66

‘The human larynx has undergone many changes in the course of evolution,

and is now a complicated organ. It is possible, however, to study its

component parts in the light of comparative anatomy and thus to derive

information as to the reasons for the various structural details’.

By V E Negus, 1937

5. Relevant anatomy of adult larynx

The ‘Larynx of man is not specialised for one function but is an

extremely versatile and efficient organ’ (Negus, 1937). It is an organ which

is suspended by the muscles and ligaments at the cranial end of the

trachea in the neck. It is lies opposite the 3rd and the 6th cervical vertebra.

The level of the larynx is slightly higher in women and in children. The

larynx and trachea is a dynamic organ and it moves up and down during

respiration, deglutination and phonation. The superior laryngeal

displacement varies from 21.1 mm to 33.9 mm (Logemann, 2000; Bingjie,

2010).

Laryngeal framework.

The basic framework of the larynx is formed by the cartilages. The

larger cartilage i.e. thyroid, cricoid and the epiglottis are the 3 unpaired

cartilages. The arytenoid, cuneiform and the corniculate are 3 paired

cartilages. These cartilages are linked by joints that are reinforced by

muscles ligaments and membranes. The intrinsic and extrinsic muscles of

the larynx move these cartilages in relation with each other.

67

Cartilages of larynx

Epiglottic cartilage: This fibroblastic cartilage is shaped like a leaf

and is seen projecting in to the pharyngeal lumen below and posterior to

the tongue and hyoid bone. The stalk of this cartilage is attached to the

thyroid cartilage at the angle between the two laminas below the thyroid

notch. The upper superior border is free. The sides of the epiglottis is

attached to the arytenoids by the aryepiglottic folds of the mucus

membrane, all these form the inlet of the larynx. The anterior surface of the

epiglottis is related to the posterior part of the tongue base and the

intervening space between the tongue and the epiglottis is the vallecular.

The epiglottis is attached to the hyoid bone by an elastic ligament the

hyoepiglotic ligament. A pyramidal space is formed between the epiglottis,

the hyoepiglotic ligament and the thyrohyoid membrane called the pre-

epiglottic space which is filled with fatty tissue. The epiglottis in man is

almost functionless as opposed to other animals in which the epiglottis has

additional olfactory function (Negus, 1937).

Thyroid cartilage: This is the largest of the laryngeal cartilage shaped

like a shield and hence known as the thyroid cartilage. The two lamina of

this cartilage meet in the midline at an angle of about 90 degree in men

and 120 degrees in women. The superior part of the fused lamina is some

γ shaped with a prominence in the midline that is easily seen and palpable

in males and is referred to as the ‘Adams Apple’. The posterior part of the

flayed lamina has elongated projections both superiorly and inferiorly called

68

the superior cornu and the inferior cornu of the thyroid cartilage

respectively.

The long curved superior cornu is attached to the hyoid bone by the

lateral thyrohyoid ligament. The smaller and thicker inferior cornu is

articulates with the cricoid cartilage inferiorly. The thyrohyoid membrane is

attached to the superior margin of the lamina on either side with thick

condensation in the midline. The cricothyroid membrane is attached to the

inferior margins of the lamina.

On the outer surface of the lamina, is a prominent oblique line that runs

from the base of the superior cornu posteriorly, to the anterior part of the

inferior margin of the thyroid lamina below. This oblique line gives

attachments to the extrinsic muscles of the larynx such as the thyrohyoid,

sternothyroid and the inferior constrictor.

The inner surface of the thyroid lamina is smooth and gives rise to

many ligamentous and muscular attachments. Thyroepiglottic ligament is

attached from the midline to the epiglottis, below this in the midline is the

attachment of the Vestibular ligament or the false vocal cord. This ligament

raises a thick fold of mucous membrane known as the vestibular fold which

is attached to the upper lateral surface of the arytenoid cartilage

posteriorly. The Vocal ligament or the true vocal cords is attached just

below the level of vestibular ligament and attached posteriorly to the vocal

process of arytenoid. This ligament raises a mucosal fold known as known

as vocal fold. This ligament is the superior condensation of the cricothyroid

ligament or the conus elasticus.

69

Arytenoid cartilages: This small paired, pyramidal shaped cartilage

articulates with the facet on the superior part of the posterior lamina of the

cricoid cartilage. The arytenoid cartilage has two projections. The anterior

projection is the vocal process which is attached to the vocal ligament

covered by mucosal fold the vocal cord. The lateral projections called the

muscularis process gives attachment to the Posterior crico-arytenoid and

lateral crico-arytenoid muscles. Between the vocal and muscular processes

is the antero-lateral surface of the arytenoid cartilage. This surface is

irregular and is divided into two fossae by a crest. The upper triangular

fossa gives attachment to the vestibular ligament and the lower to the

thyroid-arytenoid muscle or the vocalis muscle and lateral cricoarytenoid

muscles.The apex of this cartilage curves backwards and articulates with

corniculate cartilages. The posterior surface of arytenoid cartilage gives

attachment to transverse arytenoid muscle.

Corniculate and cuneiform cartilages: These two small cartilages

are seen within the superior surface of the aryepiglottic fold. The

corniculate cartilages are small conical nodules of fibro elastic cartilage that

articulates with the apex of arytenoid cartilage. These cartilages provide

rigidity to the aryepiglottic fold and prevent it from collapsing during

respiration.

Cricoid cartilage: The term cricoid is derived from the Greek word

‘krikoeides’ meaning "ring-shaped". This is the only cartilage which forms a

complete ring in the entire respiratory passage. It is shaped like a ‘signet

ring’ (Figure -8).

70

Figure – 8. Shows the oblique, posterior and superior view of the

cricoid cartilage.

From the figure - 8 it can be seen that the anterior part of the cricoid

is narrow and is called the arch of the cricoid. The posterior part of cricoid

is longer and flatter and hence called the posterior lamina of the cricoid.

The superior part of the posterior surface of the cricoid cartilage is broader

and has two facets for articulation with the arytenoid cartilage. A facet for

the articulation of the cricoid with the thyroid cartilage is present on the

lateral border of the posterior lamina. On the posterior surface of the

posterior lamina of the cricoid is a vertical ridge in the midline which gives

attachment to the longitudinal muscle of the oesophagus. On both side of

71

this ridge is the site of origin of the posterior cricoarytenoid muscles which

is attached to the muscularis process of the arytenoid. The inferior surface

of the cartilage is attached to the first tracheal ring by a thick cricotracheal

ligament. The cricothyroid muscles arises on the outer, anterior surface of

the arch of the cricoid cartilage and are attached to the inferior border of

the thyroid lamina on both sides, the thick cricothyroid membrane

separates these muscle in the midline.

The cricoid being the only complete cartilaginous ring is not

expandable. The subglottic lumen which corresponds to the cricoid

cartilage is the narrowest part of the airway in infants and children. Injury to

this region results in oedema, ulceration, perichondritis and later SGS. A 3

- dimensional histological and pathological whole organ serial sections of

adult human beings where studied and the following observations were

made. The cricoid area is located along the superior portion of the cricoid

arch on both sides. It is surrounded and limited by the perichondrium of the

cricoid cartilage on all sides. The conus elasticus and the fibrous layer of

the subglottic mucosa is more prominent on the lateral surface it contains

loose areolar tissue, that is mainly composed of adipose tissue and loose

elastic and collagenous fibres. Many vessels are present in this region with

a superficial branch of the cricothyroid artery running through it. These

vessels penetrated the antero-inferior portion of the conus elasticus and

extended into the prelaryngeal region (Sato et al., 2002). This loose areolar

tissue and increased vasculature in the subglottic region along with the

negative subglottic pressure during inspiration result in rapid development

72

of oedema with inward collapse of the subglottic tissue resulting in acute

airway obstruction.

Knowledge of size, variations in size, and configuration of the cricoid

cartilage is vital to understand, prevent and treat intubation induced SGS.

Joshi et al. (2011) in a cadaveric study from western India had observed

that 46% of adult Cricoid cartilage was ovoid, 38 % was oval in shape as

shown in table - 7.

Table - 7. The different shapes of cricoid as observed by Joshi et al.,

(2011).

Sr. No. Shape Total number n=50

Percentage %

1. Ovoid 23 46

2. Oval 19 38

3. Pear shaped 6 12

4. Narrow oblong 2 04

The dimensions of the various parameters of the adult cricoid

cartilage as reported by various authors is summarised in table - 8.

73

Table - 8. The dimensions (mm) of the cricoid cartilage in adults as reported by various authors. S

l N

o

Pa

ram

ete

r

Jo

sh

i et

al. (

201

1)

Ha

rje

et

an

d

Sa

ha

ni,

(200

2)

Ajm

an

i

(199

0)

Ec

kel

et

al.,

(199

4)

Jain

an

d D

hall,

(200

8)

Zeli

nski,

(200

1)

Ra

nd

es

tad

,

Lin

dh

olm

an

d

Fab

ian

, (2

00

0)

1. Height of arch 6.54±1.23

6.13±1.02

8.35±4.30

6.90±1.35

6.00±0.08

8.82±1.4

-

2. Height. of lamina 21.45±1.97

18.59±1.68

26.50±6.30

24.60±1.84

22.00±0.19

27.95±3.17

-

3. Transverse

diameter

18.33±2.26 16.53±1.04

29.84±6.10

26.40±2.40

17.20±0.36

- 11.6(F)

8.9-17.0(F)

15 (M)

11.0-21.5 (M)

4. Antero-posterior

diameter

19.29±2.47

20.22±1.65

28.82±4.07

30.90±3.06

19.50±0.25

-

74

The distance between the two inter articular facet of the crico-

arytenoid joint was 10.3 mm (7.4 mm to 13 mm ) in females and 12.6 mm

(8 mm to 18.2 mm) for males (Randestad, Lindholm and Fabian, 2000).

They also noted that the internal diameter and the inter-articular distance of

the cricoid cartilage sometimes is so small, especially in women that it

would not accommodate the usual 7 mm size endotracheal tube during

intubation without mucosal damage.

The larynx is divided into three parts i.e. the supraglottis, glottis and

the subglottis. The subglottic region is of great clinical relevance and its

three-dimensional region, it is cylindrical and becomes smaller in the

cranial direction following the elastic cone, cranially it extends upto the

inferior arcuate line of the vocal cord and the inferior margin is formed by

the lower border of cricoid cartilage. The ventral boundary is formed by the

median part of the cricothyroid ligament and the dorsal part by the cricoid

lamina.

In a prospective study on 52 subjects, we measured the internal

dimensions of the subglottis and upper trachea in adult Indian population

using CT scan. We had observed that the mean coronal diameter (CD) of the

subglottis and upper trachea of adult Indian male ranged from 13.18 mm to

17.68 mm. The minimum CD recorded was 9.3 mm at 5 mm below the level of

glottis and a maximum of 22.8 mm at 30 mm below the glottis. The mean

sagittal diameter (SD) ranged from 15.87 mm to 18.02 mm. The minimum SD

recorded was 10.1 mm at 5 mm below the glottis and 26.1 mm at 50 mm

below the glottis. The average intraluminal circumference ranged from 48.82

75

mm at the subglottis 5 mm below the glottis and a maximum of 54.96 mm at

30 mm below the glottis. It has been observed that the diameter and

circumference is least at the subglottis which increases up to the level of 30

mm from the glottis followed by a marginal narrowing at 40 mm both in the

coronal and sagittal plane. The level of 40 mm below the glottis is

approximately at the thoracic inlet and during deep inspiration there is

maximum pressure in this area and hence the narrowing. The mean CD of

adult Indian female ranged from 8.7 mm to 15.34 mm, with a minimum

recording of 8.2 mm at 5 mm and a maximum of 21mm at 70 mm from the

glottis. The mean SD ranged from 8.3 mm to 14.33 mm, a minimum SD

recorded was 7.7 mm at 5 mm from the glottis and a maximum of 19.8 at 70

mm. The average intraluminal circumference ranged from 36.5mm at 5 mm

and a maximum of 43.05 at 70 mm. It has been observed that the mean

subglottic and tracheal internal dimension of adult Indian population is less

than that reported in western literature. This article is attached in the Appendix

-2 for further reference (Prasanna and Ravikumar, 2014).

76

5.1. Infant larynx, how does it differ from the adult?

The advances in the management of neonatal airway problems call for

a sound knowledge of the neonatal airway. The larynx of the infants and

children differ in many respects from that of the adults. Eckenhoff (1951)

observed that direct laryngoscopy in infants was more difficult because:

a) The larynx of the infants and children is more cephalic than in

adults,

b) The infant tongue is more bulky and larger than adults and owing to

the higher level of the hyoid bone it depresses the epiglottis

posteriorly,

c) The epiglottis of the infant is short, stiff and U shaped.

Hudgins et al. (1997) observed that the paediatric larynx differed from

the adult larynx in 4 ways i.e. position, size, shape and consistency as

discussed below.

5.1.1. Position of infant larynx

At birth the lower border of cricoid is at the level of the lower border of

the 4th cervical vertebra. At 6 years it is at the lower border of the 5th cervical

vertebrae and at the 13th year it lies at the lower border of the 7th cervical

vertebrae (Piersol, 1930, pp 1828). The hyoid bone was observed to be at the

level of the 2nd and 3rd cervical vertebra in children below 2 year (Hudgins et

al., 1997). The hyoid and the larynx is suspended higher up and because of

the short thyrohyoid membrane the upper margin of the thyroid cartilage lies

77

immediate posterior or just inferior to the hyoid bone high up in the neck. Thus

the proposition of the cervical trachea in infants is more than that in adults.

About 10 rings of the trachea lie in the cervical region in infants as opposed to

about 4-6 rings in the adults (Grillo, 2004, pp. 40–59). Like the non-human

primate the tip of the epiglottis is just behind the soft palate opening directly

into the nasopharynx in infants (Laitman, 1970, pp.1164–1173). In addition

this position of the epiglottis and the inclination of the cricoid lamina place the

axis of laryngeal inlet at an angle that is away and posterior from the axis of

the trachea (Eckenhoff, 1951). This angulation and position of the larynx in

infants is useful to prevent aspiration when the infant is sucking milk and aids

in respiration. Both of these processes occur at tandem. This position also

explains why infants are obligate nasal breathers.

5.1.2. Size and shape of the infant larynx

Eckel et al. (1999) studied the morphology of the larynx in 43 children

aged 1month to 60 months old. A plastinated whole organ serial section of the

larynx was used for morphometric study. They observed that the cartilaginous

part of the vocal cord accounted for 60% - 70% of the length of the vocal

cords in children less than 2 years of age, this feature was similar to that

observed in mammals. In contrast, the vocal process of the adult larynx

accounts for 1/3rd the length of the vocal cord. The subglottis airway rapidly

increased in size up to 2 years of age and later had a linear growth and there

was no difference in the measurements between the two genders in infants

and children.

78

Fayoux et al. (2008) measured the dimension of the laryngeal

cartilages and trachea immediate post-mortem in 274 foetuses that were

between 15 weeks - 41 weeks of gestational age and in 26 infants. These

measurements were then statistically analysed to evaluate the correlation with

the biometric data such as the corrected gestational age, crown heel length,

weight and head circumference. The authors found that the correlation

coefficient for each anatomical region was close to the biometric parameter

that was considered. The correlation was more relevant for the corrected

gestational age. Cricoid diameter was always less than the tracheal diameter

and the inter-arytenoid distance. There was no significant difference between

the antero-posterior and lateral diameter of the cricoid. The antero-posterior

diameter of the trachea was found to be significantly less than the lateral

diameter. There was no significant difference for the measurements of the

subglottis and tracheal diameter, cricoid height, tracheal length, and glottis

length among male and female gender. However, there was a significant

difference for measurements of the thyroid alar height, width, median anterior

thyroid height and interarythenoid distance. Similar observation of linear

correlation between cartilaginous measurements and gestational age was

made by Schild (1984) & Kalache (1999).

The initial studies have described the infant larynx to be funnel shaped

or conical shaped and the narrowest part of the larynx is at the cricoid

cartilage, this shape becomes more cylindrical as the child grows (Bayeux,

1897; Eckenhoff, 1951; Butz, 1968; Wailoo and Emery, 1982; Hudgins et al.,

1997). Litman et al. (2003) analysing data obtained by measuring the

79

dimensions of the larynx in unparalysed sedated children using MRI observed

that the narrowest part of the larynx is at the vocal cord and immediate sub-

vocal cord level. They also observed that there is no relationship of these

dimension relative to the cricoid dimensions throughout childhood.

Dalal et al. (2008 & 2009) using video bronchoscopic technique

measured the internal dimensions and the cross-sectional area of the larynx

at the level of the glottis and the cricoid in 135 children aged between 6

months to 13 years. They observed that the overall cross-sectional area of the

cricoid was 48.9 +/- 15.5 mm2 and that at the glottis level was 30 +/- 16.5

mm2. The glottis was found to be narrower than that of the cricoid region.

There was no gender difference in the mean values. They found a positive

correlation existed between the glottis and cricoid cross-sectional area and

the age, weight and height. Masters et al. (2006) measured the large airway

size using a video bronchoscope in 125 children. They analysed the

relationship between the airway sizes with other factors that could influence

the airway dimensions in children. It was observed that the cricoid, best

correlated with the body length. The diameters of the large airway

progressively increase in size and maintained a constant relation to the cricoid

but are gender independent throughout childhood.

Savkovi et al. (2010) studied the post-mortem specimens of infant and

children larynx for morphological and histological analysis. The length of the

larynx from the superior thyroid notch up to the lower border of the cricoid was

found to be 11.9 +/-0.3 mm. The width of the larynx between the two superior

cornu of the thyroid was found to be 17.7 +/-0.5 mm. The thickness of the

80

larynx measured from the superior thyroid notch to the upper margin of the

lamina of the cricoid cartilage was 12.6+/-0.4 mm. All the above 3 parameters

correlated well with the body length of the infants. The average value of the

measurements of individual cartilages as reported by the authors is shown in

table - 9.

Table - 9. Dimensions of the laryngeal cartilage in infants (Savkoviet

al.,2010).

Dimension measured Average with SD of the measured value in mm

Ventral diameter of the thyroid cartilage

From the superior thyroid notch to the inferior part in midline

6.79 +/-0.23

Height of the arch of cricoid 2.54+/-0.6

Height of the posterior lamina of the cricoid cartilage

9.01+/-0.19

Inner sagittal diameter of cricoid cartilage

5.96 +/- 0.18

Inner transverse diameter of cricoid cartilage

6.44 +/-0.15

External sagittal diameter of cricoid cartilage

8.85+/- 0.21

External transverse diameter of cricoid cartilage

9.81 +/- 0.20

Height of the arytenoid cartilage ( right ) 5.78 +/- 0.15

Height of the arytenoid cartilage ( left ) 5.73 +/- 0.14

Basal width of arytenoid cartilage (both)

5.82 +/-0.13

Width of the epiglottis 8.92 +/- 0.3

Length of epiglottis 13.15 +/-0.44

81

Histological analysis of all the cartilage was done by the same team

(Savkoviet al., 2010). They have observed that the laryngeal cartilages are of

hyaline type at birth and differentiation occurs later in life. The thyroid cartilage

is hyaline and covered with a thin layer of mucosa. The cricoid cartilage is of

hyaline type and is covered with thicker mucosa compared to the thyroid

cartilage. The mucosa has multiple secretory glands. A layer of elastic and

collagen fibres are seen between the mucosa and the cartilage. The arytenoid

cartilage is of hyaline type. The mucosa is rich in serous glands and the

submucosa has thick layer of fibrous tissue. The epiglottis is an elastic

cartilage, has more elastin as compared to that of adults hence it is softer

(Savkovi et al., 2010).

5.2. Comparative anatomy of infant and rabbit larynx

Rabbits have been used in many experimental models for research in

the laryngotracheal airway. Some reasons why this animal is preferred to

others is that they are docile, easy to handle, ease of administering

anaesthesia and can be accommodated in common cages that do not require

too much of space. The cost of procuring and maintenance is very less

compared to the large animals. In our institution most of the animal

experimental studies are being done on small animals and our technicians are

very well trained to handle and pick up early warning signs of deterioration of

health of the animals.

82

The anatomy of the rabbit larynx is in many ways similar to that of

infants and children upto 2 years of age. The epiglottis, a valve like fold that

guards the entrance of the larynx projects into the oropharynx past the soft

palate making the rabbits an obligate nasal breather as in infants. The thyroid

cartilage (cartilago thyreoidea) is the largest cartilage which is saddle shaped

and has two lamina. It has a superior cornu which is connected to the hyoid

bone and an inferior cornu that articulates with the cricoid cartilage below.

Similar to the humans the strap muscles sternothyroid, thyrohyoid are

attached to the longitudinal ridge of the thyroid cartilage. The cricoid cartilage

(cartilago cricoidea) is annular and a complete ring. The ventral portion of the

cricoid is an arch and is situated some distance from the thyroid cartilage. The

intervening soft tissue comprises the cricothyroid membrane and the

cricothyroid muscle. The lateral part of the cricoid slants obliquely antero-

dorsal and expands into a dorsal portion, the lamina of the cricoid. The lamina

of the cricoid cartilage forms the larger part of the dorsal wall of the larynx.

The paired arytenoid cartilages (cartilaginous arytenoideae) lie obliquely one

on each side of the anterior tip of the cricoid plate, closely articulated with its

margin. Each appears curved and somewhat irregularly pear-shaped in dorsal

view, tapering to a point antero-medially. From the lateral angle of the broader

posterior end there is a prominent ventral projection for the attachment of one

end of a vocal fold. The corniculate cartilages (cartilagines corniculatae) are

minute, slender, curved bodies composed of very flexible elastic cartilage and

borne on the apices or anterior extremities of the arytenoid cartilages. Each

projects craniad and lies in the dorsal end of the fold of mucous membrane

83

extending to the edge of the epiglottis and forming the margin of the opening

from the pharynx. These cartilaginous frame works along with the ligaments

and muscles of the larynx are similar to the infants and children. The vocal

cords though rudimentary are attached to the thyroid cartilage ventrally and to

the arytenoid dorsally (Craigie, 1948).

Garrett, Coleman and Reinisch (2000) in their interspecies comparative

study of vocal fold vibrations have observed that the vocal fold layers of

canine larynx were not similar to that of humans. Though both have three

layers of lamina propria, the surface layer of the lamina propria in humans is

composed mostly of ground substance. In canines it is made up of thin sheets

of collagen and elastin above loose ground substance in their surface layer.

Thus they observed that these differences could present difficulties especially

in experimental studies that deal with tissue pathologies such as cysts or

scars. Histologically, the glottis of the rabbit has been found to be similar to

that of humans. The lamina propria and the thyroarythenoid muscle of the

rabbits are similar to the humans (Kurita, Nagata and Hirano, 1995).

Thibeault et al. (2002) has observed that the lamina propria of the rabbit

larynx is similar to humans. Rabbits have a superficial layer over an

intermediate layer of ground substance and collagen and a third deeper layer

of collagen which is similar to that of human. Thus many authors have found

the rabbit larynx useful in ex-vivo and in-vivo experimental studies of the vocal

fold. (Flint, Corio and Cummings, 1997; Thibeault et al., 2002; Hertegård et

al., 2002; Maytag et al., 2013).

84

Loewen and Walner (2001) have studied the larynx and trachea of 35

New Zealand white rabbits weighting 2.3 Kgs – 5.1Kgs. The larynx and

trachea was harvested from epiglottis above and to 2 cm below the cricoid

cartilage in most of the animals and in one animal up to the main stream

bronchi below the carina. These specimens were cut at the desired level and

the diameter of the cross-sections was measured using a slide calliper. The

measurements were taken at the level of cricoid cartilage and at the 8th

tracheal cartilage. They found the overall dimension of the airway at the level

of the cricoid was 5.81mm ventro-dorsal (range 5.25mm- 6.75mm) and

5.41mm lateral (range 3.5mm- 5.25mm).

The overall dimensions at the level of the 8th tracheal cartilage was

4.71 mm ventral-dorsal (range 3.75- 5.5 mm) and 5.92 mm laterally (range

4.75- 7.0 mm). These dimensions are similar to that of neonates. Thus these

authors have recommended that rabbits serve as a good model for

experiments relating to the subglottis and can sustain operations with little

morbidity and mortality comparable to the human counterpart.

Diagram of airway measurements taken by Loewen and Walner (2001)

described above are shown in the figure - 9. LoT- longitudinal trachea; LaS-

lateral subglottis; VDS-ventral-dorsal subglottis; LaT- lateral trachea at the 8th

ring; VDT- ventral-dorsal trachea; V -ventral; D- dorsal (Loewen and Walner.

2001)

85

Figure - 9. Diagram of airway measurements in rabbits.Adapted from

Loewen and Walner (2001).