The permanent mandibular first molar typically possesses two roots and three canals (two mesial and one distal canal). However, in certain cases, a third root may be identified on the distolingual (DL) aspect of the tooth (1, 2). This root variation is influenced by ethnicity, with a high prevalence rate ranging from 5% to 40% in Mongolia populations, whereas the frequency is generally below 5% in Caucasian populations and less than 3% in African populations (3–5). In endodontic practice, the presence of a DL root/canal poses clinical challenges (2). Conventional periapical radiographs often fail to detect the DL root due to anatomical superimposition of distal roots and surrounding alveolar bone, leading to missed diagnosis (6). This radiographic limitation, combined with the DL root's characteristic morphological variability (typically smaller diameter and distinct buccal curvature) (2, 7), creates technical difficulties during instrumentation. Such anatomical complexity predisposes to iatrogenic errors including ledging, zipping, apical/strip perforations, and instrument separation (8).

Over the past two decades, the endodontic field has witnessed the introduction of numerous nickel-titanium (NiTi) file systems, many of which have gained widespread clinical adoption (9–11) Manufacturers promote these systems as offering enhanced shaping efficiency and reduced risk of iatrogenic complications, attributing these advantages to their advanced design features, improved alloy compositions, and optimized motion kinematics (10, 11). However, substantial clinical evidence underscores that root canal anatomy remains the most critical factor influencing treatment outcomes. This reality necessitates that clinicians possess an in-depth understanding of both the operational characteristics and limitations of their selected NiTi system. Prudent instrument selection should be guided by comprehensive preoperative evaluation of root canal morphology, coupled with careful consideration of the specific instrument's design parameters, to ensure optimal procedural safety and effectiveness. The use of fewer instruments can simplify root canal preparation. Single-file NiTi systems thus provide distinct advantages, including reduced instrumentation time and minimized cross-contamination risk (12). The OneShape (OS; MicroMega, Besançon, France) system utilizes a conventional austenitic 55-NiTi alloy and consists of a single file (size 25/6%) with a passive tip and a variable cross-sectional design. This design includes a triangular cross-section with three sharp cutting edges in the apical and middle portions, which progressively transition to two cutting edges near the shaft (12). In contrast, the WaveOne Gold (WOG; Dentsply Maillefer, Ballaigues, Switzerland) system retains the reciprocating motion of its predecessor (WaveOne) but incorporates modified dimensions and geometry (13). The WOG system has an off-center parallelogram cross-section with two cutting edges and consists of four instruments: 21/6% (small), 25/7% (primary), 35/6% (medium) and 45/5% (large). This differs from the original WaveOne system, which employed a convex triangular cross-section with larger tapers and dimensions (14, 15). Additionally, WOG files undergo a proprietary gold heat treatment—a thermal process involving controlled heating and slow cooling—to enhance their flexibility and cyclic fatigue resistance. Kinematically, the WOG system operates with a 170 ° counterclockwise (cutting) motion followed by a 50 ° clockwise (release) motion, incorporating a reverse helix design and a semi-active guiding tip (16, 17). ProTaper Universal (PTU; Dentsply Maillefer, Switzerland) represents one of the most widely used rotary sequential instrumentation systems, characterized by its convex triangular cross-sectional design and progressive taper configuration. The shaping performance of PTU instruments has been frequently employed as a reference standard when evaluating newer generation NiTi systems (18, 19). To our knowledge, this study represents the first systematic comparison of these three NiTi rotary systems in the challenging anatomical context of DL canals in three-rooted mandibular first molars (3RM1s).

Three-dimensional (3D) printing technology has emerged as a valuable tool in endodontic research, enabling rapid and precise fabrication of anatomical replicas (20, 21). This innovative approach involves reconstructing digital tooth models from micro-computed tomography (micro-CT) or cone-beam CT (CBCT) scans of natural teeth, which are then exported in standard tessellation language (STL) format for 3D printing. The resulting replicas, frequently produced using advanced photo-polymerizable resins, provide numerous advantages over extracted teeth specimens, including controlled anatomical parameters, elimination of cross-infection risks, circumvention of ethical procurement challenges, and establishment of repeatable experimental conditions (22–25). Within this methodological framework, the present micro-CT study was designed to conduct a comparative evaluation of three contemporary NiTi instrumentation systems (OS, WOG, and PTU) specifically focusing on their shaping efficacy in the DL canals of 3RM1 replicas, thereby addressing critical gaps in our understanding of instrument performance in complex root canal anatomies. The null hypothesis was that there would be no significant differences among the three NiTi systems in shaping ability when preparing DL canals in standardized 3D-printed replicas of 3RM1s.

In this investigation, two permanent mandibular first molars with DL roots, representing varying degrees of curvature, were selected from a collected sample pool. Corresponding resin replicas were then fabricated using 3D printing technology. Pre-operative geometric measurements indicated no significant differences among three experimental groups (all p > 0.05) across each set of tooth replicas, indicating that the replicas possessed a consistent canal morphology. Traditional ex vivo studies evaluating root canal instrumentation have predominantly utilized extracted human teeth. However, the inherent anatomical variability of natural teeth poses significant challenges for the establishment of balanced experimental groups. Furthermore, the DL root typically exhibits severe curvature and reduced dimensions, making it particularly susceptible to fracture during the extraction process. Consequently, procuring sufficient quantities of intact DL root specimens for instrumentation studies remains problematic. Our results corroborate previous studies (22–25), demonstrating that 3D-printed tooth replicas offer advantages in terms of consistency, reproducibility, and repeatability, making them an excellent model for assessing the performance of various instruments. These replicas effectively simulate the clinical conditions and challenges involved in preparing a DL canal in 3RM1s.

The DL root typically presents with a narrow, conical canal and a pronounced buccal curvature (2, 7). This complex anatomy increases the risk of procedural errors during instrumentation. Recent advancements in NiTi file systems—particularly single-file systems—have revolutionized root canal preparation by combining flexibility and strength to optimize shaping in curved canals (12–14). These innovations address two key objectives: (1) preservation of the original canal anatomy and (2) efficient dentin removal, thereby reducing procedural time and minimizing instrument fatigue. Moreover, enhancements in alloy composition and manufacturing techniques have significantly reduced the incidence of unexpected NiTi instrument separation, a historically prevalent concern. Contemporary NiTi systems such as OS, WOG, and PTU have demonstrated satisfactory efficacy in preparing curved canals (12–16, 27). However, their geometric designs, alloy enhancements, and kinematic properties vary significantly. To date, no studies have specifically evaluated their performance in addressing the unique challenges posed by DL canal anatomy.

The present study found no occurrences of instrument separation or strip perforation across all experimental groups. However, the significantly higher incidence of procedural errors associated with the PTU system indicates that the three NiTi systems cannot be regarded as equally safe for DL canal preparation. It is well established that excessive use of rigid files in curved canals may lead to undesirable canal straightening. Instrument flexibility is determined by multiple factors such as instrument geometry, alloy composition, thermomechanical processing, and surface treatments (9). Comparative analysis revealed that the WOG system produced minimal canal straightening (Table 2). This superior performance can be attributed to its unique design features: an off-center parallelogram cross-section with dual cutting edges, a reduced 7% taper (primary), and a specialized reciprocating motion that reduces torsional stress. Additionally, the manufacturing process incorporates gold heat treatment—a precisely controlled thermal protocol involving gradual heating and cooling cycles—which substantially improves the instrument's mechanical properties. These synergistic design innovations collectively enable superior maintenance of original canal curvature during instrumentation (14–17). Conversely, the PTU system employs a progressively increasing taper design, with the final instrument (F2) reaching an 8% taper (18, 19). When combined with its conventional NiTi alloy composition and larger cross-sectional core area (featuring a convex triangular configuration), this system exhibits markedly increased rigidity. Additionally, the extended instrumentation duration required by PTU's multi-file protocol resulted in significantly greater canal straightening compared to the single-file systems. The OS system, as a first-generation single-file instrument, maintains a constant 6% taper and incorporates an innovative cross-sectional design (a triangle-shaped symmetrical three-cutting-edge in the tip area, a S-shaped symmetrical two-cutting-edge at the coronal area, and an asymmetrical progressively changing in the middle) (12, 28, 29). While its reduced core diameter and optimized geometry provide enhanced flexibility—resulting in less canal deviation than PTU—its conventional austenite alloy composition limited its performance relative to the advanced WOG system.

Table 3 reveals that instrumentation with the PTU system produced significantly greater resin removal compared to the single-file systems, creating the largest post-instrumentation canal dimensions. This outcome may be attributed to PTU's progressively increasing taper design and the crown-down preparation. While coronal flaring facilitates apical access by eliminating cervical constrictions and reduces taper lock and frictional forces (30), it inevitably alters native canal morphology, particularly in cervical regions. Such morphological changes warrant careful consideration, especially when addressing the challenging anatomy of DL canals characterized by their narrow dimensions and complex curvature. In contrast, the OS and WOG systems employ conservative taper designs (6% and 7%, respectively) without requiring coronal flaring. When maintaining identical apical preparation size (25#), the longitudinal cross-sectional area directly correlates with taper size, explaining why PTU's 25/8% (F2) instrument produced the greatest canal enlargement. The single-file systems demonstrated several clinical advantages, including reduced preparation time, enhanced flexibility, and maintained structural strength. Notably, WOG's gold heat-treated alloy exhibits superior superelastic properties that reduces cutting forces and frictional resistance while improving curvature negotiation (14–17), resulting in less aggressive dentin cutting, prevention of procedural errors, and consequently the least resin removal among all tested systems.

The comparative analysis of canal curvature cross-sections revealed significant variations among the instrumentation systems, as quantitatively demonstrated in Table 4 and visually illustrated in Figures 2, 4. Notably, the WOG system maintained optimal anatomical conservation with minimal deviation from the original canal path and least MCT at the furcation region. In contrast, the PTU group exhibited the most pronounced structural alterations. Analysis of resin removal patterns identified three primary sites of predilection: (1) the lingual aspect of the DL canal orifice, (2) the inner (buccal) wall of the curvature at the furcation region, and (3) the outer (lingual) wall of the apical canal curvature. This characteristic spatial distribution revealed the non-uniform resin/dentin removal during instrumentation, with PTU specimens exhibiting the most significant procedural errors. Previous studies have demonstrated that excessive dentin removal during root canal preparation elevates the risk of root fracture (31, 32). The DL root's pronounced buccolingual curvature and narrow dimensions increase its susceptibility to iatrogenic complications such as strip perforation and instrument fracture during endodontic procedures. Table 3 reveals that the location of MCT—a critical indicator of perforation risk—was consistently located 2–3 mm below the furcation. This region frequently corresponds to areas demonstrating both the most extensive resin removal and the thinnest RWT. While this location parallels the classical “danger zone” observed in mesial roots of mandibular molars (33), the DL root exhibits greater anatomical variability (1, 2, 34). Moreover, the morphology of the distal furcation between the two distal roots is another influence factor (7). A short root trunk combined with a pronounced divergence angle between the DB and DL roots confers a predisposition to perforation.

Recent biomechanical evidence demonstrated that the pericervical region is particularly susceptible to fracture under functional and parafunctional occlusal forces (35). Our findings indicate that this vulnerability is exacerbated by the PTU system, which utilizes an aggressive crown-down approach and large-taper design that remove excessive cervical dentin/resin. In contrast, the WOG system, representing the latest generation of single-file instrumentation, demonstrates superior flexibility. Its gold heat treatment process optimizes cutting efficiency while reducing the risk of over-preparation. Furthermore, WOG's reciprocating motion—engaging dentin counterclockwise before disengaging clockwise—minimizes torsional stress and prevents taper lock, thereby enhancing safety. These attributes allow WOG to achieve an optimal balance between efficacy and preservation, making it particularly suitable for DL canal preparation. While the OS system demonstrates intermediate performance between WOG and PTU, its smaller taper (6%) and reduced cross-sectional core area improve flexibility. However, as a first-generation single-file system made from conventional NiTi alloy, OS exhibits inferior shaping ability compared to WOG. Moreover, OS operates with continuous 360 ° rotation at 400 rpm, providing greater cutting efficiency, which may lead to more aggressive resin removal than the reciprocating system. Therefore, in cases of pronounced DL root curvature (e.g., Specimen 2), meticulous technique remains essential to minimize dentin/resin removal in the cervical and furcation regions. Overall, the significant differences observed in all evaluated parameters led to rejection of the null hypothesis. The single-file systems (WOG and OS) demonstrated superior shaping ability compared to the multi-file PTU system in the DL canals.

Clinically, these findings provide valuable guidance for managing 3RM1s—an anatomical variant with marked ethnic predisposition and a frequent source of diagnostic oversight or iatrogenic complications (2, 6). The superior conservation of canal anatomy and reduced risk of excessive dentin removal achieved with WOG, especially in severely curved DL canals, translates to a lower risk of procedural errors such as furcation strip perforation and excessive pericervical dentin removal, both of which are established predictors of vertical root fracture and compromised long-term tooth survival (31, 32, 35). By minimizing structural weakening, single-file systems like WOG and OS may enhance post-treatment prognosis and reduce the need for subsequent restorative or surgical interventions. Additionally, their streamlined workflow—requiring fewer instruments—offers practical benefits in clinical practice, including reduced preparation time and lower risk of cross-contamination due to single-use protocols, while maintaining comparable or superior shaping outcomes compared with multi-file systems (12, 25). In contrast, the more aggressive material removal associated with PTU underscores the potential drawbacks of traditional progressive-taper approaches in narrow, highly curved canals, where over-preparation could precipitate early tooth loss despite successful obturation. These results reinforce the need for individualized instrument selection informed by preoperative imaging (e.g., CBCT) to identify complex anatomies, particularly in populations with elevated prevalence of radix entomolaris. Ultimately, the adoption of advanced heat-treated reciprocating single-file systems (e.g., WOG) may contribute to higher treatment success rates and improved patient-centered outcomes in challenging endodontic scenarios, warranting further validation through randomized clinical trials.

This study has several limitations. First, the 3D-printed 3RM1s were based on replicas of only two human specimens. Although these models represent different curvature severities, their restricted sample size may introduce selection bias regarding other anatomical parameters. Second, the root curvatures were approximately confined to the buccolingual plane, whereas natural teeth often exhibit complex 3D curvatures, potentially underestimating the procedural challenges encountered in clinical practice. Third, although 3D-printed replicas offer substantial advantages over extracted teeth, including precise control of anatomical parameters, perfect standardization across experimental groups, elimination of inter-specimen variability, avoidance of ethical and logistical issues associated with tooth collection and storage, reduced risk of cross-contamination, and the ability to produce unlimited identical replicas for repeatable testing, these models remain an ex vivo simulation. Finally, the rigid photopolymer resin used has a Shore D hardness of ∼84, comparable to cured PMMA (typical range for acrylic denture teeth: 80–90), and may not fully replicate the mechanical properties of natural dentin (e.g., elasticity, fracture toughness, or tactile feedback during instrumentation), potentially influencing instrumentation outcomes and procedural errors. Therefore, although 3D-printed replicas effectively overcome key limitations of extracted teeth, particularly the challenges in obtaining sufficient intact DL roots due to their fragility and high anatomical variability, the findings from this standardized model should be interpreted cautiously and validated with natural teeth or in vivo studies before clinical extrapolation.

In conclusion, both single-file systems (WOG and OS) demonstrated satisfactory performance in shaping DL canals of 3RM1s. In contrast, the PTU system consistently exhibited excessive and asymmetrical resin/dentin removal, potentially compromising root structural integrity and increasing the likelihood of iatrogenic complications. Consequently, PTU appears less suitable for DL canal preparation compared to the more conservative single-file systems.