Ball-in-Hole Test for Microflexural Strength of 3D-Printed 5Y-TZP Restorations

Objectives: Existing flexural strength tests for dental ceramics frequently necessitate large specimens. However, for printed ceramic restorations, the surface characteristics might determine its strength. Hence, we aimed to develop a microflexural test that was validated by comparing two three-dimensional FEA models of the conventional three-point-bending device with the new ball-in-hole using solid maximum principal stresses and assess the microflexural strength of the 3D-printed 5Y-TZP specimen
Methods: The ball-in-hole device was designed to test specimens that are 12 mm in length and have a width of 1x1 mm. The device involves leaving a 10 mm free bar between the supports, with a 10 mm ball placed in the middle to load the bar. This device featured a metallic base with a 10.1 mm diameter perforation through which the specimen bar was internally positioned and stabilized by two support bases. FEA was performed considering isotropic behavior (using Young's modulus [E] and Poisson ratios [v]) and a friction coefficient between zirconia and metal of 0.45. After mesh convergence checking, the stress under a representative load application of 100 N was similar between the two devices. After this validation, we assessed the microflexural strength of the 5Y-TZP specimen manufactured by 3D printing with two printing layer orientations and milling as a control in the new ball-in-hole device. Testing was conducted on a universal testing machine (crosshead speed of 1 mm/min; Instron 6022; Instron) and the load was recorded.
Results: One-way ANOVA revealed a significant influence of zirconia material (F=5.6, P=0.009) on flexural strength. Milled zirconia (643±117 MPa) showed similar mean strength to 3D-printed parallel to the tensile plane (642±115 MPa). 3D-printed zirconia perpendicular to the tensile plane had the lowest average value (515±40 MPa).
Conclusions: 3D-printed 5Y-TZP presents similar microflexural strength to milled restorations when the printing layer orientation is considered during the manufacturing process.