## 4 Discussion  
The cross-sectional root canal shape varies, it could be round, oval, long oval or ribbon. This was the first study to analyze the cross-sectional root canal shape and its changes during its course to apex among different age groups. This study has demonstrated the changes in the root canal cross-sectional shape between the coronal, middle, and apical thirds among different age groups. The results of this study showed that root canal shape changes in two cases: (1) the presence of one or more canals in the root (with or without isthmus between them), (2) with age, as it was observed that the cross-sectional shape of the root canal becomes round with age in the apical third. This study described four types of root canal shape changes as follows: type I: un-changed cross-sectional root canal shape from coronal to apical third; type II: changed cross-sectional root canal shape in the middle and apical thirds; type IV: changed third; type III: changed cross-sectional root canal shape in the apical third. This is the main difference of our classification from other classifications, which described the canal shape just in coronal third. This study revealed that that the most common change in cross-sectional root canal shape was among premolars, in which the shape changes from oval in the coronal third to round in the apical third. The second common change was observed in mesial roots in mandibular molars, in which the shape changed from ribbon type I in the coronal third to ribbon type V in the middle and apical third, in some cases the it turned to oval in the apical third. The important aspects of root canal shape can be explained as follow: (1) the presence of different variants of root canal morphology require different methods of preparation and obturation, (2) non-round canals can't be enlarged to round without perforation which weakens the root, (3) the preparation of oval or ribbon canals cannot be prepared to a round shape, which created by \begin{table}
\begin{tabular}{c c c c c c} \hline
**Middle Group** & **Root** & **Type I** & **Type II** & **Type III** & **Type IV** \\ \hline Maximly anterior teeth & One-rooted & 100\% & 0 & 0 & 0 \\ Maximly premolars & Two-rooted & 100\% & 0 & 0 & 0 \\  & One-rooted & 49\% & 4\% & 34\% & 13\% \\  & MB & 3\% & 10\% & 46\% & 41\% \\  & DB & 71\% & 0 & 11\% & 18\% \\  & P & 77\% & 0 & 13\% & 10\% \\ Mandibular anterior teeth & One-rooted & 96\% & 3\% & 1\% & 0 \\ Mandibular premolars & One-rooted & 53\% & 2\% & 31\% & 14\% \\  & M & 58\% & 0 & 24\% & 18\% \\  & D & 60\% & 0 & 12\% & 28\% \\ \hline \end{tabular}
\end{table} 
Table 4: Changes of cross-sectional root canal shape in middle age group. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Elderly Group** & **Root** & **Type I** & **Type II** & **Type III** & **Type IV** \\ \hline Maximly anterior teeth & One-rooted & 100\% & 0 & 0 & 0 \\ Maximly premolars & Two-rooted & 100\% & 0 & 0 & 0 \\  & One-rooted & 56\% & 9\% & 24\% & 11\% \\  & MB & 6\% & 7\% & 39\% & 48\% \\  & DB & 76\% & 0 & 9\% & 15\% \\  & P & 84\% & 0 & 9\% & 7\% \\ Mandibular anterior teeth & One-rooted & 93\% & 5\% & 2\% & 0 \\  & One-rooted & 43\% & 5\% & 24\% & 28\% \\  & M & 64\% & 0 & 21\% & 15\% \\  & D & 56\% & 0 & 31\% & 13\% \\ \hline \end{tabular}
\end{table} 
Table 5: Changes of cross-sectional root canal shape in elderly age group. rotating tools, and therefore, parts of them remain unprepared, which could potentially lead to failure of endodontic treatment, (4) the taper of endodontic instruments should match the root canal taper, because if the taper does not match, the purpose of the mechanical preparation will not be achieved. While the instrument has excessive taper, this will lead to excessive removal of the dentin and weakening of the tooth structure, and at that time, the instrument will not be able to effectively clean the apical third of the canal [18].