1 Volume 22 2023 e230356 Original Article Braz J Oral Sci. 2023;22:e0356http://dx.doi.org/10.20396/bjos.v22i00.8670356 1 Assistant Lecturer in Operative Dentistry Department, Faculty of Dental Medicine, Al–Azhar University (boys-Cairo) 2 Professor of operative dentistry and Vice dean for Post Graduate affairs, Faculty of Dental Medicine, Al–Azhar University (boys-Cairo) 3 Assistant Professor in Operative Dentistry Department, Faculty of Oral and Dental Medicine, Al–Azhar University Corresponding author: Nabil Al Aggan n.a.a.elagan@gmail.com Editor: Dr. Altair A. Del Bel Cury Received: July 11, 2022 Accepted: November 4, 2022 Influence of the cervical margin relocation on stress distribution - a finite element analysis on lower first molar restored by direct nano-ceramic composite Nabil al Aggan1,* , Sameh Mahmoud Nabih2 , Abd Allah Ahmed Abd Al Hady3 Aim: Evaluate the influence of the cervical margin relocation (CMR) on stress distribution in the lower first molar restored with direct nano-ceramic composite (zenit). Methods: A 3D model of the lower first molar was modeled and used. Standardized mesio-occluso-distal (MOD) preparation consisted in two models used in this study with mesial subgingival margin in model II. (CMR) was applied in model II using flowable composite or resin glass ionomer (Riva). Both models were restored with nanoceramic composite and then subjected to six runs (2 for the model I and 4 for model II) with load (100N) as two load cases, one at (11º) and other at (45º) from the vertical axis. The stress distributions (FEA) in the final restoration and (CMR) material were analyzed using 3D models. Results: The two models recorded an equivalent Von Mises stress and Total deformation in the final restoration, regardless of the difference in the oblique angle incidence from (11º to 45º) or the type of the material used for (CMR) there was no significant difference in the (FEA) between the model with CMR (model II) and the model without CMR (model I). Conclusions: (CMR) technique seems to be biomechanically beneficial with high eccentric applied stress, (CMR) with resin glass ionomer or flowable composite resin in combination with nanoceramic composite improved the biomechanical behavior of (MOD) cavities extended below cement enamel junction (CMR) with high modulus elasticity material like (Riva) exhibits a more uniform stress distribution. Keywords: Finite element analysis. Composite resins. Glass ionomer cements. https://orcid.org/0000-0001-9635-7538 https://orcid.org/0000-0002-2371-698X https://orcid.org/0000-0002-8659-1010 2 Al Aggan et al. Braz J Oral Sci. 2023;22:e0356 Introduction Long-term clinical observations showed that even large cavities encompassing three or more surfaces and cusps of load-bearing posterior teeth can be restored success- fully with minimally invasive direct restoration techniques. However, direct restoration of deep proximal defects beyond the cemento enamel junction (CEJ) requires elabo- rate treatment techniques and considerable operator skill1,2. (CMR) technique has been proposed as a non-invasive pretreatment for the res- toration of deep Class II cavities with proximal cervical margins extending below (CEJ)3,4. (CMR) is an alternative for performing surgical crown lengthening and offers the possibility of a stepwise relocation of deep proximal margins to uplift cavity outlines for direct or indirect restorations5,6. Step one consists of placing a base of flowable or direct resin composite to elevate the margin above the (CEJ). Step two allows the practitioner to decide on whether to place a direct or an indirect restoration according to the restoration of choice under improved clinical conditions5,6. With current adhesive technology and modern composite resin materials it has become possible to restore even severely damaged teeth and undermine tooth defects using direct composite resin materials such as nano-ceramic composite6,7. Restorations should be strong enough to resist the intra-oral forces; in fact, as a result of bite forces, restored teeth are exposed to high mechanical stresses8. Therefore, biomechanical principles have an important part in the clinical success of restorative materials8. Classical methods of mathematical stress analysis are extremely limited in their scope and are inappropriate for dental structures that have an irregular struc- tural form and complex loading9. Currently, (FEA) is a numerical method for stress analysis. It involves a set of com- putational procedures to calculate the stress and strain in each component, generat- ing a model solution10. The development of technology enabled (FEA) to evolve from two-dimensional to three-dimensional modeling. The difference between 2D and 3D modeling is that 3D models are more realistic and have a closer to reality represen- tation of the biomechanical interactions in the human anatomy, restorations, and implant components10,11. Stresses acting upon the materials during function in the oral cavity are Normal or Principal stress which acts perpendicular to the cross section and causes elon- gation or compression and shear stress which acts parallel to the cross section and causes distortion (changes in original shape)11. The main advantages of (FEA) are the variables can be easily changed, simulation can be performed without the need of the patient, it offers maximum standardization, and it helps to visualize the point of maximum stress and displacement. However, it is not easy to predict fail- ure in complex designs made of different materials and complex loading varying in relation to time and point of application12. It is now considered the most theoret- ically accurate method of solving equations involving compatibility and elasticity. Finite elements are fundamental when analyzing bone and tooth failure as these 3 Al Aggan et al. Braz J Oral Sci. 2023;22:e0356 are intimately connected with stress and strain behavior10,12. Null hypothesis of the present study is that the (CMR) has an adverse effect on (FEA) of restored teeth. The aim of this study was to evaluate the influence of the (CMR) on stress distribution (FEA) on the lower first molar restored by direct nano-ceramic composite (Zenit). Materials and methods This in-vitro study was performed to evaluate and compare the influence of the (CMR) on (FEA) on the lower first molar restored by direct nano-ceramic composite (Zenit). Two models were used in this study; standardized MOD cavity preparations were performed in the two models where proximal margins were located 2 mm above (CEJ) in (moodle1) and in (model II) the mesial proximal margin located 1 mm below (CEJ)10. The generalized steps to perform a finite element analysis can be summa- rized as follows 1. Model scanning 2. Geometric model preparation. 3. Definition of the materials properties. 4. Mesh generation (nodes and elements generation). 5. Application of load, and boundary conditions. 6. Obtaining the data of resultant stresses and comparing the results10,12,13. 1. Model scanning A Three dimensional (3D) finite element model was constructed by 3D scanning of a sample tooth (lower first molar). The teeth geometry was digitized with a laser scanner (Geometric Capture, 3D Systems, Cary, NC, USA). Such a scanner produced a data file containing a cloud of points coordinates. An intermediate software was required (Rhino 3.0 - McNeel inc., Seattle, WA, USA) to trim a newly created surface by the acquired points. Then, the solid (closed) teeth geometry was exported to a finite element program as STEP file10,12,13. Standardized mesio-occlusal-distal (MOD) preparation consisted in two models used in this study with mesial subgingival mar- gin in model II. (CMR) was applied in model II using flowable composite or resin glass ionomer (Riva). 2. Geometric model preparation First, we set up the directions (top, bottom, mesial, distal, anterior, posterior). Then, we set up the mask thresholds to define the mask of enamel and the mask of dentin to define tooth tissues with its mechanical properties and finally, we calculate 3D objects10,12,13. We used a “cut orthogonal to screen” tool to cut through the tooth to reproduce the MOD of the molar, then we formed the pulpal extension part by cutting in the facial aspect and proximal surface of the molar, then the two parts 4 Al Aggan et al. Braz J Oral Sci. 2023;22:e0356 were merged to form the whole MOD10,12,13. Then all the dentin parts were merged then the enamel part was constructed to be applied in the finite element analysis test with its mechanical properties10,12,13. After applying a set of Boolean operations (add, subtract, overlap, etc.) the two models’ parts were ready for material assign- ment and meshing. Thus model I can be defined as no dentin removal under (CEJ) while model II can be defined as 1mm dentin removal under C.E.J. The 1mm dentine removed from root geometry under (CEJ) was restored by Dynamic flow flowable composite and Riva light cure glass ionomer as two case studies10. 3. Definition of the material properties For linear static stress analysis, there are two essential parameters that need to be defined; elastic (Young’s) modulus and Poisson’s Ratio, which are enough for defining the linear part of the stress strain curve of any isotropic material. The properties of the materials used in the present study were listed in Table 1 . Table 1. Material’s properties of models’ components. Materials Modulus of elasticity in MPa Poisson’ s ratio Enamel 80,350 0.33 Ref 8 Dentin 19,890 0.31 Ref 8 Zenit 18754 0.3 Ref 14 Dynamic flow 5,300 0.28 Ref 15 Riva glass ionomer 10,860 0.3 Ref 8 4. Mesh generation (Nodes and Elements generation) Each component of the model was assigned to a material property on the finite element package ANSYS Workbench version 16 (ANSYS Inc., Canonsburg, PA, USA). Then a parabolic tetrahedral element was used for meshing the model, and adequate mesh density was selected to ensure results accuracy for the discrete model10,12,13. 5. Application of load and boundary conditions After the models were meshed, two different oblique forces each of 100N14-16 were applied as two load cases, one at (11º)10 and the other at (45º)10,14-18 from the ver- tical long axis of the tooth. Each load was equally divided on 15 points represent- ing; outer, inner surface cusp tips of labial cusp, inner surface of lingual cusp, cen- tral and mesial triangular fossa distal and mesial marginal ridge18 as presented in (Figure 1). Thus, totally six runs were performed on the two models as follow- ing:-Two runs on the model1 (one at 11º and other at 45º) from vertical axis and four runs on CMR materials of model II (one run at 11º and other at 45º on Dyract Flow) & (one run at 11º and other at 45º on Riva light cure glass ionomer) from the vertical axis10,13. 5 Al Aggan et al. Braz J Oral Sci. 2023;22:e0356 Figure 1. Loading points 6. Obtaining the data of resultant stresses The resultant Von Mises stresses and Total deformations were calculated under both loading conditions and distributions as; maximal resultant values. Data anal- ysis was performed in several steps. Initially, descriptive statistics for each group results. One-way ANOVA followed by pairwise Tukey’s post-hoc tests were per- formed to detect significance between subgroups. Student t-test was done between paired groups. Two-way ANOVA was done to show the effect of each variable (main group and subgroup). Statistical analysis was performed using Graph-Pad InStat statistics software for Windows (www.graphpad.com). P values ≤0.05 are statisti- cally significant in all tests. HP Z820, with Dual Intel Xeon E5-2660, 2.2 GHz proces- sors, 64GB RAM . Results The distribution and magnitude of Von Mises stresses and Total deformation in each component of the model were calculated. In the present study Table 4 & Figure (3,5) revealed that an equivalent value of maximum Von Mises stress at 11- degree (234.7), at 45 -degree (299.8) and equivalent value of total deformation at 11- degree (0.0173), at 45 -degree (0.0526) were recorded on the final restoration of the two models. Also, there was a positive correlation between increase in the oblique angle incidence from the long axis of the tooth from (11º to 45º) and the stress received by the restorations. 6 Al Aggan et al. Braz J Oral Sci. 2023;22:e0356 The result of the present study Table 4 & Figure (7,9) revealed that both (CMR) materi- als showed nearly the same Total deformation at 11-degree (0.0109) and at 45-degree (0.0338), while flowable composite received less Von Mises stresses at 11-degree (4.7) and at 45-degree (5) than Riva at 11-degree (6.5) and at 45-degree (7.1) by about 40% in the model II. Regardless of the difference in the oblique angle incidence from (11º to 45º) or the type of the material used for cervical marginal relocation material there was no sig- nificant difference in the stress distribution (finite element analysis) between the two models where the (CMR) technique was used or not. The Von Mises stresses and Total deformation results of the six runs applied on final restorative material & (CMR) materials for (FEA) on the two models were illustrated in the Table 2 and Figures (2 - 9). Table 2. The Von Mises and Total deformation of the six runs on the two models. Total deformationVon MisesRuns 0,0173234,71- Model I –Ob 110 zenit 0,0526299,8 2- Model I -Ob 450 zenit 0,01094,7 3- Model II – Ob 110 Dyract 0,03385,0 4- Model II – Ob 450 Dyract 0,01096,55- Model II – Ob 110 Riva 0,03377,16- Model II – Ob 450 Riva Figure 2. Model II (final restoration) Von Mises stress under oblique load at 11º from vertical axis 7 Al Aggan et al. Braz J Oral Sci. 2023;22:e0356 350.0 300.0 250.0 200.0 150.0 100.0 50.0 – 234.7 299.8 234.7 299.8 234.7 299.8 Model #1 – Ob11 Model #1 – Ob45 Model #2 – D Flow – Ob11 Model #2 – D Flow – Ob45 Model #2 – Riva light – Ob11 Model #2 – Riva light – Ob45 Von Mises Stress – Restoration Figure 3. Column chart showing comparison of Von Mises of final restoration between the two models. Figure 4. Model II (final restoration) Total deformation under oblique load at 11º from vertical axis 8 Al Aggan et al. Braz J Oral Sci. 2023;22:e0356 0.0600 0.0500 0.0400 0.0300 0.0200 0.0100 – 0.0173 0.0526 0.0173 0.0527 0.0173 0.0526 Model #1 – Ob11 Model #1 – Ob45 Model #2 – D Flow – Ob11 Model #2 – D Flow – Ob45 Model #2 – Riva light – Ob11 Model #2 – Riva light – Ob45 Total Deformation – Restoration Figure 5. Column chart showing comparison of Total deformation of final restoration of the two models. Figure 6. Model II (flowable resin relocation material) maximum Von Mises stress under oblique load at 11º from vertical axis 9 Al Aggan et al. Braz J Oral Sci. 2023;22:e0356 8 7 6 5 4 3 2 1 0 4.7 5.0 6.5 7.1 Model #1 – Ob11 Model #1 – Ob45 Model #2 – D Flow – Ob11 Model #2 – D Flow – Ob45 Model #2 – Riva light – Ob11 Model #2 – Riva light – Ob45 Von Mises Stress – Cervical Figure 7. Column chart showing comparison of Von Mises of (CMR) materials in model II. Figure 8. Model II (flowable resin relocation material) maximum Total deformation under oblique load at 11º from vertical axis 10 Al Aggan et al. Braz J Oral Sci. 2023;22:e0356 0.0109 0.0338 0.0109 0.0337 Model #1 – Ob11 Model #1 – Ob45 Model #2 – D Flow – Ob11 Model #2 – D Flow – Ob45 Model #2 – Riva light – Ob11 Model #2 – Riva light – Ob45 Total Deformation – Cervical 0.0400 0.0350 0.0300 0.0250 0.0200 0.0150 0.0100 0.0050 – Figure 9. Column chart showing comparison of Total deformation of (CMR) materials in model II. Discussion (FEA) has been widely employed as an effective tool to evaluate the stress-strain distribution. It could evaluate the biomechanical characteristics of both the restored teeth and the dental restorative system. Further, the results carry signifi- cant clinical implications regarding the ability to withstand the masticatory forces in the oral cavity11. (FEA) values are divided as Von Mises stress, maximum principle stress (tensile stress), minimum principle stress (compressive stress), and shear stress. However, in most finite element studies presented in the literature, The von Mises criterion is a formula for combining three principal stresses into an equivalent stress, which is then compared to the yield stress of the material. If the “von Mises stress exceeds the yield stress, then the material is at the failure condition9,10. The 100 N load used in this study was chosen as the average chewing force, which is supposed to be one third of the maximum biting force. A restoration must resist natural forces that occur in the mouth14-16. A 45-degree angle to the long axis of the tooth was chosen to match the lateral the force (eccentric force) applied on the teeth during mastication12,17-20 while an 11-degree angle to the long axis of the tooth was chosen to match the applied perpendicular force (centric force) on the teeth (90 degrees) during mastication10. Null hypothesis that the (CMR) has an adverse effect on stress distribution (FEA) on restored teeth was rejected because in this study (CMR) by resin glass ionomer or flowable composite resin in combination with nanoceramic composite improved the biomechanical behavior of MOD cavities extended below (CEJ). The present study revealed that an increasing in the total deformation and Von Mises stresses applied on the final restoration under both models by increasing oblique angle from (110 to 450); this was in agreement with Rodrigues10 (2016) who 11 Al Aggan et al. Braz J Oral Sci. 2023;22:e0356 observed that the loads applied with a 45-degree angle incidence causes more stress accumulation on the final restoration than the load applied with an 11-degree angle incidence. In the present study, an equivalent Von Mises stress and total deformation on the final restoration of the two models can be explained by the ability of the two (CMR) materials in the model II to support the deformations and stresses excreted on final restoration as in model I; this may be dependent on the fact that the elastic modulus of restorative materials plays an essential role in stress absorption and load trans- mission21. Hence, (CMR) could work as an “absorber,” in which an intermediate layer of material with low elastic modulus reduces stress concentrations in the restoration and tooth structure22. Such findings corroborate those found by other authors10,23-25 who found that there was no significant difference in the stress distribution between the two models, CMR was not negative for biomechanical behaviors and the use of glass ionomer cement or flowable composite resin in combination with a bulk-fill composite improved the biomechanical behavior of deep class II MO cavities. How- ever, diverging from the findings of Ausiello et al.26 (2017) who found that the direct resin-based composite materials applied in multilayer techniques to large class II cav- ities produced adverse FEA stress distributions. In our study (CMR) materials showed nearly the same deformation at (110,450), while flowable composite received less stress than Riva at (110,450).This finding may be correlated to the material elasticity modulus; using restoration material with high elasticity received higher stress without differences in deformation so Riva absorbed more stresses than Dynamic Flow due to (Dynamic flow) lower in modulus of elastic- ity (5.3) than Riva (10.8) 15,29(2003). Such findings corroborate those found by other authors8,27 who showed that a restorative material with appropriate elasticity module was able to absorb more stress. In the current study, regardless the difference in the oblique angle incidence from (11º to 45º) or the type of the material used for (CMR) material there was no signif- icant difference in the (FEA) between the two models where the (CMR) technique was used or not; this may be attributed to the (CMR) technique reduce the gingival extension of the restoration, placing it in a more coronal position, which may have reduced the lever arm and consequently the restoration deflexion28. Also, the base under the resin composite restoration might have acted as a tampon layer reduc- ing the effects of stress concentration and the modulus of elasticity of Dynamic flow is close to dentin28. This finding was in agreement with Rodrigues10 (2016) who observed that there was no significant difference in the stress distribution between the two models regardless the difference in the oblique angle incidence from (11º to 45º) or the type of the material used for (CMR) material. A limitation of the present research is that several assumptions were made during designing of the models since the stress distribution pattern directly depends on the model design and the materials’ properties assigned to each layer of the model, any inaccuracy may be directly reflected in the results. Also, the magnitude and direction of the maximum bite force and masticatory bite force considered in this study are averaged values and may not match the in vivo conditions accurately. In addition, this study does not simulate the ideal structure of the tooth. Further study is needed to 12 Al Aggan et al. Braz J Oral Sci. 2023;22:e0356 allow the definition of the (Enamel, Dentin, Periodontal ligament & Cementum) in the model to mimic all dental structure related to the influence of (CMR) on (FEA). In conclusion, within the limitations of the present study, the following conclusions were drawn: (CMR) technique seems to be biomechanically beneficial with high eccentric applied stress, (CMR) by resin glass ionomer or flowable composite resin in combination with nanoceramic composite improved the biomechanical behavior of MOD cavities extended below (CEJ), (CMR) with high modulus elasticity material like (Riva) exhibits a more uniform stress distribution. Clinical significance (CMR) does not impair biomechanical behavior. Acknowledgments This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Conflict of Interest The authors have no conflicts of interest to declare. Data Availability Datasets related to this article will be available upon request to the corresponding author. 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