Putative measurement for alarming orthopantomogram relationship between mandibular third molar and inferior alveolar canal proved by CBCT: A cross-sectional study

Cone-beam computed tomography (CBCT) provides three-dimensional assessment of the relationship between mandibular third molars (MTMs) and the inferior alveolar canal (IAC). It is crucial for reducing post-operative surgical complications. However, it might not be always available. However, orthopantomograms (OPG) remain the first-line investigation and help determine the need for further CBCT evaluation. Objective — to determine possible alarming OPG radiographic features in terms of the relationship between imparted lower third molar and IAC. Materials and methods. Total 85 patients with 113 mandibular third molars were evaluated. OPG images were examined according to Rood and Shehab’s criteria. Root deflection, root darkening, and interruption of the white line were identified. Then, all cases underwent further examination with CBCT to determine the spatial relationship between MTMs and IAC and measure the distance between them. The results were classified into safe (≥2 mm) and high-risk (<2 mm) groups. Results. According to OPG results, root deflection was the most common radiographic sign (53.1%), followed by root darkening (38%) and interruption of the white line (8.8%). The IAC was most frequently positioned buccally (49.5%), followed by inferiorly (31.8%) and lingually (18.5%) in CBCT imaging. A strong connection was noticed between the OPG radiographic signs and CBCT-measured distances. Of the 113 MTMs, 59.3% were categorized as high risk (<2 mm), comprising 47.8% root deflection. The root darkening was more frequently correlated with the safe distance group (33.6%). Conclusion. Despite that OPG findings can predict the nature of the relationship between IAC and MTM, it is not useful in assessing the distance. However, it can be suggested that root deflection in the OPG radiograph might carry more nerve injury risk than other radiographic findings. This is based on the fact that distance of ≤2 mm between the MTM and the IAC would increase the risk of nerve injury.