Developmental aberration in terms of number, size, and shape may be caused by anomalies in the morpho-differentiation process of the dental lamina and the tooth germ. 1 According to where they occur, a tooth's crown, roots, and root canals, morphoanatomic changes in teeth can be classified. 2 Both the primary dentition and the permanent dentition can have irregularities or variations in the number, size, and shape of the teeth. 3 These developmental abnormalities may be caused by local as well as systemic causes.

A successful root canal procedure requires thorough understanding of the root canal system's anatomy. The degree of root canal complexity varies by race, gender, age, the existence of lateral/accessory canals, additional roots, isthmuses, the position of the teeth at the jaws, and dental abnormalities (dens invaginatus, dens evaginatus, fusion, gemination, dens in dente). When this happens, there are a number of challenges with the root canal procedure. 4

According to Slowey, mandibular premolars are considered to be the most difficult teeth to endodontically treat. 5 This is mostly because it might be challenging to distinguish between different anatomical variances in root canal morphology. The architecture of the root canals of mandibular premolars typically does not show a single, uniformly tapering canal; instead, it may show additional canals, anastomosis, and other anomalies. 6

Weine et al., Vertucci, and Gulabivala et al.in the year 1969, 1984 and 2001 respectively have classified and described the root canal system of human permanent teeth and the number of roots can be described by using Table 1 classification. ⁷

The most used classification system for the root canal configuration is Vertucci's (Figure 1), which has 8 categories such as: Type I (1), Type II (2-1), Type III (1-2-1), Type IV (2), Type V (1-2), Type VI (2-1-2), Type VII (1-2-1- 2) and Type VIII (3). 8

Developmental anomalies in teeth include fusion and gemination. Fusion was described by Pindborg as the joining of two or more independent developing teeth's dentin and/or enamel. 9 Fusion tend to occur in anterior region and the incisors are the teeth most often affected in the permanent dentition and in the primary dentition majority of cases involve anterior mandibular teeth. 9 Fusion can take place between two normal teeth that are next to each other or between a normal tooth and a supernumerary tooth. 10

This paper is reporting a rare case of dental developmental anomalies in a single patient; the fusion of the permanent right mandibular central and lateral incisor with unusual anatomy of mandibular first and second premolar with two root canals - bilaterally.

A 8-year old male patient reported to the department of Pediatric and Preventive Dentistry for routine dental check-up. The patient's medical history was ordinary; there was no evidence of paternal consanguinity or dental anomalies in the patient's family. There was no trauma history noted. Extraoral and general examinations didn't seem to be helpful.

On clinical examination a wide crown with increased width mesio-distally and one deep groove on the labial and lingual surfaces was found on the patient's adjacent permanent right central incisor. Examination of occlusion exhibited an anterior cross-bite. On the basis of the clinical findings, a preliminary diagnosis of fusion was made i.r.t 41 and 42 (Figure 1, Figure 2).

Radiographic examination revealed complete fusion of permanent mandibular central and lateral incisors with a single crown and a single root which is an uncommon finding.

A confirmed diagnosis was made using CBCT (Figure 3) which revealed fusion of mandibular permanent central and lateral incisors. The fused teeth were caries free and hence no treatment was carried out on them. The CBCT scan also identified two roots in the first and second premolars - bilaterally (Figure 5, Figure 6).

The combination of fusion of mandibular central and lateral incisors along with the presence of extra root in the mandibular first and second premolars and since it is not associated with any known systemic diseases or developmental syndrome makes this case an interesting one.

Despite the large number of cases that have been documented in the literature, the report of fusion and presence of two roots in mandibular first and second premolar was not observed.

The term fusion refers to the dentinal merger of two or more initially separate teeth.

Possible participants include both healthy teeth and extra teeth (mesiodens or supplementary tooth). 11 Two distinct tooth germs united by enamel and dentin (true fusion) and by dentin and/or cementum during the formative stage (late fusion). 12 Concrescence refers to a cementum's late fusing. If fusion starts before the calcification stage, the participating teeth completely fuse, and the crown integrates the enamel, dentin, cementum, and pulp characteristics of both participating teeth. When incomplete fusion occurs, the resulting tooth may have separate crowns, just involve the root, and have the pulp canals fused or separate. When an anomalous tooth is classified as one, fusion exposes a missing tooth. It could be either bilateral or unilateral. In the front mandibular segment, it is more frequent. 13, 14

The two epithelial structures' apical growth, which joins at the cervical loop to form a double layer of cells known as Hertwig's epithelial root sheath, starts the process of root development. Genetics controls the pattern of cell proliferation in the root sheath, which determines whether the root will be long or short, wide, or thin, straight or curved, or single or numerous. When opposing portions of the root sheath proliferate both horizontally and vertically, several roots are produced. A new root or many roots are created as horizontal segments combine and spread apically. In mandibular molars and maxillary premolars, the pattern of proliferation also affects whether the roots are distinct or connected. 15

Because of the complex anatomy of these teeth, studies on the anatomical alterations in mandibular premolars should aid dentists in understanding these teeth before doing root canal therapy. Failure to foresee and recognise the presence of the extra root canals could result in endodontic management failure, which could lead to acute flare-ups during and after the endodontic procedure and retreatment. 16

For endodontic therapy to be successful, a thorough radiographic assessment, including preoperative radiographs, is crucial. 17 Different techniques, including 2D radiography, root canal staining, hard tissue sectioning, micro-CT, and cone-beam computed tomography (CBCT) scanning, had been used to identify root canal configuration. Axial, sagittal, and coronal sections of the maxillofacial anatomy can be evaluated with CBCT, which also generates high-quality 3D diagnostic pictures free of structural overlapping. A 3D visualization of the root and canal shape, number of canals, and divergence or convergence of the canals are possible. Because of these factors, CBCT has been suggested for the precise assessment of root canal systems. ^{18, 19} Advances in imaging technology and the use of CBCT in dentistry has enabled the clinicians for the better understanding of the canals.

Racial and genetic differences may have an impact on the anatomy and morphology of the root canals. ⁶ The etiology of dental developmental anomalies (or defects) is complex and can involve multiple causative agents such as local factors (e.g., trauma), genetic influences, or environmental insults (e.g., alcohol or drug exposures during the fetal period, febrile illnesses). In the process of odontogenesis is primarily under the control of the homeobox (HOX) genes (e.g., MSX1, MSX2, SHH, PAX9) which play critical roles in multiple stages of tooth development.20

Diverse racial groupings have various canal morphology types. ⁶ Vertucci et al. report the occurrence of one-root canal in one root in 70% of the mandibular first premolars and 97.5% of the second mandibular premolars (Vertucci type I). Presence of one root with two-root canals is reported in 1.5% of the mandibular first premolars and 0% in mandibular second premolars (Vertucci type IV). 21 Hussam et al. observed in Saudi population, mandibular first premolars, the most predominant root morphology observed in this study was single-rooted (96.4%), followed by double-rooted (3.1%), and then three rooted (0.5%). 22 There are studies with an incidence of 5.5% for two root canals in the Caucasian and 16.2 and 4.8% in second premolars in the African American group of patients. 21 Rakesh et al. (2013) Indian population showed single roots in 93.5% and single canal in 93.2% of teeth and more than two roots and two canals in 6.4% and 6.8% of the teeth respectively, this might be due to genetic differences related to the racial background of the population. 16 Scott and Turner describe the accessory root of mandibular first premolar as “TOME’S ROOT” 23. They observed ethnic differences in the root morphology; and, reported the highest incidence (>25%) of accessory roots in the Australian and sub-Saharan African populations. The lowest incidence of Tome's root (0-10%) occurred in the American, Arctic, New Guinea, Jomon, and Western Eurasian populations. Sert and Bayrili ²⁴ also reported sex differences in canal morphology, reporting higher incidence (44%) of accessory roots and canals in females as compared to males (34%). 25

Double teeth are more common in primary than in the permanent dentition, the prevalence in different series ranging from 0.5% to 2.5% for the primary and 0.0 ~ 0.8% of the general population with no gender prediction for the permanent dentition. 26, 27, 28

For a prompt and accurate diagnosis and to choose the best course of treatment, it is crucial to recognise and identify developmental dental defects. Management of dental anomalies can be more complicated because they can result in failure of root canal procedure, aesthetic problems, malocclusion, and oral disorders.

The current case represents an unusual finding of fusion and an additional root with respect to mandibular premolars. CBCT interpretation revealed complex internal anatomy of the root structure that may require special attention in cases where additional roots are present, and complications can occur during root canal therapy that can cause challenges or difficulty in exploration and negotiation of the root canals due to the inability to continuously follow the root canal curvature, which may result in canal blocking, ledging, transportation, zipping, perforation, and instrumentation. 29 Other difficulties include the increased likelihood of root fracture during exodontia if multi-rooted premolar teeth are rotated during extraction. 30

Thus, a preventive strategy would be recommended to avoid the challenges during endodontic procedure of these teeth. Therefore, comprehensive knowledge of normal anatomy and common variations, careful interpretation of angled radiographs, use of 3D imaging, appropriate cavity preparation, and a thorough exploration of the interior of the tooth, ideally under magnification, followed by adequate cleaning, shaping, and obturation is crucial in endodontic success of tooth/teeth with the variations in root canal anatomy and morphology.