The Impact Of Different Occlusal Thickness And Materials On The Compressive Strength Of Full-contour Zirconia Crowns

Objective:This experiment aims to study the influences on the compressive strength of full-contour zirconium oxide crowns with different occlusal surface thickness.Compare the impacts on the crown’s compressive strength between different manufacturer and different processing methods.Meanwhile,simulate the alternated cooling and heating microenvironment in the mouth,compare the changes of the crown’s compressive strength after the thermocycling tests.Then provide a theoretical basis of the suitable restoration thickness and choices of clinical restorative materials for the full-contour zirconium oxide crowns.Methods:1 Fabrication of preparation dieChoose the standard dental models of the maxillary first molar on the right side(Nissin Dental Products),scanned the model through the Abutment Designer software in the CAD system.Refer to the basic requirements of tooth preparation in the Prosthodontics textbook,design the shape of the preparation.Abutment’s preparation standard was of 1.5mm teeth cutting for occlusal surface functional cusp and 1.0mm for non-functional cusp,8o convergence angle for four axial surface,0.5mm shallow concave shoulder in parallel with gingival margin,no undercut and stress concentration area.Pass the designed 3D preparations’ data files to the CAM milling machine.Using dedicated cutting wax to process 90 wax preparations in 1:1 ratio,following with the process of general embedding,casting,grinding and sandblasting.Then we can get 90 metal preparations.2 Fabrication of full-contour zirconium oxide crownsUsing the plasticine to fixed the preparation die on the measurement platform.Scanning the 3D data of the preparations.Import the information to the Dental Designer software in CAD system which will automatically generate a restoration with standard anatomy.Finish uniforming the occlusal surface thickness to 0.5mm,0.8mm and 1.0mm by using of a virtual knife.Transferred the final data to CAM milling machine.Using dedicated cutting wax to cut out the wax crowns according to the final data.Generally embedding and casting,so we can obtain the Co-cr metal crowns.Put these metal crowns and the prefabricated ultra-through zirconia green body(Upcera)on the manual cutting machine.Grinding in 1:1 ratio,then we can receive 6 full-contour zirconium oxide crowns in the thickness of 0.5mm,0.8mm and 1.0mm respectively.Setting these crowns for group A1,A2 and A3.Choose the prefabricated ultra-through zirconia green body fabricated by Upcera and Duselam to accomplish 72 full-contour zirconium oxide crowns.The sample groups can be illustrated as:group B1(thickness 0.5mm+Upcera),group B2(thickness0.8mm+Upcera),group B3(thickness1.0m m+Upcera),group C1(thickness0.5mm+Duselam),group C2(thickness0.8mm+Duselam),group C3(thickness1.0mm+Duselam),group D1,D2,D3(thickness0.8mm+Upcera),group E1,E2,E3(thickness 0.8mm+Duselam).Each group have 6 samples.3 The thermocycling testsPut group D and E in the thermostatic water bath under 5℃and 55℃for 30 s,the interval is 5s.Then take them out after cycling 2500 times,5000 times,10000 times respectively.4 Adhesion and compressive strength testBond the zirconium crowns to the preparation die by the Rely Xunicem self-adhesive resin cement respectively.Sustained pressing for 3 minutes.After 24 hours’ standing,take the crowns to the Universal Testing Machine having compressive strength tests.Record the force when the crown is broken.5 Statistical analysisUsing SPSS19.0 software for data analysis,mean square deviation with analysis for variance.Analyze the compressive strength of different materials and different zirconium occlusal surface thickness,when P<0.05 for difference was statistically significant.Results:1 Visual inspectionThe full-contour zirconia crowns with different thickness present the similar failure modes:rupture of the whole porcelain layer.The crack’s trend has similarity too.The thicker crown of the full-contour zirconia crowns present more fracture pieces.The crack firstly appeared in the load point and then spread along the pit and fissure.2 The compressive strength mean value of full-contour zirconia crowns group A1:(1055.95±122.45)N,group A2:(1518.55±188.29)N,group A3:(2087.43±187.39)N,group B1:(1287.52±156.14)N,group B2:(1936.37±222.11)N,group B3:(2400.68±235.47)N,group C1:(1340.65±133.03)N,group C2:(1826.50±290.78)N,group C3:(2604.95±187.61)N,group D1:(1834.00±219.09)N,group D2:(1785.03±214.53)N,group D3:(1562.22±314.82)N,group E1:(1820.37±178.58)N,gr oup E2:(1683.50±180.90)N,group E3:(1590.65±102.25)N.3 Data analysisThe crowns produced by same materia shows significant differences between different thickness(P<0.05).The comparison between any two of them has statistical significance.The crowns produced by same company,using different methods to produce,the compressive strength shows statistical significance(P<0.05).For the crowns with same thickness,the force between different resources has no significant difference(P>0.05).The compressive strength has no significant difference after the thermocycling experiment(P>0.05).Conclusion:1 The brand of zirconium have little impact on the compressive strength,the thermocycling experiment either.2 Different processing method have an impact on the compressive strength.The crowns produced by CAD/CAM are more superior than traditional ways.3 The occlusal thickness has an great influence on compressive strength of the full-contour zirconia crowns.The fracture load increased with the thickness of crowns increased.We suggested that the thickness of full-contour zirconia crowns should be equal to 1.0 mm or beyond it in the clinical use.