The Effect Of Different Optical Impression Methods And Finish Line Type On Marginal Adaption Of All-ceramic Crown

Objective:To study the effects of different optical impression methods in CAD/CAM inlabsystem and finish line type on marginal adaption of Zirconium dioxide all-ceramiccrown，and to provide references for clinical tooth preparation and impressionpattern.Methods:sixteen right mandibular first molar in the standard model were designed andprepared. Based on finish line, they were divided into4groups (n=4): shoulder(Group A), shoulder with bevel (Group B), chamfer (Group C), and chamfer withbevel (Group D); direct method, silicone rubber impression and indirect method forCAD/CAM inlab system were used to carry out optical impression and manufacture48zirconia dioxide all-ceramic crown. The marginal fit of the crowns was evaluatedwith a replica technique and measured with a travelling microscope. The mean fullcrown marginal discrepancy＜100μm was clinically acceptable. SPSS17.0was usedto conduct statistical analysis of the experimental data.Results:In the three impression groups, direct scanning (control group)＜siliconerubber scanning (silicone rubber group)＜indirect scanning (die stone group). Thefull crown marginal discrepancy of the control group was obviously lower thesilicone rubber group and the die stone group, with a statistically significantdifference (P＜0.05).Compared with impression mode, finish line showed greater influence on fullcrown marginal discrepancy. The results indicate increasing trend of mean full crownmarginal discrepancy in chamfer with bevel＜shoulder with bevel＜chamfer＜shoulder. There were significant differences in mean full crown marginal discrepancybetween the chamfer with bevel group and the other three groups (P＜0.01). Thediscrepancy thickness in the finish line area of the shoulder group was even, whilethat of the chamfer group gradually tended to increase from the corner of the die axialplane to the margin, maximizing on the margin and exceeding the clinically acceptable limit of100μm.Conclusions:①in CAD/CAM inlab system, silicone rubber impression and die stoneimpression has a certain degree of "distortion", but their accuracy are within clinicallyacceptable range.②the chamfer with bevel group has the best full crown fit, and bevel caneffectively reduce full crown marginal discrepancy.