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. 2025 Aug 23;25:1359. doi: 10.1186/s12903-025-06720-w

Complicated crown fractures in maxillary central incisors restored by fragment reattachment technique: a three-dimensional finite element analysis

Dongqi Shi 1, Weijun Zhang 1, Yao Feng 1, Qiuyue Wang 1, Zhaolun Sun 1, Xiangling Liu 1, Lingbo Zhang 1,✉
PMCID: PMC12374405  PMID: 40849648

Abstract

Objective

This study utilized finite element analysis to investigate stress distribution in fragmented teeth treated with fragment reattachment technique(FRT).

Study design

Four complex crown fracture models were constructed using maxillary imaging data. Four restoration methods were devised: direct reattachment (GA), reattachment with a fiber post (GB), complete crown restoration (GC), and post-core crown restoration (GD).

Results

Comparison of stress distribution revealed that for identical fracture lines, the GB method had a significantly higher peak von Mises stress (VMS) in dentin than the GA method, while VMS in enamel was In the GC and GD methods, the peak VMS of the complete crown exceeded that of the enamel from the GA and GB methods. Additionally, the peak VMS of the resin core in GA and GB methods was less than that of the dentin. The peak VMS of adhesive layer 2 in the GD method was lower than in the GB method, with the average VMS of the fiber post in GD being lower than in GB.

Conclusion

FRT is a viable option for restoring complicated crown fractures. Closer proximity of the fracture line to the tooth neck increases debonding risk, and GD demonstrated superior stress performance under experimental conditions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12903-025-06720-w.

Keywords: Finite element analysis, Fragment reattachment technique, Fiber post, Crown fractures, Post-core crown restoration

Introduction

Traumatic dental injury (TDI) is a prevalent dental condition, especially among adolescents, it is estimated that approximately one-quarter (24.9%) of the 6–50 year old population in the United States has at least one permanent incisor injury; Due to their prominent location, these injuries mostly affect the maxillary central incisors [1]. Crown fractures are responsible for approximately 33.1% of all injuries, with 12.3% classified as complicated fractures requiring complex treatment due to pulp involvement [2]. The fragment reattachment technique(FRT) involves reattaching the tooth fragment, offering a minimally invasive approach recommended by the International Association of Dental Traumatology (IADT) for various TDIs when the fragment is intact [3]. FRT is the process of bonding teeth fragments back onto teeth when they are available and intact with excellent results in terms of aesthetic and function. With the development of materials science, the use of nanoflow resin for bonding tooth fragments has yielded excellent aesthetics results [4].

The restoration of uncomplicated crown fractures can be easily accomplished because this method does not involve pulp treatment and can be directly repaired generally. Both FRT and direct resin restoration have achieved high success rates over a 5-year follow-up period, with no statistically significant difference in success rates between FRT and direct resin restoration [5]. The treatment of complicated crown fractures involves pulp treatment; as a fracture line deepens, it becomes increasingly difficult to repair the fractured crown. Due to this problem, some researchers have achieved good results by using fiber posts to assist during fixation and bonding, although the restoration of crown-root fracture by FRT is more challenging [6].

Tooth fractures not only affect dental aesthetics and function, they can also exert impact on the physical and psychological well-being of patients [7]. With the continuous development of dentistry, the concept of minimally invasive dentistry is being increasingly embraced by both practitioners and patients. FRT, whether with or without minimal tooth preparation, preserves natural tooth tissue and can restore the original state of teeth to a significant extent [8, 9]. Furthermore, FRT can effectively restore teeth to their original condition, and represents a rapid and affordable technique with a reduced level of technical complexity [6]. Previous studies have indicated that while resin restorations and direct reattachment result in a comparable initial strength, the resin may change color over time and wear differently from the adjacent teeth [4]. Veneers, complete crowns, and post-core crowns, are expensive and may present aesthetic challenges with neighboring teeth, thus requiring significant tooth preparation [10, 11]. Currently, FRT is considered by most researchers to be the preferred method for treating tooth fractures with a good short-term prognosis; however, some complications may occur, mainly debonding, pulp or periodontal problems in the long term [12]. However, the restorative effect of FRT, mainly the bonding effect over the long-term, can be further enhanced by the use of fiber posts [13].

Various types of tooth fractures present in the clinic with each type presenting unique challenges. Consequently, obtaining a sufficient number of maxillary central incisors with identical fractures for experimental purposes is very difficult. Finite element analysis (FEA) is a numerical analysis method that is used to solve complex engineering structures, which can be used to calculate the stress, deformation, and displacement of the structure. This technique decomposes complex structures into many small finite elements and then uses mathematical methods to calculate the stress status of each finite element, before synthesizing the stress status of each finite element to obtain the stress status of the entire structure. FEA provides numerous advantages for stomatology research, enabling precise focus on specific clinical issues and conducting biomechanical analyses by constructing three-dimensional (3D) models that closely mimic real teeth in terms of geometry and mechanics. This approach has provided valuable guidance and reference for clinical applications [14, 15]. Although there is an abundance of literature relating to the use of FRT in vitro, there is a significant paucity of biomechanical analyses relating to the restoration of complicated crown fractures by FRT, especially with regards to the combination of FRT and fiber posts for the treatment of complicated crown fractures. This study aims to compare the stress distribution of different restoration techniques, including FRT with and without fiber posts, complete crown, and post-core crown restorations, using three-dimensional finite element analysis (FEA) to identify the optimal approach for managing complicated crown fractures in maxillary central incisors.

Materials and methods

We selected a maxillary central incisor from an 18-year-old male, weighing 60 kg volunteer whose length, width, and thickness of the maxillary central incisor was close to the average level of the population. The selected tooth was not affected by caries, defects, or obvious absorption of the alveolar bone. Helsinki Declaration was read and followed the guidelines in this investigation and this study was approved by the Medical Ethics Committee of the Second Affiliated Hospital of Harbin Medical University (Ethics review approval number: YJSKY2023-340), informed consent was obtained from the volunteer.

Cone-beam Computer Tomography (CBCT, Type: SBR3D, Manufactured by SOREDEX, PaloDEx Group Oy Nahkelantie 160 FI-04300 TUUSULA, Finland) image data were obtained by scanning the head of the volunteer. Imaging scans layer thickness is 0.5 mm, a total of 378 pieces of Digital Imaging and Communications in Medicine (DICOM) images were imported into Mimics version 17.0 software (Materialise Medical, Leuven, Belgium). Mimics 17.0 can automatically recognize, process and locate the image. The Threshold tool was first used to expand the selection target region. Then, the mask tool was used to create a preliminary model of the maxillary central incisor (including enamel, dentin, and pulp), and the boundaries of the image were accurately delineated. Then, the surface of the model was optimized with the function of smoothing and coating, so that the surface of the model was closed and smooth; these models were saved as STereoLithography (STL) files. These files were then processed in Geomagic version 14.0 software (Geomagic Inc, Utah, USA) for mesh division to define the enamel and dentin, the size and number of grids are directly related to the accuracy of the results, then the hole filling, surface smoothing and surface fitting functions of the software were used for further optimization [16, 17]. The number of elements and nodes for each group is shown in Table S1. Next, we used UG version 12.0 software (Siemens PLM Software, USA), to design four fracture lines, as illustrated in Fig. 1, representing common clinical complicated crown fracture depths. Using the software, we applied several commands (stretching, rotating, offsetting, pruning, segmentation, and subtraction) to create additional geometric components, as follows. First, we used the thickening command to generate adhesive layer 1, with a thickness of 0.1 mm at the fracture plane. The fiber post prototype, modeled after 3 M Relyx Fiber Post No. 2 (3 M Company, Sao Paulo City, USA), featured a straight end diameter of 1.6 mm, a tip end diameter of 0.8 mm, a taper of 0.08 mm, a tapered length of 10 mm, and a total length of 20 mm. For experiments, the fiber post model was adjusted to a total length of 16 mm with a 10 mm taper, while maintaining the original dimensions. The adhesive layer 2 was 0.1 mm thick and surrounded the fiber post.

Fig. 1.

Fig. 1

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The positions of the four fracture lines (from the crown to the root, these lines are named as fracture lines 1, 2, 3, and 4). The first fracture line is tangential to the top of the pulp chamber. The distances between fracture lines 1, 2, 3, and 4 and the upper boundary of the alveolar bone were 6 mm, 4.8 mm, 3.6 mm, and 2.4 mm, respectively. The lines are parallel to each other and slant toward the palatal side

For the GD model, the adhesive layer 2 of the fiber post was limited to the area adjacent to the remaining dental tissue. The complete crown was 1.5 mm thick at the incisal edge and 1 mm thick at each axial plane, with adhesive layer 3 inwardly thickened by 0.1 mm. The region between adhesive layer 3 and the remaining dental tissue formed the resin core for the GC model.

The resin core of the GD model was derived by subtracting the space occupied by the coronal fiber post from the resin core. The alveolar bone was simplified into a 2 cm × 2 cm × 2 cm cube, with the upper boundary located 1.5 mm below the cementoenamel junction (CEJ). Using the software, we applied a 0.2 mm outward thickening to the teeth surrounded by the alveolar bone to simulate the periodontal membrane. Following this, 3D models of complicated crown fractures in the maxillary central incisor were developed using four distinct restoration approaches; 12 models were generated in total: A1, A2, A3, A4, B1, B2, B3, B4, C1, C3, D1, and D3, as shown in Fig. 2. The experimental groups are shown in Table S2 and were divided into a longitudinal experimental group and a transverse experimental group. The longitudinal experimental group includes GA and GB. The biggest difference between the two groups is that GB used fiber posts, while A1 and B1 can be considered as corresponding groups, and the same applies to the other groups. The longitudinal experimental group mainly studied the effect of adding fiber piles on the overall stress of the model. The transverse experimental group is a horizontal comparison between different repair methods. After the models of all groups were established, sensitivity tests and convergence tests were conducted to make the reconstructed models close to the clinical reality as much as possible, while avoiding stress concentration caused by inappropriate grid division, making the experimental results more accurate [18, 19].

Fig. 2.

Fig. 2

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Views of each group of models, as generated by Ansys version 19.1 software. The four restorations for Group A, Group B, Group C and Group D are direct FRT, FRT with fiber post, complete crown and post-core crown. A/B/C/D represents Group A/Group B/GroupC/GroupD, and 1/2/3/4 represents the fracture line 1/fracture line 2/fracture line 3/fracture line 4. Group A highlighted adhesive layer 1, Group C highlighted resin core, and Groups B and D highlighted fiber posts

In finite element analysis, all materials are assumed to be isotropic linear bodies, that is, objects are considered to have the same physical and mechanical properties in all directions. That is, the material is assumed to be uniformly and continuously distributed throughout the whole volume of the model under study. This assumption is called the uniform continuity assumption, it is also standard practice for initial biomechanical assessments. The linear body assumption assumes that the stress-strain relationship of the material under load is linear, and when the load is removed, the structural deformation is completely recovered [20, 21]. The elastic modulus and Poisson’s ratio values for various materials were input into ANSYS 19.1 software (Swanson Analysis, Canonsburg, PA, USA) to model isotropic linear bodies. The specific parameters are given in Table S3, data were derived from previous studies [22] Then, using the software, a fixed load of 100 N was applied at 45° to the tooth’s long axis, precisely at the incisal 1/3 of the lingual crown surface [23]. The bottom of the alveolar bone was constrained as fixed, as depicted in Fig. 3. Stress distribution across each model was observed and analyzed, focusing on von Mises stress (VMS) and maximum principal stress (MPS) results.

Fig. 3.

Fig. 3

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Schematic showing the methods used for mechanical loading and constraint. The red area is the load area, located on the lingual surface of the teeth. The red shear head is in the direction of the load, at a 45 degree angle to the long axis of the tooth, with a size of 100 N. The green area is the constraint area, and we assume that the bottom of the model remains stationary when subjected to force

Results

Equivalent stress results in the longitudinal experimental group

The results of the peak VMS of the longitudinal experiment group are shown in Table S4. In the longitudinal experiment, the VMS distributions in enamel for the GA and GB methods were similar, concentrated at the loading point, with higher peaks observed in A4 and B4. The VMS in dentin for the GA and GB methods remained stable overall, although the GB method showed a significantly higher peak VMS when compared to GA, with a slightly higher mean VMS (Table S5). From lines 1 to 4, the peak VMS of the corresponding group increased by 56.30%, 48.78%, 51.18%, and 46.12%, respectively. The mean VMS of the corresponding group increased by 6.94%, 0%, 0.60%, and 1.16%, respectively. In the GA method, stress was concentrated at the loading point, but was focused on the dentin and fiber post junction in the GB method.

The peak VMS in adhesive layer 1 in the GA and GB methods increased notably with deeper fracture lines, and was concentrated at the edge area. From lines 1 to 4, the peak VMS increased by 55.78%, 28.24%, and 45.57% in GA, and 35.50%, 29.95%, and 60.70% in GB. Peak stress values were similar when compared between the GA and GB methods; however, the low stress area was larger. However, the mean VMS of adhesive layer 1 for the GB method was lower than that for the GA method, with reductions of 8.02%, 1.57%, 9.79%, and 8.96%, respectively. Stress values in the remaining tooth tissue were comparable between the GA and GB methods and were concentrated at the labial surface and alveolar bone junction. The mean VMS in the remaining tooth tissue for the GB method was lower than that for the GA method, with reductions of 6.30%, 1.73%, 4.14%, and 2.56%, respectively. Figure 4 provides a VMS nephogram showing the longitudinal experimental group, including equivalent stress peak stress nephogram of the fragment enamel, fragment dentin, adhesive layer 1, remaining dentin and remaining enamel in GA and GB.

Fig. 4.

Fig. 4

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Equivalent stress nephogram for the longitudinal experimental group. The first to fifth rows represent fragmented enamel, fragmented dentin, adhesive layer 1, remaining dentin, and remaining enamel (enamel below the fracture line). A/B represents Group A/Group B, and 1/2/3/4 represents the fracture line 1/fracture line 2/fracture line 3/fracture line 4

Maximum principal stress results in the longitudinal experimental group

Table S6 presents the MPS results for the longitudinal experimental group. The notable difference between MPS and VMS for the GA and GB methods was the increasing peak MPS of dentin with deeper fracture lines. Apart from line 1, stress values in the corresponding groups were closely aligned.

Equivalent stress results in the transverse experimental group

Table S7 presents the VMS results for the transverse experimental group. For the GC and GD methods, the peak VMS of the complete crown was higher than that of the enamel for both GA and GB methods, while the dentin showed a significantly lower peak VMS when compared to the GA and GB methods. The peak VMS of fiber posts for the GB and GD methods were similar, whereas adhesive layer 2 for the GD method exhibited a lower peak VMS than the GB method. The mean VMS of fiber posts and adhesive layer 2 for the GD method was lower than that in GB, shown in Table S8. Adhesive layer 3 in GC and GD (C1) showed lower VMS compared to C3, while values were similar in other groups. Figure 5 shows a VMS nephogram for the transverse experimental group, including equivalent stress peak stress nephogram of the fragment enamel/complete, fragment dentin/resin core and remaining dentin in GA and GB, GC and GD.

Fig. 5.

Fig. 5

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Equivalent stress nephogram for the transverse experimental group. The first to thied rows represent fragmented enamel (GA and GB) or complete crown (GC and GD), fragmented dentin ((GA and GB) or resin core (GC and GD), remaining dentin. A/B/C/D represents Group A/Group B/GroupC/GroupD, and 1/2/3/4 represents the fracture line 1/fracture line 2/fracture line 3/fracture line 4

Maximum principal stress results of longitudinal experimental group

Table S9 shows the MPS results for the transverse experiment group. The peak MPS of the resin cores for the GC and GD methods were greater than those of the dentin cores in the GA and GB methods, with the former exhibiting a greater increase, while that of the GC was greater than that for the GD method. The peak MPS of the fiber post in D1 was lower than that in B1, while that for D3 was greater than that for B3. For the GD method, the peak MPS of adhesive layer 3 was less than that observed for the GC method. An equivalent stress nephogram of the fiber post and adhesive layer 2 for the GB and GD methods is shown in Fig. 6. A VMS nephogram of the resin core and adhesive layer 3 for the GC and GD methods is shown in Fig. 7. Table S10 outlines the displacement findings for each model group under the experimental loading conditions, maximum displacement was shown at the incisal end of the teeth.

Fig. 6.

Fig. 6

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Equivalent stress nephograms for the fiber Post and adhesive layer 2 in the GB and GD models. The first row represents fiber posts, and the second row represents adhesive layer of fiber post, i.e. adhesive layer 2. B/D represents Group B/GroupD, and 1/3 represents the fracture line 1/fracture line 3

Fig. 7.

Fig. 7

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Nephograms of the equivalent stress of the resin core and adhesive layer 3 in the GC and GD models. The first row represents resin core, and the second row represents adhesive layer of resin core, i.e. adhesive layer 3. C/D represents GroupC/GroupD, and 1/3 represents the fracture line 1/fracture line 3

Discussion

In this study, we focused on complicated crown fractures, a difficult but common clinical problem in dentistry. Using computer software and images scanned from a human tooth, we designed fracture lines of different depths and fragmented planes that could fully connect using 3D modeling software as well as CBCT data acquired from teeth. Then, we analyzed the stress distribution on affected teeth using different restoration methods. At present, there is no direct reattachment method that can fully restore the initial fracture resistance of teeth. According to a previous study, well-connected fragment planes may eliminate the need for additional preparation [24]. Over time, researchers have developed various methods to enhance tooth fracture resistance by creating additional adhesive areas, including enamel beveling, internal dentin grooves, over-counters, and external shoulder abutments, each contributing to different levels of reinforcement [25, 26]. In the longitudinal experimental group of the current study, stress distributions were analyzed for FRT, and FRT with fiber posts, under conditions where fragment planes were well-connected. VMS, which combines tensile, compressive, and shear forces, is a key parameter to comprehensively evaluate the stress state of a material or structure under complex loads [27]. The position of the peak VMS may be the area that is preferentially damaged by excessive external force. The area of peak VMS in adhesive layer 1 for A1 and B1 was initially concentrated at the labial edge but shifted to the lingual edge as the fracture lines deepened. The increased levels of stress in adhesive layer 1 with deeper fracture lines suggests vulnerability to initial damage at the edges under excessive external forces. Previous studies indicated that refracture tends to occur at the original site due to secondary dental trauma or excessive chewing forces [28, 29], with debonding remaining the primary cause of FRT failure. Success rates after re-reattachment did not differ significantly when compared to initial attempts [30]; thus, re-reattachment still remains viable. In the longitudinal experimental group, the peak VMS position in adhesive layer 1 shifted from the labial to the lingual edges with deeper fractures. In a previous study, Zhang et al. found that chamfering on the force application side significantly enhanced resistance to fracture [31]. However, understanding the relationship between force direction, preparation location, and the peak stress position in the adhesive layer, remains incomplete, suggesting the need to adjust preparation strategies based on specific clinical conditions. Interestingly, in the GB model, the peak VMS of adhesive layer 1 did not decrease significantly when compared to the GA model; rather, it increased slightly. Equivalent stress provides a comprehensive indicator; the mean of this parameter can reflect the average stress across the model [32]. Although the peak stress in adhesive layer 1 was concentrated in a narrow and elongated area at the edge, suggesting that the edges of the bonding layer are prone to failure, the average VMS of the GB model was lower than that of the GA model. Moreover, there was a larger area with low stress in the stress nephrogram of the GB model, and the average VMS of the remaining tooth tissue for the GB model was also lower than that for the GA model, thus demonstrating the stress-dispersing effect of fiber posts.

The peak VMS area in dentin for the GB model was localized in a narrow region at the junction of the fiber post and the dentin. The significantly higher stress produced by the GB model compared to the GA model can be attributed to differences in the elastic modulus between the fiber post and the dentin, as well as the concentration of stress at the sharp interface in the junction area. This higher stress implies that the junction area may be damaged first when subjected to excessive external force. However, the average VMS for the GB model only slightly exceeded that for the GA model, thus indicating only a minimal increase in stress. This suggests that while fiber posts increased local stress in the dentin, they primarily influenced susceptibility to damage at the junction area. Comparison of the stress nephrograms for the GA and GB models revealed a larger low-stress area for the GB model. In addition, the peak VMS in the enamel was lower for the GB model than for its GA counterpart, thus reflecting the stress-dispersing effect of fiber posts, at least to some extent. As the depth of the fracture line increased, the peak VMS in the dentin remained stable for both the GA and GB models, while MPS stress showed an increasing trend. Dentin is a brittle material that is prone to tensile fracture. In FEA, the maximum principal stress can be regarded as the maximum tensile stress, which is a reliable index to evaluate the fracture of materials. However, both peak stresses were well below the maximum tensile strength of dentin [33, 34], thus indicating that the experimental loading conditions were within safe limits based on the displacement of the model. From a mechanical standpoint, FRT meets the functional requirements for daily tooth use.

The peak VMS of the fiber post for the GB model and adhesive layer 2 remained stable with an increasing depth of fracture line and showed no significant changes. Meanwhile, the MPS of the fiber posts exhibited a decreasing trend; this compared to a general increase in adhesive layer 2. Specifically, the peak MPS of dentin in B1 increased by 18.41% compared to B2, and adhesive layer 2 increased by 20.22%. In a previous study, Tonini et al. reported that fiber posts were most effective when a crown fracture exceeded 2/3 [30]. Further analysis indicated that in B1, the peak VMS of dentin increased by 56.30% compared to A1, which was the highest of all corresponding groups; in constrast, the mean VMS increased by 6.94% compared to A1. The peak MPS of dentin in B1 showed the greatest increase at 80.83% when compared to A1; this was the largest increase recorded. The restoration effect of FRT with a fiber post to fracture line 1 was worse than other fracture lines in the GB model. Thus, caution should be exercised when using fiber posts close to the incisal end of a fracture line. This might be because the loading point is relatively close to fracture line 1.

Complete crown restoration was utilized in the GC treatment, while post-core crown restoration was applied in the GD treatment. Fracture lines 1 and 3 were applied in these two groups so that we could focus on the difference in stress distribution between traditional restorations and RFTs when fractures were in the middle 1/3 and cervical 1/3 of the crown. The distance from the edge of the complete crown in the GC and GD treatments to the boundary of the alveolar bone was set to a fixed 3 mm, thus meeting the biological width requirement [35]. Previous research has highlighted the significance of ferrules (1.5 mm in length and 1 mm in width) to ensure post-core crown restoration effects [36]. Sorensen et al. emphasized that a minimum height of 1 mm is crucial for optimal functionality [37]. In the present study, the D3 treatment applied ferrules of at least 1 mm, alongside fiber posts and resin cores. While metal post-cores provide higher strength, their greater elastic modulus when compared to dentin can lead to fractures with prolonged use [38]. In contrast, fiber posts and resin cores are favored for their superior biocompatibility and aesthetic benefits [31]. The peak VMS of complete crowns in the GC and GD treatments was greater than that of enamel in the GA and GB treatments because the elastic modulus of zirconia is much greater than that of enamel, thus resulting in more stress. Zarone et al. referred to this phenomenon as the “stress shield” effect, which protects the internal resin core [39]. Furthermore, the elastic modulus of the resin core was lower than that of the dentin, thus resulting in a significantly lower peak VMS for the resin core than for the dentin in the GA and GB models. No significant difference was observed in the peak VMS of the fiber post between the GB and GD models; however, the average VMS of the fiber post in the GD model was significantly reduced compared to the GB model. The peak VMS of adhesive layer 3 for the GD model was notably lower than in the GB model, thus suggesting that the complete crown protection reduced stress within the internal structure. External observations of the model showed that stress in the GC and GD models primarily accumulated at the loading point and the interface of the remaining dentin and the labial alveolar bone. Nevertheless, stress concentrations were identified at the labial neck region, specifically at the CEJ, in the GA and GB models. The peak stress at the labial neck area under the complete crown in the GC and GD models was located on the inner edge, with stress levels between 28.52 and 30.42 MPa, significantly lower than those observed in the GA and GB models, thus highlighting the protective role of the complete crown. Based on the above findings, we speculate that the long-term effect of post-core crown may be better when the fracture line is deeper. In the GC and GD models, adhesive layer 3, and the resin cores, exhibited stress concentrations not only at the loading point but also around the crown edge and the line angle areas; this was due to the relatively sharp areas of the model. This implies that avoiding residual sharp edges and points during clinical tooth preparation is critical, as these edges increase local stress and may cause restoration failures over the long term.

Therefore, we recommend that the tooth should be refined and polished after preparation to make the axial surface and marginal edge smooth and continuous. Additionally, it is best to further examine under a dental loupe or microscope, to avoid the appearance of acute angles and sharp edges as much as possible. For fracture line 1, the peak VMS in the resin core and adhesive layer 3 in the GC model was lower than for the GD model, yet the peak MPS was higher. With fracture line 3, the peak VMS in the resin core and adhesive layer 3 showed no significant difference when compared between the GC and GD models, although the peak MPS was higher in the GC model. This indicates superior stress performance in the post-core crown when compared to the complete crown, particularly when the cervical 1/3 of the crown has been fractured. The displacement of each model was very small, and the difference between the displacement of each model was also very small. With regards to displacement, the differences between various restoration methods were minimal.

There are various and complex cases of tooth fracture in clinical practice. Finite element mechanics analysis cannot fully simulate the stress on teeth under real conditions, nor can it simulate temperature changes and the effects of microorganisms in the oral cavity. This is a key limitation of the current research. In addition, FEA can reflect the stress on the adhesive, but cannot fully simulate the bonding effect of the adhesive; this requires further experiments in vitro. FRT has advantages that other restoration methods do not have, namely the preservation of natural dental tissue and excellent aesthetic effects. In this research, we applied FEA to analyze the stress distribution of various parts of the affected tooth after repairing complicated crown fractures with FRT. In the stress concentration area, we should preserve more dental tissue or take protective measures. This will provide mechanical reference for the clinical application of FRT and also provides a focus for in vitro experiments. However, we are not emphasizing the use of FRT instead of other restoration methods. Complete crown and post-core crown have also shown good protective effects. If the fracture line is too deep, the appropriate restoration method should be selected based on the clinical situation and the patient’s subjective wishes. Future studies should incorporate diverse anatomical models to enhance the applicability of results, and adopt loading methods that are closer to real conditions, such as cyclic loading and off-axis loads other than 45 degrees.

In addition, the simplifications of FEA introduce certain limitations. For instance, the assumption of isotropic linear materials simplifies the anisotropic and viscoelastic properties of dental tissues. Linear analyses do not account for nonlinear behaviors such as plastic deformation or fatigue under cyclic loading, which are relevant to clinical practice. The simplified geometry of the alveolar bone and periodontium further limits the ability of the model to fully replicate in vivo conditions. While conventional FEA was employed in this study to evaluate stress distribution in restored maxillary central incisors, advanced techniques such as the Extended Finite Element Method (XFEM) could further enhance the simulation of fracture behavior. XFEM allows for the modeling of crack initiation and propagation without requiring predefined mesh refinement, making it particularly suited for studying crack dynamics in heterogeneous dental tissues like enamel and dentin. The current study focused on stress analysis to guide clinical restoration strategies, but future research should explore XFEM combined with in vitro mechanical testing to investigate fracture modes.

Conclusion

Despite certain limitations of the current study, we draw the following conclusions:

  1. FEA revealed the stress performance of the FRT in response to different depths of fracture lines, and also revealed the differences between different restoration methods.

  2. FRT was determined as a viable option for restoring complicated crown fractures. The closer proximity of the fracture line to the tooth neck increased the risk of debonding under identical external forces. FRT with a fiber post elevated local stress in the dentin while reducing stress on the enamel, dentin, adhesive layer 1, and the remaining dental tissue.

  3. Post-core crown restoration is advised for cervical fractures because stress levels remain lowest.

Supplementary Information

Supplementary Material 1 (28KB, docx)

Authors’ contributions

Dongqi Shi: Writing - original draft, Methodology, Investigation, Data curation, Formal analysis.Weijun Zhang: Methodology, Investigation, Data curation, Formal analysis.Yao Feng: Writing - review & editing, Formal analysis, Project administration.Qiuyue Wang: Writing - review & editing, Software.Zhaolun Sun: Writing - review & editing.Xiangling Liu: Writing - review & editing.Lingbo Zhang: Writing - review & editing, Conceptualization, Supervision, Funding acquisition, Resources.

Funding

Not applicable.

Data availability

All data generated or analyzed during this study are included in this published article and its supplementary materials. No additional raw data are available.

Declarations

Ethics approval and consent to participate

This study was conducted in accordance with the Declaration of Helsinki. Ethical approval was obtained from the Medical Ethics Committee of the Second Affiliated Hospital of Harbin Medical University (Ethics review approval number: YJSKY2023-340). Written informed consent was obtained from all participants before participation in the study.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.

Footnotes

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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Data Availability Statement

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