Definition Of Cricoid Cartilage
The cricoid cartilage is a hyaline cartilage ring that fully encircles the trachea and composes the inferior-most boundary of the laryngeal skeleton. The term “cricoid,” (Greek, krikos meaning “ring-shaped”) refers to the signet-ring resemblance of the cricoid cartilage.
Anatomy of Cricoid Cartilage
While the majority of cartilaginous rings that surround the trachea are semi-circular, the first ring – the cricoid cartilage ring – is a complete circle and features a narrow arch anterior and a broad lamina posterior to the airway. This form is often likened to a signet ring. In the image below, the broadest side (the cricoid plate) lies in front of the esophagus and the arch faces the front.
The larynx is a complicated structure with three functions. It contains a valve that protects the airway from debris, especially when swallowing (the epiglottis), it provides structure to the otherwise soft tissue of the airway, and it contains the apparatus for vocalization.
The diagram below shows the position of the cricoid cartilage immediately below the vocal chords and in touching distance of the thyroid cartilage.
Housed within this part of the anatomy, just under the feature known as Adam’s apple (thyroid cartilage), the cricoid cartilage is more than a strengthening buttress for the trachea. It is attached to the thyroid and arytenoid cartilages via synovial joints and is so strong it can support a series of ligaments and muscles.
Vocal folds are singularly attached to a small piece of the arytenoid cartilage, which is itself attached to the cricoid cartilage by way of a ball and socket joint. The thyroid cartilage is also attached to the cricoid cartilage.
Cricoid cartilage provides an attachment point for three muscles: the lateral cricoarytenoid, posterior cricoarytenoid, and cricothyroid muscles. All of these play a role in opening, closing, and elongating the vocal chords, and are essential for voice pitch and quality.
In total, nine cartilages can be found in the larynx. These are either singular (cricoid, thyroid, epiglottis) or in pairs (arytenoids, comiculates, cuneiforms) and, together with the hyoid bone, are described as the ‘visceral skeleton’ of this region of the neck.
Cricoid Cartilage Function
Cricoid cartilage function is to provide attachments for the cricothyroid, posterior cricoarytenoid, and lateral cricoarytenoid muscles, cartilages, and ligaments involved in voice pitch and quality.
In addition, cricoid cartilage supports the soft connective tissue of the trachea together with other semi-circular cartilage bands that run along its entire length.
Cricoid cartilage is also used as an anatomical landmark for surgical cricothyrotomy (temporary) and tracheostomy (definitive) procedures that provide a viable airway in emergency settings. The diagram below shows the thyroid cartilage (1), cricothyroid ligament (2), cricoid cartilage (3), and trachea (4). Points A and B relate to cricothyrotomy and tracheostomy incision points respectively.
Cricoid pressure (Sellick’s maneuver) is occasionally implemented during intubation when there is a risk of the stomach contents rising up through the esophagus and into the trachea (and lungs) due to a paralyzed epiglottis.
Just before a patient falls asleep, light pressure is placed on the cricoid in order to flatten the soft tube of the esophagus behind it and, in theory, prevent the stomach contents from entering the airway.
As the cricoid ring is a complete circle with a broader plate facing the esophagus, such pressure will, in principle, close off the esophagus which is not protected by any cartilaginous structures. This is a controversial maneuver but still used during rapid-sequence induction of anesthesia (RSI). Extremely rarely, cricoid cartilage can fracture under pressure.
Cricoid Cartilage Structure
All cricoid cartilage is hyaline cartilage and its structure is distinguished by type II collagen and chondroitin sulfate within its extracellular matrix. This type of cartilage is durable, and it experiences very little friction due to its smooth, glass-like surface. Cricoid cartilage is surrounded by a perichondrial membrane.
Cricoid cartilage grows in a pattern of circumferential growth (cell proliferation) and interstitial growth (extracellular matrix production) but, as a cartilaginous tissue without an immediate blood supply, this process is slow.
Cricoid cartilage, as all cartilage, is aneural and avascular. However, the density of blood vessels in its immediate vicinity provides nutrients and oxygen to the perichondrium, from which these then pass via diffusion into the cartilage tissue. While cartilage tissue itself contains no nerves, the larynx is significantly innervated, as are the muscles attached to the cricoid cartilage.
In adult patients under general anesthetic where the placement of an endotracheal tube is required, the narrowest part of the trachea through which to pass the tube is between the vocal chords.
In older children and adults, the trachea remains approximately the same diameter along its length. In babies and very young children, however, the trachea is distinctly funnel-shaped, and the narrowest location is not at the vocal chords, but at the site of the cricoid cartilage itself.
The image below shows a video-laryngoscope image of an already-placed endotracheal tube fixed between the open (relaxed) vocal chords of an adult. In a young child, the comparative narrowness of the part of the trachea surrounded by the cricoid cartilage makes tube placement more complicated. Note the relaxed epiglottis at the top of the screen.
As with all joints, cricoarytenoid and cricothyroid joints can be affected by osteoarthritis. This leads to cracking or fissure-forming in the cartilage tissue, loosening of collagen structures, and loss of proteoglycans. Cricoid cartilage degeneration leads to decreased vocal quality and changes in pitch.
When intubating a patient under general anesthesia prior to surgery, the anesthesiologist will press on the cricoid cartilage to compress the esophagus behind it so as to prevent gastric reflux from occurring: this is known as the Sellick manoeuvre.
The Sellick Manoeuvre is typically only applied during a Rapid Sequence Induction (RSI), an induction technique reserved for those at high risk of aspiration.
The Sellick maneuver was considered the standard of care during rapid sequence induction for many years. The American Heart Association still advocates the use of cricoid pressure during resuscitation using a BVM, and during emergent oral endotracheal intubation. However, recent research increasingly suggests that cricoid pressure may not be as advantageous as once thought.
The initial article by Sellick was based on a small sample size at a time when high tidal volumes, head-down positioning, and barbiturate anesthesia were the rule.
Cricoid pressure may frequently be applied incorrectly.Cricoid pressure may frequently displace the esophagus laterally, instead of compressing it as described by Sellick.
Several studies demonstrate some degree of glottic compression reduction in tidal volume and increase in peak pressures. Based on the current literature, the widespread recommendation that cricoid pressure be applied during every rapid sequence intubation is quickly falling out of favor.
Gastric reflux could cause aspiration if this is not done considering the general anesthesia can cause relaxation of the gastroesophageal sphincter allowing stomach contents to ascend through the esophagus into the trachea.
A medical procedure known as a cricoidectomy can be performed in which part or all of the cricoid cartilage is removed. This is commonly done to relieve blockages within the trachea.
Fractures of the cricoid cartilage can be seen after manual strangulation also known as throttling.