Elsevier

Dental Materials

Volume 22, Issue 3, March 2006, Pages 234-242
Dental Materials

The influence of different core material on the FEA-determined stress distribution in dental crowns

https://doi.org/10.1016/j.dental.2005.04.034Get rights and content

Summary

Objectives

All ceramic restorations without metal have great advantages in their biocompatibility and aesthetic aspects. With the introduction of new core materials, the cores are sufficiently strong to produce long lasting all-ceramic restorations; however, the stresses in the veneering porcelain could still determine the longevity. The objective of this study was to evaluate, by finite element analysis (FEA), the influence of different core materials on the stress distribution in dental crowns.

Methods

The model of a multi-layer all-ceramic crown for posterior tooth 46 produced with CAD–CAM-technology was translated into a three-dimensional FEA program. This crown model was made with gold, zirconia, and alumina-based porcelain core and their matching veneering porcelains. The stress distribution due to the combined influences of bite forces, residual stresses caused by the difference in expansion coefficient of the core material and the veneering porcelain, and the influence of shrinkage of the cement was investigated.

Results

Stiffer core material does not always for various reasons result in lower stresses in the veneering porcelain.

Significance

This study indicates that the actual distribution of the tensile stresses and the design of restorations must be taken into account; otherwise, the significant contribution of stronger and tougher core materials to the performance of all-ceramic restorations may be offset by the weaker veneering porcelain.

Introduction

Despite the increased effort to prevent dental decay, there is still a need for prosthetic reconstructions. All ceramic restorations without metal have great advantages in its biocompatibility and aesthetic aspects. However, all-ceramic restorations, particularly when placed in the posterior region, have a history of being prone to brittle fracture. To overcome brittle fracture, strong ceramic core materials have been developed to support the weaker veneering ceramic materials. The bending strength of Yttria stabilized tetragonal Zirconia (Y-TPZ) ceramics [1], [2] is close to the actually used gold alloys [3]. With the introduction of these new core materials the cores are sufficiently strong to produce long lasting all-ceramic restorations; however, the stresses in the veneering porcelain could still determine the longevity. The stiffer ceramic core material might not result in lower stresses in the veneering porcelain than in crowns produced with gold alloy cores. All-ceramic restorations that are produced from these new materials are still more brittle and less ductile than metal–ceramic restorations. As a consequence, the preparation and cementation procedures are more critical for all-ceramic restorations, than for metal–ceramic restorations. [4]. For manually produced restorations, the stress distribution is difficult to predict, as the core and veneering porcelain layer shapes and sizes are operator dependent properties.

However, for computer designed and manufactured restorations, these parameters, as well as those of the preparation, are digitally available and thus more amenable to stress analysis and failure prediction. Finite element stress analysis (FEA) seems to be a proper tool for such an evaluation. FEA was originally developed in the aircraft industry [5] and has become widespread in the engineering field.

In dentistry, FEA has been used to determine stress distributions in teeth by authors like Farah et al. [6]. Many authors have been making finite element analysis of dental restorations since then. DeHoff et al. [7] studied the influence of the residual stresses due to thermal contraction mismatch between two layers forming the crown. Hojjatie et al. [8] and Palamara et al. [9] studied the influence of occlusal loads on the dentin and the restoration with a three dimensional finite element analysis, where Kamposiora et al. [10] and Shinohara et al. [11] studied specifically the effect of the cement layer on the restoration, taking into account the occlusal loads. Proos et al. did extensive work applied to margin design, different cement materials, and cement layer design [12], [13] and different core materials and thicknesses [14], [15].

Based on these experiences it was hypothesized that FEA is a proper tool for the multi causal evaluation of mechanical failure of dental restorations.

The objective of this study was to evaluate, by finite element analysis (FEA), the influence of different core materials on the stress distribution in dental crowns and the risks for failure.

Section snippets

Materials and methods

The CAD model of the crown for posterior tooth 46 of a patient produced with CAD–CAM by Cicero, Elephant Dental B.V. (Hoorn, the Netherlands) was selected to be translated into a three dimensional FEA program. The crown model was made with gold (Crown 1), zirconia (Crown 2), and alumina-based porcelain (Crown 3) core and their matching veneering porcelains. The materials used are listed in Table 1.

The crown had a chamfer with collar preparation and a uniform cement layer thickness of 0.140 mm

Stresses in the veneering porcelain at the occlusal surface

Fig. 2 shows the maximum principal stress of the combined stresses due to bite forces, difference in expansion coefficient of the veneering porcelain and the core material, and shrinkage of the cement at the occlusal surface. The stresses decrease with increasing Young's modulus of the core materials, with the exception of the tensile stress in Crown 3. The bite forces are the main component of the principal tensile and compressive stresses of the combined stresses (Table 2).

Stresses at the core–veneer interface

Fig. 3 shows the

Discussion

For the interpretation of the results of this study one has to take into account that clinically placed crowns might differ significantly from the FEA models. Possible errors are introduced by the translation from the CAD into the FEA model and are introduced during the production of the crowns; for instance, glazing of crowns will round off sharp edges. Other possible errors arise from the assumptions listed in Section 2 and the way the load was applied in the FEA model. The maximum tensile

Conclusions

The stiffer core material is for various reasons especially in the crown with alumina core not lowering the tensile stresses in the veneering porcelain. The Zirconia core is in this respect to be preferred over the alumina core, this material is combining a not to high Young's modulus with high strength. The bonding between veneering porcelain and these strong ceramic cores should be improved to exploit fully the strength of these materials [25], [26], [27], [32].

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