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

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 relatio