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Assistant Professor, Department of Oral Regenerative Medicine, Academic Centre for Dentistry Amsterdam (ACTA), Universiteit van Amsterdam and Vrije Universiteit, Amsterdam, the Netherlands
Corresponding author: Dr Amanda Maria de Oliveira Dal Piva, Academic Centre for Dentistry Amsterdam (ACTA), Universiteit van Amsterdam and Vrije Universiteit, Gustav Mahlerlaan 3004, 1081 LA Amsterdam. Noord-Holland, THE NETHERLANDS
Assistant Professor, Department of Dental Materials Science, Academic Centre for Dentistry Amsterdam (ACTA), Universiteit van Amsterdam and Vrije Universiteit, Amsterdam, the Netherlands
Assistant Professor, Department of Dental Materials Science, Academic Centre for Dentistry Amsterdam (ACTA), Universiteit van Amsterdam and Vrije Universiteit, Amsterdam, the Netherlands
Professor, Department of Dental Materials Science, Academic Center for Dentistry Amsterdam (ACTA), University of Amsterdam & Vrije Universiteit, Amsterdam, the Netherlands
Professor, Department of Oral Regenerative Medicine, Academic Center for Dentistry Amsterdam (ACTA), University of Amsterdam & Vrije Universiteit, Amsterdam, the Netherlands
Whether the replacement of a missing tooth with a fixed partial denture supported by an endodontically treated abutment could be improved with endocrowns is unclear.
Purpose
The purpose of the study was to evaluate the mechanical behavior of a fixed partial denture (FPD) according to the preparation of the abutment teeth (endocrown or complete crown) in terms of stress magnitude in the prosthesis, cement layer, and tooth.
Material and methods
A posterior model with 2 abutment teeth (first molar and first premolar) was modeled with a computer-aided design (CAD) software program for conducting a 3-dimensional finite element analysis (FEA). To replace the missing second premolar, the model was replicated in different possible FPDs according to the abutment preparation design (complete crown [Conventional], 2 endocrowns [EC]) or an endocrown on one of the abutment teeth (first molar [ECM] and first premolar [ECP]) for a total of 4 designs. All FPDs were in lithium disilicate. The solids were imported to an analysis software program (ANSYS 19.2) in the standard for the exchange of product data (STEP) format. The mechanical properties were considered isotropic and the materials to show linear elastic and homogeneous behavior. An axial load (300 N) was applied at the occlusal surface of the pontic. The results were evaluated by colorimetric stress maps of von Mises and maximum principal stress in the prosthesis, maximum principal stress and shear stresses on the cement layer, and maximum principal stress in the abutment teeth.
Results
The von Mises stresses revealed that all FPD designs behaved similarly and that, considering the maximum principal stress criteria, the pontic was the most stressed region. For the cement layer, the combined designs presented an intermediate behavior, with the ECM more suitable to reducing the stress peak. The conventional preparation allowed less stress concentration in both teeth, and higher stress concentration in the premolar was observed with an endocrown. The endocrown decreased the risk of fracture failure. Considering the risk of debonding failure for the prosthesis, the endocrown preparation was only able to decrease the failure risk when the EC design was used and when only the shear stress was considered.
Conclusions
Performing endocrown preparations to retain a 3-unit lithium disilicate FPD is an alternative to conventional complete crown preparations.
Clinical Implications
Endocrown-retained, 3-unit lithium disilicate FPDs are an alternative to the conventional complete crown preparation because of their similar mechanical behavior. The endocrown is more conservative of tissue and may decrease the risk of prosthesis failure.
The restoration of an endodontically treated tooth aims to preserve and protect the remaining tooth structure while maintaining the anatomy, masticatory function, and esthetics.
Restoration of severely damaged endodontically treated premolars: influence of the ferrule effect on marginal integrity and fracture load of resin nano-ceramic CAD-CAM endocrowns.
Restoration is not straightforward, as endodontically treated teeth have reduced tooth structure and an increased risk of secondary caries and trauma associated with biomechanical changes.
Moreover, endodontically treated teeth are often associated with difficulties in planning, design of the restoration, and clinical and laboratory procedures.
Survival rates against fracture of endodontically treated posterior teeth restored with full-coverage crowns or resin composite restorations: a systematic review.
The restoration design may include a choice between partial or complete coverage based on the structural integrity of the remaining tooth, esthetic considerations, and functional requirements.
Additionally, with improvements in adhesive dentistry, more conservative options have become popular, reducing the need for conventional post-and-cores in teeth with considerable remaining tissue.
Restoration of severely damaged endodontically treated premolars: influence of the ferrule effect on marginal integrity and fracture load of resin nano-ceramic CAD-CAM endocrowns.
Endocrowns are monobloc overlays that can partially or completely restore the missing tissue of an endodontically treated tooth with an extension inside the pulp chamber.
The purpose of the extension is to improve stability inside the cavity during the cementation process, increase the adhesive area inside the root, and increase the resistance form of the restoration.
Restoration of severely damaged endodontically treated premolars: the influence of the endo-core length on marginal integrity and fatigue resistance of lithium disilicate CAD-CAM ceramic endocrowns.
Typically, endocrowns are fabricated with computer-aided design and computer-aided manufacturing (CAD-CAM) technology, reducing surface defects and providing excellent marginal fit.
the endocrown has been reported to be a reliable alternative to post-retained restorations for molars. However, additional investigation on premolars is still required.
Restoration of severely damaged endodontically treated premolars: influence of the ferrule effect on marginal integrity and fracture load of resin nano-ceramic CAD-CAM endocrowns.
Restoration of severely damaged endodontically treated premolars: the influence of the endo-core length on marginal integrity and fatigue resistance of lithium disilicate CAD-CAM ceramic endocrowns.
The effect of a fiber reinforced cavity configuration on load bearing capacity and failure mode of endodontically treated molars restored with CAD/CAM resin composite overlay restorations.
However, in some patients, the restoration involves not only a single compromised tooth but also the replacement of a missing tooth with a fixed partial denture (FPD) with conventional complete crowns or inlay abutment preparations for a more conservative option.
As information of the mechanical behavior of endocrown FPD retainers is lacking, the present study aimed to evaluate the stress distribution in a 3-unit endocrown. The null hypothesis was that no difference would be found in the stress distribution in the teeth, cement, and prostheses, regardless of the FPD design.
Material and methods
A 3-dimensional (3D) model was designed with a CAD software program (Rhinoceros 5.0; McNeel North America) from a previously reported 3D model of a partial right jaw
exported in standard tessellation language (STL) format to the CAD software program. The geometric structure with first molar, second premolar, and first premolar was divided into network surfaces to be used as a volumetric model. Through the option “reduce mesh,” 50% of relevance was defined to create a smooth structure with all normal surfaces oriented in the same direction. Then, with the aid of the reverse engineering plugin, the nonuniform rational B-spline surfaces were reconstructed from the mesh. Finally, the 3D volumetric model of the first molar, second premolar, and first premolar was created by the curve network and automatically defined. The second premolar was defined as the pontic and the root was removed. The connector had a rounded shape with an area of 4.2 mm2 for both abutments.
The initial model was replicated in 4 models with the same design and shape, and each model received a different combination of abutment preparation (Fig. 1): conventional FPD with 2 complete crown retainers (Conventional); combination with a premolar complete crown and a molar endocrown retainer (ECM), combination with premolar endocrown and molar complete crown (ECP), and with both retainers prepared for endocrowns (EC). After the models had been assigned, the crown preparations were modeled. For complete crown abutments, the preparation had rounded corners, 6-degree axial wall convergence with 1.5-mm thickness, 1.5-mm occlusal reduction, and a shoulder finish line.
After the modeling process, the crown had a minimum thickness of 1.5 mm at the center of the occlusal surface, and the endocrown had 7 mm in the same area.
Figure 1Designs evaluated. Conventional fixed partial denture with 2 complete crown retainers (Conventional); combination with premolar complete crown and molar endocrown retainer (ECM); combination with premolar endocrown and molar complete crown (ECP) and with both retainers prepared for endocrowns (EC).
A jaw section was used for the supporting tissue. A skull model was selected from the database and exported in the standard for the exchange of product data (STEP) format to a CAD software program (Rhinoceros 5.0; McNeel North America). The jaw model contained the main anatomic characteristics of healthy bone (size, shape, and absence of pathology). The bone structure was sectioned for the construction of the virtual model. The final bone was 1-mm thick in juxtaposition with cancellous bone. For the 3D volumetric model of the bone, the surface was created by the curve network automatically generated with the reverse engineering plugin. The Boolean difference between the roots and the bone defined a shell surface that was used to model the periodontal ligament. For that, the surface was extruded with an offset of 0.3 mm.
All solids were confirmed as a volumetric structure without uneven normal or duplicate surfaces. They were exported to a computer-aided engineering software program (ANSYS 19.2; ANSYS Inc), and the mesh was determined with tetrahedral elements (Fig. 2A). A compatible mesh intersection between interfaces ensured the interelement continuity in the finite element shape functions, allowing a smooth and accurate numeric solution between contacting solids.
Figure 2A, Mesh generation. B, Fixation (purple) and 300 N load application (red). C, Load application region or area.
Stress distribution of endodontically treated teeth with titanium alloy post and carbon fiber post with different alveolar bone height: a three-dimensional finite element analysis.
Influence of alveolar bone loss, post type, and ferrule presence on the biomechanical behavior of endodontically treated maxillary canines: strain measurement and stress distribution.
Evaluation of stress distributions in mandibular molar teeth with different iatrogenic root perforations repaired with biodentine or mineral trioxide aggregate: a finite element analysis study.
were inserted into the analysis software program, and each material was assumed to be isotropic and homogeneous. The analysis was carried out considering a no-failure condition, with the materials behaving elastically. Considering an adhesive restoration, bonded contacts were used between the structures. In the boundary condition, the fixation was defined at the base of the bone tissue with fixed zero nodal displacements (Fig. 2B).
Table 1Mechanical properties of materials and structures used
Stress distribution of endodontically treated teeth with titanium alloy post and carbon fiber post with different alveolar bone height: a three-dimensional finite element analysis.
Stress distribution of endodontically treated teeth with titanium alloy post and carbon fiber post with different alveolar bone height: a three-dimensional finite element analysis.
Influence of alveolar bone loss, post type, and ferrule presence on the biomechanical behavior of endodontically treated maxillary canines: strain measurement and stress distribution.
Influence of alveolar bone loss, post type, and ferrule presence on the biomechanical behavior of endodontically treated maxillary canines: strain measurement and stress distribution.
Evaluation of stress distributions in mandibular molar teeth with different iatrogenic root perforations repaired with biodentine or mineral trioxide aggregate: a finite element analysis study.
In the postprocessing, von Mises stress (MPa) was calculated for the ceramic FPD, and tensile stress (MPa) was calculated for the ceramic FPD, cement layer, and abutment teeth. Maximum shear stress (MPa) was also computed for the cement layer.
The risk for prosthesis failure (fracture or debonding) was according to the formula: stress peak/strength. Lithium disilicate tensile strength was considered for fracture failure, while, for the debonding failure, the cement adhesive failure risk was calculated considering the cement bond strength to dentin.
Qualitative stress maps with color scales were created to evaluate the stress distribution. With the aid of autoprobe, the maximum stress peak for each structure was recorded, separately evaluating the molar and premolar abutment teeth. Considering the ceramic FPD in an isometric view, von Mises criteria (Fig. 3) revealed that all designs behaved similarly in terms of stress pattern. Considering the maximum principal stress criteria, the pontic was the region with the highest stress concentration, consistent with the load application on the pontic occlusal surface.
Figure 3Von Mises and tensile stresses generated on ceramic fixed partial denture (FPD) according to abutment preparation design. Conventional, FPD with 2 complete crown retainers; ECM, FPD supported by premolar complete crown and molar endocrown; ECP, FPD supported by premolar endocrown and molar complete crown; and EC, FPD supported by both retainers prepared for endocrowns.
Figure 4 summarizes the stress distribution in the FPD from an occlusal view and from buccal and internal views in a section plane. Regarding the mechanical behavior, the connector showed higher stress concentration, regardless of the restoration design. In general, the molar connector showed higher stress values than the premolar connector. However, the abutment design (complete crown or endocrown) modified the stress peak. This effect was greater for the molar than the premolar. In addition, a high stress peak was seen when the abutment teeth received a crown preparation instead of an EC preparation. However, despite the slight differences, the values ranged from 69.05 to 122.90 MPa for all models. The stress data are summarized in Table 2.
Figure 4Tensile stress generated on ceramic fixed partial denture (FPD) according to abutment preparation design. Conventional, FPD with 2 complete crown retainers; ECM, FPD supported by premolar complete crown and molar endocrown; ECP, FPD supported by premolar endocrown and molar complete crown; and EC, FPD supported by both retainers prepared for endocrowns.
The stress distribution on the cement layer is presented in Figure 5. Unlike prosthesis mechanical behavior, the cement layer showed a qualitative difference when the abutment preparation changed among the simulated designs. The designs that had 1 abutment with a complete crown and the other with an endocrown showed an intermediate behavior, with the molar EC showing a lower stress peak than the ECP design.
Figure 5Tensile and shear stresses generated on cement layer according to abutment preparation design. Conventional, Ceramic fixed partial denture (FPD) with 2 complete crown retainers; ECM, FPD supported by premolar complete crown and molar endocrown; ECP, FPD supported by premolar endocrown and molar complete crown; and EC, FPD supported by both retainers prepared for endocrowns.
The model with a conventional FPD showed the highest shear stress values in the cement layers of the premolar (19.66 MPa) and molar (16.38 MPa). The design with both abutments as endocrowns led to a stress reduction of approximately 21% for the molar and 9% for the premolar. Considering tensile stress peaks, at least 1 abutment with an endocrown reduced the peak in the other abutment, while the EC design decreased only the peak for the molar.
For the abutment structures, Conventional preparation resulted in less stress concentration in both teeth (Fig. 6). Higher stress concentration in the premolar was observed with the presence of an endocrown preparation. The stress peaks ranged from 6.40 to 12.12 MPa for all models.
Figure 6Tensile stress generated on teeth according to abutment preparation design. Conventional, Ceramic fixed partial denture (FPD) with 2 complete crown retainers; ECM, FPD supported by premolar complete crown and molar endocrown; ECP, FPD supported by premolar endocrown and molar complete crown; and EC, FPD supported by both retainers prepared for endocrowns.
According to the failure risk calculation (Table 3), the presence of an endocrown decreased the risk of fracture failure compared with the conventional design, with the EC design having the lowest risk. Considering the prosthesis debonding failure risk, the endocrown preparation was only associated with decreased bonding failure risk when the EC design was used and only when the shear stress was considered. For tensile stress, the presence of an endocrown preparation decreased the stress peak in one tooth over the other; however, the effect was less critical than for shear stress.
Table 3Cement and prosthesis failure risk according to abutment design
The purpose of this study was to evaluate the stress distribution in a 3-unit endocrown FPD as a new treatment concept. According to the results, the null hypothesis that no difference in stress distribution would be found in the evaluated structures (teeth, cement, and prostheses) regardless of the used design was partially accepted.
According to the colorimetric maps, the stress distribution among the evaluated designs was similar. The region with higher stress concentration was the connector between molar and premolar for all groups. The stress peaks and failure risk analysis suggested that the EC design would decrease FPD fracture failure risk and debonding of the prosthesis. However, since the stress peak is only the highest point, it is not possible to conclude that the group would fail immediately or would initiate a localized defect that over time and under fatigue would result in a failure.
reported an estimated overall 5-year survival rate of 91.4% for endocrowns in comparison with 98.3% for complete crowns. The estimated overall 5-year success rates were 77.7% for endocrowns and 94% for complete crowns. However, the authors found no significant differences in overall survival or success estimates in the assessed restorations.
The similar performance between endocrowns and complete crowns can be explained by in vitro studies that reported equivalent fatigue resistance for both types of restorations,
Based on the clinical indications and limitations of the restorative material, lithium disilicate ceramic has been principally indicated for the replacement of anterior teeth and premolars.
Microtensile bond strength and interfacial properties of self-etching and self-adhesive resin cements used to lute composite onlays under different seating forces.
Therefore a 3-unit FPD in monolithic lithium disilicate replacing a second premolar was selected as a standardized design to evaluate the effect of retainer preparation.
The endocrown FPD design led to reduced stress peaks for the molar abutment teeth, cement, and connection region, while, for the premolar, it behaved similarly to the conventional complete crown design. Therefore, the results suggest that the endocrown design might be suitable for use as a retainer preparation design for a lithium disilicate FPD. However, previous investigations determined that a monolithic endocrown restoration that extended deeply inside the root and beyond the cement-enamel junction with a pulp chamber could lead to unfavorable fracture patterns.
The effect of a fiber reinforced cavity configuration on load bearing capacity and failure mode of endodontically treated molars restored with CAD/CAM resin composite overlay restorations.
According to in vitro studies, the prevailing mode of fracture for endocrown restorations is a split vertical failure, while complete crowns had more partial cusp fractures.
According to the stiffness of the restorative material, the functional stresses transmitted to the adhesive interface and to the dental tissues can be reduced or increased. These stresses may induce debonding at the tooth-restoration interface, which can lead to microleakage, eventually to postoperative sensitivity, and to secondary caries. The longevity of a restoration has been associated with marginal integrity.
The effect of a fiber reinforced cavity configuration on load bearing capacity and failure mode of endodontically treated molars restored with CAD/CAM resin composite overlay restorations.
Finite element analyses indicated low-magnitude stress concentration areas that under loading and humidity are subjected to fatigue; these areas would be critical in initiating a failure in a restoration or cement layer. This correlation has been previously reported in silico and in vitro investigations.
However, longitudinal studies are required to determine whether stress concentration regions correspond or not with the origin of clinical failures after fatigue.
An in vitro study reported that a ferrule in the geometry of the cavity does not improve the marginal adaptation after fatigue or the fracture resistance after monotonic loading compared with endocrowns without a ferrule.
Restoration of severely damaged endodontically treated premolars: influence of the ferrule effect on marginal integrity and fracture load of resin nano-ceramic CAD-CAM endocrowns.
Microtensile bond strength and interfacial properties of self-etching and self-adhesive resin cements used to lute composite onlays under different seating forces.
Finite element analyses have been widely applied to predict the behavior of different dental restorative procedures. This mechanical tool can also be applied to investigate a new treatment before it is implemented in patients, avoiding inappropriate treatment. Based on theoretical study results, clinical trials can be designed and performed to evaluate different clinical conditions and other parameters in an endocrown FPD design such as restorative material, cementation procedure, and connector design. However, since this method does not consider defects inside the materials or the complex environment inside the oral medium, clinical studies are needed. In addition, the abutment preparation for complete crown was simulated in dentin, and the effect of different core build-up materials should be evaluated, since it can alter the final mechanical response of the prosthesis.
Based on the findings of this finite element analysis study, the following conclusions were drawn:
1.
Endocrown preparations for retaining a 3-unit ceramic FPD in lithium disilicate should be a suitable alternative to the conventional complete crown preparation.
2.
The endocrown preparation on both abutments preserved more tissue and decreased the risk of FPD fracture or debonding failure for endodontically treated teeth.
References
Rocca G.T.
Sedlakova P.
Saratti C.M.
et al.
Fatigue behavior of resin-modified monolithic CAD-CAM RNC crowns and endocrowns.
Restoration of severely damaged endodontically treated premolars: influence of the ferrule effect on marginal integrity and fracture load of resin nano-ceramic CAD-CAM endocrowns.
Survival rates against fracture of endodontically treated posterior teeth restored with full-coverage crowns or resin composite restorations: a systematic review.
Restoration of severely damaged endodontically treated premolars: the influence of the endo-core length on marginal integrity and fatigue resistance of lithium disilicate CAD-CAM ceramic endocrowns.
The effect of a fiber reinforced cavity configuration on load bearing capacity and failure mode of endodontically treated molars restored with CAD/CAM resin composite overlay restorations.
Stress distribution of endodontically treated teeth with titanium alloy post and carbon fiber post with different alveolar bone height: a three-dimensional finite element analysis.
Influence of alveolar bone loss, post type, and ferrule presence on the biomechanical behavior of endodontically treated maxillary canines: strain measurement and stress distribution.
Evaluation of stress distributions in mandibular molar teeth with different iatrogenic root perforations repaired with biodentine or mineral trioxide aggregate: a finite element analysis study.
Microtensile bond strength and interfacial properties of self-etching and self-adhesive resin cements used to lute composite onlays under different seating forces.
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