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

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.