The patient elected to proceed with nonsurgical retreatment, and verbal and written informed consent were obtained.

Visit 1 — Retreatment Access and Instrumentation

Local anesthesia was administered (two cartridges of 2% lidocaine with 1:100,000 epinephrine) and rubber dam isolation was achieved using blockout resin and a bite block. The occlusal surface of the zirconia crown was photographed under the surgical operating microscope (Zumax Medical Co., Ltd., Suzhou, China) prior to access (Figure 2A).

Access was gained through the existing zirconia crown, which remained intact. The Zumax DentSight AR integrated visualization system was used to locate the previously untreated MB1, MB2, and MB3 canals (Figure 2D). Excess gutta-percha was removed from the pulp chamber until the distobuccal and palatal canal orifices were identified, confirming their C-shaped connection. During removal of the previous obturation material from the perforation site, clinical photographs were captured through the surgical operating microscope, documenting the progressive clearance of gutta-percha and sealer remnants from the pulpal floor (Figure 2B). Gates Glidden drills (size 3) were used for coronal gutta-percha removal, and ProTaper F2 rotary files (Dentsply Sirona, Charlotte, NC, USA) were used for middle-third removal. Chloroform in conjunction with size 10 C-files was used to achieve patency in the previously treated canals.

Working lengths were established using a Root ZX electronic apex locator (J. Morita Corp., Kyoto, Japan). All five canals were negotiated and instrumented using Vortex Blue rotary files (Dentsply Sirona). The mesiobuccal canals were enlarged to size 20/.04, while the distobuccal and palatal canals were instrumented to size 30/.04, respectively. Patency was achieved in all canals. An iatrogenic perforation on the pulpal floor was repaired with EndoSequence BC Putty (Brasseler USA, Savannah, GA, USA) and Vitrebond light-cured glass ionomer liner (3M ESPE, St. Paul, MN, USA) (Figure 2C). Irrigation was performed with 6% sodium hypochlorite (NaOCl) and 17% EDTA throughout the procedure. Following cleaning and disinfection, the canals were dried with sterile paper points and dressed with Ultracal XS calcium hydroxide paste (Ultradent Products Inc., South Jordan, UT, USA) as an intracanal medicament. The access cavity was sealed with a sterile sponge and TempIT temporary filling material (Spident Co., Ltd., Incheon, South Korea). Postoperative instructions were provided, and over-the-counter analgesics were recommended as needed.

Visit 2 — Obturation

The patient returned 4 weeks later for obturation. He reported no pain since the first visit, and the sinus tract had resolved. Following the same anesthesia and isolation protocol, the temporary restoration was removed and calcium hydroxide was eliminated from all five canals. Working lengths were reconfirmed using the Root ZX apex locator. Final instrumentation was completed with Vortex Blue rotary files, with the mesiobuccal canals prepared to sizes 25/.04 (MB1 and MB2) and 20/.04 (MB3), and the distobuccal and palatal canals enlarged to sizes 40/.06 and 45/.06, respectively. 

The final irrigation protocol consisted of 6% sodium hypochlorite and 17% EDTA, followed by laser-activated irrigation using an Er,Cr:YSGG laser (2780 nm; Biolase, Irvine, CA, USA) equipped with a radial-firing endodontic tip operated at 1.5 W and 20 Hz without air or water spray. The laser tip was positioned short of working length and activated in a slow coronal withdrawal motion for 30 seconds per cycle.

Following completion of cleaning and shaping, the pulp chamber floor was photographed through the surgical operating microscope using a dental mirror, demonstrating the three mesiobuccal canal orifices (MB1, MB2, and MB3) after instrumentation (Figure 2E). A subsequent photograph captured the same view following obturation, showing gutta-percha  in all three mesiobuccal canals (Figure 2F).

A cone-fit radiograph was obtained to verify master cone adaptation in all five canals (Figure 1C). After drying with sterile paper points, master gutta-percha cones were placed in all canals, with an additional cone placed in the palatal canal, and NeoSEALER EZFlo bioceramic sealer (Avalon Biomed Inc., Bradenton, FL, USA) was placed . Warm vertical condensation was performed using the Elements obturation unit (Kerr Endodontics, Orange, CA, USA) for downpack, followed by backfilling with the same system. The pulp chamber was cleaned of excess sealer with alcohol, and EndoSequence BC Liner (Brasseler USA, Savannah, GA, USA) was placed over the canal orifices as a secondary coronal seal. Clearfil SE Protect primer and bond was applied, and a core buildup was completed using DC Core dual-cure composite (Kuraray Noritake Dental Inc.). A final postoperative periapical radiograph was obtained (Figure 1D).

The patient was provided postoperative instructions and advised to take over-the-counter analgesics as needed. He was referred back to his general dentist for definitive restoration of the access cavity through the existing zirconia crown. At the one-year recall, the patient remained asymptomatic, and clinical and radiographic examination demonstrated periapical healing (Figure 4).

Discussion

This case highlights several clinically significant findings that collectively represent an exceptionally rare anatomical presentation in a maxillary first molar: three mesiobuccal canals (MB1, MB2, and MB3), a C-shaped configuration connecting the distobuccal and palatal canals, and an iatrogenic perforation, all managed successfully through nonsurgical retreatment.

The MB3 canal is among the rarest anatomical variations reported in maxillary molars. In the present case, all three mesiobuccal canals were missed during the initial treatment, which likely contributed to persistent infection and development of a chronic apical abscess with a draining sinus tract. The successful identification and treatment of these canals highlights the importance of advanced imaging and magnification when evaluating previously treated teeth with persistent disease.

Several case reports have documented maxillary first molars containing three mesiobuccal canals and have emphasized the importance of advanced imaging, magnification, and careful exploration of the pulpal floor for identification of these uncommon anatomical variations (Kakkar and Singh, 2012; Ahmad and Al-Jadaa, 2014). However, previously reported cases generally involved otherwise conventional root morphology. In the present case, the presence of three mesiobuccal canals in conjunction with a C-shaped distobuccal-palatal canal configuration created a uniquely complex anatomical presentation.

In addition to supernumerary canals, maxillary molars may exhibit root fusion and C-shaped canal configurations, although these anatomical variations are reported more frequently in maxillary second molars than in first molars (Mashyakhy et al., 2019). Martins et al. (2016) classified five categories of C-shaped maxillary molars based on patterns of root fusion. The Type C configuration, characterized by fusion of the distobuccal and palatal roots, was identified in the present case. Sun et al. (2022) reported successful retreatment of C-shaped maxillary molars and emphasized the importance of CBCT imaging and magnification when managing these unusual anatomical configurations. The irregular canal communications, fins, webs, and isthmuses commonly associated with C-shaped anatomy present significant challenges for cleaning, shaping, and obturation.

The combination of three mesiobuccal canals and a C-shaped distobuccal-palatal configuration in a maxillary first molar has not, to the authors' knowledge, been previously reported. This finding highlights the remarkable anatomical variability that may occur even in teeth generally considered to have well-characterized root canal anatomy and underscores the importance of maintaining a high index of suspicion when evaluating previously treated teeth with persistent disease.

The iatrogenic perforation identified on the pulpal floor was likely created during the initial treatment, possibly during attempts to locate the untreated mesiobuccal canals. Perforations are recognized complications of endodontic treatment that may adversely affect prognosis if not properly managed. In the present case, the perforation was repaired with EndoSequence BC Putty, a premixed bioceramic material with favorable sealing ability, biocompatibility, and bioactive properties. Vitrebond glass ionomer liner was placed as an additional protective barrier. Clauder (2022) noted that bioactive calcium silicate materials have substantially improved the prognosis of perforation repair compared with previously available materials. 

The clinical photographs obtained during removal of the previous obturation material from the perforation site documented the extent of contamination and the progressive debridement of the defect prior to repair.

CBCT imaging was essential for diagnosis and treatment planning in this case. The axial CBCT images clearly demonstrated the three untreated mesiobuccal canals, the C-shaped distobuccal-palatal configuration, the overextended palatal obturation, and the suspected perforation, findings that could not be fully appreciated on conventional two-dimensional radiographs. Current endodontic guidelines support the use of limited-field CBCT in cases involving complex anatomy, previous treatment, and diagnostic uncertainty because of its ability to provide three-dimensional visualization of the root canal system (Sousa Melo et al., 2026). The 2025 AAE/AAOMR Joint Position Statement specifically recognizes the value of CBCT in the evaluation of previously treated teeth with persistent disease, complex anatomy, and retreatment considerations, all of which were present in this case.

In addition to CBCT imaging, image-guided visualization using the Zumax DentSight AR integrated visualization system and surgical operating microscope facilitated identification, documentation, and management of the complex anatomy encountered in this case (Figure 3). Enhanced magnification and illumination improved visualization of the three mesiobuccal canal orifices and the perforation site, while high-resolution image capture allowed precise clinical documentation. In cases involving rare anatomical variations, advanced visualization technologies may complement conventional magnification and assist clinicians in identifying otherwise overlooked anatomy.

The irrigation protocol employed in this case included 6% sodium hypochlorite, 17% EDTA, and laser-activated irrigation. Advanced irrigation activation techniques are particularly valuable in teeth with complex canal anatomy because they enhance irrigant penetration into fins, isthmuses, and irregular canal communications that may not be adequately cleaned by instrumentation alone.

Contemporary evidence supports favorable outcomes for nonsurgical retreatment, with pooled success rates ranging from approximately 78% to 87% (Sabeti et al., 2024; Olivieri et al., 2024). Nevertheless, procedural complications and complex canal anatomy may adversely influence outcomes. Resolution of the sinus tract before obturation represented an encouraging early clinical finding; however, long-term follow-up remains necessary despite evidence of periapical healing. The patient has been scheduled for routine recall examinations to monitor healing and treatment outcomes.

Patient Perspective

The patient reported satisfaction with the treatment and resolution of the sinus tract. At the completion of treatment, he remained asymptomatic and was satisfied with the outcome. The patient was informed of the need for continued clinical and radiographic follow-up and agreed to return for routine recall evaluations.

Limitations

This case report has several limitations. Although the patient remained asymptomatic and demonstrated clinical and radiographic healing at one-year, longer follow-up is still warranted. In addition, as a single case report, the findings should be interpreted cautiously and cannot be generalized to broader patient populations. Despite these limitations, this report documents an exceptionally rare combination of three mesiobuccal canals and a C-shaped distobuccal-palatal canal configuration in a maxillary first molar and highlights the value of CBCT imaging, image-guided visualization, and magnification in the management of complex endodontic retreatment cases.

Conclusion

This case demonstrates the successful nonsurgical retreatment of a maxillary first molar exhibiting an exceptionally rare combination of three mesiobuccal canals and a C-shaped distobuccal-palatal canal configuration, complicated by an iatrogenic perforation. CBCT imaging, image-guided visualization, and surgical operating microscopy were instrumental in the diagnosis and management of this complex anatomical presentation. Clinicians should maintain a high index of suspicion for untreated and variant canal anatomy in previously treated teeth and consider advanced imaging and magnification technologies when managing challenging retreatment cases.

Declaration of Competing Interests

The authors declare no competing interests.

Patient Consent

Written informed consent was obtained from the patient for publication of this case report and accompanying clinical and radiographic images.