Anatomical Study of Two Cricothyroid Approaches to the Cadaver Larynx

The purpose of this study was to measure the angle and depth at which the vocal folds are accessed via transcutaneous injection through two different approaches through the cricothyroid membrane.

Twenty-five cadavers preserved with formalin were studied. The larynges were bisected through the thyroid notch, and the skin and soft tissue were removed. A 27-gauge needle was inserted through the cricothyroid membrane, once from just inferior to the thyroid cartilage and once from just superior to the cricoid cartilage, and advanced to the midpoint of the musculomembranous vocal fold (determined as the halfway point from anterior commissure to vocal process), immediately lateral to the vocalis muscle without penetrating the epithelium. The angle of approach was determined after taking multiple measurements with a Castroviejo caliper. The depth of the needle was measured.

Ten male and 15 female cadavers were studied. Their average age was 85.5 ± 12.4 years. The mean distance from midline for an injection performed with 0° of lateral angle was 5.7 ± 0.7 mm for the males, which was significantly greater than for females (4.8 ± 0.8 mm, P = 0.012). The insertion angle for the infrathyroid approach was 22.2 ± 6.9°, which was significantly different from the supracricoid angle, which was 33.0 ± 5.2° (P < 0.001). The mean insertion depth of the subthyroid approach was 11.3 ± 1.8 mm (range 8.0-15.5 mm), which was significantly less than the depth for the supracricoid approach (18.2 ± 2.4 mm, range 11.0-22.0 mm, P < 0.001.). The infrathyroid approach angle for males was 26.0 ± 8.5°, which was significantly greater than the angle for females (19.3 ± 3.4°, P = 0.016.). The supracricoid approach angle for males was 37.5 ± 2.4°, which was significantly greater than the angle for females (29.7 ± 4.1°, P < 0.001.). The insertion depth for males was 19.6 ± 1.4 mm, which was significantly greater than that for females (17.2 ± 2.6 mm, P = 0.021).

Comparison of a supracricoid and infrathyroid approach through the cricothyroid membrane to the middle third of the vocal folds demonstrated significant differences in angle of approach. We also identified a significant difference in the angle of entry between male and female larynges in both approaches, as well as an increased depth of insertion required in males for the supracricoid approach.