Effect of CAD/CAM Bur Deterioration on the Surface Roughness of Ceramic Crowns

To evaluate the effect of repetitive milling on the surface roughness of ceramic crowns.

Twenty feldspathic ceramic crowns (VITABLOCS Mark II; Vita Zahnfabrik, Bad Säckingen, Germany) milled with hard milling protocol, and 20 zirconia crowns (5Y-TZP, inCoris TZI; Dentsply Sirona, York, PA) milled with soft milling protocol, using a 5-axis milling machine (inLab MC X5; Dentsply Sirona) were investigated. Atomic force microscopy (SPM SmartSPM™-1000) was used for surface roughness measurements (Ra, RMS). Pearson correlation between bur deterioration percentage as provided by the software of the milling machine and surface roughness was performed with an (r) range between 1 and -1. Trend analysis between the 2-half and among the 4-quarter bur lifetime was performed with linear contrast analysis and partial eta squared (η) for estimate of effect size. Analysis of mean variance between the zirconia and feldspathic ceramic groups was performed with a t-test, with α = 0.05.

Correlation analysis showed small correlation for Ra, RMS (r = -0.02, -0.16 for the feldspathic ceramic crowns and r = 0.044, 0.034 for the zirconia crowns), and no significant differences for the 2-half and 4-quarter groups (p > 0.05) for both crown types. Estimate of effect size was rated as medium for the 2-half groups (Ra = 0.420, RMS = 0.395) and large for the 4-quarter groups (RA = 0.22, F = 1.48, RMS = 0.19, F = 1.26) for the feldspathic ceramic crowns, and small effect size for the 2-half groups (Ra = 0.18, RMS = 0.20) and the 4-quarter groups (Ra = 0.035, RMS = 0.037) for the zirconia crowns. There was no significant difference (p = 0.178, F = 0.016. p = 0.088, F = 0.002) between the feldspathic ceramic and zirconia crowns with medium and large effect size (0.45, 0.57) for RA and RMS.

Milling zirconia and feldspathic ceramic crowns had small correlation between bur deterioration and surface roughness when the burs were used within their recommended lifetime. However, hard milled feldspathic ceramic crowns had higher surface roughness than the soft milled zirconia crowns. Bur deterioration had a larger effect on the surface roughness of the feldspathic ceramic crowns than the zirconia crowns.