Interaction Between the Thyroarytenoid and Lateral Cricoarytenoid Jun Yin1 Muscles in the Control Speech Production Laboratory, Department of Head and Neck Surgery, of Vocal Fold Adduction University of California, Los Angeles, 31-24 Rehabilitation Center, 1000 Veteran Avenue, and Eigenfrequencies Los Angeles, CA 90095-1794 2 Although it is known vocal fold adduction is achieved through laryngeal muscle activa- Zhaoyan Zhang tion, it is still unclear how interaction between individual laryngeal muscle activations Speech Production Laboratory, affects vocal fold adduction and vocal fold stiffness, both of which are important factors Department of Head and Neck Surgery, determining vocal fold vibration and the resulting voice quality. In this study, a three- University of California, Los Angeles, dimensional (3D) finite element model was developed to investigate vocal fold adduction 31-24 Rehabilitation Center, and changes in vocal fold eigenfrequencies due to the interaction between the lateral cri- 1000 Veteran Avenue, coarytenoid (LCA) and thyroarytenoid (TA) muscles. The results showed that LCA con- Los Angeles, CA 90095-1794 traction led to a medial and downward rocking motion of the arytenoid cartilage in the e-mail: [email protected] coronal plane about the long axis of the cricoid cartilage facet, which adducted the pos- terior portion of the glottis but had little influence on vocal fold eigenfrequencies. In con- trast, TA activation caused a medial rotation of the vocal folds toward the glottal midline, resulting in adduction of the anterior portion of the glottis and significant increase in vocal fold eigenfrequencies. This vocal fold-stiffening effect of TA activation also reduced the posterior adductory effect of LCA activation. The implications of the results for phonation control are discussed. [DOI: 10.1115/1.4028428] Keywords: vocal fold adduction, muscle activation, thyroarytenoid, lateral cricoarytenoid 1 Introduction cartilages. Contraction of both muscles is generally considered to adduct the arytenoid cartilages and approximate the two vocal A prerequisite for phonation is vocal fold adduction or approxi- folds. Another muscle, the posterior cricoarytenoid (PCA) muscle mation of the two vocal folds to close the glottis (the airspace (not shown in Fig. 1), also attaches to the arytenoid but its con- between the two folds), which enhances the glottal fluid–structure traction abducts the arytenoid cartilages and opens the glottis. In interaction and induces the vocal folds into a self-sustained vibra- addition to these three muscles, previous studies [9,10] showed tion. The resulting vibration pattern and consequently the pro- that the TA muscle also plays an important role in anterior vocal duced voice depend on the geometry and stiffness conditions of fold adduction. The TA muscle originates from the thyroid carti- the adducted vocal folds [1–3], with different vocal fold posturing lage and inserts into the anterolateral surface of the arytenoid car- producing different voice types [4–7]. Understanding the muscu- tilages and forms the main body of the vocal folds. Choi et al. lar control mechanisms of vocal fold adduction and the stiffness [10] showed that contraction of the LCA/IA alone closed the carti- and stress consequence of different adductory patterns is critical laginous portion of the glottis (the portion between the vocal proc- to understanding how humans are able to produce a variety of esses of the arytenoid cartilages and the posterior commissure) voice types. but left an opening in the membranous portion (the portion It is generally assumed that vocal fold adduction is achieved between the anterior commissure and the vocal processes of the primarily through motion of the arytenoid cartilages due to activa- arytenoid cartilages) of the glottis (Fig. 1(c)), whereas TA muscle tion of the LCA and the interarytenoid (IA) muscles. The vocal contraction completely closed the anterior glottis (Fig. 1(d)). folds are layered soft tissue structures that attach anteriorly to the Similar observation was also reported in a more recent study by thyroid cartilage and posteriorly to the arytenoid cartilages Chhetri et al. [9]. (Fig. 1(a)). The arytenoid cartilages sit on the upper posterior bor- Despite these general observations, the interaction between the der of the cricoid cartilage, forming the cricoarytenoid joint LCA/IA/TA muscles in the control of vocal fold adduction has (CAJ), which is a synovial joint of the ball-and-socket type [8]. never been systematically investigated. Furthermore, it remains The LCA muscles run between the muscular process of the aryte- essentially unknown how the stiffness and stress conditions within noid cartilage and the side of the cricoid cartilage, and the IA the vocal folds change with different vocal fold adductory pat- muscles attach to the muscular processes of the two arytenoid terns, due to the lack of reliable techniques for in vivo measure- ment of the stiffness and stress within the vocal folds (in both 1Present address: Department of Mechanics and Engineering Science, Fudan animal models or live humans). Inference of changes in stiffness University, Shanghai, China. 2 from vocal fold deformation is also difficult because in vivo ob- Corresponding author. servation of the motion of the arytenoid cartilages and the vocal Manuscript received March 24, 2014; final manuscript received August 5, 2014; accepted manuscript posted August 28, 2014; published online September 11, 2014. folds is limited to an endoscopic superior view, which can be mis- Assoc. Editor: Jonathan Vande Geest. leading. For example, although the arytenoid cartilage motion Journal of Biomechanical Engineering Copyright VC 2014 by ASME NOVEMBER 2014, Vol. 136 / 111006-1 Downloaded From: http://biomechanical.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/journals/jbendy/930797/ on 05/26/2017 Terms of Use: http://www.asme.org/about-asme/terms-of-use Fig. 1 (a) A sketch of the laryngeal framework including the vocal fold, the thyroid, cricoid, and arytenoid cartilages, and the LCA and IA muscles from a superior view. (b) Superior view of a canine larynx at resting (respiratory) position. (c) LCA activation adducts the glottis but leaves a gap in the middle-membranous glottis. (d) TA activation completely closes the anterior glottis but leaves a large gap at the posterior glottis. Figures (b)–(d) are images obtained from in vivo canine larynx experiments in Choi et al. (1993). appears to be a “pivoting” rotation about a vertical axis (an axis 2 Model perpendicular to the cricoid facet) from an endoscopic view, ana- tomical studies suggested that this pivoting rotational motion 2.1 Geometry and Boundary Conditions. The 3D finite ele- along a vertical axis may be restricted by the cricoid–arytenoid ment model included the vocal fold (with TA as the main compo- cartilage interaction in the CAJ, and the arytenoid cartilage nent, see below), the LCA muscle, the arytenoid cartilage, and the motion may be limited to either a translational sliding motion and/ cricoid cartilage (Fig. 2). The IA and PCA muscles (not shown in or a rigid-body rocking motion (rotation in the coronal plane) Fig. 2) were also included but not activated in this study. For sim- along the long axis of the cricoid facet [11–16]. In fact, the exact plicity, left–right symmetry in motion about the glottal midline arytenoid motion during vocal fold adduction, due to either LCA was assumed so that only one vocal fold and one arytenoid carti- or TA activation, is still debatable. lage were modeled in this study. The geometry of the arytenoid Due to these experimental difficulties, numerical models of and cricoid cartilages was obtained from the computer solid vocal fold adduction have been developed in previous studies e.g., model developed in Hunter and Thomson [21] based on the origi- Refs. [17–20] However, because of the highly nonlinear nature of nal magnetic resonance images (MRI) of a human laryngeal carti- muscle activation and vocal fold deformation, the interactions lage by Selbie et al.[13,14]. Note that the curves on the surface of within the CAJ and its interaction with muscle activation were the cartilages in Fig. 2 represent the curved surface of the carti- often not modeled in these studies and motion of the arytenoid lages, not the actual meshes used in the simulations. In this study, cartilages was either imposed or constrained along a predefined the cricoid cartilage was fixed. The interface between the aryte- rocking–sliding axis so that the interaction between the arytenoid noid and cricoid cartilages at the CAJ was modeled as a “contact- and cricoid cartilages in the CAJ and its influence on arytenoid sliding” interface, which allowed relative sliding motion between cartilage motion was not investigated. More importantly, because the two cartilages in the tangential direction along the interface. these previous models often assumed a rigid-body motion of the To simulate the effects of the covering soft tissues within the sy- vocal folds, the induced vocal fold tissue deformation and changes novial CAJ, a virtual elastic film (Young’s modulus of 100 kPa) in stress distribution within the vocal fold were thus not studied. was further added in between the two cartilages. The Young’s The goal of this study was to investigate the interaction modulus of this virtual elastic film was determined as the mini- between laryngeal muscles in determining arytenoid cartilage mum value required to prevent large separation of the cartilages motion, the resulting adductory pattern, and changes in the stiff- in the normal direction. ness condition within the vocal fold. Specifically, this study Because the original MRI data do not include any information focused on the interaction between the LCA and the TA muscles, that allows us to separate the different layers of soft tissue and the the two primary adductory muscles responsible for adduction of muscles, a body-cover two-layer simplified vocal fold with a uni- the membranous portion of the glottis.