Determining The Temporal Development Of Dentin-Composite Bond Strength During Curing

Light-cured resin-based composites (RBCs) was the most widely used dental materials for the repair of dental caries, crown fractures, and tooth wear. However, polymerization shrinkage, which can lead to marginal gaps and, ultimately, failure of the restoration through the development of secondary caries, remains a major drawback of RBCs.It is noteworthy that there is a competition at the composite-tooth interface between bond formation and development of shrinkage stress. Even though the final bond strength may be higher than the final shrinkage stress, it is entirely possible that debonding can still happen when, for example, shrinkage stress is developing much faster than the tooth-composite bond. To avoid debonding, therefore, the developing bond strength must be higher than the developing shrinkage stress at all times. However, there is limited studies available about the speed of bond formation.The objective of this study is to determine the time-dependent formation of the dentin-composite bond by measuring the bond strength at different time points during light curing. The possible effects on the rate of bond strength development will also be investigated.Part I Kinetics of bonding formation of dentin-cured composite assemblyIn order to exclude unwanted factors caused by uncured composite. A dentin-cured composite specimen was assembled. And its rate of bond formation was investigated. Methods:A thin layer of uncured composite was placed between a cured block of the same composite and a bovine dentin slap with a layer of adhesive pre-cured on its surface. The assembly was cured through the composite block using a LED curing light. The dentin-composite bond strength at different time intervals was measured by uniaxial tensile test. By varying the composite thickness and output from the curing light, their effects on the rate of bond formation for two different materials (a conventional and a bulk-fill composite) were also investigated. The proportions of cohesive and adhesive failure were determined by analysis of electron micrographs of the fractured surfaces.The results showed that the development of dentin-composite bond strength (S) increased with time (t). And this trend can be described by the equation:S= Smax(1-exp(-αt)), where Smax is the final bond strength and a the rate of bond formation. Initially, only composite and adhesive could be seen on the failure surfaces. With time, an increasing proportion of dentin surfaces could also be seen. The rate of development being reduced by composite thickness. And the exposed dentin on the fracture surface appeared earlier in the thin specimens than in the thick ones. Actully, for the cured model, the rate of bond formation (α) was only a function of the irradiance reaching the composite-adhesive interface:increasing initially before plateauing to a value of -0.6 s-1.Part II Kinetics of bonding formation of dentin-uncured composite assemblyAccording to the simple testing model described above. A dentin-uncured composite assembly, which was more clinical relevant, was established. Methods:Uncured composite blocks (using the same two composites described above) with different thicknesses were placed on the bovine dentin slap with a layer of adhesive pre-cured on its surface. Bond strengths of composite-dentin specimens obtained from tensile test at different curing times were used to determine the bond formation rate.The results showed that the speed of bonding formation also follow the similar developing trend found in the previous model which can be described by the same equation:S= Smax(1-exp(-αt)). But for the uncured model, the rate of bond formation was effected by both interfacial irradiance and cohesion strength of composite. With the same interfacial irradiance, a specimen composed of composite with higher cohesion strength showed higher rate of bond formation. For the composite-adhesive systems used in this study, the final bond strength was around 12 MPa for both composites. A minimum dose of -2 J/cm2 was required to achieve adequate bond strength. Prior to reaching its final value, the higher the irradiance, the lower the bond strength for a particular dose, indicating that the rule of reciprocity did not hold.Part Ⅲ Prediction of temporal development of dentin-composite bond strengthBased on all raw data obtained from the two sets of experiment described above, the numerical model which can predict the development of bond formation was established. Theory:According to the interfacial irradiance, the received energy and the parameter of efficiency could be calculated. Hence, the bond strength at particular time could be predicted.The results showed that, with the operating parameter such as power of the curing light, thikness of filled composite and translucency of composite, etc. The numerical model can described the bonding formation quite well.In conclusion, the final bond strength of composite-dental specimen was dominated by the strength of adhesive-dentin interface which was considered to be the weakest region. Both composite with higher cohesion strength and higher interfacial irradiance would contribute to the higher rate of bond formation. The predominant mode of failure changed from cohesive in the composite and adhesive to interfacial at the adhesive-dentin interface. When combined with the development of shrinkage stress, the current results will allow the problem of interfacial debonding of composite restorations to be assessed more accurately and comprehensively.