# Cricothyroid Approximation Surgery (Type IV Thyroplasty): Insights From Failure Cases and an Analysis of Contributing Factors Using Three-Dimensional Computed Tomography

## TL;DR

This paper discusses two cases where cricothyroid approximation surgery failed due to excessive shortening of the cricothyroid distance, analyzed using 3D CT.

## Contribution

The study provides insights into the anatomical and physiological factors contributing to the failure of Type IV thyroplasty.

## Key findings

- Excessive shortening of the cricothyroid distance can prevent vocal fold elongation during TP IV.
- Monitoring vocal function under local anesthesia improves surgical outcomes.
- Avoiding excessive midline traction is essential to prevent complications.

## Abstract

Type IV thyroplasty (TP IV) is commonly regarded as a simple surgical procedure to raise the pitch of the speaking fundamental frequency (SFF) by lengthening the vocal folds through the approximation of the thyroid and cricoid cartilages. However, we report two cases in which TP IV was unsuccessful due to excessive shortening of the cricothyroid distance as analyzed by three-dimensional computed tomography (3D CT). One case was performed under general anesthesia and the other under local anesthesia. In normal high-pitched phonation, the rotational movement of the thyroid and cricoid cartilages and the anterior gliding movement of the inferior horn of the thyroid cartilage play essential roles in vocal fold elongation. The anterior gliding movement shifts the inferior horn, which serves as the fulcrum of rotational motion, forward to effectively elongate the vocal folds. However, when the cricothyroid distance is excessively short, the fulcrum shifts backward, preventing adequate elongation of the vocal folds. Understanding the physiological movements involved in normal high-pitched phonation is crucial for the success of TP IV. Performing the procedure while monitoring the voice under local anesthesia allows for better assessment of vocal function, with particular attention to the movement of the oblique part of the cricothyroid muscle. Additionally, excessive traction at the midline should be avoided to prevent complications.