What structure is found midline in the trachea area just beneath the mandible?

Hyoid bone

The hyoid bone is a smallU-shaped bone in the neck between the larynx and the mandible. It has an anterior body of hyoid bone and two large greater horns, one on each side, which project posteriorly and superiorly from the body (Fig. 8.250). There are two small conical lesser horns on the superior surface where the greater horns join with the body. The stylohyoid ligaments attach to the apices of the lesser horns.

The hyoid bone is a key bone in the neck because it connects the floor of the oral cavity in front with the pharynx behind and the larynx below.

Hyoid bone and laryngeal cartilages

The hyoid bone lies in the midline at the front of the neck at the level of the fourth cervical vertebra (Badshah et al 2017). It is suspended from the styloid processes by the stylohyoid ligaments and gives attachment to the suprahyoid and infrahyoid groups of muscles. The skeletal framework of the larynx is formed by a series of cartilages interconnected by ligaments and fibrous membranes and moved by a number of muscles (Ch. 41). The laryngeal cartilages are the single cricoid, thyroid and epiglottic cartilages, and the paired arytenoid, cuneiform, corniculate and tritiate cartilages.

The larynx with the pharynx, hyoid bone and trachea

In the side view in A the lateral lobe of the thyroid gland (20) has been displaced backwards to show the part of the origin of the inferior constrictor (17) that arises from the tendinous band (18) over cricothyroid (19). The lingual artery (1) lies just above the tip of the greater horn of the hyoid bone (2) and then passes deep to the posterior border of hyoglossus (3). The internal laryngeal nerve (30) runs just below the tip of the hyoid and pierces the thyrohyoid membrane (29) with the superior laryngeal branch (28) of the superior thyroid artery (27), behind which runs the external laryngeal nerve (16). Much of thyrohyoid (12) has been removed to show part of the origin of the inferior constrictor (17) from the lamina of the thyroid cartilage (13).

In B with the thyroid gland taken away, the lowest (cricopharyngeus) part of the inferior constrictor has been removed to show the recurrent laryngeal nerve (23) passing up behind the cricothyroid joint (34).

In C all muscles, vessels and nerves have been removed to display the thyrohyoid membrane (29), the cricothyroid membrane (38 and 39), and the cricotracheal ligament (40) attached to the first tracheal ring (41, which is here unusually broad).

In D most of the internal jugular vein (43) has been removed and the common carotid artery (44) has been displaced backwards to show the inferior thyroid artery (25) and recurrent laryngeal nerve (23).

Cartilages of the larynx:

Unpaired—thyroid, cricoid, epiglottic

Paired—arytenoid, corniculate, cuneiform

Joints of the larynx: cricothyroid (B and C, 34), crico-arytenoid (page 192, E42), arytenocorniculate (page 192, E44).

Membranes and ligaments of the larynx:

Extrinsic—thyrohyoid membrane (C29), hyo-epiglottic and thyro-epiglottic ligaments, cricotracheal ligament (C40).

Intrinsic—quadrangular membrane (page 192, D37), whose upper margin forms the aryepiglottic fold (page 192, A3) and lower margin the vestibular (false vocal) fold (page 192, D28); cricothyroid ligament, whose upper margin (the vocal ligament,page 192, D32) forms the anterior part of the vocal fold (vocal cord). See notes onpage 193.

Theextrinsic muscles of the larynx (those connecting it to surrounding structures) can be divided into elevators and depressors—those directly attached to the thyroid and cricoid cartilages and which raise the larynx, e.g. during swallowing, and those that return it to the normal position:

Theintrinsic muscles of the larynx move the vocal folds and alter the shape of the laryngeal inlet, and can be classified according to their main effects on the folds or the laryngeal inlet; i.e. they alter the shape of the rima of the glottis (the gap between the vocal folds of each side), or have a sphincteric action on the inlet:

The vocalis part of the thyro-arytenoid may tighten segments of the vocal fold (as when singing a high note).

The thyroepiglottic, aryepiglottic and oblique arytenoid muscles constrict the inlet; their relaxation restores the normal shape.

Cricothyroid (B19 and 31) is the only external intrinsic muscle of the larynx; it is easily seen on the outside of the larynx in dissections of the front of the neck (as onpage 122, A9). The other intrinsic muscles are all inside the larynx and are only seen when the larynx itself is dissected (page 174).

The intrinsic muscles of the larynx are all supplied by the recurrent laryngeal nerve (A and B, 23) except for cricothyroid, supplied by the external laryngeal nerve (A and B, 16).

The mucous membrane of the larynx above the level of the vocal folds is supplied by the internal laryngeal nerve (A30), and below the vocal folds by the recurrent laryngeal nerve (A and B, 23).

The internal laryngeal nerve (A30) first enters the pharynx by piercing the thyrohyoid membrane (A29), and from there fibres spread into the larynx.

The recurrent laryngeal nerve (B23) lies immediately behind the cricothyroid joint (B34;page 192, B11) and enters the larynx by passing deep to the lower border of the inferior constrictor of the pharynx (A17).

Clinical Implications

Patients may be referred for cross-sectional imaging with either a midline neck mass ± lateral neck mass(es) from adenopathy. When protocoling such a study, it is important to remember that, if DTCa is a possible cause, consideration should be given to US, MR, or even NECT rather than CECT. Iodinated contrast can delay therapeutic ¹³¹I up to 6 months. Clinical indicators of possible thyroid cancer include young women with neck masses, particularly low neck masses and/or cystic lymph nodes, and masses associated with vocal cord paralysis.

There are 3 main considerations for a rapidly growing VS mass : (1) Hemorrhage or cystic degeneration of thyroid adenoma, (2) anaplastic thyroid carcinoma, and (3) thyroid lymphoma. The latter 2 lesions can appear quite similar on imaging, although lymphoma is more frequently a homogeneous lesion. Calcifications, cystic change, and hemorrhage are much less common in lymphoma than anaplastic carcinoma, which is typically heterogeneous and has a greater tendency to invade the trachea.

When imaging is required for preoperative evaluation of the complete extent of a multinodular goiter (MNG), 2 considerations must be kept in mind: (1) The scan is performed with the patient's arms by his or her side so as not to exaggerate the substernal extension that occurs with the patient's arms positioned over their head, and (2) up to 5% of MNGs harbor a focus of DTCa. While most often these are small foci that have not metastasized, the neck should be carefully evaluated for adenopathy and any invasive features of the thyroid contours that might make surgery complex.

Hyoid bone

Superficial layer, DCF

Thyroid cartilage

Cricoid cartilage

Thyroid gland

Visceral fascia middle layer, DCF

Manubrium

Prevertebral fascia (deep layer, DCF)

Alar fascia (deep layer, DCF)

Retropharyngeal space

Danger space

Visceral fascia (middle layer, DCF)

Esophagus

Trachea

Sagittal graphic illustrates the craniocaudal extent of the visceral space (VS) in the anterior aspect of the neck. At the hyoid bone, the superficial and middle layers of deep cervical fascia (DCF) insert. The superficial layer encloses the strap muscles of the anterior neck and sternocleidomastoid muscles of the lateral neck. These muscles surround but are separate from the VS. The middle layer of DCF surrounds the VS. The larynx and cervical trachea and the hypopharynx and cervical esophagus form longitudinal columns within this space from the hyoid to the mediastinum.

Axial graphic depicts the thyroid gland in anterior VS wrapping around the trachea

Axial CECT shows tracheal adenoid cystic carcinoma invading the thyroid gland

Clinical Implications

Small lesions of the RPS are typically not evident on clinical examination. It is only when lesions become significantly enlarged that bulging of a posterior pharyngeal wall is evident. It cannot be emphasized enough that the radiologist must consider the possibility of RPS metastatic nodal disease in all H&N cancer patients and must methodically search along the medial aspect of the cervical ICA. Nonnecrotic RPS nodes are more difficult to discern on CECT than MR, so vigilance is key.

Toxic patients with H&N infections, such as pharyngitis or tonsillitis, may be imaged to exclude the development of a, RPS abscess. This is a difficult clinical diagnosis because physical examination may not be fruitful. Thus, clinicians must rely largely on a high degree of clinical suspicion. The radiologist must exclude RPS abscess, or, if 1 is found, must delineate the entire craniocaudal extent and specifically exclude mediastinal involvement. Large abscesses may result in airway compromise, though it is rare that a patient presents with airway symptoms secondary to an RPS mass.

Hyoid bone

Superficial layer, deep cervical fascia

T2 vertebral body

Visceral fascia (middle layer, deep cervical fascia)

Esophagus

Superior limit RPS + DS: Fasciae insert to central skull base

Prevertebral fascia (deep layer, deep cervical fascia)

Alar fascia (deep layer, deep cervical fascia)

Retropharyngeal space

Danger space

Inferior limit true RPS: Alar fascia merges with visceral fascia

Sagittal graphic shows the deep cervical fascia (DCF) layers, which determine and delineate the contours of the retropharyngeal space (RPS). The anterior contour of the RPS is defined by the visceral fascia, the middle layer DCF, which separates the RPS from the pharyngeal mucosal space of SHN and visceral space of the IHN. The posterior contour is formed by the prevertebral fascia (deep layer DCF). The alar fascia (also deep layer DCF) anatomically defines an anterior true RPS and more posterior danger space (DS), although this delineation is not typically evident at imaging. Inferiorly, the alar fascia blends with the visceral fascia at approximately T3, while superiorly the middle and deep layers of the DCF insert to the central skull base.

Sagittal CECT MPR demonstrates an enhancing soft tissue mass

Publisher Summary

The hyoid bone is an intermediary between the skull and postcranial skeleton. It combines skeletal elements of the second and the third pharyngeal arches associated with the gills of primitive fish. The hyoid bone is located in the neck and can be palpated immediately above the thyroid cartilage. It is suspended from the tips of the styloid processes of the temporal bones by the stylohyoid ligaments. The hyoid gives attachment to a variety of muscles and ligaments that connect it to the cranium, mandible, tongue, larynx, pharynx, sternum, and shoulder girdle. The hyoid ossifies from six centers: two for the body and one for each of the greater and lesser horns. In most modern vertebrates, the vertebrae have replaced the notochord as the principal means of support for the central part of the body. The sacrum and coccyx are vertebrae, but they function as the parts of the bony pelvis. This chapter describes the twenty-four movable vertebrae—the seven cervical, twelve thoracic, and five lumbar vertebrae. It introduces part common to most vertebrae.

3.1 HYOID MYOTOMY (SUSPENSION)

Riley et al.22 first reported a skeletal approach to hypopharyngeal obstruction by using an adaptation of the laryngeal suspension procedure originally developed to minimize aspiration in patients who had had a supraglottic laryngectomy. In their original description, the infrahyoid musculature was removed from the hyoid bone, which was then suspended from the anterior mandibular arch using fascia lata. The technique was later simplified23 by approximating the hyoid bone antero-inferiorly to the thyroid cartilage (Figs 44.1 to 44.5). Essentially, the hyoid is releasedfrom its inferior attachments and advanced anteriorly and inferiorly over the thyroid cartilage. This applies tensionto the hyoepiglottic ligament, enlarging the hypopharynx (see figures).

1 Bones of the Larynx

The hyoid bone (Fig. 1-7) suspends and anchors the larynx during respiratory and phonatory movement. It is U shaped, and its name is derived from the Greek word hyoeides, meaning shaped like the letter upsilon. It has a body, which is 2.5 cm wide by 1 cm thick, and greater and lesser horns (cornu). The hyoid does not articulate with any other bone. It is attached to the styloid processes of the temporal bones by the stylohyoid ligament and to the thyroid cartilage by the thyrohyoid membrane and muscle. Intrinsic tongue muscles originate on the hyoid, and the pharyngeal constrictors are also attached there.4,12,34