Geometrical deformation of vocal fold tissues induced by formalin fixation

[[abstract]]Objectives/Hypothesis Many existing studies of vocal fold geometry are based on anatomical measurements made on histologically fixed laryngeal tissues using formalin. However, the validity of these geometric data is questionable because of the potentially significant tissue deformation associated with formalin fixation, particularly tissue shrinkage. Previous experiments have shown that valid geometric data may be obtained on vocal fold tissue samples quickly frozen with liquid nitrogen. Based on this finding, the present study attempted to quantify the geometric deformation of formalin‐fixed vocal fold tissues with respect to quick‐frozen tissues. Methods Six freshly harvested canine larynges were quickly frozen with liquid nitrogen and sectioned at the mid‐membranous coronal plane. Each larynx was thawed and divided into halves along the midsagittal plane, one of which was not fixed; the other was fixed with formalin, from which histological tissue sections were also prepared. Measurements of vocal fold geometry were made on digital images of mid‐membranous coronal sections of the tissue samples, based on linear dimensions of vocal fold depth and thickness defined functionally for biomechanical applications. Results Significant shrinkage of the vocal fold (particularly the vocal fold body) and considerable distortion of the vocal fold contour (particularly at the free edge) were observed for the formalin‐fixed samples and the histological sections in comparison with the unfixed samples. Conclusions Results of the present study suggested that significant geometric artifacts are induced by conventional histological fixation of laryngeal tissues using formalin. These artifacts should be carefully considered for interpreting any vocal fold geometric data obtained through formalin fixation

Three-dimensional anatomic characterization of the canine laryngeal abductor and adductor musculature

[[abstract]]The biomechanics of vocal fold abduction and adduction during phonation, respiration, and airway protection are not completely understood. Specifically, the rotational and translational forces on the arytenoid cartilages that result from intrinsic laryngeal muscle contraction have not been fully described. Anatomic data on the lines of action and moment arms for the intrinsic laryngeal muscles are also lacking. This study was conducted to quantify the 3-dimensional orientations and the relative cross-sectional areas of the intrinsic abductor and adductor musculature of the canine larynx. Eight canine larynges were used to evaluate the 3 muscles primarily responsible for vocal fold abduction and adduction: the posterior cricoarytenoid, the lateral cricoarytenoid, and the interarytenoid muscles. Each muscle was exposed and divided into discrete fiber bundles whose coordinate positions were digitized in 3-dimensional space. The mass, length, relative cross-sectional area, and angle of orientation for each muscle bundle were obtained to allow for the calculations of average lines of action and moment arms for each muscle. This mapping of the canine laryngeal abductor and adductor musculature provides important anatomic data for use in laryngeal biomechanical modeling. These data may also be useful in surgical procedures such as arytenoid adduction

Functional Definitions of Vocal Fold Geometry for Laryngeal Biomechanical Modeling

[[abstract]]Precise geometric data on vocal fold dimensions are necessary for defining the vocal fold boundaries with respect to the laryngeal framework in physiological and biomechanical models of the larynx (eg, finite-element models). In the mid-membranous coronal section, vocal fold depth can be defined as the horizontal distance from the vocal fold medial surface to the thyroid cartilage, whereas vocal fold thickness can be defined as the vertical distance from the inferior border of the thyroarytenoid muscle to the vocal fold superior surface. Traditionally, such geometric data have been obtained from measurements made on histologic tissue sections. Unfortunately, it is very difficult to obtain reliable data by this method, unless the effects of sample preparation on vocal fold geometry are quantified. Significant tissue deformations are often induced by histologic processes such as fixation and dehydration, sometimes producing shrinkages as large as 30%. In this study, reliable geometric data of the canine vocal fold were obtained by the alternative method of quick-freezing for sample preparation, using liquid nitrogen. Coronal sections of quick-frozen larynges were thawed gradually in saline solution. Images of the mid-membranous coronal sections at various thawing stages were captured by a digital camera. Measurements of operationally defined vocal fold dimensions (depth and thickness) useful for biomechanical modeling were made with a graphics software package. The results showed that geometric changes of the vocal fold induced by freezing are likely reversed by thawing, such that the measurements made on thawed larynges are reliable approximations of the actual vocal fold dimensions

Geometric characterization of the laryngeal cartilage framework for the purpose of biomechanical modeling

[[abstract]]Some new anatomic data on the laryngeal cartilage framework have been obtained for the biomechanical modeling of the larynx. This study attempted to define and measure some biomechanically important morphometric features of the laryngeal framework, including both the human and the canine laryngeal frameworks, because the canine larynx has been frequently used as an animal model in gross morphology and in physiological experiments. The larynges of 9 men, 7 women, and 9 dogs were harvested and dissected after death. Linear and angular geometric measurements on the thyroid cartilage, the cricoid cartilage, and the arytenoid cartilage were made with a digital caliper and a protractor, respectively. The results are useful for constructing quantitative biomechanical models of vocal fold vibration and posturing (abduction and adduction), eg, continuum mechanical models and finite-element models of the vocal folds