It has been reported that the incidence of DA is 6.1%-10.1%. [13, 14] As shown in Table 1, the incidence of difficult airways was slightly higher for MMT grade IV patients (26/182, 15.03%) and indirect laryngoscopy grade IV patients (21/182, 12.28%), whereas in Tables 2 and 3, no difference between MMT grade III and indirect laryngoscopy grade III patients was found in terms of DAs. If the patient has obvious anatomical or pathological abnormalities of the airway, it is necessary to determine whether DA exists before further surgery. Predictive studies are clinically significant when the structures are normal, but other factors may make glottis exposure difficult. To predict whether an airway is difficult, either a single factor model or a combination of two or more factors should be used.
MMT is widely used in DA diagnosis. In 1983, Khan et al.  proposed a method to classify DAs into three levels according to larynx structure. Nasir et al.  expanded this classification to four levels, which drew much interest among researchers, but its clinical practice has been questioned since its classification was introduced. The MMT is a comprehensive index that is mainly based on mouth opening width, tongue size and mobility, maxillary movement, other intraoral structures and cranial and cervical joint movements. The grading of glossopharyngeal structure is closely related to the ease of endotracheal intubation under direct laryngoscopy, so it has remained the primary assessment method for anesthesiologists. Adamus et al.  reported that the specificity and sensitivity of the MMT are not high, limiting its clinical use. In addition, some scholars believe that its accuracy is moderately low [18, 19]. Other reports, including meta-analyses, have shown divergent results. Lee et al.  suggested that the MMT should not be used as the sole assessment tool, a view shared by most scholars. When the MMT is used alone, there is no way to fully confirm the DA. Stud have shown that the combination of the MMT with hyperthyroidism or other airway assessments can improve the diagnostic accuracy up to 50–80% [10, 21]. El-Ganzouri et al.  analyzed 10507 cases, with MMT class III as the boundary value, and defined Cormack-Lehane grades 3 and 4 as tracheal intubation difficulties. The sensitivity of the MMT in predicting tracheal intubation difficulty was 44.7%, while its specificity was 89.0%, with a positive predictive value of 21% and a negative predictive value of 96.1% . From this perspective, the MMT has clinical value. Since the current study was a retrospective analysis in nature and did not obtain the number of specific difficult exposures during surgery, the relevant sensitivity, specificity and other indicators were not evaluated. However, our research did show a statistically significant difference between the two groups among class IV patients according to the MMT, indicating that although a class III or IV classification according to the MMT is defined as a DA, the score may not be sufficient in the single factor analysis. The results of this study suggest that the MMT is a more accurate predictor of DA among class IV patients.
The indirect laryngoscope was invented in 1854 by Spanish vocal teacher Manuel Garcia, who became the first person to observe his own throat structure. His design was subsequently improved by Professor Ludwig Turck, while the predecessor of modern laryngoscope was designed by Johann Nepomuk Gzermak in Poland in 1858. This technology has been rapidly utilized as one of the most common and simple inspection instruments in the otolaryngology field. In 1997, Yamamoto et al.  first described airway grading using indirect laryngoscopy as follows: grade I, the entire glottis is visible; grade II, the posterior commissure is visible; grade III, only the epiglottis is visible; and grade IV, no laryngeal structures are visible. Generally, grade III or above is indicative of DA. Sanchez-Morillo et al.  found that indirect laryngoscopy was an independent and reliable tool to diagnose DAs when combined with 70° rigid endoscopy and direct laryngoscopy. Additionally, Yamamoto et al.  found that the indirect laryngoscopy classification is more accurate than the direct laryngoscopy prediction, indicating that indirect laryngoscopy alone has certain clinical significance. Therefore, the use of indirect laryngoscopy alone was assessed in this study. When the indirect laryngoscopy result was grade IV, the two treatment groups showed statistically significant differences in both 5-year local control and disease-free survival rates. Additionally, the 5-year local control and disease-free survival rates based on indirect laryngoscopy were lower than those based on the MMT, indicating that indirect laryngoscopy grade IV could better diagnose DA and lead to higher diagnosis accuracy, which is consistent with the results from previous reports.
Combination of the MMT and indirect laryngoscopy to evaluate DAs
In clinical practice, the combination of multiple methods can diagnose DAs more accurately and reliably. For example, Merah et al.  combined the MMT, intercondylar distance, chest-to-twist distance, mandible length and upper and lower incisor distance to diagnose DAs. The joint diagnostic value of this combined method was the highest, but multiple shortcomings limited the application of this method. One of the shortcomings is that this combined approach is not easy to implement in practice. Therefore, our study was designed to elucidate DA patients with different exposure levels based on two assessment tools and to analyze the survival rate of patients with laryngeal cancer with AVC based on these tools. Our research found that patients with DAs (class III/IV) who underwent TLM had a lower local control rate than those who underwent OPL. Because this difference was statistically significant, combining the MMT with indirect laryngoscopy can reflect to some extent the exposure difficulties that affect prognosis. This study has clinical significance when the anatomy looks normal, but there may be difficulties in glottic exposure. After all, patients with MMT grade III or indirect laryngoscopy class III airways account for the majority of patients with laryngeal cancer with AVC. Our research showed that class III/IV and grade III/IV classifications are more meaningful when combined. In addition, these two methods are relatively simple to implement and should be used more often in clinical practice.
Strategy for difficult glottic exposure
Ohno et al.  reported that laryngoscopy and body posture are important for AVC exposure. Some scholars have found that assessing glottic exposure requires information on sex and thyroid mandibular angle (TMA). If the patient is female and the TMA is greater than 120°, the positive local control rate can reach as high as 94.6% [25, 26]. For patients who have early glottic laryngeal cancer with AVC involvement and undergo laser surgery, the AVC exposure field requirements are higher. In addition to adequate preoperative assessment, surgical techniques and surgeon experience also affect surgical outcomes . For example, in cases of poor glottic exposure, the smaller STORZ mirror can be replaced with a more rigid endoscope (30°, 45°, or 70°) to fully expose the lesion . If the target area is still insufficiently exposed, the thyroid cartilage can be pressed to shift the vocal cords before initial exposure or after the first 1/3 of the area is exposed. The goal is to remove the lesion under direct vision during surgery. If the MMT or indirect laryngoscopy is used alone, although there is a certain correlation between these classifications and DAs, surgeons should be aware of the possibility of false negative results and the inability to diagnose DAs or the possibility of unnecessary treatment when false positive results occur. It is important to balance the pros and cons and make optimal decisions for patients. For laryngeal cancer with AVC involvement, whether it is DA or another impeding factor, it is necessary to comprehensively evaluate all aspects. However, there is no indicator or method that can be used to identify appropriate exposure. Therefore, if there is still poor exposure after trying different approaches during surgery, the surgeon must be willing to adjust the treatment strategies in a timely manner.