|Year : 2005 | Volume
| Issue : 3 | Page : 32-42
|Behavioural pattern of three different bilayered restorations under tensile loading in a modified abfrfactive lesion simulated by finite element analysis
RG Balaji, V Gopikrishna, KS Karthikeyan, D Kandaswamy, S Nandini
Department of Conservative Dentistry & Endodontics, Meenakshi Ammal Dental College, Chennai-95., India
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| Abstract|| |
This study evaluates how three types of bilayered restorations behave in a modified abfractive lesion when subjected to a tensile load using a finite element model analysis. Mandibular premolars were used and an oblique load of 100 N was applied at an angle of 45 degrees to the long axis of the tooth. A wedge shaped lesion was then created on the buccal aspect of the tooth in the cervical region. It was then subjected to the same loading conditions. Then the internal line angle was rounded off and the tooth was subjected to the same loading conditions. This region then restored in four ways and subjected to the same loading conditions. The cavities were restored with a hybrid composite alone or with bilayered restorations. The stress analysis was performed on this tooth using the ANSYS solver The results were, stress was more in teeth with cervical lesions. Among the various restorations used it was seen that a combination of Z100 and Silux Plus performed the best. In conclusion it is seen that roundening of line angles is mandatory in wedge shaped cervical lesions. A bilayered restoration is to be used in which, the deeper layer has a low modulus of elasticity and a high ultimate tensile strength.
|How to cite this article:|
Balaji R G, Gopikrishna V, Karthikeyan K S, Kandaswamy D, Nandini S. Behavioural pattern of three different bilayered restorations under tensile loading in a modified abfrfactive lesion simulated by finite element analysis. J Conserv Dent 2005;8:32-42
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Balaji R G, Gopikrishna V, Karthikeyan K S, Kandaswamy D, Nandini S. Behavioural pattern of three different bilayered restorations under tensile loading in a modified abfrfactive lesion simulated by finite element analysis. J Conserv Dent [serial online] 2005 [cited 2019 Nov 12];8:32-42. Available from: http://www.jcd.org.in/text.asp?2005/8/3/32/42589
| Introduction|| |
Occlusal stress factors have gained increasing attention as causes of non carious cervical lesions. Grippo  in 1991 introduced the term abfraction to designate stress induced lesions that result from hyperfunction and parafunction and can be further exacerbated by erosion and toothbrush / dentifrice abrasion.
Abfractive lesions are mostly wedge shaped and develop over time as hard tissue defects in the cervical region of teeth commonly in the buccal aspects. They are more commonly seen in the mandibular teeth, which may be due to their lingual orientation and their anatomically smaller cervical cross section 
The exact cause of abfraction has not been conclusively proved. It has been proposed by Lee et al that when a significant lateral occlusal force acts on a tooth at a fulcrum, it will cause tension on one side and compression on the opposite side of equal magnitude. Studies by Celik  (1992) and Spears  (1993) have shown that the enamel and dentin together with the entire masticatory apparatus are superbly designed to dissipate compressive forces during function. But when a tensile stress is generated as a result of lateral forces acting on teeth, they do not dissipate but instead tend to concentrate near the cervical region. This tensile stress concentrating near the cervical region disrupts the chemical bonds of the crystalline structure of enamel and dentin. Small molecules can enter the microcracks and prevent reformation of the chemical bonds. The resultant damaged tooth structure is subsequently lost.
Restoring these defects can reduce stress concentrations in these lesions. Studies have shown that these forces can be further reduced when these areas are rounded or saucer shaped  . One of the, clinical methodologies advocated is the usage of a bilayered restoration where the deeper layer is of a lower modulus of elasticity than the superficial layer. This deeper layer is believed to alleviate and distribute stresses from occlusal forces  . In this study the materials used in the deeper layers were Ketac Bond, Vitrebond, Silux Plus.
The superficial layer used was a hybrid composite (Z 100). The stress analysis used in this study is the finite element analysis. The advantage of this technique is the ability to obtain accurately the stresses throughout the structure under consideration.
The purpose of this study is to evaluate how three types of bilayered restorations behave when subjected to a tensile load using a finite element analysis.
| Materials and Methods|| |
A three dimensional finite element analysis was used in this study to analyze the stresses occurring in various bilayered restorations in a modified wedge shaped lesion. Mandibular I premolar was selected for this study as it has been seen that abfractive lesions are most commonly seen to occur on the buccal aspect of lower premolars because of their smaller cervical cross section and lingual orientation. This tooth was then subjected to a finite element analysis.
MODELING OF A NORMAL LOWER PREMOLAR TOOTH
The first step in a finite element analysis is modeling. Modeling is the most important activity. The quality of the analysis results depends on the accuracy of the model. Various methods like CMM profiler, making photos at different levels, CT Scan, can determine the exact dimensions of the selected premolar. CMM profiler gives us only the outer profile of the tooth at different heights, but the individual profiles of enamel, dentin and pulp cannot be measured. Capturing the tooth as a photograph at different sections needs a precise mechanical setup. It needs a high quality camera to be fixed on a mechanical setup, and a surfacegrinding machine is needed to grind and create different sections. This process is not an accurate one and is time consuming.
For this analysis, a CT scan of the tooth is taken. The different cross-sections of the tooth were obtained by scanning into many cross-sections at an equal interval of 0.625mm. These cross sections were scanned and have been used to create a solid model using Pro-Engineer. The Enamel, Dentin and Pulp have been modeled individually. The individual models of enamel, dentin and pulp were inserted, and then made into an integrated model. Thus the virtual model of the lower premolar tooth had been developed [Figure 1]. The bone was created as a rectangular block. The root portion is inserted inside the bone. This one was constrained for motion in all the directions. The model was then exported to ANSYS.
MESHING (Creation of a Finite Element Model)
The next procedure is the creation of Finite Element Model (FEM). For creating a Finite Element Model the premolar was divided into several Finite Elements. The element chosen for the study is Solid 72. Solid 72 is an element well suited for irregular models. For this the preprocessor of ANSYS was used to create Finite element Model.
LOADS AND CONSTRAINTS
In this simulation the outer-elements of the bone was fully constrained. Since the application of load is oblique in XY, YZ and ZX planes, it is resolved as X, Y and Z components. An oblique load of 100 N was applied at an angle of 45 degrees to the long axis of the tooth. The point of application was determined as the working side contact [Figure 2]. The load was applied and the stress analysis was carried out using the ANSYS solver. The stress distributions have been plotted using the General Post Processor of ANSYS.
CREATION OF THE LESION
After studying the results briefly, it was seen that in the cervical part of the tooth on the buccal aspect, the ultimate tensile strength of enamel was exceeded [Figure 3] & [Figure 4]. A lesion was then created to a height of 2.5 mm and a depth of 1.5 mm (corresponding to the volume of tooth structure whose ultimate tensile strength was exceeded) [Figure 5]. The lower border corresponded to the cemento enamel junction. This model with a lesion was then meshed and it was subjected to the same loading conditions [Figure 6]. The stress analysis was performed on this tooth using the ANSYS solver. This result was studied and then the internal line angle was rounded [Figure 7]. This modified lesion was then meshed and was subjected to the same loading conditions and the stress analysis was again performed using the ANSYS solver [Figure 8].
RESTORATION OF THE LESION
After studying the results briefly, it was seen that there was a high stress concentration at the apex of the lesion. To reduce this stress concentration the region was then restored in four ways and subjected to the same loading conditions.
Model I - This model was restored completely with a hybrid composite (Z 100). No beveling was done [Figure 9].
Model II - The second model was restored with a bilayered technique. The deeper layer of the restoration was a glass ionomer cement (Ketac Bond). This deeper layer was restricted to dentin alone. This was then covered totally with the hybrid composite.
Mode III: The third model was again a bialyered restoration. The deeper layer was a resin modified glass ionomer cement (Vitrebond) while the rest of the lesion was the hybrid composite.
Model IV: In the fourth model the deeper layer was a composite (Silux Plus) while the remainder of as filled with the hybrid composite.
The stress analysis was performed on this tooth using the ANSYS solver.
| Results|| |
Principal stresses for each of the models were studied.
Normal Tooth [Figure 4]
In the normal tooth, the peak stress value was 27.4 MPa for an applied load of I00N. This peak stress value was seen in the dentin, which was able to take up most of the applied load. In enamel, the peak stress value was seen to be 18.1 MPa. The ultimate tensile strength of enamel is about 10Mpa and this high stress concentration in enamel indicates that it is susceptible to failure. Dentin being more elastic and with a higher ultimate tensile strength (105.5Mpa) is able to absorb and redistribute these forces better. It had a peak stress concentration of 22.7 Mpa
TOOTH WITH A WEDGE SHAPED LESION [Figure 6]
When a 100N force was applied on the tooth with a wedge shaped lesion, the maximum stress value obtained was 92.5 MPa. This peak stress value was present in the internal line angle on the enamel. This represents nearly a five-fold increase in the stress concentration in the cervical region. This point is prone to further failure if not restored. In dentin the peak stress value was about 34.3 MPa.
TOOTH WHOSE INTERNAL LINE ANGLE IS ROUNDED OFF [Figure 8]
The internal line angle was rounded off and the tooth was subjected to the same force of 100 N. It showed a reduction in stress concentration. This reduction in stress concentration is about 36.3 MPa. Hence the peak stress value obtained is about 56.2 MPa. This peak stress concentration is again seen in enamel, which has a higher modulus of elasticity when compared to dentin (hence stiffer). In dentin the peak stress concentration is about 29.lMpa indicating a decrease of 5.2 MPa.
The tooth whose internal line angle had been rounded off was now restored with Z 100 and the results are as follows
TOOTH RESTORED WITH Z 100 [Figure 10]
When the tooth was restored with Z 100, the peak stress concentration was seen to be about 25.7 MPa. When the stress concentration in was studied, the stress values were seen to range from .76 MPa to a peak value of 16.1 MPa. This was well within the ultimate tensile strength of Z 100, which is about 54,4 MPa. When the tooth structure around the Z 100 was studied, a peak stress value of 25.7Mpa was obtained. This indicates that when the saucer shaped area was restored with Z 100, the stress values on the tooth especially enamel falls from 56.2 MPa to 25.7 MPa, a decrease of nearly 30.5 MPa.
When a .5 mm thick deeper layer was added between the Z 100 and the dentin, the results were as follows.
TOOTH RESTORED WITH Z 100 AND KETAC BOND [Figure 11a & b]
In the first case, the .5 mm deeper layer used was Ketac Bond with modulus of elasticity of 4.7 GPa and an ultimate tensile strength of 5.8 Pa. The peak stress value observed on the tooth was 25.2 MPa. In Z 100, the stress values were found to range from 0.8 MPa to I6.7MPa. In Ketac Bond, the stress values ranged from .26 to 19.2 MPa. This peak stress value exceeded the ultimate tensile strength of Ketac bond, which 5.8 MPa. This indicates that the lining material was unable to take up the stress.
TOOTH RESTORED WITH Z-100 AND VITREBOND [Figure 12a & b]
Vitrebond has an ultimate tensile strength of 12.6 MPa and a modulus of elasticity of 1.1 GPa. When such a restored tooth was subjected to a force of I OON the peak tensile stress generated in the tooth was 24.6 MPa. In the Z 100 part of the restoration the stress value ranged from.51 MPa to 17.5 MPa. In Vitrehond the stress values were from a minimum of .08MPa to a maximum of 6.52 MPa. This is well within the ultimate tensile strength of Vitrebond thus indicating that this material was able to withstand the tensile stress generated.
TOOTH RESTORED WITH Z 100 AND SILUX PLUS [Figure 13a & b]
In this model the peak stress value generated in the tooth as such was 23.9 MPa. In Z 100 the stress values were from .55MPa to a peak value of 16.4 MPa. In Silux plus which was tried as the material used in the deeper layer the peak stress value was 19.9 MPa. It had a minimum value of.2M Pa. This value is well within the ultimate strength of Silux plus which is 40.4 MPa.
| Discussion|| |
Grippo  introduced the term abfraction in the year 1991 to describe the pathologic loss of both enamel and dentin caused by biomechanical loading forces. The abfractive lesions were caused by flexure and ultimate material fatigue of susceptible teeth at locations away from the point of loading.
Lee & Eakle in 1996  reported that the anatomically smaller cervical cross section of the mandibular teeth particularly the bicuspids might contribute to the weakness of the tooth to withstand cervical stress. These lesions are sometimes seen subgingivally. Various methodologies like articulated study models , photoelastic studies  , strain gauges  have been used to study the stress concentrations in the cervical regions.
In our study the finite element methodology was chosen. The finite element analysis is a modern technique of stress analysis, which has the great advantage of being applicable to solids of irregular geometry and composed of different materials with different properties. It is therefore ideally suited to the examination of the structural behavior of the teeth. Yettram et al in 1976  used a two dimensional FEM model and found that the enamel near the cemento enamel junction was a highly stressed area. Rees in 1998  used a finite element model to study the role of cuspal flexure in the development of abfractive lesion. Using a lower premolar he found that when a load of 100 N was applied on the buccal cusp stresses of upto 70MPa was produced. But the model used was only 2 dimensional. Moreover the force was applied in the center of the buccal cusp, which is not the working side contact. A three dimensional model produces a lesser deflection than a two dimensional model (Parameswaran et al 2000)  . Hence it is necessary to use a three dimensional model. Studies of stress distribution of a normal mandibular premolar tooth using a three dimensional analysis by Khera et al in 1988  ; found that the stress distribution near the cementoenamel junction was high. The load applied on the tooth was 17 kg. In any finite element analysis the validity of the results obtained depends on how closely the model depicts the actual structures it is intended to represent  . In this respect several aspects of the model preparation done in the study should be considered. The tooth model was a three dimensional one exactly duplicated with the help of Computed Tomographic scan images. Moreover human teeth are not symmetric in any axis. So serial sections of scan images of a mandibular premolar were duplicated on a computer. Santos  figuratively depicts the area and direction of contact of an upper and lower premolar during function. The area of contact is seen to be on the distal part in the occlusal aspect of the tooth. The direction of force application is seen to be at an angle of 45° to the long axis of the lower premolar.
In this study when a normal tooth model was subjected to a load of I00N, it was seen that a peak tensile stress 18.1 MPa was observed in enamel. This value is high and exceeds the ultimate tensile strength of enamel, which is 10 MPa. Brittle materials like enamel have a low tensile strength because of their inability to plastically deform under a load which produces a tensile stress. Dentin on the other hand though having a higher tensile stress concentration is able to handle this tensile stress due to a higher ultimate tensile strength (105.5 MPa). When a model with a wedge shaped lesion was' subjected to 100N force, it was seen that the peak tensile stress concentration was 92.5Mpa in enamel and 45.9 MPa in dentin. This tensile stress concentration was at the apex of the lesion in the internal line angle.
When this tooth with a rounded internal line angle was subjected to loading, it was seen that the tensile stress concentration in the region of the internal line angle fell sharply to value of 56.2 MPa a decrease of 36.3 MPa while in dentin, it fell to a value of 29. 1 MPa. This in itself is a high stress reduction but is still a high value. The high value may be because of the simulated loss of tooth structure in the model. Principles of biomechanics state that the presence of discontinuity in any structure causes stress concentration to occur in a structure (Kuroe et al 2000)  . Hence the need to restore this discontinuity becomes of important. Grippo in 1995  suggested that a restoration in this region would perform two main functions. The restoration may support the tooth and thus minimize flexure and abfraction. The restoration and its adjacent enamel could be more resistant to abrasion and erosion than the exposed dentin: thus restoration of the cervical lesion provided both protective and retardant values.
Kernp-Scholte and Davidson  in 1990 suggested that the use of an elastic material underneath a stiffer material might help in alleviating and redistributing the stresses from occlusal forces.
In the present study, a hybrid composite was used as the main restorative material since it has a higher tensile strength when compared to the other materials. Brackett et al in l999  and Browning et al  in 2004 showed that there was no difference in retention between a hybrid composite and a microfilled composite in a non - carious Class V lesion. This formed the basis for our choice of material. In our study a restoration of Silux plus and Z 100 performed the best. Silux plus is a microfilled composite. When compared to Z100, which is a hybrid composite the filler loading is less and the resin matrix is more. This accounts for its lower modulus of elasticity. A combination of Vitrebond and Z 100 performed the second best. The modulus of elasticity of Vitrebond which is a resin modified glass ionomer is 1.1 Gpa. The presence/of resin matrix in Vitrebond could be attributed to its low modulus of elasticity.
Ketac bond and Zl00 performed the third best. Ketac bond is a conventional glass ionomer. Due to the absence of a resin matrix its modulus of elasticity is high when compared to Vitrebond (4.25Gpa). Its ultimate tensile strength is the lowest of all the materials used. This makes it a very brittle material when compared to the other materials used. In our study the stress generated within Ketac bond was more than its ultimate tensile strength. This could lead to failure of the material. At the same point of application of load on the three different models simulated in the deeper layers it was seen that in Vitrebond which is more flexible than the other two, the area the maximum stress concentration was well within the body of the lining material. For the other two materials, which are more resistant and rigid, the area of maximum stress concentration was in line with the point of application of the load. This could be explained due to the high flexibility of Vitrebond when compared with the other two materials. Z 100 alone performed the worst. This could be due to the absence of any cushioning by an elastic material in the deeper layer.
Hence from this study we have been able to conclude that, when A tensile load is applied on a nprmal tooth, there is stress in the cervical region and can lead to abfractive lesions. During the restoration of an abfractive lesion, one needs to round off the sharp internal line angle, which leads to significant stress reduction . A bilayered restoration is to be used in which, the deeper layer has a low modulus of elasticity and a high ultimate tensile strength.[Table 1]
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R G Balaji
Department of Conservative Dentistry & Endodontics, Meenakshi Ammal Dental College, Chennai-95.
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11a & b], [Figure 12a & b], [Figure 13a & b]
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