Investigation of the Effect of Indirect Ultrasonic Force and Different Cements on the Bond Strength of Glass Fiber Posts in Teeth with Different Post Space Preparation Methods ^†

Abstract

(1) Background: The main aim of this study was to investigate the effects of different cements (Panavia V5, RelyX U200) and placement techniques (indirect ultrasonic vibration and manual method) on the bond strength of glass fiber posts in teeth with different post space preparation methods and to evaluate the failure types. (2) Methods: A total of 120 human upper central teeth were collected for the study. One week after root canal treatment, 10 mm long post space preparation was performed using post drills. Post spaces were irrigated in three different ways, namely NaOCl, NaOCl + EDTA solutions, and passive ultrasonic irrigation of NaOCl and EDTA solutions. Glass fiber posts were divided into two groups for resin cement systems, namely RelyX U200 (3M ESPE) or Panavia V5 (Kuraray Medical). Two different methods, namely the indirect ultrasonic vibration and the manual method, were applied for post placement. Then, 1.2 mm sections were taken from the middle and coronal regions of the inserted fiber posts for a push-out test. (3) Results: the Irrigation × Cement (p = 0.002), Cement × Section (p = 0.043) and Placement × Section (p = 0.049) pairwise interactions were statistically significant. (4) Conclusions: in our study, the effects of different post cavity preparations, different cement types, and different placement techniques on the bond strength of the fiber post were observed.

1. Introduction

Although many different materials have been used for posts, the similarity of their physical properties to dentin and their aesthetic properties have made fiber posts the most preferred post material today [1]. The most common problem observed in fiber post applications is the loss of retention of the post [2,3]. Studies investigating the factors affecting post retention have included post length, diameter, shape, surface, and structural properties, post cavity preparation techniques, canal shape, canal-filling materials, cement thickness, cementation technique, type of cement used, and tooth position [4,5]. The cementation process performed after preparation of the post space is one of the most important factors in ensuring retention between tooth and post, proper stress distribution, and prevention of leakage [6,7]. It has been observed that the density of dentin tubules at different levels of the root canal, the accessibility of the apical, middle, and coronal regions of the root, the type of cement used, and differences in dentin surface area can all affect the quality of the bond at the interface [8,9].

Failure of fiber post applications is usually due to inadequate bonding between post–dentin and post–cement, which is an adhesive failure [10,11]. The bonding mechanism of adhesive systems between the post surface and the dentin walls is based on hybridization between the resin tags on the demineralized surface and the side branches of the adhesive [12]. It was hypothesized that removal or modification of the smear layer, which is one of the factors affecting this micromechanical relationship, improves the micromechanical retention of resin cements [13].

Resin cements are widely used for the luting of indirect restorations. However, their bond strength is significantly affected by the cementation technique used [14].

During endodontic treatment, ultrasonics and sonics have been previously used in many areas, such as the organization of the entrance cavity, the removal of obstructions in the canal, raising the potential effectiveness of irrigation solutions, condensation of gutta-percha, and root canal preparation [15,16]. Sonic activation can be used as an adjunct for placing the resin cements to increase the bond strength [16]. At different stages of post application, ultrasonic systems have been used to activate post cavity irrigation. The application of cement and the adhesion of the post to the dentin has been investigated [17,18]. Ultrasonic activation improved the dentin tubule penetration of a self-etch adhesive system [18]. However, the effect of ultrasonic vibration to be applied during the placement of the post on the bond strength remains uncertain.

Since the push-out bond strength test is the most reliable method of measuring the bond strength between the dentin tissue and the fiber post material [8]. The bond strength was evaluated using push-out data, and then, the fracture types were examined using a scanning motorized microscope.

The purpose of our research was to investigate the impact of different cement materials and placement techniques on the bond strength of glass fiber posts in teeth with different post cavity preparation methods and to evaluate the fracture types.

2. Materials and Methods

2.1. Ethics Committee Approval and Hypotheses

The ethics committee of Ordu University approved this study, and the sample size of 120 maxillary central teeth was based on the data displayed by Rosa et al. [19]. The first hypothesis in this study is that the bond strength of the posts is better when an EDTA solution and passive ultrasonic activation are used for post space irrigation compared to NaOCl irrigation. The second hypothesis is that higher bond strength would be observed when self-adhesive resin cement material is used compared to cemented with self-etch resin cement material. The third hypothesis is that the bond strength is higher in the coronal segment of the posts than in the middle segment. The fourth hypothesis is that indirect ultrasonic vibration application would increase the bond strength values compared to manual placement.

2.2. Preparation of Samples

Endodontic treatments, post preparation, and cementation protocols were applied by a single operator to the upper central incisor. After the extraction, the attachments on the roots were removed with the aid of a crusher. The teeth were disinfected with a 2.5% sodium hypochlorite (NaOCl) solution and then washed with distilled water. The teeth with mature apex, single root, single canal, and less than a 10-degree curvature were included in the study. Teeth with root canal treatment, root defects, resorption, cracks, and fractures were not included in the study. The specimens were cut transversely at a standard length of 15 mm with a diamond disk (Dedeco International Inc., Long Eddy, NY, USA).

2.3. Preparation of Root Canals

The working length was verified as 1 mm back from the length measured with a 10 K file (Dentsply Mailfer, Ballaigues, Switzerland) seen at the apical foramen as 14 mm. The root canals were shaped using the Reciproc (VDW, Munich, Germany) system R25, R40, and R50 files, adhering to their working lengths. Irrigation with a 2.5% NaOCl was performed at each file change. Ten mL of NaOCl were used for the root canal irrigation of each specimen. Final irrigation was conducted with 5 mL 17% EDTA, 2 mL NaOCl, and 5 mL distilled water solutions.

2.4. Obturation of the Root Canals

After the root canals were dried with No. 50 paper points (Dentsply Sirona, Charlotte, NC, USA), root canal filling was completed with Reciproc R50 gutta-percha (VDW, Munich, Germany) and AH Plus (Dentsply Sirona, Charlotte, NC, USA) root canal paste using a cold lateral condensation technique. Gutta-percha numbers 20 and 25 were used as auxiliary cones. The gutta percha was cut at the level of the canal entrance with a heated tool (excavator) and condensed with Machtou Head Carrier Plugger No. 2 (Dentsply Sirona, Charlotte, NC, USA). The specimens were placed at 100% humidity and 37 °C for one week.

2.5. Preparation of the Post Spaces

Cytec Blanco (Hahnenkratt, Königsbach-Stein, Germany) 1.2 mm diameter post drills were used to complete the preparation of the post space. The working length was set as 10 mm. Different irrigation protocols were applied to remove the smear layer and dentin chips formed during the post space preparation processes. The samples were randomly split into 3 groups, with 40 in each group, according to the irrigation procedure to be applied (n = 40). The experimental study design of each group is given in Figure 1.

• NaOCl group (control) (n = 40)

After post space preparation with post drills, post space irrigation was performed with 3 mL 2.5% NaOCl using conventional needle irrigation for the removal of the formed smear layer. The total irrigation time was set at 80 s;

• NaOCl + EDTA group (n = 40)

After post space preparation with post drills, post space irrigation was carried out with 3 mL 2.5% NaOCl for 40 s, then irrigated with 3 mL 17% EDTA for 40 s using conventional needle irrigation for the removal of the formed smear layer. The total irrigation time was set at 80 s;

• (NaOCl + EDTA) + passive ultrasonic irrigation group (n = 40)

After post space preparation with post drills, 2.5% NaOCl was applied to the prepared post cavities using conventional needle irrigation for 5 s, followed by activation with a Vdw Ultra (VDW, Munich, Germany) device using Irrisafe 20# ultrasonic tips (Acteon, Merignac, France) for 5 s, with the setting at IRRI Mode. This 10 s process (5 s NaOCl irrigation + 5 s ultrasonic irrigation) was repeated for 4 cycles. The same application was performed for 17% EDTA solution (10 s process = 5 s NaOCl irrigation + 5 s ultrasonic irrigation) and was repeated for 4 cycles. The total irrigation time was 80 s. Three mL of distilled water were used as the final irrigation solution for all three groups, and the post spaces were dried with paper cones.

2.6. Cementation of the Posts

Then, each group was divided into four according to the used cement type (self-adhesive resin cement system or the self-etch resin cement system) and cement placement technique (manual placement and indirect ultrasonic vibration) (n = 10). The contents of the resin cements and the primers used in our study are presented in

Table 1. Contents of resin cements and primer used in our study.

Primer/Cement	Composition	Manufacturer
Clearfil ceramic primer plus	3-MPS(methacryloxypropyltrimethoxysilane), 10-MDP (methacryloyloxydecyl dihydrogen phosphate), Ethanol	Kuraray Dental, Tokyo, Japan
Panavia V5 tooth primer	10-MDP(methacryloyloxydecyl dihydrogen phosphate), HEMA (hydroxyethyl methacrylate), hydrophilic aliphatic dimethacrylate, Accelerators, Water	Kuraray Dental, Tokyo, Japan
Panavia V5 paste	Bis-GMA (bisphenol A diglycidyl-methacrylate), TEGDMA (triethylene glycol dimethacrylate), silanated barium glass, colloidal silica, surface-treated aluminum oxide filler, hydrophobic aromatic dimethacrylate, hydrophilic aliphatic dimethacrylate, di-camphorquinone, initiators, accelerators, pigments	Kuraray Dental, Tokyo, Japan
Rely X U200	Base: Methacrylate monomers containing phosphoric acid groups, methacrylate monomers, initiators, stabilizers, rheological additives Catalyst: Methacrylate monomers, alkaline fibers, silanated fillers, initiator components, stabilizers, pigments, rheological additives, zirconia/silica fillers. Clicker delivery system	3M ESPE, Schiller Park, IL, USA

. The self-etching resin cement was designated as Panavia V5 (Kuraray Medical, Inc., Tokyo, Japan). The self-adhesive resin cement was designated as Rely X U200 (RXU; 3M ESPE, Seefeld, Germany).

Following the manufacturer’s instructions, both cements were applied to the 1.2 mm diameter fiber posts’ (Cytec Blanco, Hahnenkratt, Königsbach-Stein, Germany) surface using mixing tips.

Subgroup 1—Panavia V5 + manual placement (n = 10).

Before Panavia V5 cementation, 35% phosphoric acid was applied to the post surface and held for 5 s. Then, it was irrigated and dried slightly. Afterward, Ceramic Primer Plus (Kuraray Medical, Inc., Tokyo, Japan) was applied to the post surface and dried slightly. Panavia V5 Tooth Primer (Kuraray Medical, Inc., Tokyo, Japan) was applied to the prepared post space for 20 s. The excess was removed with paper points and dried slightly. Panavia V5 self-etch resin cement was activated via otomix tips and applied to the post surface, and then, the post was placed into the post space until it went into the prepared post cavity with finger fissure. Excess material was removed, and the cement was light-polymerized for 40 s perpendicular to the fiber post with a Blue LED (Elipar S10, 3M ESPE, Neuss, Germany) at an intensity of 1200 mW/cm².

Subgroup 2—Panavia V5 + indirect ultrasonic vibration (n = 10).

Panavia V5 cementation procedures were applied as in subgroup 1. After the glass fiber post was manually placed in the post space, a START-X #5 ultrasonic tip (Dentsply Maillefer, Ballaigues, Switzerland) (Figure 2) was applied to the top of the fiber post indirectly, through the amalgam fulvar, for 3 s using VDW Ultra (VDW, Germany) as an ultrasonic device at REDO Mode. After indirect ultrasonic vibration, the cement was light-polymerized for 40 s perpendicular to the fiber post with a Blue LED (Elipar S10, 3M ESPE, Neuss, Germany) at an intensity of 1200 mW/cm².

Subgroup 3—Rely X U200 + manual placement (n = 10).

Relyx U200 self-adhesive resin cement was applied without any procedures to the post or dentin surfaces after post space preparation according to the manufacturer’s instructions. The cement was activated via otomix tips and applied to the post surface, and then, the post was placed in the post space until it went into the prepared post cavity with finger fissure. Excess material was removed, and the cement was light-polymerized for 40 s perpendicular to the fiber post with a Blue LED (Elipar S10, 3M ESPE, Neuss, Germany) at an intensity of 1200 mW/cm².

Subgroup 4—Rely X U200 + indirect ultrasonic vibration (n = 10).

Rely X U200 cementation procedures were applied as in subgroup 3. After the glass fiber post was manually placed in the post space, a START-X #5 ultrasonic tip (Dentsply Maillefer, Ballaigues, Switzerland) (Figure 2) was applied to the top of the fiber post indirectly, through the amalgam fulvar, for 3 s using VDW Ultra (VDW, Germany) as an ultrasonic device at REDO Mode. After indirect ultrasonic vibration, the cement was light-polymerized for 40 s perpendicular to the fiber post with a Blue LED (Elipar S10, 3M ESPE, Neuss, Germany) at an intensity of 1200 mW/cm².

Prior to the push-out tests, all of the prepared samples were kept for 24 h at 100% humidity and 37 °C.

2.7. Push-Out Test

A Microtome Mecatom T180 machine (Presi SA, Angonnes, France) was used for sectioning; 1.2 ± 0.3 mm thick slices were obtained from the cervical and middle thirds of the roots. The pushout test was performed with a cylindrical metal tip (Ø = 0.8 mm) positioned on the fiber post perpendicular to the section surface (Figure 3). The push-out test was performed on a Shimadzu universal testing device (Autograph AGS X; Shimadzu Co., Kyoto, Japan) at a crosshead speed of 1 mm/min. The bond strength data (MPa) were calculated according to Equation (1) as the ratio between the load required for specimen rupture (Newton) and the bonded surface area, Equation (2) (mm²) [16].

Equation (1)—push-out bond strength:

$$(\mathrm{MPa})={\displaystyle \frac{\mathrm{M}\mathrm{a}\mathrm{x}\mathrm{i}\mathrm{m}\mathrm{u}\mathrm{m} \mathrm{l}\mathrm{o}\mathrm{a}\mathrm{d}\left(\mathrm{N}\right)}{\mathrm{A}\mathrm{d}\mathrm{h}\mathrm{e}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{a}\mathrm{r}\mathrm{e}\mathrm{a}\left({\mathrm{m}\mathrm{m}}^2\right)}}$$

(1)

Equation (2)—the bonding area was calculated for each slice using:

$$\pi (\mathrm{R}+\mathrm{r})({\mathrm{h}}^2+(\mathrm{R}-\mathrm{r}{)}^2{)}^{0.5}$$

(2)

where π is 3.14, R is the coronal root canal radius, r is the apical root canal radius, and h is the thickness of the slice.

The samples were assessed using a stereo-motorized research microscope (Nikon SMZ25, Nikon Ltd., Tokyo, Japan) under 7× magnification. The failure type analyzed in the 4 groups is shown in Figure 4:

(1)

adhesive failure at the dentin–cement surface;

(2)

adhesive failure at the post–cement surface;

(3)

cohesive failure of the post;

(4)

mixed type, in which two or more of the other failure types occur together. Since cohesive fracture only within the cement was not observed in any sample in our study, this group was not included in the classification.

2.8. Statistical Analysis

The normal distribution of the results was analyzed by a Kolmogorov–Smirnov test. Levene’s test was used to control the homogeneity of the group variances. A four-way analysis of variance (four-way ANOVA) was applied to evaluate the result. Tukey’s multiple comparison test was applied to identify different environments, and the results were given in letter form. All statistical analyses were conducted using SPSS v27 (IBM Inc., Chicago, IL, USA), and a p-value (<0.05) was taken as statistically significant. None of the prepared specimens failed prematurely.

3. Results

The results for interactions between the groups are given in

Table 2. Interactions between the groups.

Irrigation	Cement	PM	I × C Int.
			n	Mean	SD
NaOCl	Panavia V5	MP
		IUV
		Total	40	14.026 Aa	5.645
	Rely X U200	MP
		IUV
		Total	40	11.694 Aa	4.801
NaOCl + EDTA	Panavia V5	MP
		IUV
		Total	40	17.100 Aa	4.830
	Rely X U200	MP
		IUV
		Total	40	9.370 Ba	4.032
(NaOCl + EDTA) PUI	Panavia V5	MP
		IUV
		Total	40	16.734 Aa	6.565
	Rely X U200	MP
		IUV
		Total	40	11.422 Ba	5.068

There is a difference between cement mean values that are without a common capital letter in the same irrigation method (p < 0.05). There is a difference between irrigation mean values that are without a common lowercase letter in the same cement (p < 0.05). There is a difference between cement mean values that are without a common capital letter in the same section (p < 0.05). There is a difference between the mean values of the sections that do not have a common lowercase letter in the same cement (p < 0.05). There is a difference between placement method mean values that are without a common capital letter in the same section (p < 0.05). At the same placement method, there is a difference between section means without a common lowercase letter (p < 0.05). PM: placement method, I: irrigation method, C: cement, MP: manual placement, IUV: indirect ultrasonic vibration, Mean: mean, SD: standard deviation, Int.: interaction.

and Table S2. As a result of the four-way analysis of variance performed for bond strength, the interactions of Irrigation × Cement (p = 0.002), Cement × Level (p = 0.043) and Placement × Level (p = 0.049) were found statistically significant (Table S1). An interval plot diagram of the bonding strength values of the experimental groups is given in Figure 5.

When both cement materials were statistically evaluated according to the different irrigation methods applied, no significant difference was observed in bond strength. In the NaOCl group, no significant difference was observed when comparing the two different cements. A significant difference was observed for the NaOCl + EDTA and NaOCl + EDTA + PUI groups. Panavia V5 showed higher bond strength for both irrigation types when the two cements were compared. As a cement, Panavia V5 showed significantly higher bond strength at both the coronal and middle levels. When the bond strengths of the same cement at different levels were compared, a higher bond strength value was observed for Panavia V5 in the middle level. When the bond strength of the two levels was compared for the Rely X U200 cement, the difference was not significant.

When the effects of the placement methods on different levels were analyzed, no significant difference was found between the middle and coronal levels in the manual placement group in terms of the bond strength values. The IUV group showed a higher bond strength value at the middle level compared to the coronal level, and this difference had a statistically significant effect. When the bond strengths of the IUV and MP groups were evaluated, the average bond strength value of the IUV group was higher than the average value of the MP group. However, there was no statistically significant difference between the mean MPa values.

Stereomicroscope Analysis Results

Since the failure type of six samples could not be classified, they were not included in the percentile. The most common failure was determined to be a mixed type. Mixed-type failure was seen at a higher rate at the mid-level. Type 2 adhesive failure and Type 1 adhesive failure were observed at lower rates. The fracture type showing the lowest rate was found to be Type 3 (1.28%), which is a cohesive failure within the post itself. Type 3 failure was seen only in the middle level in the Panavia V5 group. The distribution rate of the failure types according to level and cement types is given in Figure 6.

4. Discussion

In our study, the first hypothesis could not be confirmed because no significant difference was found when the effect of irrigations applied to the post cavity on the bond strength was evaluated. When self-etch resin cement material was used, a higher bond strength was observed in both sections compared to the self-adhesive resin cement material, and the second hypothesis was rejected. When the sections were compared, our third hypothesis was rejected because a higher bond strength was observed in the middle segment, and our fourth hypothesis was rejected because there was no significant difference between the placement methods.

In our study, in parallel with similar studies, no statistically significant effect of the irrigation methods applied to the post cavity on the bond strength of the post was observed when the same cement material was applied [20,21,22].

In previous studies, it was reported that the stresses transmitted during the push-out test were more uniformly distributed when the segments were in the range of 1–1.5 mm, while the stresses transmitted to the specimens were more irregular when the push-out test was applied on segments with a thickness of 2 mm [23,24,25]. Therefore, in the present study, 1.2 mm thick segments were prepared. A force was applied at a speed of 1 mm/min by means of a 0.8 mm diameter rounded metal tip positioned on the posts at the centers of the dentin discs, with reference to a study in which limitations related to push-out studies were identified [26].

Previous studies have reported that single NaOCl irrigation has a negative effect on the bond strength of fiber posts when using resin-based cements for cementation [27,28,29]. Ferrari et al. [29] reported that NaOCl’s action on organic dentinal compounds can lead to collagen breakdown, which can negatively affect the bonding ability of fiber posts to the root canal dentin. It has been shown in several studies that a prolonged exposure of dentin to high concentrations of NaOCl can have a detrimental effect on bond strength [30,31].

In a recent study, NaOCl and EDTA irrigation have been shown to increase the bond strength of post–cor systems when used in post space irrigation [32]. In their systematic review, Oliveira et al. [33] stated that the use of NaOCl + EDTA can be recommended for post space irrigation.

The lack of a statistically significant difference between the irrigation protocols used in our study could be due to the NaOCl concentration and the duration of NaOCl application. NaOCl irrigation at the appropriate concentration and duration may have prevented the adverse effects of high concentration and prolonged use of NaOCl on dentin and the adhesive surface shown in previous studies [29,30,31].

According to Kambara et al. [34], self-adhesive cements do not induce a complete demineralization of the smear layer or a decalcification of dentin. This indicates that the presence of the smear layer is not a limiting factor for adhesion, as confirmed in previous studies [35,36].

Terumi et al. [36] reported that this effect was not observed for RelyX U200, since microhardness values were not affected by NaOCl irrigation. This may be a reason why NaOCl does not have a negative effect on the bond strength of RelyX U200 cement. At the same time, EDTA application is not recommended by the manufacturer for RelyX U200 [36]. EDTA application could theoretically reduce the chemical bonding efficiency of the self-adhesive resin to hydroxyapatite [37,38,39]. Terumi et al. [36], in a study using two different self-adhesive cement materials, found that better removal of the smear layer was not associated with the bond strength of glass fiber posts when RelyX U200 was used. In our study, similar to Terumi et al. [36], NaOCl irrigation showed higher values in the U200 group compared to NaOCl and EDTA irrigation, but the difference between irrigation methods was not significant. When NaOCl is used alone for irrigation, it provides better access to the resin cement for its interaction with the hydroxyapatite structure of dentin, so that chemical bonding and sealing can be improved [36]. At the same time, NaOCl application increases dentin roughness and improves frictional bonding, which may be one of the reasons for the high bond strength data [40].

In many studies, ultrasonic devices have been selected as an auxiliary technique to remove the smear layer more effectively [41,42]. NaOCl/EDTA irrigation with ultrasonic activation performed better regarding the removal of the amount of debris and increased bond strength compared to NaOCl/EDTA irrigation without ultrasonic activation [41]. However, some studies have reported that ultrasonic irrigation has a similar effect to the conventional irrigation technique [42,43]. In addition, Hülsmann et al. [44] also reported that ultrasonic activation did not have a curative effect on root canal disinfection. The discrepancies may be due to the different experimental setups used and the differences in tested areas.

Ekim et al. [21] applied NaOCl and EDTA solutions without activation and under PUI activation on posts cemented using a self-etch resin cement and found that the PUI group showed higher bond strength but no significant difference was obtained between the groups. In our study, in the Rely X U200 group, (NaOCl + EDTA) PUI irrigation showed a higher bond strength value than NaOCl irrigation and NaOCl + EDTA irrigation, similar to the results of Akyuz Ekim et al.’s [21] study, but this difference was not significant.

In our study, total irrigation times were limited to an average of 40 s. In the PUI group, irrigation–activation cycles for each irrigation solution were performed with a standard protocol with 5 s intervals, similar to previous studies [21,45]. Previously, this protocol has been recommended by researchers to prevent severe peritubular erosion [46] and also to prevent the possible dangerous effects of temperature increase on the dentin surface due to the use of ultrasonics in the root canal [47], which are the reasons why this protocol was preferred in our study.

Since there is no study in the literature in which Panavia V5 was used for post cementation, the fact that our data are not fully parallel with the studies using different self-etch resin cements and different results may be due to the structural differences of the materials. In a study on a self-etch cement (Panavia F2.0), it was reported that irrigation with a combination of NaOCl and EDTA caused an increase in bond strength at the post and dentin interface [48]. Tagami et al. [49] compared the bond strength of two dual-cure resin cements to dentin and found that Panavia V5 showed a higher bond strength value than its previous version Panavia F2.0 at two different curing conditions, but root dentin was not tested in this study. In our study, as in the study of Moura et al. [32] NaOCl + EDTA and (NaOCl + EDTA) PUI irrigation protocols showed higher bond strength values in the groups where self-etch resin cement was applied. In the control group, this difference was not significant.

Phosphate ester monomer 10-methacryloyloxydecyl dihydrogen phosphate (MDP) has been added into various bonding and luting products, such as primers, adhesives, and resins [50]. A recent study stated that post space pretreatment with synthetic hydroxyapatite significantly increased the bond strength of fiber posts luted with 10-MDP-based adhesive techniques [51].

Serichetaphongse et al. reported that the application of Clearfil ceramic primer plus, which contains MDP phosphate monomer, increased the shear bond strength of PANAVIA™ V5 in both zirconia and titanium alloys [52]. Similar results were found in our study. Higher bond strength values were reported in Panavia V5 treated groups that were used with MDP-containing ceramic primer than in Rely X U200 treated groups without ceramic primer.

In the literature, there are many studies evaluating the effect of different root dentin regions on the bond strength of posts to root dentin, but there is no consensus on which root region has a higher bond strength [16,53,54]. It was thought that the fact that the fiber post we used in our study showed a parallel structure in the coronal and middle segments and a conical structure narrowing towards the tip in the apical segment would negatively affect the maximum force applied in the apical segment of the post in the push-out test. At the same time, it was thought that this situation would negatively affect the standardization of the push-out bond strength test. In order to minimize these factors, the sections obtained from the coronal and middle thirds of the root were used in our study, as previously used in different studies [55,56].

Some studies have suggested that the bond strength of posts is higher in the apical region than in other parts of the root canal [57,58]. There are also studies reporting a decrease in bond values from the coronal region to the apical region [21,22,32,48]. There are also studies that show the region has no effect on the bond strength of the post [16,59].

In our study, the middle section showed significantly higher bond strength in the Panavia V5 group. These data were found to be compatible with the bond strength results of self-etch resin cement (Panavia F 2.0) material in a similar study [60]. In another study, Moura et al. [32] found that the bond strength of self-etch resin cement material (Panavia F 2.0) was significantly higher in the cervical region than in the middle region for post cavities irrigated with NaOCl + EDTA and (NaOCl + EDTA) PUI. The better bond strength in the cervical segment has been explained by different theories. These theories include the effect of the micromechanical bonding mechanism with the decrease in tubule density and tubule diameter in the apical direction [61], incomplete polymerization in deeper regions of the root [62], and easier and more effective application of adhesive materials with ease of access in the coronal region [60].

In our study for the RelyX U200 group, there was no significant difference in bond strength between the sections. However, the middle group showed higher values. These data were consistent with the results of similar studies in which RelyX U200 cement was previously tested [16,59]. In another study using RelyX U200 in post cementation, the bond strength values were higher in the cervical than in the middle segment for NaOCl + EDTA-treated and PUI-treated post cavities [32].

In our study, the high bond strength values observed in the middle section may be attributed to the differences in the number, volume, and orientation of the tubules in the root dentin [63,64]. In addition, the canal paste and gutta-percha that cannot be removed from the post cavity walls may negatively affect this value [65]. Different bonding mechanisms (micromechanical or chemical), depending on the cement material applied, may also play a role in this difference [66]. Another reason may be the differences in light transfer along the fiber post and the polymerization in the middle third of the root may have increased with sufficient light transfer [67]. It has also been reported that another reason may be that the post is better adapted in the middle root third, and the lower cement thickness in this region may reduce polymerization shrinkage [67].

One of the reasons for the measured lower bonding value at the coronal section is the increase in the C-factor. The C-factor is defined as the ratio of bonded surfaces to unbonded surfaces and is one of the most important factors that influence the success of post-endodontic restorations due to the structure of the root canal system. It can be said that restorations with a higher than three-to-one C-factor ratio may have a low chance of success [68].

The fiber post system we used in our study has the same diameter along its length in the coronal and middle regions. Therefore, the resin cement is thicker in the coronal region due to the width of the canals in this region, which increases the C-factor. Although the drills of the post system have similar dimensional characteristics to the post, they may have provided more effective canal-filling removal from the middle region. This may have caused the irrigation protocols in the post cavity to be more effective in the middle region of the post and to positively affect the bond strength values.

In our study, in the NaOCl + EDTA and NaOCl + EDTA + PUI groups, self-etch cement application showed a higher bond strength than self-adhesive resin cement, but no significant difference was observed between the bond strengths of the cements in the NaOCl group. At the same time, in our study, self-etch resin cement applications had higher bond strength values than self-adhesive resin cement applications for both sections. These findings were consistent with the findings of Moura et al. [32] who used similar irrigation protocols (NaOCl, NaOCl + EDTA, and PUI (NaOCl + EDTA)) and similar cements (Panavia F 2.0 and Rely X U200) in their study. One of the reasons for this result may be that self-etch (Panavia V5) cement material showed higher bond strength values in the absence of the smear layer, which was assumed to be removed by the use of EDTA in two of the three irrigation protocols applied in our study. For Panavia V5, a significantly higher bond strength was observed in the middle section than in the coronal section. This supports our hypothesis that the irrigation protocols we applied were more effective in the middle section.

In a recent study, the effect of ultrasonic activation of different types of adhesive cements placed in the post cavity prior to fiber post cementation on the bond strength of the fiber post was evaluated [18]. In this study, Verdum et al. [18] activated different types of adhesives directly with an ultrasonic tip and reported that a significant increase in bond strength was observed when ultrasonic activation was applied only in combination with self-etch cement. However, it is unclear whether ultrasonic activation applied to the fiber post after a fiber post has been placed in the post cavity with the adhesive system will increase the bond strength.

In our study, since it is thought that direct application of the ultrasonic tip to the post may adversely affect the fiber bonds in the post structure and may have a negative effect on the fiber post by releasing high levels of heat, therefore, an amalgam fulvar was used as an intermediary. Ultrasonic vibration was applied indirectly through an amalgam fulvar placed on the top of the post to examine its effect on the bond strength of the fiber post. When the bond strengths were evaluated between the IUV group and the MP group, the IUV group showed higher values than the MP group. However, no statistically significant difference was observed between the mean bond strength values. When the medial and coronal sections were evaluated within themselves, no significant difference was found between the placement methods. In the manual method group, there was no statistically significant difference in the bond strength values between the middle and coronal sections, whereas, in the indirect ultrasonic vibration group, the middle section showed significantly higher bond strength values than the coronal section. As mentioned before, the conical anatomical form of the root canals and the parallel structure of the fiber post system may have caused the irrigation protocols in the post cavity, as well as the indirect ultrasonic vibration applied during post placement, to have a positive effect on the bond strength values by providing more effect on the middle triple region of the post.

In stereo-motorized research microscope examination, the mixed type (64.95%) was the most frequently observed failure type in both cement groups, and this result was found to be consistent with similar studies [36,48]. The lowest failure rate was Type 3 (1.28%), which was a cohesive failure observed within the post itself, for both cement groups.

When the findings of similar studies in which the bond strength of the posts were tested, it was found that the most frequently observed failures were adhesive or mixed type [18,21,69].

Erdemir et al. [70] stated that the cohesive fractures of the post indicate that the bonding between the dentin and cement and the cement and post is better than the stability of the post itself. In this case, it can be said that the sections with cohesive fractures show a better bonding than mixed fractures, and the sections with mixed fractures show a better bonding than the adhesive fractures.

In our study, when the sections were compared, it was found that a mixed-type fracture was observed at a higher rate in the middle section. A Type 3 fracture was observed only in the Panavia V5-treated group in the middle section, where the highest bond strength was observed according to our push-out test results.

5. Conclusions

Within the limitations, in the literature, there is no study that applied indirect ultrasonic vibration for fiber post placement. In this sense, the present study is the first. The results showed that:

(1)

When both cement materials were evaluated within themselves, the irrigation procedures (NaOCl, NaOCl and EDTA, passive ultrasonic irrigation of NaOCl and EDTA) had no significant effect on bond strength. However, for Panavia V5, higher bond strength values were obtained by NaOCl irrigation or passive ultrasonic irrigation of NaOCl and EDTA. Therefore, when using Panavia V5, it may be recommended to use these two procedures instead of NaOCl irrigation. In the case of Rely X U200, although there was no significant difference between the irrigation procedures. NaOCl irrigation showed higher bond strength values, in accordance with the manufacturer’s recommendations. NaOCl irrigation can be recommended for this cement material;

(2)

When NaOCl irrigation was applied, no difference was found between the bond strengths of the resin cements. However, NaOCl and EDTA irrigation or passive ultrasonic irrigation of NaOCl and EDTA provided higher bond strength values for Panavia V5 cement than RelyX U200;

(3)

When Panavia V5 was applied, higher bond strength was observed in the middle section rather than in the coronal section. When Rely X U200 was applied, there was no difference between the bond strengths of the sections;

(4)

No difference was found between manual placement and indirect ultrasonic vibration applications in terms of bond strength. No difference was stated between the sections in the manual method placement of the posts. However, when indirect ultrasonic vibration was applied, the middle section showed higher bond strength values. As the effects of the materials and techniques we applied on bond strength are more pronounced in the middle section, it can be said that indirect ultrasonic vibration has a positive effect on bond strength.

It is hoped that the findings obtained from this in vitro study will shed light on the clinical applications, and these results should be confirmed in long-term in vivo studies.