Root and root canal morphology of permanent mandibular first and second molars in a Kuwaiti population: A retrospective cone-beam computed tomography study

Abstract

Aim

Endodontic diseases typically affect the mandibular molars. Understanding the complex morphology of the root canal system and its variations is necessary for successful endodontic treatment. The aim of this study was to evaluate the morphology of roots and root canals of mandibular first and second permanent molar teeth in a Kuwaiti population using cone-beam computed tomography (CBCT).

Materials and methods

CBCT images of 651 mandibular first and second molar teeth were obtained from specialist government dental centers. The age, sex, root canal configuration, and number and type of roots were recorded. The data were statistically analyzed.

Results

The most common canal configuration in the mandibular first and second molars was type II (65.6% and 54.4%, respectively), with no significant difference seen between the sexes (p = 0.234). The canal configuration was significantly different between the mandibular first and second molars (p < 0.001). Most teeth had two roots (94.5%); split roots were common (92.6%) and the number thereof varied significantly. Radicular grooves were most common on the lingual side (4.9%). C-shaped canals were present in 43 (6.60%) teeth. Additionally, one tooth had a confluent middle mesial canal and nine (1.4%) had radix entomolaris.

Conclusion

Mandibular molars in our Kuwaiti population typically had two split roots with type II and IV canal configurations. The prevalence rates of C-shaped canals, middle mesial canals, and radix entomolaris were remarkably low.

1. Introduction

Biomechanical preparation is essential for successful root canal therapy and requires an understanding of root canal morphology. It involves biomechanical preparation, obturation, and coronal filling of the teeth. The morphology of roots and root canals has important endodontic and anthropological implications (Vertucci, 1984, Vertucci, 2005, de Pablo et al., 2010). Root canals have certain common, and several unique, morphological traits, which serve as anatomical pathways for clinical endodontics. Understanding racial and ethnic differences in tooth architecture and characteristics may aid the localization, navigation, and management of canals during root canal therapy (Ahmed et al., 2007).

Several studies have investigated the root canal system of mandibular molars. However, the morphology of the roots and canals of mandibular teeth exhibits significant variation (Vertucci, 1984), and is affected by ethnic and genetic factors. The morphological variations include distolingual root, C-shaped roots and canals, isthmus between the mesiobuccal and mesiolingual canals, and a third canal in the mesial root known as the middle mesial canal.

Studies conducted in various countries have reported variable root and canal morphology of the first and second molars. Various techniques have been used to evaluate root and canal morphology, including radiography, decalcification, tooth sectioning, replication, and computer-aided procedures. According to Patel and Rhodes (2007) tooth anatomy should be evaluated on radiographs in multiple planes before accessing the cavities. Cone-beam computed tomography (CBCT) is performed to diagnose unusual cases, including those with unusual root canal anatomy (Demirbuga et al., 2013, Patel et al., 2015). A recent CBCT study of root and canal morphology of mandibular premolar teeth in a Kuwaiti population revealed complex root canal morphology of these teeth (Alenezi et al., 2020). Two studies have evaluated the root canal morphology of mandibular molars in the Kuwaiti population (Zaatar et al., 1998, Pattanshetti et al., 2008); they used clinical examinations and conventional radiographs.

No study has explored the root and canal morphology of mandibular permanent molar teeth in a Kuwaiti population using CBCT. Knowledge of root and root canal morphology may improve endodontic treatment of mandibular permanent molar teeth, resulting in improved patient outcomes. Therefore, we evaluated the root and root canal morphology of mandibular first and second permanent molar teeth in a Kuwaiti population using CBCT.

= 98.3.

2. Materials and methods

Ethical approval

This study was approved by the Institutional Review Board (#FPGRP/2021/593/526/504) and conducted in accordance with the Declaration of Helsinki.

2.2. Sample collection

CBCT images of 250 patients who underwent scanning for diagnostic purposes were randomly collected from the radiology departments of all specialist government dental centers (Adan, Farwania, Amini, Jaber Alahmad, and Jahra) in the State of Kuwait between October 2015 and February 2020.

Images were included in our analysis if at least one mature mandibular permanent first or second molar was clearly visible. Teeth with previous endodontic treatments, canal obliteration, root resorption, or an open apex were excluded, as were CBCT images with artifacts due to full crown restorations of adjacent teeth, adjacent implants, or incorrect radiographic techniques.

The age, sex, and tooth position and laterality (i.e., unilateral or bilateral) in the arch were recorded for all patients. The patients were divided into groups A–F based on their age (9–15, 16–25, 26–35, 36–45, 46–55, and > 55 years, respectively).

2.3. Radiographic evaluation

The CBCT images were analyzed retrospectively by an endodontist and an oral and maxillofacial radiologist using RadiAnt software (Medixant, Poznań, Poland). The images were numbered and evaluated individually to avoid bias and achieve conformity. Disagreements between examiners were resolved by consensus, which sometimes required a joint meeting. Axial, coronal, and sagittal CBCT images were displayed on a 32-in LCD screen (Hewlett-Packard Inc., Palo Alto, CA, USA) with a resolution of 1,280 × 1,024 pixels. The images were interpreted in a dark room. The magnification and contrast were adjusted to aid visualization. Data reliability was assessed based on interexaminer calibration of the diagnoses based on CBCT images.

The morphology of mandibular first and second molars, numbers of roots and root canals per root, root canal configuration (according to Vertucci, 1984), C-shaped canal (according to Fan et al., 2004), radix entomolaris and radix paramolaris (according to Carlsen and Alexandersen, 1991, and Song et al., 2010), and middle mesial canal were evaluated on the CBCT images.

2.4. Statistical analysis

Statistical analyses were performed using SPSS software (version 25.0; IBM Corp., Armonk, NY, USA). Categorical variables are presented as frequencies (%) and were analyzed using the chi-square test. P < 0.05 was considered statistically significant. Interexaminer reliability was quantified using the Kappa test and intraclass correlation coefficients (ICCs).

3. Results

In total, 1,344 teeth were screened on 336 CBCT images obtained from 142 males and 194 females. We excluded 693 (51.6%) teeth. Finally, 651 (48.4%) mandibular permanent molar teeth from 266 CBCT images (115 males and 151 females) were evaluated in terms of root canal morphology and configuration. More second than first molars were examined for both sexes.

In total, 693 teeth were excluded due to previous root canal treatment, caries, open apex, external root resorption, canal calcification, or image artifacts. No significant differences were observed between males and females (p = 0.505), or among age groups (p = 0.082), in terms of root canal morphology. Similarly, the distribution of first and second molars showed no significant difference between the right and left sides (p = 0.833) (Table 1).

Table 1.

The distribution of evaluated first and second mandibular molars according to gender and age.

Variables		First Molars		Second Molars		χ2	p
		n.	%	n.	%
Gender	Male	125	45.3%	160	42.7%	0.444	0.505
	Female	151	54.7%	215	57.3%
	Total	276	100.0%	375	100.0%
Age	9–15	30	10.9%	19	5.1%	9.771	0.082
	16–25	88	31.9%	109	29.1%
	26–35	56	20.3%	88	23.5%
	36–45	56	20.3%	84	22.4%
	46–55	35	12.7%	54	14.4%
	>55	11	4.0%	21	5.6%
	Total	276	100.0%	375	100.0%
Sides	Left	137	49.6%	183	48.8%	0.045	0.833
	Right	139	50.4%	192	51.2%
	Total	276	100.0%	375	100.0%

Types I–IV canal configurations were most common in individuals aged 36–45, 46–55, > 55, and 9–15 years, respectively. The root canal configuration did not vary among the age groups (χ² = 30.606, p = 0.202). Fig. 1 presents representative CBCT images of the root canal configurations.

Fig. 1.

CBCT images of root canal configuration types A. Vertucci Type I, B. Type I with pulp stone, C and D. Type II, E. Type IV and F. Vertucci Type VI.

Table 2 presents the canal configurations in mandibular first and second molars. The most common canal configuration in mandibular first and second molars was type II (65.6% and 54.4%, respectively), followed by type IV (32.6% and 22.4%, respectively). The canal configurations were significantly different between the mandibular first and second molars (χ² = 77.107, p < 0.001). Type II and IV canal configurations were common in males (55.0% and 30.6%, respectively) and females (62.1% and 23.9%, respectively). The root canal configurations did not differ between males and females (χ² = 6.182, p = 0.234).

Table 2.

Assessment of configuration of root canal morphology of mandibular molars according to Vertucci 1984 classification.

Configuration type	First Molars		Second Molars		χ2	p*
	n.	%	n.	%
Type I	4	1.4%	76	20.3%	77.107	<0.001
Type II	181	65.6%	204	54.4%
Type III	0	0.0%	10	2.7%
Type IV	90	32.6%	84	22.4%
Type V	1	0.4%	0	0.0%
Type VI	0	0.0%	1	0.3%
Total	276	100	375	100

Most mandibular molars had two roots (94.5%); very few had fused roots (7.4%) or radicular groove (6.8%). When present, the radicular groove was most commonly located on the lingual side (4.9%). More than half of the molars showed distal angulations (56.1%). In total, 43 teeth had C-shaped canals (C1: 4 [0.6%], C2: 12 [1.8%], C3: 23 [3.5%], and C4: 4 [0.6%]) (Fig. 2), which were more common in second molars. The most common canal type in males was C3 (52.2%) followed by C1 (50%), while in females it was C4 (100%) followed by C2 (91.7%). However, no significant difference was found in the C-shaped canal morphology according to age or tooth type.

Fig. 2.

CBCT images of type of C-shaped canals. A. C1 type, B. C2 shape, and C. C3 type.

Only one confluent middle mesial canal was detected in a second molar. Nine radix entomolaris (1.4%) were observed, of which 7 (77.8%) were located in the first molar (Fig. 3). None of the teeth had paramolaris.

Fig. 3.

CBCT images of a female, 22 years old, showing Radix Entomolaris root in different sections of tooth # 46 (arrow). A. At pulp champer, B. CEJ level, C. Coronal third, D. Middle third, and E. Apical third.

Table 3 presents the sex distribution of the number of roots and tooth type. A significant difference was observed in the number of roots between the mandibular first and second molars (p < 0.001).

Table 3.

Incidence of number of roots in relation to gender and tooth type.

Variables		1 Root		2 Roots		3 Roots		4 Roots		χ2	p
		n.	%	n.	%	n.	%	n.	%
Gender	Male	7	35	258	42	6	40.0	0	0.0	1.118	0.773
	Female	13	65	357	58	9	60.0	1	100
	Total	20	100	615	100	15	100	1	100
Tooth type	First Molars	0	0.0	269	43.7	7	46.7	0	0.0	19.939	<0.001
	Second Molars	20	100	346	56.3	8	53.3	1	100
	Total	20	100	615	100	15	100	1	100

The relationships among sex, age, and tooth type were evaluated in fused and split roots. The presence of fused and split roots in mandibular molars did not differ significantly between males and females (χ² = 0.830, p = 0.362). Fused and split roots were more common in mandibular second compared to first molars (χ² = 31.012, p < 0.001).

The presence of radicular grooves and root angulations did not vary by sex (χ² = 3.024, p = 0.388 and χ² = 0.918, p = 1.00, respectively) or tooth type (χ² = 0.786, p = 0.853 and χ² = 2.265, p = 0.411, respectively).

The kappa test showed good interexaminer and intraexaminer (94%) reliability, indicating high concordance between the examiners (ICC = 0.8).

4. Discussion

Successful root canal treatment requires knowledge of the root canal morphology because it varies with age, population, ethnicity, sex, number of teeth, and study methodology (Joshi et al., 2021). Missed roots or root canals during treatment lead to infections and require retreatment in up to 42% of cases (Hoen and Pink, 2002).

Three-dimensional CBCT allows accurate clinical and laboratory evaluation of tooth anatomy. Similar to several previously published studies (Tu et al., 2009, Zhang et al., 2011, Zheng et al., 2011, Ordinola-Zapata et al., 2017, Alenezi et al., 2020), we used CBCT to evaluate tooth anatomy. A high-resolution imaging protocol was used to ensure high image quality and improve visualization of the relevant parameters.

The detailed anatomical features of the root canal were evaluated in coronal, sagittal, and axial sections. Horizontal images provide simple and reliable information regarding the presence of additional roots and root canals (Tu et al., 2009, Ordinola-Zapata et al., 2017).

In the present study, CBCT images were obtained from the archives of the radiology department of the dental centers, which include data from a large number of patients. A large sample size is essential for population-based studies. To the best of our knowledge, the present study included the largest number of CBCT images of mandibular molars from a Kuwaiti population (Zaatar et al., 1997, Zaatar et al., 1998, Pattanshetti et al., 2008).

Zaatar et al., (1997, 1998) used clinical examinations and conventional radiographs to examine teeth, while Pattanshetti et al., (2008) used the classification of Weine et al., (1969). In the present study, the classification proposed by Vertucci (1984) was used to categorize the canal configurations; this classification system is the most commonly used in morphology studies (Fan et al., 2004, Song et al., 2010, Zheng et al., 2011, Alenezi et al., 2020). A recent classification system assigns a single code according to the number of teeth, number of roots, and canal configuration (Ahmed et al., 2017); however, this system has certain limitations regarding the classification of multirooted teeth with complex configurations, leading to a long classification code.

4.1. Number of roots

Although mandibular first permanent molars typically have two roots positioned mesially and distally, an extra root located distolingually is considered a normal morphological variation. In the present study, two roots were typically found in the mandibular first (97.5%) and second (92.26%) molars, in line with the results of previous studies on different populations (Al-Nazhan, 1999, Zhang et al., 2011, Torres et al., 2015, Pawar et al., 2017, von Zuben et al., 2017).

In previous studies, few patients had a single root in the first molar (Wang et al., 2010, Torres et al., 2015, Martins et al., 2017, Al-Alawi et al., 2019). In line with this, we did not identify any first molar with a single root (Al-Nazhan, 1999).

In the present study, the prevalence rates of three roots in first molars were similar, higher, and lower than those reported by Zaatar et al., 1997, Nur et al., 2014, and Asian studies (Al-Nazhan, 1999, Tu et al., 2009, Song et al., 2010, Al-Alawi et al., 2019). These differences may be explained by ethnic differences. Three-dimensional imaging can be used for precise determination of uncommon anatomical variations, such as the distolingual root in first molars (Wang et al., 2010).

Mandibular second molars exhibit significant anatomical variations, such as the presence of supernumerary or single roots (Manning, 1990, Nur et al., 2014). In the present study, mandibular second molars exhibited variations in the number of roots (92.26%, 5.33%, 2.13%, and 1) had two, one, three and four roots, respectively, in line with the results of Gomez et al. (2021).

Despite low prevalence rates of one and three roots in mandibular molars, clinicians must consider the possibility of rare anatomical variations.

4.2. Root canal configuration

The most common type of root canal configuration in mandibular first molars was type II, followed by type IV in the mesial root and type I in the distal root. Using conventional radiographs, similar results were obtained by Zaatar et al., (1997) in a Kuwaiti population and Al-Nazhan (1999) in a Saudi population.

The findings of Madani et al., 2017, Ni et al., 2018 are in disagreement with those of the present study; both of the previous studies found that the most common canal configuration in mesial roots was type IV, followed by type II, whereas in distal roots the most common canal configuration was type I followed by type II. Similarly, Mukhaimer, 2014, Mohammadzadeh Akhlaghi et al., 2017 found that type IV and I canal configurations were the most common among mesial and distal roots in Palestinian and Iranian populations, respectively. Furthermore, de Pablo et al., (2010) performed a systematic review of the root anatomy and canal configuration of the mandibular permanent first molar, and found that the Vertucci type IV canal system was the most common type, followed by type II. These differences may be related to differences in ethnicity and study methods.

In the present study, the canal configuration was similar between the mandibular first and second molars, similar to the findings of Gomez et al. (2021). Differences in the canal configurations of mandibular first and second molars indicate anatomical variations during the development of molars. However, root canal configurations did not differ significantly by sex or age in this study, in line with Ni et al., 2018, Mashyakhy and Gambarini, 2019, Gomez et al., 2021. By contrast, significant sex differences in canal configurations were identified in the permanent teeth of a Portuguese population (Martins et al., 2017).

Fused roots are more common in mandibular second than first molars (Ahmed et al., 2007). This fusion may produce conical or square-shaped teeth and a shallow or deep radicular groove may be present on each buccal or lingual aspect of the root (Manning, 1990). These grooves may extend into the apical region, or may be present over less than half of the root length (Fan et al., 2004). A high groove-to-thickness ratio suggests the presence of a deep radicular groove, and often a C-shaped canal (Fan et al., 2004). However, a shallow groove, or no groove. can be associated with a C-shaped root canal, resulting in single broad channels (Ioannidis et al., 2011).

In the current study, 93% of mandibular molars had two separate roots, whereas 7.37% had fused roots; this is similar to the results of Miloglu et al., (2013), who reported a high proportion of separate roots in mandibular molar teeth. Additionally, the mandibular second molars in this study exhibited greater variability in external root morphology than the first molars, as shown by the large proportion of fused and separate roots. Furthermore, fused molars were more common in females than males; this is in line with Ross and Evanchik (1981), who reported an approximately 5% higher prevalence of fused molars in females than males.

Interestingly, in this study, fused roots were more common than separate roots in the mandibular molars of middle-aged individuals, which may be explained by the relatively high proportion of such participants.

4.3. Radicular groove and root angulation

In the present study, we found 44 radicular grooves, most of which were located on the lingual surface. This finding is in line with Kim et al., 2016, Alfawaz et al., 2019, and Gomez et al., (2021). Interestingly, most mandibular molar roots exhibited distal angulation. Additionally, radicular grooves and distal root angulations were more common in mandibular second molars of females compared to males.

4.4. C–shaped canal

A C-shaped canal is more common in mandibular second than first molars (Alfawaz et al., 2019, Abarca et al., 2020, Gomez et al., 2021). Of the 651 mandibular molars in this study, 6.60% had a C-shaped canal, which is a smaller proportion than in studies conducted in Saudi Arabia (9.1%), Venezuela (19.5%), Mexico (14.2%), India (13.2%), China (44%), and Korea (40%) (Alfawaz et al., 2019, Gomez et al., 2021, von Zuben et al., 2017, Pawar et al., 2017, Kim et al., 2005). However, our findings are similar to those of von Zuben et al., (2017), who reported a prevalence of C-shaped canal of 6.8% in a Brazilian population. The discrepancy between the results of the present and previous studies may be explained by differences in ethnicity, sample size, analytical approach, and statistical parameters (Gomez et al., 2021). Additionally, we found that C3 canals were the most common, followed by C2, C1, and C4 canals, based on the Fan et al., (2004) classification system. These findings are in agreement with those of Alfawaz et al., (2019). By contrast, Gomez et al., 2021, Kim et al., 2005 showed that C1 is the most common C-shaped canal configuration, while Pawar et al., (2017) found a higher prevalence of C3 in the middle and apical thirds of teeth.

In this study, C-shaped canals were present in 35% and 65% of males and females, respectively; this sex difference was significant. Although our finding disagrees with Zheng et al., (2011), in most other studies females had a higher prevalence of C-shaped canal in mandibular molars compared to males (Martins et al., 2017, von Zuben et al., 2017, Alfawaz et al., 2019, Abarca et al., 2020).

4.5. Middle mesial canal

A middle mesial canal is occasionally observed in the developmental groove between the mesiobuccal and mesiolingual canals (prevalence of 0.95–15%) (Navarro et al., 2007, Aminsobhani et al., 2010). Individuals aged < 30 years were the most likely to have middle mesial canals in first mandibular molars (Ni et al., 2018). Gaidhane et al., (2010) found three mandibular first molar teeth with five root canals in Kuwaiti citizens using conventional radiographs. Each tooth had three independent canals in the mesial root with separate orifices. However, in the present study, none of the mandibular first molars had a middle mesial canal. By contrast, a single confluent middle mesial canal was observed in a mandibular second molar. In most previous cases, the middle mesial canal joins the mesiobuccal or mesiolingual canal at the level of the apical third (Aminsobhani et al., 2010, Ni et al., 2018). In a recent CBCT study, Aldosimani et al. (2021) found 12 (0.9%) middle mesial canals: 9 in the mandibular first molar (1.3%) and 3 in the mandibular second molar (0.4%). Six canals were fused with the mesiobuccal canal and four were fused with the mesiolingual canal. No significant difference was observed in the prevalence of middle mesial canals by tooth type, tooth side, sex, or age.

Our findings suggest that endodontists must be alert to the possibility of middle mesial canals in mandibular second molars. Diagnosis of this canal requires the use of magnification tools and special access routes.

4.6. Radix molaris

In the present study, nine (1.4%) mandibular molars had radix entomolaris. Radices are most frequently observed in first molars, and least frequently in second molars. In the present study, radix entomolaris was most commonly found in the mandibular first molars of middle-aged females. However, none of the mandibular molars had radix paramolaris, similar to the findings of Bhatia et al. (2015). The prevalence of radix entomolaris was higher (4.3%) in a Saudi population (Al-Alawi et al., 2019) compared to the present study. Demirbuga et al. (2013) found a higher prevalence of radix entomolaris in second molars (3.45%) than first molars (2.06%) in a Turkish population.

The sex predilection of radix entomolaris and radix paramolaris is unclear. Several studies have investigated the prevalence of radices according to sex, but results have been conflicting; higher prevalence rates have been reported in both males and females (Yang et al., 2021), while in another study there was no sex difference (Al-Alawi et al., 2019). In the present study, radix entomolaris was more common in females than males. The anatomical variations of radix entomolaris and paramolaris among populations are mainly attributed to racial and ethnic differences (Nagaven and Umashankara, 2012).

There are several differences in root and root canal morphology between Kuwaiti and other populations, which should be considered during endodontic procedures. Detailed clinical exploration of the pulp chamber and radiographic examination of the shape and number of roots and root canals are essential for successful root canal treatment.

4.7. Study limitations

The present study included a small number of CBCT images. Moreover, the images were collected from government dental centers rather than private clinics. Hence, caution should be exercised while generalizing the results to other populations. Further large-scale research is required to fully evaluate the variations of root canal morphology in mandibular permanent molars.

5. Conclusions

We analyzed CBCT images to determine the anatomical features and variations of the root canal system of mandibular molars in a Kuwaiti population. Type II and IV canal configurations were the most common, although there were significant anatomical variations. C-shaped canals were more common in the mandibular second molars of females compared to males, and C3 was the most common type of C-shaped canal. Radix entomolaris had a low prevalence and was more common in the first molars of females than males. The presence of a middle mesial canal was rare and paramolaris was not found in the studied teeth.

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