2.1 Introduction

The dentomaxillofacial radiology covers not only stomatognathic system but also surrounding structures including oral and nasal cavity, paranasal sinuses, salivary glands, and a majority of neck structures as well as nerves and vessels that supply them. Their development starts at three and a half weeks of fetal life and is related to ectodermal oral stomodeum that unites with endoderm to form the buccopharyngeal membrane. Its rapture finally connects the future oral cavity with the foregut and at gestational sixth week the dental lamina could be seen and teeth development starts. Connection with pharyngeal arches and cranial bones finally forms a complicated apparatus that highly coordinated mastication, and is also involved in deglutition, phonation, respiration, and other activities. Since the book is dedicated for an ultrasound diagnostic, the anatomical description will focus mostly on soft tissue structures that are important for routine clinical practice. For detailed morphological and topographical explanations, proper anatomical textbooks—used to prepare the current chapter—are recommended [1–5].

2.1.1 The Skeleton of the Head and Neck

Anatomically, the skeleton of the head and neck comprises of the skull, mandible, hyoid bone, and all seven cervical vertebrae. Functionally, the upper part of thoracic cage, including manubrium of the sternum, upper ribs, and thoracic vertebrae as well as the clavicle and scapula are also added due to the close connection with major muscles of the neck. It should be also pointed out, that radiologically the computed tomography (CT) or magnetic resonance imaging (MRI) of the neck always ends on the level of the upper sternal joint (synchondrosis), not only to see the lower attachment of the cervical muscles but also due to the fact that primary lymph nodes of some of the cervical structures are located posteriorly to manubrium of the sternum, in the upper mediastinum (group VII, see below).

Since most elements of the skull and cervical vertebrae are formed by the bones they are not well-visible during the US examination, they will not be explained below. However, during an early period of human life, membranous and cartilaginous structures such as fontanelles allow to exam the neonatal brain and other intracranial elements using the modality. In adults, the access is limited only to selected so-called acoustic windows (i.e., transtemporal, submandibular, transorbital, suboccipital) but such transcranial Doppler ultrasound (TCD) is performed mostly by neurologists [6, 7].

2.1.1.1 Temporomandibular Joint

The only skeletal structure of the head that is routinely examined ultrasonographically is the temporomandibular joint (TMJ). It is a bilateral synovial joint that contains distinguish (articular surfaces, cavity, and capsule) and accessory elements (disc, ligaments). The head of the joint is formed by the head of the condylar mandibular process while (Fig. 2.1) the socket by the mandibular/glenoid fossa and articular tubercle/eminence of the squamous part of the temporal bone (Fig. 2.2). Unlike most of the synovial joints, articular surfaces of TMJ are covered by fibrous not hyaline cartilage. The border of the socket forms also the upper attachment of the capsule. The lower one is located on the neck of the condyle, slightly away from the cartilage. The capsule is formed by the external fibrous layer and internal synovial one, responsible for synovial fluid secretion. The capsule and articular surfaces surround articular cavity that contains synovial fluid and articular disc and some of the intracapsular ligaments. The disc forms a movable socket for the mandible and divides the joint cavity and synovial membrane into upper and lower compartments. The upper one (1.2 cm³) is responsible for the gliding movements protrusion and retrusion (translation between condyle-disc complex and articulate eminence), while the lower one (0.5–0.9 cm³) for hinge moments (rotation of the head). The disc itself is an avascular oval (10 × 20 mm) and biconcave fibrous plate (1–3 mm thickness). Its anterior border splits into upper and lower lamellae, that attach to the anterior border of the articular eminence and neck of the condyle, respectively. The space between them is penetrated by the tendineal fibers of the lateral pterygoid muscle. Posteriorly, the disc is attached to the fibrous capsule and limits the retrodiscal region formed by its highly vascular and innervated loose connective tissue. Both, medial and lateral borders of the disc give origin to the medial and lateral disco-condylar ligaments that are attached to proper surfaces/poles of the condylar process and stabilize the disc during protraction and retraction. The joint is also supported by capsular (lateral, medial) and extracapsular ligaments (stylomandibular, sphenomandibular, retinacular). The lateral (temporomandibular) ligament runs from the zygomatic process of the temporal bone and the articular tubercle to the lateral side of the neck of the mandible and mix fibers with fibrous layer of the articular capsule. Due to the fibrous position, it is deviated into an outer oblique portion and an inner horizontal one. It prevents excessive retraction or moving backward of the mandible. The medial ligament is only a developmental variation of the sphenomandibular ligament, that runs from the sphenoid spine bone to the mandible lingula and limits an advanced protrusion. The most medially located fibers may exit the main complex and rich the medial aspect of the condylar process to form the medial ligament that supports the joint capsule. The recently described retinacular ligament arises from the articular eminence of the zygomatic process of the temporal bone and runs to the mandible, mix fibers with masseter fascia and retrodiscal tissue, transverse facial, and wall of superficial temporal veins [8]. It has been postulated that the ligament maintains blood circulation during the masticatory movements. The joint is supplied by the auriculotemporal and masseteric branches of mandibular branch of the trigeminal nerve. The blood comes mostly from the superficial temporal artery or less commonly from deep auricular artery, anterior tympanic artery, ascending pharyngeal artery, and maxillary artery. The blood is collected by the pterygoid plexus.

2.1.2 Muscles and Fasciae of the Head and Neck

As it was pointed above, all muscles except mimic expression ones are surrounded by fascia. Just below the skin there is a superficial cervical fascia formed by the subcutaneous tissue of the neck, that is located between the dermis and the investing layers of the deep cervical fascia. It contains various superficial vessels (arterial, venal, lymphatic), superficial lymph nodes, fat, and platysma. The deep cervical consists of investing/superficial, pretracheal, and prevertebral layers [9]. The most externally located part—investing fascia—is formed by superficial and deep laminas that completely encapsulate trapezius and sternocleidomastoid (SCM) muscles, and unites together between them as well as over their distal (superior nuchal line of the occipital bone, mastoid process of the temporal bone) and proximal attachment (manubrium of the sternum, clavicle, spine and acromion of scapula, spinal processes of cervical vertebrae, nuchal ligament). On the level of inferior border of a mandible, both layers split again and form a capsule for the submandibular and parotid salivary glands. The superficial layer runs over and covers external surface of the masseter muscle (masseter fascia) and on the level of zygomatic arch it changes the name again and become a temporal fascia that covers and forms attachment for the temporalis muscle. Both layers are separated also over the jugular notch of the manubrium of the sternum and surround potential—suprasternal—space. The middle aspect of the investing fascia is attached to the hyoid bone. Much thicker is a pretracheal layer of the deep cervical fascia. On the level of the neck, there is a trapezoid-shape muscular part that runs down from the hyoid bone, surrounds all infrahyoidal muscles, including omohyoid that limits the fascia laterally and visceral layers that surround thyroid gland, trachea, esophagus and extends into the fibrous pericardium and superiorly on the pharynx as a buccopharyngeal fascia. Its fibers bilaterally form small hooks that place the intermediate tendon of the digastric muscle to the hyoid bone, as well as the carotid sheaths that surround the cervical neurovascular bundle (internal jugular vein, common and internal carotid artery, carotid sinus and its nerve, vagus nerve—CNX). The prevertebral layer of the deep cervical fascia contains the cervical part of the sympathetic trunk, and runs from the base to the skull, covers prevertebral muscles of the neck, and unites with the anterior longitudinal ligament on the level of third/fourth thoracic vertebra and laterally with endothoracic fascia. On the cervical level, laterally it forms the axillary sheaths that enclose (subclavian/axillary vessels and brachial plexus). The prevertebral layer forms a posterior wall of the retropharyngeal space, while the anterior one is limited by buccolaryngeal and visceral part of the pretracheal fascia that covers the posterior aspect of the pharynx and remaining cervical viscera. Such potential space is divided into anterior and posterior (Danger) compartments by the alar fascia that runs from the base of the skull to the level of C7 [9, 10]. The alar fascia is formed by fibers of buccopharyngeal fascia that arises from pharyngeal raphe and runs laterally to the carotid sheaths.

Secondary to fascia position, the whole neck and oral part of maxillofacial part of the head and neck are divided into various spaces [10, 11]. From the clinical point of view the most important are (Figs. 2.3 and 2.4):

• Masticatory space that is divided by the zygomatic arch into the supra- and infrazygomatic parts. It contains masticatory muscles, as well as anterior and posterior division of CNV3 (mandibular n.), mandibular and pterygoid part if the maxillary artery, superficial part of pterygoid plexus, ramus and posterior part of the mandibular body, lower dental row. The space is limited anteriorly by the buccal space, posterolaterally by the parotid space, and medially by parapharyngeal space.

• Nasopharyngeal mucosal space is limited to nasal cavity and paranasal sinuses (see below).

• Oropharyngeal mucosal space is limited to oral cavity (see below).

• Pharyngeal mucosal space is limited to the pharynx (see below).

• Retropharyngeal or retrovisceral space as explained above is divided into anterior (proper) and posterior compartment (Danger Space). It is limited anteriorly to the pharyngeal mucosal space, posteriorly by the body of vertebrae C1-Th3/4, anteromedially by the carotid space, posteromedially by the parapharyngeal space. Usually, it is a potential space, but small vessels and lymph nodes could be seen there.

• Parotid space is limited by the parotid capsule and contains the gland, terminal division of external carotid artery, and CNVII—parotid plexus, retromandibular vein, intraparotid lymph nodes. It is located below external acoustic meatus, laterally to the parapharyngeal space, medially to subcutaneous tissue of the cheek, anteriorly to the carotid space and posterior belly of the digastric muscle, posteriorly by the masticator space.

• Carotid space surrounded by carotid sheath that contains internal jugular vein, common and internal carotid artery, carotid sinus and its nerve, vagus nerve—CNX, upper part of glossopharyngeal nerve (CNIX), part of cervical neuronal ansa, carotid sympathetic plexus, and some deep cervical lymph nodes (group II, III, IV). The upper part (suprahyoid) is attached anteriorly to the masticator space and parapharyngeal space, laterally to the parotid space, posteriorly to the perivertebral space while medially to the retropharyngeal space. In the lower part (infrahyoid), it is limited anteriorly and laterally by the anterior and posterior cervical space, respectively.

• Submandibular space is limited to the area located between mandible and hyoid bone, surrounded by superficial layer of the deep cervical fascia that forms its floor. It is limited anterolaterally by the mandible, medially by the anterior belly of digastric muscles, posteriorly by muscles of the tongue, superiorly by the mylohyoid muscle, and inferiorly by the hyoid bone. Some authors add submental space to it and in such case the anterior boundary does not exist. The submandibular space contains superficial part of the submandibular gland, submandibular lymph nodes (group Ib), cervical part of the facial artery and vein, hypoglossal nerve, and fat, the upper part of anterior jugular vein, and submental lymph nodes (group Ia) that are visible on the level of submental space.

• Buccal space is located between buccinator, platysma, masticatory muscles, and posterior aspect of the body of the maxilla. It contains retromaxillary fat pat, parotid duct and accessory parotid gland, facial and buccal vessels, buccal branch of the facial nerve (CNVII), buccal nerve of the mandibular nerve (CNV₃).

• Parapharyngeal space is a narrow space located laterally to the carotid sheath, anteriorly to the retropharyngeal space, and anteromedically to masticator space, medially to the parotid space. A lower part of the space may be called a retroesophageal space. It is divided by the rudimentary stylopharyngeal fascia (Zuckerkandl/Testut aponeurosis—band from styloid process to the tensor veli palatini) into pre-styloid and post-styloid parts, that is a synonym of the suprahyoid portion of the carotid space. It contains vagus n. (CNX), upper part of glossopharyngeal (CNIX), accessory (CNXI) with hypoglossal nn. (CNXII), internal carotid artery, internal jugular vein in the carotid sheath, sympathetic trunk and its superior cervical ganglion, ascending pharyngeal artery, deep cervical lymph nodes.

• Perivertebral space is limited anteriorly by retropharyngeal/danger space; posteriorly by thoracolumbar fascia attaches to spinous process and ligamentum nuchae; laterally by posterior cervical space. It is divided into prevertebral and two almost symmetrical paraspinal compartments. The first one is limited anteriorly by prevertebral layer of the deep cervical fascia and contains deep cervical (scalene) and prevertebral muscles, vertebral vessels, phrenic nerve, and supraclavicular part of the brachial plexus. The paraspinal compartments are surrounded by the thoracolumbar fascia and contain deep/intrinsic muscle of the back (some authors include also superficial/extrinsic muscle).

• Posterior cervical space extends from the mastoid process and base of skull to the clavicle. It is located between superficial and deep layer of deep cervical fascia, between SCM and trapezius muscles, limited also by the prevertebral and carotid space. The space contains the external/spinal branch of accessory nerve (CNXI), supraclavicular part of the brachial plexus, dorsal scapular nerve, spinal accessory lymph nodes (group V), and loose connective tissue.

• Anterior cervical space is a small infrahyoid compartment, that is only partially surrounded by fascia and limited posteriorly by the carotid space, medially by the visceral space, and superiorly the hyoid bone and submandibular space.

• Visceral space is divided into upper and lower divisions. The upper one is controversial since contains pharyngeal space, nasopharyngeal and oropharyngeal mucosal spaces. However, since they are partially coved by the deep cervical fascia, some of the scientists regarded them as parts of the visceral cervical space. The proper/lower division is located inferiorly to the hyoid bone and secondarily divided by the pretracheal layer of the fascia into pretracheal and retrovisceral spaces (anterior and posterior visceral space), limited posteriorly by the alar fascia. The space contains thyroid gland, parathyroid glands, larynx, trachea, hypopharynx, esophagus, their vessels and nerves, recurrent laryngeal nerve, lymph nodes (group VI).

2.1.3 Nerves of the Head and Neck

Muscles, skin, and viscera of the head and neck have a complicated innervation since it comes not only from spinal nerves and autonomic nervous system but also from cranial nerves.

2.1.3.1 Cranial Nerves

Cranial nerves are part of the peripheral nervous system. However, the first two—olfactory and optic ones—are the extra processes of the prosencephalon not proper cranial nerves that arise from the brain stem. The terminal nerve (CN0) is a rudimentary one and usually disappears during fetal period, secondary to physiological in-utero atrophy of vomerovaginal organs. From among 12 cranial nerves, only a few are crucial in a routine dentomaxillofacial practice.

CNI—Olfactory nerves are formed by axons of cell bodies located in the so-called olfactory part of the nasal cavity. They cross the opening of cribriform lamina of ethmoidal bone and reach olfactory bulb—part of rhiencephalon.

CNII—Optic nerve is formed by efferent fibers of ganglionic cell of the retina. They cross optic canal and through optic chiasm and tracts they reach metathalamus.

CNIII—Oculomotor nerve is a mixed nerve. Its motor and preganglionic parasympathetic fibers arise from the midbrain and enter the orbit by the superior orbital fissure to innervate superior, middle, and lower rectus oculi muscles, inferior oblique, and levator palpebrae superioris as well as by ciliary ganglion the ciliary and sphincter pupillae mm.

CNIV—Trochlear nerve is a motor nerve that arises from the midbrain and by superior orbital fissure it reaches the orbit and innervates the superior oblique muscle.

CNVI—Abduces nerve is a motor nerve that arises on the junction between pons and medulla oblongata and by superior orbital fissure. It reaches the orbit where it innervates the lateral rectus muscle.

CNVII—Facial nerve emerges from the cerebellopontine angle by two roots: proper (motor fibers) and intermedian nerve (preganglionic parasympathetic and special sensory fibers). It runs across internal acoustic meatus and facial canals where gives origin for the great petrosal n., stapedian n., and chorda tympani. The first after it reaches the base of the skull forms the parasympathetic root of the pterygopalatine ganglion. Stapedial never supplies the stapedius muscle. Chorda tympani after crossing the tympanic cavity exits it through petrotympanic fissure and unite with the lingual nerve. The nerve provides preganglionic parasympathetic fibers for submandibular ganglion that innervates submandibular, sublingual, and small salivary glands of the mouth, as well as special sensory fibers for the anterior two-thirds of the tongue. The extracranial part of the CNVII exits facial canal through the stylomastoid foramen and after giving origin for the digastric (for posterior belly of digastric m.) and posterior auricular branch (for occipital belly of epicranial muscle and posterior auricular mm.), it enters the parotid capsule. On the level of the superficial part of the gland, it divides into five branches (parotid plexus): temporal, zygomatic, buccal, marginal mandibular, and cervical that innervates all muscles of mimic/facial expression, except posterior auricular and occipital belly of epicranial ones.

CNVIII—Vestibulocochlear nerve arises for cerebellopontine angle and is formed only by special sensory fibers from vestibular and cochlear ganglion.

CNIX—Glossopharyngeal nerve emerges from the medulla oblongata and contains motor, sensory, and preganglionic parasympathetic fibers. It exits the posterior cranial fossa through the jugular foramen, where it forms superior and inferior ganglia and give origin of the tympanic nerve that later becomes the lesser petrosal—the parasympathetic root of the otic ganglion that innervates the parotid gland. Then, the main trunk of CNIX enters cervical neurovascular bundle but exits the carotid sheath shortly after, gives sensory branch for carotid sinus and follows stylopharyngeus m. Finally, it splits into terminal branches including branch to stylopharyngeus m., that also supplies superior pharyngeal constrictor as well as palatine tonsil and posterior one-third of the tongue (sensory fibers), sensory branch to the posterior part of pharyngeal plexus and pharyngeal branches for the middle constrictor.

CNX—Vagus nerve like CNX, emerges from the medulla oblongata; contains motor, sensory, and preganglionic parasympathetic fibers. It exits the posterior cranial fossa through the jugular foramen where it forms superior and inferior ganglia. After the nerve enters the carotid sheath, it runs in the groove, behind internal jugular vein and internal/common carotid artery. On the level of the thorax, it splits into few esophageal branches that mixed together to form esophageal plexus and finally, anterior and posterior vagal trunks that enter the abdominal cavity. On the cranial level, the nerve gives meningeal and auricular branch—the only one that innervates skin (auricular concha and external acoustic meatus). From the cervical part emerge pharyngeal branches for a lower constrictor, superior laryngeal nerve, cervical cardiac branches, and right recurrent laryngeal nerve that gives origin for the inferior laryngeal one (the left one arises from the thoracic part). As it was pointed above motor fibers of the nerve supplies the inferior pharyngeal constrictor, all laryngeal muscle as well as skeletal muscles of the esophagus. Sensory area is much bigger and contains external ear, dura mater of the posterior cranial fossa, taste from the epiglottis and palate, as well as mucosa of the pharynx, larynx, and all thoracic viscera and digested tract until right two-third of the transverse colon. Area of visceral sensation covers also the area of parasympathetic innervation, that also includes spleen, liver, and ovaries/testes.

CNXI—Accessory nerve emerges from medulla (internal) and spinal cord (external root). Both roots unite on the level of jugular foramen then nerve splits again and most of the cranial fibers unites with the vagus nerve, while cervical ones cross the lateral cervical spaces and innervate the trapezius and SCM muscles.

CNXII—Hypoglossal nerve arises from medulla by several roots that unite together and exit the skull by the hypoglossal canal in the base of the occipital condyle. In the canal, the nerve is surrounded by the venous plexus that separates it from bone. Then, it runs inferiorly, surrounds the carotid sheath, where it is united with an upper branch of the cervical ansa. Along the posterior belly of digastric and stylohyoid mm., it reaches the external carotid artery. On this level, the nerve forms an arch and below the intermediate tendon of the digastric m. finally follows hyoglossus and mylohyoid mm. and splits into terminal branches. The nerve supplies all muscle of the tongue including intrinsic and extrinsic ones. Due to the fibrous exchanging with the cervical ansa, it also provides innervation for dura mater of the posterior cranial fossa as well as geniohyoid and thyrohyoid mm.

2.1.3.2 Spinal Nerves

The cervical part of the spinal cord gives origin for eight pairs of spinal nerves, that by the meningeal branches innervate dura mater, by posterior ones the deep muscle and skin of the back, while anterior branches form roots of cervical (C1–C4) and brachial plexus (C5–T1). Only the last nerve (C8) has a typical preganglionic branch; however, the postganglionic ones that connect spinal nerves with ganglia of cervical part of sympathetic trunk are typical for all of them.

The nearby located anterior branches unite together in a prevertebral space and form loops that give origin for sensory/cutaneous as well as short and deep muscular branches. Coetaneous branches after a piercing the prevertebral and superficial layer of the deep cervical fascia arise on the posterior border of SCM muscle on the level of C3 (Erb point) and runs radially:

• Lesser occipital nerve runs posterosuperiorly and innervates the skin of occipital and temporal region, including upper of the middle aspect of the auricle.

• Greater auricular nerve runs anterosuperiorly and innervates the skin of the cheek, lateral aspect of the auricle, and external acoustic meatus.

• Transverse cervical nerve runs anteriorly and horizontally; innervates the skin of anterior cervical triangle, limited by both SCM muscles and body of mandible.

• Supraclavicular nerve run posteroinferiorly and innervate the skin of the deltoid and upper pectoral regions as well as the lateral cervical triangle, limited by SCM, trapezius, and clavicle.

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