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| Year : 2013 | Volume
: 24
| Issue : 1 | Page : 93-97 |
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| Stress pattern generated by different post and core material combinations: A photoelastic study |
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Shaista Afroz1, Arvind Tripathi2, Pooran Chand3, Rama Shanker3
1 Department of Prosthodontics, Dr. ZA Dental College, AMU, Aligarh, India
2 Department of Prosthodontics, Saraswati Dental College and Hospital, Lucknow, India
3 Faculty of Dental Sciences, CSMMU, Lucknow, India
Click here for correspondence address and email
| Date of Submission | 29-Dec-2010 |
| Date of Decision | 28-Feb-2012 |
| Date of Acceptance | 30-Apr-2012 |
| Date of Web Publication | 12-Jul-2013 |
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 Abstract | | |
Objective: To analyze the effect of different combinations of post and core materials on stress distribution in dentin of an endodontically treated tooth.
Materials and Methods: This was an experimental stress analysis study. Models were made in photoelastic material, i.e., epoxy resin. Different combinations of post and core materials used were: Glass fiber post with composite core, stainless steel post with composite core, and cast metal post and core. Stresses generated were frozen, models were sliced and viewed under circular polariscope, and photographs were taken. Stress was calculated by counting the number of fringes.
Results: For the combination of glass fiber post with composite core, the shear stresses calculated were 1.196, 1.196, and 2.898 MPa in the apical, mid-root, and cervical region, respectively. For the combination of stainless steel post with composite core, the apical, mid-root and cervical stresses were 1.534, 0.511, and 2.557 MPa, respectively. For cast metal post and core, the apical, mid-root, and cervical stresses were 0.852, 0.511, and 1.534 MPa, respectively.
Conclusion: The cervical region of the teeth is subjected to the highest stresses irrespective of the material used. The stainless steel post with the composite core generated the highest stress concentration in different regions. A glass fiber post generated a uniform stress distribution. A cast metal post and core combination generated lesser stress than the other combinations. The vast difference in the elastic modulus of the restorative materials can lead to nonuniform stress distribution and concentration of stresses in different areas which can have deleterious effect on the survival of already compromised teeth and restoration. Such combinations should be avoided and the material which has an elastic modulus close to that of dentin should be preferred. Keywords: Composite resin, elastic modulus, fringe order, glass fiber, photoelasticity
How to cite this article:
Afroz S, Tripathi A, Chand P, Shanker R. Stress pattern generated by different post and core material combinations: A photoelastic study. Indian J Dent Res 2013;24:93-7 |
How to cite this URL:
Afroz S, Tripathi A, Chand P, Shanker R. Stress pattern generated by different post and core material combinations: A photoelastic study. Indian J Dent Res [serial online] 2013 [cited 2023 Mar 31];24:93-7. Available from: https://www.ijdr.in/text.asp?2013/24/1/93/114959 |
An endodontically treated tooth which is badly mutilated may require a post and core to rehabilitate it to proper form and function. The purpose of placing a post is to distribute the load along the long axes of the tooth and to provide retention and stability to the core which supports the final restoration. [1],[2] It is known that tooth becomes more prone to fracture after endodontic treatment; however, literature shows some controversial views. [3],[4] Brittleness of the dentin is contributed by many changes which it undergoes after endodontic treatment, like decrease in immature collagen leading to reduced hardness and resistance to shearing and dehydration causing a decrease in Young's modulus, but is mainly attributed to decreased tooth structure because of caries, access cavity preparation, or fracture. [5],[6] So, when posts and cores are used to restore extensively damaged endodontically treated teeth, the system should be selected on the basis of the effect of its shape, size (length and diameter), surface configuration, and the material on the survival of remaining tooth structure.
Stress is not a function of material, but when two materials with different elastic modulus come in contact and forces are applied, different materials behave differently under the similar loading condition, leading to stress concentration on material interfaces. Similarly, post and core with different elastic modulus will generate different stress field in different regions of the teeth, which may affect the mechanical behavior of the endodontically treated teeth. In clinical practice, the most common causes of failure in post-core restored teeth are loosening of the post and core or root fracture which may be because of stress concentration. A rigid post material increases the risk of root fracture by generating high stress concentration. [7],[8] Uniform stress distribution is desirable, whereas stress concentration in any area is harmful for the survival of the teeth. The analysis of stress distribution in endodontically treated teeth, restored with post and core, would contribute to understand the causes of failure and may help devise a post and core system with improved clinical performance. This study was performed with an aim to analyze the stress field generated by the combinations of different post and core materials commonly used in clinical practice.
| Materials and Methods | |  |
The photoelastic method was selected for this study. [9] The basic principle is that certain materials become optically, doubly refractive when subjected to load. When a transparent model is stressed and illuminated with white polarized light, a colorful pattern of lines develop because of interference of transmitted optical waves. The lines that develop are termed fringes, which are lines of constant relative retardation. These fringes are observed when a transparent model is placed in the circular transmission polariscope [Figure 1] with either white light [Figure 2] when they appear colored or monochromatic light when they appear black [Figure 3]. A fringe order consists of a sequence of color bands, including fringe line. The zero order fringes are black and indicate no stress. Stress is localized and quantified by counting the number of fringes. The larger the number of fringes, the higher the stress magnitude; the closer the fringes, the higher the stress concentration. [10],[11],[12] Shear stresses are calculated using the maximum fringe order of the combinations by counting the number of fringes [Table 1] and using the value in the formula given below for calculating shear stress: [10] | Table 1: Maximum fringe order (Nmax) in different region of the tooth with different combinations of post and core
Click here to view |
Maximum shear stress τmax = σ1 − σ2 /2 = N max Fσ/2h,
where σ is the principal stress, N max the maximum fringe order, Fσ the material fringe value = 0.410 MPa/fr/mm, and h is the thickness of the slices.
Epoxy resin (elastic modulus E 3.5-4.0 GPa) [Araldite, Petro Araldite Private Limited, Manali, Chennai.] - Adhesive CY 230 and Hardener HY 951 (aliphatic polyamine) were used for fabrication of photoelastic models of root and bone. [13] The periodontium was represented as a continuous layer with the use of light body silicone rubber impression material (Aquasil, Ultra LV Dentsply, Konstanz, Germany). [12] The restoration parts, i.e., posts and cores, were made of different combinations of materials. Three types of post and core combinations were used - glass fiber post (E = 15.0 GPa) and composite core (E = 21.0 GPa) or GFC (low elastic modulus post and core combination); stainless steel post (E = 200.0 GPa) and composite core or SSC (high elastic modulus post with low elastic modulus); and cast metal (Ni-Cr alloy) post and core or CMC (E = 150-210 GPa) (high elastic modulus post and core combination). Stress patterns in the mid-section of different models were observed after the models had been appropriately loaded. An arrangement comprising two compressive springs was devised to load the models. The springs were fully guided to prevent the horizontal deflection or drift. The load was applied by compressing the springs. The load on the model was applied with the help of a vertical bar coupled with the spring assembly. The longitudinal deflection was calibrated for variable loads. The hot air oven was used for freezing the stresses of loaded models soaked at 60°C for 4 h. Thereafter, the frozen models were sliced with diamond cutting disk to 0.6 mm thickness. The fringe patterns were observed through the analyzer of circular polariscope and then were analyzed from the photographs. The maximum fringe order was recorded around the following three regions: Region 1, apex of the post; region 2, middle third of the root; and region 3, cervical area. The zero order fringes were marked on photograph by viewing the slices in white light, where they appeared as black fringes. Then the maximum fringe order (N max ) in three different regions of the sliced model was noted by viewing the slices in sodium lamp (monochromatic light) [Figure 4], [Figure 5] and [Figure 6].
| Results | | |
For the models which were restored using the glass fiber post and composite core, the cervical stresses were calculated to be 2.898 MPa, which was the highest, whereas the stress distribution in the apical region and around the post in mid-root region was found to be uniform (1.196 MPa). As can be seen in [Figure 4], the fringes are more in number (N max = 8.5) and are more closely placed in the cervical area, indicating the zone of higher stress magnitude and stress concentration, whereas around the post in mid-root region and apical region, the fringe order (N max = 3.5) is similar and the fringes are uniformly distributed, indicating that stress is low in magnitude and there is no stress concentration. For the models which were restored using stainless steel post and composite core, the cervical stresses were again calculated as the highest to be 2.557 MPa, followed by apical 1.534 and mid-root stresses 0.511 MPa. As seen in [Figure 5], the fringe order in cervical, mid-root, and apical region is observed as N max = 7.5, 1.5, and 4.5, respectively, and the fringes are densely located in the cervical area, followed by the apical area. For the models where cast metal post and core system was used, the cervical stresses were 1.534 MPa, which was the highest, as compared to the apical 0.852 MPa and the mid-root region 0.511 MPa, but the value of cervical stress as compared to the other two systems was lower [Figure 7]. The fringe order N max observed in cervical, mid-root, and apical region was in the order of 4.5, 1.5, and 2.5, respectively, and the stress concentration as observed by the density of the fringes was not very high as compared to the other two systems.
| Discussion | | |
Corono-radicular reconstruction of root canal treated tooth should be able to protect the remaining tooth structure against future fracture by distributing the load uniformly throughout the supporting tissues, should provide retention and stability to the overlying crown, thus restoring the function of the tooth, and should not require extensive preparation leading to further loss of remaining tooth structure.
This simulation was designed to study the stress field generated in dentin by various materials of posts and cores used most commonly in clinical practice. Stress concentration occurs on two material interfaces. Therefore, the stress distribution was noted at the bottom of the post, around the post, and at the dentin-post-core interface. In this experiment, attempts were made to simulate the periodontal ligaments (uniform layer of light body addition silicone) and tooth supporting structures; thus, teeth were not embedded directly into the resin blocks. The materials were combined in such a manner that a high modulus post was used with low modulus core, a low modulus post with low modulus core, and a high modulus post with high modulus core, thus enabling to study the effect of change in modulus of post and core on stress distribution and concentration. According to the study conducted by Silva et al., the post material was found to be the more relevant factor affecting the stress distribution in dentin than the external configuration of the post. [14]
The present study confirmed that the cervical region (region 3) of the restored teeth was subjected to highest stresses regardless of the elastic modulus of the material used for restoration. This finding corroborates with those already reported by Laurent Pierrisnard et al. [15]
As depicted in GFC restored teeth, there is uniform distribution of stress in the middle third and the apical portion of the root, i.e., no stress concentration was seen around the post, but the cervical stresses were high. This finding corroborates with those reported by Pegoretti et al. [16] they suggested that the stresses at the cervical margins could be lowered using less stiff crown materials, i.e., composite resins, thus obtaining an "integrated" post-core-crown system. Similarly, Alessandro Lanza et al. in their finite element analysis (FEA) observed favorable stress distribution in case of restoration using flexible glass post. [17] They concluded that the ideal root canal post must be sufficiently elastic to accompany the natural flexural movements of the structure of the tooth, something that a rigid metal post cannot do. [18],[19] In another study using FEA, minimal stress values were recorded within the fiber composite laminate (FCL) post core system. It was observed that stress was accumulated along the cervical region of the tooth and along the buccal bone, with minimum stress along the FCL core structure. This was stated as an advantage to the restoration but disadvantage for the supporting tissues. [20]
In SSC restored teeth, high cervical as well as apical stresses were observed. Composite core material underwent deformation and resulted in localized cervical stress concentration. When the core was deformed, the load was transferred to stiffer post. This resulted into apical stress concentration due to intrusion of stiffer post. This finding is similar to other studies, where under vertical loading, the post with high modulus of elasticity resulted in high stress concentration in the apical portion of the root. [16],[19] The post with high modulus supports a large amount of vertical loading and it causes high stress at the apical portion of the post.
In the present study, in CMC restored teeth, the cervical interfaces were relatively less stressed as compared to GFC and SSC restored teeth. Here, the cervical stress was again higher when compared with the apical stress; this may be because of inhibition of the intrusion of the loaded post due to stiffer core material. This is in accordance with FEA done by Laurent Pierrisnard et al., where use of rigid core material had generated less cervical stress. [15] FEA results of another study showed approximately twice the stress concentration in the cervical region and supporting tissues in FCL post cores compared with the cast post core. [20] In the cast post system, the stress was accumulated within the cast post core and transmission of stress to supportive structures and tooth was low; this was quoted as an advantage by the authors to tooth and supporting structure. According to them, it is also possible to change the direction of the stress distribution from the apical region to the cervical region or from supporting bone structure to the post core system structure by selecting proper post and core material by understanding the demand of an individual clinical case.
| Conclusion | | |
Within the constraints of this experimental stress analysis study, the following conclusions were drawn:
- The cervical region of the teeth was subjected to the highest stresses irrespective of the materials used for restoration.
- Teeth restored with composite core (low modulus) generated more cervical stresses than those restored with cast metal core (high modulus).
- The stainless steel post (high modulus) with the composite core (low modulus) generated the highest stresses.
- A glass fiber post with elastic modulus close to that of dentin generated a uniform stress distribution in the mid-root and apical region. This is a desirable feature as it improves the longevity of the teeth.
- A cast metal post and core combination generated less stresses than other combinations. It shows that when the core is stiff and the restoration (post and core) has uniform elastic modulus, stress concentration is not very high.
The vast difference in the elastic modulus of the restorative materials can lead to nonuniform stress distribution and concentration of stresses in areas which can have deleterious effect on the survival of already compromised teeth and restoration. Such combinations should be avoided and the material which has an elastic modulus close to that of dentin should be preferred.
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Correspondence Address:
Shaista Afroz
Department of Prosthodontics, Dr. ZA Dental College, AMU, Aligarh
India
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/0970-9290.114959
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
[Table 1] |
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