To the Editor:—  I read with interest the article by Calder et al.  1showing that limitation of craniocervical extension to less than 36° limits interdental distance. They propose this as another possible mechanism for difficult airway management. However, the relation between inter-dental distance and difficult direct laryngoscopy is uncertain. Charters 2has shown in a mathematical model that extreme mouth opening actually inhibits direct laryngoscopy. Clinical experience suggests that extreme mouth opening is not practiced as part of laryngoscopy technique. Previous studies showing a correlation between interdental distance and difficult laryngoscopy have included edentulous patients in the measurements, 3,4or they have not specified whether edentulous patients were included. 5,6,7Not having maxillary teeth (which undoubtedly improves the line of vision to the glottis) is an entity different from interdental distance per se .

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