Ch. 11 Cranium, Facial Bones, and Paranasal Sinuses

Skull divided into cranial and facial bones

Cranial Bones (8)

calvarium (skullcap) and floor. The calvarium includes: frontal, right and left parietal, and occipital bones. The floor includes: right and left temporal, sphenoid, and ethmoid bones.

Calvarium (4)

1. Frontal Bone:

   • Forms the forehead and superior part of each orbit.

   • Squamous (vertical) portion: forehead.

   • Orbital (horizontal) portion: superior part of the orbit.

   • Glabella: Smooth, raised prominence between the eyebrows.

   • Supraorbital Groove (SOG): Slight depression above each eyebrow, corresponding to the floor of the anterior fossa.

   • Supraorbital Margin (SOM): Superior rim of each orbit.

   • Supraorbital Notch (Foramen): Small hole within the SOM for the supraorbital nerve and artery.

   • Frontal Tuberosity (Eminence): Larger, rounded prominence above the SOG.

   • Orbital Plate: Forms the superior part of each orbit, separated by the ethmoidal notch.

   • Articulations: Parietals, sphenoid, and ethmoid bones; also articulates with eight facial bones.

2. Parietal Bones:

   • Form the lateral walls and part of the roof of the cranium.

   • Square shape with a concave internal surface.

   • Parietal Tubercles (Eminences): Widest portion of the skull is located between these.

   • Articulations: Frontal, occipital, temporal, sphenoid, and opposite parietal bones.

3. Occipital Bone:

   • Forms the inferoposterior portion of the calvarium.

   • Squamous Portion: Rounded part forming most of the back of the head above the external occipital protuberance.

   • External Occipital Protuberance (Inion): Prominent bump at the inferoposterior portion of the skull.

   • Foramen Magnum: Large opening at the base for the spinal cord.

   • Lateral Condylar Portions (Occipital Condyles): Oval processes that articulate with the atlas (first cervical vertebra) forming the atlanto-occipital joint.

   • Articulations: Parietals, temporals, sphenoid, and atlas.

Floor

4. Temporal Bones:

   • Located between the greater wing of the sphenoid and the occipital bone, housing the organs of hearing and balance.

   • Zygomatic Process: Arch of bone extending anteriorly from the squamous portion, meeting the temporal process of the zygomatic bone to form the zygomatic arch.

   • Temporomandibular Fossa (TM Fossa): Located inferior to the zygomatic process and anterior to the EAM.

   • Styloid Process: Slender bony projection inferior to the mandible and anterior to the EAM.

   • Three primary parts:

     • Squamous Portion: Thin upper portion forming part of the skull wall; vulnerable to fracture.

     • Mastoid Portion: Posterior to the EAM, including the mastoid process (tip) with air cells inside.

     • Petrous Portion (Petrous Pyramid, Pars Petrosa): Dense portion housing hearing and equilibrium organs, including mastoid air cells; also called petromastoid portion. Includes the petrous ridge (petrous apex).

   • Internal Acoustic Meatus: Orifice on the posterior surface of the petrous pyramid, transmitting nerves of hearing and equilibrium.

   • Articulations: Parietal, occipital, and sphenoid bones; also articulates with two facial bones.

5. Sphenoid Bone:

   • Centrally located, forming the anchor for all eight cranial bones. Contains the sphenoid sinus.

   • Sella Turcica: Central depression on the body, housing the pituitary gland.

   • Dorsum Sellae: Posterior to the sella turcica.

   • Clivus: Shallow depression extending from the dorsum sellae to the foramen magnum, supporting the pons and basilar artery.

   • Lesser Wings: Triangular, nearly horizontal, ending in the anterior clinoid processes.

   • Greater Wings: Extend laterally, forming part of the floor and sides of the cranium.

   • Foramina in Greater Wings:

     • Foramen Rotundum.

     • Foramen Ovale.

     • Foramen Spinosum.

   • Anterior Clinoid Processes: Bony projections arising from the posterior aspect of the lesser wings

   • Chiasmatic (Optic) Groove: Leads to the optic canal, ending at the optic foramen.

   • Superior Orbital Fissures: Irregularly shaped openings for cranial nerves and blood vessels.

   • Lateral Pterygoid Processes: Flat extensions on the inferior surface of the body.

   • Medial Pterygoid Processes: End in pterygoid hamuli.

   • Articulations: All seven other cranial bones, and five facial bones (palatine, zygomatic and vomer).

6. Ethmoid Bone:

   • Located primarily below the floor of the cranium, situated in the ethmoidal notch of the frontal bone.

   • Cribriform Plate: Small upper horizontal portion with foramina for olfactory nerves.

   • Crista Galli: Projects superiorly from the cribriform plate.

   • Perpendicular Plate: Projects downward in the midline, forming the bony nasal septum.

   • Lateral Labyrinths (Masses): Suspended from the cribriform plate, containing ethmoid air cells/sinuses.

   • Superior and Middle Nasal Conchae (Turbinates): Thin, scroll-shaped projections extending medially and downward from the medial wall of each labyrinth.

   • Articulations: Frontal and sphenoid cranial bones and 11 facial bones.

Cranium - Sagittal View

• Ethmoid bone is anterior to the sphenoid bone.

• Crista galli and cribriform plate project superiorly, and the larger perpendicular plate extends inferiorly.

• Sphenoid bone, which contains the saddle-shaped sella turcica, is located directly posterior to the ethmoid bone.

• One of the two long, slender pterygoid processes or plates extend down and forward and ending with the small pointed process called the pterygoid hamulus.

• Inferior and slightly anterior to the sella turcica of the sphenoid bone in this sagittal view is a hollow-like body area of the sphenoid, which houses the sphenoid sinus.

• The larger frontal bone also demonstrates a cavity directly posterior to the glabella that contains the frontal sinus.

• The vomer (a facial bone) is shown as a midline structure between parts of the sphenoid and parts of the ethmoid.

Joints of the Cranium - Sutures

The articulations/joints of the cranium are called sutures which are classified as fibrous joints. In an adult, these are immovable and therefore are synarthrodial-type joints.

• Coronal Suture: Separates the frontal bone from the parietal bones.

• Sagittal Suture: Separates the parietal bones in the midline.

• Lambdoidal Suture: Separates the parietal bones from the occipital bone.

• Squamosal Sutures: Inferior junctions of the parietal bones with the temporal bones.

• Bregma: Anterior end of the sagittal suture.

• Lambda: Posterior end of the sagittal suture.

• Pterions: Points at the junction of the parietals, temporals, and greater wings of the sphenoid.

• Asterions: Points posterior to the ear where the squamosal and lambdoidal sutures meet.

Infant Cranium

In an infant calvarium is large in proportion to the rest of the body, but the facial bones are quite small. Ossification of the individual cranial bones is incomplete at birth, and the sutures are membrane-covered spaces that fill in soon after birth. However, certain regions where sutures join are slower in their ossification, and these are called fontanels. The cranial sutures themselves generally do not ossify completely until an individual is in his or her mid-to-late 20s, and some may not completely close until the fifth decade of life.

Fontanels Early in life, the bregma and the lambda are not bony but are membrane-covered openings or "soft spots." These soft spots are termed the anterior and posterior fontanels in an infant. The anterior fontanel is the largest and at birth measures about $2.5$ cm wide and $4$ cm long. It does not completely close until about $18$ months of age.

Two smaller lateral fontanels that close soon after birth are the sphenoid (pterion in an adult) and mastoid (asterion in an adult) fontanels, which are located at the sphenoid and mastoid angles of the parietal bones on each side of the head. Six fontanels occur in an infant as follows:

1. Anterior fontanel

2. Posterior fontanel

3. Right sphenoid fontanel

4. Left sphenoid fontanel

5. Right mastoid fontanel

6. Left mastoid fontanel

Sutural, or Wormian, Bones

Certain small, irregular bones called sutural, or wormian, bones sometimes develop in adult skull sutures. These isolated bones most often are found in the lambdoidal suture but occasionally also are found in the region of the fontanels, especially the posterior fontanel. In the adult skull, these are completely ossified and are visible only the sutural lines around their borders.

Anatomy Review With Radiographs

Review exercises focus on the anatomy of the eight cranial bones as labeled on the radiographs on the right. Specific anatomic parts may be more difficult to recognize on radiographs compared with drawings, but knowing locations and relationships to surrounding structures and bones should aid in identifying these parts.

• Cranial Bones - PA Caldwell Projection

• Cranial Bones - AP Axial Projection

• Cranial Bones - Lateral Projection

Facial Bones

The 14 facial bones contribute to the shape and form of a person's face, and the cavities of the orbits, nose, and mouth are largely constructed from the bones of the face. Of the 14 bones that make up the facial skeleton, only 2 are single bones. The remaining 12 consist of six pairs of bones, with similar bones on each side of the face.

Facial Bones

• Maxillae (mak-sil'-e) (upper jaw), or maxillary bones

• Zygomatic (zi"-go-mat-ik) bones

• Lacrimal (lak'-ri-mal) bones

• Nasal bones

• Inferior nasal conchae (kong-ke)

• Palatine (pal-ah-tin) bones

• Vomer (vo-mer)

• Mandible (lower jaw)

Right and Left Maxillary Bones

The two maxillae, or maxillary bones, are the largest immovable bones of the face

The right and left maxillary bones are solidly united at the midline below the nasal septum. Each maxilla assists in the formation of three cavities of the face: (1) the mouth, (2) the nasal cavity, and (3) one orbit.

Each maxilla consists of a centrally located body and four processes that project from that body. Three processes are obvious The fourth process is the palatine process, which is part of the hard palate.

The two maxillae are solidly united in the midline anteriorly. At the upper part of this union is the anterior nasal spine.

A point at the superior aspect of the anterior nasal spine is the acanthion.

The body of each maxillary bone contains a large, air-filled cavity known as a maxillary sinus.

The fourth process of each maxillary bone is the palatine process, which can be demonstrated only on an inferior view of the two maxillae. The two palatine processes form the anterior portion of the roof of the mouth, called the hard or bony palate. The two palatine processes are solidly united at the midline. A common congenital defect called a cleft palate is an opening between the palatine processes that is caused by incomplete joining of the two bones.

Note the differences between the palatine process of the maxillary bone and the separate palatine facial bones.

Articulations Each maxilla articulates with two cranial bones (frontal and ethmoid) and with seven facial bones (zygoma, lacrimal, nasal, palatine, inferior nasal concha, vomer, and adjacent maxilla).

Right and Left Zygomatic Bones

One zygoma is located lateral to the zygomatic process of each maxilla. These bones (sometimes termed malar bones) form the prominence of the cheeks and make up the lower outer portion of the orbits.

Articulations Each zygoma articulates with three cranial bones (frontal, sphenoid, and temporal) and with one facial bone (maxilla).

Right and Left Nasal and Lacrimal Bones

The lacrimal and nasal bones are the thinnest and most fragile bones in the entire body.

Lacrimal bones

The two small and delicate lacrimal bones (about the size and shape of a fingernail) lie anteriorly on the medial side of each orbit just posterior to the frontal process of the maxilla. The lacrimal bones are closely associated with the tear ducts.

Nasal bones

The two fused nasal bones form the bridge of the nose and are variable in size. Much of the nose is made up of cartilage, and only the two nasal bones form the bridge of the nose. The nasal bones lie anterior and superomedial to the frontal process of the maxillae and inferior to the frontal bone.

The point of junction of the two nasal bones with the frontal bone is a surface landmark called the nasion.

###Articulations

• Lacrimal Each lacrimal bone articulates with two cranial bones (frontal and ethmoid) and with two facial bones (maxilla and inferior nasal concha).

• Nasal Each nasal bone articulates with two cranial bones (frontal and ethmoid) and with two facial bones (maxilla and adjacent nasal bone).

Right and Left Inferior Nasal Conchae

Within the nasal cavity are two platelike, curved (or scroll-shaped) facial bones called the inferior nasal conchae (turbinates). These two bones project from the lateral walls of the nasal cavity on each side and extend medially.

Sectional drawing

The superior and middle conchae are part of the ethmoid bone, and the inferior nasal conchae are separate facial bones.

Articulations:

• Inferior nasal conchae Each inferior nasal concha articulates with one cranial bone (ethmoid) and with three facial bones (maxilla, lacrimal, and palatine).

• Palatine Each palatine articulates with two cranial bones (sphenoid and ethmoid) and four facial bones (maxilla, inferior nasal conchae, vomer, and adjacent palatine).

Nasal Septum

The midline structures of the nasal cavity, including the bony nasal septum, are shown on this sagittal view drawing. Two bones-the ethmoid and the vomer-form the bony nasal septum. The nasal septum is cartilaginous and is termed the septal cartilage.

Vomer

The single vomer (meaning "plowshare") bone is a thin, triangular bone that forms the inferoposterior part of the nasal septum. A deviated nasal septum describes the clinical condition wherein the nasal septum is deflected or displaced laterally from the midline of the nose.

Articulations The vomer articulates with two cranial bones (sphenoid and ethmoid) and with four facial bones (right and left palatine bones and right and left maxillae). The vomer also articulates with the septal cartilage.

Mandible

The last and largest facial bone is the lower jaw, or mandible. It is the only movable bone in the adult skull. This large facial bone, which is a single bone in the adult, originates from two separate bones. The two bones in the infant join to become one at approximately $1$ year of age.

The angle (gonion) of the mandible divides each half of the mandible into two main parts. That area anterior to the angle is termed the body of the mandible, whereas the area superior to each angle is termed the ramus. Because the mandible is a single bone, the body extends from the left angle around to the right angle.

Located on each half of the body of the mandible is the mental foramina.

The upper portion of each ramus terminates in a U-shaped notch termed the mandibular notch.

The process at the anterior end of the mandibular notch is termed the coronoid process.

Memory aid The coronoid process of the mandible must not be confused with the coronoid process of the proximal ulna of the forearm or the coracoid process of the scapula. One way to remember these terms is to associate "n" in coronoid with the "n" in each of the words ulna and mandible.

The posterior process of the upper ramus is termed the condyloid process and consists of two parts. The rounded end of the condyloid process is the condyle or head, whereas the constricted area directly below the condyle is the neck. The condyle of the condyloid process fits into the TM fossa of the temporal bone to form the TMJ.

The horseshoe shape of the mandible is well visualized on a submentovertex (SMV) projection

Temporomandibular Joint

The TMJ, the only movable joint in the skull, is shown on this lateral drawing and on the lateral view photograph of skull. The relationship of the mandible to the temporal bone of the cranium is well demonstrated.

The TMJ is formed by the condyle (head) of the condyloid process of the mandible as it fits into the TM fossa of the temporal bone. The TMJ is located just anterior and slightly superior to the EAM.

Joint Classifications (Mandible and Skull)

Synovial Joints (Diarthrodial) The complex TMJ is classified as a synovial type of joint that is divided into upper and lower synovial cavities by a single articular fibrous disk. A series of strong ligaments join the condylar neck, ramus, and gonion of the mandible to the lower borders of the zygomatic process of the temporal bone.

Two movements are predominant. When the mouth opens, the condyle and the fibrocartilage move forward, and at the same time, the condyle revolves around the fibrocartilage. The TMJ is classified as a bicondylar joint similar to the knee.

Fibrous Joints (Synarthrodial)

Two types of fibrous joints involve the skull:

1. Sutures between cranial bones.

2. A unique type of fibrous joint involving the teeth with the mandible and maxillae. This is a gomphosis subclass type of fibrous joint that is found between the roots of the teeth and the alveolar processes of both the maxillae and the mandible.

Radiographs of the TMJ shows the range of anterior movement of the condyle in relationship to the TM fossa.

Paranasal Sinuses

The large, air-filled cavities of the paranasal sinuses are sometimes called the accessory nasal sinuses because they are lined with mucous membrane, which is continuous with the nasal cavity. These sinuses are divided into four groups, according to the bones that contain them.

The paranasal sinuses begin to develop in the fetus, but only the maxillary sinuses exhibit a definite cavity at birth. The frontal and sphenoid sinuses begin to be visible on radiographs at age $6$ or $7$. The ethmoid sinuses develop last. All the paranasal sinuses generally are fully developed by the late teenage years.

Maxillary Sinuses

The large maxillary sinuses are paired structures, one of which is located within the body of each maxillary bone. An older term for maxillary sinus is antrum, an abbreviation for antrum of Highmore.

Each maxillary sinus is shaped like a pyramid on a frontal view. Laterally, the maxillary sinuses appear more cubic.

All the paranasal sinus cavities communicate with one another and with the nasal cavity, which is divided into two equal chambers, or fossae.

Therefore, radiographic positioning of the paranasal sinuses should be accomplished with the patient in the erect position to delineate any possible air-fluid levels.

Frontal Sinuses

The frontal sinuses are located between the inner and outer tables of the skull, posterior to the glabella; they rarely become aerated before age $6$. The maxillary sinuses are always paired and are usually fairly symmetric in size and shape; the frontal sinuses are rarely symmetric.

Ethmoid Sinuses

The ethmoid sinuses are contained within the lateral masses or labyrinths of the ethmoid bone. These air cells are grouped into anterior, middle, and posterior collections, but they all intercommunicate.

Sphenoid Sinuses

The sphenoid sinuses lie in the body of the sphenoid bone directly below the sella turcica. Because the sphenoid sinuses are so close to the base or floor of the cranium, sometimes pathologic processes make their presence known by their effect on these sinuses.

Osteomeatal complex

The pathways of communication between the frontal, maxillary, and ethmoid sinuses provide drainage between these sinus cavities. These drainage pathways make up the osteomeatal complex, Examinations of the osteomeatal complex, sometimes referred to as the osteomeatal unit (OMU), can be imaged with CT to evaluate for obstructions.

• The large maxillary sinus drains through the infundibulum passageway down through the middle nasal meatus into the inferior nasal meatus. The uncinate process of the ethmoid bone makes up the medial wall of the infundibulum passageway. The ethmoid bulla receives drainage from the frontal and ethmoid sinus cells, which drains down through the middle nasal meatus into the inferior nasal meatus, where it exits the body through the exterior nasal orifice.

Radiographs demonstrate the relative locations and relationships of each of these sinuses.

• Lateral Position

• PA (Caldwell) Projection

• Parietoacanthial Transoral Projection (Open Mouth Waters)

• SMV Projection

Orbits

The complex anatomy of the 14 facial bones helps to form several facial cavities. These cavities, which are formed in total or in part by the facial bones, include the mouth (oral cavity), the nasal cavities, and the orbits. The mouth and nasal cavities are primarily passageways and are rarely imaged. However, the orbits that contain the vital organs of sight and associated nerves and blood vessels are imaged more frequently.

Each orbit is a cone-shaped, bony-walled structure. The rim of the orbit, which corresponds to the outer circular portion of the cone, is called the base. The posterior portion of the cone, the apex, corresponds to the optic foramen, through which the optic nerve passes.

The long axis of the orbits projects both upward and toward the midline. With the head placed in an upright frontal or lateral position with the orbitomeatal line adjusted parallel to the floor, each orbit would project superiorly at an angle of $30$° and toward the MSP at an angle of $37$°.

Bony Composition of Orbits

Each orbit is composed of parts of seven bones. The circumference or circular base of each orbit is composed of parts of three bones-the frontal bone (orbital plate) from the cranium and the maxilla and the zygoma from the facial bones.

The orbital plate of the frontal bone forms most of the roof of the orbit. The zygoma forms much of the lateral wall and some of the floor of the orbit, whereas a portion of the maxilla helps to form the floor.

Summary Chart - Bones of Orbits

Cranial Bones

1. Frontal

2. Sphenoid

3. Ethmoid

Facial Bones

1. Maxilla

2. Zygoma

3. Lacrimal

4. Palatine

Openings in Posterior Orbit

Each orbit also contains three holes or openings in the posterior portion, as shown in Fig. 11-72. These openings provide for passage of specific cranial nerves (CN).

The optic foramen is a small hole in the sphenoid bone that is located posteriorly at the apex of the cone-shaped orbit. The optic foramen allows for passage of the optic nerve (CN II), which is a continuation of the retina.

The superior orbital fissure is a cleft or opening between the greater and lesser wings of the sphenoid bone, located lateral to the optic foramen. It allows transmission of four primary cranial nerves (CN III to VI), which control movement of the eye and eyelid. A small root of bone that separates the superior orbital fissure from the optic canal is known as the sphenoid strut.

A third opening is the inferior orbital fissure, which is located between the maxilla, zygomatic bone, and greater wing of the sphenoid. It allows for transmission of the maxillary branch of CN V, which permits entry of sensory innervation for the cheek, nose, upper lip, and teeth.