Apical debris extrusion associated with oval shaped canals: a comparative study of WaveOne vs Self-Adjusting File

Tooth selection and specimen preparation

The protocol was approved by the ethics committee of the Dental School of the University of Athens, and signed in-formed consent/assent was obtained from all patients. Twenty-four single-rooted, intact permanent human mandib-ular premolars were selected from a random collection of teeth that were extracted for periodontal or orthodontic reasons. Tissue remnants and external deposits were removed from the teeth by placing them in 2.5 % NaOCl solution for 5 min, during which the remnants were removed with curettes. Specimens were checked using a stereomicroscope to ensure the presence of one main apical foramen and mature apices. All teeth were checked for single-canal anatomy and for the presence of an oval canal using mesio-distal and bucco-lingual digital radiographs [14].

Only teeth with a curvature of less than 10° were used [15]. Teeth that did not fulfill all inclusion criteria were excluded. All selected specimens were stored in physiological saline solution at 4° C until use.

Group organization

The first stratification of the specimens for the organization of the groups was performed by pair-matching individual teeth based on the similarity of their physical appearance (total length, length of the root, bucco-lingual, and mesio-distal di-mensions at the cemento-enamel junction). Matching was ver-ified by comparing digital radiographs taken in two planes, to confirm the presence of one oval canal of the same size (ex-cluding long-oval ones) to ensure the creation of homogeneity and standardized groups (Student’s t tests for two independent samples, p > 0.1, Table 1).

The canals in Group A were subjected to chemo-mechanical preparation with the WaveOne system, whereas, the canals in Group B were prepared using the SAF system.

Debris collection setup

Each tooth was forced through the silicone cap of a 40-mL glass vial and stabilized so that it was held securely during instrumentation. A pre-weighed 4-mL receptor tube was sta-bilized with silicone material at the bottom of the glass vial and was used to collect debris extruded during the canal prep-aration. The apex of the root was allowed to hang within the receptor tube. The plug of the glass vial was also pierced with a 19G needle to equalize the inside of the vial to atmospheric pressure (Fig. 1). Subsequently, a rubber dam sheet along with cyanoacrylate adhesive was applied to isolate the tooth and to prevent any external leakage of the irrigant into the receptor tube.

Endodontic access cavities were prepared using an Endo Access Bur (Dentsply Maillefer) and an Endo Z bur (Dentsply Maillefer) using a high-speed handpiece, in combination with surgical suction, to aspirate the water spray from the working field. Pulp remnants were extirpated using a broach while care was taken not to push the pulp remnants in the direction of or through the apical foramen.

Table 1 Summary statistics for the distribution of length, diameter (Bucco-Lingual (B-L) and Mesio-Distal (M-D)) by group

	Groups Homogeneity
	SAF Mean (SD)	WaveOne Mean (SD)	Overall Mean (SD)	p value
Length (mm)	20.5 (1.1)	19.9 (1.3)	20.2 (1.2)	0.247
Diameter B-L (mm)	8.5 (0.6)	8.5 (0.7)	8.5 (0.6)	0.876
Diameter M-D (mm)	6.5 (0.5)	6.5 (0.6)	6.5 (0.6)	>0.999

p values provided by Student’s t tests for two independent samples

Root canal preparation

Group A WaveOne Preparation. Working length (WL) was determined by measuring the length of a size 10 K file that emerged at the apical foramen, minus 1 mm. The tip of the protruding file was observed with the aid of a dental operating microscope (magnification ×12.8, Global Protégé plus, Global Surgical Corporation, St. Louis, MA, USA). Roots were initially instrumented using a Gates Glidden bur No 3 to a level 2–3 mm apical to the cemento-enamel junction.

Fig. 1 The setup of the experimental device. An external glass vial was used to stabilize the tooth and the debris receptor tube. A needle inserted through the vial’s silicon cap was used to equalize pressure

Thereafter, irrigation with 2 mL NaOCl solution irrigation (NaviTip 27 g; Ultradent, South Jordan, UT, USA) took place, and surgical suction was placed close to the access cavity to aspirate the excess liquid. The needle was placed in the coro-nal third of the canal, and the flow rate was manually con-trolled at 4 mL/min [16]. The same flow rate and setup were applied in all of the manually delivered irrigation procedures. K files size 15, 0.2 taper, and size 20, 0.2 taper (Dentsply Maillefer) were then used to create a glide path. Only root canals with an initial apical gaging diameter of size 20, .02 taper, or less were used. In this phase, four teeth were replaced (two from each group), as the size 20 file was protruding through the apical foramen. They were replaced by ones that did not significantly differ in size, length, or in other aspect (tolerance was set at 0.5 mm), and the initial apical gage was size 20 or less, to ensure the homogenous of the groups created.

The WaveOne system (Dentsply Maillefer) was then ap-plied for the preparation of the canals, used the reciprocating motion generated by the WaveOne electric motor (Dentsply Maillefer), which was operated with a 6:1 reduction handpiece (Sirona, Bensheim, Germany). The pre-programmed motor was set for the angles of reciprocation and speed designed for the WaveOne instruments.

A WaveOne primary file (Dentsply Maillefer) with a size of

25 and a taper of 0.08 (red colored) was used in a slow in-and-out pecking motion, according to the manufacturer’s instruc-tions. After three slow in-and-out movements (pecks), 2 mL of 2.4 % NaOCl was used as an irrigant. The irrigation needle was placed into the canal as deeply as possible without resis-tance, although not deeper than the predetermined WL minus 2 mm. The flutes of the instrument were cleaned by wiping with sterile gauze after the first three pecks as suggested by the manufacturer [9].

The reciprocating file was then re-inserted for another set of three pecks and 2 mL NaOCl irrigation, as many times as necessary, to reach the WL where mechanical preparation of the canal was considered complete.

Care was taken to ensure that a total volume of 10 mL of 2.4 % NaOCl was used to irrigate each canal. Subsequently, a flushing of 10 mL of 17 % EDTA solution that was activated with an Endo-Activator size 15, .02 taper Yellow tip (Dentsply Maillefer) for 1 minute was performed. The EndoActivator tip was placed 2 mm less than the working length, according to the manufacturers’ protocol.

A final flush of 10 mL of 2.4 % NaOCl solution activated for 30 s was applied according to the manufacturers’ protocol. Each irrigation step was followed by recapitulation to ensure patency [17]. A size 10 K-file was passed 0.5 mm beyond the WL to prevent apical plugging [8, 18, 19] and in an attempt to prevent a vapor lock effect that could otherwise prevent effec-tive irrigation in the apical third of the canal [20].

Group B SAF preparation. The teeth were handled, and roots were instrumented as in the WaveOne group, until the size 20, .02 taper glide path was established. A SAF file (ReDent-Nova, diameter of 1.5 mm, length of 25 mm) was then used, following a modification of the manufacturer’s protocol [20]. The SAF file was operated using an RDT handpiece-head with the motor rotating at 5000 rpm, which resulted in 5000 in-and-out vibrations per minute. Continuous simultaneous irrigation was applied using the VATEA irrigation pump (ReDent-Nova), which was connected to the file by a polyeth-ylene tube. Irrigation was applied at a rate of 5 mL/min and alternated every minute between 2.4 % NaOCl and 17 % EDTA for 4 min was completed. To allow this mode of irri-gation, two VATEA pumps were used; one with NaOCl solu-tion and one with EDTA solution, as described by Metzger et al. [12]. Surgical suction was used to aspirate the excess irri-gation solutions.

Each canal was prepared using a new SAF instrument. Final irrigation was performed in all specimens with 10 mL of 2.4 % NaOCl, delivered with a syringe and needle that were placed into the canal as deeply as possible without resistance, although not deeper than the predetermined WL minus 2 mm. Recapitulation with a size 10 K-file was performed 0.5 mm beyond the WL after every minute (for a total of 4 min) to ensure patency, avoid apical plugging, and prevent the vapor lock effect at the apical third, similar to the WaveOne group. The design of the study ensured that equal volumes of each irrigant were used for both groups: a total of 22 mL 2.4 % NaOCl (2 mL during glide path preparation, 10 mL during instrumentation, 10 mL final flushing), and 10 mL of 17 % EDTA solutions.

All procedures were performed by a single operator, where-as the assessment of the extruded debris was performed by a second examiner who was blinded with respect to the exper-imental groups to which the teeth belonged.

Debris collection and processing

Debris collection Debris collection was performed using the technique initially described by Fairbourn et al. [21] and mod-ified by Myers & Montgomery [22]. The extruded debris from each tooth was collected in a 4-mL pre-weighed receptor tube that was stabilized with silicon material at the bottom of the external 40-mL glass vial (Fig. 1).

Once the root canal preparation was completed, the debris that adhered to the root was wiped off on the inside surface of the tube, followed by rinsing of the apical part of the root with 2 mL of bidistilled water. The tube was then removed and processed for weighing of debris.

Debris weighing The receptor tubes were stored in a dry incubator at 65 °C for 5 days to allow evaporation of all mois-ture before the dry debris was weighed. An electronic balance (AW220, Shimadzu Corp, Kyoto, Japan) with an accuracy of 0.1 mg was used to weigh the tubes containing the debris. Three consecutive weights with a difference of <0.2 mg were obtained for each tube before the mean values were calculated. The dry weight of the extruded debris was calculated by subtracting the weight of the empty tube from the weight of the tube containing the debris. During handling, one of the specimens of the SAF group was accidentally lost and was not included in the analysis.

Statistical analysis

The normality of the distribution of the weights of the extrud-ed debris in each group was tested using the Shapiro-Wilk test. Because of deviations from normality that were observed in the WaveOne group, as well as the relatively small sample size, differences in the amount of extruded debris between the two groups were assessed using a permutation-based (n = 10,000 permutations) non-parametric Wilcoxon ranksum test.

Results

The mean mass of debris that was extruded in the WaveOne group (2.18 ± 0.44 mg) was 4.4 times higher than that extrud-ed in the SAF-treated group (0.49 ± 0.33 mg). This difference was highly significant (p < 0.001) (Fig. 2).

Discussion

Debris extrusion during root canal instrumentation is an un-desirable side effect of treatment [3, 4] that is augmented by irrigation [23]. Such extrusion may lead to postoperative pain and flare-ups, and it may potentially decrease the chance of a successful outcome [1, 2]. The introduction of innovative ir-rigation systems creating a negative pressure at the apical por-tion of canals has held promise for the prevention of irrigant extrusion [24].

Clinicians should be aware that instrument design and mode of action may influence the amount of the extruded debris, as can root canal anatomy and/or the instrumentation technique. However, no instrumentation method to date has been shown to completely prevent debris extrusion [6, 25–30].

The results indicate that apical extrusion of debris occurred with both file systems tested. However, the amount of debris extrusion was greatly affected by the instrument. The reciprocating WaveOne single-file system extruded 4.4 times more debris than the hollow SAF system.

Traditional hand instruments have been associated with apical extrusion of debris [27, 31–33] caused by the piston action of the file that is used with an in-and-out filing motion in the canal [5–7].

Fig. 2 Box-plots showing the distribution of apical debris extrusion mass in the SAF (Mean 0.49 mg, SD 0.33 mg) and the WaveOne (Mean 2.18 mg, SD 0.44 mg) groups. (permutation-based Wilcoxon test p < 0.001)

Ineffective irrigation in the early stages of the procedure may be another contributing factor for debris extrusion [8, 17–19]. It may allow the accumulation of a debris plug in the apical part of the canal, which may be pushed apically by attempting to reach the WL using larger instruments [8, 17–19]. The well-adapted larger file used next, would then serve as a piston which could drive the debris plug through the foramen. It has been calculated that when attempting to push a well-adapted size 25 hand file to a WL with a force as low as 1 g, an hydraulic piston pressure as high as 200,000 Pa may be generated [34].

It has been suggested that repeated recapitulation with a small file may enhance the efficacy of irrigation in the narrow cul-de-sac of a canal [35, 36], possibly by mixing the contents of the apical part of the canal with the irrigant or by preventing the formation of a Bvapour lock^ in this area or both [20, 35, 36]. Thus, recapitulation and patency filing were applied in both groups, despite the fact that it may potentially promote apical debris extrusion.

The crown-down technique incorporated with the NiTi ro-tary systems, provides a gradual approach to the apical portion and has been shown to be associated with a reduced amount of extruded debris relative to hand files [27, 31, 32]. It is likely that the rotating flutes that have been designed to carry the debris coronally (away from the apex) resulted in the de-creased extrusion of debris. Furthermore, the sequential and gradual increase in the instrument size found in multi-file systems may also affect extrusion: The multi-file systems ap-ply a fresh instrument with clean unobstructed flutes with every sequential step, which may also contribute to the ob-served reduction in debris extrusion.

On the other hand, with the introduction of reciprocation, a single instrument is used for the entire procedure, which means that the same total amount of debris has to be contained within a single set of flutes. Neither the new WaveOne or Reciproc instruments, nor regular rotary files such as Protaper F2 or Mtwo, which have been used with reciprocation, are free from the side effects of debris extru-sion [30, 37]. Therefore, it is not surprising that the manu-facturer’s instructions indicated that the operator should manually clean the WaveOne flutes between the pecks [9], probably to reduce this side effect.

Incorporation of EndoActivator within the WaveOne prep-aration protocol is suggested by the manufacturer and coun-terbalances the active irrigation of the SAF system. Thus, following root canal preparation procedures, sonically agitat-ing an intracanal reagent into the canal space serves, among others, to move debris into solution. The suspended debris may further react with the active irrigant and then be mainly expelled via irrigation through the orifice [38]. Both devices provide sonic activation/agitation as the Endoactivator per-forms at 10 KHz [8] and the SAF at 5 KHz [12]. WaveOne and Reciproc were recently compared to a full sequence of rotary NiTi files such as Mtwo and ProTaper in order to assess their apical extrusion of debris [37]. WaveOne and Reciproc produced more debris extrusion than multi-file rotary systems, which agrees with the aforementioned potential explanation.

Therefore, the efficiency of single-file reciprocating sys-tems may be accompanied by drawbacks, at least with regard to apical extrusion of debris and especially in infected cases,  where these systems may cause more post-operative pain or interappointment emergencies [39, 40].

Comparing WaveOne apical extrusion in the present study (2.18 ± 0.44 mg), with the results of Bürklein and Schäfer [37] (0.31 ± 0.13 mg), revealed greater debris extrusion in the pres-ent study. This difference may be attributed to three factors:

(i) In the present study, NaOCl and EDTA were used as irrigation solutions, and they may have been more effective in dentine removal as compared to bidistilled water that was used by Bürklein and Schäfer, which is completely inactive. It was therefore likely that more debris was removed from all sur-faces of the root canals, especially in combination with the EndoActivator [41]. (ii) The action of the file that was used for recapitulation could also explain some of the difference.

(iii) Part of the debris weight may also represent the dried mineral residue of extruded irrigants (NaOCl and EDTA) that were used in the present study. The glide path preparation is expected to have minimal impact on the results, as the SAF system in this study (which also underwent the aforemen-tioned preparation stage) reported with the minimum extruded dry material.

The SAF that was used in the present study apparently has all of the substrates for debris extrusion. The SAF file is op-erated with an in-and-out vibration and with repeated pecking motions to the WL, with no rotation in the canal or any flutes to carry the debris coronally [12, 34].

Such a device would have been expected to cause a major extrusion of debris through the apical foramen. However, the results of the present study show otherwise. The lower debris extrusion by the SAF system may be explained by (i) the cross section of the apical part of the file, (ii) the continuous flow of the irrigant, and (iii) the continuous exchange of a fresh, fully active irrigant throughout the procedure.

When fully compressed, the cross section of the apical part of the SAF has the shape of a rectangle of 0.12 × 0.16 mm [34]. When considering the dimensions of the canal after a glide path has been prepared up to the #20 K file, the potential piston effect of the file in the apical part of the canal is negli-gible, as 38 % of the cross section of the canal is free for backflow [34].

The continuous flow of the irrigant is likely to carry the debris particles coronally, thereby avoiding the accumulation of a debris plug in the apical part of the canal. Furthermore, the continuous exchange of fresh, fully active NaOCl and EDTA throughout the procedure is likely to better dissolve and dis-perse the debris, allowing its suspension in the irrigant thus facilitating its coronal transport with the coronal back flow of the irrigant.

All of the above are likely reasons for the smaller amount of debris extruded apically when the SAF system was used. In contrast, it is likely that the solid core design of the WaveOne file allowed it to act as a plunger inside the canal, thereby extruding a greater amount of debris. The reciprocal motion that carries the file apically in the inbound non-cutting part of this motion may also have been an additional factor in the larger extrusion of debris [37].

Recently, Kocak et al. [42] evaluated in vitro, the weight of debris extruded apically from teeth using different preparation instruments: including ProTaper, Self-Adjusting File, Revo-S, and Reciproc. Despite the major differences in the two proto-cols (mainly the irrigation procedures were minimal) from both studies, it was found that the SAF does not promote apical debris extrusion, whether active or inactive irrigating solutions are used.

The results of the present ex vivo comparative study cannot be directly applied to the clinical situation. Clinical relevance of the difference between both groups remains completely unclear; maybe the amount of extrusion is harmless in both groups or similarly deleterious for the periradicular tissues in both groups or may be dose-related to the amount of the ex-truded material. No attempt was made in this study to simulate the presence of periapical tissues. In an in vivo situation, nor-mal periapical tissues may serve as a natural barrier to inhibit debris extrusion, whereas pathological periapical tissues may be more susceptible to debris extrusion. Furthermore, this study was limited to teeth with closed, fully formed apices. The results may differ when roots with apices resorbed by an inflammatory process or immature teeth with open apices are considered.

It must be mentioned that the adaptation of any new pulp space preparation system in clinical practice relies on many factors such as the ease of use, safety during operation, dura-bility of the file, cost of the system, necessary time for root canal preparation completion, retreavability of the broken seg-ment in case of fracture, apical transportation tendency, cleaning efficacy, and apical debris extrusion, among others. Single file systems are expected to be more time-efficient compared to multi-instrument systems, but still, more research on the aforementioned parameters is needed before any defi-nite clinical suggestion can be made.

Conclusions

Both WaveOne and the SAF systems were associated with apical debris extrusion. The amount of debris extruded by the WaveOne system was 4.4 times greater than that extruded by the SAF system.

Compliance with ethical standards

Conflict of interest The authors declare that they have no competing interests.

Funding None

Ethical approval This article does not contain any studies with human participants or animals performed by any of the authors.

Informed consent For this type of study, formal consent is not required.

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