Influence of endodontic access cavity design on root canal localization in mandibular first premolars using microscope and ultrasonic tips

Abstract

To identify the presence of a second canal in permanent mandibular first premolars treated on using conservative (CAC) and traditional endodontic access (TAC), aided by an operating microscope (OM) and ultrasonic tips. Seventy-two mandibular premolars were divided into two groups (n = 36): Group CAC and Group TAC. Canal localization was conducted initially without the OM, then with the OM, and finally with both the OM and ultrasonic tips. Statistical analyses included Cochran’s Q-tests and binomial tests (α = 5%). There was no significant difference in second canal detection between CAC and TAC, whether assessed without magnification, with OM assistance, or with both OM and ultrasonic tips. Detection accuracy for the second canal significantly improved across the evaluation stages for both conservative and traditional access cavity groups, with Cochran’s tests demonstrating a significant increase (CAC: p = 0.002, TAC: p = 0.049). There was no significant difference in second canal detection between CAC and TAC, whether assessed without magnification, with OM assistance, or with both OM and ultrasonic tips. Endodontists can choose the approach they prefer or that is most suitable for the clinical context, knowing that both techniques offer high reliability in detecting additional canals.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-026-43652-z.

Introduction

A critical step in endodontic treatment involves cleaning and shaping the root canal system (RCS) to remove necrotic or vital pulp contents and their by-products¹. This is achieved using mechanical instruments and antimicrobial irrigation solutions². Understanding RCS anatomy and its variability is essential for successful treatment, enabling precise localization and access to all canals³. Undetected canals can retain microorganisms, compromise disinfection effectiveness and resulting in complications such as periapical disease^4–6.

Periapical radiographs are commonly used to assess tooth and canal morphology, though they have limitations in visualizing the RCS’s complex three-dimensional anatomy⁷. Cone-beam computed tomography (CBCT) offers an advanced three-dimensional image, improving canal identification and access cavity planning compared to conventional radiography, eliminating image overlap⁸.

Proper coronal access is fundamental in endodontics, facilitating effective root canal cleaning and shaping, enhancing visibility and precision, reducing complications, and improving disinfection outcomes⁹. Conservative endodontic access cavities (CACs) are designed to preserve dentin and increase post-treatment fracture resistance, while traditional access cavities (TACs) may aid in locating more canals without magnification^10–12.

Mandibular first premolars exhibit a challenging anatomical diversity, already documented by micro-computed tomography studies^13,14. While a single root is predominantly observed (97–100%), their internal canal configurations vary widely; Vertucci’s type I is most common (36–66%), accompanied by prevalent C-shaped and type V canals^13,15. A significant proportion also features a second, often lingual canal, with global prevalence ranging from 12 to 32.7%, and an incidence of 33.3% noted in some studies^13,15–17. The canal orifice is typically oval (34–58%), though circular shapes are also common, and C-shaped canals (14–26%), often associated with radicular grooves, add to this complexity. Further ramifications include accessory canals (36–73%) and apical deltas (4–14%). This anatomical variability, driven by ethnicity, race, and gender^{13–15,18,19}. Such intricate anatomies, particularly complex apical bifurcations difficult to visualize even with magnification, so advanced diagnostic tools like CBCT are necessary for precise detection and treatment planning prior to coronal access^20,21

The operating microscope (OM) plays a critical role in detecting canals in complex anatomies, enhancing visualization and illumination to identify and treat additional canals and calcifications¹⁶. Its effectiveness improves treatment outcomes, particularly in challenging cases and retreatments²².

This study aimed to compare the efficacy of detecting lingual canals in mandibular first premolars, using either conservative or traditional endodontic access cavity designs. This detection was evaluated through a three-stage process: initial visual inspection, followed by examination with an operating microscope, and finally incorporating both the microscope and ultrasonic tips.

Undetected lingual canals in mandibular first premolars pose a significant clinical challenge, making their identification essential for successful endodontic outcomes. Building on evidence that magnification enhances canal detection, this study specifically investigated how conservative versus traditional access designs, when paired with operating microscopes and ultrasound, influence this detection.

The null hypothesis asserts that (i) endodontic access cavity type does not influence lingual canal localization and (ii) The supplementary use of the operating microscope and ultrasonic tips yields no impact.

Materials and methods

This study received approval from the local ethics committee for the use of biological materials. It was conducted following the PRILE 2021 guidelines²³ for laboratory studies (Fig. 1) and the corresponding checklist is present as Table S1 of supplementary file. Sample size calculation was based on the study’s sensitivity (0.2), specificity (1.0), and a global prevalence of 23.8% for lingual canals (LC) in mandibular first premolars¹⁷. To achieve a precision of 20% around this prevalence and a 95% confidence interval, a minimum of 65 mandibular first premolars was determined necessary. To maintain equal group sizes, each group was designed to have at least 33 first premolars.

Fig. 1.

PRILE flowchart.

Sample selection and preparation

Eighty-five extracted human mandibular permanent first premolars, selected unrelated to this study, were initially screened for the presence of a LC. Inclusion criteria required teeth with fully formed apices and no previous endodontic treatment or root caries. Exclusion criteria included teeth with resorption, cracks or fractures, prosthetic crowns, significant occlusal restorations, and those with three radiographically visible root canals.

The teeth were cleaned using ultrasonic tips (Schuster Equipamentos Odontológicos, Santa Maria, Brazil) to remove calculus and were polished with a Robson brush (American Burs, Florianópolis, Brazil), pumice (SSWhite, São Paulo, Brazil), and water. They were stored in saline until use in the study. Periapical radiographs and CBCT scans were carried out to verify inclusion and exclusion criteria. Eligible teeth were initially divided into two groups based on the presence of one or two canals, then subjected to CBCT exams (J Morita, Kyoto, Japan) to confirm canal numbers. A radiologist and endodontist analyzed axial, sagittal, and coronal CBCT images to determine canal count. Seventy-two extracted premolars were randomly selected according to criteria and divided into two groups of 36 specimens each.

Group composition and clinical set-up

In each group of 36 mandibular extracted premolars, 24 (66.7%) had a single canal, and 12 (33.3%) had two canals, as confirmed by CBCT. Teeth were anonymized and examined by another operator unaware of group assignments, specialist in Endodontics with twenty years of experience and blinded from the initial sample’s radiographs or CBCT exams. Each tooth was placed in a dental mannequin (Pronew Odonto patient simulator, São Gonçalo, Brazil) for coronal access under direct vision. The dentist was ergonomically positioned, with the mannequin head at elbow height, allowing movement from the 9 o’clock to the noon position.

Endodontic access preparation

• CAC group: Teeth were accessed centrally on the occlusal surface near the central fossa using long-shaft drills 1012 (KG Sorensen, São Paulo, Brazil). Minimal removal of mesio-distal, buccolingual, and circumferential pericervical dentin was performed. The cavity followed the pulp chamber floor design with a reduced extension, and the roof was partially removed with an inactive tip drill 3080 (KG Sorensen) for file insertion (Fig. 2A).

• TAC group: Similar initial access was made using long-shank drills 1014 (KG Sorensen). The diamond spherical bur was initially angled at approximately 45 degrees relative to the palatal surface. After penetrating about two active bur tip lengths, the angulation was adjusted to be parallel to the long axis of the tooth. The cavity’s outer contour was conical-ovoid. The pulp chamber roof was completely removed with diamond burs 3080/3081 (KG Sorensen) for straighter canal access, with cavity enlargement based on canal location (Fig. 2B).

Fig. 2.

(A) Representative image of the conservative access cavity (CAC). (B) Representative image of the traditional access cavity (TAC).

Assessment of lingual canal position

After preparing the access cavity, it was flushed with 2,5% NaOCl using a 5 mL syringe and a NaviTip 30-G needle (Ultradent Products, South Jordan, USA) and aspirated with a silicone cannula (Capillary Tips/Ultradent®, South Jordan, USA) to enhance visibility of the pulp chamber floor. Clinical evaluation involved placing the tooth in a dental mannequim (Pronew Odonto, São Gonçalo, Rio de Janeiro, Brazil), simulating a real patient scenario. Teeth underwent a three-stage examination, starting with locating the main (buccal) canal. If an explorer’s tip caught between the main canal and the lingual tooth wall, indicating a LC, a 21 mm #08 file (C-Pilot, VDW, Munich, Germany) was used to confirm the canal’s presence (Fig. 3). Regardless of initial findings, all teeth proceeded through each evaluation stage.

Fig. 3.

Traditional and conservative access cavities in mandibular premolar radiographs showing the presence/absence of lingual canal (LC).

Evaluation stages

• Stage 1: Clinical inspection of the main canal entrance and lingual area used indirect vision with a flat clinical mirror (Hu Friedy, Chicago, IL) and endodontic probes DG 16 (Hu Friedy) and C-Pilot files #08 (VDW) without magnification.

• Stage 2: Inspection repeated under 16 × magnification with a surgical microscope (Alliance Comercial São Carlos LTDA, São Carlos, Brazil) using the same tools as in Stage 1.

• Stage 3: Further inspection with the same setup as Stage 2, complemented by E2D ultrasonic diamond tips (Helse Tecnologia Odontológica, Santa Rosa de Viterbo, Brazil) coupled with an ultrasonic unit (Schuster Equipamentos Odontológicos) to selectively abrade excess dentin and calcifications. Saline irrigation followed to remove debris and improve canal visualization.

Statistical analysis

The effectiveness of each evaluation phase in detecting LCs was measured in terms of sensitivity, specificity, false-negative error rate, false-positive error rate, and accuracy (area under the ROC curve).

Cochran’s Q tests assessed differences in detection confidence across evaluation stages, independent of access cavity type. Binomial tests checked for differences in LC detection accuracy between CAC and TAC groups at each stage. Statistical analyses were conducted using SPSS 23 (SPSS Inc., Chicago, IL, USA) with a 5% significance level.

Results

Among the 36 premolars per group, 24 (66.7%) had one canal and 12 (33.3%) had two, as confirmed by CBCT. In the CAC group, sensitivity was 0.25 in Stage 1 (no magnification), 0.42 in Stage 2 (with OM), and 0.92 in Stage 3 (OM and ultrasonic tips). In the TAC group, sensitivity values were 0.33, 0.50, and 0.67 across these stages (Table 1).

Table 1.

Diagnostic indicators in the detection of lingual canals in mandibular premolars, with conservative and traditional endodontic access cavities, according to the stage of diagnosis.

Diagnostic indicator	Stage
	1 No magnification		2 Operating microscope		3 Operating microscope/ultrasonic tips
	CAC	TAC	CAC	TAC	CAC	TAC
Sensitivity	0,25	0,33	0,42	0,50	0,92	0,67
Specificity	1,00	1,00	1,00	1,00	1,00	1,00
False negative error rate	0,75	0,67	0,58	0,50	0,08	0,33
False positive error rate	0,00	0,00	0,00	0,00	0,00	0,00
Accuracy	0,63	0,67	0,71	0,75	0,96	0,83

CAC = conservative endodontic access cavity; TAC = traditional endodontic access cavity.

Specificity was 1.00 across all stages, indicating perfect single-canal detection. In the CAC group, accuracy was 0.63 in Stage 1 (poor), 0.71 in Stage 2 (normal), and 0.96 in Stage 3 (excellent). In the TAC group, accuracy ranged from 0.67 (poor) to 0.75 (moderate) and 0.83 (good) across stages.

Cochran’s tests demonstrated a significant increase in detection accuracy across stages for both groups (CAC: p = 0.002, TAC: p = 0.049, Table 2). Binomial tests indicated no significant difference in detection accuracy between access cavity types across all stages (Table 2).

Table 2.

Absolute and relative frequencies (%) of teeth with assertive detection of the lingual canal in mandibular premolars, with conservative and traditional endodontic access cavities, according to stage of diagnosis.

Stage	Access cavity		Binomial test
	CAC	TAC
1 Without magnification	3 (25,0%)	4 (33,3%)	p = 0,653
2 Operating microscope	(3) + 2 (41,7%)	(4) + 2 (50,0%)	p = 0,682
3 Operating microscope/ultrasonic tips	(5) + 6 (91,7%)	(6) + 2 (66,7%)	p = 0,132
Cohran’s Q test	p = 0,002	p = 0,049	–

* = p-values > 0.05 by binomial test. CAC = conservative endodontic access cavity; TAC = traditional endodontic access cavity. Numbers between ( ) represent canals already located in the previous step and numbers + represent canals located in the step in question.

Discussion

This study aimed to identify the presence of a second canal in permanent mandibular first premolars treated with conservative and traditional endodontic methods, utilizing a surgical microscope and ultrasonic tips. The findings indicated no significant differences in identifying the second canal between the two approaches, leading to the acceptance of the first null hypothesis.

It is important to note, however, that the specific level of root canal bifurcation was not a parameter investigated in this study. This research focused solely on the detection of the presence of a second canal, without evaluating the exact point or depth of its bifurcation. Consequently, insights into how the bifurcation level might influence detection rates or access cavity effectiveness are beyond the scope of these findings.

Clinical assessments using indirect vision without magnification showed poor accuracy in detecting the second canal in the CAC and TAC groups. The application of the OM significantly improved accuracy in both groups. Furthermore, the incorporation of ultrasonic tips enhanced accuracy to an excellent level in the CAC group and to a good level in the TAC group, thereby rejecting the second null hypothesis and corroborating findings from previous studies^24,25.

For both methods, visualization of the canals without magnification proved inferior to using the microscope, which enhances the visibility of canal details²⁴. When OM was utilized in conjunction with ultrasonic tips, canal visualization improved further²⁶. This enhancement is attributed to ultrasonic tips’ grinding action, which boosts visibility and facilitates access to difficult areas within the root canal system. This combination is particularly advantageous for CACs, where a minimally invasive approach necessitates instruments adept at navigating narrow and complex anatomical regions²⁵.

Despite an initial observation where TAC numerically detected more canals in stages 1 and 2 of the study, there were no statistically significant differences in canal detection between CAC and TAC types at any stage (Table 2). Additionally, in stage 3, when the OM and ultrasonic tips were combined, the equivalence between the groups was even more evident, with no differences emerging. This demonstrates that, regardless of the access type, the efficacy in canal localization was similar, especially with the aid of advanced technologies^11,12,27.

The root canal system of mandibular first premolars is notably complex and anatomically diverse, a characteristic extensively documented by micro-computed tomography studies. In this study, the incidence of mandibular first premolars with two root canals was found to be 33.3%, a finding that aligns with the observations of Lu et al.¹⁶, but represents a higher prevalence than that reported by Velmurugan and Sandhya²⁸. This variation is significant, as the global prevalence for a second canal, often lingual, ranges between 12% and 32.7%, highlighting the importance of advanced tools for detecting all complex anatomies, such as C-shaped configurations and Type V canals, which render endodontic treatment challenging^13,15

The mannequin was used to provide stabilization for the teeth during canal preparation, facilitating the handling of instruments, and ensuring precise technique execution. This stability simulates real clinical conditions, allowing for the standardization of procedures and minimizing execution errors²⁹.

Diagnosing variations in the root canal is vital; CBCT can assist in confirming the presence of LCs prior to coronal access^20,21. Clinically, the combination of OM and ultrasonic tips represents a non-invasive approach that provides a magnified view, aiding in the detection of canals in teeth with conservative access³⁰. While OM offers excellent illumination and magnification, it may not effectively visualize apical bifurcations.

The results of this study underscore the significance of employing tools such as OM and ultrasonic tips for detecting the second canal. OM enhances the visualization of the pulp chamber floor anatomy, facilitating the identification and negotiation of root canals^25,27. A marked increase in sensitivity for identifying the second canal was observed with these technologies, particularly when used together. Sensitivity increased significantly throughout the assessment phases—from indirect views without magnification to utilizing OM combined with ultrasonic tips, which allow for more efficient cleaning and thorough examination of the pulp chamber floor, thereby increasing the likelihood of detecting additional canals²⁴.

Although no statistically significant difference was found in canal localization between access types, a commendable “excellent” accuracy was achieved in the CAC group, compared to “good” in the TAC group at step 3 (during selective grinding with ultrasonic tips and OM). This emphasizes the essential role of microscopy and ultrasonic tips when performing conservative accesses^11,27.

The study is an ex vivo methodology, using extracted teeth, presents a key limitation as it may not fully replicate the dynamic of the clinical environment. Factors like patient cooperation, saliva, and hemorrhage were not simulated, potentially influencing the generalizability of the findings regarding operator performance and detection rates. Consequently, caution is advised when directly extrapolating these results to clinical practice, and future in vivo studies would be beneficial.

In conclusion, this study provides valuable insights into the impact of endodontic access cavity design and the use of OM and ultrasonic tips in detecting the second canal in mandibular premolars. The findings suggest that both access methods are equally effective in identifying the second canal, with adjunctive tools potentially enhancing diagnostic accuracy. These results hold significant implications for clinical practice, offering professionals effective strategies for endodontic treatment of mandibular premolars.

Overall, the localization and preparation of the second canal in bifurcated mandibular first premolars necessitate careful consideration of complex anatomical features. Employing advanced imaging techniques alongside improved visualization methods can substantially aid in locating the second canal in mandibular premolars with conservative access. Importantly, comparable numbers of canals can be discovered using conservative cavity access as with traditional cavity access.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

Conception and design were performed by M.F.G. and M.F. Material preparation, data collection, and analysis were conducted by M.F.G., A.G.L., R.F.A., É.B.M., M.P.C.F., L.P.F., W.M.N., M.F.V.M.A., and M.F. Manuscript writing and revision were carried out by A.G.L., M.F.V.M.A., M.F., and V.M. All authors reviewed and approved the final version of the manuscript.

Funding

Open access funding provided by Dr. DY Patil Vidyapeeth, Pune (Deemed to be University). This study was supported by grants from Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) (E-26/200.184/2023) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) (446114/2024-3), Brazilian Governmental Institutions.

Data availability

The data supporting the findings of this study are available upon reasonable request from the corresponding author.

Declarations

Competing interests

The authors declare no competing interests.

Ethics approval and consent to participate

The study protocol was approved by the Comitê de Ética em Pesquisa da São Leopoldo Mandic Research Institute (CAAE: 59043622.5.0000.5374). All methods were performed in accordance with relevant guidelines and regulations. Informed consent was obtained from all subjects donating extracted teeth for research use.