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Fig. 6.3 Thyromental distance.

3rd Step: Assess Thyromental Distance and Mandibular Protrusion

The third step of this test is to assess the characteristics of the patient’s mandible, which involves evaluating the thyromental distance (TMD) and the patient’s ability to protrude the mandible.

The TMD is measured with the neck (atlantooccipital junction) in full extension. In adults, the distance from the lower border of the chin (mentum) to the thyroid cartilage notch should be ≥ 3 finger breadths (or > 6.5 cm). Distances of < 6.5 cm may be associated with an anterior larynx or a small mandible. A TMD < 6.5 cm would predict difficulty in exposing the larynx with a classic Macintosh laryngoscope.

Mandibular protrusion is evaluated by asking patients to bring their lower jaw as far forward as possible or to try to bite their upper lip with their bottom teeth. With Class I, the patient is able to protrude the lower incisors anterior to the upper incisors. With Class II, the patient’s lower incisors can just reach the margin of the upper incisors. In Class III, the lower incisors cannot protrude to the upper incisors. Class II and III are associated with increased risk of difficult direct laryngoscopy.