Seeking to unpack some of the anaesthetists' "knack" for intubation, this study examines the effect of various orientations of the tracheal tube on the anterior movement of the tube tip using a computerised 3D model of intubation. The model used sets of coordinates for the upper incisor tip, lower incisor tip and vallecula extracted from mean values reported in a study of 16 volunteers predicted to have easy laryngoscopy and 16 predicted to have difficult laryngoscopy during both gentle laryngoscopy and laryngoscopy under 50N of lifting force, yielding a total of four sets of airway geometry. Tube orientation was specified with the standard aviation terms pitch, roll and yaw. Observations were repeated across permutations of tube roll (0° to 45°) and yaw (0° to 15°) in all four geometric configurations. Across all four geometries, the most favourable tip location was observed with close to 15° of yaw and 0° roll with an anterior tip movement at the level of the glottis observed between 19.2 and 26.6mm. Unsurprisingly given the curved shapes of the objects involved, incremental movement of the tip was greatest at extreme values of roll and yaw. Both yaw and roll caused posterolateral movement of the maxillary teeth contact point. The posterior motion at the mouth enables the entire tube to pitch tip up. However, rolling the tube caused the tube tip to move posteriorly and the pivot point on the laryngoscope blade to move cephalad, nearly always negating what should be a favourable change in pitch allowed by the posterolateral maxillary dentition contact point. Our analysis suggests that avoiding tube roll while maximising yaw at the time of glottic entrance may be a previously unrecognised manoeuvre to improve tracheal intubation success rate having implications for intubation teaching and simulation. Understanding the importance of posterolateral movement of the tube at the oral cavity also may provide new insights into the cause of some difficult intubations.

The authors have declared no competing interest.

This study did not receive funding.

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

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The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

Source data was published in this paper which includes the statement: "The study was approved by our ethics committee (Graduate School of Medicine, Chiba University). The aim and potential risks associated with direct laryngoscopic procedure were fully explained, and written informed consent was obtained from each individual." Kitamura Y, Isono S, Suzuki N, Sato Y, Nishino T. Dynamic interaction of craniofacial structures during head positioning and direct laryngoscopy in anesthetized patients with and without difficult laryngoscopy. Anesthesiology. 2007 Dec;107(6):875-83. doi: 10.1097/01.anes.0000291439.52483.6a. PMID: 18043055. https://pubmed.ncbi.nlm.nih.gov/18043055/

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