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.