## 4 Discussion
In this way, possible bias due to a specific panoramic unit was bypassed. The present study avoided these kinds of errors by randomly choosing the panoramic units. In addition, it is necessary to consider the patient's head positioning errors too, because even if there is some tolerance, they affect the mesiodistal tooth angulation on panoramic, as noticed in a study by [PERSON] et al. [19]. In this study, panoramic images were taken by different technicians and without a standard protocol of use, and it can be considered as a limit of the present investigation; this might have increased the likelihood of patient positioning errors in panoramic radiography. However, it was not possible to do otherwise, as patients had already undergone CBCT and panoramic investigation for individual reasons. Although the acquisition of radiographic examinations performed by different operators may represent a bias of the present study, the use of this material allowed us to conduct a clinical study without exposing patients to further radiation. The acquisition of new radiographic examinations would have been in contrast to the basic principles of radiation protection and would not have been accepted by the ethics committee. However, we must consider that patient positioning errors are related exclusively to panoramic radiography. CBCT, in fact, ensures a higher accuracy because a specific algorithm corrects the potential distortions generated in the acquisition process, before image reconstruction. According to several studies [14; 15; 27], CBCT was more reliable than panoramic radiography for root angulation measuring both at the level of maxillary and mandibular arch. [PERSON] et al. [14], after the comparison between model-panoramic radiography and model-CBCT, assumed that CBCT had a significant reliability even in linear and in angular measurements. Eventually this study by [PERSON] et al. comparing panoramic radiography with CBCT, showed not only the same limits of panoramic radiography as other studies [12; 13], but also the accuracy of CBCT as the gold standard of diagnostic device in accordance with the present investigation. Assuming CBCT as the gold standard, [PERSON] et al. [21] showed a statistically significantly difference (up to 75%) between mesiodistal root angulation measured on panoramic and mesiodistal root angulation measured on CBCT, and they could not identify a reproducible trend of alterations between these imaging devices. In accordance with [PERSON] et al., the present study showed a statistically significant difference between mesiodistal root angulation measured on panoramic radiography and mesiodistal root angulation measured on CBCT; furthermore, it identified a relevant correlation between mesiodistal root angulation measured on panoramic radiography and buccaloinual root inclination measured on CBCT. As far as this correlation is concerned, a previous study [11] reported an extensive variability among the effects of a buccolingual inclination on mesiodistal angulation, but it could not identify a standard relationship between them. However, a laboratory investigation by [PERSON] [20] showed that changes in buccolingual inclination predictably affected the appearance of mesiodistal root angulation on panoramic radiography. On the basis that CBCT is the gold standard, the use of panoramic radiographs to evaluate mesiodistal angulation and buccolingual inclination of first and second maxillary premolars should be approached with caution. A CBCT performed at the end of the treatment would be the finest way for evaluating the root parallelism, but as long as it exposes the patient to a massive radiation dose, then it is not part of standard protocol currently; at most, a CBCT evaluation can be accepted in cases of dental anomalies [39] and bone discrepancies [40] correction, or in cases of orthosurgical procedures [41].