Comparative Evaluation of Polymeric and Metal Ultrasonic Tips for the Removal of Calcium Hydroxide From Root Canals

Introduction

During root canal treatment, effective disinfection of the root canal system and elimination of bacteria are of paramount importance. Complete eradication of microorganisms from the canal system cannot be achieved solely through chemomechanical preparation [1]. Therefore, in cases in which root canal treatment cannot be completed in a single visit, the canals should be filled with an intracanal medicament to prevent the establishment of bacterial reservoirs that can allow subsequent proliferation. Owing to its potent antibacterial and anti-inflammatory properties, calcium hydroxide (Ca(OH)2) is widely used as an intracanal medicament between appointments [2,3]. However, before proceeding to the final stage of treatment, this material must be completely removed from the root canal system. Residual Ca(OH)2 has been shown to interfere with the polymerization of sealers used during obturation [4], potentially resulting in a fragile and less durable root canal filling and leading to microleakage. Furthermore, residual Ca(OH)2 in the apical region can undergo gradual resorption over time, contributing to apical leakage [4,5].

Conventional needle irrigation (CNI) is frequently used to remove Ca(OH)2 residues from root canals because of its low cost and ease of application [6]. The most commonly employed irrigating solutions in this method are sodium hypochlorite (NaOCl) and ethylenediaminetetraacetic acid (EDTA) [7,8]. Nevertheless, several studies have reported that CNI is insufficient for complete removal of Ca(OH)2 from root canals [9–11]. This limitation arises because the needle often fails to reach the working length, preventing the irrigant from effectively accessing the apical portion of the canal and thereby restricting the desired hydraulic effects of irrigation and aspiration [12,13].

To achieve more efficient removal of Ca(OH)2 residues, various irrigation activation techniques have been proposed [14]. Among these, passive ultrasonic irrigation (PUI) has gained particular attention. PUI is an activation technique in which energy generated at ultrasonic frequencies of 25 to 30 kHz is transmitted to the irrigant, producing acoustic streaming and cavitation that enhance irrigant penetration and effectiveness throughout the root canal system [15]. Acoustic streaming allows rapid and turbulent movement of the irrigant within the canal, mechanically dislodging Ca(OH)2 particles. In contrast, CNI is restricted to the 1 to 2 mm area reached by the needle tip, and apical vapor lock is frequently encountered, which further hinders effective irrigant replenishment in the apical region [16]. PUI overcomes these limitations and enhances both the chemical and physical efficacy of irrigation.

The ultrasonic tips used in PUI can be fabricated from various materials, including stainless steel, nickel-titanium, and zirconium nitride [17]. However, because of their rigid structure, metallic tips may contact the canal walls, especially in curved or narrow canals, posing a risk of unintended dentin removal or alteration of canal morphology [18]. These risks have prompted the development of softer yet efficient tip designs for irrigation activation.

The multi-visit treatment approach and the use of Ca(OH)2 as an intracanal medicament remain integral components of successful endodontic therapy. However, the removal of Ca(OH)2 represents a critical phase in the treatment process, and continued research evaluating the efficacy of various removal techniques remains essential to optimizing clinical outcomes.

The aim of this study is to compare the effectiveness of CNI and PUI using metal and polymer tips in removing Ca(OH)2 from the apical third of the root canal under standardized in vitro conditions.

Material and Methods

ETHICS APPROVAL:

This study was approved by the Harran University Clinical Research Ethics Committee (decision No: HRÜ/25.12.65, Date: 30.06.2025). Informed consent was obtained from all individuals from whom specimens were collected.

SAMPLE SELECTION:

The sample size was determined through power analysis using G*Power 3.1 (Heinrich-Heine-Universität Düsseldorf, Germany), with 80% statistical power and a 5% significance level (α=0.05). Sixty extracted single-rooted mandibular premolars were selected. The patients from whom the teeth were extracted were aged between 18 and 55 years. Teeth were extracted for periodontal, orthodontic, or prosthetic reasons. Following extraction, all soft tissue remnants, calculus, and debris were removed. Only straight canals were included, to maintain a standardized and controlled baseline for comparison. Each tooth was examined under 2.5× magnification using a dental loupe (Orascoptic, Middleton, WI, USA), and those exhibiting caries, cracks, fractures, resorption, previous root canal treatment, or restorations were excluded from the study.

TOOTH PREPARATION:

Crown portions were sectioned with a carbon disc under copious water cooling to standardize the root length at 15 mm. The working length was established 1 mm short of the apical foramen by visualizing the tip of a #20 K–file at the apex. Teeth that permitted passage of files larger than #25 at the apex were excluded to avoid apical over–enlargement bias. Root canals were instrumented using NIC Blue 40 rotary files (SLT, Shenzhen, China), with 2.5 mL of 2.5% sodium hypochlorite (NaOCl; Microvem, Sakarya, Turkey) used for irrigation after every 3 mm of instrumentation. To ensure a standardized baseline and reduce anatomical variability, only straight canals were included, defined as a Schneider curvature less than 10° on preoperative radiographs.

After canal preparation, longitudinal grooves were created along the bucco–lingual axis using a diamond disc under continuous water cooling. A thin cement spatula was inserted into the groove to facilitate separation of the root halves (Figure 1). On one-half of each root, a standardized artificial groove was prepared 2 mm from the apex, measuring 4 mm in length, 0.2 mm in width, and 0.5 mm in depth (Figure 2). The statistical unit was the root half containing the standardized artificial groove, and each tooth contributed only 1 experimental unit. The canals were then dried using sterile paper points. All procedures were performed by a single experienced operator to minimize operator–related variability.

MEDICATION APPLICATION AND STORAGE:

A Ca(OH)2 paste (Calsin, Karabağlar, Izmir, Turkey) was prepared by mixing powder and liquid in a 1: 1 ratio and was applied into the standardized grooves using paper points. The filled canals were photographed under a Zumax OMS–2380 microscope (Zumax Medical Co, Ltd Suzhou, China) equipped with a digital camera (Zumax). The root halves were reassembled and sealed with wax, including the apical foramen, to prevent leakage. The specimens were then embedded in acrylic resin within centrifuge tubes and stored at 37 °C and 100% relative humidity for 7 days to allow the material to set (Figure 3).

GROUPING AND IRRIGATION PROTOCOLS:

To minimize bias, all specimens were sequentially numbered and randomly assigned by an independent researcher not involved in the study. The teeth were distributed into 3 groups (n=20 per group) according to the irrigation technique: CNI, metal ultrasonic activation (MUA), and polymer ultrasonic activation (PUA). This procedure ensured impartial allocation and reduced operator-related bias.

Each specimen was irrigated with 6 mL of NaOCl, 6 mL of 17% EDTA (Microvem, Sakarya, Turkey), and 12 mL of distilled water. Irrigation was performed using a side-vented Diadent 30-gauge needle (DiaDent Group International, Cheongju-si, Chungcheongbuk-do, South Korea). In the ultrasonic groups, activation was performed using a UC-One device (Epdent, South Korea) for 20 seconds per irrigant cycle. Activation procedures were performed strictly according to the manufacturer’s recommended protocol for each tip to ensure safe and proper use. The MUA tip was positioned 2 mm short of the working length without contacting the canal walls, whereas the PUA tip was allowed to contact the canal walls during activation. In the CNI group, manual irrigation was performed without activation, with only needle movement applied.

IRRIGATION SEQUENCE:

The 4 steps of the irrigation sequence were as follows: (1) 2 mL NaOCl was used for 20-second activation, followed by 2 mL distilled water; (2) 2 mL EDTA was used for 20-second activation, followed by 10 mL distilled water; (3) the sequence was repeated 3 times per specimen; and (4) final drying was performed with paper points.

MICROSCOPY EVALUATION:

Residual Ca(OH)2 in the apical 5 mm of each specimen was photographed under 16× magnification and analyzed using ImageJ software (ImageJ v1.53t, National Institutes of Health, Bethesda, MD, USA) to quantify the percentage of remaining material. Images were calibrated (pixel-to-millimeter) and thresholded to isolate the Ca(OH)2 area. Re-analysis of 20% of images after 2 weeks yielded an intraclass correlation coefficient of 0.90, confirming excellent intra-observer reliability. The grooves were scored independently by 2 blinded evaluators, who viewed 20 images and demonstrated strong agreement, with a kappa value of 0.85, using the following scale (Figure 4): score of 0 indicates no Ca(OH)2 present; score of 1 indicates Ca(OH)2 present in less than 50% of the area; score of 2 indicates Ca(OH)2 present in more than 50% of the area; and score of 3 indicates groove completely filled with Ca(OH)2.

All evaluations were conducted using a double-blind protocol to ensure objectivity and minimize bias.

STATISTICAL ANALYSIS:

Statistical analysis was performed using SPSS version 25.0 (IBM Corp, Armonk, NY, USA). Quantitative data (percentage of residual area) were analyzed using one-way ANOVA and post hoc Bonferroni tests. Although the observational scores (0–3) were ordinal, the Pearson chi-square and post hoc z-tests were used to compare the proportions of residual Ca(OH)₂ in the grooves (the frequency distribution of each scoring category) among groups. Significance was set at P<0.05.

Results

:

The quantitative findings regarding the area of residual Ca(OH)2 in root canals are summarized in Table 1. One-way ANOVA revealed a statistically significant difference among the irrigation groups (P<0.001). Regarding ImageJ analysis, the area of residual Ca(OH)2 in root canals was significantly higher in the CNI group (3.05±1.30 mm2) than in both the MUA (1.00±0.59 mm2) and PUA (1.03±0.54 mm2) groups (P<0.001). According to the post hoc Bonferroni test, no statistically significant difference was found between the MUA and PUA groups (P=0.999).

QUALITATIVE ANALYSIS OF OBSERVATIONAL SCORES:

The frequency distribution of the observational scores at 2 mm and 4 mm levels is presented in Table 2.

At the 2-mm level, a statistically significant difference was observed among the irrigation protocols (chi-square=13.911; P=0.007). Post hoc z-test results indicated that the frequency of grooves completely filled with medicament (score of 3) was significantly higher in the CNI group than in the MUA and PUA groups. Notably, total removal of the medicament (score of 0) was achieved only in the activated irrigation (MUA and PUA) groups.

At the 4-mm level, significant differences in scoring distributions were recorded (P=0.007). While the CNI group demonstrated the highest frequency of completely filled grooves (score of 3), this score was not observed in either the MUA or PUA group. The most favorable cleaning results, characterized by a low amount of residual medicament (score of 1), were predominantly observed in the MUA and PUA groups.

Discussion

The prognosis of root canal treatment is directly associated with effective disinfection of the root canal system and its hermetic obturation, which together prevent microleakage and subsequent reinfection [19]. Ca(OH)2, a widely used intracanal medicament between treatment sessions, is considered the standard material, owing to its potent antimicrobial activity and its ability to reduce periapical inflammation [2]. However, complete removal of Ca(OH)2 from the canal system, particularly from the apical third of the root where canal anatomy is most complex, remains nearly impossible, potentially leading to increased microleakage and compromised long-term outcomes.

Residual Ca(OH)2 has been demonstrated to negatively influence the polymerization of root canal sealers used during obturation [4,20]. Its presence may interfere with bonding between the sealer and dentin, thereby increasing apical microleakage and reducing the overall success of endodontic therapy. Consequently, complete elimination of Ca(OH)2 from the canal system – particularly from the apical region that often contains anatomical irregularities, such as isthmuses, ramifications, and lateral canals – is a critical prerequisite before obturation [21].

The primary finding of this study was that none of the irrigation techniques evaluated achieved complete removal of Ca(OH)2 from the root canal system. This observation aligns with previous research reporting that complete elimination of Ca(OH)2 is rarely possible, regardless of the removal technique used [9,10,22].

Previous studies have shown that the combined use of NaOCl and EDTA solutions is more effective for Ca(OH)2 removal than either solution used alone [23,24]. Accordingly, both irrigants were used in combination in the present study to maximize chemical dissolution and smear layer removal.

Another important finding of this study is that PUI, whether MUA or PUA, was significantly more effective than CNI in removing Ca(OH)2. This superiority is well-documented in the endodontic literature [16,25,26] and is attributed primarily to the hydrodynamic effects of ultrasonic activation, namely acoustic streaming and cavitation [27]. Acoustic streaming allows the irrigant to circulate rapidly and turbulently within the canal, mechanically dislodging Ca(OH)2 particles from canal walls and irregularities. In contrast, the action of CNI is limited to the 1 to 2 mm area reached by the needle tip, and the formation of an apical vapor lock frequently further restricts irrigant exchange and effectiveness. PUI effectively overcomes these limitations by enhancing irrigant penetration and renewal within the canal system [16].

In this study, no statistically significant difference was observed between MUA and PUA in terms of Ca(OH)2 removal efficacy (P=0.999), indicating that both tip types performed similarly under the conditions tested. Although these findings were obtained in straight root canals, they align with previous reports demonstrating that different ultrasonic activation systems often exhibit similar cleaning performance under standardized in vitro conditions [28,29]. Prior studies have noted that rigid metal tips can cause surface alterations or smear layer formation when contacting canal walls in anatomically challenging regions [30,31], whereas polymer based tips, owing to their flexibility, can interact more gently with dentin surfaces [32]. However, the present investigation did not assess dentin surface changes or other morphological effects; therefore, any safety related distinctions between tip materials remain theoretical and should be interpreted with caution.

Alves Dos Santos et al reported that, due to its integrated negative pressure feature, the polymer-tipped iVac system removed more Ca(OH)2 than did CNI and MUA systems [13]. In contrast, our study found no significant difference between polymer and metal PUI devices, likely because the polymer system used did not incorporate a negative pressure mechanism. The current literature on the effectiveness of polymer-tipped ultrasonic instruments in Ca(OH)2 removal remains limited, and direct comparative data between polymer and metal tips are particularly scarce. Therefore, the present study contributes valuable evidence to address this gap in knowledge and to clarify the clinical relevance of polymer-based ultrasonic irrigation systems.

Evaluation of Ca(OH)2 removal from root canals is commonly performed by calculating the percentage of the residual Ca(OH)2 area from images obtained using an operating microscope or a scanning electron microscope [33]. Although scanning electron microscopy provides greater sensitivity and detailed visualization, assessments and scoring based on optical microscopy images are faster, simpler, and more practical for comparative analysis [34]. Another technique frequently used for this purpose is micro-computed tomography, which allows quantification of residual Ca(OH)2 [35]. While this technique effectively measures 2-dimensional surface areas, it is particularly advantageous for determining 3-dimensional volumetric residuals. However, its high cost and the requirement for advanced analytical expertise limit its routine use [36]. Considering these factors, the present study used image analysis from optical microscopy to evaluate the effectiveness of Ca(OH)2 removal, using scoring and area calculation methods to ensure objective and reproducible results.

This pilot in vitro study has several limitations. First, the use of straight canals allowed for maximum standardization but does not reflect the clinical advantages polymer tips may offer in curved anatomy. Second, an inherent limitation is the difference in activation modes between the groups. Following the manufacturer’s instructions, the PUA tip was allowed to contact the root canal walls, whereas the MUA tip was used in a non-contact mode. Although this physical contact in the PUA group could potentially provide an additional mechanical scrubbing effect, both ultrasonic activation groups demonstrated comparable effectiveness in Ca(OH)2 removal. This suggests that the tip material and activation mode differences did not result in a significant clinical variance under the conditions of this study. Third, statements regarding the safety of polymer tips were based on existing literature and material properties rather than direct measurements, such as dentin loss or canal morphology changes. Finally, the metal tip used was an off-label choice, selected only to standardize frequency and power output within the same device.

Future studies should validate these findings in vivo and in canals with greater anatomical complexity and curvature.

Conclusions

The findings indicate that complete removal of Ca(OH)2 from root canals remains challenging, while PUI provides higher cleaning efficiency than does CNI. Under the tested conditions, metal and polymer tips demonstrated comparable performance. To better understand the potential clinical implications of different ultrasonic tip materials, further studies involving complex root canal anatomies and assessments related to dentin preservation are warranted.