Study Protocol

Title: Anatomical Measurements of Cricoid Cartilage, Thyroid Cartilage, and Hyoid Bone Using CT Images from Radiology Department Archives in Adults for Designing Laryngeal Mask Airway

1. Introduction

The design and effectiveness of the laryngeal mask airway (LMA) can be significantly improved by incorporating precise anatomical measurements of the laryngeal structures. This study aims to analyze the dimensions and spatial relationships of the cricoid cartilage, thyroid cartilage, and hyoid bone using pre-existing CT images of adult subjects from a radiology department’s image repository to inform the design of a new LMA.

2. Objectives

2.1       Primary Objective: To obtain accurate measurements of the cricoid cartilage, thyroid cartilage, and hyoid bone in adults using archived CT imaging.

2.2       Secondary Objective: To utilize these measurements to inform and optimize the design and shape of a Laryngeal Mask Airway (LMA).

3. Methods

3.1 Study Design

A cross-sectional observational study using existing CT images from a radiology department’s image repository.

3.2 Study Population

3.2.1    Inclusion Criteria: Archived CT images of adults aged 18-65, without any known laryngeal disorders or surgical history affecting the laryngeal anatomy.

3.2.2    Exclusion Criteria: CT images where metal artifacts or other conditions impair the visualization of the laryngeal structures.

3.3 Sample Size

A total of 400 CT images of adult subjects will be selected, with equal distribution across gender and weight categories.

3.4 Image Source and Selection Criteria

3.4.1    Source: Image repository of the Radiology Department.

3.4.2    Selection Criteria: Images will be selected to ensure clear visualization of the cricoid cartilage, thyroid cartilage, and hyoid bone.

3.4.3    Weight Categories and Gender Distribution

Male: 200 images (50 - 70 kg, 71 - 90 kg, > 90 kg)

            Female: 200 images (40 - 60 kg, 61 - 80 kg, > 80 kg)

3.5 Rationale

The distribution ensures that there is an equal representation of both genders and a balanced distribution across the weight categories. This stratification will allow for the analysis of variations in anatomical measurements due to weight and gender, potentially leading to more tailored and optimized designs for the laryngeal mask airway.

3.6 Measurements 

Designing a laryngeal mask airway (LMA) that fits the anatomical contours and dimensions of the airway structures can greatly improve its effectiveness and comfort. The following measurements are crucial to inform the design of an optimized LMA. 

3D Anatomical Modelling: Three-Dimensional Reconstruction: Use CT data to create 3D models that can inform the overall contour and depth fitting of the mask airway.

3.7 Data Analysis

Software: Use image analysis software such as 3D Slicer for measuring the anatomical structures.

4. Ethical Considerations

4.1 Informed Consent: Not applicable as the study will use de-identified archived images.

4.2 Ethical Approval: The study will be reviewed and approved by the Institutional Ethical Committee of the affiliated institution, ensuring compliance with regulations regarding the use of archived medical images.

5. Literature Review

Lioa H et al, reported that air leakage is most likely to occur in the oropharynx, emphasizing the importance of proper sealing to prevent this complication. High mucosal pressure is most likely to occur in the hypopharynx, particularly where cricoid cartilage presses the mucosa, highlighting the risk of complications related to high pressure settings. Insertion difficulties are often caused by the narrow width of the hyoid bone super horn, indicating anatomical factors that can impact the successful placement of the laryngeal mask airway.

The study suggests that maintaining sufficient cuff pressure is essential to reduce the risk of air leakage and ensure a complete sealing pressure line in the contacting surface, thereby enhancing airway management. Recommendations are provided for LMA manufacturers to consider specific designs tailored to different patient groups to optimize sealing performance and minimize complications related to mucosal pressure and air leakage.

Berry et al, designed a study aimed to determine the optimal size of laryngeal mask airway (LMA) in normal adults through a randomised single-blinded trial involving 30 participants weighing less than 100 kg. The optimal size selection criteria prioritized the number of placement attempts, oropharyngeal leak pressure (OLP), fibreoptic score, and percentage of vocal cords seen.

Results showed that the size 5 LMA was optimal in 19 out of 30 participants, while size 4 was optimal in 11 out of 30, with no participants requiring size 3. Oropharyngeal leak pressure increased with larger sizes, being significantly higher for size 4 and size 5 compared to size 3. The study confirmed that selecting size 4 for females and size 5 for males was the most effective strategy, outperforming weight-based and other gender-based strategies.  External anatomical measurements did not significantly predict optimal size, height being the most useful variable. The study suggested potential new strategies like using size 5 for all adults or selecting size 5 for individuals over 165 cm and size 4 for those under 165 cm .