This study was approved by the Ethics Committee of Anhui Provincial Hospital, the First Affiliated Hospital of the University of Science and Technology of China, under ethics approval number 2019KY No. 108. It was approved by the China Clinical Trial Registration Center under registration number ChiCTR1900026775. All trial participants were informed about the entire trial process and signed informed consent.
In this trial, we enrolled patients who underwent elective surgeries with endotracheal intubation under general anesthesia and were of ASA status I-III and 18-90 years of age. We excluded patients with no teeth, with maxillofacial injuries, inability to cooperate, a thyromental distance less than three fingers wide, or limited head and neck movement (less than 80 degrees).
For all the patients enrolled in this study, during the preoperative examination one day before the operation or after the patient entered the preparation room on the day of the operation, an anesthesiologist who was skilled in the operative procedures used in this study examined whether the C-TMD could accommodate one finger width. The specific measurement procedure was as follows: the patient sat upright, and the examiner used the index fingers of both hands to locate the mandibular condyle of the mandible, instructed the patient to open the mouth as wide as possible, and felt that the condyle moved with the mouth opening movement. When the mouth opened as far as possible, the examiner then evaluated whether the distance between the condyle and the tragus could accommodate the width of one finger. The above measurement was repeated three times, and the maximum distance between the condyle and the tragus was taken（See Figure 1 for details）.
Later, another anesthesiologist, who was not aware of the evaluation results of C-TMD, measured other relevant indicators for airway evaluation. These indicators all indirectly reflected the degree of TMJ mobility:
Mallampati classification: The patient sat upright, opened the mouth wide, and extended the tongue to the maximum (no sound is made). The patient was then scored according to the pharyngeal structure that could be observed. Mallampati class > 2 was considered to be a predictive risk factor for difficult airways.
Interincisor distance (IID): The patient sat and opened the mouth as wide as possible, and then the doctor estimated IID with fingers. IID less than the width of three fingers was a predictive risk factor for difficult airways.
Upper lip bite test (ULBT) classification: The patient sat with the chin extending forward. The patient was asked to try his/her best to bite the upper lip with the lower incisors. According to the ability of the lower incisors to bite the upper lip, the test result was divided into three classes: Class 1: the lower incisors completely bit the upper lip above the vermilion border and completely covered the upper lip membrane; class 2: the lower incisors only bit half of the upper lip membrane and failed to reach the vermilion border; class 3: the lower incisor could not bite the upper lip. Classes 2 and 3 were the predictive risk factors for difficult airways.
All patients underwent routine electrocardiographic monitoring and induction of general anesthesia that started after the venous access was opened. The induction protocol utilized the following standardized recipe: midazolam 0.05 mg/kg, sufentanil 0.6 µg/kg, rocuronium 0.6 mg/kg and etomidate 0.3 mg/kg. An anesthesiologist with more than 3 years of experience, who was not aware of any preoperative airway evaluation results, conducted tracheal intubation with laryngoscopic exposure 3 min after bolus injection of rocuronium. According to the specific situation, either No. 3 or No. 4 laryngoscopy blades were used, and all patients took the head-up sniffing position. After intubation, the grading of all patients' laryngoscopic exposure, the number of intubation attempts and the time of intubation were recorded. The time of intubation defined when the laryngoscope blade tip passed the incisors until confirmation of the first wave of CO2 of the capnometer. The Cormack-Lehane classification was used to grade laryngoscopic exposure, and observations of the structure of the larynx and the glottis were divided into four classes. Class 1: the glottis structure was fully exposed, and the front and back joint structure could be seen; class 2: the glottis was partially revealed, and the rear glottal joint structure could be seen; class 3: only the epiglottis was seen; class 4: neither the glottis nor epiglottis was visible, Classes >2 were defined as difficult laryngoscopy. In our institution, no more than 3 intubation attempts via the application of conventional laryngoscope blades were permitted to ensure patient safety, and the operating time for each attempt was no longer than 1 minute. Before next intubation attempts, mask ventilation was used to ensure that the Spo2 was 98% or higher. If difficult airway appeared in the process, we followed the difficult airway treatment guidelines for the treatment and we also prepared conventional treatment tools such as a fiberoptic bronchoscope, laryngeal mask, and video laryngoscope.
To verify whether C-TMD accommodating the width of one finger can directly reflect the TMJ mobility and whether the method can accurately evaluate the condition when the patient wore protective equipment such as masks, we added two sets of reliability tests. We recruited 20 volunteers. All volunteers wore masks, and an anesthesiologist skilled in the experimental operation method evaluated whether the C-TMD of the volunteers could accommodate the width of a finger（See Figure 2 for details). After the evaluation was completed, all volunteers took off their masks, and then another anesthesiologist skilled in the operation of this experiment, who was not aware of the previous measurement results, assessed whether the C-TMD of the volunteers could accommodate the width of a finger. The difference between the two evaluation results was compared. In addition, an anesthesiologist used ultrasound to measure the maximum condylar movement distance of all volunteers, that is, the degree of condylar mobility. We then analyzed the correlation between C-TMD and the degree of condyle mobility measured by ultrasound.
Sample size calculation was performed based on a pilot study. In this study, the incidence of C-TMD ＜1 finger was 39%, using α = 0.05 and β = 0.1 and we found that a minimum of 176 participants were required in order to detect at least a 4 s difference in means in intubation times.
The SPSS 19.0 and MedCalc 19.2.0 statistical software packages were used. Measurement data were expressed as mean ± standard deviation (χ̅ ± s), and ranked or categorical variables were expressed as frequency/ratio (n/%). For univariate comparison, the independent-sample t test, rank sum test, and chi-squared test were selected, according to specific circumstances. Spearman correlation analysis was used to analyze the correlation of variables, and the results of each predictor and laryngoscopic exposure were compared with the paired chi-squared test and internal agreement tests and kappa values were calculated. The receiver operating characteristic curve (ROC curve) was used to analyze the predictive value of each observed parameter to predict difficult laryngoscopy, expressed as the area under the curve (AUC) with its 95% confidence interval (95% CI), and the odds ratio (OR), specificity, and sensitivity of each index for predicting difficult laryngoscopy were calculated. A P<0.05 indicated statistical significance.