Cricoarytenoid and cricothyroid joints

Cricoarytenoid joint

The cricoarytenoid joint is a paired synovial articulation between the cricoid and arytenoid cartilages of the larynx. There are two articular facets that form this joint: superior arytenoid and inferior cricoid facets. The proximal facet is located on the undersurface of the arytenoids. It is positioned in such a way that its short axis is parallel to the long axis of the cricoid facet. 

The arytenoids articulate with the facets located on the upper margin of the lamina of the cricoid cartilage. This facet is also elliptical in shape, tilted inferoanterolaterally and convex on both axes. The long axes of both facets meet posteriorly near the midline forming an angle of approximately 50 degrees.

The articular surfaces are enveloped in a thin and loose joint capsule that is strengthened by a joint ligament. The main ligament that limits the movements in this joint is the (posterior) cricoarytenoid ligament. Even though some sources name it "posterior" it's situated mainly on the medial aspect of the joint. The vocal ligament is attached to the anterior process (vocal process) of the arytenoid cartilage, which projects anteriorly.

Two principal movements occur in this joint: rotation and gliding. 

In rotation, the cricoarytenoid muscles turn the arytenoid cartilages in the transverse plane about the longitudinal axis of the cricoid facet. This causes the vocal processes of arytenoid cartilages to rotate outwards (dorsomediocranially) or inwards (ventrolaterocaudally). Rotation of the arytenoid cartilages is accompanied by lateral and medial swinging of the vocal processes which controls the opening and narrowing of the rima glottidis. Cricoarytenoid ligaments limit the forward movement of the arytenoid cartilage on the cricoid cartilage.The total range of motion of rotation is approximately 30°. 

Gliding of the arytenoid cartilages occurs parallel to the long axis of the cricoid facet. In this case the arytenoids move closer or further away from one another. Due to the inferior-anterolateral slant of the articular surface of the cricoid cartilage, when the arytenoids glide away from each other they also glide in an inferior direction. When they glide towards each other, the opposite occurs. Consequently, this leads to shortening or lengthening of the vocal cords during vocal adjustments.

Even though the rotation and gliding movements are described separately, they are functionally associated. Medial glide and  medial rotation happen simultaneously, causing depression and medial swinging of the vocal process and thus adduction of the vocal cords. Lateral glide is accompanied by lateral rotation, leading to the separation and elevation of the vocal processes and therefore the abduction of the cords. These movements of rotation and gliding at the cricoarytenoid joints ensure precise and synchronous movement of the vocal cords which are crucial for the acts of breathing and phonation. 

There are several muscles that act on this joint. They include the: thyroarytenoid, posterior cricoarytenoid, lateral cricoarytenoid, oblique arytenoid, transverse arytenoid, aryepiglottic and interarytenoid muscles. The main adductors of the cricoarytenoid joint are the lateral cricoarytenoid muscles, while the main abductors are the posterior cricoarytenoid muscles.

To watch our video tutorial and test your knowledge on all structures of the larynx, check out this study unit.

Cricothyroid joint

The cricothyroid joint is a paired synovial articulation that connects the cricoid and thyroid cartilages. It is formed between the inferior horn of the thyroid cartilage and the laterally located thyroid articular surface on the cricoid cartilage. 

The joint is enveloped in a fibrous capsule that is rich in elastin fibers and strengthened posteriorly by a fibrous band. The ligaments associated with this joint are the posterior, lateral and anterior cricothyroid ligaments. The posterior cricothyroid ligament extends from the posterior margin of the articular facet on the thyroid cartilage to the posterior aspect of the superior part of the cricoid lamina. The lateral cricothyroid ligament extends from the posteroinferior margin of the inferior horn of the thyroid cartilage to the posterior aspect of the lower part of the lamina of the cricoid cartilage. The anterior cricothyroid ligament spreads from the anterior border of the inferior horn of thyroid cartilage to the lateral side of the posterior part of the arch of the cricoid cartilage above its inferior border. 

The principal movement in this joint is rotation. Specially, the cricoid cartilage rotates on the inferior horns of the thyroid cartilage about a transverse axis passing through both joints. However, it can be easier to understand this movement by thinking of it as a “tilting” motion. The cricothyroid muscle contracts, pulling on the anterior aspect of the cricoid cartilage and causing it to tilt upwards by rotating around the joint. This movement results in a decrease in the anterior space between the inferior border of the thyroid cartilage and the upper border of the cricoid arch.

As the anterior aspect of the cricoid cartilage tilts superiorly, the posterior aspect of the cricoid arch correspondingly tilts inferiorly. The arytenoid cartilages sit on the posterior aspect of the cricoid cartilage, thus they are moved inferiorly also. This leads to a stretching of the vocal cords, through the vocal ligament’s attachment to the anterior process (vocal process) of the arytenoid cartilage. The result is elevation of the pitch of the voice. 

Rotation at the cricothyroid joint can occur independently, or it can occur in unison with the movements of the hyoid bone. The ligamentous attachments between the hyoid bone and the thyroid cartilage enable their synchronized movements. When the hyoid muscles pull the hyoid bone superiorly, the thyroid cartilage follows it in the same direction. Rotation then occurs as usual about the cricothyroid joint, with the thyroid tilting upwards with respect to the cricoid cartilage, increasing the anterior thyroid-cricoid space. Besides rotation, slight horizontal and vertical gliding movements are also possible.

The main muscle acting on the cricothyroid joint is the cricothyroid muscle. When activated it elevates the cricothyroid arch and depresses the thyroid lamina. This leads to lengthening and stiffening of the vocal cords.