Intracanal Medicaments Compromise Fiber Post Adhesion: Influence of Irrigation Techniques

Abstract

Aim:

This study evaluated the effect of intracanal medicaments and final irrigation techniques on resin cement penetration and the push-out bond strength of fiber posts.

Methodology:

Ninety extracted mandibular premolars were allocated into three groups: no medicament (control), calcium hydroxide (CH), and triple antibiotic paste (TAP). CH and TAP groups were then subdivided based on final irrigation techniques: Conventional needle irrigation (CNI), sonic irrigation, passive ultrasonic irrigation, and XP-endo Finisher (XPF). Fiber posts were cemented with RelyX™ U200 resin cement. Confocal laser scanning microscopy was used to evaluate cement penetration, and push-out bond strength was measured. Data were analyzed using one-way and two-way ANOVA (P < 0.05).

Results:

Compared to the control, both CH and TAP significantly decreased cement penetration and bond strength. XPF significantly enhanced these outcomes in CH-medicated canals. In TAP groups, all activation protocols improved adhesion results over CNI; however, the values remained significantly lower than those of the control. A significant interaction between medicament and irrigation protocol was observed only for maximum penetration depth.

Conclusion:

Intracanal medicaments adversely affect fiber post bonding. Activated irrigation technique, notably with XPF, enhances bonding, especially following CH treatment. Bonding outcomes in TAP-treated canals remained inferior despite the use of activation procedures.

Introduction

Endodontically treated teeth with extensive structural loss often require fiber post placement before definitive restorations.[1] However, intracanal medicaments used during root canal treatment may adversely affect the adhesion between root dentin and resin cements used for post placement.[2] Dentin affected by the medicaments or residual materials may attribute to impair bonding, which is one of the causes of fiber post failures.[3,4]

Calcium hydroxide (CH) is often used as an inter-appointment medicament due to its broad antimicrobial and biocompatibility.[5] In particular cases, antibiotic combinations such as triple antibiotic paste (TAP), which consists of ciprofloxacin, metronidazole, and minocycline, have also been used to improve disinfection.[6] However, complete removal of CH and TAP from the canal system is quite a challenge.[7,8] Remaining medicaments can impair the bonding of both sealers and resin cements, compromising the outcome of endodontics and final restoration.[2,9]

Conventional needle irrigation (CNI), commonly used, has been found to be inadequate.[10] There were many attempts to develop strategies to promote cleaning efficacy. Sonic irrigation (SI) uses high-frequency vibration, which allows safe activation of irrigants and could produce vigorous intracanal fluid agitation to remove medicament.[11] Passive ultrasonic irrigation (PUI) combines acoustic streaming and cavitation in root canal debridement. It was found to be more effective than CNI in medicament removal from the canal walls.[12,13] Nevertheless, complete removal of medicaments remains unachieved.[11,12,13]

The XP-endo Finisher (XPF; FKG Dentaire, La Chaux de Fonds, Switzerland) is a size 25/0.00 rotary file manufactured from MaxWire alloy. Due to phase transformation properties, the XPF can alter to a spoon-like shape with an expanded diameter of up to 3 mm, resulting in better physically contact to the canal walls. XPF has demonstrated increased debridement efficiency, including the inaccessible area, with conventional instruments.[14] Several studies have demonstrated the effectiveness of XPF in removing medicament from medicated canals.[12,15,16]

Residual root canal medicaments have been shown to negatively affect the bond strength between resin cement and root dentin. This study aimed to evaluate the efficacy of different irrigation protocols in removing CH and TAP by assessing the dentinal tubule penetration of resin cement and the push-out bond strength of prefabricated fiber posts. The null hypothesis was that no significant differences would be observed in cement penetration or bond strength among the different medicament types or irrigation techniques.

Methodology

Sample preparation

After approval by the Institute Human Experimentation Committee (Protocol no. 8/2564), ninety extracted mandibular premolars with mature apices were selected. Teeth with restorations, caries, or cracks were excluded. The external root surfaces were cleaned with an ultrasonic scaler and stored in 0.1% thymol solution until use.

Samples were decoronated using a double-sided diamond disc (3M ESPE, St. Paul, MN, USA), and the total root length was standardized to 16 mm. Apical patency was achieved using a size 10 K-file (Dentsply, Maillefer, Ballaigues, Switzerland). The working length (WL) was set at 1 mm short of the apical foramen. The canals were then shaped with the rotary instruments (Mtwo; VDW GmbH, Munich, Germany) up to size 40/0.06. During the instrumentation, the canals were irrigated using 2 mL of 5.25% sodium hypochlorite (NaOCl) with a side-vented 27-gauge needle (KerrHawe SA, Bioggio, Switzerland) at 2 mm from WL. Removal of the smear layer was performed using 5 mL of 17% ethylenediaminetetraacetic acid (EDTA) for 1 min and then flushed with 5 mL of distilled water and dried with paper points.

Intracanal medication

Ten samples were randomly assigned to a control group without medication. The other samples were randomly allocated into two experimental groups (n = 40 each), which were medicated with either CH or TAP.

Calcium hydroxide group

The samples were medicated with premixed CH (UltraCal; Ultradent, South Jordan, USA) using a lentulo spiral.

Triple antibiotic paste group

Equal amounts (1:1:1) of metronidazole, ciprofloxacin, and minocycline were mixed with macrogol and propylene glycol as a vehicle. The powder-to-liquid ratio was 3:1, and the mixture was blended to a creamy consistency. The lentulo spiral was used to deliver the medicament into the canals.

The access cavities were filled with Cavit (3M ESPE, Seefeld, Germany). All samples were stored at 37°C and 100% relative humidity for 1 week.

Final irrigating and obturation procedures

The samples in the CH and TAP groups were randomly divided into 4 subgroups (n = 10 per group), according to final irrigation protocols.

Conventional needle irrigation group

Canals were irrigated with 4 mL of 5.25% NaOCl using a 27G needle placed 2 mm short of the WL for 3 min. The canal was then irrigated with 2 mL of 17% EDTA for 1 min, followed by 2 mL of 5.25% NaOCl.

Sonic irrigation group

Canals were irrigated with 4 mL of 5.25% NaOCl using a 27G needle placed 2 mm short of the WL for 3 min. The canal was then irrigated with 2 mL of 17% EDTA, activated with an EndoActivator^® (6000 hertz, 160 µm amplitude at 10,000 cycles/min; medium power with a red tip 25/0.04) for 30 s. The tip was placed 2 mm from the WL. After that, the final irrigation was performed with 2 mL of 5.25% NaOCl, sonically activated in an upward-downward motion for 30 s.

Passive ultrasonic irrigation group

Similar to the SI group, canals were irrigated with 4 mL of 5.25% NaOCl using a 27G needle placed 2 mm short of the WL for 3 min. Then, 2 mL of 17% EDTA was activated for 30 s using an ultrasonic unit (Newtron P5, Acteon) with an IrriSafe tip (#25IRR; Satelec Acteon Group, Merignac, France) operating at 28,000 Hz. The tip was placed 2 mm from the WL. After that, the final irrigation was performed with 2 mL of 5.25% NaOCl, ultrasonically activated in an upward–downward motion for 30 s.

XP-endo finisher group

Canals were irrigated with 4 mL of 5.25% NaOCl using a 27G needle placed 2 mm short of the WL for 3 min. The canals were then irrigated for 1 min with 2 mL of 17% EDTA, activated by using an XPF with a torque-controlled motor operated at 1000 rpm and 1 N·cm torque. The XPF was placed 2 mm short from the WL and moved in a 5-mm vertical motion. This was followed by 2 mL of 5.25% NaOCl activated with XPF for 30 s.

All samples were finally irrigated with 3 mL of sterile saline. Each root canal was then dried and obturated using the vertical compaction technique. A size 40/0.06 gutta-percha cone (Dentsply, DeTrey, Konstanz, Germany) was used without sealer; the down pack was done at 4 mm from the WL by a heat carrier device (BandL Biotech, Gyeonggi-do, South Korea). Radiographs were taken to inspect the quality of canal obturation.

Prefabricated fiber post placement

Post spaces were prepared to a depth of 11 mm using drill size 0 provided by the RelyX™ fiber post system (3M ESPE, Seefeld, Germany). Fiber posts were trial-fitted and checked radiographically to confirm proper adaptation.

Fiber post cementation was carried out using RelyX™ U200 self-adhesive resin cement (3M ESPE, Seefeld, Germany), premixed with Rhodamine B (0.32 µg/mg). Light curing was performed, and all samples were stored at 37ºC for 24 h.

Confocal analysis of resin cement penetration

After incubation, each sample was embedded into acrylic resin parallel to their long axis using a surveyor. A 1-mm-thick transverse slice was obtained from the middle third of the root, precisely 5 mm from the WL using an IsoMet^® 1000 precision saw (Buehler, Lake Bluff, IL, USA) under water cooling. The slice thickness was measured with a digital caliper to ensure uniformity.

Resin cement penetration was evaluated using a confocal laser scanning microscope at 540/590 nm. The penetration depth of cement and the percentage of penetrated canal wall infiltrated by cement were calculated using the following formula:

The percentage of the perimeter of penetrated cement = (cement penetrated perimeter/total canal perimeter) × 100

The measurements are illustrated in Figure 1.

Figure 1.

Confocal laser scanning microscope images illustrating the measurement of resin cement penetration. (a) Percentage of penetration was calculated by dividing the cement penetrated perimeter (purple) by the total canal perimeter (purple + green) ×100. (b) Maximum penetration depth was measured from the canal wall into the dentinal tubules (green line)

A push-out bond strength test

Push-out testing was performed from the apical side of each section using a universal testing machine (Instron 5566 Universal Testing Machine, Instron Engineering Corporation, Norwood, MN, USA) at a constant crosshead speed of 0.5 mm/min. Cylindrical pluggers (0.5 mm diameter) were used. The maximum load applied to the post before displacement was recorded in Newtons (N) and converted to megapascals (MPa) according to the following formula:

The adhesion surface area (A) of each sample was determined as:

where R₁ and R₂ represent the upper and lower post space diameters, and h is the slice thickness.

After testing, the samples were visually examined under a ×40 inverted phase contrast microscope (Olympus, Tokyo, Japan) to determine the mode of failure. These were classified into five types: Type 1: Adhesive failure at the post and cement interface; Type 2: Adhesive failure at the dentin and cement interface; Type 3: Cohesive failure within the post; Type 4: Cohesive failure within the cement; and Type 5: Mixed failure involving two or more types.

Statistical analysis

One-way ANOVA followed by Duncan’s post hoc test was used to compare the penetration of resin cement and the push-out bond strength among groups. In addition, two-way ANOVA was used to assess the effects of medicament type and irrigation techniques on penetration parameters and bond strength. The level of significance was set at P = 0.05. Data were analyzed with SPSS version 26 (IBM Corp., Armonk, NY, USA) statistical software.

Results

The control group demonstrated the highest percentage of penetrated cement perimeters. In the CH groups, XPF showed significantly greater perimeter penetration than CNI, SI, and PUI (P < 0.05) and was comparable to the control. In the TAP groups, the CNI subgroup exhibited significantly lower penetration than SI, PUI, and XPF (P < 0.05).

Regarding the maximum penetration depth, the control group had the highest depth values. In the CH group, CNI showed significantly lower penetration depths compared to the other irrigation methods (P < 0.05), whereas XPF exhibited values comparable to the control (P > 0.05). In TAP groups, no significant differences were observed among methods; all subgroups showed significantly lower penetration depths compared to the control (P < 0.05). Detailed data regarding these findings are demonstrated in Table 1.

Table 1.

Mean (standard deviation) of penetrated resin cement perimeter (%) and maximum penetration depth (μm) for all groups

	Control (no med)	CH				TAP
		CNI	SI	PUI	XPF	CNI	SI	PUI	XPF
Penetrated Perimeter (%)	86.08±9.01a	59.87±22.79b	62.53±15.21b	61.11±14.09b	79.39±14.07a	30.32±16.41c	58.58±17.29b	57.97±23.09b	62.41±13.82b
Maximum depth (μm)	550.86±98.68A	320.40±71.19C	384.69±78.40BC	415.40±81.30B	527.60±60.73A	151.34±61.39D	151.02±83.06D	153.77±65.72D	200.34±76.34D

Different lowercase and uppercase letters within rows indicate statistically significant differences among groups, as determined by Duncan’s post hoc test (P<0.05). CH: Calcium hydroxide; TAP: Triple antibiotic paste; CNI: Conventional needle irrigation; SI: Sonic irrigation; PUI: Passive ultrasonic irrigation; XPF: XP-endo finisher

Push-out bond strength results are shown in Table 2. The control and CH/XPF groups exhibited the highest bond strength, with no significant difference (P > 0.05). Conversely, CNI exhibited significantly lower bond strength than other irrigation techniques in both CH and TAP groups (P < 0.05). Within the TAP groups, SI, PUI, and XPF showed comparable bond strengths, significantly lower than the control (P < 0.05). The failure mode distribution showed that type I failure was most frequently observed in the CH group, while type II failure was predominant in the TAP group [Figure 2].

Table 2.

Mean (standard deviation) of the push-out bond strength values (MPa) for all groups

	Control (no med)	CH				TAP
		CNI	SI	PUI	XPF	CNI	SI	PUI	XPF
Bond strength (MPa)	20.05±2.55a	11.29±1.96c	16.11±2.13b	17.15±2.28b	20.42±2.65a	9.74±2.30c	14.45±3.66b	16.95±3.51b	17.12±3.53b

Different lowercase and uppercase letters within rows indicate statistically significant differences among groups, as determined by Duncan’s post hoc test (P<0.05). CH: Calcium hydroxide; TAP: Triple antibiotic paste; CNI: Conventional needle irrigation; SI: Sonic irrigation; PUI: Passive ultrasonic irrigation; XPF: XP-endo finisher

Figure 2.

Failure pattern distribution (%) for all experimental groups. CH: Calcium hydroxide; TAP: Triple antibiotic paste; CNI: Conventional needle irrigation; SI: Sonic irrigation; PUI: Passive ultrasonic irrigation; XPF: XP-endo finishe

The two-way ANOVA indicated that both medicament type and irrigation method had a significant effect on penetration and bond strength (P < 0.05). An interaction effect was identified only for maximum penetration depth (P = 0.010). Details are provided in Table 3.

Table 3.

Two-way ANOVA results for the effects of medicament type and irrigation technique on cement penetration and bond strength

Outcome variable	Factor	F	P
Penetrated perimeter	Medicament	11.785	0.001
	Irrigation technique	7.383	<0.001
	Interaction	2.560	0.062
Maximum depth	Medicament	232.310	<0.001
	Irrigation technique	11.222	<0.001
	Interaction	4.065	0.010
Bond strength	Medicament	7.044	0.010
	Irrigation technique	31.601	<0.001
	Interaction	1.008	0.394

Discussion

The present study confirmed that endodontic medicaments adversely affected resin cement penetration and the bond strength of fiber posts with CH, demonstrating fewer adverse effects than TAP. These findings offer additional evidence in support of the previous studies reporting that intracanal dressings decreased the bond strength of self-adhesive cement on the canal walls.[2,17] The negative impact of intracanal medicaments observed in this study is clinically relevant, particularly when immediate post placement is planned. This highlights the essential role of effective final irrigation for optimal bonding.

Self-adhesive resin cement was chosen for this investigation due to its simplified application protocol and ability to form a chemical bond to root dentin without requiring a separate bonding step.[18] This cement contains functional monomers that allow simultaneous smear layer dissolution and chemical bonding to dentin, offering a practical advantage in clinical scenarios. In addition, self-adhesive resin cement has demonstrated comparable bond strength to conventional resin cements, even under suboptimal bonding conditions.[19]

Regarding nonactivated irrigation groups (CNI), the control group without medication demonstrated the highest resin cement penetration (86.08% ±9.01%), whereas the TAP-medicated group exhibited the lowest penetration (30.32% ±16.41%), with statistically significant differences (P < 0.05). Similarly, bond strength was significantly reduced in both medicated groups (CH: 11.29 ± 1.96 MPa; TAP: 9.74 ± 2.30 MPa) compared to the nonmedicated control group (20.05 ± 2.55 MPa) (P < 0.05). These results suggest that the use of either CH or TAP as an intracanal dressing can negatively influence resin cement penetration and the bonding of fiber posts.

Lee et al.[20] noted that although CH does not structurally alter dentin, residual CH on the canal walls is a key factor contributing to reduced bonding of luting cements. These current findings support the limited efficacy of CNI in removing CH and regaining bonding conditions. Consistent with previous studies, residual CH may interfere with the chemical interactions between dentin and resin cement.[17,20,21] Among all protocols, only XPF significantly improved both outcomes. This could be attributed to the mechanical agitation provided by XPF, effectively disrupting loosely attached CH residues on dentin walls, facilitating cement penetration, and enhancing bond strength. This finding aligns with a prior study that identified XPF as the most effective technique for removing CH and enhancing the bonding of resin-based obturation materials.[22]

In canals treated with TAP, CNI resulted in reduced cement penetration and bonding ability in comparison to CH-treated samples, revealing a greater adverse impact of TAP on resin cement properties. The inferior outcomes with TAP may result from the physicochemical characteristics of TAP components, particularly minocycline, which forms stable and insoluble calcium-chelated complexes that can embed firmly in the dentinal wall or irregular canal extensions, thereby hindering resin tag formation and reducing bond strength.[2,3,23] The strong affinity of TAP to root dentin may also contribute to impaired bonding, as Berkhoff et al.[4] demonstrated that CNI alone is ineffective in removing TAP residues. Interestingly, some studies have reported increased bond strength in TAP-treated canals when using self-adhesive resin cement compared to nonmedicated controls.[24,25] Such contradictory findings may result from variations in experimental protocols, including differences in the type of vehicle used to deliver the medicament. Those studies employed saline as the TAP vehicle, whereas the present study used macrogol and propylene glycol. This difference in vehicle composition might have enhanced TAP retention on dentin surfaces in our study, exacerbating its negative impact on bonding.[26]

In this study, activated irrigation methods (SI, PUI, and XPF) enhanced the penetration perimeter and the bond strength of fiber posts in TAP-treated canals. The results revealed that all protocols had comparable effectiveness in improving bonding conditions in the presence of TAP. However, despite this improvement, the overall penetration values and bond strength in TAP groups remained lower than those in CH-treated canals, emphasizing the difficulty of eliminating TAP even with activation.

A significant interaction between medicament type and irrigation technique was observed only for maximum penetration depth, indicating that irrigation efficacy varied with medicament type. XPF resulted in greater cement penetration depth in CH groups than TAP groups, likely due to the retentive nature of residues.[3,4] In contrast, for penetrated perimeter and bond strength, no interaction was observed, suggesting that activated irrigation consistently improved outcomes regardless of medicament type.

A key limitation of the present study was the exclusion of endodontic sealer to minimize confounding variables and isolate the effects of medicaments and irrigation techniques on bonding outcomes. While this protocol improves internal validity, it deviates from clinical practice, where residual sealer can affect fiber post adhesion.[27,28] Furthermore, Zorzin et al.[29] reported that irrigant volume impacts medicament removal; however, this study did not compare the effects of irrigant volume or activation duration, which may also affect bonding outcomes. These findings highlight the importance of effective irrigation strategies, particularly when medicaments are used prior to immediate post placement. Future research should explore the optimization of irrigation time, volume, and adjunctive activation techniques to enhance the removal of antibiotic-based medicaments and promote more reliable and consistent adhesion.

Conclusion

Based on the limitations of this study, intracanal medicaments negatively affected resin cement penetration and fiber post bond strength. Activated irrigation enhanced these outcomes compared to CNI. XPF proved greater effectiveness in CH-medicated canals. Although TAP had more adverse effects, activated techniques still improved results over CNI.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.