Difficult laryngoscopy--the "anterior" larynx and the atlanto-occipital gap.

The atlanto-occipital distance is the major factor which limits extension of the head on the neck. It varies widely in the population at large. When the posterior tubercle of the atlas is already in contact with the occiput in the neutral position, attempts to extend the head result in anterior bowing of the cervical spine, and forward displacement of the larynx. This may be a cause of difficult laryngoscopy.