The Role of Biomechanics in the Idiopathic Onset of Unilateral Vocal Fold Paralysis

Abstract

The vocal folds are important for protection of the airway during swallowing, the regulation of breathing and for voice production. Unilateral vocal fold paralysis (UVP) is caused by damage to the recurrent laryngeal nerve (RLN). Although surgery is most often linked to onset of UVP, the cause remains unknown in 12-42% of those with this disorder [1, 2]. At the level of the aortic arch the RLN branches from the vagus nerve and courses around the arch to ascend back toward the larynx. I hypothesize that an aneurysm of the aorta or alternatively changes in aortic arch compliance could impose increased stress and strain on the RLN where it is adjacent to the aorta resulting in impaired nerve function. The purpose of this research is to develop a computational model based on the biomechanical properties of the left RLN. This model is important for formulating predictions of the typical ranges of stress and strain responses of RLN tissue to forces imposed by surrounding structures (aortic arch). These predictions may be important for future investigations using an animal model to determine the amount of stretch necessary to cause onset of UVP. The first aim of this work was to identify differences in the biomechanical properties in the RLN of piglets between its location within the neck and the portion of the left RLN within the thorax, including the aortic arch region. The distal right RLN segment showed higher maximum tangential modulus (MTM) than the left. With the left nerve the proximal segment (aortic arch region) exhibited higher values of MTM and the stiffness parameter β than the distal segment. This increased stiffness of the proximal region may be in response to the pulsatile forces near the region of the aortic arch. The second aim of this work was to identify difference in the biomechanical properties in adolescent and piglet RLN specimens, between age and between the proximal and distal segments. Additionally the collagen structure of the RLN was imaged with two-photon microscopy to compare the microstructure with the biomechanical response of the RLN tissue. The tangential modulus (TM) and full width half maximum of the collagen fiber distribution (FWHM) was larger in the proximal segments than the distal segments. The strain energy and stiffness parameter α were larger in the piglet than the adolescent pigs while the stiffness parameter β was larger in the adolescent pigs. The purpose of the third aim was to use the material constants from the second aim to create a parametric computational model of the left RLN and the aortic arch. Results indicated that the parameters with the greatest sensitivity to left RLN maximum principal stress and strain are the material properties of the aortic arch. The maximum value of strain found in the RLN region of interest was 16.1%, which may indicate that some combination of aortic arch and RLN properties can elicit damage in the RLN.