Effect of Different Root Canal Preparation Sizes and Remaining Tooth Structure on the Survival of Endodontically Treated Mandibular Molars

The sample size calculation was performed using G*Power 3.1.9.2 software, based on a significance level of 5% (α = 0.05), an effect size of 0.4921, and a statistical power of 95% (1-β = 0.80). The analysis indicated that a minimum of 10 students per group would be required to detect a statistically significant effect.

3.2. Clinical Methodology Within the scope of this study, patients who applied to the University of Health Sciences, Gülhane Faculty of Dentistry, Department of Endodontics with complaints of pain in the mandibular first and second molar teeth will be evaluated based on the following inclusion and exclusion criteria.

3.2.1. Clinical Examination Clinical examinations will be performed to diagnose irreversible pulpitis. Data regarding pain onset, duration, spontaneity, nocturnal increases, and sensitivity to thermal stimuli (hot/cold) will be recorded. Pain will be evaluated using the Visual Analog Scale (VAS); only patients with moderate-to-severe (symptomatic) pain will be included.

Tooth vitality will be assessed via Electric Pulp Test (EPT) and cold tests. To ensure reliability, tests will be also applied to the contralateral tooth in the opposite arch. Teeth responding positively will be included. However, as partial necrosis can occur in multi-rooted teeth, patients will be excluded if no pulpal bleeding will be observed following access cavity preparation. Percussion and palpation sensitivity will be also evaluated.

3.2.2. Radiographic Examination Radiographic examinations will be conducted using the parallel technique with a periapical radiograph. In this technique, the image receptor is positioned parallel to the long axis of the tooth, and the X-ray beam is directed perpendicularly. This minimizes magnification and distortion, providing the most accurate anatomical view and allowing for standardized comparisons of pre- and post-treatment records.

Patients with a Periapical Index (PAI) score of <3 will be included. All images will be saved within the university's imaging system.

3.2.3. Endodontic Treatment Protocol Patients will be inferior alveolar nerve block anesthesia using 80mg/2mL+0.02mg/2mL articaine hydrochloride (Maxicaine Forte). Success will bedefined as significant lip numbness within 15 minutes. Following rubber dam isolation, tooth surfaces will be disinfected with 30% H2O2 for 30 seconds, followed by 2.5% NaOCl, which will be then inactivated with 5% sodium thiosulfate.

Caries will be removed using high-speed diamond round burs and low-speed tungsten carbide burs. Access cavities will be prepared using sterile diamond fissure burs and safe-ended burs (Endo-Z).

Teeth will be assigned to groups based on the remaining dentin walls:

Group 1: Occlusal cavity (4 walls)

Group 2: MO/OD cavity (3 walls)

Group 3: MOD cavity (2 walls)

Cusp thickness will be measured with a periodontal probe; only cases with a wall thickness of ≥1.5-2 mm were included for biomechanical safety. Patients in each group will be then randomized into two subgroups based on the file system: ProTaper Gold or TruNatomy.

Working length (WL) will be determined using a #10 K-file and an apex locator (set 1 mm short of the "0.0" point) and will be onfirmed radiographically.

ProTaper Gold Group: Shaped using Sx-S1-S2-F1-F2-F3 files in rotation. Mesial canals will be enlarged to F2, distal to F3.

TruNatomy Group: Shaped using Orifice Shaper, Glider, Small, Prime, and Medium files in rotation.

Irrigation will be performed between file changes using 2.5% NaOCl (15 mL total) via a side-vented needle (Irriflex). Final irrigation consisted of 3 mL each of: 2.5% NaOCl → Distilled water → 17% EDTA → Distilled water → 2.5% NaOCl. The final NaOCl and EDTA will be activated using the EndoActivator (3 cycles of 30 seconds).

Canals will be dried with paper points and filled with Dia-Proseal resin sealer and gutta-percha using the lateral condensation technique. Proper fit will be ensured using ProTaper Gold or TruNatomy Conform Gutta Percha. Post-obturation radiographs will be taken using the parallel technique.

Restoration and Follow-up The final restoration will be completed using GC G-ænial Posterior composite resin.