Root canal treatment aims to eliminate bacteria, necrotic debris, tissue remnants, and other irritants from the root canal system.^[1] During endodontic preparation, the piston effect of instruments can result in the extrusion of these materials to the apex.^[2] The quantity and quality of extruded debris are factors hypothesized to contribute to postoperative pain, flare-ups, delayed healing, or even endodontic treatment failure among other factors.^[3,4] Additionally, the rate of debris extrusion is influenced by various factors, including the size of the apical foramen, the type of instrument used, and the root canal preparation technique employed.^[5]

Research has demonstrated that all instruments and techniques result in some degree of debris extrusion from the apical foramen, leading to the presence of debris in the periapical region. To mitigate apical debris extrusion, it is useful to compare the efficacy of different types of files. While hand files have been traditionally used for root canal preparation, rotary files are designed to facilitate canal instrumentation with fewer procedural errors. Some studies suggest that rotary files can reduce the volume of debris extruded from the apex to the periapical region.^[6,7]

Stainless steel hand files are commonly used instruments for root canal preparation; however, due to their rigidity and limited flexibility, they are prone to causing procedural accidents such as ledge formation, transportation, and perforation.^[8] To address these issues, nickel-titanium (NiTi) rotary instruments have been introduced as a modern solution for efficient and precise root canal preparation.^[4]

Studies have shown that NiTi rotary files can effectively instrument root canals, providing optimal shaping for obturation, and are associated with significantly less debris extrusion compared to hand files.^[9] The Denco rotary file system, introduced in 2010 by Shenzhen Denco Medical Co. (China), is similar to the ProTaper system in terms of size and taper. These files are made of NiTi alloy and feature a triangular cross-section with a safe tip. A key feature of the Denco system is its variable taper along the length of the file, which may offer advantages in specific canal anatomies. The Gold type is described as being suitable for mildly curved anterior and simple posterior teeth. The blue type is claimed to have more flexibility, and fracture resistance and subsequently can be the better choice for complex root canals.^[10-14]

Various instruments, techniques, and irrigants have been investigated to reduce apical debris extrusion and improve postoperative patients’ comfort. The Denco file system is widely available in the Iranian market, yet no studies have specifically examined its performance in this context. Given the conflicting results of previous studies regarding debris extrusion with rotary versus hand files, some studies suggest that rotary files produce less debris,^[9] while others show no difference^[15] or better of hand file.^[16] This study aims to compare the apical debris extrusion of Denco Blue and Gold rotary files with that of conventional hand files. The null hypothesis posits no significant difference in apical debris extrusion between the Denco (Gold and Blue) rotary files and hand files.

This study’s methods and materials are based on previous research.^[7,17] The study protocol was approved by the local ethics committee (IR.MUI.RESEARCH.REC.1400.378) and was reported according to the PRILE 2021 checklist [Supplementary Table 1].

Using the statistical formula and recommendations from a statistical consultant, the sample size was calculated to be n = 80 n = 80. The formula used is:

In this formula z1 equals (z1 - α/2) and z2 equals (z1 - β). In medical research, α\alpha represents the type 1 statistical error (0.05) and β\beta represents the type 2 statistical error (0.1). Thus, z1z_1 equals 1.96 and z2z_2 equals 1.28. The variance (s2s²) from previous studies^[18] is 0.106, and the sampling error (d2d²) is set to 0.1. Considering these values, the sample size was calculated to be 69. Therefore, 69 mandibular first molars with poor prognosis were selected.^[14]

Using a diamond disk (DFS, Longlife, Germany), the mesial roots were selected, and the distal roots were discarded. The selected roots had two separate root canals, foramina, orifices, and closed apices with a curvature of <10° according to Schneider’s method.^[19] Roots with internal and external resorption, cracks, fractures, and caries were excluded. The root surfaces were cleaned with an ultrasonic scaler (Woodpecker UDS-K, China) and polishing brushes, then disinfected in a 5.25% sodium hypochlorite liquid for an hour, and stored in physiological serum.

According to digital radiographs (Schick CDR Dicom) with standardized parameters (70 kVp, 0.4 s), the root lengths were equalized using a diamond disk (DFS, Longlife, Germany). The teeth were positioned in an experimental setup based on the Myers and Montgomery method.^[7] The roots were placed under rubber caps of prepared flasks, and vials were placed inside for fixation. A 27-gauge needle (Ultradent, South Jordan, UT, USA) maintained internal and external pressure.^[20] Vials were weighed with a digital scale (BEL Engineering, Italy) accurate to 0.0001 g before collecting debris.

The samples were randomly divided into three groups (n = 23):

Group 1 (Denco Blue): Canals were instrumented with Denco Super Files ΙΙΙ Blue (Shenzhen Denco Medical Co, China) using the crown-down procedure (Sx, S1, S2, F1, F2, F3). After three in-and-out motions for each file, 3 mL of 5.25% sodium hypochlorite solution was used for irrigation with a 27-gauge needle syringe (Tulsa ok Dentsply, Tulsa Dental)

For all groups, preparation was conducted 1 mm shorter than the canal length. Canal patency was checked with a #15 file (MANI, Japan) to ensure a #20 file did not pass through the apical foramen. The procedures were performed by an expert endodontist.

Debris removed from the apex was transferred into vials. After rinsing the tooth with 1 mL of distilled water, the flask with vials was placed in a shaker incubator (AR.81, Pars Azma, Tehran, Iran) for 5 days at 55°C until the water evaporated. Vials were weighed twice blindly using a digital scale, and the average weight was considered the vial’s final weight with debris. The final weight was subtracted from the initial vial weight to determine the exact weight of the remaining debris [Figures 1 and 2].

The hand file group had the highest mean (SD) debris extrusion weight with 0.21 g (0.03), followed by the Denco Gold group with 0.10 g (0.31). The Denco Blue group had the lowest mean (SD) debris extrusion weight at 0.08 g (0.27). A significant difference was observed between the rotary file groups and the hand file group in terms of debris extrusion weight (P < 0.001), but no significant difference was observed between the rotary file groups (P > 0.159) [Table 1].

Instrumentation should be carried out to minimize debris extrusion into the periapical area. It is suggested that the lower weight of debris extrusion from the apex may be associated with better treatment outcomes.^[18] Although apical debris extrusion occurred in all groups, the weight of apical debris extrusion in rotary files was less than in the hand file group. Ferraz et al. determined the weight of apical debris extrusion by comparing two hand file techniques and three rotary techniques, indicating that the rotary file technique, particularly the Profile system, caused less debris extrusion than the hand file techniques.^[18] A lower amount of debris extrusion in the rotary file technique compared to the hand file technique is probably due to a higher taper of rotary files, which makes the root canal more flared, enhancing the effectiveness of the irrigating solution in removing debris from the root canal.^[20]

In contrast, a study comparing debris extrusion weight during root canal preparation among hand file techniques, Reciproc, and NiTi rotary files found the lowest weight of debris extrusion in hand file systems.^[16] Additionally, another study reported no significant difference between the rotary system and hand file techniques.^[15] As such, no definitive conclusion can be drawn, and more studies are needed to clarify these findings.

The present study indicated that the two rotary systems were similar regarding the weight of apical debris extrusion. This similarity might be related to the fact that these files were alike in terms of shape, cross-section, tip design, pitch distance, cutting angle, rake angle, and clearance surface; therefore, the heat surface treatment was the only difference that did not affect the apical debris extrusion. Other studies disclosed that all file systems were associated with apical debris extrusion, with various findings for each system in this regard [Table 2]. While a few studies showed no differences among the endodontic files,^[3,16] most of them indicated differences.^{[18,16,21,22]}

For instance, Koçak et al. compared the extrusion of debris during root canal preparation in four groups of rotary files, namely ProTaper F2, Self-Adjusting File, Reciproc, and Revo-S, and reported no differences regarding the amount of debris extrusion.^[3] In contrast, a study examining the weight of debris extrusion in four rotary systems indicated that the Reciproc and Hyflex systems extruded more and less debris, respectively, with no difference reported among the ProTaper Universal and Neolix systems.^[21] Another study found that the ProTaper Universal system extruded more debris than other systems, including ProTaper Next, WaveOne, and Reciproc.^[15] Additionally, a study presented that TruNatomy files were associated with less debris extrusion than ProTaper Next, and two other studies showed ProTaper Universal associated with less debris extrusion than SafeSiders and K-Flexofiles.^[7,9]

This heterogeneity among the findings might be attributed to the differences in file design, including the type of tip, cross-sectional design, rake angle, cutting angle, or techniques used for preparation (crown down or single length). Reciprocation files and Self-Adjusting Files each have a particular design. Some study design factors, including sample sizes, tooth types, canal shapes, irrigation solutions and methods, and operator experience, also varied among the studies, contributing to the high heterogeneity. In this study, the Myers and Montgomery evaluation was used as it was easy to conduct and suited the experimental setting. However, other evaluations, such as the assessment of colony-forming units and the use of quantitative polymerase chain reaction, should be employed in future studies to obtain more comprehensive results.^[23]

The study highlights the potential advantages of Denco rotary files in reducing extruded debris weight compared to hand files but acknowledges several limitations. These include a small sample size, a narrow focus on extruded debris weight alone, controlled laboratory conditions not reflecting clinical environments, comparison only with hand files, and lack of long-term clinical outcome evaluation. For more comprehensive and reliable conclusions, further research with larger samples, diverse performance metrics, real-world settings, and broader instrumentation comparisons is needed.

The different systems all exhibited varying amounts of apical debris extrusion. The Denco Blue and Denco Gold rotary files were associated with less apical debris extrusion compared to hand files, and these rotary systems were similar in this aspect.