EFFECT OF TWO DIFFERENT HEAT-TREATED MINIMALLY INVASIVE NICKELـTITANIUM ROTARY FILE SYSTEMS ON ROOT FRACTURE RESISTANCE (AN IN VITRO STUDY)

Background: NickelـTitanium rotary files can induce dentinal cracks on the root canal walls and over time functional load can develop and cause vertical root fracture.Aim: To evaluate the vertical root fracture (VRF) resistance of endodontically treated teeth prepared with two different heatـtreated minimally invasive rotating file systems.Materials and methods: 45 human mature mandibular first molars were selected & decoronated followed by sectioning of distal roots, obtaining mesial roots 14 mm long. The mesial roots were mounted in resin molds and randomly assigned to three equal groups; one negative control group (n=15) and two experimental groups (each n=15) according to the file system used. Group 1 ـــinstrumented by HyFlex EDM, Group 2 ـــinstrumented by Protaper Ultimate, Group 3 ـــnegative control (no instrumentation). Single cone obturation was performed with resin sealer and guttaـpercha. Subsequently, all specimens were subjected to vertical compressive load using the universal testing machine to record the force (N) needed until root fracture. Data were statistically analyzed.Results: The control group showed the greatest fracture resistance, followed by HyFlex EDM and Protaper Ultimate. No statistical difference was noted between the two tested groups (P =0.055) and between the control and HyFlex EDM (P =0.295). In contrast, a statistically substantial difference was observed between the control and the Protaper Ultimate (P =0.001). Conclusions: Minimally invasive heat-treated files could preserve more pericervical dentin (PCD), enhancing fracture resistance. HyFlex EDM files maintain tooth strength better than Protaper Ultimate system, with no significant difference.