# In Vitro Evaluation of the Strength of Dentin Replacement in Complex Posterior Tooth Restoration
## 4 Discussion  
Composite resin is currently the first choice for tooth restoration due to its aesthetic appearance and reduced financial cost. However, the procedure consists of several stages and requires sensitive techniques [28; 29; 30]. Posterior teeth are synonymous with more extensive tooth decay. In these cases, the application of dentin replacement as a base is often recommended due to its optimal marginal adaptability. Dentin replacement has been developed with the addition of fiber to improve its mechanical properties [25; 31; 32; 33]. Figure 5: Fracture patterns under SEM: (**a**) Adhesive, with 31\(\times\) magnifications, indicated by yellow arrow; (**b**) Cohesive, with 32\(\times\) magnifications, indicated by red arrow; (**c**) Mixed, with 30\(\times\) magnifications. Figure 6: Quantitative fracture pattern on fiber dentin replacement, non-fiber replacement and flowable composite group. The results of the present study indicated that the fiber dentin replacement group exhibited higher tensile strength compared with that of the non-fiber dentin replacement and flowable composite groups, although the difference was not significant. Fiber dentin replacement contains fibers, such as barium glass, glass fiber, bisphenol A-glycidyl methacrylate resin, triethyleneglycol dimethacrylate, silicon dioxide, polymethyl methacrylate (PMMA), and a photoinitiator. The content of glass fiber filler in dentin replacement can increase the material's ability towards tensile loads. Following the application of a high tensile load to the tooth structure, the fiber undergoes a modification effect by absorbing and spreading the tensile load on the tooth surface [34; 35]. A previous study reported relevant results to the findings of the current study, indicating that the fracture toughness and flexural strength of fiber dentin replacement were improved compared with those noted in the flowable composite resins [10; 25]. This was due to the ability of the glass fiber to prevent cracks, while the PMMA could absorb and distribute the force from the polymer matrix to the glass fiber [10]. Polyethylene fiber ribbons found in dentin replacement aid the strengthening of tooth restoration by increasing the modulus of the elasticity and preventing fractures [34]. Non-fiber dentin replacement is a posterior bulk-fill composite resin containing a polymerization modulator, which is the main structure of SDR(r). This polymerization modulator can reduce volumetric shrinkage by approximately 20% and polymerization force by 80%. Finally, it improves its mechanical properties [32; 36; 37]. Flowable composites are nanocomposite resins consisting of 82% nanoparticle fillers and nanofillers, in which the nanoparticles constituting the silica are categorized as non-agglomerated and non-aggregated, with 20 nm and 4-11 nm zirconia fillers. These particles enhance the mechanical and physical properties of the nanocomposite resin [32; 34; 37; 38]. In contrast to the current study, a previous study that compared the flowable composites and the non-fiber dentin replacement in the meso-oclusodistal (MOD) Class II cavities of molar teeth concluded that the tensile strength of the flowable composite resins was lower when compared with that of the non-fiber dentin replacement [39]. The differences in these results are probably associated with the adhesive system used in the study. The adhesive system plays a crucial role in maintaining the bond between the cavity wall and the restoration material. In the present study, all groups used an etch-and-rinse system that produced improved interlocking strength to increase the resistance of the fractures [15; 32; 40]. The research study conducted by [PERSON] on molar teeth with class II MOD cavities indicated higher tensile strength in non-fiber dentin replacement compared with the flowable composites [41]. The results of that study may differ since conventional incremental applications were used, while in the present study, the application techniques used were centripetal-incremental.