Hypotheses. There were three hypotheses in this study: H1) roots treated with resorcinol formaldehyde resin (RFR) are less resistant to fracture than roots with an intact pulp, H2) RFR roots are weakened by the re-treatment, and H3) retreated RFR roots are further weakened by the presence of micro cracks.
Methods. A total of 160 specimens from single roots were selected and divided into four groups: the negative control group (C1) which comprised of roots with an intact pulp, the E1 group which had RFR teeth without cracks, the E2 group which had retreated RFR teeth without micro cracks, and the E3 group which had retreated RFR teeth with micro cracks. The resistance to fracture (RtoF) was evaluated after applying a vertical compressive force (250 N/s) until a specimen fractured.
Results. Bivariate analyses (one-way ANOVA with Dunnett adjustment) showed that the resistance to fracture was significantly (P<0.001) higher in the control group than in the E1-E3 groups. The mean ± sd for C1 was (227.5±29.6 N/mm2), for E1 174.7±41.5 N/mm2, for E2 169.7±31.4 N/mm2, and for E3 it was 150.5±11.4 N/mm2. The linear multiple regression analyses with the three contrasts showed that the RtoF was affected by the RFR (P=-0.56; P<0.001), the re-treatment of the RFR roots (P=-0.17; P=0.018), and the presence of micro cracks in the RFR roots (P=-0.17; P=0.003).
Conclusion. The RtoF was substantially affected by RFR material and to a lesser extent, by the retreatment of the RFR roots and by the presence of micro cracks in the RFR roots.
