Diamond Grit Effect on Ceramic Cutting Rate and Surface Roughness

Objectives: To investigate cutting rate and surface roughness of three machinable dental ceramics using diamond burs with different grits.
Methods: Three dental ceramics used in this study: zirconia (TZ-3YSB-E, TOSOH)(ZR), lithium di-silicate (IPS e.max CAD, Ivoclar)(EM), and feldspathic porcelain (VitaBlocs TriLuxe Forte, Vita)(TF). Komet coarse and medium diamond burs were used with 1:5 NSK handpiece attachment and Midwest e-stylus electric motor at 40,000 rpm and water coolant spray. Every bur made 3 cuts, each lasting 240 sec for ZR and EM, and 120 sec for TF. The forces used were 1.96N, 1.47N and 0.98N for ZR, EM, and TF respectively. Bur cutting distance was recorded every 0.1 sec using LVDT (Omega) and cutting rate (mm/sec) was calculated later. Surface roughness (Ra) was measured by profilometer (Surftest SJ-201, Mitutoyo). Data was analyzed using two-way ANOVA, Tukey post hoc, and student’s t-test as needed at (α=0.05).
Results: Mean cutting rate and SD are presented in Graph I. Two-way ANOVA of grit and cut number indicates a significant effect of both on cutting rate of TF (p<.0001) and EM (p<0.001). However, only cut number had an effect on cutting rate for ZR (p<.0001). Tukey post hoc test revealed a significantce diference between cuts 1 vs 2 and 1 vs 3 for TF, 1 vs 3 and 2 vs 3 for EM, and between all ZR cuts. Graph II shows surface roughness results. T-test revealed no significant difference in surface roughness between the two grit sizes for TF or EM groups (p>.05). Coarse grit diamonds produced significantly rougher surface for zirconia (p<.0001).
Conclusions: Increasing diamond grit size improved cutting rate significantly in porcelain and lithium di-silicate but not in zirconia. Cutting rate decreased with repeated use. Surface roughness is material and grit dependent.