RESEARCH ARTICLE Open Access Mechanical strain to maxillary incisors during direct laryngoscopy Milo Engoren 1,2* , Lauryn R. Rochlen 1 , Matthew V. Diehl 3 , Sarah S. Sherman 3 , Elizabeth Jewell 1 , Mary Golinski 4 , Paul Begeman 3 and John M. Cavanaugh 3 Abstract Background: While most Direct laryngoscopy leads to dental injury in 25–39% of cases. Dental injury occurs when the forces and impacts applied to the teeth exceed the ability of the structures to dissipate energy and stress. The purpose of this study was to measure strain, (which is the change produced in the length of the tooth by a force applied to the tooth) strain rate, and strain-time integral to the maxillary incisors and determine if they varied by experience, type of blade, or use of an alcohol protective pad (APP). Methods: A mannequin head designed to teach and test intubation was instrumented with eight single axis strain gauges placed on the four maxillary incisors: four on the facial or front surface of the incisors and four on the lingual or back, near the insertion of the incisor in the gums to measure bending strain as well as compression. Anesthesiology faculty, residents, and certified registered nurse anesthetists intubated with Macintosh and Miller blades with and without APP. Using strain-time curves, the maximum strain, strain rate, and strain time integral were calculated. Results: Across the 92 subjects, strain varied 8–12 fold between the 25th and 75th percentiles for all four techniques, but little by experience, while strain rate and strain integral varied 6–13 fold and 15–26 fold, respectively, for the same percentiles. Intubators who had high strain values with one blade tended to have high strains with the other blade with and without the APP (all pairwise correlation rho = 0.42–0.63). Conclusions: Strain varies widely by intubator and that the use of the APP reduces strain rate which may decrease the risk of or the severity of dental injury. Keywords: Laryngoscopy, Dental injury, Strain, Intubation Background The ability to quickly, smoothly, and safely intubate the trachea is a key skill for an anesthesia provider to possess. Ideally, when using a rigid laryngoscope, the intubator gets a clear view of the larynx and inserts the endotracheal tube while avoiding contact with the maxillary teeth. However, previous studies have shown that contact is common and that the incidence of dental injury is high: 25–39% [1–4]. Injuries include avulsion and dislocation, in which the tooth is removed or loosened at the root, fractures through the dentin and pulp, and, most com- monly, fractures solely to the enamel. Fractures range from visible loss of a section of the tooth to microscopic. While many of these injuries do not get repaired and may be unrecognized by the intubator, the average cost of den- tal repair is $2000 [5]. Dental injury occurs when the forces and impacts applied to the teeth result in strains that exceed the ability of the structures to dissipate energy and stress. The rate at which the force is applied (dF/dt) directly affects strain rate of the loaded material. Higher strain rates to biological materials result in a stiffer or more brittle response, with tissue failure occurring at lower strains compared to loading at lower rates. Forces can be applied axially as a compressive load across the whole tooth or axially localized to a portion of the tooth. Forces localized to a portion of the tooth result in com- pression under the force with tension to the structures im- mediately surrounding the compression. Forces can also be applied transversely to the tooth, producing a torque * Correspondence: engorenm@med.umich.edu 1 Department of Anesthesiology, University of Michigan, Ann Arbor, MI, USA 2 Department of Anesthesiology, Mercy St. Vincent Medical Center, Toledo, OH, USA Full list of author information is available at the end of the article © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Engoren et al. BMC Anesthesiology (2017) 17:151 DOI 10.1186/s12871-017-0442-z