Fracture Strength And Stress Distribution Of Occlusal Veneers With Different Preparation Design

Objective(1)To evaluate the influence of the thickness and tooth preparation design on fracture strength and failure pattern of zirconia occlusal veneer by extraoral experiment.(2)To evaluate stress distribution in the occlusal veneer and dental structure according to the preparation design and restorative material by three-dimensional finite element analysis(3D-FEA).Through the mechanical analysis and three-dimensional finite element analysis of the occlusalveneer,to provides a reference for the clinical application of the occlusal veneer(the choice of materials and the design of the tooth preparation).Methods(1)The standard artificial molar were prepared into three groups;group A: the occlusal surface were reduced evenly in 0.5 mm;group B: the occlusal surface werereduced evenly in 0.5 mm,the axial surface were parceled for 1.5 mm;group C: the occlusal surface were reduced evenly in 0.5 mm,and prepared a cavity preparation.Each group was designed into occlusal veneers with different thickness of 0.5 mm,1.0 mm,1.5 mm by Dental Designer software in CAD/CAM system.It's 9 groups,8 in each group,a total of 72 occlusal veneers.Double-layer impressionsof three different designs of occlusal veneer preparation using silicone rubber were taken,and then replaced with self-curing resin.Each group consisted of 24 pieces,and a total of 72 pieces were prepared.The self-curing resin is used to embed the substitute so that the long axis of the tooth is perpendicular to the horizontal plane.The 3M self-adhesive resin cement was used to adhere the occlusal surface to thesubstitute.The breaking load of zirconia occlusal veneers was tested,and their failure pattern was recorded using universal material testing machine.(2)Combine the first artificial molar model and occlusal veneer with three different preparation designs built by 3Shape system and natural tooth model to form a three-dimensional model using Catia software.The standard artificial molar were prepared into three groups;group A: the occlusal surface were grinded evenly in 0.5 mm;group B: the occlusal surface were grinded evenly in 0.5 mm,the axial surface were parceled;group C: the occlusal surface were grinded evenly in 0.5 mm,and prepared a cavity preparation.Then,each model received different restorative materials(Lava Ultimate(LU)?Vita Enamic(VE)?E.max CAD(EM)?Lava(LA)),totaling 12 groups.The von Mises stress distribution in the tooth models resulting from static vertical and oblique occlusal forces(200 N)were evaluated using Abaqus software.Results(1)The average fracture strength values of all zirconia occlusal veneer with different thicknesses and different preparation forms were,1275.9±205.6 N(0.5 mm),2320.6±296.7 N(1.0 mm),4909.1±399.3 N(1.5 mm)for group A;1404.5±215.3 N(0.5 mm),2509.8±380.6 N(1.0 mm),4917.8±320.4 N(1.5mm)for group B;1279.6±248.4 N(0.5 mm),2346.6±260.2 N(1.0 mm),4575.3±458.7 N(1.5 mm)for group C.With the increase of the veneer's thickness,the fracture resistance of the occlusal veneer was also increased.In the same form of the preparation,the fracture resistance of occlusal veneer with different thickness was significantly different.However,in the same thickness but different prepare forms the fracture resistance shows no difference.The failure pattern in 0.5 mm and 1.0 mm is similar,but different in 1.5 mm.(2)The von Mises stress values in group C was less than group A and group B.The peak stress values generated by vertical occlusal force generated in occlusal veneer of BLA was 263 MPa.The peak stress values generated by vertical occlusal force generated in dental structure of BLU was 32.5 MPa.The peak stress values generated by oblique occlusal force were higher than vertical occlusal force,which in occlusal veneer of ALA was 4430 MPa,in dental structure of ALU was 221.1 MPa.The restorative material influenced stress concentration,which on occlusal veneers decreased according to the sequence LA > EM > VE > LU,but on dental structure decreased according to the sequence LU> VE > EM >LA.Conclusions(1)The occlusal veneer's thickness does influence its fracture strength.But these fracture strength ofocclusal veneer in same thickness were similar regardless of preparing modes.0.5 mm is an acceptable thickness in zirconia occlusal veneer for clinical application(2)The peak von Mises stress values of group C with captive preparation were lower than group A and group B on occlusal veneers and dental structure to vertical and oblique occlusal forces.Oblique occlusal forces is one of the risk factors of occlusal veneer and dental structure.The effect to stress distribution of restorative material is more efficient than tooth preparation design.Ceramic materials with high elastic modulus tend to concentrate the stress inside the material,thus reducing the stress transferred to the remaining dental structures.