1. Introduction {#sec1}
===============

A sound knowledge of root canal anatomy is mandatory in order to perform an adequate root canal treatment. Studies reported that failure to detect all the canals present in a root canal system was one of the causes of failure of endodontic therapy \[[@B1]--[@B6]\].

A number of studies that evaluated the anatomy of mesiobuccal roots of maxillary permanent first molars reported a wide range of anatomical variations \[[@B7]--[@B10]\].

It was hypothesized that failure to detect, debride, and fill a second mesiobuccal canal (MB2) of first permanent maxillary molars was one of the main causes of poor long-term prognosis after root canal treatment in these teeth \[[@B11], [@B12]\].

While many *ex vivo* studies investigated the presence of a mesiobuccal canal using canal staining, cross-sectioning, and dentine examination through magnification devices \[[@B13]--[@B15]\], the most used technique to investigate the anatomy of these teeth prior to an endodontic treatment is periapical radiography, which does not allow a complete detailed evaluation of the root canal anatomy \[[@B16]\].

Cone-beam computed tomography (CBCT) was developed in the 1990s with the aim of producing maxillofacial three-dimensional images using a lower radiation dose than conventional computed tomography (CT) \[[@B17], [@B18]\]. The characteristics of the CBCT scanning were described as well suited to the endodontic field because of the higher accuracy of the device in comparison to that of the standard CT \[[@B17], [@B19]\].

Despite the known limitations of the CBCT (scattering \[[@B20]\], lower resolution than conventional radiography \[[@B17], [@B21]\]), it proved to be valuable in endodontic therapy for the diagnosis and evaluation of root canal anatomy \[[@B17]\].

The aim of the present paper was to systematically review the current scientific literature about the use of CT for the investigation of the mesiobuccal roots and canals of first permanent maxillary molars.

2. Materials and Methods {#sec2}
========================

2.1. Search Strategy {#sec2.1}
--------------------

The following electronic databases were searched: MEDLINE (through PubMed interface <http://www.ncbi.nlm.nih.gov/sites/pubmed>), Scopus (<http://www.scopus.com/>), EMBASE (<http://www.embase.com/>), and the Cochrane Library (<http://www.cochrane.org/>). A search string was prepared ad hoc combining keywords with the use of Boolean operators "AND" and "OR". The search string was ("Cone Beam Computed Tomography" OR "Cone Beam CT" OR "CBCT" OR "Computed Tomography" OR "CT") AND ("endodontics" OR "endodontic diagnosis" OR "maxillary first molar" OR "fourth canal" OR "dental anatomy" OR "root canal" OR "mesiobuccal canal"). Results were limited by year of publication (from 1970 on), and the last search was performed in September 2012. In addition, a manual forward and backward search was performed in the reference lists of selected articles from the search results and of articles from the search results that were published in *Journal of Endodontics, International Endodontic Journal, Oral Surgery Oral Medicine Oral Pathology Oral Radiology and Endodontology, Journal of Dentistry, Journal of Dental Research, Clinical Oral Investigations, European Journal of Oral Sciences, Odontology, Dentomaxillofacial Radiology, Oral Radiology, Australian Dental Journal,*and*Australian Endodontic Journal.* No language restriction was posed.

2.2. Study Selection Criteria {#sec2.2}
-----------------------------

The following inclusion criteria had to be met in order to be included in the review:any study design (prospective or retrospective);*ex vivo* or *in vivo* studies;root canal anatomy evaluated using CT;at least 10 teeth analyzed;description of presence/absence of MB2 in maxillary first molars;clear description of tooth type and location.

Studies not meeting the above criteria were excluded from the review.

2.3. Data Extraction and Analysis {#sec2.3}
---------------------------------

The following parameters were recorded in an electronic form:presence/absence of the MB2 in maxillary first molars;any anatomical characteristics of the MB2, as described by Vertucci \[[@B22]\];methods for detection and CBCT machine characteristics.

Risk of bias was assessed for each of the included studies, considering the following parameters:*number of examined teeth*, posing that when less than 50 teeth were evaluated, the study had a moderate risk of bias; otherwise, the risk of bias for this parameter was judged as low;*quality of data reporting*, considering that if the authors did not report the characteristics of the root canal anatomy of MB2, but only the presence/absence of the canal, the study was judged at moderate risk of bias;*fulfillment of the aims of the study, reporting of individual data instead of frequency percentages,*and*clear description of population characteristics* were evaluated, and when missing, a moderate risk of bias was assigned.

Pooled data were analyzed through an evaluation of the weighted mean prevalence of MB2. Weighted means were calculated for the frequencies of different mesiobuccal canals morphologies following the Vertucci classification \[[@B22]\]. Moreover, a critical evaluation of differences among the various detection methods in terms of presence/absence of MB2 was performed.

3. Results {#sec3}
==========

Article selection process is summarized in the flowchart shown in [Figure 1](#fig1){ref-type="fig"}. The electronic search yielded 367 articles. After title and abstract screening, 20 articles were selected for full-text evaluation. The article selection was performed independently by two authors (SC and MDF). In case of disagreement, a joint decision was taken by discussion. A total of seven articles were finally included in the review: one article described the use of dental-CT *in vivo* \[[@B23]\], and six described the use of CBCT both in *in vivo* and in *ex vivo* studies \[[@B24]--[@B29]\]. Data about study design, population, and device used are summarized in [Table 1](#tab1){ref-type="table"}.

Each of the included studies was evaluated by two authors (SC and ST) for risk of bias analysis, whose results are presented in [Table 2](#tab2){ref-type="table"}.

In pooled analysis, the second mesiobuccal canal was present in 59.32% of the examined teeth (out of a total of 1964 teeth), and it was evaluated as completely independent (noncommunicating) in 58.45% of the teeth presenting the canal (*N* = 1165). Data about frequency of the presence of the second mesiobuccal canal with different detection methods are summarized in [Table 3](#tab3){ref-type="table"}.

Four studies presented data that could be classified according to the Vertucci classification of root canal morphology \[[@B26]--[@B29]\]. The data are summarized in [Table 4](#tab4){ref-type="table"}. More than 37% of the classified 1741 teeth included in this analysis belonged to type IV, while 36.18% belonged to type I. These two types were the most frequently detected morphologies.

Comparison of the different detection systems showed that in CBCT the MB2 was detected in 61.84% of teeth, while dental-CT showed MB2 presence in 39% of teeth.

4. Discussion {#sec4}
=============

The present study was aimed to systematically review the root canal anatomy of the mesiobuccal roots of permanent maxillary first molars as shown in CT images.

The retrieved data showed that the detection of a separate mesiobuccal canal occurred in nearly two-thirds of the cases. The common occurrence of two canals in the mesiobuccal roots has a major implication in endodontics when performing a root canal therapy in maxillary first molars.

A systematic review of the literature performed by Cleghorn and coworkers in 2006 \[[@B8]\]estimated the anatomy of the permanent maxillary first molar through a meta-analysis of data of about 8399 teeth from 34 laboratory studies and of 2576 teeth from 14 clinical studies. They reported that the incidence of two canals in the mesiobuccal root was 56.8%, which is similar to the results of the present study.

An issue to be considered is the age and gender of the subjects. Several authors found that a second mesiobuccal canal is less frequent in older subjects due to progressive calcification and obturation \[[@B8], [@B30]--[@B33]\]. As reported by other authors, there is conflicting evidence regarding differences related to gender of the subjects \[[@B11], [@B32]\].

The present study also showed that, if an MB2 was present it was mostly not merged with the primary one there were two separate apexes and two entrances without any merging (Vertucci Class IV \[[@B22]\]). This finding, which was also confirmed by previous systematic reviews \[[@B8], [@B9], [@B34]\], may have clinical relevance. The detection of a second mesiobuccal canal is mandatory for complete sterilization and filling of the root canal system, in order not to leave a pathway for bacterial migration towards the apex or a reservoir for microorganisms.

Some studies investigated the use of magnification devices (microscope or magnifying loupes) as an adjuvant for the detection of a second mesiobuccal canal \[[@B34]--[@B37]\]. One of these studies showed that the MB2 was detected in 71.1% of maxillary first molars when using a microscope, in 62.5% of these teeth when using dental loupes, and only in 17.2% of these teeth without any magnification \[[@B36]\]. These results are similar to those reported in other studies and are comparable to the frequencies reported in the present review for CT.

However, the use of CBCT is considered very important for an adequate planning of endodontic surgery because of the capability to detect anatomical variations \[[@B17], [@B38]\]. The use of CBCT enables reproduction of three-dimensional anatomy, allowing the evaluation of bone thickness and the relationships between root apices, lesions, and anatomical structures such as the maxillary sinus \[[@B38]--[@B41]\].

In the present review, the included articles seemed to be very heterogeneous in population characteristics and study methodology, limiting the external validity of the results. Also, different CBCT devices were used, which should be considered as an important confounding factor, thus limiting the ability to perform a full meta-analysis.

Despite these limitations, the present study showed that CBCT may be useful in detecting the presence of a second mesiobuccal canal in permanent first molars. This can be considered as an important adjunct in pretreatment assessment for endodontic procedures, especially in periapical surgery.

The authors declare that they have no conflict of interests.

![Article selection process.](IJD2013-614898.001){#fig1}

###### 

Study characteristics.

  Authors                          Year   Study        Device      Characteristics (machine; voxel size)   Population             Patients/teeth
  -------------------------------- ------ ------------ ----------- --------------------------------------- ---------------------- ----------------
  CBCT                                                                                                                             
   Filho et al. \[[@B24]\]         2009   *In vivo*    CBCT        i-CAT; 0.2 mm^3^                        Brazil                 NR/54
   Blattner et al. \[[@B25]\]      2010   *In vitro*   CBCT        i-CAT; NR                               ---                    NE/20
   Neelakantan et al. \[[@B26]\]   2010   *Ex vivo*    CBCT        3D accuitomo; 0.125 mm^3^               India                  NR/220
   Zhang et al. \[[@B27]\]         2011   *In vivo*    CBCT        3D accuitomo; 0.125 mm^3^               China                  269/299
   Lee et al. \[[@B28]\]           2011   *In vivo*    CBCT        Volux; 0.167 mm^3^                      Republic of Korea      276/458
   Kim et al. \[[@B29]\]           2012   *In vivo*    CBCT        Dinnova; 0.167 mm^3^                    Republic of Korea      415/814
  Dental CT                                                                                                                        
   Rathi et al. \[[@B23]\]         2010   *In vivo*    Dental CT   Somatom; 1 mm^3^                        India; age: 11--77 y   100/100

NR: Not reported; NE: Not estimable; MB: mesiobuccal.

###### 

Risk of bias evaluation.

  Authors                          Year   Study        Number of teeth   Data reporting   Aims of the study   Individual data   Population characteristics
  -------------------------------- ------ ------------ ----------------- ---------------- ------------------- ----------------- ----------------------------
  CBCT                                                                                                                           
   Filho et al. \[[@B24]\]         2009   *In vivo*    Low               Moderate         Low                 Low               Low
   Blattner et al. \[[@B25]\]      2010   *In vitro*   Moderate          Low              Low                 Low               Low
   Neelakantan et al. \[[@B26]\]   2010   *Ex vivo*    Low               Low              Low                 Low               Low
   Zhang et al. \[[@B27]\]         2011   *In vivo*    Low               Low              Low                 Low               Low
   Lee et al. \[[@B28]\]           2011   *In vivo*    Low               Low              Low                 Low               Low
   Kim et al. \[[@B29]\]           2012   *In vivo*    Low               Low              Low                 Low               Low
  Dental CT                                                                                                                      
   Rathi et al. \[[@B23]\]         2010   *In vivo*    Low               Low              Low                 Low               Low

###### 

Evaluation of the presence of second mesiobuccal canal.

  Authors                          *N*        Presence of MB   MB independent   MB1 merge MB2                                                               
  -------------------------------- ---------- ---------------- ---------------- --------------- --------- ------- ----------- --------- ------- ----------- ---
  CBCT                                                                                                                                                       
   Filho et al. \[[@B24]\]         54         21               38.89                            1         4.76                20        95.24                
   Blattner et al. \[[@B25]\]      19         11               57.89                            NR        ---                 NR        ---                  
   Neelakantan et al. \[[@B26]\]   220        99               45                               85        85.86               12        12.12                
   Zhang et al. \[[@B27]\]         299        156              52.17                            109       69.87               22        14.1                 
   Lee et al. \[[@B28]\]           458        329              71.83                            160       48.63               152       46.2                 
   Kim et al. \[[@B29]\]           814        510              62.65                            326       63.92               165       32.35                
                                                                                                                                                            
  Total                            1864       1126                              **61.84**       681               **63.93**   371               **39.68**    
                                                                                                                                                            
  Dental CT                                                                                                                                                  
   Rathi et al. \[[@B23]\]         100        39               39                               NR        ---                 NR        ---                  
                                                                                                                                                            
  Total                            100        39                                **39**          0                 **0**       0                 **0**        
                                                                                                                                                            
                                   **1964**   **1165**                          **59.32**       **681**           **58.45**   **371**           **31.85**    

  *N*: total number of teeth; *n*: number of teeth belonging to a category; NR: not reported; Cumulative %: weighted mean proportion of teeth.

###### 

Root canal morphology following the Vertucci classification \[[@B22]\].

  CBCT                                                                                                                              
  ------------------------------- ----- ----------- ----------- ---------- ----------- ---------- ---------- ---------- ---------- ----------
  Neelakantan et al. \[[@B26]\]   220   114         12          0          85          0          0          0          2          0
  Zhang et al. \[[@B27]\]         299   113         22          0          109         25         0          0          0          0
  Lee et al. \[[@B28]\]           458   129         152         0          160         11         0          0          6          0
  Kim et al. \[[@B29]\]           814   292         164         2          326         16         1          0          0          0
                                                                                                                                   
  Total                                 **648**     **350**     **2**      **680**     **52**     **1**      **0**      **8**      **0**
                                                                                                                                   
  **%**                                 **36.18**   **19.54**   **0.11**   **37.97**   **2.90**   **0.06**   **0.00**   **0.45**   **0.00**

  *N*: number of teeth.

[^1]: Academic Editor: Eyal Rosen
