## 4 Discussion
Root canal classification may be useful during diagnostic procedure and treatment planning. Over time, different root canal classifications, including their modifications, were proposed. [PERSON]'s classification is well known because of its simplicity and the fact that it is used by numerous authors [4; 49]. Despite its many advantages, it has certain limitations. Given that it represents eight types of root canals, it excludes and does not describe certain anomalies and certain morphologic forms that do not correspond to any of the described types. It also excludes the description of communications between the canals and does not describe the number of roots in the anterior teeth and premolars [4]. Nevertheless, [PERSON]'s classification was used in this study. Our results show that the Vertucci types IV (45.1%) and II (41.0%) are the most common in the subpopulation of Zagreb, which is consistent with the results of a study on the population of Saudi Arabia [29], while another study on the population of the Emirates found that, among the mesial roots of second lower molars, Type II and Type III are most commonly found [50]. The differences in the reported prevalence of specific anatomical variations in some studies could be explained by variations in the methods used in different studies. In the study of [PERSON] et al. [51] on the Indian population of Mumbai, in which the staining and clearing method was used, type IV was most frequently (63.1%) observed on lower second molars with two roots, while [PERSON] et al. [17] reported a lower percentage of type IV (45.2%) in the same population using the CBCT analysis. The \"C\"-shaped canal and certain other types according to [PERSON] (Type III, Type V, Type VI, Type VII) represent a complex and irregular space with a high potential for retaining infected soft tissue and debris after traditional root canal instrumentation. Therefore, once one of these canal forms is detected, special attention and time should be invested in planning and performing endodontic treatment. Knowledge and data from the studies of the root canal morphology, like the present study, might be helpful in recognizing this specific morphological variation [7]. Mechanical debridement of the root canal space of \"C\"-shaped root canals is difficult due to transverse anastomoses, lateral canals and apical deltas [38]. Special attention should be given to thin lingual dentine walls in curved canal due to increased risk of a stripping perforation [12; 52; 53; 54]. According to [PERSON] and [PERSON] [55], a minimum of dentine thickness of 0.2-0.3 mm is needed to resist fracture due to pressure during a root canal filling procedure which highlights the importance of careful and non-invasive root canal preparation with smaller size files, especially in the coronal third of root canal. The most common causes of failure in cases with \"C\"-shaped root canals are leakage of canals, isthmus, missing canal, overinstrumentation and iatrogenic mistakes [56]. It is also particularly important for the clinician to recognize the root canal fusion (Types II and III), which, according to this research, is quite common (41.8%) in the lower second molars, to avoid breaking the instrument in an attempt to treat both canals separately while reaching the full working length. The limitations of this study are that the ethnicity of the patients was not recorded because this information was not available to collect and analyze, and that only scans of the subpopulation of the city of Zagreb were included in the study.