Dentin Thickness at Danger Zone and Canal Morphology of Maxillary Molars.

Objectives: Dentin thickness in concave areas of the root creates risk for complications such as strip perforation during endodontic treatment. The study aims to examine dentin thickness of the danger (DZ) and safety zone (SZ), canal configuration, and the presence of isthmus in the mesiobuccal root of maxillary molars. Material and
Methods: Cone-beam Computed Tomography (CBCT) images of 1251 teeth belonging to 642 patients were retrospectively reviewed. The dentin thicknesses at DZ and SZ in maxillary molars with one (MB) or two mesiobuccal canals (MB1, MB2) were measured at the 3 mm apical to the furcation level. Vertucci's canal configurations and the isthmus rate were recorded. The Chi-square test andThe Student's t-test were performed.
Results: MB2 rate was higher in maxillary first molars (61.68%) than second molars (39.36%). Isthmus rates were 27.3% and 44.11% in first and second molars. DZ thickness was thinner than the dentin thickness in the SZ in both first and second molars with one or two mesial canals (p< 0.05). In teeth with single canal, the mean DZ thickness was 0.88mm. In teeth with two canals, the mean DZ thicknesses were 0.83mm and 0.80mm for MB1 and MB2 canals, respectively.
Conclusion: MB2 rate was higher in the first molar (61.68%), and the isthmus rate was higher in the second molar (44.11%). DZ and SZ were thinner in MB2 than in MB1 at the maxillary molars with two mesial canals. The results indicated that more conservative preparation must be applied to the MB2 canal in the maxillary molars.

Introduction

The internal anatomy of maxillary molars presents a complex configuration that compromises the success of endodontic treatment. Complicated canal anatomies including lateral canals, accessory canals, ramifications, isthmuses, and other potential inaccessible areas that occur particularly in the mesiobuccal roots of maxillary molars jeopardize the chemo-mechanical preparation due to their difficulty in detection (, ). To determine and access all canals of the root could affect the success of endodontic treatment. This highlights the clinical importance of the complex canal anatomy of maxillary molars that has the highest endodontic failure (). Numerous studies have shown that the incidence of the second mesiobuccal canal (MB2) in the maxillary molars ranges from 13% to 95% (-). Several methods in the literature were used to investigate the canal configuration and morphology of the maxillary molars including Cone-beam Computed Tomography (CBCT) (, -), micro-CBCT (, ), section analysis (, , , , ), and tooth clearing (, , ).

The complex system of the pulp is characterized not only by the presence of extra canals but also by the presence of the passage-forming anastomosis between the canals, termed the isthmus (). An isthmus can be defined as a ribbon-shaped gateway between main or accessory canals (). The isthmus acts as an organic or microbiological matter reservoir that must be removed during endodontic treatment (). In the literature, the isthmus rate in the mesiobuccal root of the maxillary molars has been reported to vary from 4.9% to 53% (, ). This complex pulpal structure jeopardizes the procedures of cleaning and shaping of all root canal systems. Thus, understanding the intricate morphology of the root canal system is substantial for the success rate of endodontic treatment ().

The dentin thickness in the distal concavity of the mesiobuccal root of the maxillary molars, which is close to the furcation, termed danger zone (DZ) is a risky area with regards to perforation caused by canal transportation during mechanical preparation (). According to the literature, the cross-sectional views reveal that canals are not located in the center of the root (). The location of the canals that are close to furcal concavities makes the amount of dentin, removed by mechanical instrumentation during endodontic treatment, important with regards to the formation of perforations, which are clinically very difficult to treat ().

Two-dimensional radiographic modalities are inadequate to evaluate intricate canal morphology or the dentin thickness of maxillary molars because of the superimpositions of dental structures or surrounding tissues (). Furthermore, a three-dimensional mechanical preparation of the root canals during endodontic treatment makes two-dimensional imaging modalities insufficient. Cone-beam Computed Tomography (CBCT) is a non-destructive technique to allow a reliable examination of the internal anatomy of the root. The accuracy of CBCT for linear measurements of maxillofacial structures has been reported to be at an excellent level by previous studies (, ). Furthermore, in the literatüre, CBCT was used for determining canal configuration (, -). Thus, the authors of the present study evaluated the dentin thickness and canal configurations of the maxillary molars using the CBCT imaging technique.

In the literature, many studies have examined the root and canal morphology of maxillary molars, but there are limited studies investigating the dentin thickness of DZ of maxillary molars (-, ). The aim of the present study was to evaluate the dentin thickness of DZ and safety zone (SZ) of mesiobuccal root at maxillary molars at the 3 mm apically to the furcation level. Our study also aimed to analyze the canal configurations and the presence of isthmus at the mesiobuccal root of maxillary first and second molars using CBCT. The null hypothesis was that dentin thickness at the DZ is lower than the SZ.

Material and methods

For the study, CBCT images of 642 patients (336 females and 306 males) aged 17-69 years (mean age 30.5 ± 3) who were referred to a dental clinic were selected and retrospectively evaluated. The present study has been approved by the Research Ethics Committee of Akdeniz University (#576). The protocol of the study was accomplished in accordance with the guidelines outlined in the Declaration of Helsinki. The CBCT data were collected from the database of the Oral and Maxillofacial Radiology Department of the University Dental Clinic from March 2018 to September 2020. All CBCT images included in the present study were obtained as a part of routine dental examination or treatment planning. Teeth with endodontic treatment, filling, post-core, periapical lesion, carious lesion, furcation lesion, vertical root fracture, crown prosthesis, external or internal resorption, under-develop roots were excluded. Also, CBCT images that had artifacts and with poor quality were excluded from the study. We selected 1251 first (n=642) and second (n=609) maxillary molars for the study. All maxillary molars in the study had three roots. Maxillary molars with two or single roots were excluded from the study.

Radiographic Image Analysis

CBCT images of subjects were obtained from Orthophos (Sirona Dental Systems, Bensheim, Germany). Imaging parameters were set as 85 kVp, 6 mA, 14.1 sn exposure time, 0.16 mm voxel size, and 80 x 40 mm field of view according to the “as low as reasonably achievable” (ALARA) principle. The data were analyzed, and the measurements were made using Horos 3.0 software (Horos Project, Annapolis, Maryland, USA) (Figure 1). Before performing measurements, to adjust optimal visualization, contrast and brightness values were regulated by image tools of the software, and all examinations were made in a dark room. All images were investigated in axial, coronal, and sagittal sections. All measurements were performed by two observers (an endodontist and a periodontist) independently blind to the patient's data. Before the measurement process, two observers were calibrated. For calibration, 10% of the images were evaluated, and the kappa score was stated (ranging from 0.92 to 0.95). Moreover, all measurements made by observers were performed twice, and the average values were accepted for statistical analysis. The measurements of three maxillary molars were performed at one time, after every three measurements, and a break was taken to eliminate eye fatigue of observers.

Figure 1

CBCT images for each maxillary molar individually in sagittal (upper left), axial (lower left), and coronal (right) planes.

The distal concave areas of mesiobuccal roots of maxillary molars close to furcation as termed danger zone (DZ) and all dentin thicknesses of DZ at mesiobuccal roots were measured at the 3 mm apically to the furcation level (Figure 2). The mesial convex areas of mesiobuccal roots of maxillary molars far from furcation, termed SZ, and all dentin thicknesses of SZ at mesiobuccal roots were measured at the level of 3 mm from the furcation (Figure 2-4). For the canal configuration, all canal systems were examined from the cementoenamel junction to the root apex. The canal configurations of the mesiobuccal roots of the maxillary molars were categorized according to Vertucci’s classification () and isthmuses of all mesiobuccal roots with two canals were recorded (Figure 5-6). Mandibular molars with obliterated canals were included in the measurement.

Figure 2

Measurements of dentin thicknesses of maxillary molar with two mesiobuccal canals in the danger zone (upper) and safety zone (lower).

Figure 3

Measurements of dentin thicknesses of maxillary molar with the single mesiobuccal canal in danger zone and safety zone.

Figure 4

Schematic view of the dentin thicknesses of the mesiobuccal root with single or two canals. MB: single canal in the mesiobuccal root, MB1; buccal canal in the mesiobuccal root with two canals, MB2; palatinal canal in the mesiobuccal root with two canals. Danger zone; x and safety zone; y (Schematized by the author (A.M.N.) of this study).

Figure 5

Vertucci’s canal configuration classification according to the morphology of the root canal system (Schematized by the author (A.M.N.) of this study).

Figure 6

Pie chart for canal configuration types of maxillary first and second molars according to Vertucci's classification.

The presence of MB2 canals and dentin thicknesses of DZ and SZ were recorded according to gender and age. Four age groups consisted of group 1 (less than 18 years), group 2 (between 18 and 34 years), group 3 (between 35 and 65 years), and group 4 (65 years or over).

Statistical Analysis

Statistical analysis was made using SPSS version 22.0 (IBM Corp., Armonk, NY, USA). The normality distribution of the data was analyzed by the Levene’s test. The Student’s t-test was used to compare the data between the dentin thickness of DZ and SZ in maxillary first and second molar. The Chi-square test was applied to examine the prevalence of the MB2 canal between the right and left molars, the first and second molars, and genders. Interclass correlation coefficient (ICC) was used for observer reliability. The level of significance was set at p<0.05 for the Levene’s, Student’s t-test, and chi-square tests. For ICC, the values with a difference of p<0.001 were considered statistically significant.

Results

Table 1 presents the incidence of MB2 and isthmus in left and right first and second maxillary molars. The distribution of the MB2 canal according to gender and age groups is shown in Table 2. According to the chi-square test, no statistical difference was found in the presence of the MB2 canal between the right and left sides in the first and second molars (p= 0.29). For left and right maxillary first and second molars, bilateral symmetries of canal configuration were 86% and 78%, respectively. There was no statistical difference in the presence of the MB2 canal in first and second molars between females and males (p=0.23). However, there was a statistical difference in the presence of MB2 between the first and the second molar, the MB2 in the first molar is higher than the second molar (p=0.0086). The incidence of MB2 in the maxillary first and second molars was 61.68% and 39.36%, respectively. There was a statistical difference in the presence of isthmus between the first (27.3%) and the second molar (44.11%), isthmuses in the second molar are higher than the first molar (p=0.034). Table 3 and Figure 4 show the distribution of Vertucci’s canal types in the left and right sides of the first and second molars.

Table 1

Incidence of MB2 canal and isthmus in maxillary first and second molar teeth (MB; single canal in the mesiobuccal root, MB2; two canals in mesiobuccal root).

		Presence of mesiobuccal canal (n), %		Presence of isthmus (n), %
		MB	MB2
First molar (n=642)	#16 (n=327)	(n=126)	(n=201)	(n=51)
		38.54%	61.46%	25.37%
	#26 (n=315)	(n=120)	(n=195)	(n=57)
		38.1%	61.90%	29.23%
	Total	(n=246)	(n=396)	(n=108)
		38.32%	61.68%a	27.3%c
Second molar (n=609)	#17 (n=315)	(n=201)	(n=114)	(n=57)
		55.96%	44.07%	50%
	#27 (n=294)	(n=192)	(n=102)	(n=39)
		65.31%	34.69%	38.23%
	Total	(n=393)	(n=213)	(n=96)
		60.63%	39.36%b	44.11%d

According to chi square test a,b statistically significant (p=0.0086); c,d statistically significant (p=0.034).

Table 2

The frequency distribution (%) of the second mesiobuccal canal (MB2) in maxillary molars is based on gender and age groups.

		Age groups (n), %					Gender (n), %
		<18	18-35	35-65	>65		Male	Female
First molar (n=642)	Without MB2	(n=6)0.93%	(n=183)28.50%	(n=50)7.78%	(n=10)1.55%		(n=102)15.88%	(n=144)22.42%
	With MB2	(n=12)1.86%	(n=228)35.51%	(n=148)23.05%	(n=5)0.77%		(n=138)21.49%	(n=258)40.18%
Second molar (n=609	Without MB2	(n=12)1.97%	(n=138)22.06%	(n=144)23.64%	(n=6)0.98%		(n=174)28.57%	(n=219)35.96%
	With MB2	(n=6)0.98%	(n=231)37.93%	(n=60)9.85%	(n=12)1.97%		(n=81)13.30%	(n=135)22.16%

Table 3

Distribution of canal types in maxillary first and second molars according to Vertucci’s classification.

		Type I (1) (n), %	Type II (2-1) (n), %	Type III (1-2-1) (n), %	Type IV (2) (n), %	Type VI (1-2-1-2) (n), %
First molar (n=642)	#16 (n=327)	(n=123)37.61%	(n=192)58.71%	(n=3)0.92%	(n=9)2.75%	0
	#26 (n=315)	(n=114)36.2%	(n=189)60%	(n=6)1.90%	(n=6)1.90%	0
	Total	(n=237)	(n=381)	(n=9)	(n=15)	0
		37.08%	59.62%	1.41%	2.32%	-
Second molar (n=609)	#17 (n=315)	(n=201)63.8%	(n=111)35.2%	0	(n=3)1%	0
	#27 (n=294)	(n=183)61.60%	(n=99)33.35%	(n=3)1.01%	(n=6)2.04%	(n=3)1.01%
	Total	(n=384)	(n=210)	(n=3)	(n=9)		(n=3)
		63.05%	34.48%	0.49%	1.47%		0.49%

The DZ and SZ thicknesses of the mesiobuccal roots with single and two canals were given in Table 4. In the first and second molars, no statistical difference was found between males and females in dentin thickness at the DZ and SZ of the mesiobuccal roots with single and two canals. In the roots with a single canal, the mean DZ and SZ thicknesses were 0.88 mm and 0.98 mm, respectively. In roots with two canals, the mean values were 0.83 mm and 0.94 mm in MB1 and, 0.80 mm and 0.89 mm in MB2, respectively. In the mesiobuccal roots with a single canal, dentinal walls were significantly thinner in the DZ than in the SZ for both first (p=0.028) and second molars (p=0.021). In the mesiobuccal roots with two canals in the first and second molars, dentinal walls were significantly thinner in the DZ than in the SZ for both MB1 and MB2 canals (p<0.05). Dentin thicknesses of DZ and SZ in maxillary first and second molars according to age and gender are shown in Table 5.

Table 4

Descriptive analysis values of the dentin thickness of danger (DZ) and safety zone (SZ) at the mesiobuccal root with single and two canals of first and second molars (MB: single canal in the mesiobuccal root, MB1; buccal canal in the mesiobuccal root with two canals, MB2; palatinal canal in the mesiobuccal root with two canals).

According to Student t-test a,b statistically significant (p=0.0088); c,d statistically significant (p=0.0099); x,y statistically significant (p=0.015); z,t statistically significant (p=0.0045).

Table 5

Mean dentin thicknesses (mm) of the danger zone (DZ) and safety zone (SZ) at the mesiobuccal canal in maxillary molars are based on gender and age groups. (MB: single canal in the mesiobuccal root, MB1; buccal canal in the mesiobuccal root with two canals, MB2; palatinal canal in the mesiobuccal root with two canals).

				Age groups					Gender
				<18	18-34	35-65	>65		Male	Female
First molar (n=642)	With single canal	MB	DZ	0.88	0.86	0.90	0.96		0.88	0.86
			SZ	1.11	0.92	1.01	1.02		0.96	0.93
	With two canals	MB1	DZ	0.93	0.80	0.83	0.93		0.82	0.81
			SZ	1.15	0.94	1.00	1.04		1.00	0.95
		MB2	DZ	0.86	0.77	0.80	0.91		0.79	0.78
			SZ	0.86	0.90	0.92	0.95		0.95	0.89
Second molar (n=609)	With single canal	MB	DZ	1.32	0.87	0.93	0.89		0.90	0.91
			SZ	1.39	0.95	1.06	1.04		1.01	1.01
	With two canals	MB1	DZ	0.92	0.83	0.73	0.79		0.83	0.78
			SZ	1.05	0.94	0.72	0.84		0.96	0.94
		MB2	DZ	0.85	0.81	0.72	0.85		0.79	0.77
			SZ	0.86	0.87	0.84	0.93		0.83	0.88

The dentin thickness in both the DZ and SZ of MB2 is significantly thinner than MB1 (p<0.05). The ICC for the measurements of the dentin thickness of maxillary molars were ICC=0.981 and ICC=0.974, respectively (p<0.001 for ICC values).

Discussion

The prevalence of MB2 of the first and second molars in our study was 61.68% and 39.36%, respectively. Endodontic failures of maxillary molars are generally related to the undetected MB2 canals (). Previous studies have reported a wide range of prevalences. The various results of previous studies in the literature can be explained by ethnic origin and different methodologies (detailed in Table 6) (- ). The results of the present study are consistent with a previous study in the same Turkish population that reported 62% and 37% MB2 canal in maxillary first and second molars, respectively (). Previous studies investigating the incidence of MB2 in different populations reported 63.59% in Korean (), 65.6% in North American (), 74.55% in Egyptian population (), and 82.62% in Croatian population () for the first molar. In addition to this, the incidence of MB2, in different populations, for the second molar was reported to be 29.7% in Chinese (), 29.4% in Indian (), 13.5% in Ugandan (), and 52.9% in the Brazilian population (). Most studies found the incidence of MB2 in the first molar higher than in the second molar. These results are consistent with our study. In the German population, a previous study using a dental microscope found the incidence of MB2 in the second molar higher than in the first molar (). Racial factors, methodology, gender, genetic factors, and sample size contribute to variations in the reported results about canal configurations (, ).

Table 6

Previous studies investigating the incidence of the MB2 canal of maxillary molars.

Study	Methodology	No. (n)		Racial origin	İncidence of MB2 (first molar)	İncidence of MB2 (second molar)	Total incidence(%)
Zhang et al.3	CBCT	509	Chinese		52%	22%	-
Pattanshetti et al.4	Loupe	110	Kuwait		42.3%	-	-
Schwarze et al.5	Dental microscope	100	German		92.3%	95.8%	-
Blattner et al.6	Section analysis	20	American		-	-	68.4%
Wasti et al.7	Clearing	30	Pakistanis		47%	-	-
Tuncer et al.8	Dental microscope	110	Turkish		-	-	78%
Degerness et al.9	Section analysis	153	American		79.8%	60.3%	-
Lima et al. 10	Section analysis	72	Brazilian		78.9%	52.9%	-
Kim et al. 11	CBCT	1400	Korean		63.59%	34.39%	-
Tian et al.12	CBCT	3097	Chinese		57.8%	29.7%	-
Guo et al.13	CBCT	637	American		65.6%	-	-
Ghobashy et al.14	CBCT	1215	Egyptian		74.55%	57.94%	-
Shetty et al.15	CBCT	100	Indian		86.3%	29.4%	-
Altunsoy et al.17	CBCT	2462	Turkish		62%	37%	-
Çalışkan et al.18	Clearing	200	Turkish		65%	55%	-
Rwenyonyi et al.19	Clearing	442	Ugandan		24.4%	13.5%	-
Šutalo20	Section analysis	443	Croatian		82.62%	-	-
Yamada et al.21	Micro-CT	90	Japanese		55.6%	-	-
Peeters et al.22	Section analysis	308	Indonesian		68.5%	-	-
Our results	CBCT	1251	Turkish		61.68%	39.36%

In the present study, apart from type I; type II, III, IV, and type VI, canal configurations were detected in maxillary molars. The overall prevalence of canal configurations other than type I was 62.92% and 36.95% in the first and second molar, respectively. With regard to the canal configuration in maxillary molars, the results of the present study were also congruent with a previous article that studied the same population and reported the prevalence of canal types other than type I. The prevalence was 62% for the maxillary first molar (). Besides, another study that used the clearing technique stated 65% and 55% of the canal configurations other than type I in the mesiobuccal root of the maxillary first and second molar in the Turkish population, respectively (). The present study demonstrated that the most common canal types were type II in the first molar and type I in the second molar. Being aware of the most frequent type of canal configurations enables a minimally invasive cavity preparation that respects healthy tissues.

CBCT enables a high-resolution three-dimensional analysis of canal system and internal anatomy of maxillary molars and is regarded as a crucial examination technique for clinical endodontics. The validity of the CBCT in the canal configuration has been reported in previous studies (-, ). Therefore, the authors of the present study have opted for CBCT for the examination of maxillary molar teeth.

Organic or bacterial remnants in the isthmus limit the action of irrigation solutions. Thus, it is fundamental to be aware of the presence of isthmus in maxillary molars that have a challenging treatment (). According to the results of the study, while the prevalence of MB2 was higher in the first molar, the prevalence of isthmus was higher in the second molar. This study reported the percentages of isthmus of 27.3% in the first molars and 44.11% in the second molars. Teixeira et al. () reported the isthmus rate in the first molars of 23.68% in their study, and the results are consistent with the present study. On the contrary, Lima et al. () found the isthmus rate in the first and second molars of 84.2% and 70.6%, respectively. These results are higher than the rates of the present study. The secondary dentin deposition, throughout the life of the tooth due to odontoblast activity, causes the volume of the isthmus to dwindle (). Different results of the literature can be explained by the fact that the data were obtained from different age groups and different populations and examined using different methods. Based on the high prevalence of isthmus stated in the present and previous studies, using sonic, ultrasonic, or laser-activated irrigation methods to properly remove organic and bacterial remnants in the isthmus during the chemomechanical preparation of maxillary molar teeth is required for the success of endodontic treatment ().

The mesiobuccal root poses a risk to structural integrity in the case of over-preparation during an orthograde endodontic treatment. Our study also examined the DZ thickness of the mesiobuccal root of maxillary molars and reported that dentin thicknesses at the DZ were lower than the SZ. The null hypothesis was accepted. However, studies in the literature measuring dentin thickness in the DZ have generally focused on mandibular molars (, ). A few studies are investigating the dentin thickness of maxillary molars (, ). Ordinola-Zapata et al. () investigated the DZ thickness of the maxillary first molar at furcation level by micro-CT. However, the root concavity decreases from the furcation towards the midpoint of the root. Therefore, clinically more risky areas remain at the coronal part of the furcation and these areas pose a risk for strip perforation. This study has examined the DZ using micro-CT in one hundred maxillary first molars and reported the dentin thickness in DZ at MB1 and MB2 was 1.24 mm and 0.99 mm, respectively and for SZ, the dentin thicknesses at MB1 and MB2 were 1.33 mm and 1.17 mm (). These results were slightly higher than those obtained in our study that report the DZ thicknesses at MB1 and MB2 were 0.82 mm and 0.79 mm, and the SZ thicknesses were 0.97 and 0.91 at MB1 and MB2, respectively. This difference can be explained by the sample size, methodology, or racial factors. Another study by Degerness et al. (), in the coronal section of the mesiobuccal root of the maxillary molar, found the dentin thickness of the distal portion (DZ) was significantly lower compared to the mesial portion (SZ). This result is similar to the present study in which the measurements were made at the level of 3 mm to the furcation level that corresponds to the coronal part of the root.

Previous studies also report thinner dentins of DZ and SZ in the MB2 canal of maxillary molars compared to the MB1 canal (, , ). These results in the literature were confirmed with the present study that demonstrates lower dentin thickness at MB2. According to the results of the present study, the DZ and SZ thickness of the MB2 is thinner, consequently, it is important to limit the orthograde preparation of the MB2 canal during endodontic treatment to prevent dentin perforation. Overpreparation of the DZ can cause root perforation, likewise, mechanical weakening also causes a decrease in the resistance of the root to fracture. Therefore, it should be considered to use preparation methods such as the anti-curvature technique and to avoid excessive use of the larger nickel-titanium instruments to protect the dentin, especially in curved roots that are prone to root fracture due to improper stress distribution ().

Limitations of our study include the low sample size and the use of CBCT to investigate dentin thickness. Further research is needed to investigate the dentin thicknesses at DZ or SZ including larger sample sizes and using the micro-CT or section analysis.

Conclusion

Within the limitations of the present study, the data showed that the prevalence of MB2 in the first molar was higher than in the second molar. On the contrary, the prevalence of isthmus was higher in the second molar. The canal ramifications and morphology of maxillary molars need to be well examined. The DZ thickness was lower in the mesiobuccal roots of both first and second molars compared to the safety zone. The DZ and SZ thicknesses in MB2 canals were lower compared to MB1 canals. These results suggest that minimally preparation should be considered for the MB2 canals where the dentin thickness is already reduced compared to MB1 during endodontic treatment of the maxillary molars.