Several studies have investigated the prevalence of AP in teeth with endodontic treatment and its risk factors [21, 22]; however, they have used 2D images, which have disadvantages compared to 3D images, since CBCT, due to its high sensitivity, can evaluate more accurately the presence of AP [23], the quality of the endodontic treatment, and the presence of missed canals [14–16]. For example, a study conducted in Colombia in 2013 found an AP prevalence of 49% using 2D images for endodontic treatments [21], whereas the current study using CBCT found a higher prevalence of AP (62.34%), similar to recent CBCT studies that found AP between 51.8% and 88% [14–16].
Furthermore, the current study found 18.61% of missed canals, similar to previous studies, which reported 12–23% [16, 18]. Also, in the present study, 95.3% of missed canals had AP, comparable with previous [14–16], that found 82.8%, 82.6%, and 98%, respectively.
Maxillary molars were the dental group with the highest frequency of missed canal 55%, similar to prior reports of 40.1% [14, 15]. Another study showed 59.55% and 40% missed canals in maxillary first and second molars, respectively [16], and the present study found similar results for the first and second maxillary molars with 61.97% and 41.18%, respectively. The reason for the high percentage of missed canals may likely be the presence of a second mesial buccal (MB) canal in 88.5% of maxillary first molars and 83.4% of second molars [24]. Additionally, the MB root has complex anatomy that usually presents type II (69.1%) [25] and type IV (48.7%) [26] Vertucci’s configurations. Likewise, the MB2 was the most frequently missed canal for maxillary molars in previous studies, with the prevalence of 65% [16], 93% [14], and 89% in the current study. The possible explanation is that the MB2 canal is usually narrow, significantly smaller than the MB1 canal, with its opening often covered by secondary dentin or calcifications [26, 27], and the curvature in the mesial root is pronounced in the apical third of both canals and more intense in the MB2 canal. Therefore, all these anatomical features may lead to greater difficulty locating and achieving a straight course to the apex [27]. Thus, the presence of a MB2 canal should be considered the rule rather than the exception.
The mandibular molars had the second-highest frequency of missed canals with 18%, similar to previous studies, which found 19.7% [15], 26% [14], and 10.42% [16]. All mandibular molars with missed canals were associated with AP in the present study. Available data have reported similar frequencies, such as 94.1% [16] and 95.5% [14]. Another study found 62% of missed canals in the mandibular first molars, with the distolingual canal being the most frequently missed canal; however, the percentage of AP in this dental group with missed canals was not mentioned [15]. Similarly, to a study that found the mandibular first molars with missed canals were distolingual canal at 21%, and mesolingual canal at 29%, and for the mandibular second molar it was mesolingual canal at 62% [14]. In the present study, the distobucal canal was the most missed in the first mandibular molars with 42.85%, followed by the mesolingual canal in the mandibular second molars with 21.05%. The possible explanation for these frequencies of missed canals in mandibular molars is anatomical variations and their location, making it difficult to access and treat all root canals [14–16].
In the present study, the probability of a tooth with missed canal having AP was 16.9 (crude OR) and 10.5 (OR adjusted to the tooth type variable). These findings could be considered higher than those of previous studies, whose reported probabilities of 4.38 [15], 6.25 [14], and 4.4 [16]. These differences may be related to the highest percentage for this study of teeth with IET. Also, the good quality of the endodontic treatment seems to have the potential to decrease the prevalence of apical periodontitis in teeth with missed canals when compared with teeth with IET and missed canals. However, further studies are necessary to confirm this hypothesis.
A relevant aspect of the results obtained in this study at a bivariate level was very similar to the above studies. Additionally, a wide confidence interval was observed for crude OR and OR adjusted for the tooth type variable. According to tooth type, an OR of 2.5 was observed for maxillary molars, similar to the 3.1 previously reported [16].
The major advantage of cross-sectional studies is that, with a large sample size, potential interpretation errors can be diluted [14]. However, the nature of cross-sectional studies may also be a limitation due to the reliability of observation. It is difficult to establish the natural history of the disease; for example, it is impossible to establish whether the AP is in the process of expansion, healing, or remains stable [1]. Many of the images classified as AP at the time of analysis could be an AP in the process of healing, reducing in size [28]. Another limitation was the CBCT method, which may show artifacts from high-density objects such as gutta-percha, which could generate false positives in some of the cases evaluated [14–16]. Future studies should also consider factors including the size of the lesion and the patient's age.