Figure 5. Comparison of the head extension angle with the angle between the occiput and C4 (C0-C4 angle). The C0-C4 angle is the angle measured between the C0 and C4 reference lines on the radiograph. Data are included for arytenoid exposure and best view for both the Macintosh and Bullard laryngoscopes. Head extension angle correlated significantly with C0-C4 angle (r²0.7). Regression line: head extension angle 27 + 1.19*C0-C4 angle. Head extension angle also was correlated with the angles at C0-C1 and C1-C2 (data not shown).