The relationship of the occurrence between three-rooted deciduous mandibular second molars and three-rooted permanent mandibular first molars in children.

BACKGROUND/
PURPOSE: This study aimed to determine the correlation between the occurrence of three-rooted deciduous mandibular second molars and three-rooted permanent mandibular first molars in children.
MATERIALS AND METHODS: Orthopantograms (n = 977) obtained from August 2008 to December 2010 were retrospectively screened and examined. Among the 977 orthopantograms, those of 591 participants (314 boys and 277 girls; mean age, 8.7 years) with bilateral deciduous mandibular second molars and permanent mandibular first molars were studied. The gender predilection and prevalence of three-rooted mandibular molars were assessed, and the pattern of concurrence was investigated.
RESULTS: The prevalence of three roots were 28.4% and 27.6% in the deciduous mandibular second molars and permanent mandibular first molars, respectively. Bilateral occurrence of three-rooted mandibular molars was almost 60% for both deciduous and permanent molars. There was a significantly greater prevalence of three-rooted mandibular molars on the right side of the mandible than that on the left side.
CONCLUSION: Deciduous and permanent molars show a similar prevalence of three roots. The presence of three-rooted deciduous mandibular second molar can strongly predict the possibility of three roots in permanent mandibular first molar.

Introduction

Knowledge of tooth and root canal anatomy is important in dental practice and for accurately identifying features of anthropologic significance. Deciduous mandibular second molars usually have two roots and three root canals, and the presence of accessory roots is rather uncommon., The prevalence of dental anomalies is lower in the deciduous dentition than that in the permanent dentition. The additional root in permanent mandibular first molar variants with three roots typically occurs lingually. Such an additional root was first described by Carabelli and was termed as ‘radix entomolaris’. This supernumerary root is typically smaller than the distal root and is usually curved, requiring special consideration during endodontic treatments.

The prevalence of accessory third roots in the permanent mandibular molars has been extensively investigated, which is not true for the prevalence of the deciduous mandibular molars. Liu reported that approximately 10% patients in a Taiwanese hospital had three-rooted deciduous mandibular second molars, detected by vertical bitewing radiographs. Recent Indian studies performed using intraoral periapical radiographs showed that 5.6–6.5% Indian children had three-rooted deciduous mandibular molars., In Taiwan, 21–27% of the surveyed patient populations had three-rooted permanent mandibular first molars., However limited information has been reported in the literature regarding the correlation between the occurrence of three-rooted deciduous mandibular second molars and three-rooted permanent mandibular first molars.

Thus, the purpose of this retrospective study was to determine the relationship between the occurrence of three-rooted deciduous mandibular second molars and three-rooted permanent mandibular first molars using orthopantograms of Taiwanese children.

Materials and methods

A total of 977 orthopantograms obtained from the Department of Pediatric Dentistry between August 2008 to December 2010 were retrospectively screened and analyzed. Ethical approval was obtained from the Institutional Review Board of Taichung Veterans General Hospital (CE18344A). The criteria for selection of participants were as follows: orthopantograms with good visual characteristics, clearly showing the deciduous mandibular second molars and permanent mandibular first molars of both sides, presences of bilateral deciduous mandibular second molars and permanent mandibular first molars, no radiolucency or furcation involvement associated with the analyzed teeth, deciduous mandibular second molars with resorption in less than one-fourth of the root area, permanent mandibular first molars with complete root formation in more than two-thirds of the root surface. To reduce radiographic misinterpretation, blurred radiographs were excluded.

Among the 977 the selected orthopantograms, those of 591 participants (314 boys and 277 girls) showing the presence of bilateral deciduous mandibular second molars and permanent mandibular first molars (1182 deciduous mandibular second molars, 1182 permanent mandibular first molars) were chosen for this investigation. Two calibrated pediatric dentists examined the orthopantograms. In this investigation, an extra root was defined as a clearly distinguishable independent extra root, evident as the crossing of translucent lines defining the pulp and periodontal ligament spaces, in accordance with the criteria described in previous studies (Fig. 1).,, Disagreements in the interpretation of radiographs were resolved by consensus between the two investigators. The kappa value was calculated to quantify the inter-examiner reliability. The overall prevalence, gender predilection, and prevalence of bilateral and unilateral right- or left-side three-rooted deciduous mandibular second molars and three-rooted permanent mandibular first molars were determined.

Figure 1

Orthopantogram showing the bilateral presence of third roots in the deciduous mandibular second molars and permanent mandibular first molars (red arrows)

Statistically significant differences in gender and side were evaluated using the chi-square test with significance set at P < 0.05. The correlation between left-side and right-side occurrences of three-rooted molars were determined by the McNemar's test. Odds ratios and their 95% confidence intervals were used to assess the statistical significance of correlations between the occurrence of three-rooted deciduous mandibular second molars and three-rooted permanent mandibular first molars. Analyses were performed using the SPSS 19.0 software (IBM Corp., Armonk, NY, USA).

Results

The kappa values for the diagnosis of three-rooted deciduous mandibular second molars and three-rooted permanent mandibular first molars were 0.89 and 0.85, indicating good inter-examiner reliability.

Detailed data on the prevalence of three-rooted mandibular molars based on tooth, gender, and side are shown in Table 1, Table 2, Table 3. The mean age of the 591 participants was 8.7 years. A total of 93 boys (29.6%, 93/314) and 75 girls (27%, 75/277) had three-rooted deciduous mandibular second molars (Table 1). The overall prevalence of these teeth was 28.4% (168/591), with these teeth representing 22.4% (265/1182) of the total number of teeth. Eighty-eight boys (28.8%, 88/314) and 75 girls (27%, 75/277) had three-rooted permanent mandibular first molars. The overall prevalence of these teeth was 27.6% (163/591), with these teeth comprising 22% (260/1182) of the total number of teeth (Table 1). The prevalence of three roots was higher in the deciduous mandibular second molars than that in the permanent mandibular first molars, but the difference was not statistically significant (P > 0.05). The chi-square test revealed that the prevalence of three roots in both types of teeth did not differ between genders (P > 0.05). The prevalence of three roots was higher on the right side than that on the left side for both types of teeth regardless of gender (P < 0.001, Table 2). The prevalences of bilateral three-rooted permanent mandibular first molars and three-rooted deciduous mandibular second molars among the three-rooted subjects were 59.5% (97/163) and 57.7% (97/168), respectively (Table 3).

Table 1

Prevalence of three-rooted mandibular molars.

	Deciduous mandibular second molars				Permanent mandibular first molars
	Total	number (%)	Teeth	number (%)	Total	number (%)	Teeth	number (%)
Male	314	93 (29.6)	628	147 (23.4)	314	88 (28.0)	628	139 (22.1)
Female	277	75 (27.0)	554	118 (21.3)	277	75 (27.0)	554	101 (18.2)
Total	591	168 (28.4)	1182	265 (22.4)	591	163 (27.6)	1182	260 (22.0)

Table 2

Distribution of three-rooted mandibular molars on the right and left sides.

	Deciduous mandibular second molars				Permanent mandibular first molars
	Total	Right no.(%)	Left no.(%)	P value	Total	Right no.(%)	Left no.(%)	P value
Male	314	82 (26.1)	65 (20.7)∗	p = 0.009	314	82 (26.1)	57 (18.2)∗	p < 0.001
Female	277	70 (25.3)	48 (17.3)∗	p < 0.001	277	74 (26.7)	47 (17.0)∗	p < 0.001
Total	591	152 (25.7)	113 (19.1)∗	p < 0.001	591	156 (26.4)	104 (17.6)∗	p < 0.001

McNemar's test.

∗p < 0.05.

Table 3

Bilateral prevalence of three-rooted mandibular molars.

	Unilateral				Bilateral
	Participants	Right no. (%)	Left no.(%)	P value	no. (%)	Total no. (%)
	Deciduous mandibular second molars
Male	314	28 (8.9)	11 (3.5)	p = 0.008∗	54 (17.2)	93 (29.6)
Female	277	27 (9.7)	5 (1.8)	p < 0.001∗	43 (15.5)	75 (27.0)
Total	591	55 (9.3)	16 (2.7)	p < 0.001∗	97 (16.4)	168 (28.4)
	Permanent mandibular first molars
Male	314	31 (9.9)	6 (1.9)	p < 0.001∗	51 (16.2)	88 (28.0)
Female	277	28 (10.1)	1 (0.4)	p < 0.001∗	46 (16.6)	75 (27.0)
Total	591	59 (10.0)	7 (1.2)	p < 0.001∗	97 (16.4)	163 (27.6)

Chi-square test.

∗p < 0.05.

The probability that a third root would be present in a permanent mandibular molar posterior to a deciduous mandibular molar with an extra root was 75.7% on the right side and 65.5% on the left side. The presence of three-rooted deciduous mandibular second molars was significantly correlated with the presence of three-rooted permanent mandibular first molars on the right (odds ratio = 30.2; 95% confidence interval 18.5–49.3; p < 0.001) and left sides (odds ratio = 28.3; 95% confidence interval 16.6–48.4; p < 0.001, Table 4).

Table 4

Correlation between the occurrence of three-rooted deciduous mandibular second molars and permanent mandibular first molars.

		Right side			Left side
K/T	19/30	no. (%)	Probability	Odds ratio95% CI	no. (%)	Probability	Odds ratio95% CI
O	O	115 (19.5)	75.7	30.2∗ (18.5–49.3) p < 0.001	74 (12.5)	65.5	28.3∗ (16.6–48.4) p < 0.001
O	X	37 (6.3)	24.3		39 (6.6)	34.5
X	O	41 (6.9)	9.3		30 (5.1)	6.3
X	X	398 (67.3)	90.7		448 (75.8)	93.7

K/T: Deciduous mandibular left/right second molar.

19/30: Permanent mandibular left/right first molar.

O: mandibular molar with an additional root.

X: mandibular molar without an additional root.

Odds ratio=(OO x XX)/(OX x XO).

CI: confidence interval.

∗p < 0.05.

Discussion

When present, the additional root in a mandibular molar is usually located distolingually. Tratman suggested that the additional root is not simply a division of the distal root, but is a true extra root with a separate orifice and apex. In the permanent mandibular first molars, an additional root is common in races of Mongoloid origin. In our study, the prevalence of three-rooted permanent mandibular first molars was investigated using orthopantograms. We found that 27.6% of all participants and 22.0% of all examined teeth showed the presence of extra roots. Previous studies have reported a prevalence of 43.6% in Aleutian, 8.0%–25.8% in Chinese,,, 10.9%–22.7% in Japanese,, and 33.1% in Korean populations.

Jorgensen reported seven cases (0.67%) of additional roots in 1041 deciduous mandibular second molars extracted from participants of a Danish population. Liu et al. evaluated the prevalence of three-rooted primary deciduous mandibular second molars retrospectively among patients in Taiwan and found an overall prevalence of 10%. The study also reported a bilateral prevalence (symmetrical distribution) of 28% for three-rooted deciduous mandibular second molars. Yang et al. determined the prevalence of three-rooted deciduous mandibular primary second molars in Chinese children as 27.52%, using cone-beam computed tomography (CBCT). In our study, we investigated the prevalence of three-rooted deciduous mandibular second molars in a Taiwanese population and found that 28.4% (168/591) of the examined patients show the presence of extra roots. The value is consistent with the result reported in the aforementioned study by Yang et al., but higher than that reported in a previous study. However the previous study used bitewing radiographs for analysis. We used orthopantograms in this study, and this could be the reason for different results.

Mayhall suggested that if a deciduous mandibular second molar has an additional root, there is a high probability that the permanent mandibular first molar will also have one. Song et al. determined the incidence of additional roots in deciduous mandibular and permanent mandibular molars and analyzed their relationship. Additional roots were present in 33.1%, 27.8%, and 9.7% of the permanent mandibular first molars, and deciduous mandibular second molars, first molars, respectively. They found that when an additional root was present in a deciduous molar, the probability of the tooth posterior to deciduous molar showing the presence of an additional root was greater than 94.3%. The simultaneous occurrence of additional distolingual roots was significant in permanent mandibular first molars with either deciduous mandibular second molars or permanent mandibular second molars (odds ratio = 36.0 and 5.3). We also found a high probability of simultaneous occurrence of extra third roots in deciduous mandibular second molars and permanent mandibular first molars. When an extra third root was present in the deciduous mandibular second molar, there was a greater likelihood of the permanent mandibular first molar to simultaneously present with an extra third root (odds ratio = 30.2 on the right side; 28.3 on the left side, p < 0.001).

Limited data have been reported in the literature regarding the exact etiological mechanisms underlying the development of an extra root. Dahlberg and Turner reported that the deciduous mandibular second molars and permanent mandibular first molars belong to the same molar field because of their same time of development and similar crown morphology. Turner also described the permanent mandibular first molar as the primary site for field-affecting genes. The similarity in the development of the permanent mandibular first molar and deciduous mandibular second molar might explain the simultaneous occurrence of extra third roots.

Song et al. found a significant gender predilection in males over females for the presence of extra roots in the mandibular molars. However we did not find any gender predilection for the occurrence of three-rooted deciduous and permanent mandibular molars (p > 0.05), which is in agreement with the results reported in Taiwanese individuals.,,

According to the results of our study, extra third roots in the deciduous mandibular second molars and permanent mandibular first molars occurred more frequently on the right side than on the left side (p < 0.001). In many previous studies, three-rooted permanent mandibular first molars were showed to have a predilection toward the right side,,,,21, 22, 23 whereas a left-side predilection has been reported less frequently.,

Studies of permanent teeth have shown high bilateral occurrences of extra roots in mandibular molars. In our study, the incidence of bilateral three-rooted permanent mandibular first molars was 59.5% (97/163). This result is similar to those of studies conducted with Asian populations, i.e., 54.6%–67%,,,,21, 22, 23, and the incidence was higher than that reported by Garg et al., who investigated the prevalence in Asian Indians (37.14%) using periapical radiographs. In our study, the incidence of bilateral three-rooted deciduous mandibular second molars was 57.7% (97/168). Song et al. determined the prevalence of bilateral three-rooted deciduous mandibular second molars using CBCT as 81.8% (9/11) in Korean children. The variation in the incidence of three-rooted mandibular molars can be attributed to multiple factors, such as the sample size, study design, method of analysis, and populations studied.

Two principle methods have been used to evaluate the prevalence of extra third roots in previous studies. Radiography is a noninvasive and practical method, which allows inter-study comparisons pertaining to gender and bilateral occurrence. The morphology of three-rooted deciduous and permanent molars can more accurately determined in extracted teeth. However it is an invasive method. According to Walker and Quackenbush, the presence of a third root is clearly evident in approximately 90% cases radiographically. In our study, we used orthopantograms, rather than extracted teeth, for analyzing larger number of teeth. However, more accurate methods for determining the prevalence of three-rooted mandibular molars are extraction and CT. Recently, Tu et al., Song et al., and Garg et al. investigated the incidence of extra roots in mandibular deciduous and permanent mandibular molars using CT. Conventional radiographs only provide a two-dimensional view of the teeth, whereas spiral CT can show detailed three-dimensional images, which can be useful for endodontic applications and morphologic analyses. In contrast to traditional CT, CBCT is characterized by the rapid acquisition of a volume image from a single, low radiation-dose scan of the patient and has a higher resolution., The use of CBCT facilitated detailed and accurate assessment of such teeth without extracting the teeth, and provides images that are more reliable than those obtained by other methods used for determining the prevalence of dental anomalies. However, the routine application of CBCT in children is not recommended due to the additional cost and radiation exposure.

Dentists should be aware of the prevalence of additional roots in deciduous and permanent teeth among Taiwanese children before starting endodontic treatment. This study provides reliable data pertaining to the prevalence of extra roots and the relationship between the occurrence of three-rooted deciduous mandibular second molars and three-rooted permanent mandibular first molars. In conclusion, approximately one fourth of the Taiwanese population exhibits three-rooted deciduous mandibular second molars or permanent mandibular first molars, and approximately 60% exhibit bilateral occurrence of third roots in their deciduous mandibular second molars or permanent mandibular first molars. The probability of the simultaneous occurrence of extra third roots in permanent mandibular first molars and deciduous mandibular second molars is significantly high. The presence of a three-rooted deciduous mandibular second molar is strongly predictive of the presence of a three-rooted permanent mandibular first molar.

Declaration of competing interest

The authors have no conflicts of interest relevant to this article.