2. Ossa Cranii

Development of the Ossa Cranii

Fig. 2.1 Bones of the skull

Left lateral view. The skull forms a bony capsule that encloses the brain and viscera of the head. The bones of the skull are divided into two parts. The viscerocranium (orange), the facial skeleton, is formed primarily from the arcus pharyngei (see pp. 6 and 7). The neurocranium (gray), the cranial vault, is the bony capsule enclosing the brain. It is divided into two parts based on ossification (see Fig. 2.2). The cartilaginous neurocranium undergoes endochondral ossification to form the base of the skull. The membranous neurocranium undergoes intramembranous ossification.

Fig. 2.2 Ossification of the ossa cranii

Left lateral view. The bones of the skull develop either directly or indirectly from mesenchymal connective tissue. The bones of the desmocranium (gray) develop directly via intramembranous ossification of mesenchymal connective tissue. The bones of the chondrocranium (blue) develop indirectly via endochondral ossification of hyaline cartilage. Note: The skull base is formed exclusively by the chondrocranium. Elements formed via intramembranous and endochondral ossification may fuse to form a single bone (e.g., the elements of the ossa occipitale, temporale, and sphenoidale contributing to the skull base are cartilaginous, while the rest of the bone is membranous).

Fig. 2.3 Suturae cranii (craniosynostoses) and fonticuli

A Left lateral view of neonatal skull.

B Superior view of neonatal skull.

The flat cranial bones grow as the brain expands; thus the sutures between them remain open after birth. In the neonate, there are six areas (fonticuli) between the still-growing cranial bones that are occupied by unossified fibrous membrane. The fonticulus posterior provides a reference point for describing the position of the fetal head during childbirth. The fonticulus anterior provides access for drawing liquor cerebrospinalis (CSF) samples in infants (e.g., in suspected meningitis).

Fig. 2.4 Sutures in the adult skull

A Left lateral view.

B Superior view.

Synostosis (the fusion of the cranial bones along the sutures) occurs during adulthood. Although the exact times of closure vary, the order (sagittalis, coronalis, lambdoidea) does not. Closure of each fonticulus yields a particular junction (see Table 2.2). Premature closure of the suturae cranii produces characteristic deformities (see Fig. 2.11, p. 22).

Skull: Lateral View

Fig. 2.5 Ossa cranii

Left lateral view.

Fig. 2.6 Skull (cranium)

Left lateral view. This view displays the greatest number of ossa cranii (indicated by different colors in Fig. 2.5). The arcus zygomaticus is formed by the processus zygomaticus ossis temporalis and the processus temporalis ossis zygomatici, which are united by an oblique suture.

Skull: Anterior View

Fig. 2.7 Ossa cranii

Anterior view.

Fig. 2.8 Le Fort classification of midfacial fractures

The framelike construction of the facial skeleton leads to characteristic patterns of fracture lines in the midfacial region (Le Fort I, II, and III).

Le Fort I: This fracture line runs across the maxilla and above the palatum durum. The maxilla is separated from the upper facial skeleton, disrupting the integrity of the sinus maxillaris (low transverse fracture).

Le Fort II: The fracture line passes across the sinus maxillaris, os ethmoidale, maxilla, and os zygomaticum, creating a pyramid fracture that disrupts the integrity of the orbita.

Le Fort III: The facial skeleton is separated from the base of the skull. The main fracture line passes through the orbitae, and the fracture may additionally involve the ossa ethmoidalia, sinus frontales, sinus sphenoidales, and ossa zygomatica.

Fig. 2.9 Skull

Anterior view. The boundaries of the facial skeleton (viscerocranium) can be clearly appreciated in this view. The bony margins of the apertura piriformis mark the start of the respiratory tract in the skull. The cavitas nasi, like the orbitae, contains a sensory organ (the olfactory mucosa). The sinus paranasales are shown schematically in Fig. 7.8, p. 175. The anterior view of the skull also displays the three clinically important openings through which sensory nerves pass to supply the face: the foramen supraorbitale, foramen infraorbitale, and foramen mentale.

Skull: Posterior View

Fig. 2.10 Ossa cranii

Posterior view.

Fig. 2.11 Premature closure of suturae cranii

The premature closure of a sutura cranii (craniosynostosis) may lead to characteristic cranial deformities:

A Sutura sagittalis: scaphocephaly (long, narrow skull).

B Sutura coronalis: oxycephaly (pointed skull).

C Sutura frontalis: trigonocephaly (triangular skull).

D Asymmetrical suture closure, usually involving the sutura coronalis: plagiocephaly (asymmetric skull).

Fig. 2.12 Hydrocephalus and microcephaly

A Hydrocephalus: When the brain becomes dilated due to liquor cerebrospinalis (CSF) accumulation before the suturae cranii ossify, the neurocranium will expand, whereas the facial skeleton remains unchanged.

B Microcephaly: Premature closure of the suturae cranii results in a small neurocranium with relatively large orbitae.

Fig. 2.13 Skull

Posterior view. The os occipitale, which is dominant in this view, articulates with the ossa parietalia, to which it is connected by the sutura lambdoidea. Ossa suturalia are isolated bone plates often found in the sutura lambdoidea. The suturae cranii are a special type of syndesmosis (i.e., ligamentous attachments that ossify with age). The outer surface of the os occipitale is contoured by muscular origins and insertions: the lineae nuchales inferior, superior, and suprema and crista occipitalis externa.

Calvaria

Fig. 2.14 Bones of the calvaria

External surface, superior view.

Fig. 2.15 The scalp and calvaria

The three-layered calvaria consists of the lamina externa, the diploë, and the lamina interna. The diploë has a spongy structure and contains red (blood-forming) bone marrow. With a plasmacytoma (malignant transformation of certain white blood cells), many small nests of tumor cells may destroy the surrounding bony trabeculae, and radiographs will demonstrate multiple lucent areas (“punched-out lesions”) in the skull.

Fig. 2.16 Sensitivity of the lamina interna to trauma

The lamina interna of the calvaria is very sensitive to external trauma and may fracture even when the lamina externa remains intact (look for cor responding evidence on CT images).

Fig. 2.17 External and internal surface of the calvaria

The external surface of the calvaria (A) is relatively smooth, unlike its internal surface (B). It is defined by the ossa frontale, parietale, and occipitale, which are interconnected by the suturae coronalis, sagittalis, and lambdoidea. The smooth external surface is interrupted by the foramina parietalia, which gives passage to the venae emissariae parietales (see Fig. 3.24, p. 67). The internal surface of the calvaria bears a number of pits and grooves:

• Foveolae granulares (small pits in the inner surface of the skull caused by saccular protrusions of the arachnoidea mater [granulationes arachnoideae] covering the encephalon)

• Groove for the sinus sagittalis superior (a sinus durae matris of the encephalon, see Fig. 3.22, p. 66)

• Sulci arteriosi (which mark the positions of the arterial vessels of the dura mater, such as the arteria meningea media, which supplies most of the dura mater and overlying bone)

• Crista frontalis (which gives attachment to the falx cerebri, a sickleshaped fold of dura mater between the hemispheria cerebri).

Skull Base: Exterior

Fig. 2.18 Bones of the skull base

External surface, inferior view. The base of the skull is composed of a mosaic-like assembly of various bones.

Fig. 2.19 Skull base

External surface, inferior view. Note the openings that transmit nerves and vessels. With abnormalities of bone growth, these openings may remain too small or may become narrowed, compressing the neurovascular structures that pass through them. The symptoms associated with these lesions depend on the affected opening. All of the structures depicted here will be considered in more detail in subsequent pages.

Skull Base: Interior

Fig. 2.20 Bones of the skull base

Internal surface, superior view.

Fig. 2.21 The fossae cranii

A Skull base, internal surface, superior view. B Skull base, midsagittal section. The interior of the skull base is deepened to form three successive fossae: the fossae cranii anterior, media, and posterior. These depressions become progressively deeper in the frontal-to-occipital direction, forming a terraced arrangement that is displayed most clearly in B.

The fossae cranii are bounded by the following structures:

• Anterior to middle: alae minores ossis sphenoidalis and the jugum sphenoidale

• Middle to posterior: margo superior partis petrosae ossis temporalis and the dorsum sellae

Fig. 2.22 Common fracture lines of skull base

Internal surface, superior view. In response to masticatory pressures and other mechanical stresses, the bones of the skull base are thickened to form “pillars” along the principal lines of force. The intervening areas that are not thickened are sites of predilection for bone fractures, resulting in the typical patterns of skull base fracture lines shown here in red. An analogous phenomenon of typical fracture lines is found in the midfacial region (see the anterior views of Le Fort fractures on p. 20).

Fig. 2.23 Skull base

Internal surface, superior view. The openings in the interior of the base of the skull do not always coincide with the openings visible on the exterior because some neurovascular structures change direction when passing through the bone or pursue a relatively long intraosseous course. An example of this is the meatus acusticus internus, through which the nervus facialis, among other structures, passes from the interior of the skull into the pars petrosa ossis temporalis. Most of its fibers then leave the pars petrosa through the foramen stylomastoideum, which is visible from the external aspect (see Fig. 4.87, p. 131, and Fig. 2.45, p. 44, for further details).

In learning the sites where neurovascular structures pass through the base of the skull, it is helpful initially to note whether these sites are located in the fossa cranii anterior, media, or posterior. The arrangement of the fossae cranii is shown in Fig. 2.21.

The lamina cribrosa ossis ethmoidalis connects the cavitas nasi with the fossa cranii anterior and is perforated by numerous foramina for the passage of the fila olfactoria (see Fig. 7.22, p. 180). Note: Because the bone is so thin in this area, a frontal head injury may easily fracture the lamina cribrosa and lacerate the dura mater, allowing liquor cerebrospinalis (CSF) to enter the nose. This poses a risk of meningitis, as bacteria from the nonsterile cavitas nasi may enter the sterile CSF.

Os Sphenoidale

Fig. 2.24 Position of the os sphenoidale in the skull

The os sphenoidale is the most structurally complex bone in the human body. It must be viewed from various aspects in order to appreciate all its features (see also Fig. 2.25):

A Skull base, exterior. The os sphenoidale combines with the os occipitalis to form the load-bearing midline structure of the skull base.

B Skull base, interior. The ala minor ossis sphenoidalis forms the boundary between the fossae cranii anterior and fossae cranii media. The openings for the passage of nerves and vessels are clearly displayed (see details in Fig. 2.45).

C Left lateral view. Portions of the ala major ossis sphenoidalis can be seen above the arcus zygomaticus, and portions of the processus pterygoideus can be seen below the arcus zygomaticus.

Fig. 2.25 Isolated os sphenoidale

A Inferior view (its position in situ is shown in Fig. 2.24). This view demonstrates the laminae medialis and lateralis processus pterygoidei. Between them is the fossa pterygoidea, which is occupied by the musculus pterygoideus medialis. The foramen spinosum and foramen ovale provide pathways through the base of the skull (see also in C).

B Anterior view. This view illustrates why the os sphenoidale was originally called the os sphecoidale (“wasp bone”) before a transcription error turned it into the os sphenoidale (“wedge-shaped”). The aperturae sinus sphenoidalis on each side resemble the eyes of the wasp, and the processus pterygoidei of the os sphenoidale form its dangling legs, between which are the fossae pterygoideae. This view also displays the fissura orbitalis superior, which connects the fossa cranialis media with the orbita on each side. The two sphenoid sinuses are separated by an internal septum (see Fig. 7.11, p. 175).

C Superior view. The superior view displays the sella turcica, whose central depression, the fossa hypophysialis, contains the hypophysis (glandula pituitaria). The foramen spinosum, foramen ovale, and foramen rotundum can be identified.

D Posterior view. The fissura orbitalis superior is seen clearly in this view, whereas the canalis opticus is almost completely obscured by the processus clinoideus anterior. The foramen rotundum is open from the fossa cranii media to the fossa pterygopalatina of the skull (the foramen spinosum is not visible in this view; compare with A). Because the ossa sphenoidale and occipitale fuse together during puberty (“tribasilar bone”), a sutura is no longer present between the two bones. The cancellous trabeculae are exposed and have a porous appearance.

Os Temporale

Fig. 2.26 Position of the os temporale in the skull

Left lateral view. The os temporale is a major component of the base of the skull. It forms the capsule for the auditory and vestibular apparatus and bears the fossa mandibularis of the articulatio temporomandibularis (TMJ).

Fig. 2.27 Os temporale in the skull

A Internal view. B Inferior view.

The os temporale develops from four centers that fuse to form a single bone:

• The pars squamosa (light green) includes the articular fossa (fossa mandibularis) of the articulatio temporomandibularis (TMJ).

• The pars petrosa (pale green) contains the auditory and vestibular apparatus.

• The pars tympanica (darker green) forms large portions of the meatus acusticus externus.

• The processus styloideus develops from cartilage derived from the arcus pharyngeus secundus and is a site of muscle attachment.

Fig. 2.28 Projection of clinically important structures onto the left os temporale

The membrana tympanica is shown translucent in this lateral view. Because the pars petrosa contains the auris media and interna and the membrana tympanica, a knowledge of its anatomy is of key importance in otological surgery. The internal surface of the pars petrosa has openings (see Fig. 2.29) for the passage of the pars petrosa, arteria carotis interna, and vena jugularis interna. A fine nerve, the chorda tympani, passes through the cavitas tympani and lies medial to the membrana tympanica. The chorda tympani arises from the nervus facialis, which is susceptible to injury during surgical procedures (see p. 131). The processus mastoideus of the pars petrosa forms air-filled chambers, the cellulae mastoideae, that vary greatly in size. Because these chambers communicate with the auris media, which in turn communicates with the pars nasalis pharyngis via the tuba auditiva, bacteria in the pars nasalis pharyngis may pass up the tuba auditiva and gain access to the auris media. From there they may pass to the cellulae mastoideae and finally enter the cavitas cranii, causing meningitis.

Fig. 2.29 Left os temporale

A Lateral view. A vena emissaria passes through the foramen mastoideum (external orifice shown in A, internal orifice in C), and the chorda tympani passes through the medial part of the fissura petrotympanica. The processus mastoideus develops gradually in life due to traction from the musculus sternocleidomastoideus and is pneumatized from the inside (see Fig. 2.28).

B Inferior view. The shallow articular fossa of the articulatio temporomandibularis, the fossa mandibularis, is clearly seen from the inferior view. The nervus facialis emerges from the base of the skull through the foramen stylomastoideum. The initial part of the bulbus superior venae jugularis is adherent to the fossa jugularis, and the arteria carotis interna passes through the canalis caroticus to enter the skull.

C Medial view. This view displays the internal orifice of the foramen mastoideum and the meatus acusticus internus. The nervus facialis and nervus vestibulocochlearis are among the structures that pass through the meatus acusticus internus to enter the pars petrosa. The part of the pars petrosa shown here is also called the petrous pyramid, whose apex (often called the apex partis petrosae) lies on the interior of the base of the skull.

Os Occipitale & Os Ethmoidale

Fig. 2.30 Position of the os occipitale in the exterior skull base

Inferior view.

Fig. 2.31 Isolated os occipitale

A Inferior view. This view shows the pars basilaris of the os occipitale, whose anterior portion is fused to the os sphenoidale. The canalis condylaris terminates posterior to the condyli occipitales, and the canalis nervi hypoglossi passes superior and opens anterior to the condyli occipitales. The canalis condylaris is a venous channel that begins in the sinus sigmoideus and ends in the vena occipitalis. The canalis nervi hypoglossi contains a venous plexus in addition to the nervus hypoglossus (CN XII). The tuberculum pharyngeum gives attachment to the raphe pharyngis, and the protuberantia occipitalis externalis provides a palpable bony landmark on the occiput.

B Left lateral view. The extent of the squama occipitalis, which lies above the foramen magnum, is clearly appreciated in this view. The internal openings of the canalis condylaris and canalis nervi hypoglossi are visible along with the process jugularis, which forms part of the wall of the foramen jugulare (see p. 27).

C Internal surface. The grooves for the sinus durae matris of the brain can be identified in this view. The eminentia cruciformis overlies the confluens sinuum. The configuration of the eminence shows that in some cases the sinus sagittalis drains predominantly into the left sinus transversus.

Fig. 2.32 Position of the os ethmoidale in the interior skull base

Superior view. The superior part of the os ethmoidale forms part of the fossa cranii anterior, and its inferior portions contribute structurally to the cavitates nasi and orbita. The os ethmoidale is bordered by the ossa frontale and sphenoidale.

Fig. 2.33 Position of the os ethmoidale in the facial skeleton

Anterior view. The os ethmoidale is the central bone of the nose and sinus paranasales. It also forms the medial wall of each orbita.

Fig. 2.34 Isolated os ethmoidale

A Superior view. This view demonstrates the crista galli, which gives attachment to the falx cerebri and the horizontally directed lamina cribrosa. The lamina cribrosa is perforated by foramina through which the fila olfactoria pass from the cavitas nasi into the fossa cranii anterior (see Fig. 7.22, p. 180). With its numerous foramina, the lamina cribrosa is a mechanically weak structure that fractures easily in response to trauma. This type of fracture is manifested clinically by CSF leakage from the nose (“runny nose” in a patient with head injury).

B Anterior view. The anterior view displays the midline structure that separates the two cavitates nasi: the lamina perpendicularis. Note also the concha nasi media, which is part of the os ethmoidale (of the conchae, only the concha nasi inferior is a separate bone), and the cellulae ethmoidales, which are clustered on both sides of the conchae nasi mediae.

C Left lateral view. Viewing the bone from the left side, we observe the lamina perpendicularis and the opened cellulae ethmoidales anteriores. The orbita is separated from the cellulae ethmoidales by a thin sheet of bone called the lamina orbitalis.

D Posterior view. This is the only view that displays the processus uncinatus, which is almost completely covered by the concha nasi media when in situ. It partially occludes the entrance to the sinus maxillaris, the hiatus semilunaris, and it is an important landmark during endoscopic surgery of the sinus maxillaris. The narrow depression between the concha nasi media and processus uncinatus is called the infundibulum ethmoidale. The sinus frontalis, sinus maxillaris, and cellulae ethmoidales anteriores open into this “funnel.” The concha nasi superior is located at the posterior end of the os ethmoidale.

Os Zygomaticum & Os Nasale

Fig. 2.35 Os zygomaticum in the skull

A Left lateral view. B Inferior view.

The os zygomaticum, or zygoma, is important in determining the width and morphology of the face and is a major buttress between the maxilla and the skull. In addition, it forms a significant portion of the floor and lateral walls of the orbita. The zygoma contains foramina that transmit the zygomaticofacial and zygomaticotemporal arteries and the nervi zygomaticofacialis and zygomaticotemporalis (from the nervus maxillaris [CN V₂]). Muscles that attach along the arcus zygomaticus include the musculus masseter, musculus zygomaticus major, and some fibers of the fascia temporalis. The tuberculum marginale, which is the attachment site for the lateral canthal tendon, is located on the zygoma. This tendon is crucial in maintaining the contour of the eye.

Fig. 2.36 Isolated os zygomaticum

A Inferior view. B Left lateral view.

The zygoma is a substantial bone but its prominent position on the face leaves it vulnerable to fracture following trauma. Trauma that transmits minimal force to the zygoma may cause a non-displaced fracture at the suture lines. Greater force, for example, following a motor vehicle accident, will result in displacement of the bone and involvement of the orbital rim and floor, the sutura zygomaticofrontalis, the zygomaticomaxillary buttress, and the arcus zygomaticus. Symptoms of zygoma fracture include pain, facial bruising and swelling, a flattened malar eminence, diplopia (double vision), trismus (lock jaw), and altered mastication (due to masseteric spasm or interference of the normal mechanism of the processus coronoideus by bony fragments), loss of sensation below the orbit (due to nervus infraorbitalis involvement), and ipsilateral epistaxis (nosebleed) (due to laceration of the mucosa of the sinus maxillaris). Nondisplaced fractures do not require treatment. Displaced fractures commonly require open reduction and fixation, with reconstruction of the orbita. Displacement of the arcus zygomaticus may be reduced by the Gillies technique, in which an incision is made over the musculus temporalis and an instrument is slid under the arch and hooked and the arch is elevated into its normal position.

Fig. 2.37 Os nasale in the skull

Anterior view.

Fractures of the ossa nasalia are common following facial trauma, for example, motor vehicle accidents, sports injuries, or fights. This is due both to the prominence of the nose and the fragility of the ossa nasalia. Symptoms of nasal fractures include pain, bruising, swelling, epistaxis (nosebleeds), and deformity of the nose. The patient may also experience difficulty breathing. Minor nasal fractures require no treatment while those that cause deformity will require manual realignment. More severe nasal fractures (for example, those involving the septum nasi or other facial bones) will require surgery.

Maxilla & Palatum Durum

Fig. 2.38 Maxilla and palatum durum in skull

A Anterior view. B Exterior of skull base, inferior view.

Fig. 2.39 Isolated maxilla

A Anterior view. B Left lateral view.

Fig. 2.40 Bones of the palatum durum

A Superior view. The upper part of the maxilla is removed. The floor of the cavitas nasi shown here and the roof of the cavitas oris (B) are formed by the union of the processus palatini of the two maxillae with the laminae horizontales of the two ossa palatina. Cleft palate results from a failed fusion of the processus palatini at the sutura palatina mediana (see p. 15).

B Inferior view. The cavitas nasi communicates with the pars nasalis pharyngis via the choanae, which begin at the posterior border of the palatum durum. The two cavitates nasi communicate with the cavitas oris via the canales incisivi (A), which combine and emerge at the foramen incisivum.

C Oblique posterior view. This view illustrates the close anatomic relationship between the cavitates oris and nasi. Note: The processus pyramidalis of the os palatinum is integrated into the lamina lateralis processus pterygoidei of the os sphenoidale. The palatine margin of the vomer articulates with the palatum durum along the crista nasalis.

Tori are bony exostoses (lumps) that can be found on both jaws. Torus palatinus occurs in the center of the palatum durum; torus mandibularis occurs in the lingual premolar or molar region of the mandibula. Tori are completely benign but may cause problems for denture retention, in which case they can be surgically excised.

Mandibula & Os Hyoideum

Fig. 2.41 Mandibula

A Anterior view. The mandibula is connected to the viscerocranium at the articulatio temporomandibularis, whose convex surface is the head of the condylus mandibularis. This “caput mandibulae” is situated atop the vertical (ascending) ramus mandibulae, which joins with the corpus mandibulae at the angulus mandibulae. The teeth are set in the alveoli dentum (pars alveolaris mandibulae) along the upper border of the corpus mandibulae. This part of the mandibula is subject to typical age-related changes as a result of dental development (see Fig. 2.43). The ramus mentalis of the nervus trigeminus exits through the foramen mentale. The location of this foramen is important in clinical examinations, as the tenderness of the nerve to pressure can be tested at that location.

B Posterior view. The foramen mandibulae is particularly well displayed in this view. It transmits the nervus alveolaris inferior, which supplies sensory innervation to the dentes mandibulares. Its terminal branch emerges from the foramen mentale. The foramen mandibulae and the foramen mentale are interconnected by the canalis mandibulae.

C Oblique left lateral view. This view displays the processus coronoideus, the processus condylaris, and the incisura mandibulae between them. The processus coronoideus is a site for muscular attachments, and the processus condylaris bears the caput mandibulae, which articulates with the discus articularis in the fossa mandibularis (glenoideus) of the os temporale. A depression on the medial side of the processus condylaris, the fovea pterygoidea, gives attachment to portions of the musculus pterygoideus lateralis.

D Superior view. This view displays the fossa retromolaris, trigonum retromolare, and buccal shelf. The fossa retromolaris is the insertion point for some fibers of the musculus temporalis. Lower dentures should be designed to avoid this area so that they are not dislodged during mastication. The buccal shelf (as a primary bearer of stress) and the trigonum retromolare are areas that are utilized to provide support for lower dentures.

Fig. 2.42 Os hyoideum

A Anterior view. B Posterior view. C Oblique left lateral view. The os hyoideum is suspended by muscles and ligaments between the oral floor and the larynx. The cornu majus and corpus of the os hyoideum are palpable in the neck. The physiological movement of the os hyoideum can be palpated during swallowing.

Mandibula: Age-related Changes & Mandibular Fractures

Fig. 2.43 Age-related changes in the mandibula

The structure of the mandibula is greatly influenced by the pars alveolaris and the dentes. Because the angulus mandibulae adapts to changes in the pars alveolaris, the angle between the corpus and ramus also varies with age-related changes in the dentition. The angle measures approximately 150 degrees at birth and approximately 120 to 130 degrees in adults, increasing to 140 degrees in the edentulous mandibula of old age.

A At birth the mandibula is without teeth, and the pars alveolaris has not yet formed.

B In children the mandibula bears the dentes decidui. The pars alveolaris is still relatively poorly developed because the dentes decidui are considerably smaller than the dentes permanentes.

C In adults the mandibula bears the dentes permanentes, and the pars alveolaris is fully developed.

D In old age with resorption of the pars alveolaris, the mandibula is edentulous (toothless).

Note: The resorption of the pars alveolaris with advancing age leads to a change in the position of the foramen mentale (which is normally located below the dens premolaris secundus, as in C). This change must be taken into account in surgery or dissections involving the nervus mentalis. The pars alveolaris is the portion of the maxilla and mandibula that supports the radices dentum. It is composed of two parts, the processus alveolaris and the supporting bone. The processus alveolaris lines the tooth sockets (alveoli). Supporting bone consists of cortical plates of compact bone on the inner and outer surfaces of the maxilla and mandibula and the intervening spongy bone between the cortical plates and processus alveolaris. The processus alveolaris is subject to resorption following tooth loss (a normal physiological process) and in certain disease states (e.g., abscess formation, cysts, osteoporosis). Basal bone is that portion of the maxilla and mandibula deep to the alveolar bone. It is not subject to resorption.

Fig. 2.44 Mandibular fracture

Anterior view. A Mandibular fracture. B Reduction and fixation of mandibular fracture. Mandibular fracture is a common injury, for example, following motor vehicle accidents, fights, or sporting accidents, due to the prominence of the mandibula and its relative lack of support. Most fractures occur in the corpus (~30%), condylus (~25%), angulus (~25%), and symphysis (~17%). To avoid misdiagnosis of the injury, the history should include not just information about the current injury but information about previous mandibular trauma or articulatio temporomandibularis (TMJ) dysfunction. Determine the patency of the airway and the presence of other injuries (facial lacerations, swellings, or hematomas). Inspect intraoral tissues for bruising, which, if present, is suggestive of a fracture of the corpus or symphysis. Palpate the mandibula from the symphysis to the angulus, noting any swelling, tenderness, or step deformities. Next palpate the condylus through the meatus acusticus externus; tenderness may indicate a fracture at this site. Note any deviation on opening the mouth. With condylar fractures, the mandibula deviates toward the side of the fracture. Note also any obstruction to mouth opening, e.g., trismus (lock jaw due to spasm of the muscles of mastication) or impaction of the processus coronoideus. Now evaluate the occlusion. If the teeth do not occlude as normal, this is highly suggestive of mandibular fracture, although this can also occur following tooth subluxation (loosening) or TMJ injury. Note any areas of altered sensation (paresthesia, dysesthesia, or anesthesia). The latter is suggestive of a fracture distal to the foramen mandibulae. Following this, the mandibula should be grasped at either side of the suspected fracture and gently manipulated to assess mobility. Confirm the diagnosis via either radiography or CT scans. Treat with antibiotics to prevent infection, followed by reduction (to the patient’s normal occlusion) and surgical fixation of the fracture. The fixation method depends on many factors including the type and site of fracture and may involve the use of bars, wires, or plates.

The double mandibular fracture shown here is treated in a two-step process. First, the fracture at the midline is fixated with metal plates followed by the angulus fracture. Note that two plates provide much more stability than a single plate.

Neurovascular Pathways through the Skull Base

Fig. 2.45 Passage of neurovascular structures through the skull base

A Cavitas cranii (interior of skull base), superior view.

B Exterior of skull base, inferior view.

This image and the corresponding table only address structures entering and exiting the skull. Many neuro vascular structures pass through bony canals within the skull (to fossa pterygopalatina, fossa infratemporalis, etc.).

Note: The nervus petrosus profundus and nervus petrosus major travel over the surface of foramen lacerum but not through it.

Muscles of the Head: Origins & Insertions

The bony origins and insertions of the muscles are indicated by color shading: origins (red) and insertions (blue).

Fig. 2.46 Origins and insertions on the skull

A Left lateral view.

B Inner surface of right hemimandible.

C Inferior view of skull base.

Note: The musculi stylohyoideus, styloglossus, and stylopharyngeus originate on the processus styloideus.