ORIGINAL RESEARCH | https://doi.org/10.5005/jp-journals-10015-1812 |
Fracture Resistance and Fracture Pattern of Maxillary Anterior Teeth Restored with Metallic and Nonmetallic Posts
1Restorative and Prosthodontics Department, Markas Angkatan Tentera Malaysia, Bahagian Perkhidmatan Pergigian, Cawangan Pergigian, Kementerian Pertahanan, Jalan Tekpi, Off Jalan Padang Tembak, 50634, Kuala Lumpur, Malaysia
2–4Prosthodontics Unit, School of Dental Sciences, Health Campus, Universiti Sains Malaysia, Kubang Kerian, 16150 Kota Bharu, Kelantan, Malaysia
Corresponding Author: Nor AA Muttlib, Prosthodontics Unit, School of Dental Sciences, Health Campus, Universiti Sains Malaysia, Kubang Kerian, 16150 Kota Bharu, Kelantan, Malaysia, Phone: +6019-9535-446, e-mail: aidaniza@usm.my
How to cite this article Baharom M, Muttlib NAA, Ariffin Z, et al. Fracture Resistance and Fracture Pattern of Maxillary Anterior Teeth Restored with Metallic and Nonmetallic Posts. World J Dent 2021;12(3):194–199.
Source of support: The Universiti Sains Malaysia Incentive Bridging Grant 304.PPSG.6316453
Conflict of interest: None
ABSTRACT
Aim and objective: This study aimed to compare the fracture resistance and fracture patterns of maxillary central incisors restored with different post systems.
Materials and methods: Thirty-two human maxillary central incisors were randomly divided into four groups (n = 8). Posts were placed according to the assigned groups: (I) Stainless Steel ParaPost, (II) Fiber White ParaPost, (III) everStick post, and (IV) No post. The cementation was carried out with self-adhesive resin cement, and nonprecious metal crowns were used for the core build-up and restoration. The thermocycling process was conducted for 500 cycles between 5°C and 50°C with a 2-second dwell time and 30-second dipping time, and the samples were subjected to a compressive load at a crosshead speed of 1.0 mm/minute and at an angle of 135° using an Instron testing machine. A Kruskal–Wallis test and Fisher’s exact test were used for the data analysis, with p %3C; 0.05.
Results: One of the samples was damaged during the testing, and thus, only 31 samples were used. The highest median load was recorded in group III (764.44 N), followed by group II (702.37 N) and group IV (657.44 N), while group I recorded the lowest median load (505.31 N). Favorable fracture patterns were recorded in groups II, III, and IV, while unfavorable fracture patterns were recorded in group I. The statistical analysis showed that there was no significant difference in the fracture resistance or fracture pattern between all the tested groups (p %3E; 0.05).
Clinical significance: This study helps in determining the prognosis of endodontically treated teeth restored with post and core.
Conclusion: Given the limitations of this study, the metallic and nonmetallic post systems had no significant influence on the fracture resistance and fracture pattern of the maxillary anterior teeth.
Keywords: Endodontics, Fiber-reinforced composite post, Post and core..
INTRODUCTION
Posts are reportedly used to restore endodontically treated teeth to their original function.1 In dentistry, it is known that the role of a post is purely to retain the restoration of the core, but it does not increase the resistance towards fracture.2 A tooth with major horizontal crown loss is indicated for post and core build-up.3
Various prefabricated post systems have been developed over the last few decades. The post-selection is crucial as it will determine the success of the restoration, thereby contributing to the longevity of the tooth.4 According to a previous study, the tooth anatomy, root width and length, amount of remaining coronal tooth structure, canal configuration, stresses, torquing force, development of hydrostatic pressure, post design and material, material compatibility, bonding capability, retrievability, core retention, esthetics, crown material, and procedures are among the factors that determine the success rate of endodontically treated teeth.5 The role of a post is to protect the area that is under higher stress and to enable that stress to be distributed evenly to the root structure.6 The elongated grain structure of a stainless steel post contributes to a high modulus of elasticity, which causes the post to be rigid. However, this type of post is subjected to corrosion of the metal base, leading to a high incidence of root fractures (72%). The reaction between the different metals used for the post and core can cause corrosion and failure.7,8 In contrast to this report, another study reported that corrosion actually occurs after root fracture rather than causes it.9
The fabrication of a nonmetallic post is based on the principle of carbon fiber reinforcement. A carbon fiber post is esthetically inferior as the black-colored post is visible with a ceramic crown. This post has been replaced by the silica fiber post, which has superior esthetic properties. This type of post is essentially a material composite.10 Fiber-reinforced composite (FRC) posts can be divided into two categories, namely, chairside fabricated posts, which are composed of either nonpreimpregnated fibers such as polyethylene weaved fiber or preimpregnated fibers such as glass fiber,11,12 and prefabricated FRC posts, which consist of either carbon fiber embedded in an epoxy matrix or S-type glass fiber embedded in a filled resin matrix. Fiber-reinforced composite posts reportedly have higher fatigue and flexural strength, a modulus of elasticity that is almost similar to that of dentine, and better esthetics.13,14 According to Abo El-Ela et al., a monoblock will be formed once the post has bonded to the dentine, enabling it to function as a single unit. This will lead to a better distribution of stress between the post system and the tooth structure, thereby reducing the risk of fracture and failure.15
A rigid post with a higher modulus of elasticity than that of the root dentine is said to cause unfavorable fractures. Various studies have reported that post and core failures due to root fractures contribute between 3% and 10% of all tooth build-up failure.16
The use of FRC posts with resin cement, which is becoming more popular nowadays, needs to be further evaluated. The use of total-etched or self-etched adhesive cement is considered as a technique sensitive that requires an additional step for the placement of a dentine-bonding agent before cementation. As not many studies have reported on the performance of FRC posts cemented without the application of a self-adhesive cement, an evaluation of the fracture resistance and fracture patterns on three different posts with the use of self-adhesive resin cement was carried out on maxillary central incisors in this study.
MATERIALS AND METHODS
Specimens Selection and Sample Size Calculation
This study used maxillary central incisors with mature apices which were extracted due to periodontal disease, caries, and/or orthodontic reasons from Government and Private Dental Clinics in peninsular Malaysia. Explanation regarding the use of their tooth tissue for this project was given to all potential candidates who came from dental extraction and informed consent was obtained. The specimens were collected after approval from The Human Research Ethics Committee of USM (JEPeM) is obtained. Maxillary central incisors were used in this study as it was reported as the most frequent teeth used as parameter to determine fracture resistance in most of the studies conducted to see the outcome of postendodontic restoration.17 The inclusion criteria were straight, single-rooted teeth that were free from caries at least 2 mm above the cementoenamel junction (CEJ). Teeth with gross caries affecting the CEJ and root caries, extra canals, cervical abrasions, calcified canals, open apices, curved roots, root resorption, and teeth with craze lines or cracks at the CEJ were excluded.
For the calculation of the sample size, the power and alpha were set at 80% and 0.05, respectively, while the standard deviation was set at 131 N, based on a study by Ozcan and Valandro.18 A total of eight samples were required for each group. With an anticipated failure of 10%, the final sample size needed was 32.
Specimens Preparation
Ten percent formalin was used to disinfect the specimens. All the specimens were restored in distilled water19 and cleaned using an ultrasonic scaler (Piezon System, EMS, Switzerland). The crowns of the teeth were cut with a diamond fissure bur (Horico, Germany) until only 2 mm of the coronal tooth structure was left at the CEJ level. The root dimensions were measured at the CEJ area with a digital caliper. Teeth with an overall root length of 13 ± 1 mm, and with a mesiodistal and buccolingual width of 7 ± 1 mm were selected. The post systems used in this study were the ParaPost XP (Coltene Whaledent, US), ParaPost Fiber White (Coltene Whaledent, US), and everStick (GC Europe).
Root Canal Preparation and Obturation
A number 10 file was inserted into the canal until it reached the apex. Then, the working length was determined by subtracting 0.5 mm from this measurement.20 The ISO file size 70 was set as the master apical file. The step-back technique was used to prepare the canal.16 The canal was irrigated with 2.5% sodium hypochlorite and 17% ethylenediaminetetraacetic acid (EDTA). Absorbent paper points (Meta Dental Corp, US) were used to dry the canal, while size-70 gutta-percha points were used to obturate the canal (Dentsply Maillefer, Switzerland). Gutta-percha accessory points were added while using the lateral condensation technique until the canal was fitted. The sealer material used was AH26 eugenol-free sealer (Dentsply, Germany). The quality of the obturation was checked with a periapical radiograph (Kodak Insight Carestream Health, US). Then, the resin sealer was allowed to set for 24 hours. All the specimens were stored in distilled water at room temperature. The posts were then prepared using size 2 and size 3 Gates-Glidden (Dentsply Maillefer, Switzerland). The apical seal was secured by leaving 4 mm of the gutta-percha at the apical area, and this was confirmed by a radiograph.
Post Space Preparation and Post Cementation
The specimens were divided into four groups. The post covered one-third of the root diameter and about three-quarters of the root length or as close as possible to this value.3 The canals for groups I, II, and III were prepared using a ParaPost XP drill (Coltene Whaledent, US) with a diameter of 1.25 mm. The specimens were irrigated with 2.5% sodium hypochlorite and dried with absorbent paper points (Meta Dental Corp, US). For group I, the post used was ParaPost XP (Coltene Whaledent, US), which represented the stainless-steel post. For group II, the post used was ParaPost Fiber White (Coltene Whaledent, US), which represented the prefabricated type of fiber post. Group III was cemented with everStick posts (GC Europe). An everStick post size of 1.5 was tried in the canal, and the adaptation with the canal was checked. Trimming was done to allow the post to fit into the canal. The space between the everStick post and the canal was filled by adding extra everStick material to the existing post. Once the post fitted the canal, it was cured outside the canal for 20 seconds before being placed back into the canal. Additional curing was then done for another 40 seconds with the post inside the canal.
A periapical radiograph was taken during the try-ins of the posts in all the groups to ensure that the posts adapted well to the canals. RelyX U200 cement (3M ESPE, Germany) was used to cement the posts. The canal was treated with 37% Scotchbond acid etching gel (3M ESPE, US) for 15 seconds, followed by the application of Adper Single Bond Adhesive (3M ESPE, US) before the cementation process. This was then light-cured for 20 seconds.
Preparation of Coronal and Tooth Supporting Structures
The core for the coronal support was built using fiber-reinforced core build-up material shade A2 (Build-It FR, Pentron Clinical, US). The material was applied to the coronal structure of the post and was light-cured for 40 seconds. Abutments measuring 5 × 2 × 3 mm were fabricated and impressions with polyvinyl siloxane light and regular bodies (Examix NDS, GC, US) were taken.
The fabrication of the crown together with the root mounting was according to Le bell-Rönnlöf et al. and Sirimai et al. with some modifications.6,21 Nonprecious metal crowns with a standard thickness of 0.7 mm were used as a final restoration. RelyX U200 (3M ESPE, Germany) was used for the cementation. The specimens were then thermocycled between 5°C and 50°C for 600 cycles with a dwell time of 2 seconds.22
Melted wax with a thickness of between 0.2 mm and 0.3 mm was used to cover the root surface from 2 mm of the root below the facial CEJ level. For the root mounting, the root surface was dipped into melted wax to a depth of 2 mm below the facial CEJ. This was to simulate the periodontal ligament thickness. A rubber mold was used to aid the mounting process. Cold-cured acrylic resin (Zartex Simple Cord Cure Powder, Malaysia) was used to mount the specimens. Once the first sign of polymerization was observed, the specimens were removed from the molds. All the remaining wax was cleaned from the root surface and the material for the light body impression (Examix NDS, GC, US) was loaded. The tooth was then placed back into the test block and left there until the impression set. All the excess material that remained was removed.
Loading Protocol
A universal testing machine (Instron Corp, UK) and mounting jig were used to evaluate the fracture resistance, based on the report by Newman et al.23 The specimens were secured at an angle of 135° with respect to the long axis of the tooth (Fig. 1). A compressive load was introduced at the center of the middle third of the palate with a crosshead speed of 1 mm/minute until failure.21,24 The failure force of each tooth was recorded in Newtons. Before a test was started on a new specimen, the machine was calibrated. The fracture pattern was assessed according to the mode of failure, where (1) fractures above and at the margin of the simulated bone level were favorable fractures, and (2) vertical and horizontal root fractures below the simulated bone level were unfavorable fractures.25 A data analysis was conducted using a Kruskal–Wallis program, with p < 0.05 being considered as significant.
RESULTS
Comparison of Median Fracture Load
One of the samples from group II was totally damaged during the testing and was removed. As illustrated in Table 1, group III showed the highest median load, while the lowest was in group I. The median failure load between the different post systems was tested using the Kruskal–Wallis test. This was because the results were not normally distributed when the assumption of normality and homogeneity tests were carried out. No significant difference was observed between all the groups when the fracture loads were compared (Table 1).
Group II and group III revealed a significantly higher median load compared to group I. A comparison between the two nonmetallic posts and the control group showed no significant difference in the median load (Table 2).
Comparison of Fracture Patterns and Restorability of the Teeth
Out of the 31 samples, 18 had favorable fracture patterns. The favorable fractures were contributed mainly by the nonmetallic posts and control group, as described in Table 3. These involved fractures above or at the simulated bone level. Among the favorable fractures, 44.4% of the samples from group II encountered fractures above the simulated bone level, followed by 33.3% from group IV, whereas 44.4% of the samples from group III encountered fractured roots at the simulated bone level. Thirteen out of the 31 samples encountered fractures below the simulated bone level. Group I showed the highest percentage (38.5%) of unfavorable fractures, while the least contributor was observed to be group II (15.4%). Group III and group IV shared a similar percentage of unfavorable fractures (23.1%). None of the samples showed evidence of crown/post structure fractures or dislodgements of the posts. The example of fracture patterns are as in Figures 2A and B.
Discussion
In this study, three types of posts were compared in terms of fracture resistance and fracture patterns.
For the evaluation of fracture resistance, group III recorded the highest median load, (764.44 N) followed by group II (702.37 N) and group IV (657.44 N), while group I recorded the lowest median load (505.31 N). This reflected that both the nonmetallic post systems had a higher fracture resistance than the metallic post. However, there was no significant difference in the fracture resistance between the nonmetallic post groups and the control group. The median fracture load among all the groups was also reported to be insignificant. The finding from this study contradicted a study by Abo El-Ela et al., in which it was reported that there was a significant difference in the mean fracture force for the ParaPost XP, Light Post, ParaPost Fiber White and everStick posts. However, the method used for their study was slightly different from that of the current study as in the former, the posts were cemented with resin-based luting cement but through the use of two different bonding systems. The everStick post group showed the highest fracture load as the post was available in an unpolymerized form, which allowed the resin cement to react with the unpolymerized resin monomer on the surface of the post, thus forming a good bond between them. The presence of a prepolymerized monomer was believed to reduce the bonding capacity, which led to a low fracture force.15 According to the manufacturer’s suggestion, there should be no preparation for the everStick,26 but this was not applied in the present study. The canal preparation was standardized for all the specimens in this study to avoid obtaining dentinal walls with different thicknesses. This may be the reason why there was an insignificant difference in the fracture load between the experimental groups in this study. Le bell-Rönnlöf et al. reported that while comparing the teeth restored with posts and intact teeth, there was a significant difference in the fracture loads that were recorded. The highest mean initial load was reported in the group with intact teeth. It was suggested that this was due to the poor bonding between the cement and post surface, which led to a reduced stress relief capacity.6
Group | Post type | N | Median (IQR) | X2a (df) | p value (0.05) |
---|---|---|---|---|---|
I | ParaPost XP (stainless steel) | 8 | 505.31 (64.13) | ||
II | ParaPost Fiber White | 7 | 702.37 (176.40) | 6.188 (3) | 0.103 |
III | everStick post | 8 | 764.44 (590.86) | ||
IV | Control | 8 | 657.44 (357.15) |
a Kruskal–Wallis test
Type of post | ParaPost XP (stainless steel) | ParaPost Fiber White | everStick post | Control |
---|---|---|---|---|
ParaPost XP (stainless steel) | p = 0.011* | p = 0.011* | p = 0.028* | |
ParaPost fiber white | p = 0.728 | p = 0.563 | ||
everStick post | p = 0.539 |
* Mann–Whitney test p < 0.05 (*significant)
Restorability (%) | |||
---|---|---|---|
Group | Favourable fracture (%) | Unfavourable fracture (%) | |
I | ParaPost XP stainless steel | 3 (16.7) | 5 (38.5) |
II | ParaPost fiber white | 5 (27.8) | 2 (15.4) |
III | everStick post | 5 (27.8) | 3 (23.1) |
IV | Control | 5 (27.8) | 3 (23.1) |
Fisher’s exact test, p =
a. Group I and group III = 0.6
b. Group II and group III = 1.0
c. Group I and group II = 0.3
Based on a previous study, the fracture resistance of teeth restored with different post systems is related to the modulus of elasticity of the post itself. The closer the modulus of elasticity of the post to that of dentine, the better it will be able to bear the fracture load. Thus, a post with a modulus of elasticity that is close to or similar to that of the dentine is the best choice for endodontic restoration.27 The canal was prepared to be parallel in all the post types in the current study. This might have caused the unnecessary removal of the remaining tooth structure, thereby influencing the result, so that there was no significant difference between them. Akkayan and Gulmez reported that among three types of nonmetallic posts that were tested, the post prepared with a double-tapered characteristic showed the highest mean fracture resistance compared to the posts with parallel-end preparation.28 In another study, a stainless steel post was also reported to have a higher stress at the interface of the post core system due to its rigidity. This contributed to a significant difference when compared to a glass fiber post. As a stainless steel post is rigid, the modulus of elasticity is higher than that of the dentine, thus contributing to a low fracture resistance.29
In the evaluation of the fracture pattern between each group, groups II, III, and IV were reported to have an equal number of samples with favorable fractures, while group I had the highest number of samples with unfavorable fractures. This finding was similar to a study to compare the fracture resistance of different FRC posts, where most of the groups showed favorable fracture patterns.30,31 However, there was no significant difference in the mode of failure among all the groups in the current study. The samples with favorable failures showed different fracture patterns between the various groups. In group II, the fracture line was located above the simulated bone, whereas in group III, it was located close to the simulated bone level. Core fractures and detachment of the core from the tooth structure were reported for group IV, whereas group I showed unfavorable fractures. While comparing the retrievability of the tooth, it was shown that the nonmetallic posts had repairable failures compared to the metallic posts clinically, even when there was no significant difference statistically in the mode of failure between these groups. The findings were almost similar to those of previous studies, where the fiber post system had a favorable pattern of fracture compared to the metallic or ceramic post systems.6,29 It was found that the absence of anchorage was the reason for the core fracture in group IV as anchorage is important for holding the core. This was similar to a study by Cagidiaco et al., where it was found that the majority of the samples experienced crown dislodgement and some had root fractures.32
The modulus of elasticity of the materials was among the factors that were considered. The higher the modulus of elasticity of a material, the less flexible and the more rigid it will be. The ideal post must be the one with a modulus of elasticity that is close to that of the dentine so that the stress will be distributed evenly to the root during functioning. As the load is evenly distributed, it will reduce the risk of root fracture.33 The stainless steel and ceramic posts were reported to have a modulus of elasticity of around 200 GPa. This was 12.3 times higher than the modulus of elasticity of the dentine. Due to the difference in the modulus of elasticity, the force was transmitted to the weaker structure, leading to unfavorable failure. On the other hand, the glass fiber post was reported to have a modulus of elasticity that was similar to that of the dentine, which resulted in a more favorable performance with a lower failure rate.34 The stress distribution pattern in the stainless steel post was reported to be at the post-core interface. Thus, fracture was induced along the post, which included the root. However, the stress distribution pattern for the glass fiber post was at the juncture with the dentine on the vestibular side. As the fracture started here, it did not spread to the root area, thus giving rise to a favorable fracture pattern.29
It was hypothesized that the use of a dentine bonding system would create internal bracing, which would preserve the critical interface between the dentine and the post.35,36 A resin cement layer would provide a cushion to allow the stress to be distributed evenly. Boschian Pest et al. reported the formation of a dentine core monoblock system resulting from the bonding between the fiber post, composite core and the bonding system, thereby allowing the stress to be distributed evenly. This prevented the tooth from experiencing catastrophic failure.37 The titanium post, however, showed a higher mean fracture load when cemented with adhesive composite luting cement compared to the same post cemented with glass ionomer cement. This suggested that the use of resin cement contributed to higher fracture resistance. However, the rigidity of the metallic post system was the one which led to its catastrophic failure. This was because the stress was concentrated in the coronal third and apical end of the canal.38
This study attempted to simulate the clinical situation in the application of all procedures. However, it is not possible to directly compare clinical situation because of the limitation that had been an encounter in the study as the following:
- The storage duration of the specimen was different among each other which may influence the result of the study.
- The quantity and quality of the root dentine among teeth were difficult to standardize even though standardize canal preparation was done.
- Application of everStick posts that requires a slight adjustment to fit the canal may influence the result.
- Application of static load instead of a cyclic load may not closely assemble clinical situation.
However, the result from this study could be used as a general guide in selecting the type of post for clinical application.
CONCLUSION
The following conclusions were made in this study, given the limitations:
- There was no significant difference in fracture resistance among the groups even though the nonmetallic posts showed a higher fracture resistance than the metallic posts.
- The metallic posts produced unfavorable fracture patterns, while the nonmetallic post groups exhibited favorable fracture patterns.
- The fracture lines for the ParaPost Fiber White posts were located above the simulated bone level, thus giving a better prognosis compared to the everStick posts, where the line was at the simulated bone level.
ACKNOWLEDGMENTS
The submission of this manuscript was supported by Universiti Sains Malaysia Incentive Bridging Grant 304.PPSG.6316453.
ETHICAL STATEMENT
The ethical clearance for this study was from The Human Research Ethics Committee of USM (JEPeM) (USM/JEPeM/270.4. (1.10)).
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