Scatter plot of the approach angle on the axial plane (y-axis) plotted against the angle of the sphenoid ridge (x-axis) in individual cases. Open circles and x-marks are for the right and left sides, respectively. The Pearson correlation coefficient (r) was significant at the 0.001 level. The diagonal line indicate the exact parallelism between the sphenoid ridge and the approach angle. Two dotted lines indicate that the difference between these two angles are ±10°. This plot illustrates the close parallelism between two angles within individuals: the most of the appropriate approach angle (89.3%; 50/56) were oriented within ±10° along the direction of the sphenoid ridge.