The Osteology of the Anterior and Middle Cranial Vault (the Frontal, Ethmoid, Sphenoid, and Temporal Bones)

The frontal bone has two components, one vertical and the other horizontal, mimicking the L-shaped architecture of the temporal bone. The vertical component resembles the hinged, bivalved, cockle shell of shellfish, and thus is referred to as the frontal boss or squama.7 The horizontal plate of the frontal bone (the frontal sinus floor) is shelf-like and the most anterior component of the cranial base. It extends laterally to overlie the medial superior orbital roof and medially to overlie the anterior ethmoid cells7 ^– 11 ( Fig. 6.2A,B ).

The frontal sinus has its greatest (anterior-posterior) depth in the midline at the level of the nasofrontal suture, and its anterior table is thickest at the midline, immediately above the nasofrontal suture. 6 ^, 7 ^, 12 ^– 14 The frontal sinus floor and the posterior table are only a few millimeters in thickness, thus subject to trauma. The posterior table marks the forward limit of the anterior cranial fossa (skull base) ( Fig. 6.3 ).

The brows and superior orbital rims are “surface markers” that suggest the level of the floors of the frontal sinus and the anterior cranial fossa. 7 The distance from the anterior table to the posterior table is abbreviated. Fractures of the anterior and posterior tables, with overlying lacerations, may therefore create a conduit to the dura and underlying brain. Cerebrospinal fluid (CSF) may be expressed through the open wound.

The frontal bone contributes to the roof of the orbit and the upper medial orbital wall. 15 The roof reaches its nadir approximately 1 cm beyond the supraorbital rim and then gradually descends toward the outlet of the optic canal (see Chapter 8). The orbital roof separates the orbit from the anterior cranial fossa laterally and from the frontal sinus medially2 ^, 7 ( Fig. 6.4A,B ).

The anterior table, posterior table, and crista galli are pictured. Attachment of the dura in the area of the crista is commonly rent asunder by fractures of the posterior table ( Fig. 6.5 ).

Like all anterior paranasal sinuses, the frontal sinus has a defined outflow tract, with three components: an infundibulum, an ostium, and a recess. 16 ^, 17

The infundibulum of the frontal sinus outflow tract (FSOFT), a funnel-like structure found in the anteromedial floor, is directed posteriorly toward the ostium and nasofrontal recess. It is on a line 8 mm off the midline and directed inwardly only some 8 to 12 mm. The nasofrontal recess (the third component) of the tract is a passive space, created by the agger nasi cells anteriorly and the ethmoid bulla posteriorly ( Fig. 6.6 ).

The boundaries of the frontal recess are created by surrounding structures such that the term “nasofron-tal duct” is a misnomer, and, according to international nomenclature, should be abandoned. A true nasofrontal duct exists in only 5% of patients, each with aberrant anatomy.14 The infundibulum and the ostium are visible from above in the anterior medial floor at surgery or upon examination of the skull; the frontal recess by comparison is visible only from below with the aid of an intranasal endoscope.

The ethmoid bone establishes the greatest depth of the anterior cranial fossa. Housed at its center are the cribriform plate (through which the two olfactory tracts pass) and fovea ethmoidalis, the roof of the ethmoid (see Chapter 7). The cribriform plate is only 1 mm thick and is delicate compared with the fovea. The fovea progressively thickens away from the midline,18 much like a miniature palatal platform (see Chapter 4) ( Fig. 6.7A ).

The cribriform plate and fovea are vulnerable to load forces and are readily involved by fractures of the floor of the frontal sinus and the medial orbital frame19 ^– 26 ( Fig. 6.7B ).

The upwardly contoured orbital roofs (created by the orbital plates of the frontal bone) bear the corrugated stigmata of contact with the gyri of the frontal lobe of the brain.7

Wedged between the frontal, ethmoid, temporal, and occipital bones is the sphenoid, an unusually shaped bone with a body and three pairs of processes: greater wings, lesser wings, and pterygoid processes. The intermediate position of the sphenoid is apparent in lateral and frontal views of the “painted skull” and equally notable when viewed at the cranial base ( Fig. 6.8 ). The span and the other features of the sphenoid at the cranial base are dramatized in the painted skull.

The sphenoid bone is strategically located and receives load-bearing stresses (facial force equilibrium circuits) (see Chapter 2) from multiple areas, notably the following five ( Fig. 6.9 ):

• The frontal boss and orbital plates of the frontal bone

• The anterior and posterior maxillary buttresses of the midface, arising from the palatal platform

• The squama and petrous pyramid of the temporal bone

• The condyles of the mandible, by way of the glenoid fossa

• The most anterior portion of the occipital bone

The planum sphenoidale and the anterior clinoid processes mark the posterior limit of the anterior cranial fossa (anterior skull base).6 ^, 7 The planum sphenoidale is a flattened surface, forming the anterior most portion of the roof of the sphenoid sinus in front of the sella turcica ( Fig. 6.10 ).

The planum connects the two lesser wings of the sphenoid as they arise from the optic strut (see Chapter 8) to create the sphenoid ridge laterally and the anterior clinoid process posteriorly.

The optic chiasm resides in a groove just medial to the anterior clinoid process.

The greater wings of the sphenoid are elevated centrally to create the sella turcica but depressed laterally to accommodate each temporal lobe at a lower plane. The greater wings, in doing so, form the superior orbital fissure and define the anterior boundary of the middle fossa.

The descent in the floor of the middle fossa puts the pole of the temporal lobe behind the thickened, deep lateral wall of the orbit (contributed by the great wing of the sphenoid). Occult temporal lobe contusion is suspected when the deep lateral wall of the orbit is displaced.27

The lateral walls, roof (planus sphenoidale), and floor of the sphenoid sinus are thick and robust, as seen in coronal section.6 ^, 28 The floor forms the roof of the nasal vault (choana) below and is twice the thickness of the bone of the posterior palate, creating, as evident in Fig. 6.11 , a tiered architecture. The first tier establishes the palatal platform, the sphenoidal platform is some 2 cm directly above, and the roof of the sphenoid sinus is 1 to 1.5 cm further superior.

The sphenoid sinus floor extends laterally to engage the dense base of the petrous pyramid, the squama of the temporal bone, and a small portion of the occipital bone. Structural continuity across the base of the skull, at the level of the sphenoid floor, is broken only by three foramina (foramen ovale, lacerum, and spinosum [OLS]) in the most lateral aspect of the greater wing of the sphenoid ( Fig. 6.12 ).

The foramina create an area of structural weakness, just lateral to the lateral wall of the sphenoid sinus. Frontobasilar, ethmoidosphenoid (basilar), temporal, temporobasilar, and occiptobasilar fractures tend to “track” toward this area, and the foramina (OLS) serve as “end points.” When front-to-back or side-to-back basilar fractures combine, the faults retain their “rights of passage” through one or more of the foramina.

The posterior boundary of the middle fossa is defined by the clivus and by the petrous ridge bilaterally.7 ^, 29 The floor (greater wing of the sphenoid) and lateral walls (squama of the temporal bone) of the middle fossa are grooved by the gyri of the temporal lobes and by the middle meningeal artery as its frontal and parietal branches course anterolaterally. The frontal branch of the artery ascends to the lateral extreme of the sphenoid ridge, before turning posteriorly. The turning point of the anterior branch of the middle meningeal artery, in terms of surface anatomy, is referred to as the pterion, a key landmark some 5.0 cm above the suture of the zygomatic arch, where the coronal, frontal, temporal, and parietal sutures intersect.

Fracture of the relatively thin bone at the pterion may injure the anterior branches of the middle meningeal artery (clinging to the undersurface of the cortex of the pterion) and provoke an epidural hematoma ( Fig. 6.13 ).

The temporal bones are situated at the sides and base of the skull. Each temporal bone consists of five parts that fuse by the end of the first year of life: the squama, petrous, tympanic, and mastoid portions, and the styloid process. Based on its pyramidal shape, the petrous segment is called the petrous pyramid, with the apex directed inwardly.

The squama is L-shaped. Its outer surface is roughened and convex, providing attachment of the temporalis muscle before the muscle descends to attach to the coronoid process. The squama of the temporal bone is not as thick as the squama of the frontal and occipital bones ( Fig. 6.14 ).

Projecting forward from the lower portion of the temporal squama is the beginning of a long “arch,” the zygomatic process (see Chapter 8). The anterior end is deeply serrated and angles downwardly to engage the upwardly serrated temporal process of the zygomatic bone 6 ^, 7 (see Chapter 8). The temporal fascia is attached at the upper border of the arch and the masseter muscle the lower ( Fig. 6.15 ).

As the zygomatic process (of the temporal bone) joins the temporal process of the zygoma, it becomes flattened and linear (see Chapter 8). With trauma, the fragments of the arch splay outwardly and may overlap, such that the linear architecture of the arch is replaced by outward bowing ( Fig. 6.16 ).

Upon impact, the body of the zygoma collapses with loss of malar projection and tends in many instances to settle 1) inferiorly, medially, and posteriorly or 2) inferiorly, posteriorly, and outwardly . Outward and downward displacement expands the orbit; medial and inward dislocation constricts the orbit and its contents and may provoke rapid intervention. In a structural sense, the zygomatic arch, zygoma, and lateral orbit are to the upper face what the palatal shelves and mandible are to the lower face, as they determine appropriate upper facial width and projection. The goal of surgery is reestablishment of the linearity of the splayed arch and the return of it and the zygoma to their anatomic position ( Figs. 6.17 and 6.18 ).

“Trimanual reduction” is best achieved with an elevator (through a small incision in the maxillary vestibule) while manual pressure is applied over the central aspect of the arch by a surgical assistant. Rigid fixation is required in the presence of instability, but exposure (of the arch) is provided by a coronal incision (see Chapter 3).

The posterior end of the zygomatic process is anchored by two roots, the anterior and the posterior roots. The posterior root is sharply chiseled as it curves backward and upward in continuity with the temporal line that defines the attachment of the origin of the temporalis muscle. The posterior root passes inferiorly to contribute to the lateral osseous aspect of the external auditory canal. Fractures of the posterior root and external canal may extend to the lateral aspect of the mastoid and trigger hemorrhage in the region ( Fig. 6.19 ).

The broad, strong anterior root of the zygomatic process ends abruptly in a rounded, cartilage-covered eminence, the articular tubercle.7 The tubercle forms the front boundary of the glenoid (mandibular) fossa for receipt of the condyle of the mandible.

After providing a fossa for registration of the condyle of the mandible and after launching its contribution to the zygomatic process, the base of the squama turns inward to interdigitate with the greater wing of the sphenoid. It is joined by the petrous-mastoid complex, as best evidenced from an inferior perspective ( Fig. 6.20 ).

The labyrinth is a closed, fluid-filled archipelago of three cavities within the petrous part of the temporal bone: the semicircular canals, the vestibule, and cochlea . The labyrinth is surrounded by extremely dense bone, the otic capsule. The semicircular canals are outermost (lateral most) and house the semicircular ducts. The vestibule is a small chamber in which there is the oval window occupied by the base plate of the stapes. The cochlear labyrinth is innermost (medial most) and contains the cochlear duct and a central core, the modiolus, about which the cochlear duct is wrapped ( Fig. 6.21 ).

Fracture-lines by high-resolution computed tomography (HRCT) either skirt or, in some cases, traverse the dense bone of the otic capsule to end near the join of the temporal bone and the greater wing of the sphenoid, where a cluster of foramina exist (OLS).

The posterior skull base houses the largest and deepest of the three cranial fossae, the posterior fossa, offering lodging for the cerebellum, pons, and medulla oblongata.7 ^, 30 The occipital bone is its largest contributor, but the petrous and mastoid parts of the temporal bone define its anterolateral borders ( Fig. 6.22 ).

Four foramina are present: the internal auditory meatus, jugular foramen, foramen for transit of the hypoglossal nerve, and the foramen magnum . A horizontal line through the anterior margin of the foramen magnum passes through the external auditory meatus but is posterior to the anterior root of the zygoma (zygomatic arch).