Effects of infraglottal implantation on voice production in type I thyroplasty: a computational study

Type I thyroplasty (TT1) is one of the primary phonosurgical interventions for unilateral vocal fold paralysis. This study investigated the effects of infraglottal implant placement in TT1 on voice production using numerical simulations. The laryngeal and implant geometries were reconstructed from computed tomography scan of a canine larynx with a hand-carved Silastic implant. Fluid-structure interaction simulations, coupling the continuum finite element vocal fold and implant models with a one-dimensional Bernoulli-based glottal flow model, were conducted to evaluate vocal fold vibration across a range of implant stiffnesses, insertion depths, and vertical positions. Effects of implant parameters on pre-phonatory shape and stiffness, glottal flow and vocal fold dynamics and acoustic outcome have been analyzed. The results revealed that infraglottal implants can enhance both sound pressure level (SPL) and vocal efficiency (VE) when the insertion depth exceeds a certain threshold. The observed enhancement in SPL and VE was attributed to an increased vertical stiffness gradient in the vocal folds induced by the inferior implant, which leads to greater force asymmetry between vocal folds opening and closing, thereby promoting more effective energy transfer from the glottal airflow to the vocal folds. The result is larger vocal fold vibrations, which leads to a larger SPL and may also contribute to the improved VE. The current simulation is based on the simulation of one single canine larynx. Future studies incorporating more subjects would help to generalize the current findings.