High-strength DLP-printed zirconia for ultra-thin dental veneers.

OBJECTIVES

To evaluate the feasibility of utilizing Digital Light Processing (DLP) 3D-printing technology to fabricate ultra-thin (0.1-0.7 mm) zirconia dental veneers.

MATERIALS AND METHODS

A high-load (80 wt%) 3Y-zirconia slurry (5 Pa·s at a shear rate of 30 s) was used to print zirconia green bodies with a custom-made DLP 3D-printer (405 nm UV light and X/Y plane resolution of 70 μm). Flexural strengths of green bodies and fully sintered zirconia printed in two orientations (0º and 90º) were evaluated using three-point bending (3PB) and biaxial flexural strength (BFS) tests, respectively. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to examine the microstructure and crystalline phases of the sintered specimens. A commercial 3Y-zirconia (UPCERA MT) was used as a control for comparison with the best-performing DLP-printed specimens. XRD and SEM were used to assess low-temperature degradation (LTD) after artificial aging (autoclave, 5 hr). Transparency of the sintered DLP-printed and conventional zirconia at 0.5 mm and 1.0 mm thicknesses was measured using a desktop spectrophotometer (400-700 nm). Resin-zirconia bonding performance was evaluated via shear bond strength (SBS) testing and failure mode analysis. SBS was measured between a self-adhesive dual-curing resin cement and the surface of sintered zirconia specimens. The coefficient of thermal expansion (CTE) and Schwickerath three-point bending strength (τ) were measured to evaluate porcelain-zirconia compatibility. Ultra-thin (0.1-0.7 mm) dental restorations were fabricated to demonstrate the practical potential application of this novel zirconia printing approach.

RESULTS

The 3PB flexural strength of green bodies printed at 0º (21.35 ± 2.19 MPa) was significantly higher (p < 0.05) than at 90º (16.98 ± 1.68 MPa). The BFS of sintered zirconia printed at 0º (1040.33 ± 236.70 MPa) was also significantly higher (p < 0.05) than at 90º (685.91 ± 139.10 MPa). Sintered specimens printed at 0º exhibited an average grain size of 440 nm and a tetragonal phase. After artificial aging, the DLP-printed group exhibited superior resistance to LTD, with a lower monoclinic phase content (40.78 %) compared to the commercial zirconia group (72.51 %). DLP-printed zirconia exhibited lower transparency than commercial zirconia at both 0.5 mm (23.22 ± 1.55 % vs. 35.67 ± 0.14 %) and 1.0 mm (12.04 ± 1.45 % vs. 28.06 ± 0.25 %) thicknesses. Although the commercial zirconia group showed higher average SBS (12.43 ± 6.66 MPa), the difference was not statistically significant compared to the DLP-printed group (7.86 ± 5.69 MPa). Adhesive failure was the predominant failure mode in both groups. CTE of DLP-printed zirconia (10.56 ×10/ºC) was comparable to conventional zirconia (10.50 ×10/ºC). The τ of DLP-printed zirconia (26.37 ± 2.37 MPa) was significantly lower (p < 0.05) than that of the conventional zirconia (33.47 ± 3.37 MPa), but both exceeded the ISO 9693:2019 minimal requirement of 20 MPa. Ultra-thin (0.1-0.7 mm) dental veneers were successfully fabricated using the DLP technique.

CONCLUSIONS

The DLP technique enables successful fabrication of ultra-thin (0.1-0.7 mm) zirconia dental veneers, with printing orientation significantly influencing the strength of both green and sintered specimens.