Comparative Study On Compressive Strength Of Different Thickness Occlusal Veneers Fabricated From Heat-pressed Ceramic Of Posterior Teeth

Objectives: To explore the relationship between thickness of the occlusal veneers and compressive strength and the clinical feasibility of ultra-thin occlusal veneers and provide theoretical basis for the clinical application of ceramic material and design of the restoration through comparing the compressive strength on different thickness of occlusal veneers fabricated from IPS e.max Press.Methods:1 Specimen preparationTwenty-four extracted human maxillary premolars were collected from Oral and Maxillofacial Surgery in Hebei medical university within a month.Standardized tooth preparations replicating a severely worn occlusal table were accomplished.Cuspal inclination was kept as constant as possible by maintaining the buccal and palatal margins at approximately 5.0 mm from the CEJ and 2.5 mm above the central groove.Flattening the surfaces in the same direction by grinding on 400#,600#,800#,1000# sandpapers after tooth preparation.The specimens were randomly divided into 4 groups based on the thickness of the restorative material.The in vitro teeth were taken impressions using silicone rubber and made superhard plaster models after preparation.The preparations were coated with plaster harden agent,coating gap and wax separation agent.Wax patterns were 0.5mm,1.0mm,1.5mm,2.0mm.Error was controlled at ±0.01 mm.Placing sprue after finishing the wax pattern.Occlusal all-ceramic restorations were fabricated from lithium disilicate ceramic blocks(IPS e.max Press).Then the embedding materials were cleaned by sand blasting using aluminum oxide particles under 0.3MPa pressure.Observing specimen surface with 10×magnifying glass in order to ensure there is no casting defects.The restorations were polished,ultrasonically cleaned and dried.Restorations were adhesively bonded with panavia F.24 extracted human maxillary premolars were embedded in autopolymerizing acrylic resin 3mm apical to the cementoe-namel junction(CEJ)using a cylindrical plastic mold,20 mm in height and 15 mm in diameter.They were kept in 37 ?constant temperature water-bath for 24 hours and spared.2 Test of compressive strengthThe specimens were located on the Instron testing machine in sequence.All specimens were loaded on occlusal surface of each veneer vertically by 30 KN pressure sensor and a 6mm pressure head,at the speed of 0.5mm per min,and then performed destructive test to determine the compressive strength,to test the most strong fracture force values,and record.3 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.If not,ranking test was chosen.Inspecting standard ?=0.05.4 The scanning electron microscope(SEM)observationSelect one specimen randomly from each group for scanning electron microscopy and observe the microscopic structure and form of the fracture surface.Results:1 The destructive values in all veneers of different groups were as follows: Group A :(1099±151)N,Group B :(1745±171)N,Group C :(1951±190)N,Group D :(2847±303)N.2 Analysis of variance(ANOVA)results indicate that there are significant differences of destruction test values on specimens of different thickness(P<0.01),multiple comparison results indicate that significant difference in destruction test values exist between group A and B,A and C,A and D,B and D,C and D(P<0.05);no significant difference between group B and C(P>0.05).3 The scanning electron microscope(SEM)observationGroup A: The surface is uneven and has no obvious crack.There are lots of funicular protuberance and concave structure,radiated distribution.A few tiny pores with diameter of 20?m are scattered.Group B: The surface is uneven and has obvious pores with diameter of 100?m.There is much ridge of protuberance of radiated distribution.Group C: The surface is uneven and has a long and irregular crack with length of 600?m and a few tiny pores of different sizes.Group D: The surface is uneven.There is much ridge of protuberance.A few tiny pores with diameter of 10?m-100?m are scattered.Conclusions:1 The compressive strength of the group of 0.5mm is much smaller than that of the other three groups(1.0mm,1.5mm,2.0mm),and the compressive strength of the group of 2.0mm is much bigger than that of the other three groups(0.5mm,1.0mm,1.5mm).There was no significant difference of the compressive strength between the groups of 1.0mm and 1.5mm.2 The ultra-thin occlusal veneers(0.5mm)can satisfy the clinical requirements.