Knowledge of the anatomical configurations of the roots and canals of each tooth is crucial in order to predict possible complications during treatment. Although the MDA has a single root and canal in most cases [19–22], clinicians should consider possible variations to remove the pulp tissue and necrotic debris effectively without negatively affecting the structural integrity of the tooth and root.
Consistent with previous studies [19, 23], Type I Vertucci (Equivalent to 1MI1−1 in Ahmed classification) was the most common type of canal morphology in MDI. The second most common type of root canal configuration for MDI was Type III Vertucci (Equivalent to to 1MI1–2−1), consistent with several studies [16,21,22]. On the other hand, in the Malaysian population Type III Vertucci (Equivalent to 1MI1–2−1) was the most common root canal configuration for MDI followed by Type I Vertucci (Equivalent to 1MI1−1) [6].
Root canal configurations in 16 (1.4%) mandibular centrals and 11 (1%) mandibular laterals teeth could not be classified with Vertucci's classification. The correspondence of all these in Ahmed classification was 1MI2−1−2−1. This was also the third most common type of root canal configuration for MDI according to Ahmed classification.
In this study, it was found that approximately 30% of MDA has two canals. It has been reported that the second canal is mostly missed by clinicians [24]. This is probably because the dentist cannot recognize the presence of the second canal, clinically it is recommended to extend the accessl cavity to the incisal edge in the form of an oval preparation with a mesiodistal width of less than 2 mm to locate the lingual canal [24]. Considering that the lingual canal merges into the labial canal before the apex and ends in a single canal, this may not compromise the outcome of endodontic treatment and the complete filling of the labial canal in the apical third [16].
Although several studies reported that MDA are single-rooted [20, 21, 26], in this study there were two-rooted laterals (0.3%) and canines (3.1%) similar to the study by Zhengyan, et al. [27]. The frequency of two-rooted mandibular canines was 0.3–1.9% in previous studies [6,28,29], compared to 3.1% in this study.
In a study of anterior teeth in a Jordanian subpopulation using canal staining and root-clearing technique, the frequency of Type 1 Vertucci (73.8%) was close to that of the present study (76%) [19]. However, different frequencies have been found for other types, possibly due to differences in the methods used. Studies have demonstrated the high reliability of CBCT in detecting root canal morphology compared to visual inspection [30, 31]. The European Society of Endodontology position statement on CBCT imaging recommends that CBCT should be used to assess complex root canal morphology [32].
According to the present study, Type I (1 MI 1−1) and Type III (1 MI 1–2−1) were found to be the most common in MDC, accounting for 90.7% and 8.2%, respectively. Several studies carried out in Iran (97.6%) [28], Israel (89.7%) [29], Portugal (90.2%) [23], Brazil (90.5%) [4], and Malaysia (87%) [6] have reported a similar frequency of Type I Vertucci. In the study by Karobari, et al. on Malaysian population [6], they came across two MDC morphologies (1MD 2−1−2−1−2−1 and 1MD 2−1−2−1) with a 0.001% rate which could not be categorized based on the Vertucci system. In this study, only one type of MDC morphology (1MI2−1−2−1) was found that could not be classified by Vertucci with a 0.4% occurrence rate.
This study, in line with the research of Geduk, et al. [33], found no significant gender-related differences in the root canal morphology of MDI. The only noticeable distinction between genders was found in MDC, the variation in root canal morphology of MDC was more diverse in females than in males. Similar to Karobari, et al. [6], females had an extra 1MI2−1−2−1 classification. Although some studies suggest that males have greater variation in canals, others found no significant difference [3, 33]. The difference in the results of studies could be attributed to factors such as sample size, methodology, or genetic traits of the individuals involved in the study.
Consistent with previous studies it seems that age plays a significant role in determining root canal morphology in MDI. There was a greater range of root canal variations among younger patients [6,27]. As we age, teeth naturally undergo tertiary dentinogenesis. This involves the deposition of dentin in response to injury, either by odontoblasts or odontoblast like cells from the pulp, depending on the extent of the injury. As a result, the root canal may transforms from a complex to a simpler configuration with age [27].
Although Vertucci classification [7] is widely used to categorize root canal morphology, it fails to consider the number of roots present in anterior and premolar teeth. Bi-rooted teeth in the anterior and premolars are classified as type IV or type V, which may mislead the clinician during root canal treatment procedures [18]. The classification by Ahmed [8] now classifies bi-rooted anterior and premolars using a single code that takes into account the number of roots and canal morphology. In complex canal variations, there is no need to memorize classifications like Vertucci system. According to a survey of senior dental students in Malaysia, more than 90% of the students found the new system to be more precise and convenient compared to Vertucci classification [34].
In the analysis using Cramer's V, there was a moderate correlation observed between the canal morphologies on the left and right sides. According to Lin, et al. [35], there is a 92.7% and 89.2% similarity in morphology between the left-right sides for MDC and MDL, respectively. In this study, even though lower values were obtained (MDS:75.42%, MDL: 67.48%, MDC: 64.84%), this correlation was noticable, thus when dealing with mandibular anterior teeth, it is important for the clinicians to take into account their morphological similarity.
The bifurcation of a root canal refers to the point where a single canal splits into two smaller ones. Previously, dentists relied on periapical radiographs to detect the presence of bifurcations through the "fast break" guideline, where the root canal suddenly narrows or even disappears [1,36]. CBCT images are now capable of visualizing the root canal bifurcation and supplying data for quantitative evaluation as well [30,36]. Martins, et al. reported that the main root canal can merge and split at any level in the root [17]. While in this study the middle third of the MDA had the highest number of bifurcations, which is similar to the findings by a previous study on MDI [35]. Clinicians should consider the possibility of second canal at the mid root level, inspection under magnification and the use of CBCT whenever needed is recomended to avoid potential complications.
The reliability of CBCT imaging depends greatly on the voxel size, with smaller size leading to better results. In this study, the CBCT voxel size used was 150 µm, which is higher than that of micro-CT systems but still reliable in identifying the number of root canals and specific root anatomy. While micro-CT can achieve voxel sizes as small as 5 µm, CBCT devices with smaller fields of view may reach up to 76 µm. Hatipoğlu, et al. reported that the voxel size (above or below 150 µm) did not affect the detection of the midle mesial canal in mandibular first molars [37].