Sequential usage of diamond bur for CAD/CAM milling: Effect on the roughness, topography and fatigue strength of lithium disilicate glass ceramic

Abstract

The aim of this study was to evaluate the effect of sequential usage (milling order) of CAD/CAM diamond burs on the surface roughness, topography and fatigue performance of a lithium disilicate glass-ceramic. Seventy-two (72) ceramic discs (Ø= 13.5 mm; thickness= 1.2 mm; IPS e.max CAD) were milled using four pairs of burs and allocated into three groups (n = 24) according to the milling sequence: 1 through 6 (1−6), 7 through 12 (7−12), and 13 through 18 (13−18). The burs were evaluated under SEM at the different milling stages to depict any degradation generated by the milling sequence. Fatigue performance was assessed by a stepwise approach (initial strength of 20 MPa for 5000 cycles; incremental steps of 20 MPa for 20,000 cycles each until fracture; frequency of 20 Hz) using the ISO 6872:2015 recommendation for piston-on-three-balls biaxial flexure strength tests. Surface roughness, topography analysis and fractography of the failed discs were also performed. Survival analysis (Kaplan-Meier and Mantel-Cox post hoc test) showed that the milling sequence had no effect on the fatigue strength (190 – 201 MPa) or the number of cycles until fracture (174,958 – 180,087 cycles). Ceramic topography and roughness (Ra, Rz and RSm parameters) were similar among the groups, even though SEM analysis depicted CAD/CAM diamond burs degraded over time. Fractography evidenced all failures starting from surface defects introduced by milling at the samples’ tensile side. The sequential usage of CAD/CAM diamond burs (milling order) does not affect the lithium disilicate surface roughness, topography or fatigue performance.