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Year : 2006  |  Volume : 9  |  Issue : 3  |  Page : 99-103
Evaluation of two post and core systems using fracture strength test and finite element analysis


Department of Conservative Dentistry and Endodontics, M. S. R. D. C,

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   Abstract 

Aim: To compare the failure load and failure modes of these two post and core systems using Fracture Strength Test and to use Finite Element Models for the comparison of pattern of stress distribution between the two post and core systems.
Methodology: In FST, 22 maxillary central incisors were selected and RCT was performed. Post spaces were prepared and specimens were divided into 2 groups: - custom cast post and light transmitting post. Posts were then cemented using GIC for custom cast and Panavia F21 for light transmitting posts followed by composite core build up. All the specimens were then mounted on acrylic resin blocks and loaded under UTM until failure. In FEM, CT scan models were used to generate 3D and 2D models of maxillary central incisor with post core assembly and supporting structures. The material properties were assigned and boundary conditions were applied and a force of 100N was applied at 45° on the palatal surface. Analysis was run and stress distribution pattern was studied.
Results: FST indicated a statistically significant difference in the 2 post core systems with light transmitting post failing at a higher load than custom cast post. Also the mode of failure was classified as being favorable for light transmitting post. FEN results indicated less stress distribution on tooth and with in post core system for light transmitting post compared to custom cast post.
Conclusion: The area of maximum stress level for post core systems and level of fracture both coincided in our study. Light transmitting post had a higher failure load and also provides a fail safe mode of failure and thus resulting in less damage to tooth.

How to cite this article:
Narang P, Sreenivasa Murthy B V, Mathew S. Evaluation of two post and core systems using fracture strength test and finite element analysis. J Conserv Dent 2006;9:99-103

How to cite this URL:
Narang P, Sreenivasa Murthy B V, Mathew S. Evaluation of two post and core systems using fracture strength test and finite element analysis. J Conserv Dent [serial online] 2006 [cited 2014 Oct 30];9:99-103. Available from: http://www.jcd.org.in/text.asp?2006/9/3/99/42354

   Introduction Top


Endodontic treatment saves the tooth and restoration of endodontically treated tooth restores the tooth hack to function. Methods of restoring pulpless teeth were described more than 100yrs ago. The restoration of root canal treated teeth, although practiced for many years, remains a major concern in dentistry. Endodontically treated tooth complicated by substantial loss of coronal tooth structure presents a multifaceted restorative challenge for the dentist. So the restoration of pulpless tooth should increase the resistance of tooth to fracture. Most of the literature concerning restoration of endodontically treated tooth focuses on the post and core unit. The post is inserted in the root canal and the core is retained by this apical extension. This supports the coronal portion that simulates the prepared tooth to sustain definite cast restoration. So a dowel and core is commonly placed to provide retention for subsequent crown when coronal tooth structure is lacking. It was believed that one of the functions of dowel is to provide reinforcement of the tooth. The current literature however seems to dispute the reinforcement potential.

Recently, fiber reinforced post systems were introduced. Goldberg and Burrstone reported that glass fiber posts were composed of unidirectional glass fiber in the resin matrix that strengthened the structure of post without compromising the modulus of elasticity. Translucent quartz fiber post systems (Light Transmitting Post) were recently introduced as an alternative to achieve optimal esthetics. Since introduced recently. there is currently less research available on them than carbon fiber posts.

Until recently, all available prefabricated posts consisted of metal alloy that resulted in a final heterogeneous combination of dentin, metallic post, cement and core material. The major disadvantage being the stresses concentrated in uncontrolled areas that were sometimes vital to the root. The restoration of endodontically treated teeth with metal free, physiochemically homogeneous material that have physical properties similar to dentin has become a major objective in dentistry.

So the aim of the study was to compare the failure load and failure modes of these two post and core systems using Fracture Strength Test and to use Finite Element Models for the comparison of pattern of stress distribution between the two post and core systems.


   Materials and Methods Top


In Fracture Strength Test. 24 maxillary central incisors with approximately similar root length (15mm) were selected and divided in to 2 groups based on the post and core systems used:

√Group I Custom Cast post

√Group 2 Light Transmitting post (Quartz fiber)

In both the groups, the canals were instrumented using the hybrid technique. During the cleaning and shaping copious amounts of 5.25% NaOCI and 15% EDTA (RC Prep) were used to irrigate the canal and to aid in instrumentation. The teeth were decoronated to 2mm above the C EJ level and butt joint shoulder preparation was done.

For the Custom Cast post, the specimens (12) were obturated with Zinc Oxide eugenol and gutta percha, and for the Light Transmitting post, the teeth were obturated with AH 26 and gutta percha. The post spaces were prepared and the custom cast posts were cemented using Glass lonomer Cement (Type 1) and Dual Cure Resin Cement (Panavia F21) was used for the Light transmitting posts. The specimens were then mounted on acrylic blocks and were loaded on the palatal surface at 45° to the long axis using UTM (Instron) with a crosshead speed of 1 mm/ min.

All the test samples were loaded until fracture and the maximum load was recorded and statistical analysis was done using "t" test.

For FEM the Finite Element Models were constructed for both the groups:

Custom Cast post

Light Transmitting post

For both the groups, 2-Dimensional and 3­Dimensional models were obtained. Initially the teeth was selected from amongst several extracted teeth such as that their dimensions matched, as closely as possible to the mentioned in the Wheeler's Textbook of dental anatomy, physiology and occlusion. The tooth was then placed in a block of wax. A CT-scan was taken and the axial sections at a distance of 1 mm from each other were obtained. These images were imported into AutoCAD and the outline of each individual layer was traced. These layers were then stacked one on top of the other. This gave a model with the outlines of the surface contours of each tooth. This model was then imported into Altair Hyper mesh version 4 as a neutral file, using the IGES (International Graphics Exchange Specification) format. The model of the surface contours of the teeth was then converted into a solid model. The periodontal ligament was modelled as a layer 0.25 mm thick around the root surface. Finally, bone was modelled around the tooth and the periodontal ligament. A force of IOON was applied to palatal surface at 45° to the long axis of the tooth.

The FEA has the following advantages: 1) It is a non­invasive technique. 2) The actual physical properties of the materials involved can be simulated. Thus this method is nearest one that could possibly get to simulate the oral environment in-vitro. 3) The actual stress experienced at any given point can be measured. 4) Reproducibility does not affect the physical properties of the involved material and the study can be repeated as many times, as the operator wants.

The finite-clement method was first used in medical orthopedics but was later introduced in other specialties, such as orthodontics, restorative dentistry, and endodontics and so on. The first finite-clement models described the tooth-bone structure two­-dimensionally, using average geometric relationships and homogeneous and isotropic material models. Three-dimensional, finite-element models were first published in 1973 . Since then, increased interest has produced a number of papers on three-dimensional models.


   Results Top


For Fracture strength test:

Differences regarding the mode of failure and fracture resistance among the 2 different post and core systems were observed and analyzed. The fracture patterns were classified as being either favorable or unfavorable. Failure threshold is defined as a point at which the loading force reached a maximum value either by fracturing the root, bending the post or debonding of cement, or core fracture.

Failure load: Mean failure load (N) for Light Transmitting post (quartz fibre) was 396.08 ± 31.40 and mean failure load (N) for Custom Cast post was 155.98 ± 18.90. The mean difference observed was statistically significant (P<0.05). So higher fracture resistance was recorded for teeth restored with light transmitting post which was almost double the load values attained for custom cast post.

Failure mode: The mode of failure in Light Transmitting post (quartz fibre) was core fracture and for Custom Cast post tooth fracture was observed for all the 12 samples tested.

The results are presented in terms of Von Mises stress value. Analyses were performed for both the 2­dimensional and 3-dimensional models. When all the structures in models were evaluated mathematically.

In the 2-dimensional and 3-dimensional models of custom cast post the maximum stress values attained were 112.06 and 49.92 respectively and for light transmitting post (quartz fibre) the maximum stress values attained were 176.34 and 69.408 respectively. When comparing the two posts and core systems results indicated that the stress value was more with light transmitting post (quartz fibre) which was compared to custom cast post.


   Discussion Top


Post and core units can be broadly classified as:- 1) Cast dowel and core system that closely reproduces morphology of root canal space, and 2) Prefabricated post systems which can be further subdivided into a) Metallic posts and b) Non-metallic posts systems which uses adhesive materials and techniques for intraradicular reinforcement of roots.

Though used most widely, custom cast post has inherent disadvantages: 1) There is an assemblage of heterogeneous materials (i.e. post, cement and core). 2) The mode of failure is unfavorable or catastrophic.

On contrary to this, Light Transmitting Posts with the latest generation bonding systems provide an integrated tooth, post and core bonded restoration. Thus results in a metal free, physiochemically homogeneous material often referred to as the monobloc type of restoration. The replacement and reinforcement of intraradicular tooth structure with materials that are elastically compatible with dentin are preferred over the morphologic dowels, which have a high elastic modulus.

Oral rehabilitation is inherently difficult due to the presence of various functional and parafunctional forces in the mouth. Stress analysis techniques help in evaluating the various stresses generated by the forces on the oral tissues and restorative materials. Objective of seeking this knowledge is to aid in predicting the clinical performance of restorative modalities. In our study, we used theoretical (FEM) and experimental (FST) techniques to evaluate the mechanical performance of two different post and core systems. Such a comparison may enable the clinician to select a design that suits best for a particular situation and thus provide the best prognosis for longevity.

In our study, it was found that the custom cast post failed at a much lower loads 155.98 ± 18.90N than light transmitting post which failed at 396.08 ± 31.40N. This was in accordance by a study done by Begum Akkayan et al which indicated that quartz fibre posts had higher failure threshold that the teeth restored with titanium, glass fiber and zirconium posts.

In another study it was reported that the Light Transmitting posts improved the structural resistance of teeth to fracture over that provided by conventional morphologic dowel restoration. Results of our study indicate favorable mode of failure for Light Transmitting Posts in comparison to Custom Cast post which resulted in a higher number of irretrievable posts and unrestorable root fractures. Similarly in a study it was reported that minimal cracking was observed in laminate resin post structure, which was attributed to the inherent crack stopping property of the fiber/ resin interface. Thus they concluded that the relative flexibility of fiber composite laminate post was responsible for minimizing crack propagation in the root.

The finite element method (FEM) is a highly precise technique used to analyze structural stress. With its application in engineering for years. this method uses the computer to solve large number of equations to calculate stress on the basis of the physical properties of structures being analyzed. Finite element analysis has been used in dentistry to investigate a wide range of topics, such as the structure of teeth, biomaterials and restorations, dental implants, and root canals.

In the present study, in FEA when the von Mises stress values were evaluated, it was observed that the maximum stresses were accumulated along the custom cast post, where as with the light transmitting post stress accumulation within the post system was low. Similar results were observed in a study where the stresses were seen to accumulate within the cast post core unit while for the fiber composite laminate post system, stress accumulation within the post system was observed to be low. The Finite element analysis in the study showed that even the transfer of stresses to the tooth and supporting structures was less for the light transmitting post as compared to the custom cast post.

Maximum stress was concentrated in cervical region near the post core junction for the Light Transmitting Post and maximum stress values attained were 176.342 (2D) and 69.408 (3D).In contrast, for the Custom cast post maximum stress was accumulated in cervical region at core-tooth junction and followed an oblique course going subgingivally. The maximum stress values were 112.06 (2D) and 49.92 (3D).

The results of the 3-dimensional model indicated maximum stress at the post-core junction for the light transmitting post, which was in accordance with the fracture strength test where all fractures were observed at the core level. Similarly for the Custom Cast post all fracture were observed in the root extending subgingivally, and in Finite Element Analysis the maximum stresses were at the same level. However further studies arc required in this regard to compare the fracture resistance and mode of failure of these post systems and to evaluate the stresses generated after crown as final restoration. Also to ascertain whether there is correlation between the maximum stress level (FEA) and mode of failure (FST).


   Conclusion Top


  1. Higher failure loads were obtained for the Light transmitting post compared to the Custom cast post which failed at much lower loads, the difference being statistically significant.
  2. Mode of failure for Light transmitting post was favorable with all the samples fracturing at core level with minimum damage to tooth structure. However, for Custom cast post the failure mode was unfavorable as all samples showed root fracture and thus resulting in teeth that are non restorable.
  3. Stress transfer was more with in the post and core unit for the Custom cast post compared to the Light transmitting post, which had low stresses distribution with in post and core unit. Also the transfer of stresses to the tooth and supporting structure was more for the Custom cast post when compared to the Light transmitting post.
  4. Maximum stresses were observed in the cervical region for both the posts systems, but at different locations. Finite Element Analysis for the Light transmitting post showed that the maximum stress concentration was at the post-core junction and all the fractures were at core level in Fracture Strength Test indicating a definite correlation between the level of maximum stress and mode of failure. Similarly for the Custom cast post all the root fractures followed an oblique course going subgingivally (Fracture Strength Test) and Finite Element Analysis showed maximum stress at the [Table 1],[Table 2],[Table 3],[Table 4].


Top
Correspondence Address:
Panna Narang
Department of Conservative Dentistry and Endodontics, M. S. R. D. C

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DOI: 10.4103/0972-0707.42354

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    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]

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1 The effect of post length and core material on root fracture with respect to different post materials
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[Pubmed]



 

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