In a previous study, a 60% increase in push-out strength was obtained in vitro with a two-step cementation of fiber posts, a procedure equivalent to the layering technique of composite restorations. The aim of this study is to find the rationale for this increase in push-out strength with finite element analysis (FEA).
FEA models were created of the push-out test set-up of fiber posts cemented according to a one-step and two-step procedure and of the complete root with post. The failure loads of glass-fiber posts cemented with RelyX Unicem as obtained in a previous study were used as the load in the push-out FEA models. For the complete root model, a load of 100 N was used. The stresses due to the shrinkage of the cement layer and the applied load were determined for the one-step and two-step procedure of the push-out test specimens and for the one-step procedure of the complete root.
Even though the load in the two-step push-out model was 60% higher compared to the one-step model, the combined stresses were comparable. The stresses due to shrinkage alone in the complete root approached or exceeded the bond strength of resin cements to dentin in the coronal and apical areas.
FEA of this test set-up explains the results of the in vitro study. Two-step cementation of fiber posts leads to a decrease in internal stresses in the restoration which results in higher failure loads and possibly in less microleakage.
Over the last years, prefabricated glass fiber posts have gained popularity. They have a number of advantages compared to metal posts . One of the main problems with any type of post, including prefabricated glass fiber posts, is the bonding of the post to the root canal walls. Retention is influenced by a number of factors, such as the type, composition and shape of the post, the cement, the bonding between the different parts, the pretreatment of the different substrates, contamination with other chemicals, etc. . The adhesion of cement to the root canal walls is mainly of micromechanical nature, based on infiltration of the demineralized dentin and the formation of a “resin dentin interdiffusion zone” . Shrinkage and contraction stress of the cement, and the unfavorable configuration factor (C-factor) within the root canal influence the quality of the bond negatively . This is illustrated by the observation that debonding is the most common mode of failure for fiber posts clinically .
A problem with composite materials in general, and with the cementation of fiber posts with resin cements in particular, is the polymerization contraction stress. The shrinkage and the accompanying contraction stress for different restorative techniques for direct resin composites have been studied extensively . The composition of the restorative materials, chemical vs. light curing, and different light curing programs on the curing devices are aimed to reduce the contraction stress within a restoration.
The contraction stress is related to the so-called C-factor, which is defined as the ratio between bonded and unbonded surface of a restoration. The higher the C-factor, the higher the contraction stresses can become. It has been shown that in high C-factor geometries, contraction stress values can exceed the bond strength and even the cohesive strength of the resin composite itself.
In intracoronal restorations, the problem of a high C-factor has been addressed by filling cavities in increments instead of using a bulk technique. This is suggested to reduce the contraction stress and microleakage, but others do not confirm these results. Using layers ( C < 1) instead of using a bulk technique ( C = 3–5) results in a more favorable configuration and, in principle, to less contraction stress . In the root canal, which in many ways can be regarded as a very deep Class I cavity, with a calculated C-factor that can exceed 200 an incremental layering technique has not been practiced until recently. In a recent study, an incremental layering cementation technique for the cementation of fiber posts (two-step cementation) has been developed and tested in vitro . This technique consists of cementing a Teflon post, with a diameter 30 μm larger than the fiber post, in the root canal. The cement is then cured and the Teflon post removed, leaving a post-space with a perfect fit for the fiber post. The corresponding fiber post can therefore be cemented with a thin and uniform cement layer. This technique significantly decreases the C-factor, since the surface of the cement in contact with the Teflon post can be considered unbonded surface. The second cement layer used for cementing the glass fiber post is so thin that the stresses in this layer will have little effect on the total restoration. In this study, a 60% increase in push-out strength was achieved with this two-step cementation procedure compared to the normal one-step procedure. Significant differences were found for all cements tested.
The aim of this study was to gain insight in the theoretical background of this two-step cementation procedure. Finite element analysis (FEA) was performed both on models representing the specimen used for in vitro push-out testing and on the entire restoration, the human canine root with a glass fiber post cemented in it.
Materials and methods
Three-dimensional finite element simplified models were created according to the dimensions and with the material properties of a previous study in which an incremental layering technique for the cementation of fiber posts (two-step cementation) was investigated . In the latter study, the difference in push-out bond strength between a conventional one-step cementation procedure where the post was placed directly, and the experimental two-step cementation procedure was determined. For the one-step procedure, the fiber posts were cemented 9 mm into the canal according to manufacturers’ instructions, and the cement was cured. With the two-step procedure a Teflon post (special design RTD; 30 μm thicker than the #2 post) was used instead. The Teflon post was removed after the recommended curing time according to manufacturers’ instructions, and freshly mixed cement was inserted into the space left by the Teflon post, followed by the definitive placement of the fiber post.
Per root, four cross-sections with a thickness of 0.7 mm were prepared. The differences in push-out strength between cementation procedure and cement were determined. Simplified models with the same geometry as these cross-sections were used as models for the finite element analysis. The experimentally determined loads for the one- and two-step cementation procedure of 45.8 N and 75.9 N, respectively, were used as input for the FE models.
Two models represented the dentin discs as used in the push-out test of fiber posts cemented in a single-rooted human canine tooth, according to a one-step and two-step cementation procedure. A third model represented a simplified model of the root portion of a fiber post restoration cemented with a conventional one-step cementation procedure. The Finite Element modeling and post processing were carried out with FEMAP software (FEMAP 10.02; Siemens PLM Software, Plano, Texas, USA), while the analysis was done with NX Nastran software (NX Nastran; Siemens PLM Software, Plano, Texas, USA).
Push-out test models
The specimens of the push-out test are symmetrical in geometry. Therefore, in order to limit the number of elements, models were realized that represented half specimens, with the nodes in the centric plane allowing for sliding in the surface only. The nodes of the bottom of the model, with the exception of the nodes in an inner circle of d = 3.28 mm, were prevented from moving in the vertical direction. The nodes in the center were prevented from moving in the horizontal plane. The models were composed of 21,448 parabolic tetrahedron solid elements; the cement layer along the post was 3 layers thick. The dimensions of the test models are shown in Fig. 1 .