Mechanical effects of foundation restoration on onlay preparations of mandibular first molars after endodontic treatment

doi:10.21203/rs.3.rs-4663360/v1

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Mechanical effects of foundation restoration on onlay preparations of mandibular first molars after endodontic treatment

https://doi.org/10.21203/rs.3.rs-4663360/v1

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Background. Foundation restorations involve the use of materials such as composite resins to partially restore defective dentin structures. They can be used to simplify the geometry of complex cavities, facilitate restoration insertion, and reduce the polymerization shrinkage of the adhesive. However, the stress distribution of teeth restored after foundation restoration placement is not fully understood.

Methods. The purpose of this finite element study was to analyze the stress distribution of onlays with composite resin foundation restorations to investigate the potential benefits of preserving the remaining dental tissue.A 3Dfinite element model of a mesial proximal defect of the mandibular first molar after endodontic treatmentwas established. Four types of onlay restorations (GA and GB: complete-coverage onlay; GC and GD: partial-coverage onlay) were designed using this model. After the models wereestablished, a fixed vertical load of 600 N and an oblique load of 200 N were applied, and the stress distributions were compared between the monolithic ceramic blockrestorations (GA and GC) and those with foundation restorations (GB and GD). The stress distributions of the onlays and models were also analyzed.

Results. Under vertical loading conditions, GD exhibited higher peak equivalent stress levels inthe onlay and adhesive layers. The maximum principal stress experienced by enamel was lower in GB and GD than in GA and GC, whereas it was greater in GB and GD than in GA and GC for the onlay and adhesive layer. Under oblique loading, the peak equivalent stress and maximum principal stress of the adhesive layer and enamel were the lowest in the GB and GD groups.

Conclusions. Foundation restorations provide good stress dispersion, which can improve the protection of the remaining dental tissue. However, onlay restorations after foundation restoration placement should be used with caution in patients with excessive occlusal forces. Oblique loading was more destructive to the adhesive layer and remaining dental tissue. Complete-coverage restorations have better stress dispersion than partial-coverage restorations.

Endodontic treatment can lead to tooth structures with compromised integrity, increasing susceptibility to fracture.¹ The extent of the structural loss varies depending on the affected area. Factors such as the thickness of the remaining tooth, type of access cavity, and instrumentation used significantly influence the amount of residual tooth tissue, thereby affecting the strength of the tooth.² Minimal endodontic instrumentation can mitigate the risk of tooth fracture, although minimal endodontic access has been reported to occur in 8% of treatments, and the traditional access cavity has been associated with reduced fracture resistance.³ In addition to accessing the cavity, teeth requiring endodontic treatment typically present with defects or defects resulting from caries removal or fracture, and the optimal restorative option should be selected to maximize prognosis.⁴

Adhesively bonded onlay restorations include complete-coverage onlays, covering all cusps, and occlusal veneers, which cover only the occlusal surface. Complete-coverage onlays extend to the lateral axial wall by 0 to 2 mm.^5,6 Complete-coverage restorations have been widely used, especially for tooth defects > 50%.^7,8 For endodontically treated teeth, onlays can be extended into the pulp cavity to provide retention as 1-piece endodontic crowns. These designs can be minimized as inlays, providing a feasible repair method for endodontically treated teeth with a 5-year survival rate of > 50%.^5,7,9,10 One-piece endodontic crowns have been defined as integral bonded ceramic structures characterized by surrounding shoulder margins that do not extend into the root canal.^11,12 The bond strength is improved by preserving the maximum enamel contact area, with reported success rates ranging from 94–100%.^13,14 The 1-piece endodontic crown technique can replace post-and-cores, especially for patients with crown tissue defects or small repair intervals, and patients with bruxism can undergo endocrown repair.¹⁵

When restoring a molar with a single-, double-, or triple-wall defect, it is essential to select the appropriate onlay form and foundation restoration to repair the affected tooth and preserve the remaining tissue. Materials such as composite resin can be used as foundation restorations to restore part of the missing dentin. However, the biomechanical mechanism underlying stress distribution in affected teeth after endodontic treatment using onlay restoration with a composite resin foundation restoration remains unclear. The remaining tooth structure and the restorative approach influence the survival rate of the tooth.^16–18 In this study, different restorative approaches were tested in an endodontically treated first mandibular molar model with a single-walled defect to determine the restoration with the most favorable stress distribution in the affected tooth. The null hypothesis was that foundation restorations would not affect the stress distribution in endodontically treated teeth.

Four restorative approaches were tested in a model of the mandibular first molar with a single-walled defect after endodontic treatment. Approval for this study was obtained from the Medical Ethics Committee of the study hospital. Cone beam computed tomography images of an intact mandibular first molar in an adult volunteer were imported into a reverse engineering software program (Mimics, 19.0; Materialise) to generate a model of the mandibular first molar region. Subsequently, the files were transferred to an inspection software program (Geomagic Studio, 17.0; Geomagic, Inc.) for hole filling, surface smoothing, and surface fitting. Next, the model was imported into a CAD software program (NX, 12.0; Siemens PLM Software) to continue production. Initially, the enamel and dentin portions were divided to orient the cementoenamel junction (CEJ). A mesial surface defect was then created 2 mm occlusal to the CEJ using the splitter function. The plane intersecting the base of the pulp chamber cylinder was shifted 2 mm from the occlusal surface to divide the pulp chamber, establishing the resin base model. Additionally, a plane parallel to the occlusal surface in the central fossa was selected and shifted apically by 2 mm. The dentin and enamel were segmented using this plane to obtain the remaining dental tissue structure. Missing parts of the teeth were integrated to form the onlay structure for GA, with GB, GC, and GD, following a similar modeling procedure. Finally, the line angles of the 4 onlays were chamfered to create rounded edges, and a 0.1-mm thick adhesive layer was applied on the tissue side of the onlays using the thickening command.^19,20

The periodontium was created by extending the root surface evenly 0.2 mm outward at the root of the tooth model. The alveolar bone, reduced to a cube, was stretched to create a 3-dimensional (3D) model. The inner part of the alveolar bone model represented cancellous bone, whereas the outer part formed a 2-mm thick cortical bone, with the upper surface positioned 2 mm apical to the CEJ.²¹ Using the segmentation function, the load application area was created at the corresponding position in the model, resulting in a 3D model of the mandibular first molar and periodontal tissue. The models of the 4 groups are shown in Fig. 1.

The 4 assembled models were imported into a finite element software program (ANSYS Workbench 18.0; Ansys Inc.), assuming that all the materials were isotropic linear bodies. The elastic modulus and Poisson’s ratio of each component are presented in Table 1. After the model was established, the grid properties were defined, and the grid of the model was divided (Fig. 2). The accuracy of the grid division directly affects the calculation accuracy and speed.^22,23 The number of delineated grids is presented in Table 2.

Table 1

Mechanical parameters for each component of the model
Material	Elastic modulus (MPa)	Poisson's ratio
Cast porcelain inlays	68900	0.31
Enamel	84110	0.33
Dentin	18600	0.31
Periodontal membrane	68.9	0.45
Resin	13000	0.30
Toothpaste tip	140	0.45
Bonding agents	82000	0.30
Cortical bone	13700	0.30
Cancellous bone	1370	0.30

Table 2

Number of nodes and units for each onlay restoration group
Group	Number of nodes	Number of units
A	1755351	1210264
B	1754367	1209161
C	1755351	1210264
D	1753044	1208272

Functional contact during mastication has been reported to be regional but not at individual loading points.²⁴ Moreover, the food texture can influence the masticatory movement of the mouth, which can be influenced by the character of the food.²⁵ Therefore, to increase the accuracy of the stress analysis of the remaining dental tissue, a round disk that simulated food was used for vertical loading. The disk covered the occlusal surfaces of the buccal and lingual cusps to simulate the cutting motion of the mastication cycle.²⁶ The vertical loading force was set as a maximum occlusal force of 600 N on the mandibular first molar during mastication. The oblique load was set to 2 Ø1.8-mm circular areas at the buccal cusp. Oblique loading at 45 degrees was used to simulate the crushing motion of the chewing cycle.²⁷ The von Mises stresses (VMS) and maximum principal stress (MPS) for all tooth structures and restoration designs were analyzed. The VMS combines tensile, compressive, and shear stresses, whereas the MPS is equivalent to tensile stress in 3D finite elements and provides a reliable index for evaluating the fracture of brittle materials.²⁸

The vertical loading condition showed differences in peak stresses among the 4 groups. The onlay in GD showed the highest peak VMS. The highest peak VMS in the adhesive layer and enamel was also observed in GD. The results of the peak VMS under vertical loading are shown in Fig. 3. The peak enamel stresses in all the models were concentrated in the distal cervical region near the CEJ, where the dental tissue is fragile. The peak VMS of resin in GB and GD was lower than that in GA and GC, but the peak VMS of the enamel and adhesive layer was greater than that in GA and GC. The stress of the onlay in GB was lower than that in GA but greater in GD than in GC.

The stress of the oblique load was greater than that of the vertical load for all the structures of the model. The peak VMS in the enamel and adhesive layer was substantially greater in the GC and GD treatments than in the GA and GB treatments. The stress in the adhesive layer in the GB was the lowest, whereas that in the GC was the highest. The highest peak VMS in the onlays and enamel was observed in GD. The difference between the peak stress values of the onlays for oblique loading was small, with the values in GA and GC being slightly lower than those in GB and GD. The peak VMS under oblique loading is shown in Fig. 4. The VMS of the resin was lower in the foundation restoration groups, whether under vertical or oblique loads. The distribution of the locations of the peak VMS regions in each part of the model is shown in Fig. 5. Overall, the dentin stress in the groups with foundation restoration was slightly lower than that in the groups with monolithic ceramic block restorations. The peak stress of the dentin was uniformly distributed on the distal surface of the mesial root. The areas of stress concentration of the dentin in the GB and GD groups were wider than those in the GA and GC groups. The peak VMS of the resin in the GB and GD groups was lower than that in the GA and GC groups, whereas the onlay stress was greater than that in the GA and GC groups. Additionally, the peak VMS of the adhesive layers and enamel parts in GB were lower than those in GA, whereas those in GD were greater than those in GC.

After standardization of the adjusted band values, the peak onlay stress in the GA under vertical loading was concentrated on the distal edge of the central retainer, and the peak stress in the adhesive layer occurred at the location corresponding to the peak stress of the onlay.²⁹ The onlay stresses in the GB were mainly distributed on the distal dental tissue surface and mesial marginal areas. The onlay stresses in GC were primarily distributed in the central retainer, with less stress in the abutting edge of the mesial plane, similar to the stress distribution in the GA (Fig. 6). The stress in the onlay in the GD was mainly concentrated on the distal dental tissue surface and mesial shoulder, similar to the stress distribution in the GB (Fig. 7). The peak stress on the onlay during oblique loading occurred at the point of loading onto the occlusal surface (Fig. 8). The stress distribution of the enamel and dentin under the oblique load was similar to that under the vertical load (Fig. 9).

The distributions of MPS and VMS were similar. Under vertical loading, the peak MPS of enamel and dentin was lower, whereas the stresses of the onlay and adhesive layer were substantially greater in GB and GD than in GA and GC. The stress of the resin of the groups with foundation restorations was slightly lower than that of the groups with monolithic ceramic blocks. Under oblique loading, a large change was not detected in the peak MPS of the onlays and dentin among all the groups. The stress in the adhesive layer and resin in the GB and GD groups was substantially lower than that in the GA and GC groups; the stress in the enamel in the GB group was lower than that in the GA group, and the stress in the enamel in the GD group was greater than that in the GC group.

The results of the stress analysis led to the rejection of the null hypothesis that foundation restorations would not affect the stress distribution in endodontically treated teeth. Four different restoration methods were used to restore mandibular first molars with mesial wall defects after endodontic treatment, and the stress distribution was analyzed to assess the protective effects of foundation restorations on the remaining tooth tissue. Onlays of GA and GC exhibited stress concentrations in the central retainer, mainly at the distal edge of the bottom of the pulp chamber, a common site for cracks in brittle materials.³⁰ Stresses concentrated in the lower third of the central retainer and marginal zone are more likely to cause restoration failure.¹² The resin used for foundation restorations replaced part of the dentin while filling the pulp cavity to support the onlay. Although the stress of the onlays and adjacent areas in the GB and GD increased to varying degrees, the stress distribution was more uniform. This increase may have been because of the high elastic modulus of the onlay material, which reduced the deformation and stress inside the onlay. The onlays in the GA and GC were thicker, resulting in lower degrees of deformation when subjected to the same external force, which enables the onlays to withstand larger impact forces,²⁸ thereby minimizing the risk of restoration fracture. The relatively low elastic modulus of the resin absorbs part of the stress during onlay deformation.³¹ However, a restorative material with a low elastic modulus increases the internal bending deformation of the onlay, causing tensile stress inside the onlay and a heightened risk of fracture, which can be shown in the increased stress in finite element analysis. The stresses of the resin were mainly concentrated in the marginal region where it was in contact with the onlay. The base resin in GA and GC was only 2 mm thick. The volume of the resin structure in GA and GC was substantially smaller than that in GB and GD, with a reduced ability to absorb stress. The peak stress of the resin structure in GA and GC was greater than that in GB and GD under any load, protecting the remaining dental tissue and enhancing the possibility of restoring it.

Oblique loads impose the most damaging force on dental tissue and restorations.¹² Under oblique loading, the peak stress concentration area of the tooth tissue and onlays was smaller, and the stress was substantially greater than that under vertical loading. In this study, stress on onlays in GA and GC was concentrated in the lower-third of the buccal retainer in the cervical region of the tooth, corresponding to the area of crack initiation.³⁰ GB and GD exhibited stress concentrations near the mesial shoulder. Stress concentrations at the margins can damage the marginal adhesive layer and lead to restoration or dental tissue fracture, causing microleakage.³²

Brittleness is a typical mechanical property of enamel and is defined as a significantly lower tensile strength than compressive strength.²⁸ In this study, the onlays in the GB and GD primarily consisted of enamel and resin as the main supporting structures. Owing to the lower elastic modulus of the resin, the enamel of GB and GD experienced greater VMS under vertical loading than did that of GA and GC. However, the peak MPS of enamel was lower and the peak MPS in GD was greater than that in GC under oblique loading, primarily because the loading area partially coincided with the edges of the onlay and enamel in GD. This result means that the restorative method used in GD would cause greater stress during mastication. Based on the VMS and MPS results for dentin, the stress in the GB and GD groups not only did not increase but was also generally lower than that in the GA and GC groups. With the same loading force, an increased stress area reduced the stress per unit area. The stress concentration areas of the dentin in the GB and GD groups exceeded those in the GA and GC groups, indicating a more uniform stress distribution in the dentin in the GB and GD groups. The elastic modulus of enamel is significantly greater than that of dentin.³³ The present findings demonstrated that foundation restorations dispersed stress and should protect weak dentin.

The extent of coverage of the occlusal surface was another factor affecting the stress distribution. Complete- and partial-coverage restorations differed mainly in the amount of tooth preparation in the crown, where less preparation increased the fracture resistance of the affected tooth. However, the complete-coverage restoration had superior stress dispersion compared with the partial-coverage restoration. Regardless of vertical or oblique loading, the findings consistently revealed the lowest stress levels in almost all parts of the model in the complete-coverage restoration group. Conversely, the highest stress levels were found in almost all parts of the model in the partial-coverage restoration group. Whether under vertical or oblique loading, the better stress dispersion effect of the complete-coverage restoration primarily manifested in the lower stress of the onlay, adhesive layer, and enamel. Under oblique loading, stress dispersion was more pronounced in the adhesive layer and enamel. The stress in the adhesive layers was lower in GB and GD than in GA and GC, indicating the improved stress dispersion of foundation restorations. Onlays in GA and GC, which are thicker than those in GB and GD, were less prone to deformation, thereby reducing stresses within the onlays and, consequently, lessening the stresses transferred to the adhesive layer.³¹ The peak stress of the enamel in all models was concentrated in the region of the distal cervical area close to the CEJ boundary. Combined with the analysis that the peak stress of GC and GD was greater than that of GA and GB, restoration failure was more likely to occur in GC and GD. For the stress distribution of the adhesive layer, the other important factor, illumination, cannot be reflected in the 3D finite element analysis. The onlay thickness of GA and GC is significantly greater than that of GB and GD, which will inevitably affect the degree of conversion of resin cement, thereby affecting aspects such as adhesive properties, mechanical properties, and shrinkage.³⁴ The type of adhesive and polymerization mode also have important effects.³⁵ The peak VMS values of the onlay, adhesive layer, and enamel under vertical loading, as well as the peak values of onlay and enamel equivalent stress under oblique loading, were consistently highest in GD, indicating that onlay restoration after foundation restorations may not be a priority for teeth with single-wall defects after endodontic treatment. Under experimental conditions, GB, featuring complete-coverage onlay restoration after foundation restoration, exhibited the most favorable stress distribution.

The relationships among tooth preparation, resistance form, and retention form to resist masticatory pressure must be considered in clinical practice. Real-world clinical restoration should consider the long-term survival rate of onlays, balancing minimal invasiveness with a thorough assessment of occlusal contact points and remaining wall resistance. Limitations of this study include that only the stress distribution at the end of the masticatory cycle under static loading was analyzed. In the modeling of GC and GD, the overlap between the loading point and the edge of the restoration was not considered, resulting in a greater stress concentration in the adhesive layer and onlay, which affected the stress analysis of the onlay and adhesive. The selection of an ideal restoration method still needs to be further explored through in vitro experiments.

Based on the findings of this finite element analysis study, the following conclusions were drawn:

Foundation restorations provide good effective stress dispersion to protect the remaining dental tissue. However, onlay restorations with foundation restoration placement should be used with caution in patients with excessive occlusal forces.

Oblique loads were more destructive to the adhesive layer and remaining dental tissue than were vertical loads.

Complete-coverage restorations have better stress dispersion ability than partial-coverage restorations.

Funding:

This research did not receive any specific grants from funding agencies in the public, commercial, or not-for-profit sectors.

Acknowledgments:

The authors thank Professor Shiqian Xu (School of Materials, Harbin Institute of Technology) for their guidance throughout the experiment. The authors also thank Editage for their English language and grammar editing.

Authors' contributions

Qiuyue Wang: Writing - original draft, Methodology, Investigation, Data curation, Formal analysis.

Dongqi Shi: Writing - original draft, Methodology, Investigation, Data curation, Formal analysis.

Xiangling Liu: Writing - review & editing, Formal analysis, Project administration.

Weijun Zhang: Writing - review & editing, Software.

Zhaolun Sun: Writing - review & editing.

Yao Feng: Writing - review & editing.

Lingbo Zhang: Writing - review & editing, Conceptualization, Supervision, Funding acquisition, Resources.

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No competing interests reported.

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Mechanical effects of foundation restoration on onlay preparations of mandibular first molars after endodontic treatment

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