|Year : 2011 | Volume
| Issue : 2 | Page : 295-302
|Effect of occlusal restoration on stresses around class V restoration interface: A finite-element study
Gaurav Vasudeva1, Poonam Bogra2, Vineeta Nikhil2, Vijay Singh2
1 Department of Conservative Dentistry and Endodontics, B.R.S Dental College and Hospital, Panchkula, India
2 Department of Conservative Dentistry and Endodontics, D.A.V Dental College and Hospital, Yanmunanagar, Haryana, India
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|Date of Submission||04-Jul-2008|
|Date of Decision||02-Aug-2010|
|Date of Acceptance||22-Sep-2010|
|Date of Web Publication||27-Aug-2011|
| Abstract|| |
Background: Causes of failures in class V restorations have always been controversial until now, since the biomechanical aspects of these restorations have been understood. Aims and Objective: This study was aimed to verify the hypothesis that eccentric forces are the cause of cuspal flexure, which lead to excessive stresses at the periphery of a class V restoration, further it identifies the amount of the stress and the flexure increase in the presence of an occlusal restoration using different restorative materials to restore (both cervical and occlusal) along with their comparison with respect to amount of stresses around the cervical restorations. Materials and Methods : The study was done by modeling a mandibular first premolar which was sectioned bucco-lingually, in the NISA, EMRC II finite element software. A 100N eccentric load was applied on the tooth structure and stresses were observed at the peripheries of the class V restoration when it was restored with composite and with glass ionomer cement. The stresses were further analyzed in the presence of occlusal restorations with different materials and configurations. Results: It was seen that the stresses at the gingival wall interface in case of class V glass ionomer cement was more than that for composite. In the presence of an occlusal amalgam restoration, the cuspal flexure was more than that of occlusal composite and the stresses at the class V peripheries were also statistically significant. Conclusion: It was suggested that lower modulus composites can flex with the tooth structure decreasing the bond failure and that the stresses are much lesser when there is absence of an occlusal restoration. Occlusal composite restorations reinforce the tooth structure and reduce the cuspal flexure as compared to silver amalgam.
Keywords: Class V restoration interface, cuspal flexure, finite-element analysis
|How to cite this article:|
Vasudeva G, Bogra P, Nikhil V, Singh V. Effect of occlusal restoration on stresses around class V restoration interface: A finite-element study. Indian J Dent Res 2011;22:295-302
The success of a restorative material depends on its property to withstand and resist occlusal forces and support the remaining tooth structure. The failure of class V restorations with various materials is well-documented. , It was always thought earlier that the restorations remote from the point of application of occlusal forces were not subjected to mechanical stresses and the only forces that dislodged the class V restorations were the pulling forces by sticky foods.  It was also stated that the shape of the class V cavity outline was of no great importance, provided that cervical margins were placed just under the gingival margins and the remainder was smooth. , Various studies have shown that the failure was confined mostly to the occlusal walls and margins and was usually seen on the buccal surfaces of lower molars and premolars. ,
|How to cite this URL:|
Vasudeva G, Bogra P, Nikhil V, Singh V. Effect of occlusal restoration on stresses around class V restoration interface: A finite-element study. Indian J Dent Res [serial online] 2011 [cited 2018 Nov 16];22:295-302. Available from: http://www.ijdr.in/text.asp?2011/22/2/295/84308
Hood (1972) found some co-relation between occlusal loading and its effect on class V restorations and suggested that excursive mandibular movements placed the buccal cusp in tension or in compression and opens up the occlusal margins.  He also placed pressure transducers in a buccal class V restoration and loaded the cusp to confirm the hypothesis given by Gabel.  Direct measurements of the changes in occlusogingival diameter of class V preparations have been made. ,, All these studies demonstrated that this dimension decreases with increasing load and suggested that extrusion and breakdown of the margins of class V restorations may be the result of occlusal loading. It was also suggested that the enamel near the cementoenamel junction is highly stressed because the reactive forces have to flow into and through this thin wedge of tissue for it to be transmitted into the root of the tooth and subsequently into the supporting alveolus. It is therefore evident that the restorations inserted into the cervical region of the teeth can be subjected to high compressive stresses even though these areas are not susceptible to direct contact stresses of mastication. 
The failure of class V restorations have also been reported to be higher with high modulus macrofilled materials and even higher in mandibular teeth, especially in patients with heavy occlusal loading identified by the presence of wear facets.  A tooth flexure theory has been suggested to explain the failure rate of class V restorations in which lateral excursive movements resulting in lateral cuspal movements generate tensile stresses along tooth restoration interface. ,
Cuspal flexure has also shown in vitro to increase as the extent of coronal preparation increases. , Furthermore, the presence of an adhesive occlusal composite restoration has been shown to reduce cuspal flexure as compared to occlusal amalgam restorations. ,, This could influence the retention of class V restorations. There have been very few studies to determine the effect of occlusal restorations in the retention rate of class V restorations.
Thus this study is an attempt to examine the effect of occlusal restorations on the stress distribution around the periphery of a buccal class V restoration using a finite-element method. The hypothesis used is that the presence of a class I restoration would increase cuspal flexure under occlusal loading, thereby increase the stresses around the periphery of class V restorations.
| Materials and Methods|| |
An extracted mandibular first premolar was sectioned buccal-lingually through the center of the tooth and ground to a very thin section such that the enamel, dentin, cemental, and the pulpal outline was clearly visible [Figure 1]a.
|Figure 1: (a) A ground section of mandibular first premolar. (b) A two-dimensional plane strain mesh of mandibular first premolar with its supporting ligament and bone from the original geometry using NISA II FINITE ELEMENT SUITE (EMRC) containing 7774 nodes and 3923 hexahedron elements.|
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The exact dimensions were determined by scanning the section including the outline of dentinoenamel junction and the pulp. The image was then modeled in the AutoCAD software. Once the model was completely prepared, it was captured and simulated by the Finite-Element Software (NISA II, EMRC).
The outline of the periodontal ligament 0.2 mm wide and the surrounding alveolar bone was generated using the outline of the tooth as a guide. The dimensions of the compact and cancellous bone forming the alveolus were derived from the standard text.  The pulp was modeled as a void since this has been shown to have no effect on the results. 
A two-dimensional finite-element plane strain mesh of the lower 1 st premolar and supporting ligament and bone was developed from the original geometry using NISA II FINITE ELEMENT SUITE (EMRC) containing 7774 nodes and 3923 hexahedron elements [Figure 1]b. The enamel was modeled as an anisotropic material with the principal elastic modulus E x = 84 Gpa and E y = E z = 20 Gpa. 
A class V cavity was modeled partly in the enamel and partly in dentin with depth of 1.25 mm and length of 1.5mm in the occlusogingival direction with 90 degree internal line angles. A two-dimensional finite- element plane strain mesh of the lower 1 st premolar and supporting ligament and bone was developed containing 7774 nodes and 3923 hexahedron elements.
Occlusal cavity was prepared in the model that was distributed into various depths and widths. The minimum depth and width was 0.5 mm inside the dentinoenamel junction and 1/4 of the intercuspal distance, respectively. The maximum depth and width was taken 1.5 mm inside the dentinoenamel junction and 1/3 of the intercuspal distance. The average amount of load is 100 N produced by normal masticatory forces, , so the analysis was done under the same load.
An initial study was done to find the point of load on the slope of the buccal cusp that produced the greatest stresses around the class V preparation. It was seen that the load 0.5 mm inside the buccal cusp tip produced the greatest stresses. Thus for all the further analysis, a load of 100 N was applied 0.5 mm inside the buccal cusp tip.
The initial load case run was with no occlusal restoration present, and the occlusal cavities prepared in the tooth model on the finite-element profile were ignored. In this, only the cervical restorations were selected, i.e., composite class V restoration and later glass ionomer cement class V restoration. This formed the baseline measurement to judge all further analysis.
In case of class V composite restoration, the first variable investigated was the effect of amalgam restoration as the coronal restorative material. Further the width and depth of the occlusal restorative material was altered. The two widths taken were 1/4 of the intercuspal distance and 1/3 of the intercuspal distance. These were called the wide and narrow preparations, respectively. Further two depths taken were 0.5 and 1.5 mm inside the dentinoenamel junction. These were called shallow and deep preparations, respectively. Maximum shear stresses were calculated with each of the width and depth of cavities, i.e., narrow-shallow, narrow-deep, wide-shallow, wide-deep.
The second variable investigated was with the occlusal composite restoration. The analysis was done same as that done for occlusal amalgam restoration with the same changes in the depth and width.
The whole analysis was repeated in case of a glass ionomer restoration in a class V cavity and the results were compared.
All the occlusal restorations were analyzed with varying the width and depths as described before. The shear stresses produced at the four interfaces (walls) of the class V cavity were calculated for each of the analysis done with varying the depths and widths. The numbers of nodes present at these interfaces are given as follows:
The maximum shear stresses at each of these nodes were calculated and the averages of these stresses were taken as the maximum shear stress at that interface.
- Occlusoenamel wall - four nodes
- Occlusodentinal wall - five nodes
- Axial wall - seven nodes
- Gingival wall - five nodes
To model the discontinuity between the amalgam/composite interface and the tooth, a layer of specialized elements known as gap elements was introduced. A gap element is a line element joining two nodes and by knowing the initial substance between two nodes, subsequent behavior can be monitored so that if contact is deleted, the element possesses an artificially high stiffness. A layer of gap elements of 1 μm was introduced around the entire tooth amalgam interface that allowed the coronal amalgam/composite restoration to move independently of the tooth. 
To overcome the problems of interpreting interfacial stresses at the class V tooth restoration interface, joint elements were placed at each node, which was common to the tooth and class V restoration. Each joint element gave a value for the stresses in compression and tension together with a shear stress. However to reproduce all these values would be impossible and, as shear stresses were numerically different for each mesh and since shear is a combination of tensile and compressive stresses acting at 45°±, only shear stresses were analyzed. This was carried out for each of the four interfaces.
Finally the results of all the groups forming the margins of the class V cavity were compared with each other and they were put into statistical analysis.
| Results|| |
A Load of 100 N was applied 0.5 mm inside the buccal cusp tip and maximum shear stresses around the periphery of the class V restoration were observed with different occlusal restorative materials with different dimensions. The results were tabulated and graphically represented.
The maximum shear stresses found around the interface of the class V restoration when there was no occlusal restoration present are shown in [Figure 2]a.
|Figure 2: (a) The maximum shear stress found around the interface of class V restoration under a 100 N load. (b) Interfacial shear stresses in (Mpa) of class V restorations with no occlusal restoration|
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A class V composite restoration was compared with a class V glass ionomer cement restoration when there was no occlusal restoration and the stresses were observed on the interfaces of these restorations [Figure 2]b.
It was seen that there is maximum stress at the gingival wall interface to the range of 31.34 Mpa seen in case of cervical glass ionomer cement restoration when it was compared to cervical composite restoration.
The comparison of the stresses produced at the interfaces of class V composite and glass ionomer cement restoration with variation in preparation width and depth of occlusal restoration is shown in [Table 1]a and b, when occlusal restorative material was amalgam and then later replaced by composite. The same in case of a class V glass ionomer cement restoration is represented in [Table 2]a and b. The mean for all the variations in the dimension of the occlusal restoration at a particular interface are also shown.
A comparative statistical analysis of interfacial shear stresses of a class V restoration with and without occlusal restoration is shown in [Table 3]. The table also shows the mean for all the variations in the dimension of the occlusal restoration at a particular interface. Each interface along with the type of restoration is shown with comparison to that of the control. The standard deviation along with the T value and P value is also shown for each interface. The maximum shear stresses were produced when amalgam was the occlusal restorative material when the class V cavity was restored by glass ionomer cement. The mean of these interfacial shear stresses produced were in the range of 21.28 and 34.41 Mpa in case of occlusal composite restoration for a class V composite at the occlusal dentin interface and occlusal amalgam restoration for class V glass ionomer cement at the gingival wall interface, respectively.
A comparative statistical analysis of interfacial shear stresses was done between occlusal amalgam and occlusal composite when composite was the class V restorative material shown in [Table 4]a and b and graphically represented in [Figure 3]a. It was seen that more stresses were produced at all interfaces when amalgam was the occlusal restorative material. In case of a class V glass ionomer cement, again the stresses were more when amalgam was the occlusal restorative material, except they were quite comparable at the axial wall interface [Figure 3]b.
|Figure 3: (a) Comparison of mean interfacial shear stresses in a class V composite restoration, with occlusal amalgam and composite resin restorations. (b) Comparison of mean interfacial shear stresses in a class V glass ionomer cement restoration, with occlusal amalgam and composite resin restorations|
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| Discussion|| |
Class V restorations are the restorations at the gingival third of the buccal or lingual surfaces of all teeth. Premature loss of restorations from cervical defects has been reported to be very common. , The failure of these restorations has been reported to be higher in composites with high elastic modulus and in mandibular teeth and also in patients with large occlusal loads identified by the presence of wear facets. ,
It was earlier thought that the only forces that dislodge the class V restorations were the pulling forces of sticky foods, while little thought was given to the biomechanics of the tooth structure. Gable was the first to consider the possibility of occlusal forces affecting class V restorations. Further, direct measurements of the changes in the occlusal-gingival diameter of class V preparations were made. , Displacement of the cavity margins and cuspal flexure were considered to be responsible for the extrusion of amalgam and the change in the cervico-occlusal width of the cavity and the magnitude of deformation was related to the amount of tooth tissue lost. It was seen that forces applied on the occlusal surface of a tooth could induce stresses in a restoration remote from the point of application of the force. Heymann et al. in 1991 have suggested a tooth flexure theory of retention to explain these findings.  They suggested that two mechanisms operate to cause failure. One is the lateral excursive movements resulting in lateral cuspal movements which generate tensile stresses along the tooth restoration interface and other are heavy forces in centric occlusion which cause vertical deformation of the tooth leading to compressive and shear stresses at tooth restoration interface. Cuspal flexure has also been shown in vitro to increase with the increase in the extent of coronal preparation. , If the tooth flexure theory is correct, then presence of an occlusal restoration may have a deleterious effect on the retention of class V restoration. Furthermore, the presence of an adhesive composite has been shown to reduce cuspal flexure compared to an occlusal amalgam restoration. ,
The presence of an occlusal restoration does indeed weaken the tooth structure and increases the stresses on a buccal class V restoration. Especially the depth of an occlusal restoration is more critical than the width. ,, Very few studies have been done on the stress analysis of a class V restoration under normal occlusal loading with and without the presence of occlusal restorations with different configurations and using different materials using finite-element methodology. Most of the previous studies on class V cavities and restorations on finite-element analysis have been done on mandibular second premolar. Heymann et al. reported that mandibular teeth undergo greater tooth flexure due to lingual inclination of clinical crowns and smaller cross-sectional dimensions in the cervical area.  It has also been reported by various authors that premolars undergo greater cuspal flexure as compared to anterior teeth. So this study was undertaken by modeling a mandibular first premolar.
The model was non-homogeneous and all the materials including that of the tooth structure were modeled as isotropic material except the enamel as it has been proved to have anisotropic properties. 
The finite elements are formed by figuratively cutting the original continuum into a number of appropriately shaped sections and retaining in the elements the properties of the original material.
It has already been suggested that cavity preparation causes cuspal flexure under simulated occlusal loads.  They also saw that the use of amalgam restorations caused static loads on the cusps of the teeth and brought about permanent deformation causing increased stresses within the tooth structure.
A class V composite restoration was compared with a class V glass ionomer cement restoration when there was no occlusal restoration and the stresses were observed on the interfaces of these restorations [Figure 2]b. The results showed that the stresses around the gingival wall were the maximum followed by around the axial wall and then the occlusal enamel wall. The stresses at the occlusal dentinal wall were the least. The reason for such high stresses at the gingival wall can be explained on the basis of simple mechanics that if we take the tooth and its supporting tissues as a "cantilever beam" in a biomechanical unit, the occlusal loads applied near the cusp tips were far away from the fulcrum point of the "beam." The fulcrum is between the cervical region of the tooth and the crest of the alveolar bone. The points closer to the fulcrum undergo greater stress as compared to the ones away from it. These results, hence, are in agreement with the photoelastic study done by Toshifumi Kuroe et al.  The enamel near the cementoenamel junction is highly stressed because the reacted forces have to flow into and through this thin wedge of tissue for them to be transmitted into the root of the tooth and subsequently into the supporting alveolus. It is, therefore, evident that the restorations inserted into the cervical region of the teeth can be subjected to high compressive stresses even though these areas are not susceptible to direct contact stresses of mastication. ,,
Occlusal enamel wall was subjected to more stresses as compared to the occlusal dentinal wall could be because of the theory suggested by Lee and Eakel.  Lateral forces create tension and compression in cervical region. There is disruption of chemical bonds between enamel rods. Small molecules enter between hydroxyapatite crystals and prevent re-establishment of bonds to make crystals more susceptible to breakage and chemical dissolution.
It was also observed that there was a very slight difference in the interfacial stresses around class V composite and glass ionomer cement restorations [Figure 2]b. The glass ionomer cement restoration showed slightly lesser stresses at the occlusal enamel and dentinal wall but showed large stresses as compared to composites at the gingival wall interface. This may be explained by the brittle nature and lesser compressive, tensile strength, and flexure strength of glass ionomer cement as compared to composites.
When interfacial shear stresses in a class V composite restoration with different widths and depths of occlusal amalgam were compared with the one having no occlusal restoration, it was seen that the stresses at the class V interfaces were more when occlusal amalgam was present. The results were statistically significant (P <0.05) on all the walls except the gingival wall. It was the same when occlusal composite restoration was present and the stresses at the occlusal dentinal and axial wall were statistically significant [Table 3].
In case of a class V glass ionomer cement restoration, when amalgam was the occlusal restorative material, the stresses at the class V interfaces were more as compared to the one, without occlusal restoration. The result was statistically significant (P <0.05) at all the walls except the axial wall interface and when composite was the occlusal restorative material, the same trend followed. The result was statistically significant at occlusal dentinal and gingival wall interface [Table 3].
So it was seen that especially when occlusal amalgam was used as a restorative material, there were significant increase in shear stresses at the occlusal enamel and dentinal interface as compared to class V restorations that had no occlusal restoration.
A comparative statistical analysis of the mean interfacial shear stresses of a class V composite restoration with occlusal amalgam and composite restorations at different widths and depths was done [Figure 3]a. It was seen that stresses were more when occlusal amalgam was used but except the occlusal enamel interface the results were statistically insignificant (P >0.05). [Figure 3]b illustrates the results with class V glass ionomer cement restoration. The results were again statistically insignificant for all the interfaces except for occlusal enamel and gingival wall interface [Table 4].
On the whole, it was seen that the maximum shear stresses were produced around a buccal class V restoration when amalgam was the occlusal restorative material that, when composite, was the occlusal restorative material. The cause of these increased stresses has been established due to the fact that there is an increased cuspal flexure due to amalgam restorations. ,, It was proposed that as the composite restoration bonds to the tooth structure, there is a decrease in the flexure of the cusps as it reinforces the tooth structure. Thus we can say that composite as an occlusal restorative material is better than the amalgam restorations, as the shear stresses at occlusal enamel walls of the class V restorations undergo less shear stress that can dislodge the restoration.
The elastic moduli of the low-flow composite would be higher than those of the high-flow composite; thus occlusal force would increase dentine leakage in class V cavities restored with flowable composites with lower modulus of elasticity.  The failure or the leakage pathway is mostly due to volumetric shrinkage in resin composites, and the type of bonding agents and surface plays an important role in the outcome and success of class V restorations. 
However, the results of this study must be viewed with a few limitations in mind, since this study is only a two-dimensional analysis, whereas teeth are three-dimensional objects. Therefore, this model was unable to model the torsional movements that may occur in the mesial-distal plane on occlusal loading, which could lead to an underestimation of the cervical stresses.
The clinical significance of the finding in this finite-element study suggests that an occlusal restoration with different morphology and the material chosen has a definitive influence on the retention of a class V restoration restored with different tooth-colored restorative materials.
| Acknowledgment|| |
I pay my deepest gratitude to Mr. Murtaza Husain, Head of the machine building division, ISGEC, who granted me permission for using the finite-element software. I acknowledge and appreciate the work of Mr. Rohit Garg and his expert engineering skills in designing the finite-element model and carried out the analysis. I would also like to thank Mr. Sunil Chawala, P.G.I, Chandigarh, for carrying out the statistical analysis for my research paper.
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Department of Conservative Dentistry and Endodontics, B.R.S Dental College and Hospital, Panchkula
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3], [Table 4]
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