Tooth morphology is always a fundamental focus in clinical dentistry, including but not limited to root canal treatment or tooth extraction. Tooth morphology involves varies aspects such as root length, canal variations, root furcation and the like. There are reports analyzing tooth characteristics within individual nations(4, 6, 12); however, limited studies focus on detailed analysis of thorough investigation on these characteristics, and it is essential to summarize data within individual nations. Therefore, we are aimed to carry out an in-depth investigation among Chinese population.
To analyze root morphology of anterior dentition, we delicately design measuring method to ensure it is perpendicular on the field of vision in determining the starting and ending point (Fig. 1). This is an original design to confirm the accuracy of the measurement, which is always been overlooked by previous research. Moreover, the root length is determined by CEJ to root apex. Since the CEJ is not a straight line but an irregular curve around neck of tooth, we calculated the average root length from both the buccal-lingual view and mesial-distal view, and took the mean as the root length. At the meantime, the subtraction of the lengths is taken for CEJ curvature interdentally.
Pervious research reported the root length in individual countries. Root length of anterior dentition in Brazil was 12mm in both maxillary and mandibular central incisors (13). It is reported that for Korean population, root length of maxillary central incisors were 12.3mm for male and 11.75mm for female, and those of maxillary canines were 15.83mm for male and 15.23mm for female(14). Indian research reported that root length of mandibular central incisor was 12.9mm, lateral incisor was 12.83mm, and canine was 14.8mm(15). In our research, we find that root length of maxillary central incisors was 13.3mm, lateral incisors were 13.4mm, and canines were 16.6mm, while those of mandibular central incisors were 12.2mm, lateral incisors were 13.4mm, and canines were 15.5mm. We may speculate that root length of Chinese anterior teeth is slightly longer than the reported data from various populations; however, research results were much influenced by different measuring methods, and no survey method is revealed in details except for our own. Therefore, we could not draw firm conclusions comparing root length among different nations yet so far.
For curvature of CEJ, the related research is comparatively scanty. It is reported to be dependent on height of crown contact area and diameter of the crown labiolingually or buccolingually. It comes to light that CEJ reaches maximum curvature in anterior teeth especially in the interproximal aspect, leaving CEJ at posterior teeth a more gentle line(16). We find curvature of CEJ is tooth location-determined. According to our research, the curvature of CEJ, expressed as subtraction of height difference between buccal-lingual view and mesial-distal view, is around 2.5mm at maxillary incisors, which is statistically significantly greater than those in maxillary canines at about 2.0mm. While no significant changes in curvature of CEJ in observed in mandibular incisors and canines with all curvature subtracted height at around 1.8mm. Our results also go accordance with the esthetic requirement of implant design for placing implant shoulder 2mm above the most apical point of vestibular CEJ of adjacent teeth(17, 18). However, it is known that CEJ curvature is an irregular and scalloping curve, with mesial aspect and distal aspect of the same tooth not on a strictly straight plane, and so it is the same with the buccal and lingual aspect(19). The maximum difference among all dentition may exist in central incisors whose mesial CEJ apex may reach 1mm even greater than that in distal CEJ apex(16). Taken together with our research result, it is necessary to compare the CEJ curvature interdentally between adjacent teeth instead of individual teeth in further investigation for periodontitis and implant esthetic research.
Root furcation is always considered in premolars and molars, however, we also observed furcation in mandibular lateral incisors (0.2%) and canines (1% for right canines and 1.5% for left canines). Previous reports described 0.2% of mandibular canines were observed with bi-roots, but lateral incisors were all single root in Malaysian population (4). 5.2% of permanent mandibular canines were also observed with two roots in Pakistani population via CBCT(20). It is also reported that 1.5% of mandibular canines in Brazilian population, 0.3% of mandibular canines in Iran population are noticed with double roots via CBCT(6, 21). However, all of them only detected bi-root in mandibular canines, leaving lateral incisors furcation undetected. Another Chinese group investigated into mandibular permanent anterior teeth and found that 0.3% of mandibular lateral incisors and 0.8% of mandibular canines with double roots(22). These research results suggested the tendency of two roots in anterior dentition in an area-dependent manner. Together with our research finding, we could speculate that Chinese may possess higher probability of root furcation in lateral anterior teeth than other populations, and which increases difficulty in clinical diagnosis and further root canal treatment.
Root canal morphology is always a focus in endodontic dentistry both clinically and theoretically. To date, dentists still adopt Vertucci’s classification to define root canal types(3). It includes eight subtypes, and even root in multi-rooted molars could also be analyzed by this classification. We observed all eight types in Chinese anterior dentition and even two more types that could not be included into any group of Vertucci’s classification. We named these two supplementary canal types as ST I (2–1–2–1) and ST II (1/O). It is worth noting that another group looked into Turkish population and also observed 2-1-2-1 canal type as we mentioned in Fig. 4 as ST I, and we find this type of root canal in mandibular canines instead of mandibular incisors(23). However, no other reports detected ST II canal type as we found in maxillary lateral incisor with two cases in our study, and we guess it may be contributed to particular tooth development mechanism in maxillary lateral incisors, which needs to be clarified in the further research.
Only maxillary right central incisors and left lateral incisors are observed with single root canal, all the other teeth in anterior dentition are noticed with double or triple root canals. We could speculate from our result that the frequency of bi-root canal is not symmetrical necessary, and may depended on growth and development of specific tooth(24). Interestingly, we find triple-root canal in mandibular canine, and only this single example is found. We did not find many documents recording triple-root canal in anterior dentition, and limited information recording triple root canal is found in Israeli population(25), therefore we may infer that triple-root canal in anterior dentition is sparsely scattered.
Except for the canal type I, we only observed type II, III and ST II in maxilla, but all eight types in mandible together with ST I. Among which, type III counts the majority in mandible except for type I (Fig. 5). However, researchers reported less variety of root canal types in anterior dentition in other populations. For example, it is reported that Vertucci’s classification type I, type II, Type III, type V and type VI were found in Iran population(6). Type I, type II, type III, type IV, type V, and type VI is noticeable in mandibular anterior teeth in Indian people via CBCT(15). The occurrence of two or more root canals in mandibular incisors is comparatively higher with 40% in Israeli population via CBCT, and Vertucci’s type I, II, III, IV, V and VIII were noticed in mandibular incisors(25). Bi-root canal occurrence rate was even higher in Turkish population with 47.6% via CBCT analysis, and Vertucci’s type I, II, III, V and 2-1-2-1 were found in mandibular incisors (23).Except for CBCT application, other traditional methods were also utilized in determining root canal types. Research group observed extracted mandibular incisors with root canals stained with Indian ink from Jordanian population, and found root canal types were type I (73.8%), type II (10.9%), type III (6.7%), type IV (5.1%) and type V (3.6%)(26). Similar method was utilized to analyze canal types in north-east Indian population, and Vertucci’s type II (7.08%), type III (22.9%) and type V (6.25%) were also noticed besides type I(27). Taken these results together, we could speculate that the overall trend of root canal variation is much more complicated in mandible than maxilla, and mandibular canine possesses much more complex variation in canal morphology among anterior dentition. However, it may be area and race-dependent.
These findings together are of great value both in clinical and theoretical dentistry. Taking the related information together would help improving dental treatment for a successful outcome. However, with the limitation of this study, we take an initial look into root morphology of anterior dentition in Chinese population. Further study is needed to explore in-depth the whole dentition in an even larger sample.