systems), an autopolymerizing interim material (BA) (Protemp 4; 3M ESPE), and a composite resin (Z350) (Filtek Z350XT; 3M ESPE) were tested for degree of color change (ΔE) (n=7) at different timepoints—24 hours after polishing/baseline (P0), 8 days after polishing (P1), and after artificial aging in water at 60 °C for 24 hours (P2)—by using a CIELab-based colorimeter; flexural strength (σ) (n=10) with a 3-point bend test; Knoop hardness (H) (n=8); and surface roughness (Ra) (n=7) with a profilometer. All specimens were polished 24 hours after polymerization, except for the additional group for surface roughness (BA) without polishing (BANP). A statistical analysis was performed by using 2-way repeated-measures ANOVA followed by the Fischer test for ΔE and 1-way ANOVA followed by the Fisher test for microhardness and surface roughness (α=.05).
The Z350 showed the highest values for σ and H, followed by PR. BA showed the lowest results for both tests ( P <.05). Considering roughness, the Z350 showed similar values to those of BA but lower than PR; PR showed similar roughness when compared with BA. PR showed the highest color variation among the groups at all timepoints, followed by BA. The Z350 was the most color stable material at all timepoints.
The 3D-printed composite resin had adequate mechanical and surface properties for an interim restorative material. It has the potential to be a low-cost workflow in dentistry, although its color stability could be a concern for long-term use.
Three-dimensional printing technology blends the digital workflow and is capable of saving time and cost. 3D-printed resins have acceptable mechanical and surface properties, although their color stability could be a concern for long-term use.
Incorporating innovative technology has accelerated the adoption of a digital workflow in many dental practices, and computer-based methods of fabricating dental restorations are a growing field in dentistry. The advantages of digital technologies include the capability to improve restorative treatment steps, including diagnostic casts, diagnostic waxing, preparation guides, trial restorations, and interim restorations, as well as the digital treatment plan, diagnosis, and fabrication of the restoration, resulting in a more efficient dental practice.
Currently, most computer-based manufacturing processes are based on subtractive manufacturing or milling. Three-dimensional printing was first developed in the 1980s, but with advances in computer-aided design (CAD) technology, these expensive and technologically limited 3D printers have improved. Currently, 3D printers are affordable, user-friendly, reliable, and capable of producing complex shapes. Most 3D printers are based on stereolithography (SLA) or digital light processing (DLP). In SLA, voxels of monomers are polymerized by a laser light track directed by a Galvano mirror scanner to build 3D structures. , In DLP, a digital projector screen flashes light through the entire layer to build the 3D structures. Compared with milling, 3D-printed appliances and restorations can improve patient acceptance, provide adequate marginal and internal fit, and reduce costs (1 liter of resin can print an average of 300 single crowns). Nevertheless, information on the properties and clinical behavior of printed materials is still lacking. ,
Interim restorations are fundamental to achieving predictable successful definitive restorations. They maintain soft-tissue health, protect the pulpal tissue, achieve an adequate emergence profile, stabilize the 3D position of the prepared tooth, enhance the acceptance of the treatment plan and patient comfort, and promote better communication among clinicians, patients, and laboratory technicians.
The most popular materials for interim restorations are polymethyl methacrylate (PMMA) and bis-acrylic resins. These materials are adequate and cost-effective for clinical use but have limitations (especially PMMA) as they cannot be directly integrated into a digital workflow and rely on free-hand fabrication. , , Despite recent research on the use of 3D-printed materials in dentistry, , , , , , , information related to their optical and surface properties, including their compatibility with the oral environment, surface roughness, and esthetics, is scarce.
Therefore, the purpose of this in vitro study was to compare the physical and surface properties of resins for 3D printing with those of resins for interim restorations. The null hypotheses tested were that no difference would be found in color stability, flexural strength, hardness, or roughness among the tested resin-based materials.
Material and methods
This in vitro study involved 1 factor (composite resins) at 3 levels: an interim material based in bisacryl resin (BA, Protemp 4; 3M ESPE); a microfilled 3D printable resin for stereolithography (SLA) printing (PR, NextDent C&B MFH; 3D Systems), and a conventional composite resin (Z350, Filtek Z350XT; 3M ESPE) with response variables of color stability (ΔE), hardness (H), surface roughness (Ra), and flexural strength (σ). Product specifications are shown in Table 1 .
|3D printable composite||Crown & Bridge, MFH (Next Dent)||A2||XK55N91||Methacrylic oligomers, phosphine oxides, microfillers|
|Nanoparticle composite||Filtek Z350XT (3M ESPE)||A2||911689||Bis-GMA, Bis-EMA, UDMA, TEGDMA, Zirconia, and silica fillers. Silane and pigments. Loading percentage by weight: 82% (5-20 nm nonagglomerated silica and 5-20 nm zirconia/silica nanoagglomerate. 0.6-1.4 μm agglomerated particles).|
|Bis-Acrylic Interim Composite Resin||ProTemp Plus (3M ESPE)||A2||3712273||Functionalized dimethacrylate polymer; Bis-GMA, zirconia and silica fillers, silane, and pigments. Loading percentage by weight: 78%|
For the degree of color change (ΔE), Knoop microhardness (H), and surface roughness (Ra) tests, disk-shaped specimens were prepared by using 2-part Teflon molds (10 mm in diameter; 2 mm in height), with 2 glass slides on the top and bottom. For the light-polymerizing materials, a wide-spectrum polywave (395 to 480 nm) light-emitting diode (LED) light-polymerizing unit (Valo Grand; Ultradent Products, Inc) with 1000 mW/cm 2 was used for 40 seconds for each specimen. The irradiance from the polymerization light was checked by using a radiometer (RD-7; Ecel Indústria e Comércio Ltda) before starting each group. The specimens were completely covered by the polymerization light tip (ø10 mm). For the BA resin, specimens were allowed to polymerize for 5 minutes before being removed from the matrix. The specimens were embedded in epoxy resin and polished with decreasing grit abrasive papers (600-, 1200-, and 2400-grit) (Buehler Ltd), followed by a polishing cloth with 0.5-μm diamond paste (Buehler Ltd) either immediately (ΔE) or 24 hours (H and Ra) after specimen preparation. Materials of similar color and opacity were chosen to standardize the conditions for all groups. To standardize specimen preparation, polishing procedures were performed by a single operator (K.C.S.) by using an automated device.
For the ΔE test (n=7), the color change was assessed at different time points by using a CIELab-based colorimeter (Vita Easyshade V; Vita Zahnfabrik). Before measurements, the spectrophotometer was calibrated according to the manufacturer’s instructions. An initial measurement (P0) was performed 24 hours after polishing; a second measurement (P1) 8 days after the polishing procedures (7 days after P0); and a third measurement after artificial aging (P2) consisting of 24 hours water storage at 60 °C. , Between P0 and P1, all specimens were dry stored at 37 °C in the absence of light. Three consecutive measurements were made in the center of each specimen until uniformity of values was observed. , ,
The ΔE was calculated based on the following equation: ΔE=√(ΔL∗)2+(Δa∗)+(Δb∗)2, where ΔL∗, Δa∗, and Δb∗ correspond to the color differences observed between the baseline (P0) and after the storage period (P1 and P2).
For the H test (n=8), 3 indentations separated by 100 μm (Knoop diamond, 0.49 N, 15 seconds) were made at the center of each specimen by using a digital microhardness tester (HMV–2000; Shimadzu). The built-in software program calculated the Knoop hardness (KHN) values based on the dimensions of the longest diagonal by using the following formula:
, where c is the load in gram force and d is the length of the longer diagonal in μm.
For Ra (n=7), including an additional group using BA without surface polishing (BA-WP), surface roughness was assessed by using a roughness tester (Hommel-Etamic W10; PCE GmbH) following ISO 4287 guidelines with a 0.80-mm cutoff, 0.0001-μm resolution (8-μm range), 0.5-mm/s speed, and total length of 4 mm. The average of the 3 measurements of the x- and y-axes was calculated and adopted as a result for each specimen.
For the flexural strength ( σ ), 10 bars with standard dimensions (10×2×2 mm) were fabricated for each material, stored at room temperature for 24 hours, and tested by using a 3-point bend test attached to a universal test machine (Instron 3342; Instron) with a cross-head speed of 0.5 mm/min. A central load was applied between supporting rods (8-mm span length). The mean σ values and the standard deviations were calculated for each of the materials using the following equation:
, where F is the loading force at the fracture point, l is the length of the support span (8 mm), b is the width (2 mm), and d is the thickness (2 mm).
The data were statistically analyzed by using a statistical software program (Minitab 18; Minitab LLC). Normal distribution was checked for all variables using the Kolmogorov-Smirnov test. The σ, H, and Ra were analyzed by using 1-way ANOVA followed by the Fischer test, and ΔE was analyzed by using 2-way repeated-measurements ANOVA followed by the Fischer test (α=.05, for all tests).