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Four-rooted permanent maxillary first and second molars in a northwestern Chinese population
archives of oral biology 60 (2015) 811–817
Available online at www.sciencedirect.com
ScienceDirect journal homepage: http://www.elsevier.com/locate/aob
Four-rooted permanent maxillary first and second molars in a northwestern Chinese population Yongchun Gu a,b,1,*, Wei Wang b,1, Longxing Ni b a
Department of Stomatology, First People’s Hospital of Wujiang District, Nantong University, Suzhou, China State Key Laboratory of Military Stomatology, Department of Endodontics, School of Stomatology, The Fourth Military Medical University, Xi’an, China b
article info
abstract
Article history:
Objectives: To detect the incidence and anatomic features of 4-rooted permanent maxillary
Accepted 24 February 2015
molars in a northwestern Chinese population by using cone–beam computed tomography (CBCT).
Keywords:
Design: A total of 725 Chinese patients with well-developed maxillary permanent first and/
Root variation
or second molars were examined by using CBCT. The number of roots and canals, shape,
Four-rooted maxillary molar
location, and division level of the roots were evaluated. Four-rooted maxillary first and
Cone–beam computed tomography
second molars were classified according to modified Versiani’s classification. The root length and cross-sectional diameters of 4-rooted molars were measured by the software Galileos Implant 1.7 (SICAT GmbH & Co. KG, Bonn, Germany). One way analysis of variance followed by a Newman–Keuls post hoc test was used to compare the groups in relation to the root length and diameters (P = 0.05). Results: Among 1365 maxillary first molars, only one tooth had four separated roots, whereas in second molars, 12 out of 1226 teeth (0.98%) exhibited an extra root; the incidences were 1.28% (7/549) for males and 0.74% (5/677) for females. All 4-rooted molars observed were unilateral. Out of a total of thirteen 4-rooted maxillary molars, six exhibited the type I root form, four exhibited the type II root form and one exhibited the type III root form; a type IV category (two molars) was added to Versiani’s classification, which described maxillary molars with three buccal roots. A transitional root form between the typical 3rooted form and double palatal roots was observed in six cases of 3-rooted molars. Measurements of root size showed that the diameter of the distopalatal roots was significantly larger than the extra mesiopalatal roots (P < 0.01). Conclusion: The prevalence of the 4-rooted maxillary first and second molars in the Chinese population is low. They usually occurred unilaterally, and the shape, location and size of the four roots may be variable. # 2015 Elsevier Ltd. All rights reserved.
* Corresponding author at: Department of Dentistry, First People’s Hospital of Wujiang District, Nantong University, Suzhou 215200, China. Tel.: +86 0512 63003231; fax: +86 0512 63422068. E-mail address: [email protected] (Y. Gu). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.archoralbio.2015.02.024 0003–9969/# 2015 Elsevier Ltd. All rights reserved.
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1.
archives of oral biology 60 (2015) 811–817
Introduction
Permanent maxillary molars typically have three roots (mesiobuccal, distobuccal and palatal).1 However, reports on variations with respect to number of roots and canals are not rare in dental literature.2,3 Previous morphological studies on maxillary molars usually focused on the presence of the fourth canal (MB2) in the mesiobuccal roots, as well as the complex canal forms in the fused roots.2–5 Only very few clinicians were aware of the 4-rooted maxillary molar due to its low incidence.6,7 The extra root and canal may lead to treatment failure if clinicians fail to identify its presence, or manage it improperly. Libfeld and Rotstein6 carried out a radiographic survey on 1200 in vivo maxillary second molars, and reported that only 0.4% of the teeth presented with four separate roots. However, the diagnostic periapical film is a 2-dimensional image of complex 3-dimensional anatomy. Because of superimposition and distortion, it is of limited value for detecting subtle anatomical structures.8 Recently, Versiani et al.,7 collected 25 extracted 4-rooted maxillary second molars in a Brazilian population, and the external and internal root structures were analyzed by using high-resolution micro-computed tomography (micro-CT). The specimens were classified into three categories according to the divergence of their roots. However, micro-CT can only scan extracted teeth, and it does not permit in vivo study on the distribution pattern of this root trait in human dentitions. In recent years, cone–beam computed tomography (CBCT) has been clinically used to evaluate the root and canal anatomy because of its high resolution and noninvasion.2,3 It differs from conventional CT imaging in that the whole volume of data is acquired in the course of a single sweep of the scanner. Therefore, the scanning time and the dose of radiation can be significantly reduced. CBCT allows for in vivo studies of the root variations in human dentitions. The purpose of this study was to investigate the incidence and anatomic features of 4-rooted permanent maxillary first and second molars in a northwestern Chinese population by using in vivo CBCT technique.
2.
Materials and methods
This study was approved by the Ethics Committee of the Fourth Military Medical University. Digital CBCT images of maxillary first and second molars were collected from 780 patients at Hospital of Stomatology, Fourth Military Medical University, Xi0 an, China, from October 2008 to December 2013. The patients underwent CBCT examination due to pathology such as impacted third molars, temporomandibular joint disorders, tooth implantation, endodontic and periodontal disease, and orthodontic reasons. The patients were scanned by a CBCT machine (Galileos, Sirona Dental Systems GmbH, Bensheim, Germany) with isotropic voxel sizes of 125 mm. Software Galileos Implant 1.7 (SICAT GmbH & Co. KG, Bonn, Germany) was used to read the images. Serial axial, coronal and sagittal CBCT images were examined by carefully rolling the toolbar.
The inclusion criteria were the following: (1) the subjects were native Chinese; (2) scans containing permanent maxillary first and/or second molars; (3) teeth without fractures, open apices or resorption; (4) good quality CBCT images available. A total of 740 patients (353 males and 387 females) were included in this investigation. The ages ranged from 10 to 86 years, with a median value of 35 years, an upper quartile of 49 years, and a lower quartile of 22 years. Among them, 719 individuals presented maxillary first molars unilaterally (73 patients) or bilaterally (646 patients), and 650 individuals presented maxillary second molars unilaterally (74 patients) and bilaterally (576 patients). The following anatomic features were analyzed: number and morphology of roots and canals, type of root form, and the size of the root (root length and cross-sectional diameter). A root with bifurcation within the level of apical 1/4 was regarded as a single root.2 The types of 4-rooted maxillary molars were determined according to modified classification of Versiani et al.7 In type I, the two palatal roots were widely divergent and often longer and more tortuous than the two buccal roots which were less divergent and often ‘‘cow-horn’’ shaped. In type II, both the two buccal roots, and the two palatal roots had blunt apices, ran almost parallel to each other, and were often shorter than type I teeth. In type III, the two palatal roots were less divergent and often shorter than the two buccal roots that were widely divergent. A type IV category was added to Versiani’s classification, which described maxillary molars with three buccal roots. The crosssectional diameters of the four roots were detected at the midpoint level between the cemento-enamel junction (CEJ) and the apex of the longest root. The buccolingual and mesiodistal diameter, and the root length (vertical distance from the apex to the CEJ along the tooth axis) were measured by the software Galileos Implant 1.7. One 3-rooted maxillary first molar with unusual mesiobuccal and palatal root was extracted after CBCT examination due to periodontal reasons. The tooth was prepared and scanned by a micro-CT (Inveon; Siemens Medical Solutions, Knoxville, TN) as previously described.9 Briefly, all scans were performed with an aluminium filter using 80 kV and 500 mA with a 500-ms exposure time. A 3608 scan of each specimen was performed around the tooth axis with voxel sizes of 15 mm 15 mm 15 mm. The data sets (DICOM format) were transferred to software Mimics 10.01 (Materialise, Leuven, Belgium). The anatomies of the teeth and root canal systems were reconstructed 3-dimensionally with a semi-automatic threshold-based segmentation approach. One way analysis of variance (ANOVA) followed by a Newman–Keuls post hoc test was used to compare the groups in relation to the root length and diameters. The level of statistical significance was set at P < 0.05.
3.
Results
The findings are summarised in Table 1. Among 1365 first molars, only one tooth (male) exhibited four separated roots, whereas in maxillary second molars, 12 out of 1226 teeth had an extra root. The incidence was 1.28% (7/549 teeth) for males
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Table 1 – The root number of maxillary first and second molars. N
1-Rooted n
Maxillary first molars Male Female Both genders
2-Rooted
3-Rooted
4-Rooted
%
n
%
n
%
n
%
641 724 1365
1 0 1
0.16 0 0.07
7 22 29
1.09 3.04 2.12
632 702 1332
98.60 96.96 97.58
1 0 1
0.16 0 0.07
Maxillary second molars 549 Male 677 Female 1226 Both genders
66 107 173
12.02 15.81 14.11
110 158 268
20.04 23.34 21.86
366 407 773
66.67 60.12 63.05
7 5 12
1.28 0.74 0.98
and 0.74% (5/677 teeth) for females; and the prevalence rate of 4-rooted maxillary second molars was 1.85% (12/650 patients) according to individual count method. Out of a total of thirteen 4-rooted maxillary molars, six exhibited type I root form (Fig. 1), four exhibited type II root form, and one exhibited type III root form. One additional root form beyond Versiani’s classification was identified in two males (one right and one left maxillary second molar), and we classified it as the type IV root form. The extra root can be located between the normal sites of the mesiobuccal and distobuccal roots, and it corresponds to an extra cuspid on the buccal surface of the crown, which is also named parastyle in dental anthropology (Fig. 2). It was separated from the tooth surface forming a free cusp tip at the occlusal third. All the 4rooted molars occurred unilaterally except in two cases, where the antimetric molars were missing (extracted). The patients’ information and anatomic descriptions of the root are summarised in Table 2. Table 3 shows the measurements of the root length and cross-sectional diameters (buccolingual and mesiodistal) of 4-rooted molars. As far as the root canal systems were concerned, we found that 4-rooted maxillary molars unexceptionally contained four separated canals (each root contained one canal). In 1 first molar and 5 second molars (Table 4), variations were manifested in the contour, size, and location of the roots, but not in the root number. Among three roots, the mesiobuccal root was the strongest one. It expanded buccolingually, occupying the whole mesial space. Whilst the palatal root was smaller than normal, and shifted to a more distal site. The lingually displaced mesiobuccal root always exhibited two longitudinal depressions (one mesial and one distal) on the proximal surfaces, and sometimes (4 out of 6 molars) divided into two apexes in the proximal view, whereas the level of root bifurcation was limited to the apical 1/4 (Fig. 3).
4.
first molars (97.58%), and more than half number of the maxillary second molars (63.05%) were typically 3-rooted. Nearly all the other sample teeth exhibited root fusion (double- or single-rooted). Kim et al.2 reported that the incidence of root fusion was 0.73% in the first molars and 10.71% in the second molars in a Korean population, which was higher than most of the previous studies in other areas, but much lower than 2.19% (30/1365 teeth) of the first molars and 35.97% (441/1226 teeth) of the second molars in this study. The incidence of 4-rooted maxillary second molars was only 0.98% (12/1226 teeth), higher than 0.42% (5/1200 teeth) reported by Libfeld and Rotstein6 in an Israeli population and 0.48% (4/821 teeth) by Kim et al.2 in a Korean population, but lower than 1.4% (7/520 teeth) reported by Peikoff et al.11 in a Canadian population. The difference may be related to the
Discussion
CBCT can provide high-resolution images in multiple planes of space while eliminating superimposition of surrounding structures, and it is a reliable method to detect root and canal variation when compared with the gold standard of physical sectioning of the specimen.10 By using this in vivo technique, we examined incidence of 4-rooted maxillary molars in a northwest Chinese population with a large sample size. A total of 1365 maxillary first and 1226 maxillary second molars were investigated. The results showed that most of the maxillary
Fig. 1 – CBCT 2D cross-section of a left maxillary second molar with double palatal roots (type I root form).
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Fig. 2 – CBCT 2D cross-sections of a right maxillary second molar with three buccal roots (type IV root form). (A) A crosssection at the level of the middle third of the root, arrow is the midbuccal root, and (B–G) cross-sections at different levels, arrow is an extra cuspid on the buccal surface of the crown ( parastyle).
methodology and sample size, as well as the genetic and racial factors. Alani12 had reported a case of bilateral 4-rooted maxillary second molars, whereas in this study, all the 13 cases were unilateral. The antimetric teeth could be 3-rooted (8 cases), 2-rooted (3 cases), and in two cases, the antimetric teeth were missing (Table 2). This finding indicates that if an extra root is found on a molar, the possibility of detecting an additional root in the contralateral molar is very low. Previous studies showed that a low occurrence rate of a morphological dental trait is more likely to correspond to a low bilateral concurrence rate.13 This rule is applicable in most instances (e.g., Carabelli’s trait, 3-rooted mandibular molar and
C-shaped mandibular molar)13 but not all. The first author and other investigators14 previously reported that the occurrence of central cusp deformity (odontome) on maxillary first premolars in a Chinese population was only 0.41% (31/7632 teeth), whereas the bilateral concurrence rate reached as high as 93.75%. It was believed that minor and random anatomic deviations from symmetry (fluctuating asymmetry) are caused by local background noise during development.13 Although the detailed mechanism is still unclear, understanding the distribution patterns may be useful for the dentists to evaluate the probability of occurrence of root abbreviation, and make a proper treatment plan.
Table 2 – Thirteen maxillary molars with four separated roots. Subject no. 1 2 3 4 5 6 7 8 9 10 11 12 13
Age (years) 26 46 39 59 25 43 39 26 41 67 50 56 23
Sex
Side
Molar type
Root type
Location of the extra root
Antimetric teeth
M F F F F F M M M M M M M
Left Right Left Left Right Right Right Left Left Left Left Left Right
1st molar 2nd molar 2nd molar 2nd molar 2nd molar 2nd molar 2nd molar 2nd molar 2nd molar 2nd molar 2nd molar 2nd molar 2nd molar
Type Type Type Type Type Type Type Type Type Type Type Type Type
Lingual side of MBR Lingual side of MBR Lingual side of MBR Lingual side of MBR Lingual side of MBR Lingual side of MBR Lingual side of MBR Lingual side of MBR Lingual side of MBR Lingual side of MBR Lingual side of DBR Between MBR and DBR Between MBR and DBR
3-Rooted 3-Rooted 2-Rooted 3-Rooted 2-Rooted 2-Rooted 3-Rooted 3-Rooted Data miss Data miss 3-Rooted 3-Rooted 3-Rooted
I I I II II III I I I II II IV IV
MBR is mesiobuccal root, DBR is distobuccal root, and ‘data miss’ means the tooth is missing.
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Table 3 – Measurement of root length, buccolingual and mesiodistal diameters of the four roots (mean W SD, mm).
Table 4 – Six 3-rooted maxillary molars with abnormal root configuration (transitional root form between a typical 3- and 4-rooted molars).
Root
Teeth Age Sex no.
Side
Molar type
1 2 3 4 5 6
Left Right Right Right Right Left
1st molar 2nd molar 2nd molar 2nd molar 2nd molar 2nd molar
Root length
BL diameter
MD diameter
Double-palatal-rooted molars (n = 11) 10.51 1.68 4.84 0.60y MB 10.89 1.75 4.23 0.54 * DB DP 11.41 2.09 4.57 0.88y MP 9.78 3.47 3.55 0.60
2.81 0.47 3.28 0.51 * 3.66 0.67y 2.73 0.32
Three-buccal-rooted molars (n = 2) MB 11.22 1.14 5.81 0.13 12.05 1.35 4.71 0.34 DB 12.90 1.28 4.32 1.36 P 12.12 1.50 4.37 0.10 Midbuccal
2.95 0.16 3.57 0.47 4.7 0.55 4.19 0.69
MB, mesiobuccal; DB, distobuccal; DP, distopalatal; MP, mesiopalatal; P, palatal. * Statistical significant (P < 0.05). y Statistical significant (P < 0.01) (comparison with the MP root).
Christie et al.15 first proposed a classification system based on a retrospective clinical study on 16 cases of maxillary molars with two palatal roots occurring in their 40 years of full-time endodontic practice. The teeth were classified into three types according to the separating level and divergence of the roots. The type I molars have two widely divergent palatal roots which are often long and tortuous, whereas the buccal roots are less divergent. The roots of a type II molar are often shorter, run parallel, and have blunt root apices. A type III molar exhibits fused roots between mesiobuccal, mesiopalatal, and distopalatal root/canal, whereas the distobuccal root appears to stand alone and may diverge distobuccally. Considering that the identification of a type II and III cases may be difficult on a radiograph because of buccolingual superimposition, Versiani et al.,7 combined them into one category (type II), and the type III molars were redefined as those cases exhibiting more divergent buccal roots than the palatal roots. Although Christie’s and Versiani’s classifications could cover a lot of situations, they neglected an important category, the maxillary molars with three buccal roots, which
25 28 22 37 70 70
M F F F M M
Number Number of canals of root apex in the MBR 2 2 1 2 2 1
3 2 2 2 2 2
had been previously reported by several scholars.2,16–18 In this study, nearly half the number of 4-rooted molars were type I cases (6/13), and type II cases approximately accounted for one third (4/13). The type IV (two cases) and III (one case) molars were even more scarce. However, the clinicians should consider all the possible variations to make sure that each root/canal is treated properly. Wide root divergence, severe canal curvature, and long and tortuous canals are characteristics (of type I and III molars) that may present challenges to the endodontic treatment. Conventional instrumentation with steel files might produce ledges, zips, elbows, or perforations.19 Nickel–titanium file can reduce the procedural errors because of super elastic behaviour, however, it might undergo unexpected fracture due to cyclic fatigue.20 Therefore, adequate coronal flaring is essential to reduce the canal curvature and gain straight access to the apical portion. Regarding the type II cases, attention should be drawn to the detection of the extra root/ canal. Additional angled radiographs or CBCT scanning is advocated to provide more information about the root anatomy. The standard triangular access form for 3-rooted maxillary molars is no longer appropriate for 4-rooted molars with four displaced canal orifices. By extending the access cavity outline form, all of the orifices can be negotiated. Jain et al.,21 recently reported two cases of maxillary second molars fused with paramolar tubercles diagnosed by CBCT. In both
Fig. 3 – Micro-CT 3D images of a right maxillary first molar exhibit transitional root form between the typical 3-rooted form and double palatal roots. OV: occlusal view; AV: apical view; BV: buccal view; LV: lingual view, arrow is cusp of Carabelli, palatal root fractured during root extraction; MV: mesial view, arrow is apical bifurcation; DV: distal view.
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cases, pulpal communication was detected between the paramolar tubercle and the mesiobuccal canal, and this may pose challenge to thorough cleaning and filling of the complex root canal system. Whereas in this study, the canal in the midbuccal root of type IV molars was independent to other canals. While the aetiology of this anomaly is still unknown, the size, location, and division level of the extra root, as well as the configuration of the internal root canals may provide some clues. Hess and Zurcher22 pointed out that the extra root was divided from the mesiobucal root. The present findings support this viewpoint. The additional root was usually located at the palatal side; dentine connection was commonly observed between the mesiobucal and mesiopalatal roots (Fig. 1), whereas the distobuccal root is usually stand alone (a type III molar in Christie’s classification is a good example). As long as two palatal roots were presented, the mesiopalatal root was always smaller than the distopalatal (Table 3, Fig. 1). The type IV molar was a more confusing phenomenon. The root located between the original sites of the mesiobuccal and distobuccal root was usually considered as the extra root (midbuccal root), whereas measurement of root size indicated that this root was not always the smallest one among the three buccal roots (Table 3, Fig. 2). In two cases of this study, the midbuccal root was concurrent with a supernumerary cuspid on the buccal surface of the crown (Fig. 2), which is named as parastyle or paramolar tubercle in dental anthropology.13 This finding seems to support some other scholars who believe that the 4-rooted maxillary molar represents tooth fusion with a supernumerary molar ( paramolar).23 However, in regard to the internal root canal morphology, we found that no matter which molar type it belongs to, a 4-rooted molar always contains four independent canals (one root contained one canal). Therefore, we would rather believe that the mesiobucal root, typically having two canals, was now divided into two roots with two separated canals. However, the shape, location and size of the four roots may still be variable, and in some occasions, it would be difficult to determine which root was the extra one. The six 3-rooted maxillary molars listed in Table 3 were quite unusual (Fig. 3). The mesiobuccal root expanded buccolingually and occupied the whole mesial space, whereas the palatal root was relatively slim and tiny, and looked like it was being pushed distally by the mesial root. In some occasions, the size of the palatal root was even smaller than the distobuccal root, and this root form is quite similar to that of the 3-rooted mandibular molar. The robust ‘mesial’ root usually exhibited deep longitudinal depressions at the proximal surfaces, as well as an apical bifurcation, suggesting a trend (incomplete form) of root division. We regard these molars as the transitional form between the typical 3-rooted molars and double palatal roots, whereas in Versiani’s study, they were classified into 4-rooted molars with fusion roots.7 There is no report on proper management of this type of root variation. Theoretically, the strategy may be similar to that of a type II molar. Attention should be put on the abbreviated location and size of the two palatal canals. In summary, the prevalence of the 4-rooted maxillary first and second molars in the northwestern Chinese population was very low. They usually occurred unilaterally, and the shape, location and size of the four roots may be variable.
Clinicians should be aware of the occurrence of the extra root during endodontic treatment, and neglecting this anatomical variation can compromise the therapy outcomes.
Funding This research was supported by The First People’s Hospital of Wujiang District, Suzhou, China.
Competing interests None of the authors had any personal or financial conflicts of interest relevant to the conflict of interest policy expressed in this editorial.
Ethical approval Ethical approval was given by the Medical Ethics Committee of The First People’s Hospital of Wujiang District and The Fourth Military Medical University.
references
1. Scheid RC, Weiss G. Woefel’s dental anatomy. 8th ed. Philadelphia: Lippincott Williams & Wikins; 2012. 2. Kim Y, Lee S, Woo J. Morphology of maxillary first and second molars analyzed by cone–beam computed tomography in a Korean population: variations in the number of roots and canals and the incidence of fusion. J Endod 2012;38(8):1063–8. 3. Zhang R, Yang H, Yu X, Wang H, Hu T, Dummer PM. Use of CBCT to identify the morphology of maxillary permanent molar teeth in a Chinese subpopulation. Int Endod J 2011;44(2):162–9. 4. Hartwell G, Bellizi R. Clinical investigation of in vivo endodontically treated mandibular and maxillary molars. J Endod 1982;8(12):555–7. 5. Kulild JC, Peters DD. Incidence and configuration of canal systems in the mesiobuccal root of maxillary first and second molars. J Endod 1990;16(7):311–7. 6. Libfeld H, Rotstein I. Incidence of four-rooted maxillary second molars: Literature review and radiographic survey of 1200 teeth. J Endod 1989;15(3):129–31. 7. Versiani MA, Pecora JD, de Sousa-Neto MD. Root and root canal morphology of four-rooted maxillary second molars – a micro-computed tomography study. J Endod 2012;38(7):977–82. 8. Gelfand M, Sunderman EJ, Goldman M. Reliability of radiographical interpretations. J Endod 1983;9(2):71–5. 9. Gu Y, Zhang YP, Liao ZG. Root and canal morphology of mandibular first premolars with radicular grooves. Arch Oral Biol 2013;58(11):1609–17. 10. Blattner TC, George N, Lee CC, Kumar V, Yelton CD. Efficacy of cone–beam computed tomography as a modality to accurately identify the presence of second mesiobuccal canals in maxillary first and second molars: a pilot study. J Endod 2010;36(5):867–70. 11. Peikoff MD, Christie WH, Fogel HM. The maxillary second molar: variations in the number of roots and canals. Int Endod J 1996;29(6):365–9.
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12. Alani AH. Endodontic treatment of bilaterally occurring four-rooted maxillary second molars: case report. J Can Dent Assoc 2003;69(11):733–5. 13. Scott RG, Turner II CG. The anthropology of modern human teeth. Cambridge: Cambridge University Press; 1997. 14. Liu F, Gu Y. Investigation of central cusps on premolars in the Han Chinese middle school students in Urumqi city. J Pract Stomatol 2012;28(6):779–82. [in Chinese]. 15. Christie WH, Peikoff MD, Fogel HM. Maxillary molars with two palatal roots: a retrospective clinical study. J Endod 1991;17(2):80–4. 16. Fahid A, Taintor JF. Maxillary second molar with three buccal roots. J Endod 1988;14(4):181–3. 17. Zmener O, Peirano A. Endodontic therapy in a maxillary second molar with three buccal roots. J Endod 1998;24(5):376–7. 18. Jafarzadeh H, Javidi M, Zarei M. Endodontic retreatment of a maxillary second molar with three separate buccal roots. Aust Endod J 2006;32(3):129–32.
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19. Peters OA. Current challenges and concepts in the preparation of root canal systems: a review. J Endod 2004;30(8):559–67. 20. Pruett JP, Clement DJ, Carnes Jr DL. Cyclic fatigue testing of nickel–titanium endodontic instruments. J Endod 1997;23(2):77–85. 21. Jain P, Ananthnarayan K, Ballal S, Natanasabapathy V. Endodontic management of maxillary second molars fused with paramolar tubercles diagnosed by cone beam computed tomography – two case reports. J Dent (Tehran) 2014;11(6):1–7. 22. Hess W, Zurcher E. The anatomy of the root–canals of the teeth of the permanent dentition. London: John Bale, Sons & Danielsson; 1925. 23. Friedman S, Stabholtz A, Rotstein I. Endodontic management of molars with developmental anomalies. Int Endod J 1986;19(6):267–76.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: http://www.elsevier.com/locate/aob
Four-rooted permanent maxillary first and second molars in a northwestern Chinese population Yongchun Gu a,b,1,*, Wei Wang b,1, Longxing Ni b a
Department of Stomatology, First People’s Hospital of Wujiang District, Nantong University, Suzhou, China State Key Laboratory of Military Stomatology, Department of Endodontics, School of Stomatology, The Fourth Military Medical University, Xi’an, China b
article info
abstract
Article history:
Objectives: To detect the incidence and anatomic features of 4-rooted permanent maxillary
Accepted 24 February 2015
molars in a northwestern Chinese population by using cone–beam computed tomography (CBCT).
Keywords:
Design: A total of 725 Chinese patients with well-developed maxillary permanent first and/
Root variation
or second molars were examined by using CBCT. The number of roots and canals, shape,
Four-rooted maxillary molar
location, and division level of the roots were evaluated. Four-rooted maxillary first and
Cone–beam computed tomography
second molars were classified according to modified Versiani’s classification. The root length and cross-sectional diameters of 4-rooted molars were measured by the software Galileos Implant 1.7 (SICAT GmbH & Co. KG, Bonn, Germany). One way analysis of variance followed by a Newman–Keuls post hoc test was used to compare the groups in relation to the root length and diameters (P = 0.05). Results: Among 1365 maxillary first molars, only one tooth had four separated roots, whereas in second molars, 12 out of 1226 teeth (0.98%) exhibited an extra root; the incidences were 1.28% (7/549) for males and 0.74% (5/677) for females. All 4-rooted molars observed were unilateral. Out of a total of thirteen 4-rooted maxillary molars, six exhibited the type I root form, four exhibited the type II root form and one exhibited the type III root form; a type IV category (two molars) was added to Versiani’s classification, which described maxillary molars with three buccal roots. A transitional root form between the typical 3rooted form and double palatal roots was observed in six cases of 3-rooted molars. Measurements of root size showed that the diameter of the distopalatal roots was significantly larger than the extra mesiopalatal roots (P < 0.01). Conclusion: The prevalence of the 4-rooted maxillary first and second molars in the Chinese population is low. They usually occurred unilaterally, and the shape, location and size of the four roots may be variable. # 2015 Elsevier Ltd. All rights reserved.
* Corresponding author at: Department of Dentistry, First People’s Hospital of Wujiang District, Nantong University, Suzhou 215200, China. Tel.: +86 0512 63003231; fax: +86 0512 63422068. E-mail address: [email protected] (Y. Gu). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.archoralbio.2015.02.024 0003–9969/# 2015 Elsevier Ltd. All rights reserved.
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Introduction
Permanent maxillary molars typically have three roots (mesiobuccal, distobuccal and palatal).1 However, reports on variations with respect to number of roots and canals are not rare in dental literature.2,3 Previous morphological studies on maxillary molars usually focused on the presence of the fourth canal (MB2) in the mesiobuccal roots, as well as the complex canal forms in the fused roots.2–5 Only very few clinicians were aware of the 4-rooted maxillary molar due to its low incidence.6,7 The extra root and canal may lead to treatment failure if clinicians fail to identify its presence, or manage it improperly. Libfeld and Rotstein6 carried out a radiographic survey on 1200 in vivo maxillary second molars, and reported that only 0.4% of the teeth presented with four separate roots. However, the diagnostic periapical film is a 2-dimensional image of complex 3-dimensional anatomy. Because of superimposition and distortion, it is of limited value for detecting subtle anatomical structures.8 Recently, Versiani et al.,7 collected 25 extracted 4-rooted maxillary second molars in a Brazilian population, and the external and internal root structures were analyzed by using high-resolution micro-computed tomography (micro-CT). The specimens were classified into three categories according to the divergence of their roots. However, micro-CT can only scan extracted teeth, and it does not permit in vivo study on the distribution pattern of this root trait in human dentitions. In recent years, cone–beam computed tomography (CBCT) has been clinically used to evaluate the root and canal anatomy because of its high resolution and noninvasion.2,3 It differs from conventional CT imaging in that the whole volume of data is acquired in the course of a single sweep of the scanner. Therefore, the scanning time and the dose of radiation can be significantly reduced. CBCT allows for in vivo studies of the root variations in human dentitions. The purpose of this study was to investigate the incidence and anatomic features of 4-rooted permanent maxillary first and second molars in a northwestern Chinese population by using in vivo CBCT technique.
2.
Materials and methods
This study was approved by the Ethics Committee of the Fourth Military Medical University. Digital CBCT images of maxillary first and second molars were collected from 780 patients at Hospital of Stomatology, Fourth Military Medical University, Xi0 an, China, from October 2008 to December 2013. The patients underwent CBCT examination due to pathology such as impacted third molars, temporomandibular joint disorders, tooth implantation, endodontic and periodontal disease, and orthodontic reasons. The patients were scanned by a CBCT machine (Galileos, Sirona Dental Systems GmbH, Bensheim, Germany) with isotropic voxel sizes of 125 mm. Software Galileos Implant 1.7 (SICAT GmbH & Co. KG, Bonn, Germany) was used to read the images. Serial axial, coronal and sagittal CBCT images were examined by carefully rolling the toolbar.
The inclusion criteria were the following: (1) the subjects were native Chinese; (2) scans containing permanent maxillary first and/or second molars; (3) teeth without fractures, open apices or resorption; (4) good quality CBCT images available. A total of 740 patients (353 males and 387 females) were included in this investigation. The ages ranged from 10 to 86 years, with a median value of 35 years, an upper quartile of 49 years, and a lower quartile of 22 years. Among them, 719 individuals presented maxillary first molars unilaterally (73 patients) or bilaterally (646 patients), and 650 individuals presented maxillary second molars unilaterally (74 patients) and bilaterally (576 patients). The following anatomic features were analyzed: number and morphology of roots and canals, type of root form, and the size of the root (root length and cross-sectional diameter). A root with bifurcation within the level of apical 1/4 was regarded as a single root.2 The types of 4-rooted maxillary molars were determined according to modified classification of Versiani et al.7 In type I, the two palatal roots were widely divergent and often longer and more tortuous than the two buccal roots which were less divergent and often ‘‘cow-horn’’ shaped. In type II, both the two buccal roots, and the two palatal roots had blunt apices, ran almost parallel to each other, and were often shorter than type I teeth. In type III, the two palatal roots were less divergent and often shorter than the two buccal roots that were widely divergent. A type IV category was added to Versiani’s classification, which described maxillary molars with three buccal roots. The crosssectional diameters of the four roots were detected at the midpoint level between the cemento-enamel junction (CEJ) and the apex of the longest root. The buccolingual and mesiodistal diameter, and the root length (vertical distance from the apex to the CEJ along the tooth axis) were measured by the software Galileos Implant 1.7. One 3-rooted maxillary first molar with unusual mesiobuccal and palatal root was extracted after CBCT examination due to periodontal reasons. The tooth was prepared and scanned by a micro-CT (Inveon; Siemens Medical Solutions, Knoxville, TN) as previously described.9 Briefly, all scans were performed with an aluminium filter using 80 kV and 500 mA with a 500-ms exposure time. A 3608 scan of each specimen was performed around the tooth axis with voxel sizes of 15 mm 15 mm 15 mm. The data sets (DICOM format) were transferred to software Mimics 10.01 (Materialise, Leuven, Belgium). The anatomies of the teeth and root canal systems were reconstructed 3-dimensionally with a semi-automatic threshold-based segmentation approach. One way analysis of variance (ANOVA) followed by a Newman–Keuls post hoc test was used to compare the groups in relation to the root length and diameters. The level of statistical significance was set at P < 0.05.
3.
Results
The findings are summarised in Table 1. Among 1365 first molars, only one tooth (male) exhibited four separated roots, whereas in maxillary second molars, 12 out of 1226 teeth had an extra root. The incidence was 1.28% (7/549 teeth) for males
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Table 1 – The root number of maxillary first and second molars. N
1-Rooted n
Maxillary first molars Male Female Both genders
2-Rooted
3-Rooted
4-Rooted
%
n
%
n
%
n
%
641 724 1365
1 0 1
0.16 0 0.07
7 22 29
1.09 3.04 2.12
632 702 1332
98.60 96.96 97.58
1 0 1
0.16 0 0.07
Maxillary second molars 549 Male 677 Female 1226 Both genders
66 107 173
12.02 15.81 14.11
110 158 268
20.04 23.34 21.86
366 407 773
66.67 60.12 63.05
7 5 12
1.28 0.74 0.98
and 0.74% (5/677 teeth) for females; and the prevalence rate of 4-rooted maxillary second molars was 1.85% (12/650 patients) according to individual count method. Out of a total of thirteen 4-rooted maxillary molars, six exhibited type I root form (Fig. 1), four exhibited type II root form, and one exhibited type III root form. One additional root form beyond Versiani’s classification was identified in two males (one right and one left maxillary second molar), and we classified it as the type IV root form. The extra root can be located between the normal sites of the mesiobuccal and distobuccal roots, and it corresponds to an extra cuspid on the buccal surface of the crown, which is also named parastyle in dental anthropology (Fig. 2). It was separated from the tooth surface forming a free cusp tip at the occlusal third. All the 4rooted molars occurred unilaterally except in two cases, where the antimetric molars were missing (extracted). The patients’ information and anatomic descriptions of the root are summarised in Table 2. Table 3 shows the measurements of the root length and cross-sectional diameters (buccolingual and mesiodistal) of 4-rooted molars. As far as the root canal systems were concerned, we found that 4-rooted maxillary molars unexceptionally contained four separated canals (each root contained one canal). In 1 first molar and 5 second molars (Table 4), variations were manifested in the contour, size, and location of the roots, but not in the root number. Among three roots, the mesiobuccal root was the strongest one. It expanded buccolingually, occupying the whole mesial space. Whilst the palatal root was smaller than normal, and shifted to a more distal site. The lingually displaced mesiobuccal root always exhibited two longitudinal depressions (one mesial and one distal) on the proximal surfaces, and sometimes (4 out of 6 molars) divided into two apexes in the proximal view, whereas the level of root bifurcation was limited to the apical 1/4 (Fig. 3).
4.
first molars (97.58%), and more than half number of the maxillary second molars (63.05%) were typically 3-rooted. Nearly all the other sample teeth exhibited root fusion (double- or single-rooted). Kim et al.2 reported that the incidence of root fusion was 0.73% in the first molars and 10.71% in the second molars in a Korean population, which was higher than most of the previous studies in other areas, but much lower than 2.19% (30/1365 teeth) of the first molars and 35.97% (441/1226 teeth) of the second molars in this study. The incidence of 4-rooted maxillary second molars was only 0.98% (12/1226 teeth), higher than 0.42% (5/1200 teeth) reported by Libfeld and Rotstein6 in an Israeli population and 0.48% (4/821 teeth) by Kim et al.2 in a Korean population, but lower than 1.4% (7/520 teeth) reported by Peikoff et al.11 in a Canadian population. The difference may be related to the
Discussion
CBCT can provide high-resolution images in multiple planes of space while eliminating superimposition of surrounding structures, and it is a reliable method to detect root and canal variation when compared with the gold standard of physical sectioning of the specimen.10 By using this in vivo technique, we examined incidence of 4-rooted maxillary molars in a northwest Chinese population with a large sample size. A total of 1365 maxillary first and 1226 maxillary second molars were investigated. The results showed that most of the maxillary
Fig. 1 – CBCT 2D cross-section of a left maxillary second molar with double palatal roots (type I root form).
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Fig. 2 – CBCT 2D cross-sections of a right maxillary second molar with three buccal roots (type IV root form). (A) A crosssection at the level of the middle third of the root, arrow is the midbuccal root, and (B–G) cross-sections at different levels, arrow is an extra cuspid on the buccal surface of the crown ( parastyle).
methodology and sample size, as well as the genetic and racial factors. Alani12 had reported a case of bilateral 4-rooted maxillary second molars, whereas in this study, all the 13 cases were unilateral. The antimetric teeth could be 3-rooted (8 cases), 2-rooted (3 cases), and in two cases, the antimetric teeth were missing (Table 2). This finding indicates that if an extra root is found on a molar, the possibility of detecting an additional root in the contralateral molar is very low. Previous studies showed that a low occurrence rate of a morphological dental trait is more likely to correspond to a low bilateral concurrence rate.13 This rule is applicable in most instances (e.g., Carabelli’s trait, 3-rooted mandibular molar and
C-shaped mandibular molar)13 but not all. The first author and other investigators14 previously reported that the occurrence of central cusp deformity (odontome) on maxillary first premolars in a Chinese population was only 0.41% (31/7632 teeth), whereas the bilateral concurrence rate reached as high as 93.75%. It was believed that minor and random anatomic deviations from symmetry (fluctuating asymmetry) are caused by local background noise during development.13 Although the detailed mechanism is still unclear, understanding the distribution patterns may be useful for the dentists to evaluate the probability of occurrence of root abbreviation, and make a proper treatment plan.
Table 2 – Thirteen maxillary molars with four separated roots. Subject no. 1 2 3 4 5 6 7 8 9 10 11 12 13
Age (years) 26 46 39 59 25 43 39 26 41 67 50 56 23
Sex
Side
Molar type
Root type
Location of the extra root
Antimetric teeth
M F F F F F M M M M M M M
Left Right Left Left Right Right Right Left Left Left Left Left Right
1st molar 2nd molar 2nd molar 2nd molar 2nd molar 2nd molar 2nd molar 2nd molar 2nd molar 2nd molar 2nd molar 2nd molar 2nd molar
Type Type Type Type Type Type Type Type Type Type Type Type Type
Lingual side of MBR Lingual side of MBR Lingual side of MBR Lingual side of MBR Lingual side of MBR Lingual side of MBR Lingual side of MBR Lingual side of MBR Lingual side of MBR Lingual side of MBR Lingual side of DBR Between MBR and DBR Between MBR and DBR
3-Rooted 3-Rooted 2-Rooted 3-Rooted 2-Rooted 2-Rooted 3-Rooted 3-Rooted Data miss Data miss 3-Rooted 3-Rooted 3-Rooted
I I I II II III I I I II II IV IV
MBR is mesiobuccal root, DBR is distobuccal root, and ‘data miss’ means the tooth is missing.
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Table 3 – Measurement of root length, buccolingual and mesiodistal diameters of the four roots (mean W SD, mm).
Table 4 – Six 3-rooted maxillary molars with abnormal root configuration (transitional root form between a typical 3- and 4-rooted molars).
Root
Teeth Age Sex no.
Side
Molar type
1 2 3 4 5 6
Left Right Right Right Right Left
1st molar 2nd molar 2nd molar 2nd molar 2nd molar 2nd molar
Root length
BL diameter
MD diameter
Double-palatal-rooted molars (n = 11) 10.51 1.68 4.84 0.60y MB 10.89 1.75 4.23 0.54 * DB DP 11.41 2.09 4.57 0.88y MP 9.78 3.47 3.55 0.60
2.81 0.47 3.28 0.51 * 3.66 0.67y 2.73 0.32
Three-buccal-rooted molars (n = 2) MB 11.22 1.14 5.81 0.13 12.05 1.35 4.71 0.34 DB 12.90 1.28 4.32 1.36 P 12.12 1.50 4.37 0.10 Midbuccal
2.95 0.16 3.57 0.47 4.7 0.55 4.19 0.69
MB, mesiobuccal; DB, distobuccal; DP, distopalatal; MP, mesiopalatal; P, palatal. * Statistical significant (P < 0.05). y Statistical significant (P < 0.01) (comparison with the MP root).
Christie et al.15 first proposed a classification system based on a retrospective clinical study on 16 cases of maxillary molars with two palatal roots occurring in their 40 years of full-time endodontic practice. The teeth were classified into three types according to the separating level and divergence of the roots. The type I molars have two widely divergent palatal roots which are often long and tortuous, whereas the buccal roots are less divergent. The roots of a type II molar are often shorter, run parallel, and have blunt root apices. A type III molar exhibits fused roots between mesiobuccal, mesiopalatal, and distopalatal root/canal, whereas the distobuccal root appears to stand alone and may diverge distobuccally. Considering that the identification of a type II and III cases may be difficult on a radiograph because of buccolingual superimposition, Versiani et al.,7 combined them into one category (type II), and the type III molars were redefined as those cases exhibiting more divergent buccal roots than the palatal roots. Although Christie’s and Versiani’s classifications could cover a lot of situations, they neglected an important category, the maxillary molars with three buccal roots, which
25 28 22 37 70 70
M F F F M M
Number Number of canals of root apex in the MBR 2 2 1 2 2 1
3 2 2 2 2 2
had been previously reported by several scholars.2,16–18 In this study, nearly half the number of 4-rooted molars were type I cases (6/13), and type II cases approximately accounted for one third (4/13). The type IV (two cases) and III (one case) molars were even more scarce. However, the clinicians should consider all the possible variations to make sure that each root/canal is treated properly. Wide root divergence, severe canal curvature, and long and tortuous canals are characteristics (of type I and III molars) that may present challenges to the endodontic treatment. Conventional instrumentation with steel files might produce ledges, zips, elbows, or perforations.19 Nickel–titanium file can reduce the procedural errors because of super elastic behaviour, however, it might undergo unexpected fracture due to cyclic fatigue.20 Therefore, adequate coronal flaring is essential to reduce the canal curvature and gain straight access to the apical portion. Regarding the type II cases, attention should be drawn to the detection of the extra root/ canal. Additional angled radiographs or CBCT scanning is advocated to provide more information about the root anatomy. The standard triangular access form for 3-rooted maxillary molars is no longer appropriate for 4-rooted molars with four displaced canal orifices. By extending the access cavity outline form, all of the orifices can be negotiated. Jain et al.,21 recently reported two cases of maxillary second molars fused with paramolar tubercles diagnosed by CBCT. In both
Fig. 3 – Micro-CT 3D images of a right maxillary first molar exhibit transitional root form between the typical 3-rooted form and double palatal roots. OV: occlusal view; AV: apical view; BV: buccal view; LV: lingual view, arrow is cusp of Carabelli, palatal root fractured during root extraction; MV: mesial view, arrow is apical bifurcation; DV: distal view.
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cases, pulpal communication was detected between the paramolar tubercle and the mesiobuccal canal, and this may pose challenge to thorough cleaning and filling of the complex root canal system. Whereas in this study, the canal in the midbuccal root of type IV molars was independent to other canals. While the aetiology of this anomaly is still unknown, the size, location, and division level of the extra root, as well as the configuration of the internal root canals may provide some clues. Hess and Zurcher22 pointed out that the extra root was divided from the mesiobucal root. The present findings support this viewpoint. The additional root was usually located at the palatal side; dentine connection was commonly observed between the mesiobucal and mesiopalatal roots (Fig. 1), whereas the distobuccal root is usually stand alone (a type III molar in Christie’s classification is a good example). As long as two palatal roots were presented, the mesiopalatal root was always smaller than the distopalatal (Table 3, Fig. 1). The type IV molar was a more confusing phenomenon. The root located between the original sites of the mesiobuccal and distobuccal root was usually considered as the extra root (midbuccal root), whereas measurement of root size indicated that this root was not always the smallest one among the three buccal roots (Table 3, Fig. 2). In two cases of this study, the midbuccal root was concurrent with a supernumerary cuspid on the buccal surface of the crown (Fig. 2), which is named as parastyle or paramolar tubercle in dental anthropology.13 This finding seems to support some other scholars who believe that the 4-rooted maxillary molar represents tooth fusion with a supernumerary molar ( paramolar).23 However, in regard to the internal root canal morphology, we found that no matter which molar type it belongs to, a 4-rooted molar always contains four independent canals (one root contained one canal). Therefore, we would rather believe that the mesiobucal root, typically having two canals, was now divided into two roots with two separated canals. However, the shape, location and size of the four roots may still be variable, and in some occasions, it would be difficult to determine which root was the extra one. The six 3-rooted maxillary molars listed in Table 3 were quite unusual (Fig. 3). The mesiobuccal root expanded buccolingually and occupied the whole mesial space, whereas the palatal root was relatively slim and tiny, and looked like it was being pushed distally by the mesial root. In some occasions, the size of the palatal root was even smaller than the distobuccal root, and this root form is quite similar to that of the 3-rooted mandibular molar. The robust ‘mesial’ root usually exhibited deep longitudinal depressions at the proximal surfaces, as well as an apical bifurcation, suggesting a trend (incomplete form) of root division. We regard these molars as the transitional form between the typical 3-rooted molars and double palatal roots, whereas in Versiani’s study, they were classified into 4-rooted molars with fusion roots.7 There is no report on proper management of this type of root variation. Theoretically, the strategy may be similar to that of a type II molar. Attention should be put on the abbreviated location and size of the two palatal canals. In summary, the prevalence of the 4-rooted maxillary first and second molars in the northwestern Chinese population was very low. They usually occurred unilaterally, and the shape, location and size of the four roots may be variable.
Clinicians should be aware of the occurrence of the extra root during endodontic treatment, and neglecting this anatomical variation can compromise the therapy outcomes.
Funding This research was supported by The First People’s Hospital of Wujiang District, Suzhou, China.
Competing interests None of the authors had any personal or financial conflicts of interest relevant to the conflict of interest policy expressed in this editorial.
Ethical approval Ethical approval was given by the Medical Ethics Committee of The First People’s Hospital of Wujiang District and The Fourth Military Medical University.
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