Fig. 4. (  A ) and (  B ): The incisor-carina neutral length (Y axis) is plotted against the incisor-MSJ extension length (X axis) for all 100 adults and 50 children, respectively (○= male; •= female). The formula of the regression line is Y = 0.868 × X + 4.260 (solid line), and Y = 1.009 × X + 0.468 (solid line), respectively. The coefficient of determination (r²) is 0.88 and 0.98, respectively (  P < 0.001). (  C ) and (  D ): The difference between the incisor-carina neutral length and the incisor-MSJ extension length (Y axis) is plotted against the eye-ear lines’ angle (X axis) for all 100 adults and 50 children, respectively (○= male; •= female). The incisor-carina neutral length is the length from the upper incisor to the carina after intubation with a flexible fiberoptic bronchoscope through the endotracheal tube in neutral position. The incisor-MSJ extension length is the straight length from upper incisor to the manubriosternal joint (MSJ) in extended position.