Control of vocal fold cover stiffness by laryngeal muscles: a preliminary study.

OBJECTIVES To perform preliminary measurements of the shear modulus of the vocal fold cover layer during intrinsic laryngeal muscle contraction. STUDY DESIGN Shear modulus was measured in an in vivo canine larynx and an ex vivo human larynx. METHODS Shear stress was applied to the transverse axis of the vocal fold using a modified linear skin rheometer (LSR) via an attached suction probe. The probe displacement in response to the applied force was measured at various levels of laryngeal muscle contraction. The force-displacement data were used to derive the shear modulus using a simple shear model. In the ex vivo human larynx, lateral cricoarytenoid (LCA) muscle and cricothyroid (CT) muscle activity was simulated with arytenoid adduction and cricothyroid approximation sutures, respectively. In the in vivo canine, adductor muscle and CT muscle contraction was induced with graded stimulation of the recurrent laryngeal nerve (RLN) and the superior laryngeal nerves (SLN), respectively. RESULTS : Baseline shear modulus was between 1,076 and 1,307 Pascals. In the ex vivo human larynx, the shear modulus increased gradually to a maximum of 1.6 times baseline value with graded arytenoid adduction and 3.7 times baseline value with cricothyroid approximation. In the in vivo larynx, the shear modulus increased to a maximum of 1.6 times baseline value with RLN stimulation and 2.5 times baseline value with SLN stimulation. CONCLUSIONS Findings are in agreement with the cover-body model in that cricothyroid muscle activity generates a greater change in cover stiffness than laryngeal adductors. The role of the individual laryngeal adductors (thyroarytenoid [TA] vs. LCA) in control of vocal fold cover stiffness remains to be further elucidated.