# In Vitro Evaluation of the Strength of Dentin Replacement in Complex Posterior Tooth Restoration
## 4 Discussion
Centripetal-incremental approaches exhibit improved marginal adaptation and lower shrinkage polymerization, which can in turn enhance the mechanical properties of the restoration [37; 42]. The difference in mechanical properties occurs due to the composite restoration material having different organic content. The small filler size and higher nanofiller volume can improve the mechanical properties of composite resins, such as tensile strength, compressive strength, and fracture resistance [34]. A previous study indicated that the tensile strength obtained in the non-fiber dentin replacement was higher than that noted in other composite resins due to differences in the composition of the polymer matrix and filler size [37]. Based on the fracture pattern observations noted in the present study, the most common dentin fiber replacement pattern was the adhesive pattern, which indicated optimal mechanical properties. High tensile strength indicated that the fiber dentin replacement ability was not deformed when a tensile force was applied. Bondable reinforcement fibers such as polyethylene fibers and glass fibers can increase the fracture resistance of the tooth structure. Therefore, when a tensile force is applied, a fracture occurs between the restoration and the tooth, which indicates that the fracture is more affected by the bonding ability of the adhesive material [37]. The use of fiber dentin replacement as a sub-structure under conventional composites has certain advantages, such as significantly increased strength of the restoration and the ability to prevent spread cracks and alter the fracture pattern [8]. In case a fracture is restored, the fracture direction will change and approach the coronal direction of the tooth [29; 38]. The most common fracture in the non-fiber dentin replacement and flowable composites groups was a mixed pattern fracture since the material was less able to withstand cracks and plastic deformation under tensile pressure. An additional study demonstrated that the most frequent fracture pattern in the non-fiber dentin replacement was the adhesive fracture pattern in premolars with class II MOD cavities, which was not in line with the results reported in the current study and may be affected by differences in cavity extension [40]. The present study used non-fiber dentin replacement and flowable composites in complex cavities so that the tooth structure that received a load distribution was diminished. Non-fiber dentin replacement possesses a low elastic modulus so that when a force is applied, the load will be transferred on to the underlying structure [43]. The cohesive fracture pattern only occurs in the fiber dentin replacement group since, in the non-fiber dentin replacement and flowable composite groups, all the cohesive fracture patterns that occurred were accompanied by fractures on the tooth structure and were consequently grouped into mixed fracture patterns [43]. Thermocycling is a combination of hydraulic and thermal degradation that simulates extreme temperature changes in the oral cavity [44]. The thermocycling method is carried out to simulate the same conditions as those of the composite resin restoration, which is affected by the oral cavity [45]. In the present study, thermocycling was not performed. Therefore, the effects of the thermal stress on these three restoration groups were unidentified. Based on the present study, the results indicated that the addition of fiber to dentin replacement did not show a significant increase in tensile strength compared with non-fiber dentin replacement. Accordingly, dentin replacement of both fiber and non-fiber material can be used as an alternative base material for the complex restoration of the posterior tooth.