Root and canal specific features of maxillary first molars with fused roots

Background / Aim. Maxillary first molars are one of the most challenging
 teeth for endodontic therapy. There are certain disparities in number of
 roots and canals, and canal inter-relationship within the same root,
 particularly in those with fused roots. The aim was to evaluate ex vivo
 features of roots, root canals and canal walls in maxillary first molars
 with fused roots. Methods. Sixty-four extracted maxillary first molars with
 fused roots were included for cone-beam computed tomographic and microscopic
 studies. Tooth dimensions at the level of pulp chamber floor, number,
 location and distance between orifices, number and canal morphology, canal
 wall thickness, and features of apical foramina were examined and measured.
 Results. Incidence of maxillary first molars with root fusion was 17.5%, of
 which 60.0% was palatal fused to disto-buccal root. At the level of pulp
 chamber floor bucco-palatal dimension was significantly larger with 10.4 mm
 than mesio-distal with 7.0 mm. Four canal orifices were detected in 65.6%,
 with the shortest distance of 1.95 mm between mb1 and mb2. In fused roots
 most frequently were two or three canals strongly correlating to the number
 of major apical foramina. No fusion of canals was found in fused roots. The
 thinnest canal wall in mesio-buccal and disto-buccal fused root was mesial
 with 1.25 mm, and distal with 1.31 mm, while for the palatal root the
 thinnest was palatal wall with 1.97 mm. Conclusion. The most frequent type
 of fusion was between palatal and disto-buccal root. Bucco-palatal dimension
 at the level of pulp chamber floor was significantly larger than
 mesio-distal, with the shortest inter-orifice distance between mb1 and mb2
 orifice. Number of canals was either two or three, strongly coinciding with
 the number of major apical foramina. There was no fusion of the canals in
 fused roots. The thinnest canal wall was either mesial or distal.


Introduction
Beside adequate and thorough knowledge about usual external and internal root canal morphology and its possible variations 1 , it is of utmost importance to evaluate each individual case for aberrant anatomy and to identify any morphological variation before and during endodontic procedure of such teeth 2,3 . Clinicians often have to treat teeth with unusual anatomy of their root canal system and with atypical configurations, what is constant challenge for diagnosis and management of such teeth 4 . Maxillary first molars are one of the most complex and challenging teeth in endodontology and endodontic practice as "possibly the most treated, least understood, posterior tooth" 5 . There are certain disparities and aberrations in their root morphology and configuration of the canal system, particularly in maxillary first molars with fused roots, mostly presented in various case reports, experimental studies, but less in clinical evaluations or retrospective assessments [6][7][8] . Those variations have been attributed to differences in either ethnic, i.e. national background, gender, or to differences in study design, evaluation method, or sample size and structure [9][10][11][12] .
The very beginning of the 21 st century brought Cone-Beam Computed Tomography (CBCT), or Digital Volumetric Tomography (DVT) into endodontic practice, which provides noninvasive and nondestructive way for three-dimensional images 13 . Importantly, it has been proved as a more accurate method for precise and detailed detecting of the root canal morphology in clinical conditions, especially in the maxillary region 9,14,15 . A literature that deals with the use of CBCT for revealing root canal anatomy presents and describes wide variations in morphological features of maxillary first molars, but reports are mostly focused on the number and configuration of mesio-buccal root canals, supernumerary roots or root canals [16][17][18][19] .
There are quite a few articles on maxillary first molars that have been targeting those with fused roots, with or without C-shaped canals, describing often their endodontic treatment and being generally confined to a case reports [20][21][22] . However, incidence, type of root fusion, root and root canal relation to other anatomical parameters that may influence and interfere with endodontic treatment of maxillary first molars with fused roots, have been presented in a few studies and literature reviews 6,7,[23][24][25] .
The aim of this study was to evaluate ex vivo anatomo-morphological characteristics of the roots, root canals and dentin canal walls in maxillary first molars with fused roots with the aid of cone-beam computed tomography and light microscopy.

Methods
Material for this study were human maxillary first molars collected from individuals of both genders, 25-60 years of age and from both sides of the jaw. According to the Approval of the Ethic Board of the School of Dental Medicine, University of Belgrade, Serbia, after each patient has signed the written consent, teeth were extracted due to advanced periodontal disease, prosthetic or orthodontic demands, or to extremely poor prognosis for endodontic treatment. Teeth with cracked or fractured roots, apical root resorption, massive coronal destruction or restorations, as well as having endodontic 7treatment, were excluded from the further procedure. Tooth samples were then stored in 3% NaOCl solution (Parcan, Septodont, Saint-Maur-des-Fossés, France) for one hour to dissolve periodontal ligaments. After cleaning the root surface all teeth were stored in saline solution with 0.2% thymol at 4 0 C temperature until examining procedure.
From total number of 366 collected maxillary first molars only teeth with two or all roots entirely fused from the cement-dentinal junction (CDJ) to the very apical portion were included in this study. Coronal preparation, trepanation and removing of the entire pulp chamber roof were done using high-speed round diamond bur with water spray as a coolant. Occlusal walls were flattened using diamond cylindrical bur (F011 series; Dentsply/Maillefer, Ballaigues, Switzerland), and lateral walls refined using conical carbide bur with passive tip (EndoZF.G; Dentsply/Maillefer). Ultrasonic tips Start-X1 and Start-X2 (Dentsply/Maillefer) were used to remove dentin deposits interfering with canal orifices, which then were identified and marked using 3.5 x loupes and Micro Opener tip #1 (Dentsply/Maillefer) with neither widening nor reshaping from the original. Respecting the original root canal diameter K-Reamers size .06, .08, and .10 (C-Pilot, VDW GmbH, Munich, Germany) were used to establish patency of each canal until the tip of the instrument was visible at the anatomical foramen under 3.5 x magnification.
After completing this procedure four teeth were placed with their roots in a round block of impression paste Zeta Plus (Zermak, Rovigo, Italy) with the pulp chamber floor parallel to the horizontal plane and mounted at the CBCT device with the aid of laser positioner. CBCT examining was performed using Scanora 3DX (Soredex, Tusuula, Finland) with small field of view 50x50 mm, with voxel size of 100 µm, 90 kVp, 10 mA.
All data were analyzed in OnDemand 3D Application computer program (CyberMed, Seoul, South Korea). Images were processed and analyzed from axial, sagital, and coronal plane. All measurements for each tooth sample and at each pre determined point along the roots from the coronal to the apical portion of each canal were conducted and recorded by two independent examiners, both endodontic specialists and trained for CBCT techniques.
At the level of the pulp chamber floor quadrangle was drawn around each cross section of the scan tangenting most prominent spot on the mesial, buccal, distal and palatal border line. Dimensions were measured in four directions: a) mesio-distal /M-D/; b) buccopalatal /B-P/; c) mesiopalatal-distobuccal /MP-DB/ and d) distopalatal-mesiobuccal /DP-MB/ (Fig. 1a). Centers of each consecutive orifice were connected by straight lines which formed multi-angle, presenting specific "dentin map" at the pulp chamber floor. Number, shape, distance between centers of the orifices and "dentin map" for each tooth were recorded on CBCT scans. Distance between two neighboring centers was measured with the precision 0.01mm (Fig. 1b). Angle between two lines connecting three consecutive orifices was expressed in degrees (Fig. 1c). This enabled to determine precise orientation and localization of the canal orifices. Statistical analysis contained correlation between the Statistical methods contained descriptive analysis and Spearman analysis of correlation that reflects the level of agreement. Inter-rater reliability was analyzed with Cohen's kappa-test for two examiners. All data were computed using software package SPSS 20 (IBM Corporation).

Results
Of total 366 maxillary first molars 294 (80.3%) had three distinctive roots, 64 (17.5%) were with fused roots, and eight teeth (2.2%) were with four separate roots. Types of fusion are presented in Table 1 and (Fig. 2). Most frequently was palatal (P) fused to disto-buccal (DB) root (Fig. 2a), followed by P to mesio-buccal (MB) (Fig. 2b) and MB to disto-buccal (DB) root (Fig. 2c), with significant differences among all those types (Table   1; p<0.001). Consecutively, the strongest tendency towards fusion showed P, and least MB root. Of 64 fused rooted maxillary first molars only two had all roots fused into one (Fig. 2   d).
At the level of the pulp chamber floor BP dimension was significantly larger than MD ( Table 2; p<0.05). Diagonal dimension MB-DP was larger compared to MP-DB with significant difference (Table 2; p<0.05). Analysis of the orifice shape showed that circular or oval shape was found to be absolutely dominant, with a few crevice-like orifices. Four canal orifices were found in two thirds of fused rooted maxillary first molars with high statistical difference from other variations ( Table 3; p<0.001). Sides and angles of quadrangle formed by connecting those four orifices were measured. The longest distance was between p-db and p-mb2 orifice, while the shortest distance was between mb1-mb2 orifice ( Table 4; p<0.005). The largest angle was between neighboring lines connecting centers of mb1-mb2 with mb2-p canal orifices, and the smallest between mb2-p and p-db sides ( Table 4; p<0.001).
Either two or three canals were found in the same percent with dominant prevalence over four or five, with high statistical difference (Table 5; Table 5 presents distribution of configuration types with significant difference between type IV , VI and VIII (p<0.01).
In 40% of fused roots pulpo-periodontal communications (PPC) were detected at different levels of root canals ( Fig. 4 a, b, c, d). Table 6 shows that the thinnest canal wall of the P fused root was mesial, followed by distal and palatal, but with no significant difference (p>0.1). Greatest thickness had the buccal wall with highly significant difference from the other three (p<0.001).
The thinnest canal walls of DB fused root were mesial and distal, with no mutual differences (Table 7; p>0.1). Buccal wall was slightly thicker from the later two (Table 7, p<0.05), while palatal wall was the thickest at all three levels with high significant difference from the other three (p<0.001).
The smallest values of dentin wall thickness for mb1 canal in MB fused root at all three levels were found for the distal one (Table 8). There was no statistical difference between values for distal, mesial and buccal walls ( Table 6; p>0.1). The thickest wall was the palatal at all three levels with significant differences from the other three (Table 8; p<0.005).
Mesial and distal walls of mb2 canal were significantly thinner than the other two at all three levels of MB fused root ( Table 9). The thickest wall was the buccal one with significant difference from the later two (Table 9; p<0.001). Values for the palatal wall of mb2 canal were significantly different from the distal and mesial (p<0.05), as well as from the buccal one (p<0.01).
Half of fused roots had two, and approximately one third had three major apical foramina (Table 10; Fig. 5 a, b). There was strong direct correlation between number of major foramina and number of canal in fused roots (Table 10 and Table 5; Rho=0.509; p<0.003). Table 10 also presents that more than half of fused roots had no minor, i.e. accessory foramina, and ¼ had one accessory foramen (Figure 5 a, c).

Discussion
Material for this ex vivo study was primarily chosen in respect to the previous study in the same population 26,27 aiming to reveal wide scope of different morphological features and variables that characterize first maxillary molars with fused roots. There are quite a few research articles dealing with this topic, as well as case presentations, mostly accompanied with endodontic treatment or retreatment of those teeth 8,21,28,29 .
CBCT and scanning technique used in this study enabled to detect and register not only anatomical details of the main canal, but also the presence of accessory canals, intercanal communications, their dividing, deviations, to follow their entire paths along different roots and to measure the thickness of the canal walls from different aspects at various levels of the root. All those anatomical features are more complex and specific in teeth with fused than in teeth with three distinctive, i.e. separate roots. Micro-CT even proved as the most precise method in presenting morphological details of the root canal, is however limited only on extracted teeth 30,31 . Results of several retrospective studies of the morphology of maxillary first molars and quite a few case reports showed high precision of CBCT in revealing tinny details of their root canal anatomy both ex vivo, and more importantly in vivo, in clinical conditions [32][33][34] . Bauman et al. 35 showed that the voxel size has great impact on the accuracy in detecting multiple canals of first maxillary molars, stressing that only 60.3% canals were detected when voxel size was 400 µm, and 93.3% with voxel size of 125 µm for the same group of teeth. Therefore, the CBCT technique with voxel size of 100 µm was used in this study to detect and describe important and specific morphological details in fused roots of maxillary first molars with root fusion.
The term fused root is defined as two or more roots that are united either through the deposition of cementum from the cement-enamel junction to the root apex 9  This study showed that of total number of examined maxillary first molars 17.5% was found to be with fused roots. Researches that have been conducted using different methods, review articles and case reports showed wide range in incidence of this anatomical entity from none 15,36,37 raising up to 23.9% 9 . The incidence of fused roots was found to be significantly lower in the first than in second maxillary molars from an extreme difference of 0.7% versus 10.7% 23 , or 1.4% versus 23.9% in Chinese patients 9 . With almost the same values but with less reciprocal differences of 7% vs. 21%, and of 7.1% vs. 25.2% was found in Saudi Arabians 24 and Portuguese individuals 34 , respectively.
Marcano-Caldera et al. 38 found in Columbian patients extremely high incidence of root fusion in maxillary molars of 23.3% in first vs. 57.7% in second, with lower difference ratio between the two. High percentages and variations, even within the same population, could be attributed to the fact that different authors presumably applied different criteria for defining three-rooted first maxillary molars. Silva et al. 39 stated that differences could also be found due to an erroneous assessment method of morphological details. All authors pointed out the impact of ethnic, i.e. race origin on the prevalence and anatomical characteristics of fused roots in maxillary first molars, what was one of the main reasons to conduct this study specifically on a Serbian population.
Age factor may affect detection of the root canals and their morphology and therefore range of 25-50 old patients were included in this study, as in previous survey 26 , revealing no influence of age on examined characteristics of maxillary first molars with fused roots. Mohara et al. 40 used individuals from 18-45 years old, similar to this study, while Naseri et al. 41 included patients with very wide age range from 10-70 years old, and both found no statistical difference between patients' age. The late result may be attributed to the sample size and higher concentration of individuals in particular age groups.
However, most of the studies generally showed that as the age progresses, the number of detected mb2 canals decreases 42 .
Considering influence of gender on the incidence of fused roots this study showed no difference between patients' sex, what coincides with results by Naseri et al. 41 and Lee et al. 42 . Conversely, Ross and Evanchik 43 reported for 13% higher incidence of root fusion in females than in males in multi-national group, supported by findings of Martins et al. 34 for Portuguese individuals. Marcano-Caldera et al. 38 found in Latin Americans that 64.1% of all fused roots belonged to women, similarly to results by Al-Shehri 44 with 71.4% of root fusion in females in Saudi population.
No significant difference between left and right sided teeth was found in this study, confirmed by Zheng et al. 45 and previously cited authors. Zhang et al. 15 found that 84% maxillary molars had perfect symmetry in the root and canal morphology of homonym teeth on the opposite side, similarly to Felsypremila et al. 32 with 77.5% of bilateral symmetry of root fusion. Mashyajkhy et al. 24 in Saudi Arabians found no statistical difference between patients' sex, left-and right-sided teeth in fused rooted maxillary molars. On the contrary, Al-Shehri 44 in the same subpopulation reported significantly more prevalence of fused roots in the right sided teeth. Those findings support statements that anatomical variations between different and within the same morphological group of teeth could be affected beside ethnic factor, by the sample characteristics or by the varieties in methodology.
Most of the articles that have studied root fusion paid either no attention on the type of fusion 25,41,43,46 , or just have presented rare cases 8,22,47 . Since the palatal root dominates on a periapical radiography it is clear why fusion between massive palatal with one of the buccal roots is very hard to detect. Thus, the CBCT technique with voxel size of 100 µm was used as it revealed the entire anatomy from all three scanning planes enabling detection of many tiny details. The most frequent fusion was found between P and DB root (Table 1). Marcano-Caldera et al. 38 confirmed this result with frequency of 58.9%, while Mashyajkhy et al. 24 in Saudi Arabians and Martins et al. 34 in Portuguese found even higher incidence of P/DB fusion with 66.7% and 85.3%, respectively. On the contrary, Al-impact of ethnic foundation of root morphology, and furthermore justifies the use of specific national population in this study.
Of 64 maxillary first molars with root fusion only two had all roots fused into one conical shaped (Table 1). Single rooted maxillary first molars are considered as an extreme anatomical feature or certain root anomaly, and have been presented as rare cases 29,49-51 , or with no incidence of such entity 24,52 . Conversely, Marcano-Caldera et al. 38 found 16.1% maxillary first molars with all three roots joined into a single cone-shaped, and when the authors added teeth with all three fused roots associated with one or more lateral grooves, the percentage raised up to 21%. This is in an enormous discrepancy with the result from the present study, as well as with findings by other authors 23 Results showed that regular oval shape of the canal orifice was found in absolute majority of cases, and the rest were crevice-like or a combination of those two, with no information in available literature on these characteristics of maxillary first molars with fused roots. Information on the number of orifices in maxillary first molars with fused roots may be found in few case reports mostly associated with their endodontic management 8,28,47,51 .
Considering inter-orifice distances the most intriguing and clinically important is the one between mb1 and mb2. There are a few reports for maxillary first molars with three separate roots, and values varied from 1.20 mm detected by Spagnuolo et al. 54 , to 2.90 mm presented by Magat et al. 55 . Keçeci et al. 56 measured mb1-mb2 distance of 1.97 mm which strongly coincides with 1.95 mm found in this study (Table 4). Differences in those results have been attributed to variations in race, sample and voxel size, and/or experimental methods. The review of the current literature revealed no study on special geometry formed by canal orifices in maxillary first molars with fused roots. Presented results have clinical relevance when dentist tends to negotiate canal orifices in maxillary first molars with fused roots, stressing on great importance of having proper insight onto a "dentin map", particularly on mb1 and mb2 relation, which is the first instance that practitioner meets when approaching root canal instrumentation.
Roots formed by fusion of two or more roots showed specific morphological features different from a single root, and thus they were considered as a separate anatomical unit. Complex morphology complicates and hinders canal instrumentation and thus decrease success rate of endodontic therapy, proved by many case reports 17,40,41,51 .
The same incidence of either two or three canals was detected in fused roots, and no case was foundith one single canal ( Table 5), indicating that fusion of the roots is not associated with fusion of the canals. This is confirmed by Tian et al. 57 and Mashyajkhy et al. 24 with only 4.5% and 8.3% of merged canals in DB-P type of root fusion, respectively. On the contrary, Martins et al. 34 found multiple merging canals in 25%, where confluence position was usually between DB root and the palatal canal. Several case reports presented two rooted maxillary first molars with two canals, where buccal orifice was the large one, most likely C-shaped, and another was regular single palatal canal 8,56,58 . All those authors estimated that root fusion is not always accompanied with merging of the canals confirming the results from this study.
Of all multiple canals detected in fused roots 40% couldn't be classified according to Vertucci types, what emphasizes their complexity (Fig. 3 a,b). Interestingly, pulpoperiodontal communications (PPC) were revealed in significant incidence of 40% of all fused root canals, irrespective to a type of fusion. Pulpo-periodontal communications have always been detected on the furcation aspect of fused root, meaning that any ingress of noxious stimuli through PPC will inevitably cause either inter-radicular bone lesion, or vice-versa, pulp pathology. Depending on location and diameter PPCs could complicate and cause failure of endodontic and peri-radicullar treatment, particularly due to the lack of their precise revealing and detection on the periapical radiographs. Therefore, CBCT should be applied whenever there is a hint of PPC's presence. Those findings couldn't be discussed since there is no information in available literature.
At the same apex of the fused root there were foramina of various diameters and numerical threshold for the major, i.e. large foramen was defined to be 0.3 mm and over 59 , while below that value they were classified as small, i.e. accessory foramina. On the apices of fused roots more than half were with two, and 1/3 with three large anatomical foramina.
Degree of correlation showed direct and strong correlation between number of canals and number of major apical foramina in fused roots (Rho=0.509; p<0.003), indicating that the larger number of major foramina the larger number of canals was in the "curtain-shaped" fused root. Importantly, no foramen coincides with the anatomic apex. As for accessory foramina, more than half of the apices where without any, quarter was with one, and the rest were with two, three or four small foramina. No analysis of this kind was found in the available literature. Spearman's correlation coefficient showed no statistically significant difference between number of canal orifices and total number of the apical foramina in maxillary first molars with fused roots (Rho=0.285; p=0.114). There was tendency that higher number of orifices was associated with higher number of foramina, but with low correlation and with no significant differences between those two anatomical entities. Therefore, in clinical situations practitioner might predict number of the apical foramina upon the clear insight on the number of canal orifices when treating maxillary first molars with fused roots.
During root canal preparation with manual or engine-driven instruments certain amount of paracanal dentin is removed, which may often lead to either extreme thinning of walls, or to worse complication in form of strip perforation at any level of the root canal, often followed by micro-cracks or vertical fractures 60,61 . The main intention of measuring the canal walls thickness in this study was to reveal critical zones, i.e. critical instrumentation areas for the specific root canal in a fused root, which would help to prevent excessive instrumentation and consequences of such endodontic preparation 6,60,61 .
In the fused P root buccal wall of the canal was three to four time thicker at all three levels than other three walls, since the palatal root was always fused with one of the buccal roots with huge inter-canal dentine layer. Mesial and distal walls were the thinnest along its entire length, and therefore it is important to bear in mind that this area is potentially risk zone for extreme thinning, in spite of massiveness of the palatal root.
Slightly different situation was with canal walls in a fused DB root as it was fused most frequently with P root, with three to five greater dentine thickness for the palatal wall.
The thinnest wall was mesial at all three levels, with no statistical difference compared to the distal, but significantly thinner than the buccal one. Oval canal shape in DB fused root with smaller M-D dimension stresses on mesial and distal walls as potentially prone to weakening and strip perforation during mechanical instrumentation. No data of such measuring on the palatal and disto-buccal root canals in maxillary first molars with fused roots were found in the available literature.
Considering the canal complexity in MB fused root, dentin walls thickness of mb1 and mb2 canal has been put in focus. Regardless either MB root was fused to P or to DB root, the thinnest walls around mb1 and mb2 canals were distal and mesial, thus they could be considered as a dangerous zone, or "critical instrumentation areas", and most prone to procedural errors during their mechanical instrumentation. In contrast to those two, palatal wall of mb1 and buccal wall of mb2 canal were several times thicker with very similar values at all three levels. No article was found that has been particularly dealing with measuring and assessing canal walls thickness in maxillary first molars with fused roots.
There are a few studies presenting dentin thickness from different aspects around the mb1 and mb2 canal, however, only in maxillary first molars with three separate roots. Matus et al. 62 found that mesial and distal walls for mb1 and mb2 canals were the thinnest with mean values ranging from 0.81mm to 1.28 mm, what correlates to the values for a fused MB root in this study. Furthermore, the same authors showed the palatal and buccal walls of similar thickness to the values presented in this study. Degerness and Bowles 63 measured mesial and distal walls as the thinnest towards coronal portion of MB root and emphasized that the average canal wall thickness decreases for 1/3 on the distal aspect, suggesting this area as a "danger zone" for maxillary molars at the level where MB root joins the crown of the tooth. This statement corroborates findings by Yoo et al. 64 pointing on distal wall as the thinnest one for both mb canals, and that dentin walls around mb1 are generally thicker than around mb2 canal, what corresponds to the results from this study.
Previous authors also found that palatal wall of mb1 and buccal wall of mb2 canal were approximately three times thicker, what also coincides with the results from this study.
Respecting results from this and other articles, weakening of distal and mesial wall in MB root of maxillary first molars should be avoided, particularly in those with fused roots.
Thus, there is a little room for procedural errors with increasing possibility for strip perforation, which might lead to vertical root cracks and fractures. Knowledge and awareness of presented discrepancy in the wall thickness between distal and misial on one, versus buccal and palatal canal walls on the other side, would help clinicians to keep in mind that real thickness is always less than what appears in intra-oral radiographs.
Generally, mesial and distal walls of all fused roots are more sensitive to thinning at mid-root and coronal third due to greater tapered design of endodontic instruments, and specific "brushing motion" during canal preparation with rotary files. This is particularly important for moderately curved canals and in situation where canal orifice has to be dislocated away from furcation. Therefore, combination of variously designed canal instruments during preparation sequences could significantly decrease producing of "dangerous zone" and thus, increase the final success of entire endodontic treatment.
For judging inter-rater reliability regarding all conducted measurings and calculated data, both from CBCT scans and micro-photographs Cohen's kappa test was used 65 .
Results showed 94% of agreement between two examiners (κ-test>0.90). This high interrater reliability is to be expected due to standardized and reproducible levels and locations for detection of each anatomical entity and for each of the measurements. Calculating program was calibrated to precision of l/10,000 of unit (four decimals), and final score was shown with two decimals (1/100) in order to present data in less complicated and confusing manner, with no effect on the accuracy and significance of each value.
It should be emphasized that there were neither studies, nor reviews or reports that have been focusing on the anatomical details in such variety and on such morphological specificities of maxillary first molars with fused roots as it was presented in this study. The collected number of extracted maxillary first molars from the patients of Serbian origin was representative when correlated with various studies on other ethnic groups. Due to those facts only a few comparisons with findings of other authors have been discussed. However, results from this study may be of a great help for endodontic practice and should facilitate clinical diagnosis when one aims to predict which of those canal variations exist in the specific case. To recognize and reveal major anatomical aberrations using all available recourses, like CBCT and operating microscope, is the first step towards more predictable root canal preparation and higher long-term success of endodontic therapy.

Conclusion
Of total number of maxillary first molars collected from patients of Serbian origin 17.5% were with fused roots. The most frequent type was P fused to DB root, and significantly less P to MB and MB to DB root. At the level of the pulp chamber floor bucopalatal dimension was significantly larger than mesio-distal. Number of canal orifices was four in 2/3 of teeth, with the shortest mb1-mb2 orifice distance, and the longest between pdb orifices. There were either three or two canals in fused roots with strong correlation to the number of major apical foramina. In vast majority there was no inter-canal communications. In MB and DB fused root the thinnest canal wall was either mesial or distal, while in the P fused root the thinnest wall was palatal. Those walls are considered as critical areas during mechanical instrumentation. CBCT scanning technique with 100 µm voxel size enabled detection of tiny details and precise measurements. Comparing data from available literature with the results from this study certain specificities of the anatomical characteristics were shown in maxillary first molars with fused roots within Serbian population. Table 1 Type of fusion (%)  Table 3 Distribution of the number of orifices at the pulp chamber floor Table 4 Distance between orifice centers (mm) and angles formed by sides of a quadrangle ( 0 )  Table 5 Distribution of number of canals in a fused roots and Vertucci-type classification