Effects of Asymmetric Scarring in a Lumped-Mass Vocal System

Vocal fold (VF) scarring following phonosurgical intervention alters tissue stiffness, damping, and inter-mass coupling, but whether these changes destabilize phonation through elevated stiffness or through inter-fold mechanical asymmetry has not been determined. Distinguishing these two mechanisms is clinically important because they implicate different therapeutic targets.

A bilateral two-mass lumped VF model was used to simulate three conditions: a healthy symmetric baseline, a unilaterally scarred condition in which right-fold parameters (k, r, k_c, m) were modified to reflect fibrotic remodeling, and a bilaterally scarred condition in which both folds were symmetrically modified. The bilateral condition served as a key control, preserving elevated stiffness throughout while restoring inter-fold symmetry. For each condition, subglottal pressure (P_S) was swept from 0 to 35 cm H₂O. Vibratory dynamics were characterized using time-domain waveforms, bifurcation diagrams, and largest Lyapunov exponent (LLE) analysis, and collision and vibrational stresses were calculated across conditions.

The healthy and bilateral scar conditions exhibited stable periodic oscillation with LLE below the 0.01 chaos threshold across all P_S values. The unilateral scar condition produced chaotic aperiodic oscillation across P_S ≈ 20 to 34 cm H₂O, with LLE intermittently exceeding threshold and peaking at approximately 0.077 per ms. Within this chaotic range, the scarred fold exhibited collision stress elevations of approximately 4.1 kPa and vibrational stress elevations of approximately 57 kPa, a threefold increase over baseline, while the contralateral unscarred fold showed reduced rather than elevated vibrational stress. Bilateral scarring elevated vibrational stress approximately 2.5-fold but preserved proportional pressure-stress scaling without chaotic spikes.

Chaotic phonation concentrates mechanical loading on the scarred fold rather than distributing it to the contralateral fold, establishing a candidate mechanism for a self-perpetuating postsurgical injury cycle. The absence of chaos under bilateral symmetric scarring demonstrates that inter-fold mechanical asymmetry, rather than elevated stiffness alone, is the primary driver of chaotic phonation following VF scarring.