| Objective:To study the influences of different zirconium materials with different occlusal surface thickness to the compressive strength of three units fixed bridge on the posterior teeth, and then provide theoretical basis for the choice of clinical restorative materials and suitable restoration thickness.Methods:1 Teeth preparationThe standard dental models(Nissin Dental Products, Inc., Japan) were scanned, designed and prepared suing the CAD system(3 Shape Abutment Designer). The prosthodontic treatment was defined as the right upper first molar missing with a treatment plan of three-unit fixed bridge. Abutments preparation standard was of 1.5mm teeth cutting for occlusal surface functional sharp and 1.0mm for non-functional sharp, 8o convergence angle of four axial surface, 0.5mm shallow concave shoulder in parallel with gingival margin, no under cuts and stress concentration area.2 Fabrication of preparation dieThe designed 3D teeth preparation data files were imported to the CAM milling machine. The dedicated cutting wax were used to process 60 wax preparatory models in a 1: 1 ratio, following with the process of general embedding, casting, grinding and sandblasting.3 Fabrication of restorationThe preparation die were fixed on the measurement platform. The scanned 3D preparation data were imported to 3Shape Dental Designer of CAD system which will automatically generate restoration standard anatomy. Based on the form description of right maxillary second premolar, first molar and second molar in the book of Oral Anatomy and Physiology, a virtual knife trimmed the crown with different thickness of 0.5mm、0.8mm and 1.0mm required in this study. The data were transferred to CAM cutter bar and 18 full-zirconium fixed bridge were fabricated by Upcera, Adit and Du selam prefabricated ultra-through zirconia green body respectively. The sample groups can be illustrated as:group A1(thickness 0.5mm+Upcera),group A2(thickness 0.8mm+Upcera),group A3(thickness 1.0mm+Upcera);group B1(thickness 0.5mm+Adit),group B2(thickness 0.8mm+Adit),group B3(thickness 1.0mm+Adit);group C1(thickness 0.5mm+Du selam), group C2(thickness 0.8mm+Du selam) group C3(thickness 1.0mm+Du selam).The control group(D) consists of 6 cobalt-chromium ceramic fixed bridges with the occlusal surface thickness of 1.5mm. Six substrate fixed bridge wax models were fabricated with the same method mentioned above, then embed and cast cobalt-chromium ceramic fixed bridge stent, paste opacifier, dentine porcelain and enamel porcelain following with grinding, polishing and glazing to complete the fabrication process.4 Compressive strength testThe zirconium fixed bridge was fastened to the preparation die by the Rely Xunicem self-adhesive resin cement, and the cobalt-chromium alloy ceramic fixed bridge was fixed by the 3M glass ionomer cement. After 24 hours’ standing, the compressive strength of test-pieces would be tested using a Universal Testing Machine.5 Statistical analysisThe SPSS19.0 statistical software was used to analyze the experimental data. The method of “mean square deviation” was used to analyze the compressive strength of different materials and different zirconium occlusal surface thickness.Results:1 The data of compressive strength of zirconium fixed bridge groups were summarized belowA1 group:(3085.90 ±475.69)N,A2 group:(4382.77 ±262.13)N,A3 group:(5629.88±321.53)N,B1 group:(2904.60±439.01)N,B2 group:bridge and the cobalt-chromium ceramic, we recommend a occlusal thickness being equal or greater than 1mm as the appropriate choice for pure zirconium fixed bridge.2 The types of zirconium have little impact on the compressive strength, while the occlusal thicknesses play a key role on the compressive strength. There is a linear positive correlation between the pure zirconium fixed bridge and the compressive strength.3 The fracture mechanism of pure zirconium fixed bridge differs from that of cobalt-chromium ceramic fixed bridge. The connector is the weak point of pure zirconium fixed bridge.(4236.07±273.63)N,B3 group:(5441.58±321.85)N,C1 group:(3012.13±299.19)N,C2 group:(4301.70±370.40)N,C3 group:(5595.13±314.49)N.(The compressive strength of cobalt-chromium alloy ceramic fixed bridge:(5527.35±320.52)N).2 Data analysisFor any brand of zirconium, significant differences were found betweenthe crown thickness and the compressive strength(group A vs group B,P<0.05; group A vs group C, P<0.05; group A vs group C, P<0.05). Thecompressive strength significantly increased in fixed bridges with thickerocclusal surface. Three kind of zirconium involved in this study did notexpress significant differences when were analyzed on the same thickness.The 0.5mm and 0.8mm thickness zirconium exhibited lower compressivestrength than the control(P<0.05) which had same strength with the 1.0mmzirconium(P>0.05).3 Fracture observationFor the 18 0.5mm thickness zirconium, the mesiodistal longitudinal fracture occurred in three, pontic micro crack in three and connector fracture in 12. In the 0.8mm group, two of 18 samples with mesiodistal longitudinal fracture, two with pontic micro crack, and 14 with connector fracture were seen. There were two mesiodistal longitudinal fracture, two pontic micro crack, and 14 connector fracture were observed in all 18 1.0mm zirconium samples. While in all the 6 samples of control(cobalt-chromium ceramic fixed bridge), only porcelain layer crack occurred in each.The full-zirconium fixed bridge always presented all-ceramic layer damage. When it happened, the crack firstly appeared in the lower part of the connector, and then extended to the load point resulting in fracture damage finally. The cobalt-chromium metal ceramic fixed bridge showed only decorative porcelain destruction. The crack originally occurred beneath the load-point and then spread away from the load-point along the pit and fissure.Conclusion:1 One the basis of a contrastive analysis to the pure zirconium fixedbridge and the cobalt-chromium ceramic, we recommend a occlusal thickness being equal or greater than 1mm as the appropriate choice for pure zirconium fixed bridge. 2 The types of zirconium have little impact on the compressive strength, while the occlusal thicknesses play a key role on the compressive strength. There is a linear positive correlation between the pure zirconium fixed bridge and the compressive strength. 3 The fracture mechanism of pure zirconium fixed bridge differs from that of cobalt-chromium ceramic fixed bridge. The connector is the weak point of pure zirconium fixed bridge. |
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