Introduction

For cleaning and disinfection of infected root canals, a combination of preparation, irrigation, and medication has been proposed with calcium hydroxide being the most widely used medicament for temporization [1–3]. Before definite obturation of the root canal system, the dressing should be removed completely to improve the seal of the root canal filling. Remaining calcium hydroxide could be washed out by body fluids resulting in apical leakage and reduced bond-strength of AH plus sealer (Dentsply, Konstanz, Germany) [4]. A decrease of bond-strength of a resin-based sealer [5], or a silicone-based sealer to root dentine [6] may be consequence of remaining medicament. Also, increased apical leakage after use of zinc-oxide eugenol-based sealers [7], or Real Seal (SybronEndo, Orange, USA) and Endofill sealers (Produits Dentaires, Vevey, Switzerland) [8], but no reduction in the sealing ability of Resilon (Pentron Clinical Technologies, LLC, Wallingford, USA) [9] have been reported. Margelos et al. [10] and Calt and Serper [11] demonstrated incomplete penetration of the sealer into radicular dentine due to the precipitation of calcium eugenolate, which results from the contact between calcium hydroxide and eugenol. Calcium hydroxide accelerates setting of some sealers, challenging the obturation [10]. Ricucci and Langeland [12] in a case report showed how remaining calcium hydroxide was resorbed from the apical part of a root canal by time resulting finally in treatment failure. Remaining medicament also has been demonstrated to influence negatively the accuracy of electronic working length measurement [13].

Removal of calcium hydroxide has been investigated in several experimental studies using different irrigants and removal techniques. Irrigation with sodium hypochlorite only was not effective in removal of the medication [14, 15], therefore a combination of chemical and mechanical removal seems to be necessary. A widely used removal technique uses irrigants and hand instruments inserted onto working length [16]. Ultrasonics in several experimental studies has been used with good success, but was not able to remove the calcium hydroxide completely [17–20].

Recently, several advanced techniques and devices for root canal irrigation and disinfection have been introduced [21]. Some of these new instruments, such as the Self Adjusting File (ReDent Nova, Ra’anana, Israel) [13, 22, 23], photon-initiated photoacoustic streaming [24, 25], CanalBrush (Coltene/Whaledent, Altstätten, Switzerland) [13, 20], laser systems [26], EndoActivator (Dentsply, Tulsa, USA.) [13, 19, 24, 25, 27–29], EndoVac (Kerr Dental, Orange, USA) [13, 22, 23, 28], RinsEndo (Dürr-Dental, Bietingheim-Bissingen, Germany) [30], Gentle Wave system (Sonendo, Laguna Hills, USA) [29], and XPEndo finisher (FKG, Chaux-de-Fonds, Switzerland) [31], have been used for removal of intracanal medicaments. The results concerning the efficacy of these devices and techniques are controversial.

It was the aim of this study to compare four different techniques for removal of calcium hydroxide from root canals: (1) passive ultrasonic irrigation (PUI), (2) hydrodynamic irrigation using RinsEndo, (3) sonic irrigation using the EndoActivator, (4) motor-driven plastic brush (CanalBrush), and (5) manual irrigation with a syringe as the control group.

Materials and methods

The present study basically used the study design suggested by Lee et al. [32] and van der Sluis et al. [17]. One-hundred and ten extracted human central and lateral maxillary incisors, canines, and premolars with straight single roots, intact root tips, and a length of at least 19 mm were selected. All teeth were shortened to a total length of 19 mm.

Following preparation of an access cavity and removal of pulp tissue, the root canals were enlarged to ISO-size 02/50 at a length of 18 mm using FlexMaster NiTi files (VDW, Munich, Germany). Apical patency was checked after each instrument with a reamer ISO-size 10 (VDW, Munich, Germany).

Irrigation was performed with a syringe and a gauge 30 needle (Navitip; Ultradent, Munich, Germany) and 2 mL 1% NaOCl following each instrument [33]. Following final irrigation with 5 mL 17% EDTA, the root canals were dried with paper points. A gutta-percha cone ISO-size 50 (VDW, Munich, Germany) was introduced into the root canal, two longitudinal grooves were prepared on the root outside and the roots split longitudinally in a mesio-distal direction. Under a microscope (MOTIC Ergonomic Trinokular Zoom Stereo Mikroskop, Motic, Wetzlar, Germany), it was checked whether both root halves could be reassembled without gaps.

Two grooves (4 mm long, 0.2 mm wide, and 0.5 mm deep) were prepared into the roots, one in the apical third with a distance of 2–6 mm from the apex at the buccal side, the second into the opposite oral root half with a distance of 10–14 mm from the apex (Fig. 1).

Fig. 1
figure 1

Schematic drawing of size and locations of the grooves [16, 17, 32]

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Both cavities were placed into an individual silicone mold (Silaplast, Dentax, Ettlingen, Germany) for each root half and photographed using a microsope with 40× magnification. The lateral cavities were filled with Calxyl (OCO, Dirmstein, Germany) under the microscope and photographed again with 40× magnification (Fig. 2).

Fig. 2
figure 2

Lateral groove filled with calcium hydroxide (magn. ×40)

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The root halves were reassembled, fixed with wax, and the apical foramen also covered with wax to simulate a closed system [34]. The roots were embedded in plastic tubes with silicone (Silaplast) [35] and were filled with calcium hydroxide (Calxyl) using a Pasteinject size ISO 40 (Micro-Mega, Besançon, France). Following radiographic control, the specimens were stored in an incubator (Climacell 111, MMM Medcenter, Munich, Germany) at 37 °C and 100% humidity for 7 days (Fig. 3).

Fig. 3
figure 3

Radiograph of a root embedded in silicone and filled with calcium hydroxide

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The roots were randomly divided into five groups and the medicament was removed as follows.

Hydrodynamic irrigation with RinsEndo (Duerr-Dental, Bietigheim-Bissingen, Germany)

The device was used as recommended by the manufacturer with 3.5 bar. A syringe filled with 3.1 mL distilled water was adapted and the system activated for 30 s. The needle was introduced into the root canal for 16 mm. This was repeated 6 times resulting in a total of 3 min irrigation time with a total of 18.6 mL irrigant.

Mechanical activation with canalbrush (Coltene/Whaledent, Altstätten, Switzerland)

The brush was used in a handpiece with a rotational speed of 600 rpm at 16 mm working length. The brush was activated for 30 s with 3.1 mL distilled water applied with a syringe. A new brush was used for ech root canal. This was repeated 6 times resulting in a total of 3 min irrigation time with a total of 18.6 mL irrigant.

Sonic activation with EndoActivator (Dentsply, Tulsa, USA)

The irrigant was activated with 10,000 cycles per minute with a tip size 0.04/35. A new tip was used for each root canal.

During a 30 s activation, 3.1 mL distilled water was applied with a syringe. Irrigation was repeated 6 times resulting in a total of 3 min irrigation time with a total of 18.6 mL irrigant.

Ultrasonic activation with Piezon Master 400 (EMS, Nyon, Switzerland)

The device was used as recommended by the manufacturer in the “ENDO’’-mode with a file size 15 inserted. The volume of irrigant was 3.1 mL distilled water during a 30 s activation period. This was repeated 6 times resulting in a total of 3 min irrigation time with a total of 18.6 mL irrigant.

Manual passive irrigation with a syringe

A 5 mL syringe with a gage 30 needle (Navitip, Ultradent, Munich, Germany) was used at 16 mm working length. 3.1 mL distilled water was applied over 30 s. This was repeated 6 times resulting in a total of 3 min irrigation time with a total of 18.6 mL irrigant.

Ten roots served as controls: five roots were only prepared and split with the grooves left empty, 5 roots were prepared and filled with calcium hydroxide but not irrigated.

The root canals were dried with paper points size 50, the root halves removed from the molds, and separated and photographed again under identical conditions.

The amount of calcium hydroxide remaining in the grooves was independently scored by two calibrated investigators under a microscope with 40× magnification using the scoring system of Lee et al. [32]. Calibration included the repeated scoring of 50 specimens by each of both raters. The final rating was performed some days later after blinding the specimens. One week later all 220 specimens were rated by both investigatiors again.

The following scores were used:

0:

Empty groove

1:

Less than half of the cavity is filled with calcium hydroxide

2:

More than half of the cavity is filled with calcium hydroxide

3:

The cavity is completely filled with calcium hydroxide

Statistics

For intraindividual reproducibility and interrater agreement, Cohens Kappa was calculated. Results of scoring were analyzed using a non-parametric test [33]. Post hoc pairwise comparisons were used for comparison of irrigation techniques. Level of significance was set at α = 0.05.

Results

For intraindividual reproducibility Cohen’s kappa was 0.949 for the first and 0.934 for the second rater, indicating good reproducibility.

Regarding both scoring procedures, interobserver agreement was not lower than 0.883, indicating good interobserver agreement. Differences between both observers in no case exceeded one score.

All the grooves in the teeth which were only prepared and did not receive further treatment showed completely empty grooves; all roots which received a filling with calcium hydroxide but no attempt of removal showed grooves completely filled with the medicament.

The relative frequencies of the scores for the apical and coronal groove are presented in Figs. 4 and 5. Between irrigation technique and localization of the groove, a statistically significant interaction was shown (P = 0.013). The coronal grooves overall showed more remaining calcium hydroxide than the apical grooves, although this was evident only in two of five groups. Pairwise comparisons between the irrigation techniques showed significant differences between some devices (Fig. 6). Ultrasonics performed significantly better than all other groups in the apical groove. Significant differences also were found between RinsEndo and Canal Brush (P = 0.019) and Canal Brush and syringe irrigation (P = 0.000). In the coronal groove, again ultrasonics performed significantly superior compared to all other groups. Hand irrigation performed significantly worse than all other groups in the coronal groove of the root canal.

Fig. 4
figure 4

Relative frequency of scores for cleanliness of the apical groove

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Fig. 5
figure 5

Relative frequency of scores for cleanliness of the coronal groove

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Fig. 6
figure 6

Results of statistical analysis, significant differences are printed in bold. 1 = RinsEndo, 2 = CanalBrush, 3 = EndoActivator, 4 = ultrasonics, 5 = syringe irrigation

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Discussion

The design of the present study was the same as used in previous investigations [17, 30, 32]. In some other studies, cleanliness of the complete root canal wall was investigated [14, 36–38]. The advantage of the groove model is the standardized size and location of the grooves. Grooves also may be regarded as typical irregularities of a root canal which are most difficult to clean from the medication, although the complexity of root canal anatomy cannot be completely replicated by this model. Scoring of cleanliness of the grooves probably will be easier and more reproducible than scoring of such a large area as the complete root canal wall. Both interobserver agreement between the two observers and intraobserver reproducibility showed good results with no statistically significant interaction between observers and results for cleanliness.

Preparation was performed to size 50 with a 2% taper as suggested by the manufacturer for use of the FlexMaster system. A more tapered preparation probably could have allowed better exchange of irrigant, but in all roots it was possible to introduce the irrigation systems/needles to working length minus 2 mm without friction.

Chemical dissolution of the medication was prevented as distilled water is not able to dissolve calcium hydroxide [10, 14]. Clinically, irrigation using EDTA will assist in removal of calcium hydroxide. Irrigation time and volumes of the irrigant were identical for all groups. Continuous exchange of the irrigant probably could have enhanced cleanliness but was possible only for ultrasonics and RinsEndo, for CanalBrush and EndoActivator the irrigant could be applied only intermittently.

An influence of the consistence of the carrier material of the calcium hydroxide suspension or paste has been demonstrated: mixtures with oily or viscous substances could be removed significantly worse compared to mixtures with aqueous solutions [37, 39, 40]. In the present study, Calxyl was used, containing 42% calcium hydroxide in an aquaeous solution with barium sulfate as a contrast medium.

In straight root canals, additional recapitulation with the Master Apical File (MAF) improved removal when compared to irrigation alone [15]. Goldberg et al. [41] reported on straightening of curved root canals as a sequaela of calcium hydroxide removal with hand instruments, but they removed the medicament with stainless steel files size 30. In curved root canals, the removal of the medication using NiTi instruments resulted in less straightening [14].

Ultrasonic activation proved to be significantly more effective than the other techniques used in this study. In a previous study passive ultrasonic irrigation with 2% NaOCl was more effective than manual irrigation in removal of calcium hydroxide from simulated lateral grooves [17]. In a microtomographic study, Wiseman et al. [19] investigated the removal of calcium hydroxide from curved root canals using ultrasonic or sonic activation for 3 × 20 s combined with rotary instrumentation. Best results were reported for ultrasonic activation combined with rotary preparation. CanalBrush and PUI in combination with NaOCl removed significantly more calcium hydroxide than irrigation alone [34]. Nandini et al. [40] reported on good results for citric acid in the removal of Metapex paste (Meta Biomed, Chalfont, USA) and EDTA for removal of calcium hydroxide powder, both activated with ultrasonics (PUI).

In a similar study, using the same design RinsEndo removed dentinal debris more effectively than syringe irrigation, independently from preparation size [42], but ultrasonic irrigation performed significantly better. Activation of the irrigant with a well-fitted gutta-percha cone removed biofilm significantly more effective from the root canal wall than RinsEndo [36]. Additionally, there has been some concern regarding apical extrusion of the irrigant, which has been reported by Hauser et al. [43] to occur in vivo in up to 80% of the specimens.

Regarding the efficiency of the EndoActivator, controverse results have been published. Ma et al. [29] did not detect significant differences between PUI and the EndoActivator when used for calcium hydroxide removal from C-shaped root canals. Syringe and EndoActivator removed similar amounts of different intracanal paste medicaments, among these Ledermix (Riemser, Riems, Germany) and Pulpdent (ADS, Vaterstetten, Germany) [27]. In the study of Topcuoglu et al. [13], PUI and SAF performed significantly superior compared to the EndoActivator, EndoVac, EndoBrush, or syringe irrigation. Photon-initiated photoacoustic streaming was shown to remove calcium hydroxide completely from lateral grooves, whereas ultrasonics and the EndoActivator left significantly more paste remnants [24]. Although the ultrasonic system in the present study showed the best results, a potential benefit of the EndoActivator could be its non-invasive mode of action which especially in curved root canals could prevent ledging.

In the study of Taşdemir et al. [34], a 30 s application of the CanalBrush improved cleanliness and removal of compared to syringe irrigation, nevertheless there was no difference to ultrasonics.

Conclusions

Complete removal of calcium hydroxide from the root canal could not be achieved with any of the techniques investigated. The highest degree of cleanliness resulted from the use of ultrasonics.