Effect of CAD-CAM tool deterioration on the trueness of ceramic restorations.

STATEMENT OF PROBLEM: Tools will gradually deteriorate with repetitive milling. However, tool lifetime can vary depending on the type of milling machine, the hardness of the ceramic material, and the size of the restoration. Studies evaluating the effect of tool deterioration on the trueness of milled restorations are lacking. PURPOSE: The purpose of this study was to evaluate the effect of tool deterioration on the trueness of milled restorations. MATERIAL AND METHODS: A patient requiring a ceramic crown was recruited. Repetitive milling of zirconia crowns (inCoris TZI; Dentsply Sirona) with tungsten carbide rotary instruments and glass-ceramic crowns (VITABLOCS mark II; Vita Zahnfabrik) with diamond rotary instruments was performed by using a 5-axis milling machine (inLab MC X5; Dentsply Sirona) until the machine software program indicated replacing the largest tool. The percentage of deterioration for the largest tool was recorded before each milling. The trueness for different crown areas was evaluated by using a 3-dimensional evaluation software program (Geomagic Control X; 3D systems), and the crowns were clinically evaluated. The crowns were grouped into 4 quadrants as per the deterioration percentage. The Pearson correlation (r) between the deterioration percentage and trueness and linear contrast analysis (partial eta squared [η]) among the groups were performed. RESULTS: Glass-ceramic crown areas had low correlation except for mesial (high, r=0.63, high, η=0.37) and distal (medium, r=0.42, high, η= 0.34) areas, whereas zirconia crowns had low correlation for inner and internal areas and medium to high correlation for mesial (r= -0.31, η=0.33), distal (r= -0.53, η=0.36), occlusal (r= -0.32, η=0.164), external (r= 0.69, η=0.52), and marginal (r= -0.44, η=0.19) areas. Zirconia crowns had excellent marginal fit and occlusal contact and light to excellent proximal contact, whereas glass-ceramic crowns had excellent marginal fit, minor to major occlusal correction, and heavy proximal contact. CONCLUSIONS: The effect of tool deterioration on trueness depended on material type and crown area. Only proximal areas were affected in the glass-ceramic crowns, whereas external, proximal, occlusal, and marginal areas were affected in the zirconia crowns. Trueness did not necessarily reflect the clinical quality of the crown, as both crown types were clinically satisfactory. Although zirconia crowns were overmilled in comparison with the reference design and glass-ceramic crowns, they had better clinical quality.

Effect of Milling Protocols on Trueness and Precision of Ceramic Crowns.

PURPOSE: To investigate the effect of different milling protocols for different ceramic materials on the trueness and precision of milled ceramic crowns. MATERIALS AND METHODS: A definitive impression from a patient requiring a ceramic crown was used to fabricate forty glass ceramic crowns (VITABLOCS mark II, Vita Zahnfabrik) milled with wet, hard milling protocol, and 40 zirconia crowns (inCoris TZI, Dentsply, Sirona) milled with dry, soft milling protocol, using a 5-axis milling machine (inLab MC X5; Dentsply Sirona). Trueness and precision for different crown areas were evaluated with a 3D evaluation software (Geomagic Control X, 3D systems). Statistical analysis was performed between the 2 crown types with independent t-test, and analysis of variances followed by Bonferroni tests among crown areas for each crown. RESULTS: All areas of the glass ceramic crowns had higher trueness than the zirconia crown areas (p < 0.05). Crown areas of each crown type had significantly different trueness values (p < 0.001, F = 175.17 for glass crowns, p < 0.001, F = 35.04 for zirconia crowns). Glass crowns had 3.78 µm precision, while zirconia crowns had 4.12 µm precision, with a precision difference range between the 2 types of crowns from 0.32 µm for the inner surface to 6.5 µm for the marginal surface. CONCLUSIONS: Type of ceramic material and milling protocol affected trueness of the milled crown, with higher trueness for the hardmilled glass crowns than the soft milled zirconia crowns. Soft milled zirconia crowns were generally overmilled in comparison with the reference design and glass crowns. However, glass crowns exhibited more undermilling in thin and deep areas. The crown area affected trueness, with external areas having better trueness than internal areas. The 5-axis milling machine had high precision, which was minimally affected by the ceramic type, milling protocol, or crown area.