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

Electrical Stimulation of Vocal Fold Adduction Triggered by Laryngeal Electromyography Using a Custom Implant

PURPOSE

Medialization procedures for unilateral vocal fold (VF) paralysis generally improve voice but do not fully replace dynamic VF adduction. Paralyzed VFs typically experience synkinetic reinnervation, which makes it feasible to elicit movement through electrical stimulation. We tested a novel laryngeal pacing implant capable of providing closed-loop (automatic) stimulation of a VF triggered by electromyography (EMG) potentials from the contralateral VF.

METHOD

A custom, battery-powered, microprocessor-based stimulator was tested in eight dogs with bipolar electrodes implanted for recording EMG from the left VF and stimulating adduction of the right VF. A cuff electrode on the left recurrent laryngeal nerve (RLN) stimulated unilateral VF adduction, modeling voluntary control in anesthetized animals. Closed-loop stimulation was tested in both acute and chronic experiments. Synkinetic reinnervation was created in two animals by right RLN transection and suture repair to model unilateral VF paralysis.

RESULTS

In all animals, left VF activation through RLN stimulation generated a robust EMG response that rapidly triggered stimulation of contralateral thyroarytenoid and lateral cricoarytenoid muscles, causing nearly simultaneous bilateral adduction. Optimal triggering of VF stimulation from elicited EMG was achieved using independent onset and offset thresholds. Real-time artifact blanking allowed closed-loop stimulation without self-perpetuating feedback, despite the proximity of recording and stimulation electrodes.

CONCLUSIONS

Using a custom implant system, we demonstrated real-time closed-loop stimulation of one VF triggered by the activation of the contralateral VF. This approach could potentially restore dynamic glottic closure for reflexive behaviors or phonation in cases of unilateral VF paralysis with synkinetic reinnervation.

SUPPLEMENTAL MATERIAL

https://doi.org/10.23641/asha.24492133.

Effects of cricothyroid and thyroarytenoid interaction on voice control: Muscle activity, vocal fold biomechanics, flow, and acoustics

An MRI-based three-dimensional computer model of a canine larynx was used to investigate the effect of cricothyroid (CT) and thyroarytenoid (TA) muscle activity on vocal fold pre-phonatory posturing and glottic dynamics during voice production. Static vocal fold posturing in the full activation space of CT and TA muscles was first simulated using a laryngeal muscle mechanics model; dynamic flow-structure-acoustics interaction (FSAI) simulations were then performed to predict glottal flow and voice acoustics. The results revealed that TA activation decreased the length and increased the bulging, height, and contact area of the vocal fold. CT activation increased the length and contact area and decreased the height of the vocal fold. Both CT and TA activations increased the vocal fold stress, stiffness, and closure quotient; and only slightly affected the flow rate and voice intensity. Furthermore, CT and TA showed a complex control mechanism on the fundamental frequency pattern, which highly correlated with a combination of the stress, stiffness, and stretch of the vocal fold.

A three-dimensional vocal fold posturing model based on muscle mechanics and magnetic resonance imaging of a canine larynx

In this work, a high-fidelity three-dimensional continuum model of the canine laryngeal framework was developed for simulating laryngeal posturing. By building each muscle and cartilage from magnetic resonance imaging (MRI), the model is highly realistic in anatomy. The muscle mechanics is modeled using the finite-element method. The model was tested by simulating vocal fold postures under systematic activations of individual as well as groups of laryngeal muscles, and it accurately predicted vocal fold posturing parameters reported from in vivo canine larynges. As a demonstration of its application, the model was then used to investigate muscle controls of arytenoid movements, medial surface morphology, and vocal fold abduction. The results show that the traditionally categorized adductor and abductor muscles can have opposite effects on vocal fold posturing, making highly complex laryngeal adjustments in speech and singing possible. These results demonstrate that a realistic comprehensive larynx model is feasible, which is a critical step toward a causal physics-based model of voice production.

Interaction between the thyroarytenoid and lateral cricoarytenoid muscles in the control of vocal fold adduction and eigenfrequencies

Although it is known vocal fold adduction is achieved through laryngeal muscle activation, it is still unclear how interaction between individual laryngeal muscle activations affects vocal fold adduction and vocal fold stiffness, both of which are important factors determining vocal fold vibration and the resulting voice quality. In this study, a three-dimensional (3D) finite element model was developed to investigate vocal fold adduction and changes in vocal fold eigenfrequencies due to the interaction between the lateral cricoarytenoid (LCA) and thyroarytenoid (TA) muscles. The results showed that LCA contraction led to a medial and downward rocking motion of the arytenoid cartilage in the coronal plane about the long axis of the cricoid cartilage facet, which adducted the posterior portion of the glottis but had little influence on vocal fold eigenfrequencies. In contrast, 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.

Magnetoelastic sensors as a new tool for laryngeal research

CONCLUSIONS

The use of custom-built magnetoelastic sensors designed for insertion in the larynx to measure the force of transverse deformation of the thyroarytenoid and cricothyroid muscles was examined.

OBJECTIVES

The availability of sensors that transduce deformation force into an electrical current (measurable in millivolts) suggested that such sensors could be used to study muscle function in the canine larynx. This study aimed to obtain information about the force exercised by larynx muscles on the sensor in an experiment in vivo.

MATERIALS AND METHODS

The results of six surgical interventions in three dogs using four magnetoelastic sensors are described. The sensor and insertion technique were atraumatic. One dog underwent surgery four times to study the morbidity, reliability and reproducibility of the technique. An electromyographic recording was made at the same time that sensor response was registered to compare results with a standardized technique.

RESULTS

We obtained 584 regular responses: 256 deglutitions (150 thyroarytenoid and 106 cricothyroid recordings) and 328 phonations (223 thyroarytenoid and 105 cricothyroid recordings). No signs of injury or functional deficit were observed after any intervention. In the dog that underwent four interventions, results were consistent after each intervention.

A muscle controlled finite-element model of laryngeal abduction and adduction

A three-dimensional finite-element model was developed to simulate the complex movement of the laryngeal cartilages during vocal fold abduction and adduction. The model consists of cricoid and arytenoid cartilages, as well as the intralaryngeal muscles and vocal folds. The active and passive properties of the muscles were idealised by one-dimensional elements based on the Hill theory. Its controlling input value is a time dependent stimulation rate. Optimisation loops have been carried out for the arrangement of the individual stimulation rates. Since in vivo measurements are not feasible, the developed biomechanical model shall be used to analyse the force distribution within the laryngeal muscles during phonatory manoeuvres. Simulations of abduction and adduction in different pitches of voice lead to realistic tensions of the vocal folds. The model is a first step to analyse motional vocal fold diseases and to predict the consequences of phonosurgical interventions.

Refinements in modeling the passive properties of laryngeal soft tissue

The nonlinear viscoelastic passive properties of three canine intrinsic laryngeal muscles, the lateral cricoarytenoid (LCA), the posterior cricoarytenoid (PCA), and the interarytenoid (IA), were fit to the parameters of a modified Kelvin model. These properties were compared with those of the thyroarytenoid (TA) and cricothyroid (CT) muscles, as well as previously unpublished viscoelastic characteristics of the human vocal ligament. Passive parameters of the modified Kelvin model were summarized for the vocal ligament, mucosa, and all five laryngeal muscles. Results suggest that the LCA, PCA, and IA muscles are functionally different from the TA and CT muscles in their load-bearing capacity. Furthermore, the LCA, PCA, and IA have a much larger stress-strain hysteresis effect than has been previously reported for the TA and CT or the vocal ligament. The variation in this effect suggests that the connective tissue within the TA and CT muscles is somehow similar to the vocal ligament but different from the LCA, PCA, or IA muscles. Further demonstrating the potential significance of grouping tissues in the laryngeal system by functional groups in the laryngeal system was the unique finding that, over their working elongation range, the LCA and PCA were nearly as exponentially stiff as the vocal ligament. This paper was written in conjunction with an online technical report (http://www.ncvs.org/ncvs/library/tech) in which comprehensive muscle data and sensitivity analysis, as well as downloadable data files and computer scripts, are made available.

A two-dimensional biomechanical model of vocal fold posturing

The forces and torques governing effective two-dimensional (2D) translation and rotation of the laryngeal cartilages (cricoid, thyroid, and arytenoids) are quantified on the basis of more complex three-dimensional movement. The motions between these cartilages define the elongation and adduction (collectively referred to as posturing) of the vocal folds. Activations of the five intrinsic laryngeal muscles, the cricothyroid, thyroarytenoid, lateral cricoarytenoid, posterior cricoarytenoid, and interarytenoid are programmed as inputs, in isolation and in combination, to produce the dynamics of 2D posturing. Parameters for the muscles are maximum active stress, passive stress, activation time, contraction time, and maximum shortening velocity. The model accepts measured electromyographic signals as inputs. A repeated adductory-abductory gesture in the form /hi-hi-hi-hi-hi/ is modeled with electromyographic inputs. Movement and acoustic outputs are compared between simulation and measurement.

Active and passive properties of canine abduction/adduction laryngeal muscles

Active and passive characteristics of the canine adductor- abductor muscles were investigated through a series of experiments conducted in vitro. Samples of canine posterior cricoarytenoid muscle (PCA), lateral cricoarytenoid muscle (LCA), and interarytenoid muscle (IA) were dissected from dog larynges excised a few minutes before death and kept in Krebs-Ringer solution at a temperature of 37 degrees C +/- 1 degree C and a pH of 7.4 +/- 0.05. Active twitch and tetanic force was obtained in an isometric condition by applying field stimulation to the muscle samples through a pair of parallel-plate platinum electrodes. Force and elongation of the samples were obtained electronically with a dual-servo system (ergometer). The results indicate that the twitch contraction times of the three muscles are very similar, with the average of 32 +/- 1.9 ms for PCA, 29 +/- 1.6 ms for LCA, and 32 +/- 2.4 ms for IA across all elongations. Thus, PCA, LCA, and IA muscles are all faster than the cricothyroid (CT) muscles but slower than the thyroarytenoid (TA) muscles. The tetanic force response times of these muscles are also similar, with a maximum rate of force increase of 0.14 N/ms.

A three-dimensional model of vocal fold abduction/adduction

A three-dimensional biomechanical model of tissue deformation was developed to simulate dynamic vocal fold abduction and adduction. The model was made of 1721 nearly incompressible finite elements. The cricoarytenoid joint was modeled as a rocking-sliding motion, similar to two concentric cylinders. The vocal ligament and the thyroarytenoid muscle's fiber characteristics were implemented as a fiber-gel composite made of an isotropic ground substance imbedded with fibers. These fibers had contractile and/or passive nonlinear stress-strain characteristics. The verification of the model was made by comparing the range and speed of motion to published vocal fold kinematic data. The model simulated abduction to a maximum glottal angle of about 31 degrees. Using the posterior-cricoarytenoid muscle, the model produced an angular abduction speed of 405 degrees per second. The system mechanics seemed to favor abduction over adduction in both peak speed and response time, even when all intrinsic muscle properties were kept identical. The model also verified the notion that the vocalis and muscularis portions of the thyroarytenoid muscle play significantly different roles in posturing, with the muscularis portion having the larger effect on arytenoid movement. Other insights into the mechanisms of abduction/adduction were given.

An investigation of cricoarytenoid joint mechanics using simulated muscle forces

Rotational and translational stiffnesses were calculated for arytenoid motion about the cricoarytenoid joint. These calculations were obtained from measurements on five excised human larynxes. For each larynx, known forces were applied to the arytenoid cartilage, and three markers were tracked as a function of applied forces. Assuming rigid body motion, arytenoid translations and rotations were computed for each applied force. Translational stiffnesses were obtained by plotting force versus displacement, and rotational stiffnesses were calculated by plotting torque versus angular rotation. A major finding was that the translational stiffness along the anterior-posterior direction was three times as great as the translational stiffnesses in the other two directions. This nonisotropic nature of the stiffnesses may be an important consideration for phonosurgeons who wish to avoid subluxation of the cricoarytenoid joint in patients. The computed rotational and translational stiffnesses currently are being implemented in 2D and 3D models. These stiffness parameters play a vital role in prephonatory glottal shaping, which in turn exerts a majorinfluence on all aspects of vocal fold vibration, including fundamental frequency, voice quality, voice register, and phonation threshold pressure.

Developing an anatomical model of the human laryngeal cartilages from magnetic resonance imaging

The purpose of this work was to construct a three-dimensional anatomical framework of the cartilages of the human larynx. The framework included representative surface models of the four laryngeal cartilages and estimated attachment points for the intrinsic laryngeal muscles. High-resolution magnetic resonance imaging (MRI) was used to scan one female and four male human cadaveric larynges. The cartilages were segmented manually from the MRI volume for analysis. Two of these larynges were subsequently dissected and the landmark distances on the cartilages measured for comparison with the MRI measures and previous studies. The MRI measures were 8% smaller than the anatomical measures and 12% smaller than data reported in the literature. A laryngeal coordinate system was defined using the plane of symmetry of the cricoid cartilage. Measures of cricoid cartilage symmetry had less than 3% difference between the two sides for a series of measures. An algorithm for registering larynges that minimized the root-mean-square distance between the surface of a reference cricoid cartilage and the surfaces of nonisotropically scaled candidate cricoid cartilages was evaluated. This study provided an anatomical framework for registering different larynges to the same coordinate space.

Functional definitions of vocal fold geometry for laryngeal biomechanical modeling

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

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.