## 4 Discussion  
The root canal morphology of mandibular second molars is of special interest due to the high incidence of distal caries lesions reported for second molars [24; 25] which might progress over time and end up with dental pulp involvement and endodontic treatment. The potential problem is that mandibular second molars are known for the large number of possible root canal morphology variations and detailed knowledge of all possible variations is needed for successful endodontic treatment. The higher incidence of anomalies in the second lower molar in comparison to the first one could be explained by evolutionary and environmental theories. According to [PERSON]'s evolutionary Field theory, the jaws are divided in four morphogenetic fields (incisors, canines, premolars and molars) and the more distal tooth is in this field, the less genetically stable it is considered, except for the lower incisors group. Other theories are based on environmental conditions affecting dental lamina during sepcific areas of tooth development, such as the fusion of the bones for the upper lateral incisors or during the development of the tooth's innervation [26]. Root and root canal morphology of different types of teeth has been analyzed in different populations, such as the Indian, Chinese, Korean, Israeli, Brazilian and Saudi Arabian populations, and the results obtained indicate that certain variations in root and root canal morphology are genetically determined [17; 18; 19; 20; 27; 28; 29]. Furthermore, these observations can be used for various epidemiological studies or the study of the ethnic origin of an individual. CBCT analysis has proven to be a useful method in these studies because it is non-invasive, provides a three-dimensional view of anatomical structures and is increasingly being used for the diagnosis of various conditions in the oral cavity [30]. Therefore, it is not surprising that numerous studies investigating anatomical variations in different types of teeth using CBCT scans have been published [31], but there is still no data for the Croatian population. In this study, an analysis of the morphology of endodontic space of the lower second molars in the population of the Zagreb area using CBCT showed that \"C\"-shaped root canals were found in 9.1% of cases. This is not in agreement with a study by [PERSON] et al. [21] which was conducted on extracted teeth, and \"C\"-shaped canals were found in 12.5% of lower second molars. A possible explanation for the different results might be the different methodologies, i.e., there is a greater possibility that, among the extracted teeth, more teeth will have certain anatomical variations, because dentists will be more inclined to extract such teeth in contrast to teeth with normal morphology that will receive the treatment needed and thus be preserved. Interestingly, large variations can be found in the occurrence of \"C\"-shaped root canals in different countries. Countries where, as in Croatia, the frequency of \"C\"-shaped canals is up to 10% are Serbia (5.53%), Israel (4.6%), Brazil (3.5%), the Middle East (7.9%) and Pakistan (10%), while the frequency is high in Jordan (12%), Egypt (12.8%) India (13.1%), Venezuela (19.5%) and Iran (21.4%) and an extremely high frequency is found in Asian \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Vertucci** & **Type I** & **Type II** & **Type IV** & **Type V** & **Type VI** & **Type VII** & **Type VIII** \\ \hline Frequency & 12.0\% & 41.0\% & 0.8\% & 45.1\% & 0.4\% & 0.5\% & 0\% & 0.1\% \\ \hline \end{tabular}
\end{table} 
Table 2: Frequency of Vertucci types in mesial roots of lower second molars. populations, where studies show results between 36.8% and 41.3% [5; 6; 17; 19; 20; 27; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42]. Moreover, studies have reported the influence of ethnicity and race on the root canal morphology of mandibular second molars [7; 31] which highlights the importance of studies of root canal morphology in different populations.