Evaluation of the stress induced by post-and-core systems: finite element analysis.

Vitalariu, Anca Mihaela ; Comaneci, Radu

1. INTRODUCTION

During masticatory function the stress distribution into a tooth reconstructed with a post-and-core system can cause root fissure or fracture.

Controversy exists between specialists regarding what post is the best choice for the longevity of the restoration: metallic post or non-metallic post? (Pegoretti et al., 2002; Pierrisnard et al., 2002; McAndrew & Jacobsen, 2002).

The studies performed in vivo and in vitro showed that fracture resistance and clinical longevity of endodontically treated teeth reconstructed with post-and-core systems are significantly influenced by the post characteristics, such as design, dimensions and, mostly, the material (Rosentritt et al., 2004; Cormier et al., 2001).

This paper consists in a simulation study using the Finite Element Method (FEM) designed to evaluate the behaviour of the teeth reconstructed with different posts (titanium, ceramic, carbon fibre and glass fibre post) under a compression similar to occlusal forces and to compare the influence of the different post materials on stress distribution into the dental tissues.

2. MATERIAL AND METHOD

The Finite Element Method is the best method for evaluating the direction, nature and intensity of stress. This method consists of decomposing an object into as many elementary volumes as possible. Each volume takes on the mechanical properties of the part in which it is situated. At this point, it is possible to apply a load with known direction and intensity to any part of this object and to study the behaviour of each elementary volume. In the biomedical field FEM is an important method since it can avoid the necessity of traditional specimens and it is fundamental in studies that investigate stresses generated in restored teeth.

Finite element analysis steps (Figure 1):

--Identification of the physical model

--Realization of the 3D model

--Discretization of the 3D model

--Establishing the analysis type and adding the material characteristics

--Simulation of the physical phenomenon

--Results interpretation

--Conclusion

[FIGURE 1 OMITTED]

The study model was a sound maxillary central incisor with the following dimensions: L = 25.2 mm, Mesio-Distal diameter =8.5 mm (incisal) and 6.2 mm (cervical), Buccal-Oral diameter = 1.6 mm (incisal) and 5.1 mm (cervical).

The tooth was sectioned perpendicular to the long axis. Every slide was photographed and the digital images were transferred in AutoCAD program (AUTODESK Inc.), to construct the outlines of the morphologic elements in parallel plans and to define the isocline curves. The isocline curves were vertically superposed in the ALGOR program for obtaining the mesh and after that, the domain is structured in finite elements (Figure 2).

[FIGURE 2 OMITTED]

After the 3D model of an intact tooth was made, the root was separated in order to construct the 3D model of the tooth with a post-and-core system

The model has different components (enamel, dentin, titan post, carbon fibre post, glass fibre post and ceramic post) that designate the finite element groups. Every finite element group was ascribed with the physical and mechanical characteristics of the component represented by the group (Modulus of elasticity and Poisson's ratio). The biomechanical properties of materials used in this study were adopted from those available in the literature (Pegoretti et al., 2002; Pierrisnard et al., 2002)

The load (25 daN) was applied on the oral surface of the crown, under an angle of 45 degrees to the long axis of the tooth, evenly distributed to the loading area. The deformation limits are determined to ensure the equilibrium of the structure. The conditions differ between the two 3D models (intact tooth and reconstructed tooth with different post).

3. RESULTS

The results show the intensity and distribution of stresses into the root and in the whole tooth reconstructed with different posts.

3.1 The von Mises stresses recorded in the root

The root of the intact tooth showed the highest mechanical resistance from all, as was expected. The root with carbon fibre post had a lower resistance than intact root, but higher than the rest. The behaviour of the root with glass fibre post is close to that with carbon fibre post because of the similarities between mechanical properties of these two kind of posts. In case of a rigid post, like titan and ceramic posts, the root resistance is lower than that of a root reconstructed with a fibre (carbon or glass) reinforced post.

The von Mises stresses recorded in the root reconstructed with carbon fiber post are closer to that recorded in the root without post, comparing with the other posts studied (titan and ceramic posts)(Figure 3).

[FIGURE 3 OMITTED]

3.2 The von Mises stresses recorded in the tooth

Regarding the whole post/tooth complex, the stress distribution in the tooth with fiber reinforced post is quite similar with that in the tooth without post. The titan post and ceramic post produces the greatest stress concentration at the post/dentin interface (Figure 4), which predispose to vertical irreparable root fractures.

[FIGURE 4 OMITTED]

4. CONCLUSIONS

Post material has a significant effect on the stress concentration. The rigid posts (metallic and ceramic) produce the greatest stress concentration at the post-dentin interface, which predispose to vertical irreparable root fractures.

The fibre reinforced post shows the lowest peak stresses inside the root because of its stiffness that is much similar to dentin. Because of their low Young's modulus, the nonmetallic posts made from resin composite reinforced with carbon and glass fibres have a protective effect on the dental supporting tissues by reducing the risk of root fracture and therefore increasing the longevity of the restoration. Except for the force concentration at the cervical margin, the fibre reinforced post induces a stress field quite similar to that of the intact tooth.

The practical significance of these conclusions is that knowing the advantages and disadvantages of different type of post, the dentist will be able to avoid a post that predispose to irreparable root fractures.

The evaluation of post material influence on stress concentration only is a limitation of our paper. Therefore, the next step in our future research is to evaluate the influence of post dimensions and cement type on stress distribution into dental tissues.

5. REFERENCES

Cormier, C.J.; Burns, D.R. & Moon P. (2001). In vitro comparison of the fracture resistance and failure mode of fiber, ceramic and conventional post systems at various stage of restoration. J. Prosthodont, Vol.10, No. 26, 2001, pp.26-36

McAndrew, R. & Jacobsen, P.H. (2002). The relationship between crown and post design on root stress-A finite element study. Eur J Prosthodont Rest Dent, No.10, 2002, pp.9-13

Pegoretti, A,; Fambri, L; Zappini, G. & Bianchetti, M.(2002). Finite element analysis of a fiber reinforced composite endodontic post. Biomaterials, No.23, 2002, pp. 2667-2682

Pierrisnard, L.; Bohin, F.; Renault, P. & Barquins, M. (2002). Corono-radicular reconstruction of pulpless teeth: a mechanical study using finite element analysis. J Prosthet Dent, No. 88, 2002, pp. 442-448

Rosentritt, M.; Sikora, M.; Behr, M. & Handel, G. (2004). In vitro fracture resistance and marginal adaptation of metallic and tooth-colored systems. J Dent Rehabil, No. 31, 2004, pp. 675-681