Dimensioning Initial Preparation for Dental Incisal Reconstruction with Biomaterials

Received for publication, February 3, 2014

Accepted, March 1, 2014

ANDREEA ANGELA ŞTEŢIU1, VALENTIN OLEKSIK2, Mircea ŞTEŢIU*2, rADU PETRUSE2, MIHAI BURLIBAŞA3, MIHAELA CERNUŞCĂ-MIŢARIU1, ILEANA IONESCU4

1Faculty of Medicine, „Lucian Blaga” University, Sibiu, Romania; 2Faculty of Engineering, „Lucian Blaga” University, Sibiu, Romania; 3Faculty of Midwifery and Nursing , University of Medicine and Pharmacy „Carol Davila” Bucharest, Romania; 4Faculty of Dental Medicine , University of Medicine and Pharmacy „Carol Davila” Bucharest, Romania

*Address correspondence to: Faculty of Engineering, „Lucian Blaga” University, 4 Emil Cioran street, 550025 Sibiu, Romania.

Tel: +40269217928; Fax: +40269212716; TM: +40744391063; Email:

Abstract

Our goal is to determine the minimal width section value of the incisal preparation of the frontal human teeth area to be restored. We've used a model of a 31(FDI) incisor to be cured with photopolymerisable composite materials or ceramic. The scanned model of a 31 natural tooth is processed in CATIA software. A model of the preparation at different section widths (0.5 mm and 1.5 mm) is generated. In order to obtain a reliable restoration with minimum tooth sacrificed substance the minimum width section value was determined. A 30 N constriction force was considered as a regular clenching force in mastication. A finite element analyse is made in ANSYS software. Shapes of tensions, deformations and safety factor of the modelled tooth are presented, pointing the dangerous areas and sections both for photopolymerisable and ceramic materials. The results shows a dangerous area at the periphery of the restoration. This means that a good marginal adaptation can avoid any risks of fracture and an in-time reliability of the restoration. The results demonstrates that a minimum width of 1.5 mm for photopolymerisable materials and 0.5 mm for ceramic increases the safety factor and provides a stabile reconstruction.

Keywords: incisor reconstruction, dental materials, composites, ceramic, finite element.

1. Introduction

For dental restorations in anterior region there is mandatory to take into consideration the dental and morphological pathology. The incisal destruction of lower jaw incisors can affect the mastication and occlusion (1, 2). For occlusal cavities the shape of the preparation should follow the shape of teeth’s morphology in order to confer fracture resistance to the teeth (3). For incisor's restoration the dentist has to create his own shape, based on the shape of adjacent teeth or general morphology (4).

The preparation of these shapes has to be done with a minimal healthy dental substance sacrifice but in order to obtain a maximum resistance of the restoration.

The mechanical properties of the composites used for restoration depends on the filling type, on the efficacy of the resin-filling coupling process and on the material’s porosity degree after curing. A photopolymerisable conventional composite generally has a 260 MPa compression resistance and the chemically activated equivalent material, with 3% porosity, seems to be 210 MPa resistant at compression (5, 6, 7, 8, 9).

The ceramic blocks are resistant enough and have properties suchlike the dental enamel, with modulus of elasticity round 185 GPa for glass infiltrated oxide ceramics; 210 GPa for polycrystalline oxide ceramics and 45 GPa for feldspar structures (9, 10).

As for the force to be used in simulations, a human bite in regular mastication has been tested to a force of an average of 15 to 40 N (11). As for the maximum bite force, Waltimo and Könönen (12) have reported that the bite forces in the 113–1692 N range could be recorded with good reliability with quartz force transducer in the molar region, these values decreasing for the frontal region (8, 9, 11, 12).

2.Materials and Methods

2.1.In our research we have used:

A)A 31 (FDI numberring system) natural human teeth to be scanned and mathematically modelled.

B)SCAN 3D ENGINE - non contact scanner for scanning the human tooth.

C)CATIA software (Dessault Systems) to generate solid model of the scanned tooth.

D)Particular materials to be investigated – photopolymerisable composites (7) and ceramic blocks (10).

E)ANSYS V12 software (ANSYS Inc. Pennsylvania, USA) for finite element analysis of strain and deformations of the modelled teeth restoration(13).

2.2.We considered the following steps:

  • Study of the shape of a real preparation of the incisor 31 and modelling the shape of the preparation;
  • Mathematical model of a natural 31 incisor to be tested with finite element method considering a uniform load of 30 N at two levels of the preparation (0.5 mm and 1.5 mm);
  • The use of Finite Element Method to prove the critical areas of the restoration taking into consideration different materials and different levels for preparation;
  • Determining the minimal area and level for a good and reliable preparation.

Figure 1 ilustrates an intact 31 human incisor, on the plate scan of the Next Engine 3D Scanner. There is pointed the initial capture as a 3D scan model to be used with the CATIA design software (8).

Figure. 1.Experimental device. A) Lingual view of a 31 incisor; B) Distal view of a 31 incisor; C) Scan 3D engine platform with the 31 incisor positioned; D) Acquired 3D image of the scanned 31 incisor.

The geometric patterns obtained by scanning as shown in figure 1D, were processed with CATIA software and transformed in solid corps (fig. 2 A, C, D). The 3D scanned and designed model has different shapes for enamel, dentine and pulp such as a real teeth looks like (figure 2 B). Figure 2 shows the modelled 31 incisor.

Figure. 2. Modeled 31 inciisor. A) Enamel model; B) Real view of incisor region; C) 3D model; D) Cross section.

In figure 3 presents the 31 incisor prepared in CATIA for reconstruction with the obtained shape considering a 0.5 mm level and 1.5 mm level. A uniform load of 30 N masticator force, as a linear contact, was simulate (Figure 3C).

Figure. 3. Modeled 31 inciisor. A) At 0.5 mm level; B) Area of the shape at 0.5 mm level; C) At 1.5 mm level; D) Area of the shape at 1.5 mm level; E) Preparation for load FEM calculus and analyze.

The ANSYS (13) analyse method for the deformations, tensions, strain and stress analyze is based on the discrete principle and modal analysis using the finite element method (FEM). So we determine the tension and deformations state when loading the model in static regime (14, 15). The characteristics of the biomaterials is provided by our tests (9). Figure 4 presents the preparation of the model for investigation.

Figure. 4. Ansys 12 investigation of the model. A) Mesh of the model; B) Uniform distributed force simulation.

3.Results and Discussions

To analyse the tensions distribution the equivalent tension von Mises (ecv) is used, and the ANSYS 12 program calculates it as quadratic average of normal tensions at the base, middle or top of the finite elements. The results for two different materials type (photopolymerisable composites and ceramic blocks) at two different section levels (0.5 mm and 1.5 mm) are presented. The deformations and safety factor provides information about the dangerous zone where fractures can appear after reconstruction.

Figure 5 there presents the finite element analysis in Ansys software of the designed teeth with the restorative material – Zmack composite (producator) – in the modelled shape of 31 incisor. Displacements and safety factor wer set at 0.5 mm level for a shape area of 4.02 mm2, meaning a 0.5mm width preparation.

Figure 6 presents the same material – Zmack composite – at 1.5mm level, for an area corresponding to 10.53 mm2, meaning a preparation of 1.5mm width preparation.

Figure 6 and 7 reveals, or the studied photopolymerisable composite, that at a level of 0.5 mm the stress and deformations are greater at teeth-restoration interface than for a 1.5 mm level. This means that an optimum preparation must be created greater than 0.5mm width level. The preparation is to be created with preserving as much as possible health teeth tissue parts; therefore we recommend a 1.5 mm width preparation

Figure. 5. Finite element method investigation in ANSYS software of a composite biomaterial (ZMACK) for a reconstruction at 0.5mm. A) Filling material displacements; B) Safety factor of the reconstruction; C) Teeth displacements; D) teeth safety factor.

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Figure. 6. Finite element method investigation in ANSYS software of a composite biomaterial (ZMACK) for a reconstruction at 1.5mm. A) Filling material displacements; B) Safety factor of the reconstruction; C) Teeth displacements; D) Teeth safety factor.

As for ceramic block (figure 7A) the safety factor chart reveals that there is a small surrounding zone of the shape posed in the external zone of the area, among the interface enamel-restoration. More obvious area is revealed for the displacements of the restoration in figure 7B. As for teeth section, figure 7C shows that in the common area restoration-teeth there are transversal solicitation that could damage the restoration.

Fig. 7. Finite element method investigation in ANSYS software of a ceramic block restoration for a reconstruction at 0.5mm. A) Safety factor; B) Filling material displacements; C) Teeth displacements.

Figure 8 for ceramic block restoration made at 1.5 mm level, reveals that the dangerous area is decreased and the prognostic of the restoration is improved, meaning a 1.5mm width preparation offers a minimum safety zone to obtain a maximum reliability of restoration.

Figure. 8. Finite element method investigation in ANSYS software of a ceramic block restoration for a reconstruction at 1.5mm. A) Safety factor; B) Filling material displacements; C) Teeth displacements.

As for the stress analyse we present the VonMisses equivalent for photopolymerisable composite at 0.5 mm level (figure 9); 1.5 mm level (figure 10).

Figure. 9. VonMisses equivalent stress, for photopolymerisable composite at 0.5 mm level.

A) Restoration chart; B) Dentin chart; C) Enamel chart.

Figure. 10. VonMisses equivalent stress, for photopolymerisable composite at 1.5 mm level.

A) Restoration chart; B) Dentin chart; C) Enamel chart.

It is obvious that the influence on dentin (fig 9B, 10B) and cervical enamel region of the teeth (fig. 9C, 10C) is equal, not depending of the initial preparation. Some critical problems are supposed to appear at the border between preparation and restoration. Same observation is revealed when analysing figures 11 and 12, where ceramic block were considered for the incisor reconstruction.

Figure. 11. VonMisses equivalent stress, for ceramic at 0.5 mm level.

A) Restoration chart; B) Dentin chart; C) Enamel chart.

Figure. 12. VonMisses equivalent stress, for ceramic at 0.5 mm level.

A) Restoration chart; B) Dentin chart; C) Enamel chart.

4. Conclusions

The reliability of the dental reconstruction is in direct correspondence with the width of the incisor preparation.

The safety factorfor photopolymerisable composites where 0.817 (fig. 5B) at a width section of 0.5 mm of increasing to 1.4823 (fig. 6B) at a width section of 1.5 mm. Comparing to ceramic restorations where the safety factor for 0.5 mm width is 1.137 (fig. 7A) increasing at 1.99 (fig. 8A) for 1.5 mm width .

The minimum width we recommend for photopolimerisable materials is 1.5 mm in order to obtain a minimum safety factor greater than 1.0

The minimum width we recommend for ceramic reconstructions is 0.5 mm in order to obtain a minimum safety factor greater than 1.0 therefore more dental structure can be preserved comparing to photopolimerizable materials.

The incisal reconstruction does not affect internal dental structures such as dentin as the maximum stress charts indicates the same maximum values of 11.9 MPa ( fig. 9B, 10B, 11B, 12B).

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