An Experimental Study On The Fit Of Co-Cr Basal Crowns Fabricated By Different Technology

Objective: This study aims to compare the marginal and internal fit of Co-Cr basal crowns fabricated by four technologies,traditional casting technology,CAD/CAM cutting wax casting technology,CAD/CAM cutting metal technology and direct metal laser sintering technology.Methods:1 Dies preparationUsing three-dimensional parameterized modeling software Solid Works designed data model of mandibular first premolars preparation.The details of design:the height of the crown core was 4 mm,the diameter was 8 mm,and the height of the tooth preparation was 6 mm,and the occlusal convergence angle was 6°,and the shoulder was 135°chamfer margin,The width of the shoulder was 1mm,no sharp edges.Design a 1mm diameter,0.5mm deep groove in the axile wall as the logo of cutting position.Import the model data into the rapid prototyping systerm(Projet 3510 MP, 3D Systerm, American),and laser curing 24 resin dies.Divide 24 dies into A,B,C,D groups with completely random method.Each group has 6 dies numbered A1-A6,B1-B6,C1-C6,D1-D6.2 Co-Cr basal crowns fabricationCo-Cr basal crowns of group A were made by traditional casting technology.Dies A1-A6 coated paraffin oil as a separating agent.Wax models were made on the six dies with BEGO wax,and the thickness was 0.5mm. Place sprue on the axis angle of crowns,and marked according to A1-A6.Then use investing and casting method to make six Co-Cr basal crowns.Then the embedding materials were cleaned by sand blasting.Co-Cr basal crowns of group B were made by CAD/CAM cutting wax and casting technology.Spray powder on the surface of dies B1-B6 evenly,then collect the optical impression and design the corresponding three-dimensional data models of basal crowns.The thickness of the edge gap material was 10μm,the thickness of the gap material above the neck edge was 40μm,the thickness of the basal crown was 0.5mm.According to the data models of B1-B6,the CAD/CAM systerm cut Wieland wax block and then use investing and casting method as group A to make six Co-Cr basement crowns.Then the embedding materials were cleaned by sand blasting.Co-Cr basal crowns of group C were made by CAD/CAM cutting metal technology.Spray powder on the surface of dies C1-C6,then collect the optical impression and design the data models as group B.According to the data models of C1-C6,the CAD/CAM systerm cut BEGO Co-Cr block and made six Co-Cr basal crowns.Co-Cr basal crowns of group D were made by direct metal laser sintering technology.Spray powder on the surface of dies D1-D6,then collect the optical impression and design the data models as group B.According to the data models of D1-D6,EOS M270 systerm laser sintering BEGO Co-Cr powder and made six Co-Cr basal crowns.Four groups of basal crowns blasted bym the 100-μm aluminum oxide particles for 20 s under 0.2MPa pressure,the distance between the nozzle and the crown was 2cm.Then all crowns were ultrasonically cleaned in distilled water sample 30 s and made them dry fully.3 The cementation of basal crownsAccording to the product instructions standard spatulated glass ionomer cement,cemented the crowns to the corresponding dies,remove excess cement and these crowns were stored at room temperature for 24 hours.4 Basal crowns and dies embedding and cuttingThe basal crowns and dies were embedded in methyl acrylic resin,the size was 1.2×1.2×1.2cm,and then were cut open along a long axis of the dies.The sectioned surfaces were polished by consecutive grinding with 240#,400#,600 #, and 800# silicon carbide paper.5 Stereoscopic microscope measuring the thickness of the cement layerThe thickness of the cement layer on the section was measured with a stereoscopic microscope at ×100 magnification.The measurement points were as follows:the margin of the crown,0.1mm distance from the margin,the center point of the shoulder,the center point of the axial wall,and the occlusal.Each point was measured 3 times by a single investigator and the mean value was determined6 Statistical analysisExperimental data were analyzed with SPSS13.0 statistical software.ANOVA test was chosen if the data was normally and the variances were the same.Ranking test was chosen if the data was not normally or the variances were not the same.Inspecting standard ɑ=0.05.Results:1 Marginal gap measurement results of four groups:Group A:66.08±3.90μm,Group B:53.42±2.76μm,Group C:29.21±3.31μm,Group D:24.96±2.99μm,there was statistically significant difference in marginal fit between four groups(P<0.01).2 Internal gap measurement results of four groups Internal fit:Group A:114.10±27.66μm,Group B:105.26±37.99μm,Group C:73.69±31.31μm, Group D:79.89±33.63μm,there was statistically significant difference in marginal fit between group A and C,A and D,B and C,B and D(P<0.01),and there was no statistically significant difference in marginal fit between group A and B,C and D(P>0.05).3 Different position measurement results of the same group:The largest is the occlusal gap,secondly is the axial gap,thirdly is the shoulder gap,the margin gap is minimum.Conclusions:1 The group of DMLS technology has the best marginal fit,the group of CAD/CAM cutting Co-Cr metal technology has the best internal fit.The marginal and internal fit of the group of traditional casting technology is the worst.2 The marginal fit of all crowns is less than 120μm,the internal fit of all crowns is less than 200μm.All of them satisfy the clinical requirements.