Teeth that have undergone endodontic treatment are biomechanically weakened compared to vital teeth due to factors such as extensive carious lesions, previous restorations, loss of marginal ridges and pericervical dentin, and hard tissue removal during access cavity preparation and root canal instrumentation. Additionally, the quantity and quality of the remaining coronal tooth structure, the presence of marginal ridges, the ferrule effect, and the integrity of pericervical dentin directly influence both fracture resistance and the long-term success of restorations.

Following endodontic access cavity preparation, the fracture resistance of the tooth decreases by approximately 5%. This reduction increases to around 20-30% in the case of unilateral marginal ridge loss (mesio-occlusal [MO] or disto-occlusal [DO] cavities), and can reach up to 63% when both marginal ridges are lost (mesio-occluso-distal [MOD] cavities). This condition may lead to cusp deflection under occlusal forces and, particularly in the absence of appropriate post-endodontic restoration, may result in failures such as vertical root fractures.

It has been reported that, after caries removal and access cavity preparation, the amount of remaining sound coronal tooth structure may play an important role in determining the extent of mechanical preparation during treatment. The goal of mechanical preparation of the root canal system is to remove infected dentin and biofilm as much as possible and to create a continuously tapered canal shape that allows effective delivery of irrigants to the apical region. Although conventional preparation principles may provide higher levels of disinfection, they have been associated with reduced tooth strength, particularly due to the loss of resistance in the pericervical area. This has led to the development of more conservative root canal preparation strategies using instruments with smaller apical sizes and tapers.

Previous studies investigating the relationship between preparation size and fracture resistance have shown considerable heterogeneity. Moreover, the predominantly in vitro nature of these studies limits their ability to accurately simulate clinical conditions. In addition, factors such as the amount of remaining tooth structure after treatment and the effectiveness of the final restoration are often overlooked. Therefore, it is important to support in vitro findings with clinical studies. To date, there is no long-term clinical study in the literature evaluating the combined effects of conventional and conservative preparation strategies on clinical survival and periodontal health in teeth with varying degrees of coronal tissue loss.

The aim of this study is to clinically and radiographically evaluate the survival of mandibular molars with different amounts of remaining tooth structure (O, MO/DO, and MOD) following conventional and conservative root canal preparation. The null hypothesis is that different preparation strategies and the amount of remaining tooth structure have a similar effect on tooth survival.

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.

Inclusion Criteria: Patients with mandibular molar teeth diagnosed with symptomatic irreversible pulpitis according to the pulp disease classification recommended by the American Association of Endodontists

Patients with active caries in the relevant tooth that would create O, MO/OD, or MOD type cavitations.

Patients with periodontally healthy teeth (Grade I mobility or <4 mm pocket depth).

Patients with no systemic diseases (ASA I or II).

Patients between the ages of 18-60.

Teeth where rubber dam isolation could be achieved.

Cases where the remaining sound tooth structure allowed for a direct composite restoration.

Exclusion Criteria:Teeth with non-vital pulp and/or observable periapical lesions.

Teeth that were not periodontally healthy (Grade II/III mobility or >3 mm pocket depth).

Teeth with complex root canal anatomy (curved canals, C-shaped canals, calcified/obstructed canals, etc.).

Patients with systemic diseases, or those who were pregnant or breastfeeding.

Patients with parafunctional habits such as bruxism.

Teeth without an opposing tooth in the maxillary arch.

Teeth that could not be restored due to advanced crown destruction or those with existing prosthetic restorations.

Patients whose endodontic treatment had already been initiated elsewhere.

Patients who experienced complications during the treatment procedure.

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Within the scope of the necessity of protecting the individual information of the students and patients, it is considered that the recorded data will not be shared.

Patients who will be treated and the assessor who will assess the periapical healing and restoration survival will be blinded regarding which file system was used.