The overall outcome of IAN injury after IMTMs extraction
In this study, 21 cases of IAN injury with numbness of the lower lip occurred after IMTM extraction. The incidence of IAN injury was 3.90% (21/539). The IMTMs information was shown in Supplementary Table S1.
Comparison of the incidence of IAN injury among different groups of IMTMs
In terms of impaction depth, except that no IAN injury was found for IMTMs with complete eruption (0/25), the incidence of IAN injury for partial eruption, mucosal impaction, and osseous impaction, was 0.79% (1/127), 2.81% (5/178), and 7.18% (3/209), respectively.
In terms of root orientation, except that no IAN injury was found for the encircling group (0/15), the incidence of IAN injury for the buccal group, upper group, and lingual group was 7.62% (16/210), 1.92% (2/104), and 1.43% (3/210), respectively.
In terms of contact degree of root-IAC, the incidence of IAN injury for level I, II, III, and IV, was 1.18% (2/169), 3.08% (5/130), 4.11% (6/146), and 8.51% (8/94), respectively.
Taken together, there were significant differences in the incidence of IAN injury after IMTM extraction among different types of impaction depth (p = 0.006), root orientation(p = 0.008), and contact degree of root-IAC (p = 0.02). The results are shown in Table 1.
Correlation analysis of root orientation with impaction depth/root orientation
As demonstrated above, IMTMs with the root located on the buccal side of IAC were more prone to IAN injury. However, it was unclear whether the difference was attributed to the distribution unevenness of impaction depth and contact degree of root-IAC among different groups of root orientation. Therefore, the chi-square test of the contingency table was used to analyze the correlation between root orientation and impaction depth/contact degree of root-IAC, and the lambda coefficient was 0.276(p < 0.001) and 0.102(p = 0.003), respectively (the results are shown in Table 2). The proportion of level III and IV of contact degree in the buccal group was 70.1% (149/210), which was higher than that of 28.6% in the non-buccal group (94/329). Furthermore, the proportion of mucosal and osseous impaction in the buccal group was 80.5% (169/210), which was higher than that of 66.3% in the non-buccal group (218/329).
The correlation analysis revealed the necessity of reasonable statistical methodologies to balance the distribution of impaction depth, contact degree of root-IAC, and other confounding factors, among different groups of root orientation, for a better insight into the independent effect of root orientation on IAN injury.
PSM of the buccal and non-buccal type of root orientation
Age, sex, impaction depth, and contact degree of root-IAC were considered confounding variables. Binary logistic regression was used to model and calculate the propensity score of each case. Using nearest neighbor matching with no replacement and setting the caliper to 0.2, the buccal group and non-buccal group were matched at the ratio of 1:1. There were 21 cases with IAN injury and 518 cases without IAN injury before PSM and 9 cases with IAN injury and 257 cases without IAN injury after PSM. The case number of buccal/non-buccal was 210/329 before matching and 133/133 after matching. The standard mean difference (SMD) of age, sex, impaction depth, and contact degree of root-IAC, was respectively 0.066, 0.457, 0.357, and 1.245 before PSM, and was respectively 0.05, 0.051, 0.122, and 0.101 after PSM, as shown in Table 3. It was demonstrated that PSM significantly reduced the interference of confounding factors. In addition, the plot of the Kernel density of the PS score before and after PSM was shown in Fig. 3, indicating that group-to-group comparability was significantly improved after PSM.
Kernel density of the PS score before and after PSM
The effect of root orientation on IAN injury before and after PSM
Before PSM, the incidence of IAN injury was 7.62% in the buccal group and 1.52% in the non-buccal group, and the difference was statistically significant (p <.001). The odds ratio (OR) was 5.344 and the risk ratio (RR) was 5.013. After PSM, the incidence was 6.02% in the buccal group and 0.75% in the non-buccal group, and the difference was statistically significant (p=.04), as shown in Table 4. It is evident that after balancing confounding factors, the risk of IAN injury of IMTMs whose root was located on the buccal side of IAN was significantly higher than that of non-buccal counterparts. The odds ratio (OR) was 8.448 and the risk ratio (RR) was 8.
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