In Response

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Excerpt

We read the letter by Liang et al1 with intense interest for which, in addition to their insightful comments, we thank them. The anatomical and physiological mechanisms underlying difficult laryngoscopy remain poorly understood. Mandibular condylar mobility, mouth opening, and angle of mouth opening may be influenced by one another2 and may have influence on laryngoscopy.3,4 However, the most dominant factor among the 3 remains undetermined. As Liang et al have deduced, with the same angle of mouth opening, the larger condylar translation results in a smaller mouth opening. However, whether the angles are equal during this process has yet to be ascertained. The occurrence and degree of extra condylar translation during laryngoscopy remain unknown. In addition, whether condylar translations remain the same when the mouth is maximally opened, or lower jaw protruded (without mouth opening), or during laryngoscopy has yet to be determined. We conducted an observational study to address these questions.
The mouth opening and the angles of mouth opening of 10 volunteers were measured under the following conditions: (i) with the lower jaw actively withdrawn and the mouth opened; and (ii) with the mouth maximally opened (Figure). Condylar translations were measured when the mouth was in a maximally opened position and when the lower jaw was protruded (without the mouth opening). We also observed 20 patients and measured their mouth opening when their mouths were opened voluntarily to the maximum and when they were undergoing laryngoscopy. We asked the volunteers to protrude their lower jaw under the same degree of mouth opening as in laryngoscopy (a thick tube was kept between incisors, and the thickness was equal to the average distance of mouth opening of the patients). We observed the maximum condylar translations of the volunteers under this condition. The data were tested by 1-way analysis of variance and least significant difference testing (Table).
Interestingly, we observed a larger angle when the mouth was opened maximally. The condylar translation distances varied when the mouth was opened under different conditions. The condylar translation distances were slightly different when the mouths of the volunteers were in a maximally opened position and under a simulated mouth opening (27 mm) as during laryngoscopy. The condylar translation distances of the volunteers were limited when the lower jaw was protruded without opening the mouth.
The present data revealed that mandibular condylar mobility, mouth opening, and angle of mouth opening can be influenced by one another. Without condylar translation, the largest angle of mouth opening cannot be obtained. Likewise, without mouth opening, the largest condylar translation distance cannot be obtained. Data from the previous study2 indicate a significant positive correlation (r = 0.41, 99% confidence interval, 0.31–0.50) between the mouth opening and the condylar translation distance, even though the correlation was not high. This finding can be used to support the aforementioned statement. This result may partly explain the concern raised by Liang et al, that is, with anterior or inferior condylar translation, the angle of mouth opening can increase. The present data also reveal that the condylar translation distance under simulated mouth opening as during laryngoscopy is similar to that under the condition of a maximally opened mouth. However, the question remains whether the mandibular condyle can achieve further translation when patients are undergoing laryngoscopy and are paralyzed. Unfortunately, we did not accurately measure the condylar translation distance under the laryngoscopy condition. The measurement results were inconclusive because probe stability during laryngoscopy was difficult to maintain. To the best of our knowledge, further condylar translation is not likely to occur when patients are undergoing laryngoscopy and paralyzed, except for the patients suffering from temporomandibular joint pain.
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