Comparative evaluation of stress distribution in maxillary premolar restored with onlay fabricated with different restorative materials – A three-dimensional finite element analysis study

Abstract

Aim:

The aim of this study was to evaluate stress distribution in maxillary premolar onlay fabricated of four different restorations.

Methodology:

A three-dimensional (3D) maxillary premolar model of onlay was simulated using SolidWorks software. The fabricated 3D onlay model was then restored using four different restorative materials, namely fiber-reinforced direct composite, indirect composite, pressable ceramic, and zirconia using the parameters of Young’s modulus and Poisson’s ratio. The models were subjected to an axial load of 200 N. Von Mises stresses and strains were calculated. The highest stresses and strains generated in four different restorative materials were observed when subjected to vertical force on the palatal cusp, one on each marginal ridge, and one on the central fossa.

Results:

Maximum von Mises stresses generated with indirect composite, direct composite, IPS Empress ceramic, and zirconia onlay were 453.83 MPa, 441.36 MPa, 376.82 MPa, and 368.82 MPa, respectively, in the maxillary premolar onlay group.

Conclusion:

The results concluded that zirconia is a preferred choice of material for complex premolar cavities due to its superior stress distribution, enhancing durability, and protecting tooth structure. Conversely, composites may elevate failure risk under occlusal loads in large cavities.

INTRODUCTION

In the oral cavity, teeth are consistently subjected to varying degrees of masticatory forces. The structurally compromised teeth, which involve large restorations, are more susceptible to fracture under masticatory forces.[1] Hence, it is crucial to assess the stress concentration in such teeth, which will enable the clinicians to opt for the best possible restoration, which will not only restore the lost structure and function but will also improve their longevity over time.[2]

Partial coverage adhesive restorations, such as onlay, offer an advantage over complete coverage crowns by allowing more conservative preparations, thereby increasing the longevity of teeth.[3]

Different materials have been used for the fabrication of partial coverage restorations, and the continued research in material sciences have led to the development of materials which are capable of successfully restoring cavities involving significant loss of tooth structure. The ceaseless upgradation of materials utilized in dental restorations has facilitated the enhancement and fine-tuning of their mechanical and optical properties.[4]

Various studies such as structure analysis, contact analysis, and model analysis have been conducted to evaluate such stress patterns. The merger of biological and engineering sciences has enabled researchers and practitioners to evaluate the biocompatibility and mechanical performance of dental restorations as well as the oral components, via computational analysis, which is difficult to assess in vivo.[2]

One such engineering breakthrough is the finite element modeling, which has emerged as a valuable research tool in dentistry, and is utilized to scrutinize the impact of different elements and techniques.[5,6]

The maxillary premolars and molars endure maximum masticatory load which include both axial and shear stresses, making them susceptible to fractures.[7] The premolars, in particular, pose a challenge to the clinician owing to their varying morphology, small occlusal table, and steep cusps resulting in aberrant stress distribution.[8]

It is a challenge to achieve long-term successful rehabilitation of such teeth with significant loss of tooth structure. Partial coverage adhesive restorations, such as onlays, offer a more conservative option compared to complete coverage crowns.[1,9]

The present study, therefore, aimed to quantify and analyze the stress distribution in maxillary premolar teeth restored with onlay fabricated with four diverse materials with a three-dimensional (3D) finite element model analysis.

METHODOLOGY

An intact, caries-free maxillary first premolar was utilized as a reference to acquire a cone-beam computed tomography (CBCT) scan (Carestream Dental CS 8200 3D). A 3D Computer-aided design (CAD) model of the maxillary first premolar was then generated from the CBCT data using finite element analysis (FEA) software (Geomagic software). The resulting surface contours and meshes were imported into SolidWorks 2015 software, where a 3D solid model of the scanned tooth was created using the “SCANto3D” add-in module. The scanned model was then converted into a virtual solid model using the same software. The surface discretization obtained was then imported into SolidWorks software. It was imported into ANSYS Workbench 18.2 version for FEA simulation. The generated surface meshes were imported into SolidWorks Software 2019 (Dassault Systemes SolidWorks Corp., Waltham, MA, USA). 3D solid model of the scanned tooth was generated using a “SCANto3D” add-in module and imported. Interfacial surface between pulp chamber and dentin and interfacial surface between dentin and enamel were made by lofting technique of CAD program according to the anatomy of natural tooth. Following 3D volumes of enamel, dentin, and pulp, Boolean operations were used to ensure congruence between relevant interfaces.

Three-dimensional cavity design preparation

In the solid maxillary premolar model, mesio occluso distal (MOD) onlay cavity was designed [Figure 1a]. The buccolingual dimension at the occlusal margin was fixed at 3 mm and at the pulpal floor area was set at 2.5 mm; the cavity had an occlusal divergence of 2°–5° with the occlusal box having a depth of 2 mm. The dimension of proximal box was fixed at 3 mm occlusally and 2.5 mm gingivally. The width of gingival seat from axial wall to cavosurface margin was 2 mm, and at the gingival seat, it was placed 1.5 mm above the cement-enamel junction. No cavosurface bevels were incorporate. The residual dentin thickness from the pulpal and axial walls to the pulp was 1.0 mm. The occlusal anatomy was reshaped by reducing the functional cusps by 2 mm and nonfunctional cusps by 1.5 mm [Figure 1b].

Figure 1.

(a) Mesio-occluso-distal cavity was created on the three-dimensional CAD model of the maxillary premolar tooth, (b) Onlay fabricated from the maxillary second premolar tooth

The materials used to restore the restoration had the following three similar properties:

1. Linear elasticity: response to stress being proportional to the amount of deformation up to a certain point[10]

2. Homogeneity: Same properties at all points[10]

3. Isotropicity: The same properties that do not depend on the direction of the force applied.[10]

The 3D models of the teeth were broken down into tiny pyramid-shaped pieces (tetrahedral elements). The number of these pieces and connection points (nodes) varied depending on the onlay tooth model (between 36,372 and 40,148 elements and 53,620–59,284 nodes). To prevent the entire model from moving freely, the bottom nodes (representing the tooth root) were fixed in all directions (X, Y, and Z). This stopped the whole tooth from floating around the simulation. The elastic properties (Young’s modulus [E] and Poisson’s ratio [μ]) used in the models were taken from the literature [Table 1].[11]

Table 1.

Properties of tooth components and materials used in virtual model of premolar restored with onlay[11]

	Young’s modulus	Poisson’s ratio
Enamel	93	0.20
Dentin	18.6	0.30
Fiber-reinforced composite	2.09	0.32
Indirect composite	6.2	0.30
Pressable ceramic	40	0.26
Zirconia	25	0.24

To simulate biting forces, a vertical load of 200 Newtons (N) was applied vertically on the palatal cusp as well as to each marginal ridge and the central fossa of the restoration. The von Mises stress gradient was utilized to assess the stress distribution within the restorative materials. This method was chosen due to its effectiveness in comparing stress levels across different geometric configurations. The analysis primarily concentrated on identifying the peak stress value, serving as a benchmark to understand the overall stress distribution within the restoration. The findings from the FEA are presented in the form of colorimetric graphs [Figures 2–4]. These graphs use color to represent the magnitude of the maximum principal stress (in megapascals, MPa) across the restoration. This allows for a quick visual understanding of where the highest stresses are located. These graphs plot the actual values of the maximum principal stress (MPa) across different locations on the restoration. This provides a more detailed picture of the stress distribution.[11] To pinpoint the areas experiencing the most stress, and to analyze how stress was distributed throughout the different parts of a tooth, namely enamel, dentin, and any buildup material, the automatic maximum value detection feature in ANSYS Workbench 18.2 software was utilized. This identified the region with the highest stress concentration. Instead of relying on color-coded maps (colorimetric maps), the stress data for this specific region were then exported to a more convenient format: a. stp file. The data were then organized based on its distribution and overall pattern. Finally, bar graphs were created to visualize the peak stress levels for each tooth component.[11]

Figure 2.

Maximum von Mises stresses generated in restorations with various dental materials. Stress magnitudes in restorations restored with various dental materials showed the following difference

Figure 4.

Stress concentration for IPS Empress ceramic depicting von Mises stress values localized on palatal cusp, marginal ridge, and central fossa

RESULTS

The study found that various dental restorative materials, when subjected to vertical occlusal load of 200 N, produced comparable stress distribution patterns within all onlay groups. This applied load was distributed across the functional cusp, marginal ridges, and central fossa. The findings from the FEA are presented in the form of colorimetric graphs [Figures 3-6]. These graphs use color to represent the magnitude of the maximum principal stress (in megapascals, MPa) across the restoration. Among the restorative materials evaluated for maxillary premolar onlays, indirect composite exhibited the highest maximum von Mises stress (453.83 MPa, Figure 6), followed by direct composite (441.36 MPa, Figure 5), IPS Empress (376.82 MPa, Figure 4), and zirconia, which showed the lowest stress value (368.82 MPa, Figure 3) among all materials tested. In terms of stress distribution, elevated von Mises stress values were localized on the restoration surfaces, primarily in the region subjected to direct occlusal loading, as depicted in Table 2.

Figure 3.

Stress concentration for zirconia depicting von Mises stress values localized on palatal cusp, marginal ridge, and central fossa

Figure 6.

Stress concentration for direct composite resin restorations depicting von Mises stress values localized on palatal cusp, marginal ridge, and central fossa

Figure 5.

Stress concentration for indirect composite resin restorations depicting von Mises stress values localized on palatal cusp, marginal ridge, and central fossa

Table 2.

Maximum von Mises stress values for the respective restorative materials

Group name	Load value (N)	Maximum von Mises stress
Zirconia	200	368.28
Pressable ceramic	200	376.82
Direct composite	200	441.36
Indirect composite	200	453.48

DISCUSSION

Posterior teeth endure functional and parafunctional forces of different magnitudes and directions, with intraoral loads ranging from 10 N to 430 N. Abnormal stress concentration can lead to clinical failures, such as tooth fractures, cement seal ruptures, and fractures of the restorative material.[11]

Maxillary premolars, among posterior teeth, are subjected to the highest axial and shear stresses, rendering them particularly vulnerable to vertical fractures.[12] An experimental study by Mondelli et al. confirmed that the palatal cusp and marginal ridges of premolars are more frequently implicated in fracture occurrences.[13]

Currently, onlay restorations have emerged as the treatment of choice for complex cavities in premolars, as they optimize the preservation of tooth structure while simultaneously enhancing its strength and resilience.[13]

When designing an onlay for a MOD cavity in a premolar, it is essential to strike a balance between preserving healthy tooth tissue and creating a strong and durable restoration. In this study, simplified 3D model of maxillary premolar with onlay was subjected to loading conditions that may mimic loads generated during the final clenching stage of the chewing cycle.[14]

To analyze the stress distribution in premolars with MOD onlay, the 3D FEA study was employed wherein von Mises stress was used as a measure of stress concentration which is the measure of the overall stress level in a material, often used to predict the likelihood of fracture in FEA.[15]

For evaluating the long-term performance of dental restorations, FEA offers a suitable approach which, by simulating the effects of continuous loading, can help identify potential weaknesses in the restoration, thereby influencing treatment decisions.[15]

FEA allows researchers to compare the performance of different restorative materials and designs under controlled conditions, providing valuable insights into their potential for failure (Soares et al., 2015; Boschian Pest et al., 2006).[15]

Among the composite materials simulated in the present study, the indirect composite was observed to exhibit the high von Mises stress values (453.48 MPa) compared to the fiberfill direct composite (441.36 MPa). These findings align with a study conducted by Mendonça et al., which compared direct and indirect composite resins, in which they concluded that direct composite restorations were more effective in reducing stress transmission to the tooth than indirect composite onlay and hence were better suited to restore extensive cavities in teeth.[16] In contrast to our findings, Azeem and Sureshbabu reported that indirect composite onlay outperformed direct composite restorations in posterior teeth due to their superior anatomical form, wear resistance, marginal seal, and reduced microleakage.[17]

In the present study, the ceramics outperformed the composites, generating lower stresses with the least stress values being observed in the zirconia (368.8 MPa) onlay followed by IPS Empress ceramics (376.82 MPa). This difference may be attributed to the higher elastic modulus of the zirconia which may be responsible for transmitting minimum stress to the tooth structure. The fatigue load capacity of zirconia indicates its high fracture resistance, making it a robust choice for restorative materials. Specifically, zirconia exhibits a flexural strength ranging from 1.0 to 1.2 GPa, which is significantly greater than the 160 MPa typically associated with IPS Empress ceramics.[18] The findings of this study are consistent with a study conducted by Kobayashi et al.,[19] which demonstrated that zirconia generates lower stress on teeth compared to other restorative materials, owing to its superior mechanical properties, particularly its fracture resistance, rendering it a desirable choice for dental restorations.[19]

In another 3D FEA conducted by Boschian Pest et al.,[15] the stress distribution in onlay restored with IPS Empress ceramic and composite materials was examined. The results showed that IPS ceramic onlay was more effective in dissipating stress, while indirect composite onlay transmitted a significant amount of stress to the tooth (Boschian Pest et al., 2006).[15] These results are in accordance with the present study highlighting the differing mechanical behaviors of these two restorative materials under load, emphasizing the importance of material selection in dental restorations to optimize stress distribution and minimize potential damage to the underlying tooth structure.

The choice of restorative material is crucial for optimizing outcomes, and ceramic materials, in particular, can help to reduce tooth deflection by concentrating stress within the restoration itself.[20]

It is, however, challenging to incorporate all variables present in the oral environment into computer simulations. Furthermore, higher von Mises stress concentrations which are measured under static conditions may not reliably predict failure patterns under dynamic conditions and are likely associated with the fracture of either the restoration or the abutment tooth. The oral environment is highly dynamic and complex, making it challenging to accurately simulate with computer models. Future studies should address these limitations to improve the predictive power of simulations. Additionally, both experimental and clinical research are necessary to corroborate the findings.[21]

CONCLUSION

Within the limitations of this study, zirconia appears to be the preferred material for restoring complex cavities in premolars, where its superior stress distribution can contribute to enhanced durability and structural integrity, protecting the underlying tooth structure. In contrast, composite materials may increase the risk of failure in large cavities when subjected to occlusal loads.[22]

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.