Head Trauma

Skull: Anterior and Lateral Aspects

The anterior, or facial, aspect of the skull is composed of the frontal part of the calvaria (skullcap) above and the facial bones below. The facial contours and proportions are largely determined by the underlying bones, and it is a commonplace observation that they show considerable variations associated with age, sex, and race. The outer surface of the frontal bone underlies the brow. The facial skeleton is irregular, a feature accentuated by the presence of the orbital openings, the piriform aperture, and the superior and inferior dental arches of the oral cavity.

The convex anterior surface of the frontal bone is relatively smooth, but there are frontal tuberosities, or elevations, on each side. In early life, a median suture separates the two halves of the developing bone. This suture normally fuses between ages 6 and 10 years but occasionally persists as the metopic suture. The two orbital openings are roughly quadrangular and have supraorbital, infraorbital, medial, and lateral borders. The supraorbital notch, or fissure, carries the corresponding nerve and vessels. The infraorbital foramen, located about 1 cm below the infraorbital margin, transmits the nerve and vessels of the same name. The orbits are somewhat pyramidal in shape, with the quadrangular openings, or bases, directed forward and slightly outward, whereas the apexes correspond to the medial ends of the superior orbital fissures.

The superior wall (roof) separates the orbital contents from the brain and meninges in the anterior cranial fossa. Anteromedially, it is hollowed out by a variably sized frontal sinus, and anterolaterally, there is a shallow lacrimal fossa for the orbital part of the lacrimal gland. Posteriorly, the optic canal (foramen) lies between the two roots of the lesser wing of the sphenoid bone, just above the medial end of the superior orbital fissure; it transmits the optic (II) nerve and ophthalmic artery.

The inferior wall (floor) is formed mainly by the orbital surface of the maxilla, which separates the orbit from the maxillary sinus (antrum). A groove for the infraorbital nerve and vessels ends in the infraorbital foramen.

The thin medial wall separates the orbit from the ethmoidal air cells, the anterior part of the sphenoidal sinus, and the nasal cavity. At its anterior end, the lacrimal fossa is continuous below with the short nasolacrimal canal that opens into the inferior nasal meatus. The thicker lateral wall separates the orbit from the temporal fossa anteriorly and from the middle cranial fossa posteriorly. The orbital surface of the zygomatic bone shows a foramen for the zygomatic nerve, which bifurcates within the bone to emerge on the cheek and temporal fossa as the zygomaticofacial and zygomaticotemporal nerves, respectively.

The lateral wall and roof are continuous anteriorly but diverge posteriorly to bound the superior orbital fissure , which lies between the greater and lesser wings of the sphenoid bone and opens into the middle cranial fossa. The fissure transmits the oculomotor (III) and trochlear (IV) nerves, the lacrimal, frontal and nasociliary branches of the ophthalmic nerve, the abducens (VI) nerve, the ophthalmic veins, and small meningeal vessels.

The lateral wall and floor of the orbit are also continuous anteriorly but are separated posteriorly by the inferior orbital fissure , most of which is located between the greater wing of the sphenoid bone and the orbital surface of the maxilla. The inferior orbital fissure connects the orbit with the pterygopalatine and infratemporal fossae. The maxillary nerve passes from the pterygopalatine fossa into the orbit through the inferior orbital fissure and continues forward as the infraorbital nerve. Anastomotic channels between the orbital and pterygoid venous plexuses, and orbital fascicles from the pterygopalatine ganglion, also traverse this fissure.

The anterior nasal (piriform) aperture is bounded by the nasal and maxillary bones. The nasal bones articulate with each other in the midline, with the frontal bone above and with the frontal processes of the maxillae behind. The irregular lower borders of the nasal bones give attachment to the lateral nasal cartilages.

The lower face is supported by both the maxillary alveolar processes and the mandible. The inferior margin of each maxilla projects downward as the curved alveolar process, which unites in front with its fellow to form the U -shaped alveolar arch containing the sockets for the upper teeth. The roots of the teeth produce slight surface elevations, the most obvious of which are produced by the canine teeth. The upper border of the body of the mandible is called the alveolar part and contains sockets for the lower teeth, whose roots also produce slight surface elevations.

Viewed from the side, the skull is divided into the larger ovoid braincase and the smaller facial skeleton. The two are connected by the zygomatic bone, which acts as a yoke (zygon) between the temporal, sphenoid (greater wing) and frontal bones, and the maxilla. Other features on the lateral aspect of the skull include parts of the sutures between the frontal, parietal, sphenoid, and temporal bones (which form most of the braincase), and the sutures between such facial bones as the nasal, lacrimal, ethmoid, and maxilla. Clearly seen are the parts of the mandible and the temporomandibular joint, the external acoustic meatus and the various foramina that transmit nerves and vessels of the same name. Not readily visible are the foramen ovale and the foramen spinosum.

Certain features deserve particular mention. The curved superior and inferior temporal lines arch upward and backward over the frontal bone from the vicinity of the frontozygomatic suture, pass over the coronal suture and the parietal bone, and then turn downward and forward across the temporal squama to end above the mastoid process. The superior and inferior temporal lines provide attachments, respectively, for the temporal fascia and the upper margin of the temporal muscle, which occupies most of the temporal fossa . This fossa is bounded above by the superior temporal line, and below it is bounded by the infratemporal crest, separating the greater wing of the sphenoid bone from the pterygoid processes. The anteroinferior corner of the parietal bone usually fills the angle between the greater wing of the sphenoid and the frontal bone, although sometimes the squamous part of the temporal bone may extend forward to articulate directly with the frontal bone, thus excluding the sphenoid. This area is the pterion , and its internal surface is deeply grooved by the anterior branches of the middle meningeal vessels. It is situated about 3.5 cm behind the frontozygomatic suture (usually palpable as a slight ridge) and 4 cm above the zygomatic arch. As the most common site of damage to these vessels from a skull fracture, it is a surgical landmark.

The infratemporal fossa is an irregular space lying below the infratemporal crest. It is continuous above with the temporal fossa through the gap between the crest and the zygomatic arch. It is bounded medially by the lateral plate of the pterygoid process and the infratemporal surface of the maxilla, and laterally, by the ramus of the mandible. It communicates through the pterygomaxillary fissure with the pterygopalatine fossa.

Skull: Midsagittal Section

The rigid braincase is formed by the bones of the calvaria (see Plate 14-4 ) and the base of the skull (see Plate 14-5 ), which is divided into anterior, middle, and posterior cranial fossae (see Plates 14-6 and 14-7 ). These divisions are less visible on a sagittal section of the skull.

The occipital bone bounds most of the posterior cranial fossa. It is pierced by the foramen magnum, through which the medulla oblongata and spinal cord, surrounded by their meninges, become continuous; it also transmits the vertebral arteries, a few small veins, the spinal roots of the accessory (XI) nerves, and the recurrent meningeal branches from the upper spinal nerves. The occipital condyle articulates with the homolateral superior atlantoarticular process. The hypoglossal (XII) nerve passes through the corresponding canal. The jugular foramen lodges the superior bulb of the internal jugular vein (in which the sigmoid and inferior petrosal sinuses end); the glossopharyngeal (IX), vagus (X), and accessory nerves pass through it anteromedial to the bulb, and it provides an entry for the recurrent meningeal branches of the vagus and small meningeal branches of the ascending pharyngeal and occipital arteries. The basilar part of the occipital bone unites with the body of the sphenoid to form a sloping platform anterior to the pons and medulla oblongata.

The squamous part of the temporal bone is grooved by the posterior branches of the middle meningeal vessels and the sulcus along the superior border of its petrous part is for the superior petrosal sinus. The inferior petrosal sinus lies in the sulcus between the petrous temporal and occipital bones. The internal acoustic meatus is a canal about 1 cm long, ending in a cribriform septum that separates it from the internal ear. It transmits the facial (VII) nerve and its nervus intermedius, the vestibulocochlear (VIII) nerve, and the internal auditory (labyrinthine) artery.

The sphenoid bone has a central body from which two greater and two lesser wings and two pterygoid processes arise. The body contains two air sinuses separated by a septum that is often incomplete. Its concave upper surface, the sella turcica, houses the pituitary gland. The optic canal transmits the optic (II) nerve and the ophthalmic artery.

The nasal cavity is roofed over mainly by the cribriform plate of the ethmoid bone, augmented anteriorly by small parts of the frontal and nasal bones, and posteriorly, by the anteroinferior surface of the sphenoidal body. Its floor is formed by the palatine processes of the maxillae and by the horizontal plates of the palatine bones. The incisive canal transmits the nasopalatine nerves and branches of the greater palatine arteries. Each lateral wall is formed above by the nasal surface of the ethmoid bone that covers the ethmoidal labyrinth and supports thin, shell-like projections, the superior and middle nasal conchae . These overhang the corresponding nasal meatuses. Below, each lateral wall is formed by the nasal surface of the maxilla, the perpendicular plate of the palatine bone and the medial pterygoid plate. The maxillary and palatine bones articulate with a separate bone, the inferior nasal concha , overhanging the inferior nasal meatus. The sphenoidal air sinuses open into the nose through the sphenoidal aperture in the sphenoethmoidal recess posterosuperior to the superior concha. The frontal and maxillary air sinuses open into the middle meatus through a semilunar hiatus , and the multiple air cells forming the ethmoidal labyrinth open into the superior and middle meatuses. The lower opening of the nasolacrimal duct is near the anterior end of the inferior meatus. The sphenopalatine foramen behind the middle concha transmits the nasopalatine nerve.

The nasal cavity is subdivided by a more-or-less vertical septum formed by the perpendicular ethmoidal plate and the vomer. The triangular gap between them anteriorly is filled in by the nasal septal cartilage (not shown in the illustration).

Calvaria

The calvaria , or skullcap, is the roof of the cranium and is formed by the frontal, parietal, and occipital bones. It is ovoid in shape and widest toward the posterior parts of the parietal bones, but there are individual variations in size and shape associated with age, race, and sex; thus minor degrees of asymmetry are common.

The anterior part, or brow, is formed by the frontal bone, which extends backward to the coronal suture between the frontal bone and the parietal bones. The latter bones curve upward and inward to meet at the midline sagittal suture . Posteriorly, the parietal bones articulate with the triangular upper part of the occipital squama along the lambdoid suture . The meeting points of the sagittal suture with the coronal and lambdoid sutures are termed, respectively, bregma and lambda . In the fetal skull, they are the sites of the anterior and posterior fontanelles. The vertex , or highest point, of the skull lies near the middle of the sagittal suture. Parietal foramina are usually present; they transmit emissary veins passing between the superior sagittal sinus and the veins of the scalp.

The deeply concave internal, or endocranial, surface of the calvaria is made up of the inner aspects of the bones, sutures, and foramina mentioned above. The bones show indistinct impressions produced by related cerebral gyri, more evident grooves for dural venous sinuses and meningeal vessels, and small pits, or foveolae , for arachnoid granulations. Thus there is a median groove in the frontal, parietal, and occipital bones extending backward from the frontal crest to the internal occipital protuberance; it increases in width posteriorly and lodges the superior sagittal sinus . The frontal crest seen in the midline is produced by the coalescence of the anterior ends of the lips of the groove for the superior sagittal sinus. There are other, narrower grooves for meningeal vessels. The largest of these, the middle meningeal arteries and veins , leave their imprints in particular on the parietal bones, and the channels containing them may become tunnels where the anteroinferior angles of the parietal bones meet the greater wings of the sphenoid bone. The skull varies in thickness, and the area around the pterion is thin. It is relatively easily fractured by a blow to the side of the head, with possible tearing of the middle meningeal vessels. The resulting hemorrhage can be serious if it is not recognized and treated promptly.

The cut edge of the skullcap reveals that the constituent bones possess outer and inner laminae of compact bone separated by the diploë , a layer of cancellous bone. The outer lamina is thicker and tougher than the more brittle inner lamina.

External Aspect of Skull Base

The inferior surface of the base of the skull, the norma basilaris , is formed anteriorly by the arched hard palate, fringed by the maxillary alveolar processes and teeth; posteriorly by the wider occipital squama, pierced by the foramen magnum; and, in between, by an irregular area comprising several bony processes for muscular and tendinous attachments, articular and other fossae, and many foramina. The bones and fissures shown in the illustration need no added description, but the nerves and vessels traversing the foramina will be listed.

The incisive foramen transmits the terminal branches of the nasopalatine nerves and greater palatine vessels. The major and minor palatine foramina are traversed by the corresponding arteries and nerves. The choanae are the posterior nasal apertures.

The foramen ovale pierces the greater sphenoidal wing near the lateral pterygoid plate and the sulcus for the auditory tube; the mandibular nerve, the accessory meningeal artery, and communications between the cavernous sinuses and pterygoid venous plexus pass through it. The foramen spinosum , anteromedial to the sphenoidal spine, transmits the middle meningeal artery and the meningeal branch of the mandibular nerve.

The foramen lacerum is an irregular canal between the sphenoidal body, the apex of the petrous part of the temporal bone and the basilar part of the occipital bone. The upper end of the carotid canal opens into it, and the internal carotid artery with its nerves and veins, on emerging from the canal, turn upward to enter the cavernous sinus. Meningeal branches of the ascending pharyngeal artery and emissary veins from the cavernous sinus pass through the foramen lacerum, and the deep and greater petrosal nerves unite within it to form the nerve of the pterygoid canal.

The anterior part of the mandibular fossa articulates with the mandibular head and belongs to the temporal squama, but the posterior nonarticular part is derived from the tympanic plate. The tympanosquamous fissure between them is continued medially as the petrotympanic fissure , through which the chorda tympani nerve emerges. The stylomastoid foramen behind the root of the styloid process transmits the facial (VII) nerve and the stylomastoid branch of the posterior auricular artery.

The lower opening of the carotid canal is anterior to the jugular fossa , which lodges the superior bulb of the internal jugular vein. The canal bends at right angles within the petrous part of the temporal bone, and its upper end opens into the foramen lacerum. The tympanic canaliculus pierces the ridge between the carotid canal and the jugular fossa and conveys the tympanic branch of the glossopharyngeal (IX) nerve to the tympanic plexus. The mastoid canaliculus opens on the lateral wall of the fossa and transmits the auricular branch of the vagus (X) nerve. The jugular foramen in the depth of the fossa may be partly or completely divided into three parts by bony spicules. The anteromedial compartment transmits the inferior petrosal sinus and a meningeal branch of the ascending pharyngeal artery; the intermediate part transmits the glossopharyngeal, vagus, and accessory (XI) nerves; and the posterolateral part conveys the sigmoid sinus to the superior bulb of the internal jugular vein. Often seen near the posterior border of the mastoid process is a mastoid foramen , which is traversed by an emissary vein from the sigmoid sinus and a meningeal twig from the occipital artery. The anterior end of the hypoglossal canal (for the hypoglossal [XII] nerve and some small meningeal vessels) is above the anterior end of the occipital condyle. Behind the condyle is a shallow condylar fossa, usually pierced by a condylar foramen conveying an emissary vein between the sigmoid sinus and cervical veins.

The posterior part of the base of the skull is formed predominantly by the occipital squama; these are marked by nuchal lines, occipital crest, and so forth, which serve mainly for muscular and ligamentous attachments. However, the most notable feature is the foramen magnum , through which the medulla oblongata and spinal cord become continuous. The vertebral arteries, spinal roots of the accessory nerves, and recurrent meningeal branches from the upper cervical nerves ascend through the foramen magnum, while down through it pass the anterior and posterior spinal arteries.

Bones, Markings, and Orifices of Skull Base

The internal surface of the base of the skull has adapted its shape to the configuration of the adjacent parts of the brain. It consists of three cranial fossae, the anterior, middle, and posterior, which are separated by conspicuous ridges and increase in size and depth from front to back.

The anterior cranial fossa is the shallowest of the three fossae and lodges the lower parts of the frontal lobes of the brain. The sulci and gyri of the lobes are mirrored in the irregularities of the bony surfaces. It is limited anteriorly and laterally by the frontal bone. On each side, the floor is formed by the slightly domed and ridged orbital plate of the frontal bone , which supports the orbital surface of the homolateral frontal lobe of the brain and its meninges and separates them from the orbit. Posterior extensions from the frontal air sinuses may expand the orbital plates for varying distances, and the medial parts of these plates overlie the ethmoidal labyrinths.

On each side of the midline crista galli are the grooved ethmoidal cribriform plates that help to form the roof of the nasal cavity, lodge the olfactory bulbs, and provide numerous orifices for the delicate olfactory nerves. A small pit exists between the frontal crest and the crista galli, the foramen cecum , which occasionally transmits a tiny vein from the nose to the superior sagittal sinus. The crista galli and frontal crest give attachment to the anterior end of the falx cerebri.

Posterior to the ethmoid and frontal bones, the floor of the anterior cranial fossa is formed by the anterior part of the body of the sphenoid bone, the jugum sphenoidale , and on each side, by the lesser wings of this bone. These lesser wings slightly overlap the anterior part of the middle cranial fossa and project into the stems of the lateral cerebral sulci, thus forming the upper boundaries of the superior orbital fissures.

The medial ends of the posterior borders of the lesser wings end in small, rounded projections, the anterior clinoid processes , which provide attachments for the anterior ends of the free border of the tentorium cerebelli. Each anterior process is grooved on its medial side by the internal carotid artery, and each may be joined to the inconstant middle clinoid process by a thin osseous bar, thus forming a narrow bony ring around the artery as it emerges from the cavernous sinus.

The middle cranial fossa is intermediate in depth between the anterior and posterior fossae. It is narrow and elevated medially but expands and becomes deeper at each side to lodge and protect the temporal lobes of the brain. It is bounded anteriorly by the posterior borders of the lesser wings of the sphenoid bone and the anterior margin of the prechiasmatic sulcus; posteriorly by the superior borders of the petrous parts of the temporal bones, which are grooved by the superior petrosal sinuses and by the dorsum sellae of the sphenoid; and laterally by the greater wings of the sphenoid, the frontal angles of the parietal bones, and the temporal squamae.

The floor in the median area is formed by the body of the sphenoid bone , containing the sphenoidal air sinuses. The lesser wings of the sphenoid are attached to its body by two roots, separated from each other by the optic canals that transmit the optic (II) nerves and ophthalmic arteries. Behind the prechiasmatic sulcus is a median elevation, the tuberculum sellae , and the hypophyseal fossa housing the pituitary gland. The fossa is limited behind by the dorsum sellae , an upward-projecting bony plate with a concave upper border expanding laterally into the posterior clinoid processes . Lateral to the sellae is a shallow, sinuous groove for the internal carotid artery ; at its anterior end on the medial side may be a small tubercle, the middle clinoid process .

The lateral parts of the middle fossa are related in front to the orbits, on each side to the temporal fossae, and below to the infratemporal fossae. The middle fossa communicates with the orbits through the superior orbital fissures.

Various other, more-or-less symmetric openings exist on each side. The foramen rotundum pierces the greater wing of the sphenoid bone just below and behind the inner end of the superior orbital fissure, and then it opens anteriorly into the pterygopalatine fossa. The foramen ovale also penetrates the greater sphenoidal wing posterolateral to the foramen rotundum and leads downward into the infratemporal fossa. The smaller foramen spinosum lies posterolateral to the foramen ovale and opens below into the infratemporal fossa close to the sphenoidal spine; the sulcus for the middle meningeal vessels starts at this foramen. The foramen lacerum is an irregular aperture between the body and greater wing of the sphenoid bone and the apex of the petrous part of the temporal bone; it marks the point of entry of the internal carotid artery into the cavernous sinus. Behind the foramen lacerum is the shallow depression for the trigeminal (semilunar) ganglion on the anterior surface of the petrous temporal bone, and lateral to this are two narrow grooves leading to the hiatuses for the lesser (minor) and greater (major) petrosal nerves.

The arcuate eminence is produced by the superior semicircular canal of the internal ear. Anterolateral to this eminence is a thin plate of bone, the tegmen tympani , forming the roof of the tympanic cavity and mastoid antrum and extending forward and medially to cover the bony part of the auditory (pharyngotympanic) tube.

The posterior cranial fossa is the largest and deepest of the cranial fossae and lodges the cerebellum, pons, and medulla oblongata. It is bounded anteriorly by the dorsum sellae, the back of the body of the sphenoid bone, and the basilar part of the occipital bones; posteriorly by the squama of the occipital bone below the sulci for the transverse sinuses and the internal occipital protuberance; and laterally by the petrous and mastoid parts of the temporal bones, the mastoid angles of the parietal bones, and the lateral parts of the occipital bone.

The posterior fossa is pierced by a number of foramina and is grooved by various dural venous sinuses. A large median opening in the floor of the fossa, the foramen magnum , penetrates the occipital bone. The medulla oblongata and spinal cord and their surrounding meninges become directly continuous immediately below the foramen. The petrous part of the temporal bone and the occipital bone are separated by the petro-occipital fissure and the sulcus for the inferior petrosal sinus ; the fissure ends behind, in the jugular foramen . The inferior petrosal and sigmoid sinuses pass through the anterior and posterior parts of this foramen, respectively, while the glossopharyngeal (IX), vagus (X), and accessory (XI) nerves occupy an intermediate position as they leave the skull.

Two canals are associated with the occipital condyles: the hypoglossal canal , for the twelfth cranial nerve, and the condylar canal .

Above the jugular foramen, the internal acoustic meatus tunnels into the petrous part of the temporal bone. It is about 1 cm long and is separated laterally from the internal ear by a thin bony plate pierced by many apertures for fascicles of the facial (VII) and vestibulocochlear (VIII) nerves. Behind the orifice of this meatus is the slitlike opening of the vestibular aqueduct , which lodges the blind end of the endolymphatic duct.

The internal opening of the inconstant mastoid foramen is close to the sulcus for the sigmoid sinus , which winds downward from the transverse sinus to the jugular foramen, where it ends in the superior bulb of the internal jugular vein. The internal occipital protuberance is related to the confluence of the superior sagittal, straight, occipital, and transverse sinuses. The margins of the sulci for the transverse sinuses give attachment to the tentorium cerebelli.