Evaluation of stress patterns in teeth with endodontic treatment and periapical lesions as abutments for fixed prosthesis: a finite element analysis study
Periapical lesions, commonly referred to as apical granulomas, are immune-mediated responses initiated by host cells to provide immunological defence. The purpose of this response is to inhibit the spread of bacterial infections from the root canal system to the surrounding periapical tissues . However, simulated inflammatory activity can lead to increased bone resorption and the development of radicular cysts . The oral cavity comprises several components that aid in alleviating stress or strain, including the periodontal ligament (PDL), alveolar bone, tooth fibrous joint, and their interfaces . The PDL, serving a protective function, distributes occlusal forces to the supporting components of the tooth. Furthermore, the PDL is subjected to loads that remain within its physiological limits, thus ensuring optimal space for the PDL . However, in cases of periapical pathological conditions, alterations in neighbouring mineralised tissues can impede the space of the PDL, thus hindering the proper transmission of occlusal stresses to the surrounding tissues . In cases involving endodontic problems, root canal treatment (RCT) is necessary, which is considered a contributing risk factor for vertical root fractures (VRFs) . The incidence of VRFs in teeth that have undergone RCT is significantly higher compared to teeth with healthy pulps. The fracture is likely caused by factors such as the design of the access cavity, the protocol for instrumentation and irrigation, the presence of pre-existing fractures, and the biochemical effects of loss of vitality and structural integrity . The remaining tooth structure should be carefully assessed when deciding on post-endodontic restoration for teeth with RCT. Postponing the finalisation of post-endodontic restoration throughout the healing period can lead to a greater probability of tooth fracture. Hence, the positioning of a stable coronal restoration promotes the regeneration of tissues around the apex of the tooth . Post-core systems, which rely on support from the root canal, are commonly utilised to restore structurally compromised teeth and enhance retention . Fiber-posts, commonly used in such scenarios, possess comparable characteristics to dentin and effectively transmit occlusal stresses to the root . In prosthetic restorations, durability is improved by pairing dental porcelains with a strong framework. porcelain-fused-to-metal restorations have been utilized for years because their metal framework strengthens them and reduces the likelihood of external force-induced porcelain fractures. Alternative materials have been developed due to allergic responses, corrosion, galvanic currents, radiopaque appearance, rising aesthetic expectation . Zirconia is a contemporary ceramic repair material with biocompatibility, fracture hardness, and durability . Compared to metal-based restorations, it transmits more light, creating more natural results . In addition, E-max crowns, a full-ceramic alternative that mimics real tooth tissues, are durable and appealing. Although many ceramic systems are available, literature evaluations and clinical outcomes suggest that no single material is best for all situations . Furthermore, these materials behave differently depending on loading methods such as occlusal static or dynamic load . Tooth fracture prediction often relies on stress analysis and the identification of stress concentrations from a biomechanical standpoint. This approach helps determine the likelihood of failure . In a previous finite element stress analysis (FEA) study, changes in the behaviour of biomechanical properties of teeth were reported due to the presence of bone loss . In teeth with RCT and periapical lesions, the process of complete healing for both hard and soft tissues take time, and the biomechanical response may vary during this period . Clinicians may consider it risky to use a tooth with lesions and RCT as a bridge abutment. However, it is essential to consider when replacing the molar area with a bridge, particularly in cases of multiple lost teeth. In the literature, there are some studies evaluating the stress distribution of teeth with RCT . However, it would not be wrong to say that literature does not provide a proper study about investigating the stress distribution occurring in teeth with lesions and the surrounding bone tissue when used as abutments. Consequently, clinicians face a clinical problem in utilising teeth with lesions and RCT as bridge abutments due to insufficient data on stress distribution in these teeth and the surrounding bone tissue. The aim of this study was to evaluate the equivalent stress distributions and structural deformation behaviour of endodontically treated mandibular premolars with periapical lesions and fiber-posts used as abutments in four-member fixed prostheses. The null hypothesis was that the stress distributions occurring in an endodontically treated mandibular premolar with a periapical lesion and fiber-post used as a four-member bridge abutment were not different from those in a vital healthy premolar without any restoration, and in the premolar with endodontic treatment, periapical lesion, fiber-post, and metal crown restoration. 

Fundamental FEA procedure To explore relevant biomechanics in dentistry, a FEA investigation incorporates a specific numerical method-based simulation procedure, which adheres to the accepted standard Finite Element Method (FEM) strategy. This procedure was employed in this study to investigate the biomechanical response of mandibular premolars under various conditions, including healthy premolars, premolars with endodontic treatment, periapical lesions, fiber-post and single-crown restoration, and premolars with endodontic treatment, periapical lesions, and fiber-posts used as abutments in a four-member bridge. The procedure essentially follows pre-processor steps, including solid model preparation for analysis, and necessary parameter definitions such as material properties, boundary conditions and meshing boundaries for the FEA setup. Subsequently, the solving procedure and the post-processor procedure involve extracting visual and numerical outputs. In this study, the Structural module of the ANSYS Workbench FEA code was utilised. Specific details regarding the user interface and application can be found in the software's user guide. In addition, this study was completed according to SQUIRE reporting guidelines . Master model and identifying the structural components The foundational CAD model data for the human mandible structure and associated teeth models were sourced from open-access CAD data provided by projects aimed at developing 3D educational resources for the University of Dundee's School of Dentistry in Dundee, Scotland (Accessible at: https://sketchfab.com/DundeeDental/models ). The final CAD models used in this study for FEA analysis were created and refined using SolidWorks (SolidWorks Corp., Massachusetts, USA), a 3D parametric solid modelling software. Subsequently, the FEA procedure was conducted in ANSYS Workbench (ANSYS, Canonsburg, USA), a commercial FEM software. Realistic three-dimensional (3D) tooth models served as references to evaluate the impact of periapical lesions on stress distributions in mandibular premolars restored with fiber posts. The simulation of RCT and fiber-post restoration was created using the 3D CAD model, and structural analysis was completed using the finite element code. The following assumptions were made during these processes: (1) The geometry of the restored mandibular premolar tooth with a fiber-post and porcelain-fused-to-metal restoration is idealised by modelling all sub-components to obtain axial symmetry along the vertical line through the centre of the tooth, (2) Dentin and cementum were treated as two separate components, (3) all components are interconnected perfectly with no gaps between them, (4) The outer surface and related parts of the alveolar bone were structurally rigid and prevented displacement, (5) the influence of pre-stress due to root canal treatment was disregarded, (6) all the materials chosen for this study were isotropic, linear, and homogeneous. Furthermore, all FEA scenarios were performed using an implicit analytical approach, which is more appropriate for dealing with static or slowly dynamic situations characterised by low strain rates. First, a model without any lesions was generated (Model 1). The dentin component was synthesised alongside the gutta-percha, PDL, post, core, and crown. In the model constructed following the RCT and preceding the insertion of the fiber-post, a 4 mm segment of gutta-percha was intentionally retained at the apical region . The apical end was positioned 0.5 mm in front of the gutta-percha end, which is considered the working length. The periapical lesion was determined to be a score of 4 according to the PAI index (indicating probable present periapical destruction of bone . The average thickness of the PDL was assumed to be 0.2 mm. Trabecular and cortical bone, PDL, root, gutta-percha, post and adhesive cement, core, and crown cement tissue were also modelled separately. The diameter and length of the modelled fiber-post (ParaPost Fiber White, Coltène/Whaledent, Mahwah, USA), which was designed to taper towards the end and to be parallel, were 1.5 mm and 15 mm, respectively. The thickness of the cement layer, composed of dual-cure resin cement (Ivoclar Vivadent, Schaan, Liechtenstein), between the dentin of the post and the core-crown, was established as 0.25 mm, 0.1 mm, and 0.2 mm, respectively. The ferrule diameter was determined to be 1.5 mm. A chamfer-style step with a 135° angle was made at the gingival margin for the purpose of core restoration. This was achieved assigning resin composite material (ParaPost Fiber White, Coltène/Whaledent, Mahwah, USA). Three clinical scenarios were established to evaluate the mechanical response of the tooth under applied load, following the completion of a healthy model without any periapical lesions: Model 1: Mandibular premolar teeth without any restoration and periapical lesion; Model 2: Mandibular premolar teeth with endodontic treatment, periapical lesion, fiber post, and crown restoration; Model 3: Mandibular premolar teeth with endodontic treatment, periapical lesion, and fiber post used as an abutment in a four-member bridge. Assigned boundary conditions During the configuration of the FEA, contact relationships between all components were established using bonded contact definitions, assuming complete contact. Furthermore, the bone structure was fully fixed. Each tooth model received a 300 N occlusal static load at 45 degrees to the long axis of the tooth at the lingual inclination of the buccal cusp . Since the incoming forces in the marginal ridge-tubercle bite relationship are also met by the contact tooth, the forces were applied as three-point contact in accordance with Okeson's bite criteria and the tubercle-fossa relationship was taken as the basis . The contact relations between all components were allocated bonded contact definitions during FEA setup assuming 100% contact and not allowed to separate. Definition of the material properties The model of homogenous isotropic linear elastic material was assumed in the present study. Elastic modulus, Poisson’s ratio and density values of materials were defined according to the previous literature (Table ) [ , , – ]. Creating mesh structure The ANSYS Workbench meshing interface was employed to generate the mesh structure (FE Model) for the FEA. In order to accurately describe the geometry of the structures in the models and provide the most precise contact connections, smaller element sizes were chosen for the contact areas to achieve the highest level of quality. The quality of a model's mesh structure plays a critical role in determining the accuracy of finite element analysis (FEA) simulations. Consequently, verifying the mesh structure is an essential step in the simulation process. Among the primary quality metrics used for this purpose is skewness, which evaluates how closely a face or cell in a finite element model approaches its ideal shape. Skewness measures the degree of deviation from an equilateral shape, providing a reliable indicator for mesh quality verification. In the context of FEA, skewness values range from 0 to 1, where a value of 0 represents an equilateral (optimal) cell, and a value of 1 indicates a fully degenerate (least desirable) cell. In this study, the skewness metric was used to assess and verify the quality of the mesh structure, ensuring its suitability for accurate simulation results. FE models with an average skewness metric value of 0.234, indicating excellent level of the FE model, were obtained . Related details of the FE models for Models 1, 2, and 3 is shown in Fig. . 

To explore relevant biomechanics in dentistry, a FEA investigation incorporates a specific numerical method-based simulation procedure, which adheres to the accepted standard Finite Element Method (FEM) strategy. This procedure was employed in this study to investigate the biomechanical response of mandibular premolars under various conditions, including healthy premolars, premolars with endodontic treatment, periapical lesions, fiber-post and single-crown restoration, and premolars with endodontic treatment, periapical lesions, and fiber-posts used as abutments in a four-member bridge. The procedure essentially follows pre-processor steps, including solid model preparation for analysis, and necessary parameter definitions such as material properties, boundary conditions and meshing boundaries for the FEA setup. Subsequently, the solving procedure and the post-processor procedure involve extracting visual and numerical outputs. In this study, the Structural module of the ANSYS Workbench FEA code was utilised. Specific details regarding the user interface and application can be found in the software's user guide. In addition, this study was completed according to SQUIRE reporting guidelines . 

The foundational CAD model data for the human mandible structure and associated teeth models were sourced from open-access CAD data provided by projects aimed at developing 3D educational resources for the University of Dundee's School of Dentistry in Dundee, Scotland (Accessible at: https://sketchfab.com/DundeeDental/models ). The final CAD models used in this study for FEA analysis were created and refined using SolidWorks (SolidWorks Corp., Massachusetts, USA), a 3D parametric solid modelling software. Subsequently, the FEA procedure was conducted in ANSYS Workbench (ANSYS, Canonsburg, USA), a commercial FEM software. Realistic three-dimensional (3D) tooth models served as references to evaluate the impact of periapical lesions on stress distributions in mandibular premolars restored with fiber posts. The simulation of RCT and fiber-post restoration was created using the 3D CAD model, and structural analysis was completed using the finite element code. The following assumptions were made during these processes: (1) The geometry of the restored mandibular premolar tooth with a fiber-post and porcelain-fused-to-metal restoration is idealised by modelling all sub-components to obtain axial symmetry along the vertical line through the centre of the tooth, (2) Dentin and cementum were treated as two separate components, (3) all components are interconnected perfectly with no gaps between them, (4) The outer surface and related parts of the alveolar bone were structurally rigid and prevented displacement, (5) the influence of pre-stress due to root canal treatment was disregarded, (6) all the materials chosen for this study were isotropic, linear, and homogeneous. Furthermore, all FEA scenarios were performed using an implicit analytical approach, which is more appropriate for dealing with static or slowly dynamic situations characterised by low strain rates. First, a model without any lesions was generated (Model 1). The dentin component was synthesised alongside the gutta-percha, PDL, post, core, and crown. In the model constructed following the RCT and preceding the insertion of the fiber-post, a 4 mm segment of gutta-percha was intentionally retained at the apical region . The apical end was positioned 0.5 mm in front of the gutta-percha end, which is considered the working length. The periapical lesion was determined to be a score of 4 according to the PAI index (indicating probable present periapical destruction of bone . The average thickness of the PDL was assumed to be 0.2 mm. Trabecular and cortical bone, PDL, root, gutta-percha, post and adhesive cement, core, and crown cement tissue were also modelled separately. The diameter and length of the modelled fiber-post (ParaPost Fiber White, Coltène/Whaledent, Mahwah, USA), which was designed to taper towards the end and to be parallel, were 1.5 mm and 15 mm, respectively. The thickness of the cement layer, composed of dual-cure resin cement (Ivoclar Vivadent, Schaan, Liechtenstein), between the dentin of the post and the core-crown, was established as 0.25 mm, 0.1 mm, and 0.2 mm, respectively. The ferrule diameter was determined to be 1.5 mm. A chamfer-style step with a 135° angle was made at the gingival margin for the purpose of core restoration. This was achieved assigning resin composite material (ParaPost Fiber White, Coltène/Whaledent, Mahwah, USA). Three clinical scenarios were established to evaluate the mechanical response of the tooth under applied load, following the completion of a healthy model without any periapical lesions: Model 1: Mandibular premolar teeth without any restoration and periapical lesion; Model 2: Mandibular premolar teeth with endodontic treatment, periapical lesion, fiber post, and crown restoration; Model 3: Mandibular premolar teeth with endodontic treatment, periapical lesion, and fiber post used as an abutment in a four-member bridge. 

During the configuration of the FEA, contact relationships between all components were established using bonded contact definitions, assuming complete contact. Furthermore, the bone structure was fully fixed. Each tooth model received a 300 N occlusal static load at 45 degrees to the long axis of the tooth at the lingual inclination of the buccal cusp . Since the incoming forces in the marginal ridge-tubercle bite relationship are also met by the contact tooth, the forces were applied as three-point contact in accordance with Okeson's bite criteria and the tubercle-fossa relationship was taken as the basis . The contact relations between all components were allocated bonded contact definitions during FEA setup assuming 100% contact and not allowed to separate. 

The model of homogenous isotropic linear elastic material was assumed in the present study. Elastic modulus, Poisson’s ratio and density values of materials were defined according to the previous literature (Table ) [ , , – ]. 

The ANSYS Workbench meshing interface was employed to generate the mesh structure (FE Model) for the FEA. In order to accurately describe the geometry of the structures in the models and provide the most precise contact connections, smaller element sizes were chosen for the contact areas to achieve the highest level of quality. The quality of a model's mesh structure plays a critical role in determining the accuracy of finite element analysis (FEA) simulations. Consequently, verifying the mesh structure is an essential step in the simulation process. Among the primary quality metrics used for this purpose is skewness, which evaluates how closely a face or cell in a finite element model approaches its ideal shape. Skewness measures the degree of deviation from an equilateral shape, providing a reliable indicator for mesh quality verification. In the context of FEA, skewness values range from 0 to 1, where a value of 0 represents an equilateral (optimal) cell, and a value of 1 indicates a fully degenerate (least desirable) cell. In this study, the skewness metric was used to assess and verify the quality of the mesh structure, ensuring its suitability for accurate simulation results. FE models with an average skewness metric value of 0.234, indicating excellent level of the FE model, were obtained . Related details of the FE models for Models 1, 2, and 3 is shown in Fig. . 

Upon examining three model situations, it was revealed that the stresses resulting under predefined loading conditions were focused on approximately the same locations for all models. The stress and deformation outputs obtained from the FEA are displayed in Figs. , and (Models 1 to 3, respectively). The FEA has provided numerical results for deformation and equivalent stress, which are categorized depending on their components. These numerical results related to maximum stress and deformation were presented in Table by components. The stress was shown to be focused in the buccal and lingual cervical regions of the tooth, as well as in the interfacial areas, the buccal areas of the cervical and middle thirds of the root, and the region where the occlusal force was applied (Fig. ; Model 1). The maximum deformation was measured as 0.091 mm on the buccal cusp of the crown. Upon evaluating the stress distributions in Model 2, it was observed that stress was concentrated in the buccal and palatal areas outside the interface in the cervical region of the tooth. Additionally, stress was found in the middle and cervical thirds of the root, as well as in the buccal area and the area where the occlusal load is applied in the crown. Furthermore, stress was observed on the surface of the fiber-post that is suitable for the cervical buccal part of the tooth root. The enamel-cement junction area in the buccal part of the cervical region of the tooth experienced the most stress, measuring 133.87 MPa (Fig. ). The maximum deformation was 0.098 mm in the buccal cusp of the crown. The distal marginal ridge, the buccal cusp of the first premolar, the location where force was applied to the bridge, and the connection areas were found to be the sites of concentration for stress when the stress distributions determined in Model 3. A stress of 110.18 MPa was recorded as the highest in the connector between the first premolars and the pontic (Fig. ). The maximum deformation value recorded was 0.064 mm, observed on the buccal area of the first premolars and in the connectors of the bridge. When comparing Model 2 with Model 3, no difference was observed in the localization of the tooth components and the distribution of stress in the lesion area. Across all models, the equivalent stress values in cortical bone were greater than those observed in trabecular bone (Fig. ). The greatest equivalent stress was consistent for all models and localized in the cortical bone adjacent to the cervicobuccal region of the tooth. Model 3 had the lowest value, while Model 1 had the highest value. The values of the models with periapical lesions (Model 2, Model 3) showed minimal variation, and the lesion area had a low-stress distribution. All models showed a decrease in stress on the alveolar bone from the tooth's cervical line to its apex. The study found that the stress distribution values were lower in the dentin when fiber-posts were used (specifically in Model 2 and Model 3). Furthermore, the stress distribution in the fiber-post was greater in Model 2 compared to Model 3. Both models exhibited greater stress distribution in the central third of the fiber-post, namely on the surface corresponding to the cervicobuccal region of the tooth. 

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 . 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 . 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 . Compared to their counterparts, teeth that had a crown or other permanent restorations were linked to a far greater survival rate . 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 . These findings were consistent with the present studies . 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 . 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 . A study by Ng et al.  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 . 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 . Furthermore, Vogler et al.  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 . Gomes et al.  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 . 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 . Kırmalı et al.  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 . 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 . 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 . 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 . 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 . Therefore, treatment plans should incorporate tactics to reduce these pressures. Consistent with other FEA investigations, [ , , ] 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 . One limitation of this FEA analysis is that the force is only applied at a 45-degree angle. Belli et al.  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 . Testing factors that may contribute to clinical fatigue are also not feasible . 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 . 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 . 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. 

The current FEA study investigated the impact of the force applied to components of a healthy single crown and a four-membered bridge abutment with apical lesions and fiber-posts on the tooth. After analyzing three model circumstances (Model 1, 2, and 3), it was found that the stresses caused by occlusal load applications were concentrated in identical regions in all models. Equivalent stresses from occlusal load in simulated models were lower in teeth with root canal treatment, fiber-post, and prosthetic abutment (Model 2, Model 3) compared to the healthy tooth model (Model 1). In models with a periapical lesion (Model 2, Model 3), low stress and total deformation distribution were detected in the lesion area where softer tissues absorb stresses and exhibit less stiffness compared to other tissues like trabecular bone. Additionally, high-stress levels in the cortical bone decrease as they move from the cervical line of the tooth towards the apical region. This observation indicates a possible therapeutic benefit by alleviating pressure in the periapical lesion region. This study highlights that premolar teeth with periapical lesions, when reinforced with fiber posts, can serve as abutments for four-unit bridges, emphasizing the importance of conservative approaches in clinical practice. It is essential to acknowledge that this is a FEA study with previously stated limitations, and further in vivo studies are required. 

