Validating conventional 2D IMPA against 3D one: is it still a trustworthy angle? A retrospective CBCT-based...

This study tested whether conventional 2D incisor mandibular plane angle (IMPA) can be used interchangeably with a rigorously defined 3D one.

This single-centre retrospective study included 169 CBCT scans. Synthetic lateral cephalograms were generated from DICOM data to calculate the conventional 2D IMPA between the Go-Me mandibular plane and the lower central incisor long axis. The 3D IMPA used the axes of teeth 3.1 and 4.1 projected three-dimensionally to the orthogonal plane to GoL-GoR-Me and the higher angle was selected. Normality of paired differences was assessed using the Shapiro-Wilk test. Systematic bias was evaluated with a paired t-test. Association between 2D and 3D IMPA was assessed using Pearson's correlation and linear regression. Agreement was quantified by Bland-Altman analysis. Classification agreement within the 85-95° range was evaluated using Cohen's kappa and McNemar's test.

Mean 2D and 3D IMPA were 95.29±8.98° and 95.75±9.42°, respectively, with a mean paired 3D-2D difference of 0.46° (paired t-test, P=0.084). Correlation was strong (r=0.931), and linear regression yielded R²=0.867. Bland-Altman bias was 0.46° (95%LoA -6.29° to +7.21°). Classification agreement across the 85-95° range was 83.43% (κ=0.636; McNemar P=0.345).

2D and 3D IMPA showed negligible mean bias, excellent association, and substantial diagnostic concordance. Routine cases may rely on 2D IMPA without material loss of information, whereas in torque-critical, anatomically constrained, or borderline cases the observed ±7° dispersion argues for 3D assessment.