UV-Curing Assisted Direct Ink Writing of Dense, Crack-Free, and High-Performance Zirconia-Based Composites With Aligned Alumina Platelets.

Additive manufacturing (AM) of high-performance structural ceramic components with comparative strength and toughness as conventionally manufactured ceramics remains challenging. Here, a UV-curing approach is integrated in direct ink writing (DIW), taking advantage from DIW to enable an easy use of high solid-loading pastes and multi-layered materials with compositional changes; while, avoiding drying problems. UV-curable opaque zirconia-based slurries with a solid loading of 51 vol% are developed to fabricate dense and crack-free alumina-toughened zirconia (ATZ) containing 3 wt% alumina platelets. Importantly, a non-reactive diluent is added to relieve polymerization-induced internal stresses, avoid subsequent warping and cracking, and facilitate the de-binding. For the first time, UV-curing assisted DIW-printed ceramic after sintering reveals even better mechanical properties than that processed by a conventional pressing. This is attributed to the aligned alumina platelets, enhancing crack deflection and improving the fracture toughness from 6.8 ± 0.3 MPa m0.5 (compacted) to 7.4 ± 0.3 MPa m0.5 (DIW). The four-point bending strength of the DIW ATZ (1009 ± 93 MPa) is also higher than that of the conventionally manufactured equivalent (861 ± 68 MPa). Besides homogeneous ceramic, laminate structures are demonstrated. This work provides a valuable hybrid approach to additively manufacture tough and strong ceramic components.

3D printing and milling accuracy influence full-contour zirconia crown adaptation.

OBJECTIVES: To correlate trueness and cement-space characteristics of crowns milled chairside and in the laboratory with those of inkjet 3D-printed crowns, and to assess whether 3D-printing accuracy meets the clinical standard. METHODS: Thirty crowns were either (1) milled using a chairside Cerec MCXL unit from Cerec Zirconia Mono L (Dentsply Sirona), (2) milled using a LX-O 5-axis (Matsuura Machinery) industrial machine from Initial Zirconia HT (GC), or (3) 3D-printed using an inkjet Carmel 1400 (Xjet) printer (n = 10). Crown trueness determined by comparing the original CAD with each visible-light digitized crown was correlated with the 3D cement-space characteristics recorded by micro-CT. Statistics involved Kruskal-Wallis testing and Spearman correlation. RESULTS: Crown trueness at the intaglio marginal area positively correlated with the marginal and axial cement-space characteristics. 3D-printing revealed data in-between those of the two milling systems with undercut values being not statistically different from those recorded for chairside milling and a low overcut level that was statistically similar to that obtained by laboratory milling. Laboratory milling revealed a significantly better marginal accuracy with a consequently lower cement-space thickness. A higher overcut level was recorded for the chairside-milled crowns in the marginal/occlusal thirds, resulting in the significantly highest occlusal cement-space thickness and cement-volume percentage with a cement thickness above 120 µm (limit considered as clinically acceptable). No statistical difference in trueness was found for the external crown dimensions. SIGNIFICANCE: The 3D-printed zirconia crowns provided sufficient manufacturing accuracy for clinical use. Accurate milling and printing of the crown's intaglio marginal area is primordial.

3D printed zirconia dental implants with integrated directional surface pores combine mechanical strength with favorable osteoblast response.

Dental implants need to combine mechanical strength with promoted osseointegration. Currently used subtractive manufacturing techniques require a multi-step process to obtain a rough surface topography that stimulates osseointegration. Advantageously, additive manufacturing (AM) enables direct implant shaping with unique geometries and surface topographies. In this study, zirconia implants with integrated lamellar surface topography were additively manufactured by nano-particle ink-jetting. The ISO-14801 fracture load of as-sintered implants (516±39 N) resisted fatigue in 5-55 °C water thermo-cycling (631±134 N). Remarkably, simultaneous mechanical fatigue and hydrothermal aging at 90 °C significantly increased the implant strength to 909±280 N due to compressive stress generated at the seamless transition of the 30-40 µm thick, rough and porous surface layer to the dense implant core. This unique surface structure induced an elongated osteoblast morphology with uniform cell orientation and allowed for osteoblast proliferation, long-term attachment and matrix mineralization. In conclusion, the developed AM zirconia implants not only provided high long-term mechanical resistance thanks to the dense core along with compressive stress induced at the transition zone, but also generated a favorable osteoblast response owing to the integrated directional surface pores. STATEMENT OF SIGNIFICANCE: Zirconia ceramics are becoming the material of choice for metal-free dental implants, however significant efforts are required to obtain a rough/porous surface for enhanced osseointegration, along with the risk of surface delamination and/or microstructure variation. In this study, we addressed the challenge by additively manufacturing implants that seamlessly combine dense core with a porous surface layer. For the first time, a unique surface with a directional lamellar pore morphology was additively obtained. This AM implant also provided strength as strong as conventionally manufactured zirconia implants before and after long-term fatigue. Favorable osteoblast response was proved by in-vitro cell investigation. This work demonstrated the opportunity to AM fabricate novel ceramic implants that can simultaneously meet the mechanical and biological functionality requirements.

Accuracy of additively manufactured zirconia four-unit fixed dental prostheses fabricated by stereolithography, digital light processing and material jetting compared with subtractive manufacturing.

OBJECTIVE: To evaluate the manufacturing accuracy of zirconia four-unit fixed dental prostheses (FDPs) fabricated by three different additive manufacturing technologies compared with subtractive manufacturing. METHODS: A total of 79 zirconia FDPs were produced by three different manufacturing technologies, representing additive (one stereolithography [aSLA] and one material jetting [aMJ] device, two digital light processing [aDLP1/aDLP2] devices) and subtractive manufacturing (two devices [s1/s2]), the latter serving as references. After printing, additively manufactured FDPs were debound and finally sintered. Subsequently, samples were circumferentially digitized and acquired surface areas were split in three Regions Of Interest (ROIs: inner/outer shell, margin). Design and acquired data were compared for accuracy using an inspection software. Statistical evaluation was performed using the root mean square error (RMSE) and nonparametric Kruskal-Wallis method with post hoc Wilcoxon-Mann-Whitney U tests. Bonferroni correction was applied in case of multiple testing. RESULTS: Regardless the ROI, significant differences were observed between manufacturing technologies (P < 0.001). Subtractive manufacturing was the most accurate with no significant difference regarding the material/device (s1/s2, P > 0.054). Likewise, no statistical difference regarding accurary was found when comparing s2 with aMJ and aSLA in most ROIs (P > 0.085). In general, mean surface deviation was< 50 µm for s1/s2 and aMJ and< 100 µm for aSLA and aDLP2. aDLP1 showed surface deviations> 100 µm and was the least accurate compared to the other additive/subtractive technologies. SIGNIFICANCE: Additive manufacturing represents a promising set of technologies for the manufacturing of zirconia FDPs, but not yet as accurate as subtractive manufacturing. Methodological impact on accuracy within and in between different additive technologies needs to be further investigated.

Mechanical properties, aging stability and translucency of speed-sintered zirconia for chairside restorations.

OBJECTIVE: To evaluate the performance of zirconia ceramics sintered in a speed sintering induction furnace by comprehensive understanding of their optical and mechanical properties, microstructure, phase composition and aging stability, in comparison to ceramics sintered in a conventional furnace. METHODS: Speed sintered (SS) Katana STMLSS (Kuraray Noritake) (total thermal cycle/sintering time/dwell temperature: 30min/16min/1560°C) and CEREC Zirconia (CEREC ZrSS) (Dentsply Sirona) (15min/2min/1578°C) were compared to conventionally sintered (CS) Katana STMLCS (6.8h/2h/1550°C) and inCoris TZICS (4h/2h/1510°C). The translucency parameter (TP) and contrast ratio (CR) were measured with a spectrophotometer. The chemical composition of the materials was determined by XRF and phase composition was characterized using XRD. Hydrothermal aging behavior was evaluated by measuring the tetragonal-to-monoclinic ZrO2 phase transformation after accelerated hydrothermal aging in steam at 134°C. The indentation fracture toughness, Vickers hardness and biaxial strength of the sintered ceramics were assessed. RESULTS: Speed and conventionally sintered zirconia revealed similar density, microstructure, average strength and hydrothermal aging stability. Both Katana STMLSS/CS 5Y-PSZ ceramics were characterized with a higher content of cubic phase (≈53wt%), which resulted in a higher amount of Y2O3 in the remaining tetragonal ZrO2 phases compared to the 3Y-TZP CEREC ZrSS and inCoris TZICS (8 and 20wt%, respectively). The sintering program did not affect the hydrothermal aging behavior of Katana STMLSS and CEREC ZrSS. TP of Katana STMLSS (TP≈32) was not affected by speed sintering, while the translucency of CEREC ZrSS (TP=14) was significantly reduced. Hardness, fracture toughness and Weibull characteristic strength of Katana STMLSS and CEREC ZrSS also reached the optimal level, but speed sintering substantially lowered their mechanical reliability. SIGNIFICANCE: Speed sintering of 3Y-TZP and 5Y-PSZ in a speed sintering induction oven appeared suitable for clinical applications. However, further studies should focus on improving of translucency and mechanical reliability of the speed-sintered zirconia ceramics.