The Effect Of Different Occlusal Thickness On The Compressive Strength Of Two Lithium Silicate Glass Ceramics

Objective: This experiment mainly study the different occlusal thickness influence on the compressive strength of no decoration porcelain disilicate Lithium glass ceramic crowns,at the same time simulated oral environment,compare the changes of the lithium silicate glass ceramic crown's compressive strength after thermal cycling.To provide a theoretical basis and guidance of the suitable occlusal restoration thickness.Methods:1 Fabrication of preparation dieThe maxillary first molar on the left side(Nissin Dental Products)was chose as the standard dental models,using CAD Abutment Designer software scan the system model.According to the sixth edition of prosthodontics textbook in posterior crown reserve requirements and Ips e.max CAD preparation of tooth preparation : 1.5 mm to 2.0mm teeth cutting for occlusal surface,four axis surface each grinding 1.5mm,and gathered 2 to 5 degrees.The inner edge of the concave shoulder prepared a rounded,shoulder width 1mm,line angle obtuse,remove the stress concentration zone.In order to ensure the consistency of the occlusal thickness and better simulate clinical,the experimental die prepared into two inclined planes.Then the designed preparation of 3D model data transfer to CAM cutting machine,with the ratio of 1:1 cutting casting special wax,processing to form 36 preparation of wax type,then the lost wax casting finished 36 cobalt chromium metal model preparation.2 Fabrication of full-IPS e.max CAD/CAM crownsThe experiment is divided into two parts:the first part is to test the change of the compressive strength caused by the change of the thickness of the full crowns,according to the pre-experimental force value,the 18 all ceramic crowns were divided into A,B,and C three groups,six samples in each group;the second part is based on the first part of the effective value of the occlusal surface thickness 1.5mm as the standard,the 18 all ceramic crowns were divided into D,E and F three groups,six samples in each group,carries on 10 thousand times,20 thousand times,30 thousand times thermal cycling.Scan 36 metal preparation generation respectively with inEos Blue scanner,transmit the data to the CAM system software,control the minimum thickness of crowns,then Dental Designer software in CAD system which will automatically generate a restoration with standard anatomy,adjust the thickness of crown with virtual carving knife,finish uniforming the occlusal surface thickness to 1.0mm?1.5mm and 2.0mm crowns.Puting the IPS e.max CAD LT A3/C14 low permeability blocks in Sirona inLab MC XL ceramic cutting system,finished 36 all ceramic crowns.3 The bonding of e.max CAD ceramic crownsThe prepared crowns by Varolink N resin adhesive bonded to the generation die.Operate strictly according to the manufacturer's instructions:HF acid etching,silane coupling,resin adhesiving,adding in 37? saline preserved 24 h set aside.4 The thermocycling testsThe parameters of thermal cycling machine is set to 5 ?and 55 ?,every water bath stop 30 seconds.Then D,E,F three groups of crowns were placed in thermal cycling machine swing bucket were cycled 10 thousand times,20 thousand times,30 thousand times and respectively equivalent in oral 1 year,2 years,3 years.5 Compressive strength testThe e.max CAD crowns are successively placed into the Universal Testing Machine.With a diameter of 6mm spherical loading head loaded on the crown at the tip of the function(Buccal tip),the loading speed set 1mm/min,having the maximum loading strength test.Record the force until the crown is broken.6 Statistical analysisThe single factor analysis of variance in SPSS 21 software was used to analyze the compressive strength of all ceramic crowns in all groups,comparison between the two groups using LSD test,when P<0.05 for difference was statistically significant.Results: 1 Fracture force value Group A:(1570.17±271.16)N,group B:(2445.00±354.95)N,group C:(2646.00±250.06)N,group D:(2288.33±188.30)N,group E:(2151.00±199.19)N,group F:(2037.50±113.85)N;2 Fracture mode The model of full crown fracture was divided into groups by Burke method.The e.max CAD full crown fracture form can be divided into the following forms:?,slight fracture or apparent crown fracture,4 crowns;?,less than 1 /2 crown fracture,12 crowns;?,1 /2 crown displacement or loss,9 crowns;?,greater than 1 /2 crown loss,8 crowns;?,severe fracture or complete fracture of crown,3 crowns;3 Data analysis The IPS e.max CAD crowns have a significant difference in the experimental force value with different thickness(P<0.05).LSD test can be known: group A compared with the group B and C,the difference was statistically significant,but there was no significant difference in the experimental force value between B and C groups.The same thickness,0 times,10 thousand times,20 thousand times,30 thousand times after the thermal cycling experiment,the compressive strength has a significant difference(P<0.05).LSD test can be known: there was no significant difference between the 0 and 10 thousand experiments,0 times and 20 thousand times,0 times and 30 thousand times the value of the experiment was statistically significant difference,there was no significant difference between the 10 thousand and the 20 thousand and the 30 thousand experimental values.Conclusions:1 The different occlusal thickness of e.max CAD crowns has a great influence on compressive strength.As the thickness increases,the compressive strength increases.According to the experimental force value and the reference data can be known: the e.max CAD with the occlusal thickness of 1.0mm and the force value of 1570 N can be used for the crown of anterior teeth and premolar.But when repairing posterior teeth,it is suggested that the thickness of occlusal surface is not less than 1.5mm.2 A certain number of thermocycling have a significant effect on the compressive strength of the material,with the increase of the number,the compressive strength of the material decreases gradually.