The influence of cavity configuration and irrigation activation on root canal smear removal—an in vitro study

Introduction

Root canal therapy aims to minimize microbial presence sufficiently to promote periapical tissue healing, as total sterilization is not feasible under clinical conditions (Al-Helou et al., 2023; Arıcıoğlu, Çıkman & Babacan, 2021). Successful disinfection relies not only on mechanical instrumentation and chemical irrigation but also on optimal access to the canal system (Bago et al., 2023; Boutsioukis & Arias-Moliz, 2022).

Access cavity design plays a critical role in facilitating effective chemo-mechanical debridement. The conventional access cavity (ConvAC) provides straight-line access to canal orifices by removing the roof of the pulp chamber, allowing enhanced visibility and instrument maneuverability (De-Deus et al., 2017). In contrast, the ultraincisal access cavity (UincAC), a form of minimally invasive access cavity design, aims to preserve pericervical dentin and sound coronal tooth structure, potentially enhancing long-term tooth integrity and resistance to fracture (De Gregorio et al., 2009). However, this conservative design may limit irrigant distribution, particularly in the apical third.

During instrumentation, a smear layer composed of dentin particles, pulp tissue, and microbial elements may form, which complicates canal disinfection. Its presence can obstruct dentinal tubules and hinder the penetration of irrigants and sealers, thereby compromising the disinfection process (Fiegler-Rudol et al., 2025; Gulabivala & Ng, 2023). NaOCl and EDTA are commonly paired due to their complementary abilities to address both organic and inorganic debris during smear layer removal (Gündüz & Özlek, 2023). To enhance the efficacy of irrigants, various activation techniques have been developed. Sonic activation (SA) utilizes low-frequency agitation to improve fluid dynamics, while passive ultrasonic irrigation (PUI) employs high-frequency oscillations that create acoustic streaming and cavitation, facilitating deeper irrigant penetration (Krishan et al., 2014; Lebbos et al., 2024). SWEEPS, utilizing Er:YAG laser pulses, creates sequential shock waves via bubble collapse, enhancing the cleaning potential in complex apical anatomy (Mancini et al., 2013; Mancini et al., 2021). Although numerous studies have compared these irrigant activation methods in terms of cleaning efficacy, limited data exist on the potential interaction between access cavity configuration and irrigation technique (Miguéns-Vila et al., 2022; Nair, 2004). As minimally invasive approaches gain popularity in modern endodontics, it is essential to understand whether the geometry of access cavities affects the outcome of different activation regimens. Therefore, the aim of this in vitro study was to compare the smear layer removal efficacy of four irrigant activation methods—SWEEPS, PUI, SA, and syringe irrigation (SI)—across two access cavity designs: ConvAC and UincAC. The null hypothesis was that neither the access cavity configuration nor the type of activation technique would significantly influence smear layer removal at any level of the root canal.

Materials and Methods

This in vitro study was conducted using 80 human maxillary incisors extracted for periodontal reasons, all with fully formed roots, intact crowns, and no previous restorations. The study was approved by the Institutional Ethics Committee from the Research Ethics Istanbul Medipol University with protocol number E-10840098-772.02-3156; each tooth was taken after receiving written informed consent from the patients to use their teeth for dental research purposes.

After extraction, the teeth were cleaned of soft tissue remnants and stored in 0.9% saline at 4 °C. Preoperative radiographs (Kodak RVG 5200; Carestream Health, NY, USA) were taken in buccolingual and mesiodistal directions to confirm the presence of a single canal and to exclude teeth with internal/external resorption, calcification, cracks, fractures, or canal curvatures exceeding 10° (Fig. 1).

Access cavity preparation

Under 12.5× magnification provided by a dental operating microscope (OMS2380; Zuma Medical Co., Ltd., Suzhou, China; Fig. 2), access cavities were prepared using a high-speed handpiece with water-cooling to prevent thermal damage.

• Conventional access cavity (ConvAC): Created using a 1.6 mm rosehead bur.

• Ultraincisal access cavity (UincAC): Prepared using the EndoGuide™ Bur EG1A (SS White Burs, Inc Lakewood, NK, USA) with a 0.33 mm tip diameter and 2.5 mm head length, designed for conservative entry (Plotino et al., 2017).

After completing the access cavities, the working lengths of the teeth were calculated to be one mm short of the radiographic apex using a #10 K-type canal file (Maillefer, SA CH-1338, Ballaigues, Switzerland). The root canals were chemomechanically prepared using the VDW.ROTATE Ni-Ti rotary instrument system (VDW, Munich, Germany) with instruments of sizes 15/0.04, 20/0.05, 25/0.04, and 35/0.04 sequentially, according to the manufacturer’s instructions.

Although irrigation protocols varied among the groups, irrigation parameters such as volume and duration were standardized. A 30-gauge side-vented endodontic irrigation needle (Endo-Eze, Ultradent, South Jordan, UT, USA) was used to deliver 2.5 mL of 2.5% sodium hypochlorite (NaOCl; Wizard, Rehber Kimya, Istanbul, Turkey) at a flow rate of 1.5 mL/min between each instrumentation step. Following completion of root canal preparation, each canal was irrigated with five mL of 17% EDTA, five mL of 2.5% sodium hypochlorite (NaOCl), and finally four mL of distilled water, each for 1 min. Irrigation activation was performed prior to tooth splitting in intact samples. The respective devices for each group (SWEEPS, PUI, SA, SI) were applied according to manufacturer protocols. Subsequently, the canals were dried using suitable paper points.

Results

A total of 80 maxillary incisors were analyzed across all experimental groups. Smear layer scores were evaluated separately for the coronal, middle, and apical thirds of the root canals.

Smear layer scores in coronal and middle thirds

No statistically significant differences were observed among the eight groups in the coronal (p = 0.142) and middle (p = 0.087) thirds of the root canals (Kruskal–Wallis test). (Tables 2 and 3; Fig. 3—top and middle rows) Regardless of cavity configuration or activation method, smear layer scores in these regions remained relatively consistent across all groups.

Table 2:

Comparison of smear layer scores in the coronal third among experimental groups.

Group	n	Mean ± SD	Median (Min–Max)
Group 1 (SWEEPS–ConvAC)	10	1.9 ± 0.1	1.9 (1.7–2.0)
Group 2 (SWEEPS–UincAC)	10	1.9 ± 0.1	1.9 (1.8–2.0)
Group 3 (PUI–ConvAC)	10	2.3 ± 0.1	2.3 (2.1–2.5)
Group 4 (PUI–UincAC)	10	2.5 ± 0.1	2.5 (2.4–2.8)
Group 5 (SA–ConvAC)	10	3.3 ± 0.2	3.3 (2.9–3.5)
Group 6 (SA–UincAC)	10	3.8 ± 0.1	3.8 (3.7–4.1)
Group 7 (SI–ConvAC, Control)	10	4.2 ± 0.2	4.2 (3.9–4.4)
Group 8 (SI–UincAC, Control)	10	4.7 ± 0.1	4.7 (4.5–4.8)
Conventional Access (Groups 1–3–5–7)	40	2.9 ± 0.9	2.7 (1.7–4.4)
Ultraincisal Access (Groups 2–4–6–8)	40	3.2 ± 1.1	3.3 (1.8–4.8)

DOI: 10.7717/peerj.19678/table-2

Notes:

Comparison of smear layer scores in the coronal third among experimental groups.

Mean ± standard deviation and median smear layer scores for each group in the coronal third.

Statistically significant differences were observed between Groups 1–5, 1–6, 1–7, 2–6, 2–7, 2–8, and 3–8 (Bonferroni-adjusted Mann–Whitney U test, p < 0.0017). No significant difference was found between Conventional and Ultraincisal access cavity designs (Mann–Whitney U test, p = 0.142). The smear layer was scored using the Hülsmann 5-point SEM classification, ranging from Score I (no smear layer, open dentinal tubules) to Score V (dense, inhomogeneous smear layer with no visible tubules).

Table 3:

Comparison of smear layer scores in the middle third among experimental groups.

Group	n	Mean ± SD	Median (Min–Max)
Group 1 (SWEEPS–ConvAC)	10	1.8 ± 0.1	1.8 (1.6–1.9)
Group 2 (SWEEPS–UincAC)	10	1.9 ± 0.1	1.9 (1.8–2.1)
Group 3 (PUI–ConvAC)	10	2.3 ± 0.1	2.3 (2.1–2.5)
Group 4 (PUI–UincAC)	10	2.6 ± 0.2	2.6 (2.4–2.9)
Group 5 (SA–ConvAC)	10	3.4 ± 0.2	3.5 (3.1–3.7)
Group 6 (SA–UincAC)	10	3.9 ± 0.2	3.9 (3.7–4.2)
Group 7 (SI–ConvAC, Control)	10	4.2 ± 0.1	4.2 (3.9–4.4)
Group 8 (SI–UincAC, Control)	10	4.7 ± 0.2	4.7 (4.4–4.9)
Conventional Access (Groups 1–3–5–7)	40	2.9 ± 1.0	2.8 (1.6–4.4)
Ultraincisal Access (Groups 2–4–6–8)	40	3.3 ± 1.1	2.9 (1.8–4.9)

DOI: 10.7717/peerj.19678/table-3

Notes:

Mean ± SD and median values for each group. Statistically significant differences were observed between the following pairs: 1–5, 1–6, 1–7, 2–6, 2–7, 2–8, 3–8 (Bonferroni-adjusted Mann–Whitney U test, p < 0.0017). No significant difference was observed between Conventional and Ultraincisal access groups (Mann–Whitney U test, p = 0.142). The smear layer was scored using the Hülsmann 5-point SEM classification, ranging from Score I (no smear layer, open dentinal tubules) to Score V (dense, inhomogeneous smear layer with no visible tubules).

Comparison of cavity designs

When comparing ConvAC and UincAC cavity designs regardless of activation method, no statistically significant difference was found in apical smear layer scores (Mann–Whitney U test, p = 0.089). The median apical score for ConvAC was 2.9, compared to 3.3 for UincAC, suggesting a non-significant trend toward improved cleaning in conventional access designs (Fig. 4).

Ranking of irrigant activation techniques

The images demonstrate progressive differences in smear layer removal efficacy depending on both activation technique and cavity configuration. Groups activated with SWEEPS and PUI showed cleaner canal walls, particularly in the apical third, compared to sonic and syringe irrigation, where residual smear and debris were more pronounced (Figs. 4 and 5). The inclusion of coronal and middle third images was made in response to reviewer recommendations, ensuring full visualization along the root canal length. The final rows emphasize the limited effectiveness of non-activated syringe irrigation, especially in ultraconservative access designs (Group 8).

Discussion

Effective root canal treatment generally begins with appropriate access cavity preparation, which ensures adequate visibility and a direct pathway for cleaning and obturating the root canal system. While access cavity design is crucial for preserving tooth structure, it may also influence irrigant flow dynamics and smear layer removal, particularly in the apical third, where cleaning is most challenging due to anatomical complexity and limited space (Rödig et al., 2010; Silva et al., 2022).

This study investigated the combined effects of access cavity configuration and irrigant activation techniques on smear layer removal efficacy across different canal levels. Our findings revealed that although cavity design alone did not significantly influence smear layer scores, activation technique had a substantial impact—particularly in the apical third. Among the techniques tested, SWEEPS was the most effective, followed by PUI, SA, and SI. These trends and the presence or absence of statistical significance across canal thirds are detailed in Table 6. Despite the absence of statistically significant differences among groups in the coronal and middle thirds (Table 2: p = 0.142; Table 3: p = 0.087), pairwise comparisons revealed meaningful differences between specific group combinations. These trends were consistent across all canal thirds, with the SWEEPS–ConvAC group demonstrating the lowest smear layer scores and the SI–UincAC group exhibiting the highest. The apical third (Table 4), however, showed statistically significant differences among groups (p < 0.001), highlighting the increased sensitivity of this region to activation technique and cavity design. These findings were visually confirmed by SEM observations, which revealed progressive differences in smear layer accumulation across groups and canal thirds depending on both activation technique and cavity configuration (Fig. 3).

Consistent with prior studies, full smear layer removal was not achieved in any group across all root canal levels (Miguéns-Vila et al., 2022; Nair, 2004; Siqueira Jr & Rôças, 2008). The apical third remained the most resistant region to debridement due to its complex morphology, limited irrigant exchange, and restricted access (SS White Burs Inc, 2024). In this study, root canals were enlarged to size 35/0.04 to optimize irrigant penetration without excessive dentin removal, which aligns with current recommendations balancing efficiency and structural preservation (Tong et al., 2023).

Regarding cavity design, the comparison between Conventional Access Cavity (ConvAC) and Ultraincisal Access Cavity (UincAC) revealed no statistically significant differences in smear layer removal. These findings are in line with those of Gündüz et al. (Torabinejad et al., 2002), who also reported that conservative access designs did not significantly alter debris or smear layer accumulation. They emphasized that activation technique—particularly laser-based methods—had a more pronounced effect than access geometry, which supports our results.

The present study is among the first to evaluate SWEEPS in both conventional and ultraconservative cavity configurations. SWEEPS employs a dual-pulse Er:YAG laser protocol that enhances the collapse of laser-induced vapor bubbles, thereby generating shock waves that travel deep into the canal system (Tsotsis et al., 2021). Unlike traditional photon-induced photoacoustic streaming (PIPS), SWEEPS accelerates cavitation and generates more powerful shock waves that improve smear layer removal, even in apical regions (Van der Sluis et al., 2007). This mechanism was confirmed by Violich & Chandler (2010), who found that SWEEPS outperformed both PIPS and ultrasonic irrigation in removing intracanal debris.

In our comparison of activation techniques, PUI also demonstrated strong performance, especially in conventional access groups. PUI’s acoustic streaming and cavitation effects facilitate deeper irrigant penetration and dislodge debris from canal walls (Virdee et al., 2018). However, its efficiency was inferior to SWEEPS, possibly due to limitations in energy transfer in the narrower regions of ultraconservative cavities.

SA and SI showed the lowest efficacy among the tested techniques. This finding aligns with prior literature, which suggests that although sonic agitation improves fluid movement, it lacks the intensity required for effective smear layer removal in apical areas (Wigler et al., 2023).

While the current study strengthens the case for SWEEPS as a superior irrigation method, it is important to note its limitations. The in vitro setting may not fully replicate clinical conditions such as complex root morphologies or variable canal curvatures. Moreover, although SEM provided high-resolution images for qualitative assessment, quantifying smear layer distribution remains challenging.

Conclusion

The present in vitro study revealed that the type of access cavity—whether conventional (ConvAC) or ultraincisal (UincAC)—did not significantly influence smear layer removal efficacy across root canal thirds. However, the choice of irrigant activation technique had a marked impact, particularly in the apical third.

Among the four activation protocols evaluated, shock wave enhanced emission photoacoustic streaming (SWEEPS) demonstrated the highest smear layer removal efficacy, especially when combined with the conventional access design. Conversely, the ultraincisal access cavity with syringe irrigation (Group 8) exhibited the greatest residual smear layer, highlighting the limited effectiveness of syringe irrigation in conservative cavity configurations.

These findings underscore the clinical relevance of selecting an effective activation technique—particularly SWEEPS—for enhanced apical cleaning. Further in vivo studies are warranted to validate the superiority of SWEEPS and its implications for treatment outcomes and long-term bonding performance.

Supplemental Information