Abstract
BACKGROUND: In clinical restorative applications, different materials and techniques are used to replace lost tooth tissue and compensate for the fracture resistance that are essential for function. Further investigation is required to ascertain the effect of polyethylene fiber application, which is one of these interventions, on the fracture resistance, fracture location, reparability and crack propagation of premolar teeth restored with direct composite resin restorations. This in-vitro study aimed to evaluate fracture resistance and crack propagation of two bulk-fill composites (BRC) with/without polyethylene-fiber in extensive Class I resin composite restorations. METHODS: Cavities were prepared on 40 mandibular premolars. In half of the samples (n = 20), polyethylene-fiber (Ribbond Fiber/RF) coated with adhesive resin was placed on cavity floor. Half of the teeth were restored with SonicFill 3 and the other half with Charisma Bulk Flow ONE. The groups were as follows: SonicFill 3 + Fiber(SF + RF), SonicFill 3(SF), Charisma Bulk Flow ONE + Fiber(CO + RF), and Charisma Bulk Flow ONE(CO). Following 20,000 thermo-cycles fracture resistance (FR) was determined. Crack number/orientation were assessed with transillumination using two different wavelengths, before (t(0)) and after preparation (t(p)), and following fracture test (t(f)). Visual examination was performed with micro-CT to evaluate the adaptation. Data were analyzed using Independent samples T-test, Fisher’s Exact, Fisher Freeman Halton, Pearson Chi-Square and Cochran’s Q Tests (P < .05). RESULTS: No statistically significant difference was found between the FR of groups (P = .994) with and without RF (P = .167) according to BRC. The mean FR values in decreasing order were CO (761.09 ± 224.32) > SF + RF (671.08 ± 150.51) > CO + RF (669.95 ± 358.44) > SF (580.7 ± 269.04). A reduction in the formation of irreparable fracture patterns was detected in RF groups. Crack number/orientation did not differ according to BRC and RF application. Statistically significant increase was observed in crack formation over examination periods (t(f)> t(p)> t(0); P < .001). CONCLUSIONS: The reinforcement of large cavities with polyethylene fibers changed the fracture pattern of the restorations towards repairable, whereas had no effect on fracture resistance and crack formation. Furthermore, the placement of polyethylene fibers may have an adverse effect on the adaptation of the cavity. In clinical applications, both wavelengths can be used for transillumination testing in crack inspection. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12903-025-07297-0.