Introduction
The life span of endodontically treated teeth has been upgraded with advancements made in endodontic and restorative procedures (1). Fabrication of post and core becomes part of components in endodontically treated teeth as it gives better outcome to the restoration and facilitates crown retention and support (2).
In order to ensure that the placement of post is beneficial to the tooth, studies were carried out to see how it contributes to the strength of the restoration and preservation of the remaining tooth structure. There were multiple studies on performance of post and cores in relation to fracture resistance (3, 4), materials used (2, 5-10), shape and designs (11-14) and many more were conducted to evaluate the outcome of different systems used. A study on the amount of microleakage between the system and the tooth was crucial for the success of the final restoration (9).
Microleakage is defined as the “diffusion of the bacteria, oral fluids, ions and molecules into the tooth and the filling material interface” or “the clinically undetectable passage of bacteria, fluids, molecules or ions between tooth and the restorative or filling material” (15). The presence of 3 to 5mm apical seal was reported to be “questionable” in terms of preventing microleakage (12).
Fatigue that happens in the structures subjected to dynamic stress was reported to cause microleakage clinically. This factor needs to be considered as microleakage will later cause secondary caries formation that will affect the survival rate and cause failure of the treatment itself (3). Other study reported that bacteria and endotoxins had the ability to penetrate the obturating materials in the post prepared root canals (13). The penetration of bacterial toxins, oral fluids, and other ions will lead to marginal discoloration, secondary caries and marginal fractures. Microleakage had been reported to be the most frequent cause of failure for the post and core system because of the separation of the post from the internal root surface (9). This loss of retention between the post and the root may lead to vertical root fracture, which was the most severe cause of failure with irreversible consequences (16).
A study by Jung et al. (2007) found that there was a difference in the amount of microleakage between group with the dye penetration test performed after dynamic loading and during the repeated loading. The group with repeated loading showed the adhesive failure between the tooth structure and the core where the dye penetrated into a larger area of root dentin. Fogel (1995) evaluated several prefabricated post and core systems with fluid filtration microleakage test and found that none of the systems tested were capable of consistently achieving fluid-tight seal.
In the past, the options for post and core were limited and cast metal post was the only practicable choice for the restorative treatment of severely damaged teeth (17). At present, there was lack of evidence with regard to superiority of the post and core systems related to microleakage under gradual loading. Thus, the aim of this experimental research was to compare the difference in marginal dye penetration between three types of posts under gradual loadings (everStick post, Parapost XP, Parapost fiber white) and control groups.
Materials and methods
The study was conducted at Craniofacial Laboratory of the School of Dental Sciences, Health Campus University Sains Malaysia (USM). The sample size was calculated using PS software (Dupont and Plummer, 1997) based on the standard deviation (σ) of the mean gamma count at 7.33 (8) with 80% power and alpha of 0.05. Fifteen teeth were needed in each study group. With anticipation of 10% of the samples from each group which could pose problems during the procedure, 17 teeth were included in each group, to make a total of 68 teeth in this study. All teeth were collected from government and private dental clinics in Peninsular Malaysia.
The human permanent maxillary incisors with straight single-rooted canal, non-carious or carious teeth without pulpal involvement and that limited to 2mm incisal to cementoenamel junction (CEJ) with mature apices or the teeth that were extracted due to periodontal problem and those free from cracks and defects were included in this study. Exclusion criteria included teeth with extra canal, open apices, calcified canal, curved roots, and resorbed roots. The posts used in this study are as shown in Table 1. Rely X U200 (3M ESPE, USA) cement was used to cement the posts and crowns in this study.
Specimens Preparation
Root canal preparation
All external debris was removed with an ultrasonic scaler and the teeth were stored in normal saline solution. The coronal section of the teeth were amputated horizontally to the long axis using water cooled diamond fissure bur (Horico, Germany) in air turbine handpiece (Bien Air Dental SA, Switzerland), leaving only 2mm of the coronal part incisal to the cementoenamel junction (CEJ) from the buccal surface.
Straight-line access into the coronal third of the canal was made using water cold diamond fissure bur (Horico, Germany) in air turbine handpiece (Bien Air Dental SA, Switzerland). All root canals were instrumented at a working length using the step-back technique. The working length of the teeth was obtained by inserting number 10 file into the canal until it appeared at the apex of the root, then 0.5mm was subtracted from the total length. Intermittent irrigation was made with 2.5% sodium hypochlorite, rinsed with saline solution between file sizes and the canals were dried with paper points. The canals were prepared until Master Apical File size of 45 for Parapost XP, Parapost fiber white and control groups, whereas for everStick group the Master Apical File the size was 55. The canal preparation for all groups was continued using 3 sizes of larger files with substraction of 1 mm between each file. Obturation was done using the gutta percha (Meta Biomed, South Korea) and AH26 eugenol free sealer (Dentsply, USA). A radiograph was taken for each specimen to ensure there was no air bubbles present in the canal. After 24 hours of the initial preparation, the gutta percha points in Parapost XP, Parapost fiber white and everStick groups were removed using Gates Glidden (Dentsply, Switzerland) size 2 and size 3 leaving only 5mm at the apex of the root as an apical seal.
The post spaces were prepared for each system with Parapost XP drill and Parapost Fiber White (PF160) drill (Coltane Whaledent, USA). Post space preparation for everStick post was done with Parapost Fiber White (PF160) drill. The posts for Parapost XP and Parapost Fiber White (PF160) were placed in the canals following the canal length prepared with additional of 2mm above the coronal margin. The total length was 4mm from CEJ for each specimen. The recommendation from Fernandes and Dessai (2001) was followed in order to standardize the size and length of the post where the post lengths used were three-quarters of the total root length for each specimen, and the post size was smaller than one-third of the root diameter or as close as possible to this value. The procedure based on report from Le Bell-Rönnlöf et al. (2011) was applied to the everStick group. The bundle fibers of 1.2mm were fitted in the canal, both end were cut to fit leaving 2mm of fiber incisal to the coronal opening. The fibers were then light-polymerized using a light curing device for 40 seconds (Mini L.E.D OEM, France). Next, the fibers were removed and light-cured again outside the canal for another 40 seconds. After that, the additional bundles were fitted next to and attached to the individually formed 1.2mm bundle until the posts were properly fitted in the canal and light cured. The fitting of the posts inside the canals was confirmed with a radiograph. Cementations of all types of posts were performed using self-adhesive resin luting cement (Rely X U200, USA). The resin cement application was based on the manufacturer’s guidelines.
For the control group, no post space preparation was carried out. The core was prepared with 2mm of sound tooth structure, which was included at the margin of the preparation.
Crown Fabrication
A core built up for each specimen was done using composite resin (Filtek™ Z350 XT, USA). The horizontal surfaces of the coronal area of each specimen were firstly etched using 32% phosphoric acid (Scotchbond™ Universal Etchant, USA) for 15 seconds and then washed and dried using triplex syringe. Each tooth was continued with application of bonding agent (3M ESPE Adper Single Bond Plus, USA) and light-cured for 10 seconds followed by composite resin placement incrementally where each layer were light-cured for 40 seconds. Laboratory fabricated metal crowns were used as final restorations and cemented using self-adhesive resin luting cement (Rely X U200, USA) application based on the manufacturer’s guidelines.
Material Testing
Each specimen was coated with 3 layers of commercial nail varnish from the apex to the CEJ level to prevent microleakage at the apex. All groups were subjected to thermal cycling of 2000 cycles in 5º-55ºC water bath with a dwell time of 20 seconds in each bath. The everStick post, Parapost XP, Parapost fiber white groups were positioned at 135 degrees to the long axis of the tooth and were loaded for gradual loading from 0N to 50N for 100 cycles (Instron, United Kingdom) (20). The gradual loading test was not performed for the control group.
Marginal leakage testing
After the loading, the everStick post, Parapost XP, Parapost fiber white and the control group were immersed in 2% methylene blue dye for 24 hours at room temperature (21) followed by rinsing with tap water to remove excess dye. All teeth were sectioned transversely from the tooth core interface towards the apex (Exact, Germany) and observed under 30x magnification stereomicroscope (Leica, Germany). Descriptive measurements were made from the photograph of the sectioned tooth followed by measuring the depth of dye penetration along the post preparation area in mm in order to obtain the sign and depth of the leakage. Ratios of dye penetration to entire section of the roots were measured and dye penetration percentages were recorded.
Statistical Analysis
Statistical Package for the IBM Social Sciences (SPSS) version 22.0 was used for data entry. Non parametric analysis using Kruskal-Wallis test was performed to determine and compare the mean depth of dye penetration between everStick, parapost XP, parapost fiber white and control groups while a two-way ANOVA was performed to determine and compare the percentage of dye penetration these groups. The mean score with significant P-value was tested using multiple comparison Post-hoc tests (Scheffe’s procedure). The p-value was set as significant at p<0.05.
Results
The dye penetration at the marginal area for each specimen in each group was analyzed using Kruskal-Wallis and the result showed there was no significance of the median between the groups (p-value =0.193). The median with interquartile range for each group was as stated in Table 2. It showed that everStick group had the highest median while Parapost fiber white group had the lowest median.
| Group | Dye penetration at the margin (μm) Median (IQR) | p-valuea |
|---|---|---|
| Parapost XP | 1884.23 (1477.360) | 0.193 |
| Parapost fiber white (PF160) | 892.43 (1816.050) | |
| everStick | 2018.01 (1746.130) | |
| Control | 1919.14 (573.560) |
a Kruskal-Wallis test
A two-way ANOVA was used to analyze the percentage of dye penetration between everStick, Parapost XP, Parapost fiber white, and control groups. As shown in Table 3, there was a significant difference of the mean of the percentage of dye penetration between the groups with p-value of = 0.009 and the F-statistic value was 4.194 with degree of freedom (df) 3. The mean and standard deviations for each group were as shown in Table 3. The highest mean was presented in the control group while Parapost Fiber White showed the lowest mean.
| Group | Percentage of Dye Penetration at Margin Mean (SD) | F-stat (df) | p-value |
|---|---|---|---|
| Parapost XP | 82.97 (48.793) | 4.194 (3) | 0.009 a |
| Parapost fiber white (PF160) | 49.87 (38.514) | ||
| everStick | 84.71 (28.310) | ||
| Control | 94.68 (19.173) |
a post-hoc shows significant difference between Fiber White post and Control group.
Based on the result obtained, there were two samples in everStick group where the penetration of the dye reached the canal area by 60.63% and 42.81%. There was also dye penetration in canal of a sample in Parapost XP group which was by 38.05%, two samples in Parapost Fiber Fhite group which were by 22.97% and 27.41%, and also a sample in the control group which was by 93.64%. All samples in everStick and Parapost XP groups were presented with dye penetration at the marginal area, whereas 15 out of 17 samples in Parapost Fiber White group and 16 out of 17 samples in the control group presented with marginal dye penetration.
Discussion
There were no significant differences in depth of dye penetration between everStick, Parapost XP, Parapost Fiber White, and control groups in this study. These results may be influenced by the presence of 2mm sound tooth structure that acted as a ferrule for each specimen, thus resulting in no difference in the outcome. It had been suggested that the presence of ferrule as part of post and core system would resist functional lever forces and the wedging effect of tapered post (4). The presence of at least 1.5mm ferrule in single rooted teeth was reported to have better resistance to failure. Ferrule provides less impact to the post and core system, luting cements and the final restorations (18-22). This finding was similar to a study by Libman and Nicholls (1995) where the presence of 1.5-2mm ferrules provides better resistance to failure.
The everStick group showed the highest median depth of dye penetration compared to other groups in this study. This result may be influenced by the properties of the everStick material itself. The material itself has sticky behavior in a polymerized state and the fibers have a tendency to separate (23, 24). These factors may cause contamination and alteration to the material itself that prevent the formation of a monoblock with the dentine (25). Another factor that leads to deeper leakage in the everStick group may also be the effect of polymerization shrinkage. As everStick was cured after it was adapted into the canal, polymerization shrinkage may have caused gap formation between the post and the dentinal wall (11). This may have caused the separation of cement when introduced to gradual loading in this study.
Torbjörner et al. (1996) reported that degradation and hydrolysis of the organic matrix occurred in fiber reinforced composite (FRC) post as a reaction to the contact between moisture and resin based material in the FRC itself. The reaction caused the matrix to swell, which later resulted in debonding and/or cracking of the fiber interface. This will increase the water absorption by the FRC post and simultaneously decrease their mechanical properties. An increase in fibers to a polymer matrix will significantly increase the fracture toughness, stiffness and fatigue resistance of the post (26, 27). This statement may explain the reason why Parapost Fiber White post had lower microleakage compared to an everStick post.
Flexural properties of a post play a major role in stress distribution and survival of a post-restored tooth. Our finding was however contradicted with a study by Lassila et al. (2004) where the result from the three-point bending test unexpectedly showed that everStick had the highest flexural strength compared to other tested FRC posts which include Parapost fiber white (28). This may occur due to the difference in polymer matrix of everStick post where there is existence of polymethyl methacrylate (PMMA) chains which have the ability to plasticize the cross-linked Bisphenol A Glycidyl Methacrylate (bisGMA) based matrix which reduces stress formation at the fiber-matrix-interface during deflection.
We found that Parapost XP had lower microleakage pattern compared to everStick group, which is similar to the finding by Reid et al. (2003). They reported that a nonmetallic group had significant increase in microleakage compared to the metallic groups due to thermocycling process. This might be due to degradation of the polymer that hold the fibers together and/or the fibers themselves were susceptible to stress during thermocycling (2). However, there was no evidence found regarding flexural strength specifically comparisons between the post types and brands.
Based on the two-way ANOVA analysis, there was a significant difference in percentage of dye penetration between the groups where the post-hoc test showed a significant difference between Parapost Fiber White and control groups. The result showed that the group treated with post and core had lower dye penetration within the group compared to the control group that had not been restored with post. In order to reinforce an endodontically treated tooth, post system is used to provide stabilization and fasten the restoration to the remaining tooth structure (29). Thus, it can be concluded that endodontically treated teeth without post placement causes the core to be less stable and at the same time increases the microleakage of the teeth.
This study was performed under gradual loading as an alternative to cyclic loading. Cyclic loading caused flexion to posts and resulted in adhesive failure between the tooth structure and the post and/or the core (3), which explained why some of our samples in each group had dye penetration that reached the canal. Based on study by Naumann et al. (2005), gradual loading test was able to provide the information that was clinically relevant. The gradual loading used in this study is a modification of a testing model previously introduced which was chewing simulation and linear compressive loading. This modification allowed us to do material testing procedure for endodontically treated teeth in a time-saving and inexpensive manner. Linear compressive loading is less favorable for post materials testing since it may result in unequal load capacities not appearing in either dynamic method. Thus, gradual loading had been used following the recommendation by the study.
Although gradual loading could produce results that are clinically relevant, it would be more accurate if the material testing could be done under chewing simulation in which it will be able to mimic the real occlusal forces distribution.
