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 in coronal, middle and apical thirds 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 follow: 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 third; type III: changed cross-sectional root canal shape in the middle and apical thirds; type IV: changed cross-sectional root canal shape in the apical third. This is the main difference of our classification from other classifications, which consider the cross section at only one level, or describe the unchanging shape of the canal along its entire length.
It is known that non-round canals can’t be enlarged to round without perforation which weakens the root. The presence of different variants of root canal morphology require different methods of preparation and obturation. The preparation of oval and ribbon canals cannot be prepared to a round shape created by rotating tools, and therefore, they remain unprepared, which could potentially lead to failure of endodontic treatment. The requirements for the preparation of the conical shape of the root canal include requirements for irrigation, evacuation of debris and subsequent obturation. But if the taper of the tool used for cleaning and shaping the channel does not match the taper of the canal, the purpose of the mechanical preparation will not be achieved. If the instrument has excessive taper, this will lead to excessive removal of the coronal 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.
Mechanical preparation of the root canals can be performed using hand tools or rotary instruments where the canals are formed by rotation or reciprocal movement. Whether the entire wall of the main canal will be properly cleaned and shaped accordingly 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 - the choice of preparation technique and the choice of instrument depend on and are determined by the shape of the canal. It is very difficult to adequately clean the canals only with tools, which increases the importance of choosing the right solution for irrigation with a sufficient volume and method of irrigation. Most often used tools create a round canal shape, leaving isthmuses, narrowing unprepared and filled with a greased layer and debris. Ultrasound activation of irrigation can better clean lateral and additional canals [5].
Mehrvarzfar et al. studied the ribbon shape in the apical third according to Kim & Peroca 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 [9]. 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 Fumes 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 [19].
In 2016 (China), Hu et al. [20] 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 [21] 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 in agreeable with the results of this study.
Kacharaju 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%. In the middle third of the canal, the oval shape was detected in 36% of cases, round in 21% of cases, irregular in 30% of cases, ribbon with isthmus in 7% of cases, and ribbon without isthmus in 6% of cases. In the apical third, the canal shape changed; in 22% an oval shape was detected, in 37% it was round, in 25% it was irregular, in 8% it was ribbon with an isthmus and in 8% it was ribbon without an isthmus [8].
This variability of the root canal shape is important to understand in order to complete the endodontic treatment stage—mechanical treatment (preparation of the root canal). The choice of instrument and technique for preparing the canal will determine the effectiveness and correctness of this stage, and therefore a favorable prognosis of treatment.
According to the presented classification, the most changes were observed in the middle and apical thirds of the root canal in each age group.