This study confirms that evaluating whether the C-TMD can accommodate the width of one finger can relatively effectively predict difficult laryngoscopy, and it is better than other indicators, as indicated by Spearman’s correlation, the agreement test, the AUC value, and OR value.
The differences in the Mallampati classification, ULBT, IID and C-TMD were significant between the difficult laryngoscopy group and the nondifficult laryngoscopy group, which shows that they are all indicators predicting difficult laryngoscopy. The difference in age between the two groups was also significant. The most common age range of patients in the difficult laryngoscopy group was 44–70 years. This result is consistent with the recent study of Schnittker R et al. [10].
The airway is evaluated before anesthesia mainly by two methods: accurate measurement and finger width estimation. Finger-width estimation is more commonly used because of its simplicity and speed, and it is more advantageous in large-scale top-tiered hospitals with a high surgery volume and fast pace. Yao et al. [4] used ultrasound to measure the distance moved by the condyle before and after the opening of the mouth to evaluate the degree of condyle mobility and applied it to the prediction of difficult laryngoscopy. The resulting AUC value of the ROC curve was 0.934, which is higher than those of the accurate measurement methods IID, ULBT grading, and Mallampati classification. The current method used the tragus as a reference line and used the width of the finger to estimate the maximum distance between the condyle and the tragus. This method can avoid the constraints of objective conditions, such as the availability of ultrasound, and is more convenient. Its AUC value was 0.699, which was higher than the AUCS from the finger-width estimation of IID, ULBT grading, and Mallampati classification. These results are basically consistent with those of Yao et al.
C-TMD had the highest correlation and agreement with the laryngoscope classification. This may have been because C-TMD can directly reflect the degree of TMJ movement. Reliability testing results showed that C-TMD was highly correlated with the maximum movement distance of the condyle measured by ultrasound. The maximum movement distance of the condyle measured by ultrasound can directly reflect TMJ mobility, meaning C-TMD can directly reflect the degree of TMJ mobility as well. The process of laryngoscopic exposure is actually the process of mandibular opening and forward movement, in which the condyle is the pivot point of the entire movement [11]. The wider the range of motion of the condyle, the greater the potential for mandibular movement. Sójka A et al. [12] also showed that the degree of TMJ mobility was closely correlated with the range of motion of the condyle at the maximum mouth opening. Taking the tragus as a reference, C-TMD can reflect the maximum mobility of the condyles. Therefore, C-TMD < 1 finger width may be an independent risk factor for difficult laryngoscopy.
The results of this study show that the specificity of this predictive index of C-TMD < 1 finger width was 0.929, and the positive predictive value was 0.676, higher than those of other indices, indicating that the misdiagnosis rate and missed diagnosis rate of this index are lower than those of other related indicators. In predicting difficult airways, the indicator of IID < 3 finger width, which is used most frequently in our clinical practice, only had a positive predictive value of 0.467, in line with the findings of China AK et al. [13]. These data further support the advantage of C-TMD < 1 finger width in predicting difficult laryngoscopy.
The upper lip bite test has always been considered a useful indicator for predicting difficult laryngoscopy. One advantage is its simple operation, as high ULBT classes can be easily detected by clinicians. However, its misdiagnosis rate is high, and many patients who do not have difficult laryngoscopy are misdiagnosed because of higher ULBT classes [14]. Our results are consistent with this observation and showed that the sensitivity of high ULBT class was 0.796, while its positive predictive value was only 0.295. We believe that the reason why many patients can not bite the upper lip above the vermilion border with the lower incisors could be due to thick lips rather than reduced TMJ mobility. Whether the factor of lip thickness is one of the reasons for the low positive predictive value of ULBT classification needs to be further investigated.
At present, COVID-19 has broken out all over the world, the situation is still very grim, and we can not lower our guard. While adopting protection for ourselves and our patients, we can improve the traditional diagnosis and treatment methods to reduce the cross-infection rate between medical staff and patients [15]. For patients undergoing elective surgery in the new environment, preoperative airway assessment is essential, but it also carries high risk. It is meaningful to find a way to balance the effectiveness and safety of airway assessment. The assessment of C-TMD can be completed even while the patient wears personal protective equipment such as a mask, and it has high predictive value. During this pandemic, it can be used as a safe method to assess airways instead of IID, Mallampati classification, and ULBT classification.
This study still has some limitations. From the perspective of methodology, the method of evaluating the condyle mobility depends on the estimation using finger width. Although it has higher predictive value than other related indicators, whether it will have lower predictive value than accurate measurement still needs to be confirmed. The sample size of this study was small. In clinical practice, limited TMJ mobility in some patients is due to fractures and inflammation. For this type of patient, we cannot tell whether they are unable to open their mouths because of pain. To eliminate such interference, this study excluded this type of patient. In some patients, the condyles can not be accurately identified by touching due to obesity, which may affect the test results.
The width of the index finger of a normal adult is approximately 1.2 cm. Whether this means that C-TMD less than 1.2 cm is a high-risk factor for difficult laryngoscopic exposure still needs to be explored through visualization techniques such as ultrasound. In future studies, we will use ultrasound to locate the condyle to measure C-TMD, in order to calculate the error rate of finger positioning, eliminate the impact of individual finger-width differences on the prediction results, and compare the advantages and disadvantages of ultrasound positioning and finger positioning.