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