|Year : 2012 | Volume
| Issue : 2 | Page : 120-123
Cervical stress due to normal occlusal loads is a cause for abfraction? - A finite element model study
Kartik Reddy1, Sesha Reddy2, Bheemalingeswar Rao3, Durga Kshitish4, Satheesh Mannem2
1 Department of Periodontics, Hazari Bagh Institute of Dental Sciences, Demotant, Hazari Bagh, Jharkhand, India
2 Department of Periodontics, Vishnu Dental College, Bhimavaram, Andhra Pradesh, India
3 Department of Prosthodontics, Vishnu Dental College, Bhimavaram, Andhra Pradesh, India
4 Department of Periodontics, Dr. HARSM Dental College and Hospital, Hingoli, Maharashtra, India
|Date of Web Publication||17-Jan-2013|
Department of Periodontics, Vishnu Dental College, Bhimavaram, West Godavari District - 534 202
Source of Support: None, Conflict of Interest: None
Background: Through the years the dental profession has held a variety of theories about the causes of abfractions, including chemical wasting of the teeth, the effects of tooth brushing, and lateral forces. Acidic and abrasive mechanisms have been well documented as an etiologic factor but the stress theory remains controversial. Materials and Methods: In this study using a three-dimensional finite element model (FEM) of a maxillary central incisor with its supporting structures the normal compressive stress occurring in the tooth is plotted for a normal occlusal load of 24 kgs at an angle of 50° to the long axis of the tooth. Results: The results showed an increased stress concentration at the cervical region, which may be susceptible to cracking that, could eventually contribute to the development of cervical lesion (abfraction). Conclusion: The results of the study demonstrate higher stress values in the cervical region of the tooth for normal occlusal load. The cumulative effect of these stresses would result in abfraction as the age advances along with other wasting diseases of teeth.
Keywords: Abfraction, cervical stress, finite element model
|How to cite this article:|
Reddy K, Reddy S, Rao B, Kshitish D, Mannem S. Cervical stress due to normal occlusal loads is a cause for abfraction? - A finite element model study. J Orofac Sci 2012;4:120-3
|How to cite this URL:|
Reddy K, Reddy S, Rao B, Kshitish D, Mannem S. Cervical stress due to normal occlusal loads is a cause for abfraction? - A finite element model study. J Orofac Sci [serial online] 2012 [cited 2019 Sep 16];4:120-3. Available from: http://www.jofs.in/text.asp?2012/4/2/120/106204
| Introduction|| |
A new classification for non-carious dental lesions has evolved from the dental literature. The name given to these lesions, dental "abfractions" is a theory propounding tooth fatigue flexure and deformation through biomechanical loading to tooth structure, primarily at the cervical regions of the dentition. These lesions are typically wedge shaped with sharp line angles but occlusal abfractions have been observed as circular invagination. Abfraction, a non-carious cervical tooth loss lesion is typically wedge shaped cavity with sharp line angle. The primary causative factor of the lesion is tooth flexure and secondary factors are abrasion and acid erosion. Tooth sensitivity, gingival inflammation, dental caries, pulp exposure, pulp necrosis and tooth fracture may be the associated problems of abfraction. A tooth flexure mechanism has been proposed over the past 15 years to explain non-carious cervical tooth loss. 
Lateral forces can create stresses that disrupt hydroxyapatite crystals in the enamel, allowing small molecules such as those of water to penetrate and render these crystals more susceptible to chemical attack and further mechanical deterioration. This article reviews the literature on this topic and analysis the stresses developed in the cervical area of the tooth under normal occlusal loading by finite element model (FEM), which could be a cumulative, factor that produce abfraction (from abfraction perspective).
Review of literature
In 1991 Grippo coined the term abfraction to distinguish this type of cervical lesion associated with cuspal flexure.  Xhonga found a significant higher prevalence of the lesions in patients with bruxism.  According to Burke et al. there is evidence to support the cuspal flexural theory on the formation of cervical lesions.  The lesions occur in the teeth subjected to lateral loads, but the adjacent teeth not undergoing the load remain affected  lesions are rarely seen in the lingual side of teeth and  lesions may occur subgingivally which would not be the case for erosion or abrasion.  Lee and Eakle  reported these tensile stresses are known as the primary etiologic factor for non-carious cervical lesions.  Braem et al. noted that the location and distribution of abfractive lesions support this tensile stress induced biomechanical theory.  Goel et al. found that enamel and dentin have dissimilar mechanical responses to both compressive and shearing forces at the dentino-enamel function in response to occlusal loading forces.  Yettram et al. using engineering principles studied forces applied within a tooth when external loads were placed on it using finite mathematical stress analysis and explained why abfractions could occur even gingival to the margins of crowns.  In 1995 McCoy discussed vertical and horizontal forces as related to "dental compression syndrome" and stated that vertical forces were less harmful because they provided axial stimulation to the teeth and bone.  Horizontal forces, however, were extremely damaging because they subjected the teeth and bone to torquing and off-loading.
| Materials and Methods|| |
Finite element method
In complicated structures, it is difficult to achieve an accurate analytic solution. Numeric methods such as the finite element method of analysis (FEA) can be considered a practical approach. Finite element analysis divides the problem domains into a collection of smaller parts (elements). An overall approximated solution to the original problem is determined. In this method, solutions for each element are combined to obtain a solution to the whole body. Among various methods of assessing deformations produced in different structures, the FEM has proven its efficacy in many ways, from the normal situations concerning the nature of tooth movements under orthodontic loads, to special solutions like alveolar bone loss, extra oral force systems and many other fields.
A three-dimensional FEM of a maxillary central incisor was designed consisting of 3457 elements and 7695 nodes based on the analytical model of an extracted maxillary central incisor. A 3D hexahedral element was chosen to construct the model. The model contained the tooth, pulp, periodontal ligament and alveolar bone. The cementum was considered as a too thin a layer to be adequately modeled in finite element simulations.  Various periodontal ligament widths were taken from the data of Coolidge.  Occlusal force of 24kgs was applied in a palato-labial direction at an angulation of 50° to the long axis of the tooth at the level of middle third region of the crown (contact of mandibular central incisor to the maxillary central incisor in class I malocclusion) [Figure 1]. Boundary condition is an important factor in the FEM, reflecting the manner of movements occurring at the nodes and their relationships. All the nodes at the base of the model were fixed so as not to move when subjected to force systems. The material characteristics were taken from the data available in the literature, , [Table 1]. The stress was analyzed at 8 sampling points positioned along the tooth.
|Table 1: Material constants of tooth, alveolar bone, periodontal ligament and pulp|
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| Results|| |
The criterion used to evaluate a structure from the stress perspective is the von-mises stress. The results show maximum stress intensity in the cervical region of the tooth. The results are summarized in [Table 2]. The results are depicted in [Figure 2], [Figure 3] shows maximum stress intensity in the cervical area.
|Figure 2: The stress concentration in the tooth and the color coding system for calculating the stress|
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| Discussion|| |
Any type of stress (tensile, compressive or shearing), when sufficient in magnitude, can inflict damage on the tooth structure. Tooth flexure has been described as a lateral or axial bending under occlusal loading, tooth flexure produces tensile or compressive strains causing a disruption of the bonds between hydroxyapatite crystals leading to the formation of cracks in the enamel and the eventual loss of enamel and underlying dentin. Lambrecht's et al. reported frequent findings of enamel cracks at the cervical enamel under tensile stress. Stereomicroscopic studies, clearly demonstrated evidence of hydroxyapatite crystal disruptions caused by the stress. 
Abfraction can be commonly found on any tooth that has an exceptionally heavy occlusion marking on an inclined plane. Abfraction are also found quite frequently on patients with slight anterior bites for the same reason - guidance among from the bicuspids rather than the cuspid. Abfraction are rarely seen on teeth with prehistoric cultures, and lesions that are found can usually be explained by the customs of that culture. Mc Evoy et al. noted cervical lesions in two prehistoric populations and stated that the lesions were smaller and had rougher surfaces that the modern lesions. 
Khan et al. found that cervical lesions of all types (96%), occurred in teeth with occlusal attrition or erosion.  Conversely in the absence of occlusal tooth tissue loss, very few (4%) had cervical lesions. Thus a very strong association was found between axial and occlusal pathology.
Rubin et al. tried to analyze the stress distribution on a human mandibular first molar, without modeling the PDL.  Although using a 3D model is superior to a 2D model, ignoring PDL can influence results seriously. The model presented in the present study is as similar as possible to the real situation.
The results of this study showed maximum stress at the cervical region of the tooth similar to the findings of Allahyar Geramy conducted a study using a 3D FEM model of maxillary central incisor applying loads apart from occlusal load and showed that CEJ junction undergoes maximum of von mises stress and stress intensity.  Darendeliler et al. demonstrated stress distributions in maxillary central incisor on application of a load 450 N, 26° to the longitudinal axis applied to the incisal margin of tooth showed which maximum stress around the cervical line.  Richard A. Reinhardt in a 2D study using occlusal loading had shown the cervical region as the maximum stress area.  Others developed numerical evidence for the stress theory with finite element analysis and reported that the development of a subgingivally located lesion suggested that the principal influences are the initiation of wedge shaped cervical defects were unstable occlusal forces which is in complete agreement with the findings of this study. The presence of large values of the strains in the enamel adjacent to the CEJ is explained with the following reasons.  It is thinner than the other regions.  The enamel rod arrangement is less interviewed near the CEJ than in other regions and  the weaker bond between the enamel and dentin in the cervical area also may contribute to the occurrence of high strains in the enamel. Braem et al. described the development process of the stress induced lesion from the initial cracking stage of inter cervical enamel to subsequent advancement of the destructive process in the dentin. 
| Conclusion|| |
The results of the study demonstrate higher stress values in the cervical region of the tooth for normal occlusal load. The cumulative effect of these stresses would result in abfraction as the age advances along with other wasting diseases of teeth.
| References|| |
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2]