Tabriz University of Medical Sciences Journal of Dental Research, Dental Clinics, Dental Prospects 2008-210X 19 1 2025 03 31 Exploring the impact of remaining tooth structure and preparation size on the fracture resistance of endodontically treated mandibular premolars 23 28 10.34172/joddd.025.42125 EN Sıla Nur Usta https://orcid.org/0000-0001-9513-0841 Hilal Tekkanat Yiğitcan Sağlam Cumhur Aydin Journal Article 10.34172/joddd.025.42125 2025 01 17 2025 03 08 Background. This study evaluated the influence of the combined effects of remaining dentin walls and mechanical instrumentation with taper sizes of 0.04 and 0.06 on the fracture resistance of endodontically treated mandibular premolars. Methods. Seventy single-canal mandibular premolar teeth with similar dimensions were selected and divided into one control group and three main experimental groups: control group: intact teeth, group 1: four remaining walls, group 2: three remaining walls, and group 3: two remaining walls. Each group was further divided into two subgroups in terms of preparation size (0.06 or 0.04 taper). The teeth were restored with composite resin after creating restorative models and performing endodontic treatments. The fracture resistance of teeth was measured by the push-out test. Data were analyzed using two-way ANOVA and post hoc tests and the square of Spearman’s linear coefficient (P<0.05). Results. The control group exhibited the highest fracture resistance compared to the experimental groups (P<0.05). Regardless of the taper size, group 3 showed the lowest fracture resistance values compared to groups 1 and 2 (P<0.05). Preparation sizes similarly affected the fracture strength of teeth (P>0.05). A significant positive association was found between the remaining dentin walls and fracture resistance (P<0.05). Conclusion. This study emphasized the importance of preserving coronal dentin for fracture resistance in endodontically treated teeth (ETTs). Conservative instrumentation did not provide any advantages over traditional preparation in increasing fracture resistance.