INTRODUCTION
============

Non-carious cervical lesions are characterized by a loss of hard tissue at the cemento-enamel junction.[@b1-dent04_p0160] These lesions are generally wedge-shaped and were previously termed idiopathic cervical erosion lesions, now referred to by Grippo[@b2-dent04_p0160] as abfractions.

A cervical lesion changes the distribution of stress within a tooth. Grippo suggests that if the lesion were left unrestored, the stress concentration caused by the cervical lesion would facilitate further deterioration of the tooth's structure, and hypothesizes that restoration of the lesion will decrease the concentration of the stress and progression of the lesion.[@b3-dent04_p0160]

These lesions were restored with mostly resin-based esthetic restorative materials, such as composite or resin-based glass ionomer. Many failures were seen in the cervical composite restorations,[@b4-dent04_p0160],[@b5-dent04_p0160] researchers report a greater loss of retention of these restorations among older patients.[@b6-dent04_p0160],[@b7-dent04_p0160] Lee states that this may occur due to either fewer teeth bearing the occlusal load in older patients, or to the protective mechanisms of natural dentition, such as cuspid guidance wearing down and allowing for greater lateral forces to be transmitted to the teeth.[@b8-dent04_p0160]

Elastic composites have been suggested to restore cervical lesions in order to accommodate bending and flexing.[@b6-dent04_p0160] A possible alternative for accommodating the stresses between restorations and dentin is the application of an elastic resin layer (low viscosity composites, filled adhesives) of a sufficient thickness.[@b9-dent04_p0160]--[@b11-dent04_p0160] Van Meerbeek et al[@b12-dent04_p0160] proposed that an adhesive layer and an intermediate layer may have a strain capacity sufficient to relieve stress between the shrinking composite restoration and rigid dentin substrate. Uno and Finger[@b13-dent04_p0160] speculate that the hybrid layer might play a role in absorbing stress in the dentin bonding procedure.

In complicated structures, it is difficult to achieve an accurate analytical solution. Numeric methods, such as the finite element method of analysis, can be considered a practical approach. Finite element analysis divides the problem domains into a collection of smaller parts (elements) and an overall approximated solution to the original problem is determined. In this method, solutions for each element are combined in order to obtain a solution for the entire problem.[@b14-dent04_p0160]

The aim of this particular finite element stress analysis (FEM) study is to evaluate the effect of a hybrid layer and its thickness on the distribution and intensity of shear stress formed under occlusal loading in a premolar tooth which had a wedge shaped cervical lesion restored with composite. The null hypothesis of this FEM study was that the hybrid layer and its thickness would alter the shear stress concentration in wedge-shaped cervical lesions restored with a resin composite.

MATERIALS AND METHODS
=====================

In this study, a three-dimensional finite element model simulating the cross-section of a first mandibular premolar tooth was used; the mathematical model was based on the anatomical measurements suggested by Wheeler.[@b15-dent04_p0160] The model included simulations of cortical bone, spongy bone, periodontal membrane, enamel, dentin, composite, adhesive layer, hybrid layer, and pulp tissue. In this model, the cortical bone level was formed through to the root, according to a natural tooth. All materials were homogenous, isotropic, and linearly elastic; the elastic properties of these tissues and materials were determined from the literature ([Table 1](#t1-dent04_p0160){ref-type="table"}). The thickness of the composite restorations was 2 mm and the width of the adhesive layer was 30 μm. The width of the hybrid layer was defined as 1.5 μm in model 2, and 3 μm in model 3.[@b16-dent04_p0160] Each mathematical model was composed of 966 nodes and 726 solid elements. The analysis was performed using a computer (Sony Vaio UGN-FZ320EIB 1.66 GHz Intel Core 2 Duo) and an SAP 2000 structural program (Computer Structures Inc. Berkley, CA).

Three different restoration models were evaluated:

Model A: Wedge-shaped non-carious cervical lesion, restored with a composite resin and without a hybrid layer.

Model B: Wedge-shaped non-carious cervical lesion, restored with a composite resin and a hybrid layer measuring 1.5 μm thick.[@b16-dent04_p0160]

Model C: Wedge-shaped non-carious cervical lesion, restored with a composite resin and a hybrid layer measuring 3 μm thick.[@b16-dent04_p0160]

All models were loaded from the central fossa and buccal tubercule vertically to the tooth axis with a total of 200 N of loading force (divided into 100 N per point) ([Figure 1](#f1-dent04_p0160){ref-type="fig"}).[@b1-dent04_p0160],[@b15-dent04_p0160] The final elements on the X and Y axis for each model were assumed to be fixed for boundary conditions. Shear stress distribution and amounts were calculated using Von Misses stress criteria.[@b17-dent04_p0160] The outputs were transferred to the SAPLOT program, which displayed the resulting shear stress.

RESULTS
=======

Model A showed the lower premolar tooth model with a wedge-shaped non-carious cervical lesion restored with composite resin ([Figure 2](#f2-dent04_p0160){ref-type="fig"}); the model did not include an adhesive and hybrid layer. When a 200 N load was applied to the buccal tubercule and central fossa (100 N each), additional shear stress concentration was observed on the gingival margin of the composite restoration (dark blue; 46.2--50.0 MPa). Composite restoration was illustrated mesio-distally in [Figure 2a](#f2-dent04_p0160){ref-type="fig"}. The shear stress values seemed to decrease in the occlusal area, especially when compared to the gingival area in this illustration (light yellow; 23.1--26.9 MPa). Bucco-lingual composite restoration was illustrated in [Figure 2b](#f2-dent04_p0160){ref-type="fig"}, and the shear stress intensity seemed to decrease gingivally towards the lingual area.

Model B showed the shear stress distribution in a lower premolar tooth model with a wedge-shaped non-carious cervical lesion restored with composite resin ([Figure 3](#f3-dent04_p0160){ref-type="fig"}). A 1,5 μm thick hybrid layer was added to the model ([Figure 3c](#f3-dent04_p0160){ref-type="fig"}); although the shear stress distribution appeared to be similar to that of Model A, a decrease in shear stress intensity was observed in model B in all composite surfaces (gingival, axial, occlusal). Composite restoration was illustrated mesio-distally in [Figure 3a](#f3-dent04_p0160){ref-type="fig"}, and bucco-lingually in [Figure 3b](#f3-dent04_p0160){ref-type="fig"}. According to these illustrations the gingival margin of the composite was the most intense surface (light blue; 42.3--46.2 MPa).

When a 3.0 μm thick hybrid layer was included in Model C ([Figure 4c](#f4-dent04_p0160){ref-type="fig"}), shear stress distribution at the gingival, occlusal and cervical margins of the restoration decreased ([Figures 4a, b](#f4-dent04_p0160){ref-type="fig"}). This decrease was observed, at the gingival margin of the restoration especially when compared to Model A (the lightest blue; 38.5--42.3 MPa).

DISCUSSION
==========

Developing hybridization with acidic solutions and bonding systems is the most important mechanism of bonding to tooth structures. The hybrid layer is created by the practitioner, and its thickness can be altered by using various procedures (i.e, prolonging acid etching time, additional phosphoric acid usage before self-etch primer) and adhesive systems (total-etch, self-etch).[@b16-dent04_p0160],[@b18-dent04_p0160]--[@b20-dent04_p0160] The chemical and physical features of a hybrid layer are quite different from the normal tooth structure, because it has been partially demineralized then infiltrated with resin.[@b21-dent04_p0160] Van Meerbeek et al[@b12-dent04_p0160] evaluated the hardness and elasticity of the resin-dentin bonding area and reports that the elastic structure of a hybrid layer formed by collagen tissue and resin can tolerate the stress of the resin-dentin bonding area.

During normal functional and parafunctional movements, the teeth are exposed to acute occlusal stress which is distributed throughout the entire tooth.[@b1-dent04_p0160] Many stress analysis methods are being used to determine this stresses within the tooth; among these methods FEM is most advantageous because of improvements in computer technology.[@b14-dent04_p0160],[@b22-dent04_p0160] In addition, the load direction and amount can be ideally applied, and stress type and localization within the tooth can be clearly determined using this method.[@b23-dent04_p0160] It is because of these advantages, that the FEM was used in the present study.

Various studies have used FEM to determine the stress concentration in the cervical area due to occlusal forces;[@b24-dent04_p0160]--[@b27-dent04_p0160] the majority of these studies generally aim to understand the etiology of noncarious cervical lesions.[@b24-dent04_p0160]--[@b26-dent04_p0160] The results of previous studies suggest that, all occlusal forces cause stress concentration in the cervical area; however, not enough knowledge exists about the effect of the stress concentration in cervical restorations. Grippo[@b3-dent04_p0160] reports that stress concentration in the cervical area does not only cause cervical lesions, but is also an important factor that adversely affects restoration bonding in this area.

In the present study, FEM was used on lower premolar tooth because studies on the frequency of non-carious cervical lesions indicate that lower premolar teeth are the most affected tooth group.[@b28-dent04_p0160] Holmreng et al[@b29-dent04_p0160] reports that, the stress concentration in the cervical area of premolar teeth is greater than that of other tooth groups because of the differences in crown and root morphology. To simulate centric occlusion, in the current study, a total of 200 N were applied vertically on the buccal tubercule and central fossa of the premolar tooth models. Centric occlusion can be standardized to chewing functions generated by complex movements and is the location with the lowest controversy in force localization, amount, and direction. For this reason, centric occlusion was taken as a standard, and occlusal forces were applied on the tooth model.

When a force is applied on the occlusal surface of restored teeth, the stress is not only distributed in structures such as enamel and dentin, it is also concentrated in areas such as composite and adhesive layers.[@b30-dent04_p0160] To ensure successful restorative procedure, the bonded interface area must be capable of withstanding occlusal stresses. Belli et al[@b30-dent04_p0160] analyzed the effects of a hybrid layer on the amount and distribution of stress generated by occlusal forces in premolar teeth restored with a composite and ceramic inlay, and concludes that the hybrid layer reduces stress distribution. Rees et al[@b31-dent04_p0160] suggests that forming a thick hybrid layer for Class V restorations is necessary to prevent stress from forming in the bonding layer after polymerization shrinkage. Previous FEM studies report that the hybrid layer serves as a cushion to both polymerization shrinkage stress and the stress generated by occlusal forces in Class II cavities.[@b31-dent04_p0160]--[@b33-dent04_p0160] In the current study, we want to determine the effect of the hybrid layer and its thickness on stress intensity of cervical restorations. To do this, we used the same adhesive and composite in all models, but with different hybrid layer thicknesses. When the models were compared, the presence of a hybrid layer altered the shear stress distribution; by adding this layer, the amount of shear stress decreased in all layers of the composite restoration. Although we can conclude that a hybrid layer absorbs stress, our results, further suggest that another adhesive system or composite with different elastic property will effect stress distribution in the cervical region differently.

Unlike previous studies, the current study was conducted on cervical lesions and determined that the degree of shear stress due to occlusal forces was compensated by the hybrid layer in cervical composite restoration. In addition, the current study also evaluated the effect of the hybrid layer thickness on shear stress distribution; by increasing the thickness, shear stress distribution was reduced, especially in the gingival margin of the composite, which may avoid the cervical defects of the restoration, such as fractures or cracks. These results support the null hypothesis: based on these findings, it is suggested that the hybrid layer serves to provide both bonding to tooth structures and a cushion for absorbing shear stresses in cervical restorations.

Studies indicate that clinical success was considerably low in cervical areas when compared to restorations in other areas.[@b1-dent04_p0160],[@b2-dent04_p0160],[@b6-dent04_p0160] In addition to sclerotic layers that do not allow for sufficient bonding resistance and formation of hybrid layer in cervical lesions, the negative effects of occlusal forces in this area should not be ignored.[@b4-dent04_p0160],[@b5-dent04_p0160],[@b34-dent04_p0160] However, a previous study examined the effects of changing the hybrid layer thickness, in non-carious cervical lesions with different adhesive systems.[@b16-dent04_p0160] In this study, hybrid layer thickness was increased when surface dentin was removed with a bur before the adhesive systems. Within the limitations of the current study, it is concluded that, techniques allowing for a thick and flexible hybrid layer are beneficial in increasing the success rate of cervical area restorations. Further long-term clinical studies must be performed, and the effects of various hybrid layers and adhesives on restoration success must be investigated.

CONCLUSIONS
===========

The hybrid layer reduces the intensity of shear stress formed under occlusal loading in premolar teeth that have a wedge-shaped cervical lesion restored with composite resin. Furthermore, hybrid layer thickness affects the intensity and distribution of the shear stresses in cervical composite restorations.

![The lower premolar tooth model that was loaded with 200 N from the buccal tubercule and central fossa is shown with green. Boundary conditions are indicated with blue.](page_162_fig_1){#f1-dent04_p0160}

![Wedge-shaped noncarious cervical lesion was restored with composite resin without a hybrid layer. a) Mesio-distal composite restoration is seen. b) Bucco-lingual composite restoration is seen.](page_163_fig_2){#f2-dent04_p0160}

![Wedge-shaped noncarious cervical lesion was restored with composite resin with a thin hybrid layer. a) Mesio-distal composite restoration is seen. b) Bucco-lingual composite restoration is seen. c) Thin hybrid layer (1.5 μm).](page_163_fig_3){#f3-dent04_p0160}

![Wedge-shaped noncarious cervical lesion was restored with composite resin with a thick hybrid layer. a) Mesio-distal composite restoration is seen. b) Bucco-lingual composite restoration is seen. c) Thick hybrid layer (3 μm).](page_163_fig_4){#f4-dent04_p0160}

###### 

Elastic properties, Young modulus (E) and Poissons ratio (μ) of the structures used in the FEM models.

  **Material**           **Young's modulus (GPa)**   **Poisson's ratio (V)**   **References**
  ---------------------- --------------------------- ------------------------- ---------------------------------
  Enamel                 84                          0.33                      Wheeler[@b15-dent04_p0160]
  Dentin                 17.5                        0.31                      Menicucci G[@b35-dent04_p0160]
  Pulp                   0.0021                      0.45                      Mac Gregor[@b36-dent04_p0160]
  Periodontal membrane   0.069                       0.45                      Reinhardt RA[@b37-dent04_p0160]
  Cortical bone          13.7                        0.30                      Holmes DC[@b22-dent04_p0160]
  Spongiose bone         1.37                        0.30                      Holmes DC[@b22-dent04_p0160]
  Composite              16.4                        0.28                      Belli S[@b30-dent04_p0160]
  Adhesive layer         10.5                        0.28                      Belli S[@b30-dent04_p0160]
  Hybrid layer           15.5                        0.28                      Belli S[@b30-dent04_p0160]
