Cricoid Confusion

I was recently emailed a question regarding the function of the cricoid cartilage, in relation to swallowing and respiration. When trying to appreciate the functional anatomy of the cricoid cartilage, we need to frame it in the context of the larynx, as the cricoid cartilage functions in conjunction with other structures that make up the larynx. The topography and functional anatomy of the larynx can often be a little confusing. I’ve therefore decided to split the topic into two posts, this one will focus on the topography of the larynx regarding its cartilages and ligaments. The second will focus on the muscles and cord actions. I’d strongly recommend you also look at Acland’s Atlas of Anatomy, in particular, his video on the laryngeal cartilages.

pyriform-6

Figure 1 Posterior View of The Pharynx Opened Up To Show The Larynx , Adapted From Netter

So a quick reminder, the larynx opens from the anterior wall of the pharynx, with the piriform recesses on either side of the laryngeal inlet see Figure 1. The larynx acts as a passage for the flow of air, allows manipulation of pitch & volume and behaves as a sphincter protecting the rest of the respiratory system below. The larynx connects the trachea with the oral and nasal cavities. The larynx consists of 9 distinct cartilages (thyroid, cricoid, epiglottis, 2 arytenoids, 2 corniculate and 2 cuneiform cartilages), connected by various connective tissue membranes (referred to as ligaments) and muscles. Remember that the larynx is suspended from the hyoid bone. Hence the larynx moves when the hyoid moves. The basic structure is illustrated in Figure 2. From this diagram its obvious that the thyroid and cricoid cartilages are the largest. The thyroid cartilage has a prominent anterior ridge known as the laryngeal prominence – commonly called the Adam’s apple – typically more apparent in men. The Thyroid cartilage is open posteriorly like the cartilaginous rings of the trachea, the exception being the cricoid.

Figure 2 Anterior/oblique and Posterior View of the Laryngeal Cartilages and Ligaments, Adapted From Gray’s Anatomy, The Anatomical Basis Of Clinical Practice

The thyroid cartilage

This is the largest of all the laryngeal cartilages, it has a shield like shape with an angled prominence (Adam’s apple) anteriorly. In males the angle of the laryngeal prominence is acuter, 90 vs 120 degrees in women. In men, this results in vocal cords of a greater length and hence a lower (deeper) pitch in voice. Remember the longer a piece of string the lower the pitch and with regards to tension, the greater the tension, the higher the pitch. Look at Figure 2, here you can see the two surfaces which form the laryngeal prominence are called laminae (lamina – singular). Posteriorly on both sides, you have the superior and inferior cornua (cornu – singular). Each inferior cornu articulates with the lamina of the cricoid cartilage to form the cricothyroid joint; this allows the thyroid cartilage to tilt forwards on the cricoid (discussed further below).

The thyroid cartilage (the superior edges of the laminae and the front of the superior cornua) articulates with the superior margin of the body and greater cornua of the hyoid bone via the thyrohyoid membrane. This membrane has three thickenings, a median and two lateral thickenings see Figure 2. On both sides of this membrane is an important perforation, through which the superior laryngeal neurovascular bundle travels through. A common misconception is that the thyroid gland sits immediately anterior to the thyroid cartilage; in fact the thyroid isthmus is anterior to the second, third and forth tracheal rings, the lateral lobes may reach the oblique line of the thyroid lamina. A pyramidal lobe if present, may ascend towards the laryngeal prominence.

The cricoid cartilage

The cricoid cartilage is a complete ring unlike the thyroid cartilages and tracheal rings. It marks the demarcation point between the larynx and the trachea, at the level of C6. The cricoid cartilage is narrower anteriorly (known as the arch) and broader posteriorly (known as the lamina), hence the shape of the cricoid is often said to resemble a signet ring (Figure 3).

cricoid1s

Figure 3 Lateral View of The Cricoid And Thyroid Cartilage And A Superior View Of The Cricoid And Arytenoid Cartilages. Adapted From Gray’s Anatomy, The Anatomical Basis Of Clinical Practice

The cricoid is attached to the thyroid cartilage above by two synovial cricothyroid joints and a very important membrane. This membrane is often a source of confusion as it is referred to by a few names. In some texts, it is called the cricothyroid membrane and in others the conus elasticus – see Figure 2. The important concept is to understand that this membrane has three parts, a median (anterior) thicker part and the thinner lateral aspects (one each side). In some texts, the median (anterior) part is called the cricothyroid membrane (or the medial part of the conus elasticus) and the lateral aspects are called the conus elasticus. The lateral parts (both sides) – the conus elasticus ascend superiorly and medially, creating two free edges (left and right), the anterior part of each free edge is attached to the inside of the laryngeal prominence, the posterior part is attached to the vocal process of their respective arytenoid cartilages. The rest of the free edge of the conus elasticus (between the anterior and posterior attachment) is essentially the vocal cord (true cord) – see Figure 5.

Above these vocal cords (true cords) are the vestibular ligaments (false cords) – these are the inferior free edges of the quadrangular ligament. The vestibular ligament is attached anteriorly to the inside of the front of the thyroid cartilage, like the true cords, but their posterior edge is attached the anterior lateral aspect of the arytenoid cartilages, not the vocal processes of the arytenoid cartilages, as in the vocal cords (true cords). The anterior-superior edge of each (left and right) quadrangular membrane attaches to the lateral edge of the epiglottis. It’s posterior-superior edge is another free edge and forms the aryepiglottic ligament. The inferior most point of each aryepiglottic ligament attaches to the apex of the arytenoid cartilage on each side – see Figure 5.

The laryngeal ventricle

On both sides of the larynx, between the vestibular ligament (false cord) and the free edge of the conus elasticus (true vocal cord) is a mucosal lined pocket known as the ventricle, this lines the entire left and right lateral aspects of the larynx. The anterior part of this pocket opens into a diverticulum known as the saccule of the ventricle – see Figure 4. This saccule ascends in a cranial direction, sandwiched between the vestibular ligament and the thyroid cartilage. The saccule is lined with mucous glands, producing mucous to lubricate the cords. The saccule can be  a source of acquired and congenital saccular cysts, presenting with hoarseness, dysnoea or a mass in the neck.

cricoid6

Figure 4 Coronal section of the larynx and trachea, Adapted From Gray’s Anatomy, The Anatomical Basis Of Clinical Practice

An important clinical point to note, is that the mucosa covering the true cords is avascular (hence they appear pearly white) and lacks a submucosa, unlike the false cords. So thankfully when oedema of the larynx occurs – the true cords are spared. Otherwise, there would be rapid impending airway closure.

The epiglottis

The epiglottis is a leaf-shaped cartilage which is attached by its “stem” to the inside surface of the anterior aspect of the thyroid cartilage (posterior aspect of the laryngeal prominence) and the tongue facing surface is attached by the hyoepiglottic ligament to the hyoid bone. During ingestion of food, the epiglottis is flattened, the larynx is lifted up and under the tongue, hence the base of tongue passively pushes the epiglottis down along with the weight of the food bolus, thus closing the laryngeal inlet. Interestingly even when the epiglottis is removed in the treatment of certain cancers, patients are still  able to swallow without aspiration in most cases. The likely explanation is the fact that the laryngeal inlet (aditus) is moved out of the way of incoming food, by being lifted up (by the hyoid) and under the tongue during swallowing and the aryepiglottic ligaments and muscles act like purse strings, closing the laryngeal inlet.

The arytenoids

The paired arytenoids and their articulation with the cricoid cartilage helps explain the movements of the vocal cords, but also how the aryepiglottic folds close during swallowing. Looking at Figure 3, one can see how the arytenoids are perched on top of the cricoid cartilage. Each arytenoid is pyramidal in shape, with the base articulating with the cricoid. The anterior corner of the pyramid is the vocal process, while the lateral corner is the muscular process. The apex of the pyramid points in a cranial direction and articulates with the corniculate cartilages – here its worth re-looking at Acland’s video on the laryngeal cartilages. The arytenoids while perched on the cricoid can swivel towards and away from the mid line, as well as slide towards and away from one another. Remember that the vocal cords posterior attachment is the vocal processes, so swivelling or sliding of the arytenoids outwards abducts the cords, while swivelling inwards and sliding towards one another adducts the cords. The specifics of cord movements and the muscles involved will be discussed in a future post.

The regions of the larynx

We now have an overview of the main laryngeal cartilages and some of the key membranes, therefore we can move on to defining how the space of the larynx is divided up into different regions. These are terms you will come across in your clinical training and they will hopefully make more sense if you appreciate the anatomy. The larynx is divided into distinct regions: laryngeal inlet, vestibule, rima glottidis and subglottis. The inlet, also known as the aditus is where the hypopharnyx opens into the larynx, it is defined by the following boundaries, anteriorly the superior edges of the epiglottis, laterally the aryepiglottic folds and posteriorly the mucosa between the arytenoid cartilages. The vestibule is the space between the aditus and the vestibular ligaments, the rima glottidis is the opening between the true cords and arytenoid cartilages. The subglottis is the region between the true cords and the cricoid cartilage.

CRICOID4S

Figure 5 Sagittal Section Of The Laryngeal Cartilages and Ligaments, Adapted From Gray’s Anatomy, The Anatomical Basis Of Clinical Practice

Functional importance

Swallowing: The cricoid per say has no unique function associated with swallowing, rather in this respect its function should be considered as part of the larynx as a whole, which on swallowing is lifted up by the hyoid bone. Remember the suprahyoid and thyrohyoid muscles elevate the larynx and bring it slightly forward, tucking it under the tongue and out of the way of food which is being swallowed. The elevation of the larynx narrows the laryngeal inlet, improves contact with the epiglottis, and also contributes to pharyngeal dilation. The laryngeal muscles, interarytenoid, aryepiglottic and thyroepiglottic also contract, helping to close the laryngeal inlet (these muscles will be discussed in more detail in the second post).

Voice production: As discussed above the cricoid cartilage is an important structure in airflow and phonation as the vocal cord influences the passage of air through the larynx. The vocal cords arise from the free edge of the conus elasticus ligaments and because the cords are affected by the movement of their attachment sites, the shape of the rima glottidis can be altered. The vocal cords are attached posteriorly to the arytenoid vocal processes and anteriorly to the inside surface of the laryngeal prominence. The movements of the arytenoids allow abduction and adduction of the cords, aiding in phonation. The specific movements of the cords will be discussed in a future post. Remember the cricoid is attached to the thyroid cartilage anteriorly by the cricothyroid membrane and posteriorly by the two cricothyroid joints. Both cricothyroid joints are synovial and allow the thyroid cartilage to tilt forwards (via the action of the cricothyroid muscles) – see Figure 3, this movement has the effect of tensing the vocal cords and increasing the pitch of the voice.

Clinical correlations

The Sellick manoeuvre is still used by some anaesthetist during rapid sequence induction (RSI – intubation of patient using an endotracheal tube in an emergency situation). The manoeuvre aims to prevent regurgitation of stomach contents into the oesophagus/pharynx and aspiration via the larynx. The anaesthetist applies downward pressure on the cricoid, which should in theory compress the oesophagus behind it, preventing regurgitation into the upper airway. In an emergency situation, artificially ventilation prior to intubation causes stomach insufflation and increases the risk of regurgitation. In elective (non-emergency) intubation, the patient stops breathing only once the anaesthetic drugs start to take effect, the patient is then intubated immediately, and artificial ventilation commences. As the airway has been secured before artificial ventilation, the risk of insufflation is mitigated therefore in elective cases the Sellick manoeuvre is not necessary.

Emergency cricothyrotomy (often incorrectly called an emergency tracheotomy – think about the anatomy): In an emergency situation, where the oral and nasal cavity cannot be used as a patent airway (cannot intubate or ventilate situation); an emergency airway must be created. This involves identifying the laryngeal prominence, tracing your finger down to a depression between the thyroid and cricoid cartilages. This depression is the cricothyroid membrane, a vertical incision is made through the superficial layers (skin, superficial fascia, anterior jugular veins, investing fascia and infrahyoid muscles) until the cricothyroid membrane is reached. Then a horizontal incision is made through the cricothyroid membrane allowing the passage of a temporary tube or an endotracheal tube through the incision. This will allow temporary ventilation until the patient is transferred to a hospital, where a more permanent airway such as a tracheotomy can be considered.

References

  • Clinical Anatomy By Regions, Richard Snell
  • Gray’s Anatomy, The Anatomical Basis Of Clinical Practice, Susan Stranding

Leave a Reply