Prevalence of 3-rooted first permanent molars among aShanghai Chinese populationYa Yang, PhD,a Lu-Dong Zhang, MD,a Jian-ping Ge, MD,b and Ya-qin Zhu, DDS, PhD,a

Shanghai, ChinaSHANGHAI JIAO TONG UNIVERSITY SCHOOL OF MEDICINE AND TONGJI UNIVERSITY

Objectives. This study aimed to assess the prevalence of 3-rooted mandibular first molars among a Shanghai Chinesepopulation by using periapical radiographs.Methods. A total of 510 patients possessing both mandibular first molars were selected. The radiographs of these caseswere evaluated under optimal conditions using double magnifying glasses. The incidence of 3-rooted mandibular firstmolars and the correlation between occurrences in males and females and between the left and right sides wererecorded and analyzed. A total of 1020 mandibular first molars were evaluated.Results. A total of 165 patients were found to have a 3-rooted mandibular first molar: 92 males and 73 females(P � .106). The overall incidence of patients with 3-rooted mandibular first molars was 32.35%. Of all the patientswith 3-rooted molars, 67.27% (111/165) of cases occurred bilaterally. The incidence of teeth showing an extra rootwas 27.06%. The occurrence of such permanent 3-rooted mandibular first molars also did not differ significantlybetween the right side (n � 142, 13.92%) and the left side (n � 134, 13.14%; P � .573).Conclusions. The prevalence and bilateral incidence of 3-rooted mandibular first molars among the Shanghai Chinesepopulation was high and such variation varied among Chinese populations living in different areas. (Oral Surg Oral

Med Oral Pathol Oral Radiol Endod 2010;110:e98-e101)

The mandibular first molars are the earliest permanentmolars to erupt. With multiple pits and curves on theocclusal surface, this tooth group, in China, is the onemost frequently affected by caries, periodontitis, pulpi-tis, and periapical lesions, and also the most commonone to need root canal therapy.1

The main objective of root canal therapy is tothoroughly clean and completely obdurate the entirepulp space. All root canals should be accessed,cleaned, and shaped. Incomplete cleaning, shaping,and obturation of any root canal will lead to almostcertain root canal treatment failure. One of the mainreasons associated with endodontic failure is thepersistence of microbial infection in the root canalsystem caused by the “missed” canal or canals. Amandibular first molar usually has 2 roots and 3canals.2 The major variant is the occurrence of athird root, which is called radix entomolaris.3

This work was supported by Shanghai Leading Academic Discipline Project(Project Number: S30206). The authors deny any conflicts of interest.aDepartment of General Dentistry, Ninth People’s Hospital, ShanghaiJiao Tong University School of Medicine, Shanghai Key Laboratoryof Stomatology, Shanghai, China.bDepartment of Endodontics, Tongji Hospital of Stomatology, TongjiUniversity, Shanghai, China.Received for publication Apr 3, 2010; returned for revision May 9,2010; accepted for publication May 19, 2010.1079-2104/$ - see front matter© 2010 Mosby, Inc. All rights reserved.

doi:10.1016/j.tripleo.2010.05.068

e98

It was reported that the frequency of this addi-tional root was rare in European populations. Overallincidence of 3-rooted mandibular first molars in theGerman population was 1.35%,4 and about 2.8% inAfrican populations5; whereas in populations withMongoloid traits (American Indians, Eskimos, andChinese), the prevalence of the radix entomolariswas much higher (21.1%-26.9%)6-8 and could bemore than 30%.9 Tu et al.9 reported the prevalence ofpermanent 3-rooted mandibular first molars in Tai-wanese Chinese was as high as 33.33%. Thereafter,this additional root is considered to be a normalmorphologic variant and is of a certain anthropologicsignificance.10-12

As this additional root of mandibular first molarsis usually curved and smaller than the distobuccalroot,13,14 it may contribute to root canal treatment fail-ure because of a “missed” canal or incomplete rootcanal preparation and obturation. In view of the factthat mandibular first molars are the most common teethaffected with periodontitis, pulpitis, and periapical lesionsin Chinese people, knowledge of the occurrence and sig-nificance of these teeth in the Chinese population is ex-tremely important for dentists. According to the results ofprevious studies, incidence of 3-rooted mandibular firstmolars varied not only between European and Chinese,but also varied among different Asian subjects. This factprompted us to verify the incidence of this kind of teeth in

the Shanghai Chinese population.

31

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This study was aimed to evaluate the frequency ofoccurrence of 3-rooted mandibular first molars in theShanghai Chinese population with periapical radio-graphs.

MATERIALS AND METHODSA total of 510 patients who visited the Department of

General Dentistry, Ninth People’s Hospital, School ofMedicine, Shanghai Jiaotong University, China, fromOctober 2008 to February 2010 were collected. Thesepatients possessed at least one mandibular first molarthat needed root canal therapy. Each of these patientshad to have radiographs taken of mandibular first mo-lars both on the left and right sides, have no dentalcrowding or torsiversion, and had to be of Chineseorigin. Personal details including age, sex, and race ofall these patients were recorded. This study involved258 men and 252 women whose ages ranged from 15 to75 years.

Each tooth was radiographed at the orthoradial po-sition and at the mesial 30-degree position. The peria-pical radiographs were taken using a dental x-ray set(MAXF1, 70 kV, 10 mA, Kyoto, Japan) and Kodak 3 �4 cm films (Kodak, Rochester, NY). The films weredeveloped, fixed, and dried in an automatic processor(Dürr, Bietigheim-Bissingen, Germany).

The radiographs were placed on a viewing box, andthe light surrounding the radiograph was blocked. Eachradiograph was separately evaluated by 3 observers,who were endodontic dentists with at least 10 years ofendodontic experience each. The criteria for the indi-cation of an extra root were adopted from recent stud-ies, and the presence of an extra root was indicated bythe crossing of the translucent lines defining the pulpspace and periodontal ligaments. Observers were toldthe purpose and criteria of the study. If disagreementexisted, a joint evaluation of the 3 observers was madeuntil a consensus was reached.

The incidence of 3-rooted mandibular first molarsand the prevalence of the bilateral appearance of suchteeth were assessed. The ratio of such teeth and the

Table I. Number and percentage of 3-rooted mandibultotal occurrence for 510 Shanghai Chinese individuals

No. of patients

Unilatera

Left

n %

Male 258 10 3.88Female 252 13 5.16Total 510 23 4.51No. of total teeth 1020 23 2.25

comparison of the occurrence between genders and the

occurrence on the right or left sides were also esti-mated.

Comparison of the incidence and the correlationsbetween males and females and left- and right-sideoccurrences were analyzed by using the Pearson chi-square test with SPSS (11.0; SPSS Inc., Chicago, IL).

RESULTSA total of 510 patients, 252 females and 258 males,

aged between 15 and 75 years were considered in thisstudy. A total of 1020 mandibular first molars wereevaluated. One hundred and sixty-five patients werefound to have a 3-rooted mandibular first molar: 73females and 92 males. The overall incidence of patientswith 3-rooted mandibular first molars was 32.35% (Ta-ble I). The incidence did not differ between men (n �92, 35.66%) and women (n � 73, 28.97%; �2 test: P �.106) (Table I). The prevalence of teeth showing anextra root from all teeth examined was 27.06%. In67.27% (111/165) of cases, the permanent 3-rootedmandibular first molars occurred bilaterally. Of the 54unilaterally occurring teeth, 23 occurred on the leftside, and 31 occurred on the right side. Of the malepatients with 3-rooted mandibular first molars, 70.65%(65/92) were bilateral, 10.87% (10/92) were unilateralon the left side, and 18.48 % (17/92) were unilateral onthe right side. Of the female patients with 3-rootedmandibular first molars, 63.01% (46/73) were bilateral,17.81% (13/73) were unilateral on the left side, and19.18 % (14/73) were unilateral on the right side (TableII). The bilateral occurrence of permanent 3-rootedmandibular first molars did not differ between malepatients (n � 65, 25.19%) and female patients (n � 46,18.25%; �2 test: P � .058) (Table I). The occurrence ofsuch permanent 3-rooted mandibular first molars alsodid not differ significantly between the right side (n �142, 13.92%) and the left side (n � 134, 13.14%; �2

test: P � .573) (Table I).

DISCUSSIONIn the present study, the occurrence of 3-rooted man-

t molars by gender, unilateral and bilateral status, and

Bilateral Totalight

% n % n %

6.59 65 25.19 92 35.665.56 46 18.25 73 28.976.08 111 21.76 165 32.353.04 222 21.76 276 27.06

ar firs

l

R

n

171431

dibular first molars in a sample of Shanghai Chinese

OOOOEe100 Yang et al. November 2010

people was 32.35% of all patients and 26.96% of allteeth examined, respectively. This finding is in agree-ment with the previous study by Tu et al.9 by usingcone-beam computed tomography (CBCT) of Taiwan-ese subjects. According to the present results, there wasnot a significant difference between men and women(P � .106) or the side of occurrence (left vs right side,P � .573). This finding is similar to a recent report ona German population but is contrary to some previousreports on Chinese people. According to the reports ofTu et al.9 as well as Jayasinghe and Li,15 the extradistolingual root for the permanent mandibular firstmolar occurred more frequently on the right side thanon the left side. However, there are also studies show-ing that such 3-rooted mandibular first molars occurredmore frequently on the left side in Singaporean Chi-nese.16,17 We found in the present study that the inci-dence of bilateral permanent 3-rooted mandibular mo-lars was 67.27% (111/165 individuals), which is higherthan the percentage found by Tu et al.9 by using CBCTin a Taiwanese (Chinese) population (53.65%) but is inconcordance with results of the previous study by Tu etal.,6 which was based on periapical radiographs of aTaiwanese population. Our results are in accordancewith that of several other studies involving Japaneseand Hong Kong Chinese populations. In these studiesthe incidence of bilateral permanent 3-rooted mandib-ular molars was between 56.60% and 68.57%.18,19

These contrary results indicated that incidence of3-rooted mandibular first molars varied among Chinesesubjects living in different areas. Such variation mightbe attributable to differences in sample sizes and meth-ods. Further larger sample study is encouraged to dis-close these features in Chinese people and whether diethabits or any other possible causes could contribute tothis variation.

In previous studies, full-mouth periapical radio-graphs20-22 and extracted teeth23-25 were the 2 mainmethods used to assess the prevalence of 3-rooted man-dibular molars. Tu et al.9 used a 3-dimensional (3D)imaging method to determine the occurrence of perma-nent 3-rooted mandibular first molars in a Taiwanese

Table II. The unilateral and bilateral status of 3-rootedwith 3-rooted mandibular first molar

No. of patients with3-rooted mandibular first

molar

U

Left

n %

Male 92 10 10.87Female 73 13 17.81Total 165 23 13.94

(Chinese) population and found that 33.33% of the

examined patients had an extra DL root. This percent-age was higher than a number of studies on Asiansubjects and by using extracted teeth or radiographs ofAsian subjects.16,19,20,24,26,27 The authors thought thereason could be attributed to the use of 3D imageanalysis, which was thought to be able to provide moreaccurate determination. In the present study, noninva-sive and inexpensive periapical radiographs were used.These periapical radiographs were taken from at least 2different horizontal angles (one of these taken in theorthoradial position and the other taken either 30° me-sially or distally). This method ensured proper identi-fication of 3-rooted mandibular molars. Our result(32.25%) was in accordance with that of Tu et al.(33.33%).9 This result indicates that periapical radio-graphs are not inferior to 3D image analysis whenassessing the prevalence of this aberration. As Walkerand Quackenbush19 pointed out, the accuracy of a cor-rect diagnosis of a 3-rooted mandibular molar is about90% even when using only panoramic radiographs. Butthere is some limitation for the assessment of rootconfiguration. A 3D radiographic technique is moresuitable for such kinds of investigations.

Based on the present results, with a prevalence ofmore than 30%, a supernumerary root must be expectedwhen treating Chinese patients. As the occurrence of asupernumerary root is bilateral in 67.27% of cases, thepossibility of an extra root should always be consideredand looked for when the mandibular first molar on theother side is already found to have such variation.Preoperative angled periapical radiographs facilitatedisclosing the extra root and will help to avoid“missed” canals. In such case, the conventional trian-gular access cavity must be modified to a trapezoidalform to create a straight access.

CONCLUSIONSOur results indicate that prevalence and bilateral

incidence of 3-rooted mandibular first molars among aShanghai Chinese population was high. The incidencedid not differ between men and women, or right and leftsides. And such variation varied among Chinese pop-

ibular first molars among the male and female patients

al

BilateralRight

n % n %

17 18.48 65 70.6514 19.18 46 63.0131 18.79 111 67.27

mand

nilater

ulations living in different areas.

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Reprint requests:

Ya-qin ZhuDepartment of General DentistryNinth People’s HospitalShanghai Jiao Tong University School of MedicineShanghai Key Laboratory of Stomatology639 Zhizao Ju RoadShanghai 200011, China

zyq1590@163.com