Recent Advances in Applied & Biomedical Informatics and Computational Engineering in Systems Applications

Stress Distribution in with MOD Cast Metal Restorations

LILIANA SANDU, FLORIN TOPALĂ, SORIN POROJAN School of Dentistry “V. Babeş” University of Medicine and Pharmacy 9 Revolutiei 1989 Blv., 300072 Timişoara ROMANIA [email protected]

Abstract: Little information is available on the stress analysis of cast metallic inlays and onlays. Modern design and valuation in order to obtain an adequate framework strength involves numerical simulations. The aim of the paper was to evaluate, the effect of MOD cavity design for cast metal inlays and onlays on stress distribution in teeth structures and restorations. The study was performed on an upper first , using a finite element analysis. The first step of the study was to achieve 3D models in order to design and analyze teeth and cast metal MOD inlays and onlays. Eleven 3D models of maxillary first premolars, prepared for MOD inlays and onlays with different tapers were generated. The mesh structure of the solid 3D model was created using the computational simulation of Ansys finite element analysis software. An occlusal load of 200 N was conducted, and stresses occurring in the inlays, onlays and teeth structures were calculated. For all cavity designs, stresses in the restoration, enamel, and dentin were evaluated separately. The highest stress values were exhibited in the restoration in the most cases. Stresses in the enamel were significantly lower in the teeth prepared for onlays, compared to the teeth prepared for inlays. The MOD restoration on premolars has to distribute the load over a wide surface. Covering the occlusal surface with the restoration, using an MOD onlay, may prevent failure and produce a lower stress values than an MOD inlay.

Key-Words: premolar, MOD cavity design, cast metal restoration, inlay, onlay, 3D model, stress analysis.

1 Introduction techniques on stress distribution. FEA is deemed as Large restorations have been considered the an effective tool to evaluate the biomechanical principal factors to predispose a to fracture, characteristics of these dental restorative materials and adequate preparation guidelines are important. and systems, whereby the results carry significant Irrespective of the cavity preparation design, clinical implications [5,6]. preparations weaken teeth. To reduce loss of tooth Little information is available on the stress analysis tissue and to improve biomechanical results, inlay of cast metallic inlays and onlays. Modern design and onlays restorations are good treatment choices and valuation in order to obtain an adequate for extensive cavities in posterior teeth [1, 2]. framework strength involves numerical simulations. Inlays restore central cavities in teeth. Onlays Analyses in this field continuously advance both in restore one or more cusps and may completely cover three-dimensional modeling, computer aided design the occlusal surfaces, resulting in a favorable and structural analyses simulation methods. distribution of stresses in teeth and a decreased risk of fractures. However, preparation for onlays requires additional tooth reduction relative to inlay 2 Purpose restorations [3,4]. Cast metal inlays and onlays can The aim of the paper was to evaluate, by means of be used on premolars requiring a MOD restoration three-dimensional finite element analysis, the effect instead and offer a durable alternative. of MOD cavity design for cast metal inlays and Because it is known that MOD inlays may increase onlays on stress distribution in teeth structures and the susceptibility to fracture, it is important to restorations. ensure optimal performance in selection of the adequate preparation design to reduce stresses in teeth structures and also in the restorations [3]. 3 Materials and Method Finite element analysis (FEA) has been widely Sophisticated three-dimensional models are required employed in many researches to investigate the to better understand the mechanical behavior of impact and effect of dental materials and restorative teeth structures and prosthetic dental restorations.

ISBN: 978-1-61804-028-2 356 Recent Advances in Applied & Biomedical Informatics and Computational Engineering in Systems Applications

The first step of the study was to achieve 3D models the preparation. Inlay and onlay cavities designs in order to develop applications for basic research were created using literature data. use, to design and analyze teeth and cast metal MOD inlays and onlays. The study was performed on an upper first premolar, using a finite element analysis. Surfaces were modeled according with anatomical dimensions. The nonparametric modeling software (Blender 2.57b) was used in order to obtain the shape of root, enamel, dentin and pulp structures (Fig. 1).

a

Fig. 1. Premolar surfaces modelled according to the anatomical dimensions. b Fig. 3. Volume of the prepared and restored The collected data were used to construct three premolar: a. with inlay, b. with onlay. dimensional models using Rhinoceros (McNeel North America) NURBS (Nonuniform Rational B- The mesh structure of the solid 3D model was Splines) modeling program. These points were used created using the computational simulation of Ansys to extrapolate the shape of the object, a process finite element analysis software (Fig. 4). called reconstruction (Fig. 2). Reconstruction involves finding and connecting adjacent points in order to create continuous surfaces. Nonuniform rational B-spline (NURBS) is a mathematical model commonly used in computer aided design, manufacturing and engineering. CAD is mainly used for detailed 3D models but it is also used throughout the manufacturing process, from conceptual design, through strength and dynamic simulation analyses.

a

Fig. 2. Volume of the premolar.

Twenty-two 3D models of maxillary first premolars, b prepared for MOD inlays and onlays with different Fig. 4. Mesh structure of the restored premolar: tapers (between 0 and 10 degree from the cavity a. with inlay, b. with onlay. base to the surface) were generated (Fig. 3). In the onlay cavity, cusps were reduced and included in

ISBN: 978-1-61804-028-2 357 Recent Advances in Applied & Biomedical Informatics and Computational Engineering in Systems Applications

An occlusal load of 200 N was conducted, and Table 2. Von Mises equivalent stress values in the stresses occurring in the inlays, onlays and teeth restoration and in the inlay and onlay restored structures were calculated. premolars. An oblique loading of 200 N was applied in 5 Sample Taper Von Mises equivalent stress [Pa] points: to the mesial and distal marginal ridge, and [degree] Restoration Enamel Dentin buccal (3 points). At each selected loading 1 0 5.65E+08 4.57E+08 2.72E+07 point, an oblique loading of 50 N was applied 2 1 4.93E+08 5.03E+08 3.71E+07 (Fig. 5). 3 2 4.85E+08 4.98E+08 3.80E+07 4 3 4.98E+08 4.30E+08 3.83E+07 5 4 6.40E+08 5.29E+08 2.55E+07 6 5 5.06E+08 5.23E+08 1.89E+07 7 6 5.31E+08 4.34E+08 2.72E+07 8 7 4.53E+08 4.95E+08 3.52E+07 9 8 5.55E+08 4.62E+08 2.66E+07 10 9 5.50E+08 5.50E+08 2.59E+07 11 10 4.96E+08 4.69E+08 3.64E+07 12 0 5.93E+08 3.75E+07 3.56E+07 13 1 5.90E+08 3.83E+07 2.63E+07 14 2 5.96E+08 3.91E+07 2.76E+07 a 15 3 5.52E+08 3.80E+07 2.82E+07 16 4 5.75E+08 3.84E+07 2.86E+07 17 5 5.40E+08 4.00E+07 2.46E+07 18 6 5.41E+08 3.97E+07 2.55E+07 19 7 5.44E+08 4.16E+07 2.60E+07 20 8 6.22E+08 3.60E+07 2.83E+07 21 9 5.76E+08 3.64E+07 2.47E+07 22 10 5.89E+08 3.88E+07 2.71E+07

b Fig. 5. Points selected for loading on the restored premolar: a. with inlay, b. with onlay.

In making the finite element models, the characteristics of a tooth structures and gold alloy used for the cast restoration were entered into the computer program (Table 1).

Table 1. Elastic properties of the isotropic materials. a Material/component Elastic Poisson’s modulus ratio [GPa] Dentin 18.6 0.32 Enamel 84.1 0.33 Gold 70 0.30

3 Results and Discussions To analyze stress distribution and location, all the created structures were isolated from the rest of the model. For all cavity designs (sample 1-11 for b inlays, sample 12-22 for onlays), stresses in the Fig. 6. Von Mises equivalent stress values in the restoration, enamel, and dentin were evaluated restored premolar: a. with inlay, b. with onlay. separately (Table 2, Fig. 6).

ISBN: 978-1-61804-028-2 358 Recent Advances in Applied & Biomedical Informatics and Computational Engineering in Systems Applications

The highest stress values were exhibited in the restoration in the most cases, excepted for the tooth prepared for inlay with a taper of 7 degree. In this case the maximal stress was located in the enamel. Stresses in the enamel were significantly lower in the teeth prepared for onlays, compared to the teeth prepared for inlays. Even if the distribution of the stresses is more extended for the teeth prepared for onlays, the values 10 times lower do not affect the strength of a the tooth structures. Occlusal load on an MOD restored tooth produces stress surrounding the contact areas in the restorations (Fig. 7).

b Fig. 8. Von Mises equivalent stress in the enamel of the restored tooth: a. with inlay, b. with onlay.

a

b a Fig. 7. Von Mises equivalent stress in the restoration: a. inlay, b. onlay.

In the enamel stresses are located around the contact areas for inlays prepared teeth and distributed in the functional cusp and around the buccal cervical area for the onlays prepared teeth (Fig. 8). For onlays restotred teeth the variations for different tapers were smaller than for inlays restored teeth. Regarding the distribution of stresses in dentin, they are located in the cervical areas, oral for inlays b prepared teeth and around the teeth for onlays Fig. 9. Von Mises equivalent stress in the dentin of prepared teeth (Fig. 9). the restored tooth: a. with inlay, b. with onlay. In enamel the stresses were significant lower in all cases when an onlay was used for the MOD restoration compared to those when an inlay was used.

ISBN: 978-1-61804-028-2 359 Recent Advances in Applied & Biomedical Informatics and Computational Engineering in Systems Applications

4 Conclusion diferent post, core and materials. Dental Within the limitations of this study, the following Materials 2007, 23:983–93. conclusions were drawn: [8] Lanza A, Aversa R, Rengo S, Apicella D, 1. Cast metal inlays transferred more functional Apicella A. 3D FEA of cemented steel, glass stress to the teeth structures if they are prepared and carbon posts in a maxillary incisor. Dent for inlays. Mater 2005;21:709–15. 2. The taper of the preparation has no significant influence on the stress values for all the studied cases. 3. The MOD restoration on premolars has to distribute the load over a wide surface. Covering the occlusal surface with the restoration, using an MOD onlay, may prevent failure and produce a lower stress values than an MOD inlay.

4 Acknowledgements This work was supported by CNCSIS-UEFISCSU, project number PN II-RU TE_217/2010.

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