We thank Drs. Lee and Mashour for their interest in our article. We established that there is a link between mouth opening and craniocervical position in conscious volunteers. When the subjects were prevented from extending from the neutral position, we found that the upper  95% confidence limit of interdental distance was 37 mm, which is the same as the lower  limit of normal interdental distance in young people. 1 

The motions of the temporomandibular joint include depression, elevation, protrusion, retraction, and side-to-side grinding movements. 2From the airway management point of view, although almost infinite protrusion should be advantageous, we agree with Dr. Lee that excessive depression can be inimical to our interests.

Dr. Mashour mentions the Mallampati examination. It was the unexpected success of the Mallampati examination in patients with cervical spine disease 3that led us to perform our investigation. As far as we know, the positive predictive value of the Mallampati examination has only exceeded 50% (the same as tossing a coin) in our study 3and Dr. Mallampati’s original series. 4Unfortunately, the Mallampati examination has not proved to be a useful predictor of difficult direct laryngoscopy in any other patient population. 5 

Our findings may not be reproduced in anesthetized patients. Nevertheless, we suspect that one of the reasons that the sniffing position is popular for direct laryngoscopy 6is that it produces craniocervical extension and facilitates mouth opening.

Mouth opening is a complex phenomenon, and we have complicated its analysis because the craniocervical junction must now be included in the list of factors involved. We hope that some of those involved in airway management or oropharyngeal surgery will find it useful to know that active mouth opening can be facilitated by craniocervical extension and impeded by flexion.

Landtwing K: Evaluation of the normal range of mandibular opening in children and adolescents with special reference to age and stature. J Maxillofac Surg 1978; 6:157–62
Soames RW: Skeletal system, Gray’s Anatomy, 38th edition. Edited by Williams PL. Edinburgh, Churchill Livingstone, 1995, pp 425–736
Calder I, Calder J, Crockard HA: Difficult direct laryngoscopy and cervical spine disease. Anaesthesia 1995; 50:756–63
Mallampati SR, Gatt SP, Gugino LD, Desai SP, Waraksa B, Freiburger D, Liu PL: A clinical sign to predict difficult tracheal intubation: A prospective study. Can Anaesth Soc J 1985; 42:429–34
Yentis SM: Predicting difficult intubation: Worthwhile exercise or pointless ritual? Anaesthesia 2002; 57:105–9
Horton WA, Fahy L, Charters P: Defining a standard intubating position using “angle finder.” Br J Anaesth 1989; 62:6–12