The goal of this observational study is to evaluate thyromental distance (TMD) and mandibular triangle perpendicular length (MTPL); to know which correlates better with the incisor vallecula distance (IVD). Based on the direction and strength of the correlation measure, a multiple regression equation can then be created to first predict the IVD with external linear measure, and second, based on IVD length prediction, the appropriate laryngoscope blade size can be ascertained.

Preoperative evaluation of the upper airway is vital to the success, safety, and quality of airway access. The evaluation primarily aims to anticipate the course of direct laryngoscopy and intubation (DLI) of the trachea. While assessment of mouth opening, neck extension, and dentition forms the preliminary step of knowing whether DLI is feasible; there are certain other parameters whose evaluation facilitates understanding as to how the actual DLI will turn out. Among the secondary parameters, the linear thyromental distance (TMD) is an important measure that determines how the direct laryngoscopy aspect of conducting DLI will go - in terms of ease-of-laryngoscopy to gain the desired visual access to the view to the vocal cords. The ease of laryngoscopy involves as to how the laryngoscope blade would be able to displace tongue to one side, create forward space enhancement, and reach the vallecula at the base of the tongue; before lifting the epiglottis to view the cords. Though in use for over 4 decades, the TMD suffers from weakness, including, low sensitivity and specificity, not relating to the actual and direct teeth vallecular distance, and getting confounded by neck dimensions (thickness, length), state of dentition, and the tongue size.

The present study is built on the premise that it's the mandibular space to which the tongue is displaced at the time of DLI, the bony mandibular space is more exclusive of the confounders, and that the teeth (lower incisors) vallecular distance represent the actual distance that needs to be predicted.

The present study aims to evaluate TMD and mandibular triangle perpendicular length (MTPL); to know which correlates better with the incisor vallecula distance (IVD). Based on the direction and strength of the correlation measure, a multiple regression equation can then be created to first predict the IVD with external linear measure, and second, based on IVD length prediction, the appropriate laryngoscope blade size can be ascertained.

Inclusion Criteria:

1. Age 18-65 years
2. Males and females
3. ASA (American Society of Anaesthesiologists) physical status I and II
4. Patients scheduled for elective surgery with controlled ventilation through orotracheal tube conduit

Exclusion Criteria:

1. Smokers (those who have smoked ≥100 cigarettes in their lifetime), chewing tobacco dependence or substance abuse

2. Submucous fibrosis: progressive fibrosis of the submucosal tissue leading to rigidity, relative trismus, and progressive decrease of mouth opening

3. Anticipated difficult airway

4. Patients requiring intubation with MacIntosh blade #4 or #5

5. Dental issues:

   • missing tooth (upper/lower incisors, upper/lower premolar)

   • crowded dentition: - irregular, misaligned, or overlapping teeth within a relatively small oral cavity

   • presence of artificial teeth and/or dental implants

     • Mandible/mandibular structure related issues:

   • Retrognathia: - posterior displacement of the mandible relative to the maxilla or cranial base

   • Prognathia: - anterior displacement of the mandible relative to the maxilla or cranial base

6. Emergency Surgery

7. Those requiring urgent rapid-sequence intubation due to the presence of a full-stomach status secondary to gastro-esophageal reflux disease, diaphragmatic hernia, among others.

8. Refusal to informed consent

Patients undergoing elective surgery under general anaesthesia