The study revealed a decrease in equivalent stress levels in trabecular and cortical bone, crown, cementum, and PDL under occlusal force, from Model 1 to Model 3. Furthermore, the stress distributions on the tooth, surrounding bone tissue, and lesion varied between groups in Model 2 and Model 3. The study's hypotheses were rejected by the findings.

The FEA method is commonly favoured in the field of structural analysis and material investigations [28]. For instance, this technique enables the visualization of stress patterns and associated magnitudes throughout the entire modelled structure. It facilitates the analysis of materials with intricate and uneven surface properties, allows for comparisons by modifying the properties of the applied force, permits repeated analyses, and easily simulates the interaction between dental posts, surrounding tissues, and restoration materials [29]. Given the numerous benefits, the FEA method was chosen as the optimal approach in this investigation.

Restoring teeth permanently within ninety days after receiving RCT increased the survival rate [30]. Compared to their counterparts, teeth that had a crown or other permanent restorations were linked to a far greater survival rate [31, 32]. Based on the findings, restorations of teeth that have undergone RCT should be promptly performed, even in the presence of a periapical lesion. Furthermore, a previous research study examined the impact of several crown options on tooth structures in relation to root canal treatment, apical lesions, and fiber posts, utilizing the FEA approach. Applying various restorative treatment approaches to root canal-treated teeth with periapical lesions can influence the stress in the alveolar bone and the biomechanical response of the tooth. It has been shown that high cortical bone stress values at the tooth's cervical area decrease toward the apical area. The authors emphasized that this finding indicates a possible therapeutic benefit by decreasing force in the periapical lesion area [33]. These findings were consistent with the present studies [33].

A previous meta-analysis found that the survival rate of teeth with post-retained restorations following RCT was slightly lower than that of teeth without such restorations, although the difference was not statistically significant [33]. Teeth that do not serve as fixed or removable prosthesis abutments had a higher survival probability compared to those that did function as fixed-prosthesis abutments [33]. A study by Ng et al. [33] discovered that using a cast post and core for restoration retention decreased tooth survival, contradicting previous research which found that retention posts did not significantly affect tooth survival [32, 34]. However, the investigation did not categorize by the type of post and core material. In the present study, fiber-posts were utilized to construct the models. Fiber-posts have enhanced mechanical characteristics. Their modulus of elasticity, comparable to dentin, is expected to decrease the occurrence of irreversible failures [35, 36]. Furthermore, Vogler et al. [37] reported a 7.3% reduction in the percentage of tooth fractures compared to their study. This finding was attributed to the presence of fiber-reinforced post materials in the investigation that possess an elastic modulus comparable to dentin [38].

Gomes et al. [39] found that stress was localised in the area with the highest modulus of elasticity when a structure was loaded, distributing the load to neighbouring structures with greater density. Materials with a higher Young's modulus exhibited greater stress absorption. This indicates a favourable outlook for teeth that are repaired using fiber-posts [39]. The current study found that stresses were consistently located at the enamel-cementum junction in the buccal cervical region of the crown in all load-applied models. This finding was also consistent with previous study stated that the maximum stress concentration is at the cervical third of the tooth with fiber post [40].

Kırmalı et al. [41] used FEA to study the stress distribution in auto transplanted molars. The study noted a reduction in stress in the alveolar bone and tooth root as the tooth moved from the cervical to the apical in all models. The decrease was remarkable, although it varied depending on the direction of force application and tooth structure. The stress values in the cortical and trabecular bone of teeth with lesions were lower compared to the healthy tooth [42]. The present study found that the order of maximum equivalent stress level from high to low in cortical and trabecular bone was Model 1, Model 2, and Model 3, respectively.

A previous study suggests avoiding the use of teeth with RCT as abutments for prostheses or for providing occlusal assistance in excursive movements [7]. This previous study showed that teeth used as prosthetic abutments had lower survival rates. However, the sample size of teeth acting as abutments was insufficient to establish a statistically significant impact in this study [7]. It is believed that this finding may be linked to the unequal and unfavourable distribution of occlusal forces on abutment teeth. In this FEA study, when the occlusal force was applied to all three models under ideal conditions, the models with periapical lesions (Model 2, Model 3) exhibited minimal variability in values and had a low-stress distribution in the lesion area.

Each restoration should be able to handle both functional and non-functional forces that are applied to it in the mouth [42]. This aspect must be considered during the production of fixed partial dentures. Exceeding the physiological tolerance threshold of the supporting bone due to loads applied to dental abutments might result in bone loss and treatment failure [43]. Therefore, treatment plans should incorporate tactics to reduce these pressures. Consistent with other FEA investigations, [14, 16, 20] a 300 N occlusal force was exerted in the current study. This force was applied at a 45-degree angle to the long axis of the premolar to accurately represent the various forces experienced during functional activities. During the usage of the dental arch, the area of the mandibular premolars experiences several stresses, including tension, compression, rotation, and shear forces [16]. One limitation of this FEA analysis is that the force is only applied at a 45-degree angle.

Belli et al. [20] observed that endodontic-periodontal lesions impact stress distribution, hence the choice of post material should be determined by the degree of the lesion. In the primary endodontic lesion model, the fiber-post exhibited uniform stress distribution and elevated stress levels compared to the tooth model without lesions. The study found that despite a lesion in the periapical area of the tooth, the glass fiber-post was able to protect the root structure because of its elastic qualities, which are comparable to dentin. Similar data were collected in this study, showing low and uniform stress in the core structure and root, but at values lower than dentin in both models with a fiber-post.

An inherent constraint of this study is that the complex nature of intraoral circumstances cannot be precisely replicated using FEA experiments. Furthermore, the pressures present in the oral environment possess both static and dynamic characteristics, and the temperature changes that arise from oral activities can be regarded as supplementary constraints. Although there may be variations in the anatomical structures of teeth among individuals, this research focuses on analyzing and standardizing the optimal tooth forms and structures. FEA investigations involve the conversion of numerical data into models; however, they cannot be completely validated [43]. Testing factors that may contribute to clinical fatigue are also not feasible [10]. Due to these factors, while the FEA results provide an anticipated understanding of structural deformation behaviour, the data acquired from the study should be carefully interpreted for clinical applications. Because of the limitations of this study, it can be corroborated by clinical in vivo investigations that involve extended monitoring of several treatment options.

Furthermore, aside from the outcomes of this study, which involved the creation of three distinct clinical scenarios (a healthy premolar and the need for post-core restoration after crown destruction), it is unavoidable that certain clinical situations necessitate more conservative treatment approaches. In such cases, complex surgical procedures cannot be employed due to the administration of diverse medications that may lead to the development of osteonecrosis. For these individuals, more cautious approaches are necessary to ensure both safety and efficacy in dental restorations. Furthermore, it has been documented that performing tissue-level filling procedures following root canal treatments and endodontic restoration is a secure method for treating extensively damaged teeth, particularly in individuals who are using bisphosphonates [44]. This approach is especially advantageous for patients on long-term bisphosphonate therapy, as it minimizes invasive interventions while preserving functionality. Patients with compromised general health may not be suitable candidates for implant surgery, or the patient may prefer a more cost-effective option than an implant. In these cases, a dental bridge is a frequently encountered solution in clinical practice. Currently, clinicians may find it intriguing that a tooth with lesions and root canal treatment can be chosen as a bridge abutment. This finding opens new possibilities for conservative and cost-effective treatment planning, especially in challenging clinical situations. However, it is crucial when restoring the molar area with a bridge, especially when there are several missing teeth. Implant placement serves as an alternative to extraction; however, it is crucial to carefully consider contraindications, including systemic medical conditions such as bisphosphonate therapy, head and neck radiotherapy, and tobacco addiction. Clinicians must provide the most robust evidence while achieving consensus with patients and caregivers. However, the patient's decision regarding implant placement may be affected by financial constraints, cultural considerations, treatment duration, and possible complications. Financial constraints may arise from inadequate insurance coverage for implant surgery and the patient's socioeconomic status, while extended healing periods for implants may prompt patients to decide for fixed or removable prostheses [45].

Based on these results, premolar teeth with periapical lesions and fiber-posts can be utilised as abutments in a four-unit bridge. It is important to note that this is an FEA study, and additional in vivo studies are necessary.