A micro–computed tomographic analysis of the root canal systems in the permanent mandibular incisors in a Chinese population
Authors: Tang Ying , Wu Yihan , Pei Fan , Liu Chao , Qiu Yinfeng , Yang Tao , Gu Yongchun 
Published: March 2023

Abstract
Background
Comprehensive understanding of the root canal system complexity is critical important for successful root canal therapy. A double root canal system may be present in permanent mandibular incisors with a variable incidence in different ethnic populations. Ignorance or improper management of this canal variation can lead to treatment failure. This in vitro study aimed to identify the anatomic features of root canal systems in the mandibular incisors in a Chinese population by using micro-CT.

Methods
A total of 106 permanent mandibular incisors (53 central incisors and 53 lateral incisors) were collected from a native Chinese population. The teeth were scanned by a micro-CT scanner and then reconstructed three-dimensionally. The canal configurations were detected by Vertucci’s classification, and the number and location of the accessory canals were also identified. The long (D) and short diameters (d) of the main and accessory canals were measured and D/d ratio was calculated at different root levels (cemento-enamel junction [CEJ] level, mid-root level and 1, 2, 3 and 4 mm from the apex). The root canal curvatures in the double-canaled mandibular incisors were measured at the proximal view by using modified Schneider’s method. Chi-square test or Fisher's exact test was used for comparison of occurrence rates. Comparison of means from multiple groups was performed by using one-way ANOVA and LSD post-hoc test.

Results
In regard to the occurrence of double root canals, gender difference was neither detected in the mandibular central (16.0% [male] vs 14.3% [female]; p = 0.862), nor in the mandibular lateral incisors (26.9% [male] vs 33.3% [female]; p = 0.611). Age group difference was also not detected in the mandibular central (p = 0.717) and lateral incisors (p = 0.521). The incidence of double root canals was 15.1% (8/53) in the central incisors, and 30.2% (16/53) in the lateral incisors, but the difference did not reach statistical significance (p = 0.063). The most frequent non-single canal type was the type III (1–2-1) (18.9% [20/106]), and the other types identified included 1 case of type II (2–1) and 3 cases of type V (1–2). The incidence of accessory canals was 17.9% (19/106), with a mean level of 1.92 ± 1.19 mm from the apex. The frequency of long-oval (2 ≤ D/d < 4) and flattened canals (D/d ≥ 4), as well as the mean value of D, d and D/d ratio increased from the apical 1 mm to the apical 4 mm level (the D/d ratio increased from 1.9 to 2.9 for the single canals, from 1.4 to 3.3 for the buccal canals and from 1.2 to 2.3 for the lingual canals), and the D/d ratio reached the peak at the mid-root level. Double curvatures were detected in 33.3% (8/24) of the buccal canals and 37.5% (9/24) of the lingual canals, and the difference has no statistical significance (p = 0.063). The degrees of the primary curvatures were 21.5 ± 7.1 degrees for the buccal and 30.1 ± 9.2 degrees for the lingual canals, and the degrees of secondary curvatures were 27.0 ± 11.4 degrees for the buccal and 30.5 ± 12.5 degrees for the lingual canals in the double curvatures. The degrees of the single curvatures were 14.2 ± 6.3 degrees for the buccal and 15.6 ± 6.0 degrees for the lingual canals. Significant difference was detected among above 6 groups of canal curvatures (p = 0.000), and severe curvatures (≥ 20 degrees) were more frequently detected in the double curved canals.

Conclusions
Double-canaled mandibular incisors were not uncommon in the Chinese population, and type 1–2-1 was the most frequent non-single canal type. Gender and age did not significantly impact the occurrence of a second canal in mandibular incisors. Long-oval and flattened canals were very common at different root levels and their incidence increased from apex to the mid-root level. Severe curvatures were frequently detected in the double canal systems, especially in those canals with double curvatures.