Effect Of Trimming With Diamond Burs Of Different Grits On The Bond Strength And Internal Adaptation Between Full Crown And Prepared Tooth

With the development of dental prosthodontics,the requirements from dentists and patients increased gadually.Fixed restorations,especially porcelain fused to metal crown/ bridge(PFM),have been widely applied in clinic restoration,accounting for 65%in all kinds of restorations.However,in cases of restoration failure,33.3%～41.8%of restorations are caused by bad marginal seal and poor retention.Therefore,marginal seal and retention have come to be of the two main factors inducing the failure of fixed restorations.There are numbers of methods have been applied to improve the retention and internal adaptation of fixed restorations.Among these,changing dentin surface roughness by diamond burs with different grit sizes may be an effective and simple way.Most of the relative have been focusing on the effect of etching on restorations,the effect of different luting agents on marginal adaptation between dentin and crown,and surface treatments that may improve crown retention.The effect of trimming with diamond burs of different grit sizes on the bond strength and internal adaptation between full crown and dentin has seldom been reported. The purpose of the present study is to evaluate the effect of trimming with diamond burs of different grit sizes on the shear and tensile bond strength, internal adaptation between dentin and luting agents;retention between dentin and based crowns after cementation.It may be helpful to improve the retention and adaptation of fixed restorations in clinic.The total study was divided into four experiments:Experiment 1:All the dentin plates were divided into four groups which were prepared with diamond burs of different grit sizes.The dentin surface roughness was measured and observed with SEM,respectively.Results showed that:dentin surface roughness was affected by the grit size of diamond burs applied.The smaller the grit size of diamond bur was,the lower the dentin surface roughness would be.Experiment 2:After being prepared with diamond burs of different grit sizes, all the dentine plates were divided into three subgroups and bonded with polycarboxylate cement,glass-ionomer cement,and resin-modified glass ionomer cement respectively.Specimens were then subjected to shear bond strength test. The fracture mode of each specimen was also recorded.Results showed that:the smaller the grit size of diamond bur was,the lower the shear bond strength was; with the same dentin surface roughness,the use of glass-ionomer cement and resin-modified glass ionomer cement could obtain higher shear bond strength than polycarboxylate cement.Experiment 3:According to the type of the final diamond bur used for the finishing of tooth preparation,all the prepared teeth were divided into 4 groups. All the teeth were prepared using a set of diamond burs with decreasing grit size equipped on a self-made parallel muller.A Ni-Cr alloy crown was individually fabricated.Glass ionomer cement was used to cement the crowns.Specimens were soaked into 0.5%fuchsine solution for 24h.The bonded specimens were evaluated for microleakage and internal adaptation.Results showed that:as the grit size of diamond bur for preparation decreases,the adaptation between dentin and Ni-Cr based crown increased;when using the brand of MANI diamond bur to prepare teeth ruth decreasing-sized bur until a red code,the internal adaptation between abutment and Ni-Cr based crown could meet clinical needs.Experiment 4:The specimens were prepared according to the methods mentioned in experiment 3.The impressions were made and Ni-Cr alloy crowns with rings were fabricated.Glass ionomer cement was used to cement the crowns. The tensile bond strength and the fracture mode were observed for each bonded specimen.Results showed that:no significant effect was found on the tensile bond strength between teeth prepared with diamond bur of different grit sizes and Ni-Cr based crown when bonded with glass ionomer cement;regardless of other related factors,the correlation between fracture mode and dentin surface roughness after the separation of the cemented specimens is little.Based on the results above,it could be briefly concluded that:when a final trimming with diamond burs of smaller grit size was applied to dentin surface after the regular tooth preparation,as the grit size of diamond bur for preparation decreases,(1) dentin surface roughness would decrease;(2) the shear bond strength between cements and dentin decreases;with the same dentin surface roughness,glass-ionomer cement and resin-modified glass ionomer cement obtain higher shear bond strength than polycarboxylate cement;(3) the adaptation between dentin and Ni-Cr based crown increased; (4) no significant difference was found on tensile bond strength between prepared teeth by diamond bur with different grit sizes and Ni-Cr based crown when bonded with glass ionomer cement.Based on these conclusions mentioned above,considering the retention and internal adaptation of full crown,it is suggested that when using the MANI diamond bur to prepare teeth following the grit size order to red code (53～63μm),the retention and internal adaptation between abutment and Ni-Cr based crown could meet clinical needs.