## 4 Discussion
While recently, new rotary instruments such as (SAF, XP-endo Shaper, and TRU Shape) were used in the preparation oval root canals and presented a higher percent of touched walls without excessive removing of dentine, which is so effective in endodontic treatment [17; 23; 24]. Mechanical preparation of root canals can be performed using hand tools or rotary instrument. Cleaning and shaping the entire wall of the main canal depends on a number of factors, such as canal morphology, canal wall thickness, canal taper, type of tool used and its size. Among all factors, the root canal morphology can be considered the most important since all other factors, like the choice of preparation technique and the choice of instrument depend on and are determined by the shape of the canal. It is also very difficult to adequately clean the canals only with tools, because most often tools create a round canal shape, leaving isthmuses, narrowing unprepared and filled with debris, which increases the importance of choosing the right solution for irrigation with a sufficient volume and method of irrigation [18; 25]. A few studies have evaluated the root canal shape, as [PERSON] et al. studied the ribbon shape in the apical third according to Kim and Pecora classification in the mesial roots of mandibular molars, it was observed at 6 mm of the apex type I in 8.3%, type II 40%, type III 8.3%, type IV 18.3%, and type V 25%. At 4 mm of the apex the prevalence of types I, II, III, IV, and V was 11.7%, 16.7%, 15%, 10%, and 46.8%, respectively, and at 2 mm of the apex the prevalence of types I, II, III, IV and V was as follow 30%, 10%, 5%, 3.3%, and 51.7%, respectively [7]. This study demonstrates the change of root canal shape in the apical third, which coincide with the results of our study. In Brazil, a micro-CT study of [PERSON] et al. was conducted to study the shape of the canals of the first and second molars in the apical area. For mandibular first molars, a rounded shape was detected in the mesial root in 30%, oval in 10%, ribbon in 50%, and irregular in 10% of cases, and in the distal root a round shape in 30%, oval in 20%, ribbon in 40%, and irregular in 10% of cases. For the mandibular second molars in the mesial root revealed a round shape in 30%, ribbon in 60%, and irregular in 10% of cases, and in the distal root, a round shape in 40%, oval in 20% and ribbon in 40% of cases. For maxillary first molar, a round shape in 10%, oval in 30%, ribbon in 50% and irregular in 10% of cases were revealed in the mesio-buccal root, for the disto-buccal root, an oval shape was observed in 100% of cases, and in the palatal root round shape was found in 20% and oval in 80% of cases. For the maxillary second molars in the mesio-buccal root revealed a rounded shape in 20%, oval in 30%, ribbon in 40% and irregular in 10% of cases, in the disto-buccal root revealed an oval shape in 100% of cases, and in the palatal root a rounded shape in 30%, oval in 70% of cases [26]. In 2016 (China), [PERSON] et al. [21] studied morphological changes in the root canals of the maxillary first premolar using CBCT in different age groups and found that the morphology of the sections in most age groups was ribbon (48.5%) and oval (28.2%). In 2019, the researchers [22] studied the anatomy and morphology of the second maxillary premolar using CBCT and differences with age, and found that in most groups, the ribbon shape was higher in 57.8% of cases and the oval form in 31.3%. Researchers concluded that with age, the percentage of the round shape gradually increased, while the ribbon and oval shapes decreased, these results are expected, as it is known that the physiological deposition of tertiary dentin along the life of human beings will change the morphology of root canals, resulting in narrowing and rounding. [PERSON] et al. studied the morphology of the root canals of mandibular premolars in the Malaysian population. The study revealed that in the coronal third, an oval shape was detected in 52%, round in 1% and irregular in 46%.