Table 2 summarizes the results of the Bland–Altman plots. The largest range of inter-rater variation was found for the NSBa parameter, where 95% of the measurement differences between the two raters were in a range of −3.91mm and 3.44mm. Considering the other parameters, a low inter-rater variability could be assumed.
The averaged skeletal configuration of the patients showed a bimaxillary orthognathic configuration, with sagittal and vertical neutral jaw relation, neutral maxillary and mandibular base inclination, neutral skull base inclination, and neutral index and jaw angle (Table 1). The mean inferior bone height was 25.39 mm, and the mean vertical nerve position ranged from 10.26 mm (MN1) to 12.30 mm (MN3).
Significance of cephalometric parameters for nerve localization
Table 3 presents the results of the applied t-test to analyze the correlations between the cephalometric measurements of the LC/OPT images and the buccal or lingual IAN position according to the CBCT scans. The cephalometric NSBa angle (LC) and the mandibular angle (Mand.Ang.; OPT) correlated significantly with each other (p = 0.039 and p = 0.010, respectively). Thus, a decreased NSBa angle or an increased mandibular angle predicted a lingual IAN position, while an increased NSBa angle and a decreased mandibular angle indicated a buccal nerve position. The inclination of the mandibular angle (GnGoAr) and the extent of the Hasund score measured in the LC confirmed this relationship with a strongly significant expression (p = 0.003; p = 0.006). Moreover, a lingual nerve position was also correlated with an increased jaw angle and a reduced Hasund score, whereas a reduced jaw angle and an increased Hasund score resulted in a buccal nerve position.
Relevance of jaw angle and bite relation for nerve localization
Table 4 summarizes the significant results of the Pearson correlation analysis between the LC and CBCT measurements. All measurement points of the vertical nerve position in the CBCT scans were significantly negatively correlated with the sagittal mandibular position according to the LC (SNB). In retral mandibular positions (decreased SNB), increased distances between the nerve position and the occlusal cortical bone could be determined. Accordingly, the nerve position was caudal in retral located mandibles and cranial in anterior located mandibles. The negative correlation was significant for the measurement points MN1 and MN3 (p = 0.040 and p = 0.012, respectively) and strongly significant for measurement point MN2 (p = 0.006). The correlation analysis of the skeletal configuration confirmed the previously described relationship according to which increased deviations of the mandibular bases in the sagittal plane (ANB) were associated with an increase of the measurement points MN1 to MN3 (Figure 6). The positive correlation was strongly significant for measurement points MN1 and MN2 (p = 0.003; p = 0.001) and significant for point MN3 (p = 0.044). Measuring point MN3 was negatively correlated with the jaw angle (p = 0.004) and positively correlated with the Hasund score (p = 0.002). A decreased (increased) jaw angle and an increased (decreased) Hasund score thus allowed caudal nerve position to be detected and vice versa.
Relationship between mandibular base and nerve localization
The RT7 value (OPT) was positively correlated with a vertical IAN position in the CBCT scans (MN1, MN2, and MN3: p = 0.002, p = 0.001, and p = 0.013, respectively) (Table 5, Figure 7). Thus, an increased RT7 distance suggested an increase in MN1–MN3 readings on CBCT, suggesting a more caudal nerve localization. This observation was supported by the negative correlation of the mandibular angle (Mand.Ang.) and the most posterior measurement point MN3 (CBCT). With reduced jaw angles, significantly increased distances, and thus more caudal nerve positions were found (p = 0.013).
Relevance of mandibular structure for nerve localization
The influence of all parameters on the observed correlations of the nerve position was tested using logistic regression analysis with a backward removal algorithm (Table 6). For a buccal nerve position, the analysis revealed an odds ratio of 1.186 for the Hasund score. Thus, the relative chance of the IAN being positioned in the buccal half of the mandible at the position examined increased by 18.6% when the Hasund score increased by 1 (p = 0.020).
Correlation between LC and OPT measurements
A strong positive correlation between the jaw angles measured in LC and OPT could be observed (r = 0.89; p = < 0.001; Table 4, Figure 8).