Gentlemen, damn the sphenoid bone!
Oliver Wendell Holmes (1809-1894),
Opening of anatomy lectures at Harvard Medical School
The complex anatomy of the cranium and facial bones can be intimidating. However, with three-dimensional (3D) imaging and multiple imaging planes, the task of identifying these structures can be simplified. It is important to understand the normal sectional anatomy of the cranium and facial bones to identify pathologic disorders and injuries that may occur within this area (Fig. 2.1). This chapter demonstrates the sectional anatomy of the structures listed in the outline.
FIG. 2.1 3D CT of skull. Trauma resulting from a gunshot wound.
OBJECTIVES
• Differentiate between the three cranial fossae.
• Identify the location and unique structures of each cranial and facial bone.
• Identify the structures of the external, middle, and inner ear, and describe their functions.
• Identify the cranial sutures.
• Describe the six fontanels in the infant cranium.
• Describe the structures that constitute the temporomandibular joint.
• Identify the location of each paranasal sinus and the meatus into which it drains.
• Identify the structures of the osteomeatal unit.
• Identify the bones that form the orbit and their associated openings.
• Describe the structures that constitute the globe of the eye.
• List the muscles of the eye, and describe their functions and locations.
CRANIUM
The cranium is composed of eight bones that surround and protect the brain. These bones include the parietal (2), frontal (1), ethmoid (1), sphenoid (1), occipital (1), and temporal (2) (Figs. 2.2-2.5). The cranial bones are composed of two layers of compact tissue known as the internal (inner) and external (outer) tables. Located between the two tables is cancellous tissue or spongy bone called diploe (Figs. 2.6-2.9). The base of the cranium houses three fossae called the anterior, middle, and posterior cranial fossae. The anterior cranial fossa (frontal fossa) is composed primarily of the frontal bone, ethmoid bone, and lesser wing of the sphenoid bone and contains the frontal lobes of the brain. The middle cranial fossa (temporal fossa) is formed primarily by the body of the sphenoid and temporal bones and houses the pituitary gland, hypothalamus, and temporal lobes of the brain. The posterior cranial fossa (infratentorial fossa) is formed by the occipital and temporal bones and contains the cerebellum and brainstem (Figs. 2.6 and 2.7). For additional details of the contents found within the cranial fossa, see Table 2.1. Each cranial bone is structurally unique, and thus identification of the physical components can be challenging.
FIG. 2.4 3D CT of anterior skull.
FIG. 2.5 3D CT of lateral skull.
TABLE 2.1 Contents of the Cranial Fossae
|
Fossa |
Contents |
|
Anterior cranial fossa |
Frontal lobes of cerebrum; olfactory nerve (I) |
|
Middle cranial fossa |
Temporal lobes of cerebrum, pituitary gland, cavernous sinus, trigeminal ganglion, internal carotid artery, hypothalamus, and the following cranial nerves: optic nerves (II) and chiasm, oculomotor (III), trochlear (IV), trigeminal (V), abducens (VI) |
|
Posterior cranial fossa |
Cerebellum, pons, medulla oblongata, midbrain, and the following cranial nerves: facial (VII), vestibulocochlear (VIII), glossopharyngeal (IX), vagus (X), accessory (XI), hypoglossal (XII) |
Parietal Bone
The two parietal bones form a large portion of the sides of the cranium. Prominent markings and grooves that are found within the inner surface of the cranium are formed by corresponding meningeal vessels and cerebral gyri and sulci (Figs. 2.8 and 2.9). The parietal bones articulate with the frontal, occipital, temporal, and sphenoid bones. The superior point between the parietal bones is the vertex, which is the highest point of the cranium (Figs. 2.9 and 2.10). Each parietal bone has a central prominent bulge on its outer surface termed the parietal eminence (Fig. 2.4). The width of the cranium can be determined by measuring the distance between the two parietal eminences.
Frontal Bone
The frontal bone consists of a vertical and a horizontal portion. The vertical or squamous portion forms the forehead and anterior vault of the cranium (Figs. 2.2-2.5). The vertical portion contains the frontal sinuses, which lie on either side of the midsagittal plane (Figs. 2.8, 2.9, 2.11, and 2.12). Two elevated arches, the supraciliary arches, are joined to one another by a smooth area termed the glabella (Figs. 2.2 and 2.4). The horizontal portion forms the roof over each orbit, termed the orbital plate, and the majority of the anterior cranial fossa (Figs. 2.6, 2.7 and 2.13). Located in the superior portion of each orbit is the supraorbital foramen, or notch, which exists for the passage of the supraorbital nerve (Figs. 2.2 and 2.11). Between the orbital plates is an area termed the ethmoid notch, which receives the cribriform plate of the ethmoid bone (Figs. 2.6 and 2.7).
Ethmoid Bone
The ethmoid bone is the smallest of the cranial bones and is situated in the anterior cranial fossa. This cubeshaped bone can be divided into four parts: horizontal portion, vertical portion, and two lateral masses (labyrinths) (Figs. 2.14-2.17). The horizontal portion, called the cribriform plate, fits into the ethmoid notch of the frontal bone (Figs. 2.6 and 2.7). This plate contains many foramina for the passage of olfactory nerve fibers (Figs. 2.14 and 2.15). The crista galli, a bony projection stemming from the midline of the cribriform plate, projects superiorly to act as an attachment for the falx cerebri, which is the connective tissue that anchors the brain to the anterior cranial fossa (Figs. 2.16 and 2.17). The vertical portion of the ethmoid bone, called the perpendicular plate, projects inferiorly from the cribriform plate to form a portion of the bony nasal septum (Fig. 2.16). The lateral masses (labyrinth) incorporate thin-walled orbital plates (lamina papyracea), which create a portion of the medial orbit (Figs. 2.15 and 2.17). Contained within the lateral masses are many ethmoid air cells (ethmoid sinuses), one of the largest being the ethmoid bulla (Figs. 2.14-2.16). Projecting from the lateral masses are two scroll-shaped processes called the superior and middle nasal conchae (turbinates) and the uncinate process. Between the uncinate process and ethmoid bulla is a narrow groove called the infundibulum, which is an important landmark of the paranasal sinuses (Figs. 2.16 and 2.17).
FIG. 2.14 Superior view of ethmoid bone.
The naso-orbitoethmoid (NOE) complex is the union of the ethmoid sinuses, frontal bone and sinuses, anterior cranial fossa, orbits, and nasal bones. Fractures of the NOE may cause symptoms that include nasal and forehead swelling, diplopia (double vision), and cerebrospinal fluid (CSF) rhinorrhea (leakage of CSF into the nose).
FIG. 2.16 Anterior view of ethmoid bone.
Sphenoid Bone
The butterfly-shaped sphenoid bone extends completely across the floor of the middle cranial fossa (Figs. 2.6 and 2.7). This bone forms the majority of the base of the skull and articulates with the occipital, temporal, parietal, frontal, and ethmoid bones. The main parts of the sphenoid bone are the body, lesser wings (2), and greater wings (2) (Fig. 2.18). Located within the body of the sphenoid bone is a deep depression called the sella turcica, which houses the hypophysis (pituitary gland). Directly below the sella turcica are two air-filled cavities termed sphenoid sinuses (Figs. 2.15 and 2.19). The anterior portion of the sella turcica is formed by the tuberculum sellae, and the posterior portion by the dorsum sellae. The dorsum sellae give rise to the posterior clinoid processes (Figs. 2.18 and 2.20-2.22). The triangularshaped lesser wings attach to the superior aspect of the body and form two sharp points called anterior clinoid processes, which, along with the posterior clinoid processes, serve as attachment sites for the tentorium cerebelli (Figs. 2.18, 2.20, and 2.22). The optic canal is completely contained within the lesser wing and provides passage of the optic nerve and ophthalmic artery (Fig. 2.22).
FIG. 2.20 Lateral view of sphenoid bone.
The optic canal is separated from the superior orbital fissure by a bony root termed the optic strut (inferior root) (Fig. 2.2, see bony orbit). The superior orbital fissure is a triangular-shaped opening located between the lesser and greater wings that allows for the transmission of the oculomotor, trochlear, abducens, and ophthalmic division of the trigeminal nerves, as well as the superior ophthalmic vein (Figs. 2.2, 2.22, 2.24, also see bony orbit). The greater wings extend laterally from the sides of the body and contain three paired foramina—rotundum, ovale, and spinosum—through which nerves and blood vessels course (Figs. 2.6, 2.18, and 2.23-2.25; Table 2.2). Extending from the inferior surface of each greater wing is a pterygoid process, which is divided into medial and lateral pterygoid plates. The pterygoid plates serve as attachment sites for the pterygoid muscles used in movements of the lower jaw. The medial section is longer and has a hook-shaped projection on its inferior end termed the pterygoid hamulus, which provides support for the gliding motion of the tendon of the tensor veli palatine muscle as it opens the eustachian tube (Figs. 2.20, 2.24, and 2.25). At the base of the pterygoid process is the pterygoid (vidian) canal, an opening for the passage of the petrosal nerve (Figs. 2.23-2.25). The pterygoid processes articulate with the palatine bones and vomer to form part of the nasa cavity.
The sphenoid bone is considered the keystone of the cranial bones because it is the only bone that articulates with all the other cranial bones.
TABLE 2.2 Foramina and Fissures of the Skull
|
Bone |
Foramen/Fissure |
Major Structures Using Passageway |
|
Frontal |
Supraorbital foramen (or notch) Frontal foramen (or notch) |
Supraorbital nerve and artery Frontal artery and nerve |
|
Ethmoid |
Cribriform plate |
Olfactory nerve (I) |
|
Sphenoid |
Foramen rotundum Foramen ovale Foramen spinosum Pterygoid canal Optic canal Superior orbital fissure |
Maxillary branch of trigeminal nerve (V2) Mandibular branch of trigeminal nerve (V3) Middle meningeal artery Petrosal nerve Optic nerve (II) and ophthalmic artery Ophthalmic vein and the following cranial nerves: oculomotor (III), trochlear (IV), ophthalmic branch of trigeminal (V1), abducens (VI) |
|
Sphenoid and maxillary bone |
Inferior orbital fissure |
Maxillary branch of trigeminal nerve (V2) |
|
Occipital |
Foramen magnum Hypoglossal canal |
Medulla oblongata and accessory nerve (XI) Hypoglossal nerve (XII) |
|
Temporal |
Carotid canal External auditory meatus Internal auditory canal Stylomastoid foramen and facial nerve canal |
Internal carotid artery Air in canal conducts sound to tympanic membrane Vestibulocochlear nerve (VIII) and facial nerve (VII) Facial nerve (VII) |
|
Temporal and occipital bone |
Jugular foramen |
Internal jugular vein, glossopharyngeal nerve (IX), vagus nerve (X), and accessory nerve (XI) |
|
Temporal, sphenoid, and occipital bones |
Foramen lacerum |
Fibrocartilage, internal carotid artery as it leaves carotid canal to enter cranium, nerve of pterygoid canal and a meningeal branch from the ascending pharyngeal artery |
|
Maxillary |
Infraorbital foramen |
Infraorbital nerve and maxillary branch of trigeminal nerve (V2) |
|
Lacrimal with maxilla |
Lacrimal groove, nasolacrimal canal |
Lacrimal sac and nasolacrimal duct |
|
Mandible |
Mental foramen |
Mental artery and nerve |
Occipital Bone
The occipital bone forms the posterior cranial fossa and the inferoposterior portion of the cranium. On the inferior portion of the occipital bone is a large oval aperture called the foramen magnum located at the junction of the brainstem and spinal cord (Fig. 2.26). The occipital bone can be divided into four portions: occipital condyles (2), basilar portion (1), and squamous portion (1) (Fig. 2.27). The occipital condyles project inferiorly to articulate with the first cervicalvertebra (atlas), forming the atlantooccipital joint (Figs. 2.28 and 2.29). Located obliquely at the base of the condyles and anterolateral to the foramen magnum are the hypoglossal canals through which the hypoglossal nerve (CN XII) courses (Figs. 2.8, 2.27, 2.28, and 2.30; Table 2.2). The basilar portion forms the anterior margin of the foramen magnum and slopes superiorly and anteriorly to meet with the dorsum sella of the sphenoid bone to form the clivus (Figs. 2.8, 2.27, and 2.29-2.32). The squamous portion curves posterosuperiorly from the foramen magnum to articulate with the parietal and temporal bones (Fig. 2.3). Located on the inner surface of the squama is a bony projection termed the internal occipital protuberance, which marks the site where the dural venous sinuses converge (Figs. 2.6, 2.31, and 2.33). The external occipital protuberance is a midline projection on the external surface of the squamous part of the occipital bone. The highest point of the external occipital protuberance is termed the inion (Figs. 2.27 and 2.31).
Temporal Bone
The two temporal bones contain many complex and important structures. They form part of the sides and base of the cranium, and together with the sphenoid bone, they create the middle cranial fossa (Figs. 2.3 and 2.6). The temporal bone can be divided into four portions: squamous, tympanic, mastoid, and petrous (Figs. 2.34 and 2.35). The thin squamous portion projects upward to form part of the sidewalls of the cranium (Fig. 2.3). Extending from the squamous portion is the zygomatic process, which projects anteriorly to the zygoma of the face to form the zygomatic arch (Figs. 2.23, 2.25, 2.30, 2.34, and 2.36). At the base of the zygomatic process is the articular eminence that forms the anterior boundary of the mandibular fossa.
The mandibular fossa is the depression that articulates with the condyloid process of the mandible, creating the temporomandibular joint (Figs. 2.34 and 2.37). The tympanic portion lies below the squama and forms the majority of the external auditory meatus (Figs. 2.33-2.35 and 2.37). Just posterior to the tympanic portion is the mastoid portion, which has a prominent conical region termed the mastoid process (Figs. 2.34 and 2.37-2.39). The mastoid process encloses the mastoid air cells and mastoid antrum. The mastoid antrum is located on the anterosuperior portion of the mastoid process. It is an air-filled cavity that communicates with the middle ear (tympanic cavity) (Figs. 2.37-2.39).
The petrous portion of the temporal bone is pyramidal in shape and situated at an angle between the sphenoid and occipital bones (Fig. 2.35). The posterior surface of the petrous pyramid forms the anterior bony limit of the posterior fossa (Fig. 2.6). Near the center of this surface is the opening to the internal auditory canal, which transmits the seventh and eighth cranial nerves (Figs. 2.35 and 2.39). Other openings associated with the posterior surface of the petrous pyramid are the jugular foramen and the carotid canal, which provide passage for the internal jugular vein and the internal carotid artery (Figs. 2.36 and 2.38-2.41; Table 2.2). An enlargement of the jugular foramen is the jugular fossa (Fig. 2.42). Continuous in front of the jugular foramen is the petro-occipital fissure that separates the petrous portion of the temporal bone from the foramen magnum of the occipital bone (Fig. 2.40). The carotid canal courses superiorly at its lower segment, then changes direction and is seen coursing posterior to anterior (Figs. 2.33 and 2.38-2.41. See also Chapter 3, internal carotid arteries). Superior to the carotid canal is an indentation on the petrous portion called Meckel’s cave (Fig. 2.41). Also known as the trigeminal cistern, Meckel’s cave is located between two layers of dura and encloses the trigeminal ganglion. It is filled with CSF and is continuous with the pontine cistern and subarachnoid space (see also trigeminal nerve in Chapter 3). Between the apex of the petrous pyramid, the body of the sphenoid bone, and the basilar portion of the occipital bone is a jagged slit termed the foramen lacerum, which contains cartilage and allows the internal carotid artery to enter the cranium (Figs. 2.6 and 2.40; Table 2.2). The inferior surface of the petrous pyramid gives rise to the long slender styloid process that is attached to several muscles of the tongue and ligaments of the hyoid bone (Figs. 2.8, 2.29, and 2.34). The stylomastoid foramen is situated between the mastoid process and the styloid process. This foramen constitutes the end of the facial nerve canal (Figs. 2.38, 2.42, and 2.50-2.58; Table 2.2). The interior of the petrous pyramid houses the delicate middle and inner ear structures.
A basilar skull fracture is a fracture of the bones that form the base (floor) of the skull and typically involves the occipital, sphenoid, temporal, and/or ethmoid bones. Basilar skull fractures can cause tears in the meninges, the membranes surrounding the brain. Subsequent leakage of CSF into the nasopharynx may cause the patient to experience a salty taste. Other clinical signs of a basilar skull fracture may include bruising behind the ears or around the eyes; loss of hearing, smell, or vision; and possible nerve damage resulting in weakness of the face.
FIG. 2.38 Coronal view of temporal bone.
Structures of the External, Middle, and Inner Ear
The structures of the ear can be divided into three main portions: external, middle, and inner (Figs. 2.43-2.59).
The external ear consists of the auricle and the external auditory meatus. The external auditory meatus is a sound-conducting canal that terminates at the tympanic membrane of the middle ear (Figs. 2.40 and 2.43).
The narrow, air-filled middle ear, or tympanic cavity, communicates with both the mastoid antrum and the nasopharynx. Air is conveyed from the nasopharynx to the tympanic cavity through the eustachian tube (auditory tube) (Figs. 2.40 and 2.43). The middle ear consists of the tympanic membrane and three auditory ossicles (malleus, incus, and stapes) (Fig. 2.43B). The tympanic membrane transmits sound vibrations to the auditory ossicles. The auditory ossicles, which are suspended in the middle ear, conduct sound vibrations from the tympanic membrane to the oval window of the inner ear (Figs. 2.43, 2.49-2.59).
The middle ear can be subdivided into the epitympa- num, mesotympanum, and hypotympanum. The epitympanum, also called the attic, is located superior to the tympanic membrane and contains the head of the malleus and body of the incus. It communicates with the mastoid air cells through a narrow opening called the aditus ad antrum (mastoideum), a potential route for the spread of infection from the middle ear to the mastoid air cells (Figs. 2.50 and 2.51). The roof of the epitympanum is separated from the middle cranial fossa by a thin layer of bone termed the tegmen tympani. Two other important landmarks of the epitympanum include the scutum and the Prussak space. The scutum is a sharp, bony spur on the lateral wall of the tympanic cavity and the superior wall of the external auditory meatus (Figs. 2.43B and 2.57). The scutum provides the superior attachment site for the tympanic membrane. The Prussak space (lateral epitympanic recess) is bordered laterally by the tympanic membrane, superiorly by the scutum, medially by the neck of the malleus, and inferiorly by the lateral process of the malleus (Figs. 2.43 and 2.57). The boundaries of the Prussak space limit the spread of infection to other compartments of the middle ear.
The mesotympanum is the portion of the middle ear that is medial to the tympanic membrane and contains the stapes, the long process of the incus, the handle of the malleus, and the oval and round windows (Figs. 2.43B and 2.56).
The hypotympanum is the portion of the middle ear that is located inferior to the lower border of the tympanic membrane and is the site of the tympanic opening for the eustachian tube (Figs. 2.43B and 2.49-2.59).
The inner ear, or bony labyrinth, contains the vestibule and semicircular canals, which control equilibrium and balance, and the cochlea, which is responsible for hearing (Figs. 2.43-2.48). The vestibule is a small bony compartment located between the semicircular canals and the cochlea. Two openings of the vestibule are the oval window (Fig. 2.44) for the footplate of the stapes and the vestibular aqueduct, which contains the endolymphatic duct (Fig. 2.47). The semicircular canals are continuous with the vestibule and are easily identified because of their three separate passages (superior [anterior], posterior, and lateral) that are at right angles to each other (Figs. 2.42-2.44). The three interconnected semicircular canals are lined with microscopic hairs called cilia and are filled with fluid known as endo- lymph. Every time the position of the head changes, the fluid moves against the cilia, creating a kind of motion sensor. This helps the brain create a sense of balance. The cochlea is a spiral-shaped structure with a base that lies on the internal auditory canal (Figs. 2.43 and 2.45).
Located within the basilar turn of the cochlea is the round window, which allows the fluid of the inner ear to move slightly for propagation of sound waves and nerve impulses to be sent to the brain (Figs. 2.44 and 2.47). Within the bony labyrinth is a complicated system of ducts called the membranous labyrinth, which is filled with endolymph, a fluid that helps with the propagation of sound waves (Fig. 2.47). Extending from the vestibule is a slender endolymphatic duct that terminates as the endolymphatic sac, which is located between two dural layers on the posterior wall of the petrous pyramid (Figs. 2.47 and 2.48). The endolymphatic duct and sac are thought to be responsible for the reabsorption of endolymph and may contribute to vestibular dysfunction. Figs. 2.49-2.59 provide sequential computed tomography (CT) images through the external, middle, and inner ear in the axial and coronal planes, respectively.
Meniere disease is a disorder of the membranous labyrinth that results from a failure of the mechanism controlling the production and elimination of endolymph. In advanced cases, there is an increased accumulation of endolymph volume, resulting in an abnormal distention of the membranous labyrinth (endolymphatic hydrops). Meniere disease is most common in middle age and may become bilateral in up to 50% of affected patients. Symptoms include episodic vertigo accompanied by nausea, fluctuating hearing loss, and a feeling of fullness in the affected ears. The success of surgical intervention in relieving Meniere disease depends a great deal on the ability to image and evaluate the vestibular aqueduct and endolymphatic duct and sac.
Cholesteatomas are epidermoid cysts of the middle ear that can be acquired or congenital. The lumen of the cyst is filled with debris. As a cholesteatoma enlarges, it destroys the ossicles and adjacent bony structures. Cholesteatomas are usually associated with chronic infection, aural discharge, and conductive or mixed deafness. The Prussak space is the most common site of acquired cholesteatomas within the tympanic cavity.
Sutures
The cranial bones are joined by four main articulations termed sutures. The squamous suture, which is located on the side of the cranium, joins the squamous portion of the temporal bone to the parietal bone. The coronal suture runs across the top of the cranium and is the articulation between the frontal and parietal bones. The sagittal suture provides the articulation between the parietal bones along the midsagittal plane. The lambdoidal suture is located posterior in the cranium and joins the occipital and parietal bones (Figs. 2.3 and 2.60-2.63). Sutures corresponding to the mastoid portion of the temporal bone include the occipitomastoid suture between the occipital bone and mastoid portion of the temporal bone and the parietomastoid suture between the parietal bone and mastoid portion of the temporal bone. The asterion is a point on the skull corresponding to the posterior end of the parietomastoid suture (Figs. 2.3 and 2.60). Sutures corresponding to the sphenoid bone include the sphenosquamosal suture between the sphenoid bone and squamous portion of the temporal bone, the sphenofrontal suture between the greater wing of the sphenoid bone and the frontal bone, and the sphenoparietal suture located between the greater wing of the sphenoid bone and the parietal bone. The region surrounding the sphenoparietal suture where the parietal, sphenoid, temporal, and frontal bones meet is termed the pterion, an important landmark because it is considered the weakest part of the skull and is also the site of the anterolateral (sphenoid) fontanel in neonates (Figs. 2.3 and 2.60). The frontal (metopic) suture divides the frontal bone into halves as it extends from the anterior fontanel or sagittal suture to the nasion in infants and children and typically disappears by the age of 6 (Fig. 2.64).
The sutures in neonates are not fully closed, allowing for growth of the head after birth. Craniosynostosis, which is the result of premature ossification of one or more of the cranial sutures, causes abnormal growth of the cranium and can limit the growth of the brain.
The pterion is known as the weakest part of the skull and is located over the anterior division of the middle meningeal artery. A severe blow to the side of the head causing a fracture and rupture of the middle meningeal artery may result in an epidural hematoma. A favored site of access for performing a bur hole to drain the hematoma is at the pterion.
Fontanels
Within the neonatal cranium are six areas of incomplete ossification called fontanels. The largest is the anterior fontanel located at the junction of the upper parietal and frontal bones termed the bregma (Fig. 2.64). This fontanel remains open until the age of 2. Located at the lambda, the junction of the parietal and occipital bones is the posterior fontanel (Fig. 2.65). The posterior fontanel typically closes between the first and third months after birth. On the sides of the cranium are four additional fontanels, two anterolateral (sphenoid) and two posterolateral (mastoid) (Figs. 2.65 and 2.66). The anterolateral fontanels are located between the parietal and greater wing of the sphenoid bones. The posterolateral fontanels are located at the junction of the occipital, temporal, and parietal bones. The anterior and posterolateral fontanels ossify at approximately 2 years of age, whereas the posterior and anterolateral fontanels close between 1 and 3 months after birth.
Bulging of the anterior fontanel may indicate increased intracranial pressure, whereas a sunken fontanel may indicate dehydration.
FACIAL BONES
The face is made up of 14 facial bones. The facial bones can be difficult to differentiate because of their relatively small size and irregular shape. They consist of the nasal (2), lacrimal (2), palatine (2), maxilla (2), zygoma (2), inferior nasal conchae (2), vomer (1), and mandible (1) (Figs. 2.67-2.85).
FIG. 2.67 Anterior view of facial bones.
FIG. 2.68 Sagittal view of facial bones.
Nasal Bones
The two nasal bones form the bony bridge of the nose and articulate with four bones: the frontal and ethmoid bones of the cranium and the opposite nasal bone and maxilla (Figs. 2.67, 2.68, 2.70, 2.71, and 2.73).
Lacrimal Bones
Posterior to the nasal bones and maxilla are the lacrimal bones, which are situated on the medial wall of each orbit (Fig. 2.70). The junction between the lacrimal bones and the maxillae forms the lacrimal groove, which accommodates the lacrimal sacs that are part of the drainage route for excess lacrimal fluid (tears) (Figs. 2.67, 2.68, 2.70, and 2.71).
Palatine Bones
The palatine bones are slightly L-shaped and are located in the posterior aspect of the nasal cavity between the maxilla and the pterygoid process of the sphenoid bone (Fig. 2.70). The palatine bones consist of a horizontal portion and a vertical portion. The horizontal portion of the palatine bones joins anteriorly with the palatine process of the maxilla to form the hard palate (Figs. 2.8, 2.69, 2.74, and 2.75). The vertical portion extends to form a segment of the lateral wall of the nasal cavity and the medial wall of the orbit (Fig. 2.70). The pterygopalatine fossa is a gap between the pterygoid process of the sphenoid bone, maxilla, and palatine bones. The pterygopalatine fossa contains the maxillary nerve V2 (second division of the trigeminal nerve), the pterygopalatine ganglion, and the third part of the maxillary artery (Figs. 2.30, 2.70, and 2.77).
Maxillary Bones
The largest immovable facial bones are the maxillary bones, which fuse at the midline to form a pointed process termed the anterior nasal spine (Figs. 2.68, 2.70, and 2.71). An opening on the anterior aspect of the maxilla is the infraorbital foramen, which transmits the infraorbital nerve and blood vessels (Figs. 2.67 and 2.72). The maxillary bones contain the large maxillary sinuses and four processes: the frontal process, zygomatic process, alveolar process, and palatine process (Figs. 2.67 and 2.722.76). The frontal and zygomatic processes project to articulate with the frontal bones of the cranium and the zygomatic bones of the face (Figs. 2.71-2.73). The inferior border of the maxilla has several depressions that form the alveolar process, which accepts the roots of the teeth (Figs. 2.67, 2.71, 2.75, and 2.76). The palatine process of the maxilla extends posteriorly to form three- fourths of the hard palate. The posterior one-fourth of the hard palate is created by the horizontal portion of the palatine bones (Figs. 2.69, 2.74 and 2.75).
FIG. 2.72 Coronal CT of maxilla and zygoma.
Zygomatic Bones
The zygomatic bones (zygoma or malar) create the prominence of the cheek and contribute to the lateral portion of the bony orbit (Figs. 2.67, 2.71, 2.72, 2.77, and 2.78). They articulate with the maxilla and temporal, frontal, and sphenoid bones. The temporal process of the zygomatic bone extends posteriorly to join the zygomatic process of the temporal bone to form the zygomatic arch (Figs. 2.68, 2.69, 2.71, 2.74, and 2.77).
Le Fort fractures are a result of direct anterior facial injuries. They are classified into three groups according to the facial bones that are traumatized. Type I: The alveolar process of the maxilla and the hard palate are separated from the superior part of the skull. Type II: The alveolar, zygomatic, and frontal processes of the maxilla along with the nasal bones are separated from the frontal and zygomatic bones. Type III: Virtually the entire facial skeleton, including the maxillae, nasal bones, and zygomatic bones, is separated from the frontal bone above it.
FIG. 2.76 Axial CT of alveolar process of maxilla.
FIG. 2.77 Axial CT of facial bones.
Inferior Nasal Conchae
The inferior nasal conchae (inferior nasal turbinates) arise from the maxillary bones and project horizontally into the nasal cavity (Figs. 2.67, 2.72, and 2.77). They can be identified by their scroll-like appearance. These conchae in conjunction with the superior and middle nasal conchae of the ethmoid bone divide the nasal cavity into three openings or meati, termed superior, middle, and inferior (Figs. 2.72, 2.79, and 2.80).
Vomer
The vomer is an unpaired facial bone located on the midsagittal line. The vomer forms the inferior portion of the bony nasal septum as it projects superiorly to articulate with the perpendicular plate of the ethmoid bone (Figs. 2.8, 2.9, 2.67, and 2.72).
Mandible
The largest facial bone is the mandible. This bone is composed primarily of horizontal and vertical portions (Figs. 2.81 and 2.82). The angle created by the junction of these two portions is termed the gonion. The curved horizontal portion, called the body, contains an alveolar process (similar to the maxilla) that receives the roots of the teeth of the lower jaw. The mental foramina extend through the body of the mandible and allow passage of the mental artery and nerve (Figs. 2.81 and 2.82). The vertical portion of the mandible is called the ramus (Figs. 2.71 and 2.81-2.83). Each ramus has two processes at its superior portion: the coronoid process, anteriorly, and the condyloid process (condyle), posteriorly (Figs. 2.81, 2.82, 2.84, and 2.85). They are separated by a concave surface called the mandibular notch. The coronoid process serves as an attachment site for the temporalis and masseter muscles, whereas the condyloid process articulates with the mandibular fossa of the temporal bone to form the temporomandibular joint (TMJ) (Figs. 2.82 and 2.86).
TEMPOROMANDIBULAR JOINT
The TMJ is a modified hinge joint that allows for the necessary motions of mastication.
Bony Anatomy
The mandibular fossa and articular eminence of the temporal bone form the superior articulating surface for the condyloid process of the mandible (condyle). The articular eminence creates the anterior boundary of the joint, preventing the forward displacement of the mandibular condyle (Figs. 2.86 and 2.87).
FIG. 2.86 Lateral view of temporomandibular joint.
Articular Disk and Ligaments
The articular disk, frequently called the meniscus, is shaped like a bowtie and is interposed between the mandibular condyle and fossa to act as a shock absorber during jaw movement (Figs. 2.86, 2.88, and 2.89). The anterior and posterior portions of the meniscus are referred to as the anterior and posterior bands, respectively. The anterior band attaches to the lateral pterygoid muscle, and the posterior band has fibrous connections to both the temporal bone and the posterior aspect of the condyle (Figs. 2.86 and 2.88). The articular disk is not tightly bound to the fossa but moves anteriorly with the condyle. Several ligaments help maintain the position of the articular disk. The articular disk is attached to the medial and lateral surfaces of the condyle by the collateral ligaments (Figs. 2.89 and 2.90). Lateral stability is provided by the temporomandibular ligament (lateral ligament), which extends from the articular eminence and zygomatic process to the posterior aspect of the articular disk and the condylar head and neck (Fig. 2.91). Additionally, this ligament restricts the posterior movement of the condyle and articular disk.
FIG. 2.91 Sagittal view of temporomandibular joint and lateral ligament.
Muscles
The cooperative actions of four muscles located on each side of the TMJ provide the movement of the mandible and are collectively referred to as the muscles of mastication (Fig. 2.92). The fan-shaped temporalis muscle originates on the temporal fossa, inserts on the coronoid process and anterior ramus of the mandible, and elevates the mandible. The masseter muscle is the strongest muscle of the jaw, arising from the zygomatic arch and inserting on the ramus and angle of the mandible. Its actions include elevation of the mandible (Figs. 2.92 and 2.93). The pterygoid muscles (medial and lateral) originate from the pterygoid processes of the sphenoid bone and insert on the angle of the mandible and condylar process, respectively. The medial pterygoid muscle acts to close the jaw, whereas the lateral pterygoid muscle opens the jaw and protrudes and moves the mandible from side to side (Figs. 2.92, 2.94, and 2.95).
PARANASAL SINUSES
The paranasal sinuses are air-containing cavities within the facial bones and skull that communicate with the nasal cavity. The nasal cavity is lined by nasal mucosa and is responsible for filtering airborne particles as it warms and humidifies air going into the lungs. The sinuses are named after the bones in which they originate: ethmoid, maxillary, sphenoid, and frontal. There is great variance in the size, shape, and development of these sinuses within each individual (Figs. 2.96 and 2.97).
Ethmoid
The ethmoid sinuses are contained within the lateral masses (labyrinths) of the ethmoid bone and number in the adult between 3 to 18 cells. They are present at birth and continue to grow and honeycomb into a varying number of air cells through puberty. The ethmoid sinuses are divided into anterior and posterior groups by the basal lamella of the middle conchae (turbinates). The basal lamella is the lateral attachment of the middle nasal conchae to the lamina papyracea (Figs. 2.80 and 2.98). The anterior group drains into the middle nasal meatus, and the posterior group drains into the superior nasal meatus (Figs. 2.79, 2.80 and 2.96-2.100; Table 2.3).
TABLE 2.3 Paranasal Sinus Drainage Location
|
Sinus |
Drainage Location |
|
Ethmoid: anterior |
Middle nasal meatus |
|
Ethmoid: posterior |
Superior nasal meatus |
|
Maxillary |
Middle nasal meatus |
|
Sphenoid |
Sphenoethmoidal recess |
|
Frontal |
Middle nasal meatus |
Maxillary
The paired maxillary sinuses (antrum of Highmore) are located within the body of the maxilla, below the orbit and lateral to the nose. These triangular cavities are the largest of the paranasal sinuses in adults but are just small cavities at birth. Their growth stops at approximately the age of 15. The roots of the teeth and the maxillary sinuses are separated by a very thin layer of bone. Often it is difficult to differentiate between the symptoms of sinusitis and infection of the teeth. The maxillary sinuses drain into the middle nasal meatus (Figs. 2.96, 2.97, 2.100, and 2.101; Table 2.3).
Sphenoid
The sphenoid sinuses are present at birth but contain red marrow and are therefore devoid of air. Pneumatization of the sphenoid sinuses may be seen as early as 2 years of age. Major growth of the sinuses occurs in the third to fifth year, and they typically assume adult configuration between 10 and 14 years of age.
Sphenoid sinuses are normally paired and occupy the body of the sphenoid bone just below the sella turcica. Each sphenoid sinus opens into the sphenoethmoidal recess directly above the superior concha and drains into the superior nasal meatus (Figs. 2.80, 2.96-2.99, 2.102, and 2.103; Table 2.3).
Frontal
The frontal sinuses are located within the vertical portion of the frontal bone (Figs. 2.96, 2.97, and 2.99). These sinuses are typically paired and are separated along the sagittal plane by a septum (Fig. 2.104). The frontal sinuses are rarely symmetric, vary greatly in size, and can contain numerous septa. These sinuses do not form or become aerated in the frontal bone until approximately age 6, making them the only paranasal sinuses that are absent at birth. The frontal sinuses drain into the middle nasal meatus (Figs. 2.99 and 2.100; Table 2.3).
Osteomeatal Unit
Drainage of the paranasal sinuses occurs through various openings, or ostia. The major drainage pathways and structures of these osteomeatal channels form the osteomeatal unit (OMU) (Figs. 2.105-2.107). There are two osteomeatal channels: the anterior OMU and posterior OMU. The anterior OMU includes the ostia for the frontal and maxillary sinuses, frontal recess, infundibulum, and middle meatus. The anterior OMU provides communication between the frontal, anterior ethmoid, and maxillary sinuses. The posterior OMU consists of the sphenoethmoidal recess and the superior nasal meatus, which communicate with the posterior ethmoid air cells. The sphenoethmoidal recess lies just lateral to the nasal septum, above the superior nasal concha, and drains the sphenoid sinuses. Key OMU structures to identify include the infundibulum, middle meatus, uncinate process, semilunar hiatus, and ethmoid bulla. The infundibulum is a narrow oblong canal that serves as the primary drainage pathway from the maxillary sinuses into the middle meatus. The medial wall of the infundibulum is created by the uncinate process. The uncinate process is a thin, hook-shaped bony plate that arises from the floor of the anterior ethmoid sinuses and projects posteriorly and inferiorly, ending in a free edge. The free edge of the uncinate process forms the semilunar hiatus, which opens directly into the middle meatus. The semilunar hiatus is a gap located between the ethmoid bulla and uncinate process that forms the opening of the infundibulum. Also draining into the middle meatus is the ethmoid bulla, located superior and posterior to the infundibulum, which receives drainage from the anterior ethmoid air cells (Figs. 2.105-2.107).
ORBIT
Bony Orbit
The bony orbits are cone-shaped recesses that contain the globes, extraocular muscles, blood vessels, nerves, adipose and connective tissues, and most of the lacrimal apparatus. The junction of the frontal, sphenoid, and ethmoid bones of the cranium and the lacrimal, maxillary, palatine, and zygomatic bones of the face forms the orbit (Figs. 2.108 and 2.109). Each orbit presents a roof, floor, medial wall, lateral wall, and an apex. The roof of the orbit is composed of the orbital plate of the frontal bone and the lesser wing of the sphenoid bone. On the anterolateral surface of the roof is the lacrimal fossa where the lacrimal gland is located (Figs. 2.108 and 2.109). The medial wall is exceedingly thin and is formed by a portion of the frontal process of the maxilla, the lacrimal bone, the ethmoid bone, and the body of the sphenoid bone (Figs. 2.106-2.111). On the anterior surface of the medial wall is the lacrimal groove for the lacrimal sac (Figs. 2.108-2.110). The floor of the orbit, which is also the roof of the maxillary sinus, is made up of the maxilla, zygoma, and palatine bones. The lateral wall is the thickest wall and is formed by the greater wing of the sphenoid bone and the zygoma (Figs. 2.106, 2.108, and 2.111). The posterior portion of the orbit or the apex is basically formed by the optic canal (optic foramen) and the superior orbital fissure. The optic canal and the superior and inferior orbital fissures allow various structures to enter and exit the orbit and establish communication between the orbit and middle cranial fossa. The optic canal forms an angle of about 37 degrees with the sagittal plane of the head; it is bound medially by the body, superiorly by the lesser wing, and inferiorly and laterally by the optic strut (inferior root) of the sphenoid bone (Figs. 2.108-2.112). Coursing through the optic canal are the ophthalmic artery and optic nerve. The superior orbital fissure, a triangular opening located between the greater and lesser wings of the sphenoid bone, allows the passage of cranial nerves, oculomotor (III), trochlear (IV), ophthalmic branch of the trigeminal (V), and abducens (VI), as well as the ophthalmic veins (Figs. 2.108, 2.109, 2.111, and 2.112). At the orbital apex, the inferior and lateral walls of the orbit are separated by the inferior orbital fissure through which the maxillary branch of the trigeminal nerve (V) courses (Figs. 2.108 and 2.113). The medial lip of the inferior orbital fissure is notched by the infraorbital groove, which passes forward in the orbital floor to become the infraorbital canal that opens on the anterior surface of the maxilla as the infraorbital foramen (Figs. 2.106 and 2.108-2.110).
Soft Tissue Structures
The globe of the eye has an irregular, spherical shape and sits in the socket of the bony orbit. The globe is divided into anterior and posterior compartments (Fig. 2.114). The anterior compartment is a small cavity located anterior to the lens. It contains the cornea and iris and is filled with aqueous humor that helps maintain intraorbital pressure. The larger posterior compartment is located behind the lens and is surrounded by the retina. The retina consists of layers of tissue that include the photoreceptors responsible for vision. The posterior chamber contains a jelly-like substance called the vitreous humor that helps maintain the shape of the globe (Figs. 2.114-2.125).
Direct trauma to the globe will commonly result in a blowout fracture of the orbit. These fractures most commonly involve the floor of the orbit and cause orbital content herniation, which results in diplopia. A medial blowout fracture involving the orbital plate of the ethmoid bone is much less common but may cause open communication between the frontal and ethmoid sinuses and the orbit.
Optic Nerve
The optic nerve is the nerve of sight. It commences at the posterior surface of the globe and courses posteromedially to exit the orbit through the optic canal and is entirely surrounded by dura mater, which is continuous with the meninges of the brain (Figs. 2.114-2.116). The ophthalmic artery courses adjacent to the optic nerve as it exits through the optic canal. The superior ophthalmic vein is located inferior to the superior rectus muscle and courses obliquely from the medial orbit through the superior orbital fissure, where it drains directly into the cavernous sinus (Figs. 2.117-2.119 and 2.122-2.124). Retroorbital fat surrounds the muscular and vascular structures within the orbit, which allows for better visualization of structures in cross-sectional imaging (Figs. 2.114-2.121).
Muscles of the Eye
Six major muscles work together to control the movement of the eye. The rectus muscle group consists of four muscles that arise from a common tendinous ring surrounding the optic nerve and is located at the medial portion of the superior orbital fissure. The superior, inferior, medial, and lateral rectus muscles act to abduct and adduct the globe (Figs. 2.114-2.117 and 2.120-2.125). Two oblique muscles, superior and inferior, abduct and rotate the globe. The superior oblique is located medial to the superior rectus muscle, and the inferior oblique lies below and anterior to the inferior rectus muscle. The upper eyelid is controlled by the superior levator palpebrae muscle, which originates from the orbital roof near the origin of the superior rectus muscle (Figs. 2.120-2.125).
Lacrimal Apparatus
Each lacrimal apparatus consists of a lacrimal gland, lacrimal canaliculi, lacrimal sac, and nasolacrimal duct and is responsible for the production and distribution of tears. Tears are important for keeping the eye moist and clean, removing waste, preventing bacterial infections, and providing nutrients and oxygen to portions of the eye. The almond-shaped lacrimal gland is located in the lacrimal groove, superior and lateral to the globe, where it provides most of the tear volume (Figs. 2.117-2.119, 2.125, and 2.126). On blinking, tears collect in the area of the medial canthus and subsequently empty into small canals termed lacrimal canaliculi that lead to the lacrimal sac (Fig. 2.126). The lacrimal sac, found within the lacrimal groove of the orbit, continues inferiorly to form the nasolacrimal duct, which passes through the nasolacrimal canal of the maxillary and lacrimal bones to empty into the inferior nasal meatus (Figs. 2.115, 2.116, 2.126, and 2.127).
REFERENCES
Abrahams, P. H., Marks, S. C., Jr., & Hutchings, R. T. (2003). McMinn’s color atlas of human anatomy (5th ed.). St. Louis: Mosby.
Frank, G. (2012). Merrill’s atlas of radiographic positioning and procedures (12th ed.). St. Louis: Mosby.
Harnsberger, H. R. (1995). Handbook of head and neck imaging (2nd ed.). St. Louis: Mosby.
Mosby. (2008). Mosby’s dictionary of medicine, nursing, and health professions (8th ed.). St. Louis: Mosby.
Som, P. M., & Curtin, H. D. (2011). Head and neck imaging (5th ed.). St. Louis: Mosby.
Standring, S. (2016). Gray’s anatomy: the anatomical basis of clinical practice (41st ed.). New York: Elsevier.