|Year : 2018 | Volume
| Issue : 1 | Page : 74-80
|Influence of light transmission through fiber posts: Quantitative analysis, microhardness, and on bond strength of a resin cement
Luís Fernando Dos Santos Alves Morgan1, Marcos Barbosa Pinotti2, Fernanda Morais Ferreira1, Giovana Mongruel Gomes3, Guilherme Costa Silva1, Rodrigo De Castro Albuquerque1, Allyson Nogueira Moreira1
1 Department of Restorative Dentistry, Federal University of Minas Gerais, Belo Horizonte, Brazil
2 Department of Mechanical Engineering, Federal University of Minas Gerais, Belo Horizonte, Brazil
3 Department of Restorative Dentistry, State University of Ponta Grossa, Ponta Grossa, PR, Brazil
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|Date of Web Publication||12-Feb-2018|
| Abstract|| |
Context: Light transmission (LT) into deeper areas of the dentin root is limited. Aim: The aim of this study is to perform a quantitative investigation of the radial transmission of light (LT) through different fiber posts and its influence on the Knoop hardness number (KHN) and bond strength (BS) of a dual-cure self-adhesive resin cement at 3 different depths. Materials and Methods: Four types of fiber posts (2 translucent and 2 conventional) were used. LT and KHN analyses were performed in a specially designed matrix, which allowed measurements at 3 different depths. LT was measured using a volt-ampere meter while KHN tests were performed in a microhardness tester. For BS analysis, endodontically treated bovine roots were divided into 4 groups, each group receiving one type of post. After cementation, cross sections of the root were tested for resistance to displacement using a universal testing machine. Statistical Analysis Used: Statistical analysis was performed by using this ANOVA and Tukey's test. Results: For LT, translucent posts showed significantly higher values at all depths compared to the conventional ones. For all posts, LT decreased at the deeper depths. The KHN results showed no statistical differences among the different posts, regardless of depth. For BS, a translucent post showed the highest values, and comparative analyses between the different depths of posts also showed statistically significant differences while comparisons among the different depths of the same post showed no differences. Conclusions: LT depended on the type of post and on depth. The type of post did not significantly influence the cement KHN. A translucent post showed higher BS in pooled data.
Keywords: Bond strength, fiberglass post, light transmission, microhardness
|How to cite this article:|
Alves Morgan LF, Pinotti MB, Ferreira FM, Gomes GM, Silva GC, Albuquerque RD, Moreira AN. Influence of light transmission through fiber posts: Quantitative analysis, microhardness, and on bond strength of a resin cement. Indian J Dent Res 2018;29:74-80
|How to cite this URL:|
Alves Morgan LF, Pinotti MB, Ferreira FM, Gomes GM, Silva GC, Albuquerque RD, Moreira AN. Influence of light transmission through fiber posts: Quantitative analysis, microhardness, and on bond strength of a resin cement. Indian J Dent Res [serial online] 2018 [cited 2020 Nov 25];29:74-80. Available from: https://www.ijdr.in/text.asp?2018/29/1/74/225252
| Introduction|| |
Endodontically treated teeth often present extensive structure loss. Thus, the use of posts and cores is usually necessary to improve the retention of restorations , In cases in which there is a minimal dentin remaining, prefabricated fiber posts may present clinical advantages in their use over metal posts since they have a modulus of elasticity similar to that of the dentin and chemical characteristics compatible with resins commonly used in adhesive procedures. However, because prefabricated fiber posts do not present precise fit into root canal preparation, the cementation process is critical to ensure adequate postretention and stability.,,
Resin luting agents are recommended for cementing fiber posts, and they are available in 3 curing systems: self-cured, light-cured, or dual-cured. The use of self-cured materials has provided more reliable cementation of intraradicular posts since there is evidence that light does not properly polymerize the cement into the root, particularly in the deeper areas.,, However, considering the different techniques of cementation and types of cements, the self-adhesive dual-cure resin cement appears to be less sensitive, with fewer clinical steps, being easier to apply than conventional resin cement associated with an etch and rinse adhesive, and thus being a viable alternative for cementation of intraradicular fiber posts.
Besides the use of self-adhesive resin cements, another factor that may maximize clinical effectiveness of fiber postcementation is utilization of translucent posts. They may increase light transmission (LT) into deeper areas of the dentin root, improving the polymerization of light-or dual-cure resin cements. Manufacturers have recommended the use of light- or dual-cured resin cements in association with translucent fiber posts., The ability of translucent posts to transmit light has been investigated.,,,,,,, Most studies showed that light intensity decreased as root depth increased, resulting in loss of mechanical properties of the cement. Undesirable effects such as incomplete polymerization of resin cements, biological toxicity,,,,, and low-bond strength (BS) values ,,, have also been described in the literature. On the other hand, the use of translucent posts has already shown positive results in cement polymerization. Thus, the effectiveness of the use of translucent posts in association of dual-cured resin cements is still controversial.
Thus, the aim of this study was to quantitatively investigate LT through fiber posts and the effect of this LT on the Knoop microhardness number (KHN) and the BS of a dual-cure self-adhesive resin cement bonded to root dentin at 3 different depths. The research null hypothesis is that there will be no statistically significant difference on LT, KHN, and BS for the different depths.
| Materials and Methods|| |
Four different fiber posts of 2 types (translucent and conventional) and one self-adhesive resin cement were used in the study [Table 1].
|Table 1: Composition, type, and batch number of the materials used (posts and resin cement)|
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The translucent (T) types, which had similar compositions but different amounts of chemical components, were (T1) white post-DC (FGM, Joinville, SC, Brazil) and (T2) DT Light Post (Bisco, Inc., Schaumburg, IL, USA). The conventional opaque types (C), presenting different compositions, were (C1) Exacto and (C2) Reforpost (both from Ângelus, Londrina, PR, Brazil). For each group, one post was used for LT, 5 for KHN, and 8 for BS. Before the tests, all the posts were cut on the coronal portion under cooling at the standard height of 16 mm by a precision machine (Isomet 1000, Buehler, Lake Bluff, IL, USA).
LT was evaluated using a volt-ampere meter (Nova, Ophir, Hicksville, NY, USA) at 3 depths (thirds): cervical third (CT) at 4.1–8 mm depth; middle third (MT) at 8.1–12 mm depth; and apical third (AT) at 12.1–16 mm depth.
To assess the 3 postdepths, a metal matrix device was designed and manufactured to support the posts, the digital power meter, and the tip of the curing light unit. It also obstructed the influence of external sources of light. It had two parts: a nonreflective internal frame, which contained the posts and the volt-ampere meter, and an external cylinder, which enveloped the first part and guided the curing light tip at its top. Metal blocks assisted in determining the position of the volt-ampere meter sensor for each third of the evaluated post [Figure 1], patent pending]. Furthermore, the metal matrix allowed that the curing light unit touches the top of the post.
|Figure 1: (a) frame; (b) external cylinder; (c) volt-ampere meter; (d) post; (e) metallic blocks; (f) light curing tip; (g) set of apparatus|
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The postspace frame was manufactured with the exact dimensions of each post by means of electroerosion machining (EDM Global, Mason, OH, USA). Since the tested posts had different shapes, one frame was manufactured for each type of post. Aiming at standardizing the quantitative radial reading, the frame had a 120-degree lateral opening for each third of the posts.
The measures of each depth were taken separately. For an accurate separate assessment of each third, strategically positioned 4 mm-thick metallic blocks determined the internal position of the volt-ampere meter in relationship to the posts.
A 60-s light exposure was used (Curing Light 2500, 3M ESPE, St. Paul, MN, USA), and the luminous intensity was recorded by the volt-ampere meter at 2 and 59 s to establish the mean. The curing unit was preheated with five 60-s cycles before the first measure, and between each measurement, the light source was left at rest for 1 min and 30 s, which was the time necessary for the cooling fan to turn off.
Equations 1 and 2 were used to calculate the luminous intensity per unit of area at each depth. Equation 1 provided the area of exposure from a trunk of cone to the volt-ampere meter scanner (considering a 120° side opening). In this equation, “A” was the area (mm 2), “g” (m) was the generatrix, “R” (m) was the large base radius, “r” (m) was the small base radius, and “h” (m) was the height of the truncated cone.
Equation 2 represented the luminous flux per unit of area, IR(W. m −2). In this equation, A (mm 2) derived from Equation 1, and “I” (W) was the total luminous flux from a given section of a post of height “h” (m), the measurement particularly targeting one-third of the surface area of this section (120° side opening).
Knoop hardness number measurements
KHN of the cement was measured at 3 depths of posts: CT at 4.1–6.8 mm depth; MT at 8.8–11.5 mm depth; and AT at 13.5–16 mm depth.
The metallic device for KHN tests consisted of four parts, including a new part designed to support the resin cement [Figure 2]b and [Figure 2]c. Its internal structure provided the separation of each third, which allowed the resin to polymerize in blocks, separately [Figure 2]c. The other parts, (a) a frame, the main structure which contained the posts, (d) post, (e) light curing tip, and (f) an external cylinder which holds the other part while also incorporating the tip of curing light unit, were the same as those used for LT tests.
|Figure 2: Metallic matrix: (a) a frame, the main structure which contained the posts, (b) a support to standardize the position and volume of resin cement, (c) a support to standardize the length of each three third deep postregions, (d) post, (e) light curing tip, and (f) an external cylinder which holds the other part while also incorporating the tip of curing light unit|
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The cement (RelyX Unicem Aplicap, 3M ESPE, St. Paul, MN, USA) was manipulated per manufacturer's specifications and was inserted directly into the projected spaces (Elongation tips, 3M ESPE, St. Paul, MN, USA), followed by the post. The photoactivation was the with the light tip point down the axis of the tooth root for 40 s (Curing Light 2500). After 10 min, the cement specimens were removed from the matrix and immediately included in premolds (Buehler, IL, USA) with a black pigment crystal resin, which was poured into the device using a cast N'vac (Buehler, IL, USA). After the crystal resin had cured, the specimens were removed from the premolds and stored dry and away from light for 7 days. The surface to be analyzed was sequentially polished with #320 to #1200-grit SiC papers and filled with diamond polish paste (Buehler, IL, USA). The control group used the same post as the T1 group and the same method but did not include the photopolymerization step.
KHN measurements were performed by a hardness tester (Micromet 5104, Buehler, Tokyo, Japan) using a static load of 50 g for 10 s. Three indentations were performed for each third of each group. The values were obtained from the average reading of the 3 indentations oriented along the axis of the post on each third.
Bond strength measurements
The crowns of 32 permanent bovine incisor teeth with mature roots were removed with a precision machine (Isomet), leaving a 19 mm-long root (approved by Ethics Committee of Animal Experiments #19-2010) and resulting in 8 roots allocated to each of the 4 groups. The preparation of root canals was standardized using ISO size 110 Gates-Glidden drills (Dentsply Maillefer SA, Baillaigues, Switzerland). The root canals were obturated using cold lateral compaction of gutta-percha with the sealer 26 (Dentsply, Tulsa, OK, USA). After 7 days, 14 mm deep postspaces were prepared with drills (Dentsply Maillefer SA) ISO size 30, 70, and 110, except for Group T1, in which root canals were prepared using drills provided by the manufacturer's kit. After preparation, root canals were dried with endo paper points (Dentsply, Tulsa, OK, USA). Roots were then filled with cement (RelyX Unicem Aplicap) from bottom to top (Elongation tips). Then, previously cleaned (70% alcohol) posts were placed into the root canals. Finally, sets were photoactivated (Curing Light 2500) for 40 s with light curing unit tip point down the axis of the tooth root. The teeth were in storage in wet conditions. One week after cementation, the roots were embedded in acrylic resin (Duralay, Reliance, Worth, IL, USA), confined into tubes of polyvinyl chloride, and each one was sectioned transversely by diamond disk (Isomet) to produce discs of one millimeter: 2 discs of the coronal third (CT) at 2.5 and 4.0 mm; 2 of the MT at 6.5 and 8.0 mm; and 2 of the AT at 10.5 and 12 mm. Thus, 16 specimens were made for each posttype for each of the 3 root thirds (coronal, middle, and apical). The specimens were stored in sterile distilled water at room temperature for 1 week.
The dimensions of the discs were calculated to obtain the bonding area in mm 2 by applying the formula:P(R + r) [(h2+ (R − r) 2] 0.5, whereP = 3.14, “R” represented the coronal radius (mm), “r” was the apical radius (mm) and “h” was the disc thickness (mm).
The specimens were subjected to compressive loads on the post in the apical-coronal direction of its longitudinal axis by a universal testing machine (AG-I, Shimadzu Autograph, São Paulo, SP, Brazil) at a crosshead speed of 0.5 mm/min until the moment of displacement.
For LT, ANOVA, and Tukey's statistical tests were applied to the results (P< 0.05) to compare the thirds and the groups.
The KHN and BS results were pooled from the three root sections and were used to assess differences among groups. Data gathered from each section of the root were used to verify differences within the sections (one-way ANOVA). Later, the results were analyzed for each post according to the different thirds to evaluate the presence of interaction effects (two-way ANOVA and Tukey's test; P < 0.05).
| Results|| |
[Table 2] shows within-group means, standard deviations, and statistical analyses of the amount of radially LT through the fiber posts at the different depths. For AT, the cross-group analysis indicated a higher luminous energy in T2, followed by T1. Groups C1 and C2 showed significantly lower values and had no significant differences between them. For MT, the analysis revealed significantly higher values for groups T1 and T2, with significant differences between them. Groups C1 and C2 presented statistically lower values. For CT, Group T2 showed higher luminous energy than T1 while groups C1 and C2 once again presented significantly lower values. Finally, within-group analyses of the different thirds showed significant reductions in the luminous intensity as a result of increased depths.
|Table 2: Within-group means, standard deviations, statistical tests of the light transmission in mW/cm2|
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KNH showed no statistical difference in the cross-group analysis and in the within-group analysis of the different thirds (one-way ANOVA). The analysis of the results for each post, depending on the different thirds and the presence of interaction effects, showed no significant differences (two-way ANOVA). The Tukey posttest showed no individual differences [Table 3].
|Table 3: Means and standard deviations of experimental and control groups of the different thirds and posts for Knoop hardness number|
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In BS analysis, there was a significant difference within each post type, with groups T1 and C1 showing the highest values. Comparative analyses among the same third of each post type revealed significant differences (one-way ANOVA). The analysis of the results for each post, depending on the different thirds and the presence of interaction between the effects, did not differ significantly (two-way ANOVA) (P< 0.05) [Table 4].
|Table 4: Means and standard deviations of experimental groups of different thirds and posts for bond strength (MPa)|
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| Discussion|| |
Although studies have revealed unfavorable results concerning the amount of luminous energy transmitted through translucent posts,,,,,, some of the manufacturers promise a sufficient transmission of light for the polymerization of both light- and dual-cured resin cements. The LT and characteristics of the root dentin ,, are closely related to the cementation quality and the stability of the adhesives interfaces when light- or dual-cured cements are used.,,, Consequently, studies are still necessary to validate this clinical recommendation.
Special attention was given to developing a valid methodology. The designed metal matrix used for LT and KHN tests prevented the introduction of any light wavelengths along the optical path of the post that had not been axially introduced through the upper end of the post. Furthermore, the matrix provided a fixed distance between the top of the post and curing light tip, guaranteeing the same amount of light energy reaching the posts. The electroerosion machining of the internal frames that supported the posts resulted in a standardized fit for all different posts, minimizing the effect of the adaptation, and cement thickness layer on the results. Another adopted care was related to the calculus. The investigated posts were different in diameters and in shapes. Assuming that the amount of light transmitted through the post is directly related to its diameter, the data calculation for each post was based on the standardized area that was exposed (120°) to the volt-ampere meter for LT, or on the standardized area that was in contact with the resin cement for KHN, as explained through Equation 1. In addition, the choice of an self-adhesive dual-cure cement was based on the fact that it did not use adhesive systems, eliminating this variable. Moreover, it is a type of cement recommended for post cementation due to its simple use and its partially chemical cure. However, the mechanism of chemical reaction of the cement, besides being dependent on the light energy, is also related to the presence of dentin., The evaluations of the KHN of the present samples were made without the presence of dentin, therefore enabling the evaluation of the effect of LT alone.
In the present study, the values obtained for LT revealed higher results for translucent posts, which was expected since translucent posts may present luminescent agents and have less opacifiers than conventional posts, resulting in better LT. Low luminous intensity was also shown across all investigated depths, which is in accordance with the previous studies., The reduced luminosity, as a result of the increased depth, was expected and can be explained by the principles of transmittance, reflectance, and absorbance. During its path along the post, light loses energy. The T2 results for the MT depth were unexpected. The MT showed less LT than the AT. At MT region, this post is remarkably conic, and the longitudinal orientation of the fibers appears to be the most likely feature to explain this phenomenon, i.e., the MT region of this post has fewer translucent fiber endings, which act as waveguides, so it is acceptable that smaller amounts of light were recorded. The null hypothesis was rejected since translucent posts revealed higher LT, and statistically lower values of LT were found on deeper areas.
The KHN test is a common method to evaluate changes that can be attributed to the amount of polymerization of the resin-base materials. It is frequently used to evaluate the physical properties of these materials.,, In the present study, for KHN, the null hypothesis tested was accepted since there was no difference in the microhardness of the tested resin cement between the different posts and control at different depths. A previous study  also found similar and uniform values for the same cement in combination with a translucent fiber post. Since the translucent posts presented higher LT than the conventional ones, it seems that the amount of energy effectively transmitted was unable to result in a better cement monomer conversion compared to that found in the conventional opaque posts.
For the evaluation of the BS of fiber posts to root dentin, a pushout test is a reliable method. The perpendicular sectioning of root-post sets into 1-mm-thick sections in this study allowed a uniform force application, with less interference of tensile forces.,, The results for BS showed statistical differences among the posts, for the pool of the three thirds, and among the different thirds of the posts. A translucent post, T1, revealed better-pooled BS values, as observed in another study. However, these differences may be due to the postadaptation and not only to the LT according to Pirani et al., 2005. Group T1, which showed the highest values of BS, was the only root canal preparation made with the drill provided by the manufacturer × s kit, which allowed a more precise fit of the post into the root canal. Regarding the results of the comparative analysis between the different postthirds, the highest values obtained by CT and MT for C2 can also be explained by the adaptation of this post, which has a cylindrical geometry similar to the drill used to format the root space. This hypothesis is supported by the data obtained for KHN test, in which there was no difference in the results between the control group and the others. This may show that clinical success in the cementation of fiber posts may also be related to the frictional retention achieved by a precise fit., The null hypothesis was rejected since the differences were found in BS analysis.
Despite the low rates for LT, the present results proved that the posts do in fact transmit light radially, but in most cases, not enough to improve KHN and BS values. The fit of the post into root canal may play an important role in BS. Future tests may consider the behavior of BS for self-adhesive cements after aging.
| Conclusions|| |
- The amount of radial transmission of luminous energy depended on the type of post
- There was a decrease in the amount of radial transmission at deeper depths
- The amount of light transmitted through the post did not significantly influence the cement's microhardeness or the BS of the different posts and thirds evaluated, except for one translucent post (T1) in BS tests.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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Prof. Luís Fernando Dos Santos Alves Morgan
Avenida Silva Lobo 1730, Belo Horizonte, MG
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3], [Table 4]