Mechanical Effect of Vocal Fold Nodules on Phonation: Physical and Computational Modeling Approach

BACKGROUND: Vocal fold (VF) nodules are bilaterally symmetrical thickenings that grow in the middle part of the VFs and disrupt VF oscillations. Yet the physical mechanisms underlying their impact on phonation remain poorly understood. METHODS: We used an magnetic resonance imaging (MRI)-based physical model of the VFs to study how the size and stiffness of VF nodules affect phonation. The MRI model was based on geometry scanned from a real human larynx, providing a realistic representation of human vocalization. Left and right VFs were fabricated independently to account for their slightly asymmetrical movements. The VF nodules were located on the midpoints of the left and right membranous folds. To elucidate experimental results on flow-induced oscillations of the MRI model, a modified two-mass model was simulated by attaching VF nodules to lower masses of the VFs. RESULTS: The presence of VF nodules increased phonation threshold pressures, with larger nodules producing progressively higher pressures. Nodule stiffness, on the other hand, did not substantially alter phonation threshold pressures or other measured quantities. High-speed imaging combined with laryngotopography revealed that nodules locally suppressed vibratory motion of the VFs, with nonoscillating regions expanding as the nodule size increased. The modified two-mass model demonstrated that the elevated phonation threshold pressures arose from glottal aerodynamics altered by nodule geometry, rather than from collision dynamics of nodules. CONCLUSIONS: VF nodule size plays a more critical role than stiffness in disrupting phonation, primarily through its effect on glottal aerodynamics. Future studies should measure subglottal air pressures and average glottal airflows in patients with VF nodules to examine how nodule size and location influence these quantities.