Fiberglass intraradicular posts are frequently used to restore endodontically treated teeth since the 1990s [1]. In many cases, the removal of these posts is indicated for endodontic retreatment, considering that the removal of the fiberglass pins may not occur completely, with residues remaining in the dentinal walls, as well as the presence of cement and fiber debris on the walls of the elements, thus compromising the success of endodontic retreatment [2-4]. On the other hand, when the need for removal is related to biomechanical problems or fractures requiring new cementation of a fiberglass pin, it is important to have dentin walls without residues so that adhesiveness and correct imbrication is achieved. However, some factors may increase the difficulty of this removal, such as material, shape and length of the pin, type of cement used, the interrelationship of the pin with the canal walls, accessibility, the skill of the professional and the technical resources available [5].

The methods used to remove fiberglass posts are not completely safe, leading to the initiation and propagation of fractures, lateral perforations, loss of dentin structure in the apical region and deviations of the root axis [6-9]. Among the techniques for removing intraradicular pins are the removal by drills, by ultrasound, and the multi-laminated drill kit system [4,8,10]. In association or isolation, such techniques can lead to a significant loss of dentin and the formation of microcracks in the most apical region, resulting in the fragility of the dentinal element [11,12].

Thus, the removal of fiberglass pins in a safe, effective and efficient manner is not yet fully ensured, requiring the development of new techniques that can detach themselves from the mechanical issue and seek in the physical-chemical scope of the materials involved ways to release or break the adhesive bond that is reached by the cement-dentin-pin union. The study aimed to investigate the degrading potential of different solutions and acids at the cementing resin interface between intraradicular fiberglass posts and root canal walls and their quality.

The 66 human single-rooted premolars adopted in this study, obtained from the tooth bank of the research institution, presented similar diameter and root length, with straight roots and complete root development. The project was approved by the Human Research Ethics Committee under number: CAAE 58114922.8.0000.5283.

Preparation and Filling of Root Canals

Initially, the crowns were sectioned at the cementoenamel junction and endodontically treated using the staggered technique, under irrigation with 2.5% sodium hypochlorite (NaOCl). At the end, the root canals were dried with paper cones and filled with lateral and vertical compaction with gutta-percha and Sealer 26 endodontic cement (Dentsply-Sirona, Konstanz, Germany), sealed with temporary cement and stored in an oven in saline solution for 48 hours at 37.5 °C.

The root canal obturator material was removed with Wide #2 or #3 drills and Gates Glidden #2 or #3 (Dentsply Maillefer, Joshnson City, Switzerland), maintaining 3 mm to 4 mm of gutta-percha in the apical region of the root canal. Then the root canals were irrigated with 5 ml of distilled water and dried with absorbent paper tips (All Prime #45, Meta Biomed Co. Ltd, Chungcheongbuk-South Korea). Fiberglass pins #1 and #2 (White Post DC, FGM, Joinville, Brazil) were used according to the best adaptation to the characteristics of each root anatomy.

We adopted the protocol for surface treatment of the fiber pin by Pegoraro.^[13] The fiberglass pin was cleaned with 70º alcohol, with subsequent application of 37% phosphoric acid for one minute, washing with running water for the same time, air-dried, application of silane for one minute (Lot.371974M. Denstply, Sirona, Konstantz, Germany), waiting one minute to air-dry, and subsequent application of light-curing adhesive (Adper ScotchBond multipurpose, 3M ESPE, Maplewood, U.S.), being light-activated for 30 seconds with light-curing (Cordless Valo, K20629, South Jordan, EUA). Pin cementation was performed with Rely-X U200 resin cement (Lot. 8180358, 3M ESPE, Maplewood, Estados Unidos) and photoactivated for one minute perpendicularly to the fiberglass pin with photopolymerizer.

In the end, the 66 teeth were radiographed using two different radiographic views, mesiodistal and vestibulo-lingual, in digital X-ray, using 0.1 Kvt/s, to verify possible gaps, lacunas, and the correct positioning of the fiberglass pins. The defects present in the cementing interfaces, in the different radiographic incidences, were categorized and analyzed visually, by the same evaluator, and through digital image processing through the FIJI program (ImageJ 1.53q, National Institutes of Health, USA).

After 48 hours of the intraradicular pins cementation, 28 teeth were transversely sectioned with a diamond disc (American Burss), ensuring the formation of a proof body (PB) for each tooth with two surface areas containing the dentin interface, resin cement, and fiberglass pin (Den/CRes/PFibr). Then, 18 teeth were reserved for analysis in a SkyScan 1172 microtomograph (µCt) (Bruker-μCT, Kontich, Belgium) for the intraradicular degradation step.

The initial parameters of the PBs were determined to measure the sorption and solubility properties, used to evaluate the durability, degradation, and dissolution of resin materials [14,15].

Initially, the mean initial mass of each PB (M₁) was obtained, weighed in triplicate, on a scale with millisimal precision (Precision Analytica Scale, Bioscale, Brazil). Then, a manual micrometer (Mitutoyo, Model E-347 0-25mm) measured the thickness of each PB at five points, with four measurements taken at the ends, arranged on opposite faces, and the fifth measurement taken at the center, to obtain the average height (h) of each PB. From the radius (r) and h of each PB, the volume (V) in mm³ was calculated using the formula below (Eq. 1).

Degradation Test on Root Discs with Fiberglass Pins

The PBs formed seven groups: control (saline solution) (n = 4); 37% phosphoric acid (n = 4); 9% (n = 4) and 10% hydrofluoric acid (n = 4); Xylol (n = 4); Eucalyptol (n = 4) and EDTA (n = 4). We immersed the PBs in 1.5 ml Eppendorf tubes containing one milliliter of the solutions preheated in a water bath at 37.5 ºC for 20 minutes, on a heating plate (Magnetic Stirrer, XMTD204), in a laminar flow hood (Quimis, 0216-11). Immediately after the degradation test, three baths of distilled water for ten minutes stopped the chemical reactions of the PBs.

Sorption and Solubility Properties

Following the degradation test, the PBs were dried with absorbent paper and weighed to determine the mass immediately after the action of the solutions (M₂). Subsequently, after storage in a dissector (Sterilifer, 1.2 DTMC) at 37.5 °C for 24 hours, PBs were weighed in triplicate until they reached a maximum difference of 0.0001 g, and M₃ was defined by obtaining an average of the weightings; otherwise, they would return to a new dissector storage cycle. Then, the formulas below, with the data obtained, determined the Sorption (S) and solubility (SL) (µg/mm³) of the PBs (Eq. 2).              

 Mass Loss

The percentage of mass loss (ML) consisted of evaluating the final mass in relation to the initial mass, in grams, with the following formula, taking into account the masses obtained from steps M₁ and M₃, used in the property of sorption and solubility (Eq. 3).

Intra-Radicular Degradation

This step counted with 18 teeth previously analyzed in µCt. The parameters used for image acquisition in μCT were: voltage 50 Kvp; Source Current 800 μA, flat-field correction, Al filter 0.5, Image Pixel size 21.98 μm; Exposure 4000 ms; Rotation Step 0.5 and Frame Averaging 3.

After scanning, the teeth formed six groups, containing three teeth each: 37% phosphoric acid; 9% and 10% hydrofluoric acid; xylol; eucalyptol and EDTA. The teeth were placed in 1.5 ml Eppendorf tubes, over 1 ml of distilled water, and inoculated with 0.2 ml of the respective group solution in the root canal. The groups were conditioned for 20 minutes in a water bath at 37.5 ºC. Immediately afterwards, the teeth were submitted to three ten-minute baths in distilled water, to interrupt the chemical reactions, and the root canal of each tooth was dried with a sterile paper cone for 30 seconds.

Then, the teeth experienced a new analysis in µCt, following the same parameters used in the previous acquisitions. The NRecon program (SkyScan, Kontich, Belgium) reconstructed and processed the images obtained by adjusting the parameters. Subsequently, the images were segmented in the CTan program (SkyScan, Kontich, Belgium), delimiting the regions of interest (ROI) of the samples. Each tooth presented two ROIs, whereas the first ROI is the tooth without the cementing material, and the second ROI presented the intraradicular cementing material, delimiting the analysis sections from the dentin cement-enamel junction to the interface region fiberglass pin and resin cement with the upper region of the gutta-percha.

The ROI of the intraradicular cementitious material of each tooth followed for the process of thresholding and binarization of the images, adjusting the histogram to evidence the artifacts suggestive of voids or bubbles, resinous cementitious material or fiberglass pin residues, obtaining the 3D integrated analysis of all objects in the volume of interest of the root canal of each tooth. The Percentage of object volume after injection of the different solutions (Obj. V/TV% final) was subtracted from the initial Obj. V/TV% of teeth resulting in the Percentage of degraded object volume (Obj. V/TV% degraded) of each condition. Ultimately, we visually evaluated the 3D images in DataViewer and CtVox software (SkyScan, Kontich, Belgium).

Radiographic Evaluations

The defects found were visually quantified in each radiograph, categorized into gaps or lacunas, where the gaps are smaller spaces and the lacunas as spaces that ran through larger areas in the resin cementing interface [16-18].

The radiographs were compared between the PBs and the different radiographic incidences: mesiodistal (G.M-D) and vestibular-lingual (G.V-L). When comparing the different radiographic incidences, different magnitudes were found regarding the presence of these defects, suggesting that the radiographic incidence is decisive in their characterization. The distribution of defects or interfaces considered perfectly sealed were different when evaluating the same PB in the directions G.M-D or G.V-L (Figure 1).

Figure 1: Comparative analysis between the radiographic images in the direction G.M-D and G.V-L. Radiography in the direction G.M-D (A.1) revealing areas of gaps (yellow arrows) and area of lacunas (blue arrow) while the same PB in the radiographic take in the direction G.V-L (A.2) revealed the incidence of lacunas (blue arrow) and absence of gaps. B.1 and B.2 revealed the same cementing condition in the two incidences performed.

Then, the defects present in the G.M-D and G.V-L radiographic analyses were analyzed using the non-parametric Mann-Whitney test, with a confidence interval of 0.95%, using GraphPad Prism 5 software (Graph Pad Software Inc). The two types of defects, gaps and lacunas, are higher in G.M-D than G.V-L, revealing a statistical difference between G.M-D and G.V-L in gaps defects (p = 0.0260) (G.M-D, 0.848 ± 0.749, and G.V-L, 0.560 ± 0.500) and lacunas (p < 0.0001) (G.M-D, 1.151 ± 0.898, and G.V-L, 0.560 ± 0.725) (Figure 2).

Figure 2: Comparative analysis between the different defects and between the images suggestive of complete cementation.

           = Statistical difference.

Was analyzed the association of gaps defects and lacunas in the same radiographic image and categorized the defects according to the number of occurrences of the events in association, revealing no statistical difference (p = 0.1262). However, we verified a greater number of these associations in G.M-D (0.348 ± 0.480) compared to G.V-L (0.227 ± 0.422) (Figure 2). At the same time, regarding the number of radiographs suggesting complete cementation of the intraradicular pin, without the occurrence of defects, thus indicating success in the procedure, the analysis revealed a significant difference (p = 0.0202) of radiographs suggesting complete cementation in G.V-L (0.2428 ± 0.431) compared to G.M-D (0.090 ± 0.289) (Figure 2).

Image Processing Analysis

Once gaps and lacunas in the radiographs were verified, FIJI program processed and analyzed the images. All images followed the same macro for image processing and analysis: conversion to 8 Bits; Selection of the ROI, delimiting the radicular region surrounding the dentin union interface and intraradicular pin; Enhance contrast (0.35%, normalize and equalize); Gaussian Blur (sigma 2.00); Unsharp mask (5.0 pixel sigma and Mask Weight 0.60); FFT Bandpass Filter (filter large structures down 40 pixels, filter small structures up 3 pixels, suppress Horizontal Stripes and 5% tolerance of direction) and Threshold (Otsu filter). In the end, the particle analysis was performed, accounting for the area of gaps and lacunas in the images.

Corroborating the visual radiographic analysis, the means of the total areas of gaps and lacunas were significantly lower (p = 0.0002) in G.V-L (G.V-L 0.083 ± 0.110 pixels/mm²) when compared to G.M-D (G.M-D 15.001 ± 86.774 pixels/mm²). Particle unit (pcs) analysis also followed the same pattern, revealing a statistically significant difference (p < 0.0001) between G.V-L (G.V-L 1.287 ± 1.133 pcs) and G.M-D (G.M-D 2.666 ± 1.629 pcs) (Figure 3).

Figure 3: Analysis by digital image processing. Comparative analysis between the areas of gap and lacuna defects and between particle units.

Δ = Statistical difference.

Sorption and Solubility Properties

The data were analyzed in the GraphPad Prisma 5.01 software, using the analysis of variance method (One-Way - ANOVA) and complemented by the Tukey post-test, with a significance level of 5% (p < 0.05).

The sorption property showed no statistically significant difference (p = 0.6685) between the groups. However, the PBs exposed to saline, as the control group, presented the highest sorption property (0.292 ± 0.339 µg/mm³), followed by 9% hydrofluoric acid (0.188 ± 0.060 µg/mm³); 37% phosphoric acid (0.181 ± 0.104 µg/mm³); xylol (0.179 ± 0.123 µg/mm³); EDTA (0.142 ± 0.091 µg/mm³); eucalyptol (0.088 ± 0.015 µg/mm³) and 10% hydrofluoric acid (0.084 ± 0.041 µg/mm³).

The solubility property showed a statistically significant difference (p = 0.007), indicating greater solubility in the PBs exposed to 37% phosphoric acid (0.175 ± 0.073 µg/mm³), followed by saline, as the control group (0.056 ± 0.048 µg/mm³), xylol (0.042 ± 0.012 µg/mm³), EDTA (0.042 ± 0.018 µg/mm³), 9% hydrofluoric acid (0.036 ± 0.078 µg/mm³), eucalyptol (0.018 ± 0.004 µg/mm³) and 10% hydrofluoric acid (0.004 ± 0.007 µg/mm³).

The comparative analysis showed statistically significant differences between the 37% phosphoric acid group (0.175 ± 0.073 µg/mm³) and the groups: xylene (0.042 ± 0.012 µg/mm³); eucalyptol (0.018 ± 0.004 µg/mm³); saline solution (0.056 ± 0.048 µg/mm³); EDTA (0.042 ± 0.018 µg/mm³); 9% hydrofluoric acid (0.036 ± 0.078 µg/mm³) and 10% hydrofluoric acid (0.004 ± 0.007 µg/mm³) (Figure 4).

Figure 4: Analysis of mass loss and sorption and solubility properties in the removal of intraradicular fiberglass pins.

Δ ?  • ° ? ? =  Statistical difference between groups.

Weight Loss

The mass loss showed a statistically significant difference (p < 0.0001), indicating a greater mass loss in PBs exposed to 37% phosphoric acid (- 0.83 ± 0.072%), followed by xylol (- 0.272 ± 0.033% µg/mm³), EDTA (- 0.247 ± 0.072 µg/mm³); saline solution (- 0.205 ± 0.0.151 µg/mm³), 9% hydrofluoric acid (- 0.145 ± 0.324% µg/mm³); eucalyptol (- 0.132 ± 0.061% µg/mm³) and 10% hydrofluoric acid (- 0.0025 ± 0.059% µg/mm³), respectively (Figure 4). While the comparative analysis between  the groups showed a statistically significant difference for 37% phosphoric acid compared to the other solutions (Figure 4).

Intra-Radicular Degradation

The analysis of this step revealed no statistically significant difference (p = 1.297) between the groups. However, the microtomographic analysis revealed a higher percentage of volume loss of objects, intraradicular cement material, in teeth exposed to 37% phosphoric acid  (-1.688% ± 0.01 Obj.V/TV%) followed by EDTA (-0.843% ± 0.007 Obj.V/TV%), 9% hydrofluoric acid (-0.549 ± 0.005 Obj.V/TV%), 10% hydrofluoric acid (-0.484 ± 0.003 Obj.V/TV%), xylol (-0.465 ± 0.001 Obj.V/TV%) and eucalyptol (-0.363 ± 0.003 Obj.V/TV%) (Figure 4). The 3D reconstructions revealed a greater loss of cementing material along the walls of the root canal, and the detachment of particles of different sizes moving to the apical region of the root canal is also visible (Figure 5, A.2-B.2, C.2-D.2 and E.2-F.2). In the interface region, between the endodontic material (gutta-percha) and the cementing material (intraradicular fiberglass pin and resin cement) it was possible to verify a partial degradation (Figure 5, C.2-D.2 and E.2- F.2).

Figure 5: 3D reconstructions of the root canals. 3D reconstructions before (A.1, A.2, C.1, C.2, E.1 and E.2) and after injection of solutions (B.1, B.2, D.1, D.2, F.1 and F.2). Image A.1, blue arrow, demonstrates the displacement of larger volume particle (B.1, blue arrow) after the action of the xylol solution. The yellow arrow (B.1) demonstrates the detachment of the resin interface, previously adhered to the dentin wall (A.1). Images C.2 and F.2, white arrows, demonstrate the areas that suffered volume reduction after contact with 37% phosphoric acid (D.1, D.2, F.1, and F.2). Image E.2 (green arrow) indicating the area with a volume reduction at the cementing interface above the resin cement and gutta-percha interface (F.2, green arrow).

The correct adaptation of the fiberglass post depends on the correct choice of diameter and cementation process [17]. In this process, cementation techniques can produce different results, resulting in the formation of gaps and gaps within the root canals, which can affect the longevity of restorations and the clinical treatment itself [18]. Radiographic analyzes, with different incidences, allowed the identification of these failures in the cementing interface, when the incidence of X-rays was changed. These results suggested that vestibulo-lingual radiography, as a clinical standard, could mask existing flaws in cementations, requiring further studies on the incidences using the Clark technique.

The results obtained through digital image analysis corroborated the results of the visual radiographic analysis, suggesting that the images obtained in the vestibulo-lingual direction presented smaller gaps and lacunas, after measuring the areas of the defects, as well as the number of particles evaluated in each direction.

The sorption property of resin cements corresponds to the durability of the restoration since they should be, ideally, impenetrable resisting dissolution [15]. The results presented for sorption suggested that the solutions used are not different as they cannot penetrate the resin cement interface to generate statistical differences. For the higher sorption of the PBs exposed to the saline solution, compared to the other solutions, the water is considered a weak solvent [15], and the high incidence of pre-existing defects in the dentin interface, resin cement, and fiberglass pin demonstrated in the radiographic analysis, may have acted in the action of this solution, facilitating the penetration in the cementing interfaces with a higher incidence of previous defects. Marghalani [15] already reported this mechanism of water sorption through the free volume theory, where water molecules diffuse through microvoids and structural defects in these interfaces. Also, the phosphate in the chemical composition of the resin cement used in this study, Rely-X U200, can form a hydrogen bond with water [15] which respond to the higher sorption of PBs exposed to saline when compared to the other solutions employed.

When evaluating the solubility property, the revealed data should be interpreted according to the number of unreacted residues, particles, and released ions, reflecting a loss of mass of the resinous material [15]. Monomers that have not reacted, stuck in the gaps or pre-existing defects in the dentin interface, resin cement, and fiberglass pin have a greater propensity to solubility [15]. These arguments corroborated the finding of higher solubility of the PBs exposed to 37% phosphoric acid since the pre-existing defects demonstrated in this study may have positively influenced the solubility of these interfaces, chemically reacting with the confined monomers. The rapid absorption of solvents can result in softening of the polymer [15], so the 37% phosphoric acid is expected to facilitate the removal of the resin cement at the cementing interface when necessary. The microtomographic analysis corroborated these data, demonstrating the partial degradation of the resin cement and the detachment of particles from the cementing material displacing in the interior of the root canal.

In addition, the acid pH of 37% phosphoric acid can act on dentin, as it removes the smear layer, increasing the diameter of dentin tubules and causing dentin demineralization.^[19] Within, the chelating action of the acid may be responsible for the high SL of 37% phosphoric acid breaking the union of the resin cement with the dentin matrix, thus helping in its removal.

The data found in the mass loss corroborated with the highest solubility found in 37% phosphoric acid among the other groups. Its major chelating action was active in both the cementation interface and dentin, as shown in the results of the solubility property and the mass loss. Once the PBs are immersed in solutions, the chelating action was evidenced in the xylol and EDTA groups, leading the control group to lower scales when compared to the action scale in the sorption property, suggesting a greater action on the dentin matrix than on the resin cementation interface.

As reported in other studies, cementation protocols using 37% phosphoric acid for 30 and 60 seconds exposed irreversible damage to fiberglass integrity [20]. When exposed to 35% phosphoric acid for periods of one to five minutes, the occurrence of areas of fracture or dislodgement of the outermost fibers in the fiberglass pins was reported, while the innermost fibers remained intact [21]. Previously, D`Arcangelo et al. [22] concluded that the pins conditioned with 30% phosphoric acid and 9.5% hydrofluoric acid for 15 seconds presented morphological changes in their structure, presenting little or no resin matrix and even fractured fibers.

Condition the fiberglass pins in hydrofluoric and 9.6% hydrofluoric acid for 60 seconds also promote microporosities, resulting from the loss of the outermost matrix of the pin fiber [23,24]. In addition, conditioning with 4.5% hydrofluoric acid for above 60 seconds demonstrated dislodged fibers on the surface, leaving only grooves in the matrix [25]. Such acids will act selectively, reaching the epoxy resin matrix, which, in turn, will alter the surface roughness, increasing the available area and creating more micro-retentive spaces [26]. Therefore, the possibility of using this degrading action to assist in the removal of resin cements used in the cementation of fiberglass pins may be a new perspective in the clinical arsenal for removing these posts.

This innovative work opens a new perspective for possible alternatives in removing fiberglass pins and the resin cement layer in root canals. Comparing the results of solubility, mass loss, and microtomographic, 37% phosphoric acid has a high potential to degrade the resin interface of the cemented fiberglass pins. However, the sorption result showed low penetration in this interface, demonstrating the need for further studies to enhance this penetrating action, such as the possible association of ultrasonic inserts or the local thermal elevation.

This study demonstrated that the incidence of vestibulo-lingual radiography, as clinical pattern, can mask existing failures in intraradicular cementations, requiring further studies at different incidences to achieve a correct correlation between images and cementing resin interface.

The 37% phosphoric acid solution presented the greatest degrading potential in the cementing interface of the fiberglass pin, in the resin cement and dentin.

Conflict Of Interest

The authors have no conflicts of interest to declare.

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