Three-Dimensional Analysis of Root Anatomy and Root Canal Curvature in Mandibular Incisors Using Micro-Computed Tomography with Novel Software

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Article Three-Dimensional Analysis of Root Anatomy and Root Canal Curvature in Mandibular Incisors Using Micro-Computed Tomography with Novel Software

1, 2, 3 4 5 JongKi Lee †, Shin-Hoon Lee †, Jong-Rak Hong , Kee-Yeon Kum , Soram Oh , Adel Saeed Al-Ghamdi 6, Fawzi Ali Al-Ghamdi 7, Ayman Omar Mandorah 8, Ji-Hyun Jang 5,9 and Seok Woo Chang 5,9,*

1 Private Practice, Changwon 51495, Korea; [email protected] 2 Eunpyeong Appletree Dental Clinic, 19, Jingwan 2-ro, Eunpyeong-gu, Seoul 03306, Korea; [email protected] 3 Department of Oral and Maxillofacial Surgery, Gaon Dental Hospital, 129, Dongseo-daero, Seobuk-gu, Cheonan-si, Choongcheongnam-do 31109, Korea; [email protected] 4 Department of Conservative Dentistry, Dental Research Institute, National Dental Care Centre for Persons with Special Needs, Seoul National University Dental Hospital, Seoul National University School of Dentistry, Seoul 03080, Korea; [email protected] 5 Department of Conservative Dentistry, Kyung Hee University Dental Hospital, 23 Kyungheedaero, Dongdaemun-gu, Seoul 02447, Korea; [email protected] (S.O.); [email protected] (J.-H.J.) 6 King Abdulaziz Hospital, Jeddah 22421, Saudi Arabia; [email protected] 7 Director of Dental Services, Ministry of Health, P.O.Box 109196, Jeddah 21351, Saudi Arabia; [email protected] 8 Department of Endodontics, Restorative and Dental Materials, Faculty of Dentistry, Taif University, Taif 26571, Saudi Arabia; [email protected] 9 Department of Conservative Dentistry, Kyung Hee University School of Dentistry, 23 Kyungheedaero, Dongdaemun-gu, Seoul 02447, Korea * Correspondence: [email protected] Contributed equally to this study as ﬁrst authors. † Received: 22 May 2020; Accepted: 24 June 2020; Published: 26 June 2020

Abstract: Root canal treatment of mandibular incisor is diﬃcult because of the narrow pulp space and apical curvature. The aim of this study was to measure the anatomical indicators of the mandibular incisors in Koreans using micro-computed tomography (MCT) with novel software (Kappa 2). The MCT-scanned data from 27 mandibular incisors were reconstructed and analyzed. For each canal, 3-dimensional (3D) surface models were re-sliced at 0.1 mm intervals perpendicular to the central axis of the root canal. Root canal width, dentine thickness, and direction and degree of root canal curvatures were measured automatically on each slice. Measurements were analyzed statistically with Bhapkar test, Friedman test, and Wilcoxon signed rank test. Labial and lingual dentine thicknesses were signiﬁcantly larger than mesial and distal thicknesses (p < 0.001). The thinnest dentine was mainly located on the mesio-lingual side of the canals in the apical third. The mean narrowest and widest canal width in the apical sixth were 0.22 mm and 0.40 mm, respectively. The canal curvature abruptly increased in the apical 0.5-mm portion. MCT with novel software provided useful anatomical information for root canal instrumentation.

Keywords: apical curvature; canal width; dentin thickness; mandibular incisor; micro-computed tomography

Appl. Sci. 2020, 10, 4385; doi:10.3390/app10124385 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4385 2 of 11

1. IntroductionAppl. Sci. 2020, 10, 4385 2 of 11

1.For Introduction successful endodontic treatment, an understanding of the complex root canal anatomy as well as surrounding root dentine architecture is essential. Although the mandibular incisors usually For successful endodontic treatment, an understanding of the complex root canal anatomy as have a single canal (72% (Leoni et al., 2014) [1], 91.1% and 82.5% (Liu et al., 2014) [2]), they pose the well as surrounding root dentine architecture is essential. Although the mandibular incisors usually greatesthave dia singlefficulty canal in access (72% (Leoni preparation et al., 2014) owing [1], to91.1% the and narrowness 82.5% (Liu of et the al., crown 2014) [2]), and they pulp pose space the in the mesio-distalgreatest difficulty direction in access [3]. Furthermore, preparation owing mandibular to the narrowness incisors often of the have crown mesial and pulp and space distal in curvesthe in theirmesio-distal apical third direction [3], which [3]. makesFurtherm theore, root mandibular canal preparation incisors often in this have area mesial even and more distal di fficurvescult. in theirIn terms apical of third root [3], canal which preparation, makes the root the canal root preparation canal width, in dentinthis area thickness even more around difficult. the root canal, and 3-dimensionalIn terms of root (3D) canal direction preparatio andn, the degree root canal of the width, root dentin canal thickness curvature around are relevant. the root canal, However, previousand 3-dimensional studies on the (3D) root direction canal anatomy and degree of the of mandibularthe root canal incisors curvature were are mainly relevant. focused However, on the root canalprevious type, the studies number on the of rootroot canals,canal anatomy and the of shape the mandibular of root canals incisors [1,2 ].were These mainly factors focused provide on the valuable root canal type, the number of root canals, and the shape of root canals [1,2]. These factors provide information on the root canal anatomy itself. However, more clinically relevant information for root valuable information on the root canal anatomy itself. However, more clinically relevant information canalfor preparation root canal preparation is the narrowest is the narrowest and the and widest the widest root root canal canal width, width, the the dentin dentin thickness, thickness, and and the directionthe direction and degree and ofdegree the root of the canal root curvature. canal curv Thereature. have There been have no been previous no previous studies investigatingstudies the 3Dinvestigating direction the or degree3D direction of root or degree canal curvature of root canal of curvature human mandibular of human mandibular incisors. incisors. Micro-computedMicro-computed tomographytomography (MCT) (MCT) with with math mathematicalematical modeling modeling is an accurate is an and accurate non- and non-destructivedestructive tool tool for forstudying studying root rootcanal canalanatomy anatomy [4]; it provides [4]; it provides detailed information detailed information on root canal on root canaldimensions, dimensions, dentin dentin thickness, thickness, and anddirection direction and anddegree degree of the of root the canal root canalcurvature. curvature. Additionally, Additionally, imageimage analysis analysis software software is essential is essential to analyzeto analyze the the information information obtained obtained via via MCT. MCT. Our Our previous previous studies usedstudies custom-made used custom-made Kappa 2 software, Kappa 2 software, which produced which produced detailed detailed 3D measurements 3D measurements of various of various aspects of aspects of canal anatomic parameters [5–7]. This method used slices of images perpendicular to the canal anatomic parameters [5–7]. This method used slices of images perpendicular to the central axis of central axis of the root canal (Figure 1A), not the long axis of the tooth, which can reduce distortions the rootof measurements canal (Figure in1 A),areas not where the longa curvature axis of is the a pr tooth,esent (Figure which can1B). reduceOur previous distortions studies of measuring measurements in areasthe 3D where root canal a curvature anatomy is of a presentthe lateral (Figure incisor1 B).[8],Our maxillary previous first studiesmolar [5,9], measuring and mandibular the 3D rootfirst canal anatomymolar of[6] the confirmed lateral incisor the validity [8], maxillary of this firstcustom-made molar [5 ,9image], and analyzing mandibular software first molar (Kappa [6 ]2) confirmed in the validitycoordination of this with custom-made MCT. image analyzing software (Kappa 2) in coordination with MCT.

Figure 1. Slice of reconstructed image. (A) When sliced perpendicular to the long axis of the root canal, no distortions are produced. (B) When sliced perpendicular to the long axis of the tooth, distortions are Figure 1. Slice of reconstructed image. (A) When sliced perpendicular to the long axis of the root canal, produced in the images of the root canal and surrounding dentin. no distortions are produced. (B) When sliced perpendicular to the long axis of the tooth, distortions are produced in the images of the root canal and surrounding dentin. The aim of this study was to determine the minimum and maximum root canal widths, dentin thickness, and 3D direction and degree of the root canal curvature in extracted human mandibular incisors. Appl. Sci. 2020, 10, 4385 3 of 11

The aim of this study was to determine the minimum and maximum root canal widths, dentin thickness, and 3D direction and degree of the root canal curvature in extracted human mandibular Appl. Sci. 2020, 10, 4385 3 of 11 incisors.

2. Materials2. Materials and and Methods Methods ThisThis study study was was approved approved by bythe the institutional institutional review review board board of ofSamsung Samsung Medical Medical Center, Center, Seoul, Seoul, KoreaKorea (SMC (SMC 2014-08-088). 2014-08-088). One-canalled One-canalled human human mandibular mandibular incisors incisors (n (=n 27)= 27) with with mature mature apices apices and and intactintact crowns crowns without without caries caries or orprevious previous root root canal canal treatment treatment were were collected. collected. To To get get rid rid of ofthe the calculus calculus and adhering soft tissues, the root surfaces of collected teeth were cleaned. They were stored (4 °C) and adhering soft tissues, the root surfaces of collected teeth were cleaned. They were stored (4 ◦C) in in0.5% 0.5% sodium sodium azide azide solution solution and and scanned scanned usin usingg MCT MCT (SkyScan (SkyScan 1172; 1172; Bruker-microCT, Bruker-microCT, Kontich, Kontich, 3 Belgium)Belgium) to toacquire acquire detailed detailed im imagesages (voxel (voxel size size = 31.8= 31.8 μmµm). 3 ).

2.1.2.1. Image Image Analysis Analysis ImagesImages were were analyzed analyzed as aspreviously previously described described [5–7 [5–]:7 5–15]: 5–15 image image slices slices that that spanned spanned each each canal canal werewere selected selected (Figure (Figure 2).2 ).The The canal canal width, width, dentin dentin thickness, thickness, and and canal canal curvatures curvatures were were measured measured on on re-slicedre-sliced planes planes perpendicular perpendicular to tothe the central central axis axis of ofthe the canal. canal.

Figure 2. Overall process of the automatic measurement of root and canal dimensions. From the MCT Figure 2. Overall process of the automatic measurement of root and canal dimensions. From the MCT images of mandibular incisors, 3D surface models of root canals were constructed and the central axis images of mandibular incisors, 3D surface models of root canals were constructed and the central axis was plotted using V works 4.0; Cybermed, Seoul, Korea. Subsequently, surface models were sectioned was plotted using V works 4.0; Cybermed, Seoul, Korea. Subsequently, surface models were sectioned perpendicular to the canal axis (at 0.1 mm intervals). Pre-deﬁned anatomic parameters were computed perpendicular to the canal axis (at 0.1 mm intervals). Pre-defined anatomic parameters were in each section. computed in each section. The central axes of the canals were plotted and visualized (Figure3 O1 ). The cross-sectional plane thatThe is perpendicularcentral axes of the to the canals central were axis plotted was visualized.and visualized The (Figure canal (Figure 3①). The3 O2 cross-sectional) and root (Figure plane3 O 3 ) thatoutlines is perpendicular were marked to the as central shown. ax Theis was shortest visualized. distance The betweencanal (Figure these 3 outlines,②) and root referred (Figure to 3 as③ the) outlinesdentin were thickness, marked was as measured shown. The in the shortest mesial, di distal,stance labial, between and lingualthese outlines, directions referred (Figure to3 O4asO5 theO6 O7 ). The thinnest dentin thickness and direction (Figure3 O8 ) were obtained in the cutting plane as mesiolabial, dentin thickness, was measured in the mesial, distal, labial, and lingual directions (Figure 3④⑤⑥⑦ ). mesiolingual, distolabial, or distolingual. The narrowest (minimum) and widest (maximum) canal Thewidths thinnest were dentin measured. thickness and direction (Figure 3⑧) were obtained in the cutting plane as mesiolabial,To measure mesiolingual, the curvature distolabial, of the rootor distolingual. canal, the axis The of the narrowest canal was (minimum) equally divided and intowidest three (maximum)parts (S1, canal apical widths third; S2,were middle measured. third; and S3, coronal third). Subsequently, S1, S2, and S3 were further equally divided into apical and coronal sections, denoted as S1A, S1C, S2A, S2C, S3A, and S3C, respectively (Figure4) . Appl. Sci. 2020, 10, 4385 4 of 11

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Figure 3. Graphical visualization of the measurements. Left image is the representative 3D root model, and the right image is its section; ① the central axis (red curve) of the canal; ② the root canal; ③ root outline; ④ labial dentin thickness; ⑤ lingual dentin thickness; ⑥ mesial dentin thickness; ⑦ distal dentin thickness; ⑧ the thinnest dentin thickness and its direction.

FigureTo measure 3. Graphical the curvature visualization of ofthe the root measurements. canal, the axis Left of image the iscanal the representative was equally 3Ddivided root model, into three partsandFigure (S1, the apical3. right Graphical imagethird; visualization is S2, its section;middle ofO 1third; thethe me central andasurements. S3, axis coronal (red Left curve) image third). of is the theSubsequently, canal; representativeO2 the root S1, 3D canal; S2, root and Omodel,3 root S3 were furtheroutline;and equallythe rightO4 labial dividedimage dentin is itsinto thickness;section; apical ① Oand 5thelingual centralcoronal dentin axis sect (red thickness;ions, curve) denoted ofO6 themesial canal;as S1A, dentin ② S1C,the thickness; root S2A, canal; S2C,O7 ③distal S3A,root and S3C,dentinoutline; respectively thickness; ④ labial (Figure Odentin8 the 4) thinnestthickness;. dentin ⑤ lingual thickness dentin and thickness; its direction. ⑥ mesial dentin thickness; ⑦ distal dentin thickness; ⑧ the thinnest dentin thickness and its direction.

To measure the curvature of the root canal, the axis of the canal was equally divided into three parts (S1, apical third; S2, middle third; and S3, coronal third). Subsequently, S1, S2, and S3 were further equally divided into apical and coronal sections, denoted as S1A, S1C, S2A, S2C, S3A, and S3C, respectively (Figure 4).

FigureFigure 4. 4.Axial Axial levellevel of of the the root root canal canal 2.2. Statistical Analysis 2.2. Statistical Analysis Diﬀerences in the dentin thickness and canal curvature among directions (mesial, distal, labial, Differences in the dentin thickness and canal curvature among directions (mesial, distal, labial, and lingual) or levels (S1A–S3C) were examined with Bhapkar’s test. The minimum dentin thickness, and lingual) or levels (S1A–S3C) were examined with Bhapkar’s test. The minimum dentin thickness, minimum and maximum canal widths, and canal curvature diﬀerences between six sections (S1A–S3C) minimum and maximum canal widths, and canal curvature differences between six sections (S1A– were analyzed with the Friedman and Wilcoxon signed rank test (α = 0.05). S3C) were analyzed with the FriedmFigurean 4.and Axial Wilcoxon level of thesigned root canalrank test (α = 0.05). 3. Results 2.2. Statistical Analysis 3.1. Dentin Thickness Differences in the dentin thickness and canal curvature among directions (mesial, distal, labial, and lingual)The mean or lengthlevels (S1A–S3C) of the root were canals examined between wi theth apex Bhapkar’s and the test. cementoenamel The minimum junction dentin (CEJ)thickness, was 11.40minimum0.96 and mm. maximum The dentin canal thickness widths gradually, and canal decreased curvature from differences S3C to S1Abetween in all six mesial sections and (S1A– distal ± S3C) were analyzed with the Friedman and Wilcoxon signed rank test (α = 0.05). Appl. Sci. 2020, 10, 4385 5 of 11

3. Results

3.1.Appl. Dentin Sci. 2020 Thickness, 10, 4385 5 of 11 The mean length of the root canals between the apex and the cementoenamel junction (CEJ) was 11.40directions ± 0.96 (p mm.< 0.05). The The dentin mean thickness (SD) thickness gradually of the decreased mesial dentin from wasS3C 1.50to S1A (0.19) in mmall mesial in the S3Cand sectiondistal directionsand 0.66 (0.14) (p < mm0.05). in The the S1Amean section. (SD) thickness The mean of (SD) the thicknessmesial dentin of the was distal 1.50 dentin (0.19) was mm 1.50 in (0.16) the S3C mm sectionin the S3Cand section0.66 (0.14) and mm 0.66 in (0.12) the S1A mm section. in the S1A The section. mean (SD) The thickness dentin thickness of the distal gradually dentin decreased was 1.50 (0.16)from S2Cmm toin S1Athe S3C in labial section and and lingual 0.66 directions(0.12) mm (inp < the0.05) S1A (Figure section.5). The The dentin mean (SD) thickness thickness gradually of the decreasedlabial dentin from was S2C 2.28 to (0.23) S1A mmin labial in the and S3C lingual section directions and 1.13 (0.23) (p < mm0.05) in (Figure the S1A 5). section. The mean The mean(SD) thickness(SD) thickness of the of labial the lingual dentin dentinwas 2.28 was (0.23) 2.71 mm (0.23) in mm the inS3C the section S3C section and 1.13 and (0.23) 1.27 (0.29)mm in mm the in S1A the section.S1A section The mean (Table 1(SD)). The thickness dentin thicknessof the lingual was greaterdentin was on labial 2.71 (0.23) and lingual mm in sides the S3C compared section toand mesial 1.27 (0.29)and distal mm in sides the S1A in all section six sections (Table (S1A–S3C) 1). The dentin (p < thickness0.001) (Table was1 ,greater Figure on6). labial The mesial and lingual and distal sides comparedthicknesses to had mesial no statisticallyand distal sides signiﬁcant in all six diﬀ sectionserence in (S1A–S3C) any of the ( sixp < sections. 0.001) (Table 1, Figure 6). The mesial and distal thicknesses had no statistically significant difference in any of the six sections.

Figure 5. Measurements of dentin thicknesses. The average dentin thickness was measured on the Figure 5. Measurements of dentin thicknesses. The average dentin thickness was measured on the mesial, distal, labial, and lingual sides, and the thinnest dentin thickness (MinDist), narrowest canal mesial, distal, labial, and lingual sides, and the thinnest dentin thickness (MinDist), narrowest canal width (Canal width (min)), and the widest canal width (Canal width (max)) were plotted along the width (Canal width (min)), and the widest canal width (Canal width (max)) were plotted along the entire length of the canals. entire length of the canals. Table 1. Mean values of dentin thicknesses, canal widths, and canal curvatures. Table 1. Mean values of dentin thicknesses, canal widths, and canal curvatures. S1A S1C S2A S2C S3A S3C S1A S1C S2A S2C S3A S3C Thinnest dentin thickness (mm) 0.56 0.12 0.86 0.13 1.00 0.15 1.14 0.14 1.30 0.15 1.41 0.21 Thinnest dentin thickness (mm) 0.56± ± 0.12 0.86± ± 0.13 1.00± ± 0.15 1.14 ±± 0.14 1.30 ±± 0.15 1.41 ± ±0.21 Mesial dentin thickness (mm) 0.66 0.14 0.91 0.14 1.07 0.17 1.20 0.16 1.37 0.16 1.50 0.19 Mesial dentin thickness (mm) 0.66± ± 0.14 0.91± ± 0.14 1.07± ± 0.17 1.20 ±± 0.16 1.37 ±± 0.16 1.50 ± ±0.19 Distal dentinDistal thickness dentin (mm) 0.66 0.12 0.98 0.14 1.11 0.13 1.23 0.13 1.36 0.13 1.50 0.16 0.66± ± 0.12 0.98± ± 0.14 1.11± ± 0.13 1.23 ±± 0.13 1.36 ±± 0.13 1.50 ± ±0.16 Labial dentinThickness thickness (mm) (mm) 1.13 0.23 1.81 0.22 2.06 0.23 2.22 0.24 2.29 0.23 2.28 0.23 ± ± ± ± ± ± LingualLabial dentin dentin thickness thickness (mm) (mm) 1.27 1.130.29 ± 0.23 1.97 1.810.28 ± 0.22 2.32 2.06 ±0.36 0.23 2.57 2.22 ± 0.440.24 2.29 2.68 ± 0.230.34 2.28 2.71 ± 0.230.23 Lingual dentin thickness (mm) 1.27± ± 0.29 1.97± ± 0.28 2.32± ± 0.36 2.57 ±± 0.44 2.68 ±± 0.34 2.71 ± ±0.23 Narrowest canal width(mm) 0.22 0.05 0.25 0.05 0.27 0.08 0.28 0.13 0.31 0.15 0.30 0.16 Narrowest canal width ± ± ± ± ± ± Widest canal width (mm) 0.400.220.11 ± 0.05 0.50 0.250.17 ± 0.05 0.70 0.27 ±0.27 0.08 0.78 0.28 ± 0.360.13 0.31 0.70 ± 0.150.50 0.30 0.59 ± 0.160.38 (mm) ± ± ± ± ± ± 1 Canal CurvatureWidest canal (mm − ) 0.50 0.22 0.13 0.05 0.09 0.05 0.10 0.07 0.11 0.08 0.12 0.06 0.40± ± 0.11 0.50± ± 0.17 0.70± ± 0.27 0.78 ±± 0.36 0.70 ±± 0.50 0.59 ± ±0.38 width (mm) (mean standard deviation). Canal Curvature (mm−1) 0.50 ± 0.22 0.13± ± 0.05 0.09 ± 0.05 0.10 ± 0.07 0.11 ± 0.08 0.12 ± 0.06 (mean ± standard deviation). Appl. Sci. 2020, 10, 4385 6 of 11 Appl. Sci. 2020, 10, 4385 6 of 11 Appl. Sci. 2020, 10, 4385 6 of 11

FigureFigure 6. 6.Average Average dentin dentin thickness thickness (mm)(mm) inin mesial,mesial, distal, labial, labial, and and lingual lingual directions. directions. Figure 6. Average dentin thickness (mm) in mesial, distal, labial, and lingual directions. 3.2.3.2. Measurement Measurement of of the the Thinnest Thinnest Dentin Dentin 3.2. Measurement of the Thinnest Dentin TheThe distance distance of theof the thinnest thinnest dentin dentin (MinDist) (MinDist) gradually gradually decreased decreased from from the the CEJ CEJ towards towards the the apex, andapex, then and decreasedThe then distance decreased abruptly of the abruptly inthinnest the apical in dentin the 0.5 apical mm(MinDist) 0.5 of canals.mm gradually of Thecanals. mean decreased The (SD) mean valuefrom (SD) of thevalue MinDist CEJ of towards MinDist was 1.41 the (0.21)wasapex, mm 1.41 at(0.21)and S3C then mm section decreased at S3C and section 0.56abruptly (0.12)and 0.56in mm the (0.12) atapical S1A mm 0.5 sectionat mmS1A ofsection (Table canals. 1(Table, FigureThe 1,mean 7Figure). The(SD) 7). direction value The direction of MinDist of the MinDistof thewas MinDist was 1.41 increasingly(0.21) was mm increasingly at locatedS3C section located on theand on mesial0.56 the (0.12) mesial side. mm side. The at The frequenciesS1A frequencies section (Table of cross-sectionsof cross-sections1, Figure 7). inThe in which whichdirection the MinDisttheof MinDist the was MinDist located was located was in increasingly the in mesial the mesial directionlocated direction on were the were mesial 44.45%, 44.45%, side. 51.85%, The 51.85%, frequencies and and 70.37% 70.37% of cross-sections in in the the S3, S3, S2, S2, inand and which S1 parts,S1 parts,the respectively MinDist respectively was (Figure located (Figure8). in 8). the mesial direction were 44.45%, 51.85%, and 70.37% in the S3, S2, and S1 parts, respectively (Figure 8).

FigureFigure 7. 7.The The thinnestthinnest dentin thickness thickness (mm). (mm). Figure 7. The thinnest dentin thickness (mm). Appl. Sci. 2020, 10, 4385 7 of 11

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Figure 8. The relative frequency (%) of the directions with the thinnest dentin. S1: apical third; S2: Figuremiddle 8.third;The S3: relative coronal frequency third. (%) of the directions with the thinnest dentin. S1: apical third; S2: Figure 8. The relative frequency (%) of the directions with the thinnest dentin. S1: apical third; S2: middle third; S3: coronal third. middle third; S3: coronal third. 3.3. Canal Width 3.3. Canal Width 3.3.The Canal mean Width value of the narrowest dimension of the canal width gradually decreased from S3C The mean value of the narrowest dimension of the canal width gradually decreased from S3C (0.3 mm) to S1A (0.22 mm) (Figures 5 and 9). The mean value of the widest canal width ranged from (0.3 mm)The to S1Amean (0.22 value mm) of (Figuresthe narrowest5 and9 dimension). The mean of value the canal of the width widest gradually canal width decreased ranged from from S3C 0.4 mm (S1A) to 0.78 mm (S2C) (Figure 10, Table 1). 0.4(0.3 mm mm) (S1A) to to S1A 0.78 (0.22 mm mm) (S2C) (Figures (Figure 510 and, Table 9). The1). mean value of the widest canal width ranged from 0.4 mm (S1A) to 0.78 mm (S2C) (Figure 10, Table 1).

Figure 9. The narrowest canal width (mm). Figure 9. The narrowest canal width (mm). Appl.Appl. Sci. Sci.2020 2020, 10,, 438510, 4385 8 of8 11 of 11 Appl. Sci. 2020, 10, 4385 8 of 11

Figure 10. The widest canal width (mm). Figure 10.10. The widest canal width (mm). 3.4.3.4. Canal Canal Curvature Curvature 3.4. Canal Curvature CanalCanal curvature curvature increased increased fromfrom the S3C S3C section section (0.12 (0.12 ± 0.06 0.06mm−1 mm) to the1) S1C to the section S1C (0.13 section ± 0.05 Canal curvature increased from the S3C section (0.12 ± 0.06 ±mm−1) to the− S1C section (0.13 ± 0.05 (0.13mm−0.051) and mm abruptly1) and increased abruptly increasedto the S1A to section the S1A (0.50 section ± 0.22 (0.50 mm−10.22) (Figure mm 11,1) (Figure Table 1). 11 , Table1). mm−1±) and abruptly− increased to the S1A section (0.50 ± 0.22 mm−1)± (Figure 11,− Table 1).

1 FigureFigure 11. Average11. Average degree degree of canal of canal curvatures curvatures (mm (mm− ). −1). Figure 11. Average degree of canal curvatures (mm−1). Appl. Sci. 2020, 10, 4385 9 of 11

4. Discussion To avoid procedural errors in root canal preparation, information on the root canal anatomy—especially data regarding the remaining dentin thickness, narrowest and widest canal widths, and 3D direction and degree of canal curvature—is relevant. Notwithstanding the large number of root canal anatomy studies of the mandibular incisors [10–13], these variables have scarcely been measured, possibly because of the lack of customized image analyzing software. In the present study, anatomical indicators were measured using MCT combined with custom-developed software, Kappa 2, which provided 3D surface models of the roots and canals and an imaginary central axis in each cross-sectional image. Based on our measurement results, clinicians can determine the initial apical file size when root canal treatment of mandibular incisor is performed. In the present study, the narrowest canal width was 0.22 0.05 mm in the S1A section, and the widest canal width was 0.40 0.11 mm in the S1A ± ± section. These results coincided with the finding reported by Milanezi de Almeida et al. that the medians of the buccolingual diameter in the apical 1 mm portion were 0.36 and 0.41 mm, respectively, in the Vertucci type I root canal and type III root canal [14]. Therefore, apical preparation should be performed at least up to the size of 0.40 mm for debridement of the necrotic pulp and removal of the infected dentin. The widest canal width of S2C (0.78 0.36 mm) was greater than that of S3C ± (0.59 0.38 mm). This interesting finding might result from root canal calcification in S3C and imply ± that these dentin collars should be removed for efficient root canal preparation. Concurrently, detailed measurement of 3D canal curvature was obtained. It was the greatest in the apical region, followed by the coronal regions. The 3D canal curvature was the straightest in the mid-root region. The overall curvature of mandibular incisors measured in this study (0.18 0.27 mm 1) ± − was less than that of the mesiobuccal canal of the maxillary first molar (0.22 0.06 mm 1)[9] and the ± − mesiobuccal (0.26 0.23 mm 1) and mesiolingual canals (0.22 0.18 mm 1) of the mandibular first ± − ± − molar [7]. These results imply that root canal preparation in mandibular incisors is not as difficult as in the mesiobuccal canal of the maxillary first molar or the mesiobuccal and mesiolingual canals of the mandibular first molar. Knowledge of the thinnest dentin, particularly in the apical part, is useful for preventing apical perforation when root canal shaping is performed. The mean (SD) value of MinDist was 0.56 (0.12) mm in the S1A section, which is susceptible to apical or strip perforation and weakening of the tooth structure. While the average mesial and distal dentin thicknesses were both 0.66 mm, the average labial and lingual dentin thicknesses were 1.13 and 1.27 mm, respectively. Therefore, instrumentation should mainly be directed toward the buccal and lingual directions, rather than the mesial and distal directions. Furthermore, the apical third (S1) has the most severe curvature, a sharp decrease in dentin thickness, and an increased frequency of the thinnest dentin being directed mesially. These features may lead to procedural errors such as transportation, ledge formation, loss of patency, and perforation. Owing to the development of the nickel-titanium rotary instruments, these iatrogenic events could be minimized by the use of flexible instruments. Previous studies reported that root canal preparation with nickel-titanium rotary files enabled the maintenance of the original canal anatomy with less canal transportation and better centering ability compared to root canal preparation with stainless-steel K files [15–17]. In the present study, detailed anatomical indicators were obtained at high precision using MCT with Kappa 2 software [18]. Nonetheless, clinical application of MCT to patients is not allowed because of the high radiation dosage and long scanning time [19,20]. A limitation of this study was the small sample size. Further studies are required in order to examine a large number of specimens.

5. Conclusions Anatomical variables of the mandibular incisors, such as dentin thickness, direction of the thinnest dentin, root canal width, and root canal curvature, were analyzed using MCT with Kappa 2 software. The distance between the root canal wall and the root outline was the thinnest on the mesiolingual side Appl. Sci. 2020, 10, 4385 10 of 11 in the apical third. The canal width decreased gradually from the CEJ to the apex, and the root canal curvature was most severe in the apical third. Based on these measurements, clinicians can perform successful root canal preparation from a biologic perspective, without any procedural error.

Author Contributions: Conceptualization: J.L. and S.W.C.; Methodology: S-H.L., J.-R.H., and S.W.C.; Software: J.L., J.-R.H., K.-Y.K., and J.-H.J.; Validation: K.-Y.K., S.O., A.S.A.-G., and A.O.M.; Formal Analysis: J.L., S.O., A.S.A.-G., F.A.A.-G., and A.O.M.; Resources: J.-R.H. and A.S.A.-G.; Writing—Original Draft Preparation: J.L. and S.-H.L.; Writing—Review and Editing: S.O., SW.C., and K.-Y.K.; Visualization: A.S.A.-G. F.A.A.-G. A.O.M., and J.-H.J.; Supervision: S.W.C. and K.-Y.K.; Project Administration: J.L. and S.-H.L. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors have no conﬂict of interest.

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