External skull

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CHAPTER 26 External skull

The skull is the bony skeleton of the head. It shields the brain, the organs of special sense and the cranial parts of the respiratory and digestive systems, and provides attachments for many of the muscles of the head and neck. Movement of the lower jaw (mandible) occurs at the temporomandibular joint.

The skull without the mandible is called the cranium, and may be subdivided into two regions. The cranial vault or neurocranium encloses the brain, cranial meninges and cerebrospinal fluid, while the facial skeleton or viscerocranium hangs down from the front of the neurocranium and houses the organs of special sense.

The skull is the most complex osseous structure in the body: the young adult skull is composed of an average of 28 separate bones, many of which are paired, although some in the median plane are single. Most of the vault bones are flat, and consist of two tables or plates of compact bone enclosing a narrow layer of relatively dense cancellous marrow (diploic bone). The marrow within the skull bones is a site of haemopoiesis, at least in the young individual. These bones form by intramembranous ossification of a highly vascular connective tissue membrane and have often been referred to as ‘dermal’ in deference to their alleged ancient phylogenetic origin. The inner table is thinner and more brittle while the outer tends to be thicker and more resilient: this is important to remember when examining fractures to the skull caused by either blunt or sharp trauma. The skull bones vary in thickness in different regions, tending to be thinner where they are covered by muscles, e.g. in the temporal region, and thicker where muscles attach, e.g. in the occipital region. The thinner regions are more prone to fracture.

The majority of bones in the skull are held together firmly by fibrous joints termed sutures: in the developing skull sutures allow for growth. The three main sutural morphologies are dependent upon the magnitude of strains placed upon them. Thus the margins of adjacent bones of a suture may be smooth and meet end-to-end, giving a simple (butt-end) suture (e.g. median palatine suture); bevelled, so that the border of one bone overlaps the other (e.g. parietotemporal suture); or present numerous projections that interlock, giving a serrated appearance (e.g. sagittal suture). The complexity of serrated sutures increases from the inner to the outer surface. Fusion across sutures (synostosis) can start early in the third decade, although its variability precludes using this information to assess age at death in skulls with any degree of accuracy. The process of fusion starts on the internal surface of the cranium first and proceeds externally; the sagittal suture is often one of the first affected. By middle age, many of the larger sutures will show evidence of synostoses, although there are some which rarely show fusion, e.g. the zygomaticofacial. Premature fusion of sutures during the early growth phase of the skull will result in various abnormalities.

The bones forming the base of the skull develop mainly via endochondral ossification, and also play an important part in the overall growth of the face and the neurocranium. The joints between bones in the skull base are primary cartilaginous. One of the most important is the spheno-occipital synchondrosis between the body of the sphenoid anteriorly and the basilar part of the occipital bone posteriorly: fusion is completed between 13 and 18 years of age.

There are only two sites of synovial articulation associated with the exterior of the skull – the temporomandibular joint with the mandible and the atlanto-occipital joint between the condyles of the occipital bone and the superior articular facets of the atlas. Rotation of the skull does not directly involve any joints of the skull, but occurs at the atlanto-axial joint between the first and second cervical vertebrae.

Many important nerves and vessels pass in and out of the skull via openings (foramina). The skull is a prime site for fractures resulting from trauma, which means that these structures can be damaged as a result of head injury. In addition to main foramina, irregular emissary foramina allow veins situated externally on the face and scalp to communicate with those lying intracranially. The spread of infection along these routes can have serious clinical consequences.

In the account of the skull that follows, only generalized standard views of the skull will be considered. A more detailed account of each individual bone will be found associated with the relevant regional text.

FRONTAL (ANTERIOR) VIEW

Viewed from the front, the skull is generally ovoid in shape and is wider above than below (Fig. 26.1). The upper part is formed by the frontal bone which underlies the forehead region above the orbits. Superomedial to each orbit is a rounded superciliary arch (more pronounced in males) between which there may be a median elevation, the glabella. The glabella may show the remains of the inter-frontal (metopic) suture, which is present in about 9% of adult skulls. Above each superciliary arch is a slightly elevated frontal tuber or eminence which is generally more obvious in the female. Below, where the nasal bones meet the frontal bone, is a depression marking the root of the nose. The frontal bone articulates with the two nasal bones at the frontonasal sutures; the point at which the frontonasal and internasal sutures meet is the anthropometric landmark known as the nasion.

The upper part of the face is occupied by the orbits and the bridge of the nose. Each orbital opening is approximately quadrangular (see Ch. 39). The upper, supraorbital, margin is formed entirely by the frontal bone, interrupted at the junction of its sharp lateral two-thirds and rounded medial third by the supraorbital notch or foramen, which transmits the supraorbital vessels and nerve. The lateral margin of the orbit is formed largely by the frontal process of the zygomatic bone and is completed above by the zygomatic process of the frontal bone; the suture between them lies in a palpable depression. The infraorbital margin is formed by the zygomatic bone laterally and maxilla medially. Both lateral and infraorbital margins are sharp and palpable. The medial margin of the orbit is formed above by the frontal bone and below by the lacrimal crest of the frontal process of the maxilla.

The central part of the face is occupied mainly by the two maxillae, separated by the anterior nasal aperture. Each maxilla contributes to the upper jaw, the floor of the orbital cavity, the lateral wall of the nose, the floor of the nasal aperture and the bone of the cheek. Medially, the maxilla forms the nasal notch which is the floor and inferolateral border of the anterior nasal aperture. The prominent anterior nasal spine surmounts the intermaxillary suture at the lower margin of the anterior nasal aperture and is palpable in the nasal septum. The infraorbital foramen, which transmits the infraorbital vessels and nerve, lies about 1 cm below the infraorbital margin. The maxillary alveolar process bears the upper teeth. The short, thick zygomatic process of the maxilla has an oblique upper surface that articulates with the zygomatic bone at the zygomaticomaxillary suture. The frontal process of the maxilla ascends posterolateral to the nasal bone to articulate with the frontal bone.

The anterior nasal aperture is piriform in shape, and is wider below than above and bounded by the nasal bones and maxillae. The upper boundary of the aperture is formed by the nasal bones while the remainder is formed by the maxillae. In life, various cartilages (septal, lateral nasal, major and minor alar) help to delineate two nasal cavities (see Ch. 32). The defleshed skull presents a single anterior nasal aperture because these cartilages are lost during preparation or decomposition. However, the shape of these bones can be used quite successfully to predict the shape of the cartilaginous nose.

The lower part of the face, below the nose, is formed from the alveolar arch of the maxillae and the upper dentition, and the body of the mandible, the alveolar process of the mandible and the lower dentition. In the midline the mental protuberance produces the characteristic prominence of the chin. The mental foramen, which transmits the mental nerve and accompanying vessels, lies in the same vertical plane as the supraorbital and infraorbital foramina.

AP radiographs of the skull clearly show the central location of the paranasal air sinuses in the frontal bone, the maxilla and the ethmoid. These can be particularly useful indicators of identity when postmortem images are compared with premortem clinical films.

POSTERIOR VIEW

The parietal, temporal and occipital bones form the entirety of the posterior view (Fig. 26.2). The superolateral region is occupied by the parietal bones, the mastoid part of the temporal bones comprises the inferolateral regions and the central portion is occupied by the occipital bone, which is why this aspect is also referred to as the occipital view. The parietal bones articulate with the occipital bone at the lambdoid suture and extend inferiorly into the occipitomastoid and the parietomastoid sutures behind and above the mastoid process respectively. Sutural or wormian bones are islands of bone that are commonly found within the lambdoid suture. They may arise from separate centres of ossification and they appear to have no clinical significance, being of genetic rather than pathological aetiology. A large interparietal bone is not uncommon and is sometimes referred to as an Inca bone (Fig. 26.2).

The external occipital protuberance is a midline ridge or a distinct process on the occipital bone which can become particularly well developed in the male. Superior nuchal lines extend laterally from the protuberance and represent the boundary between the scalp and the neck. Inferior nuchal lines run parallel to, and below, the superior nuchal lines; a set of highest nuchal lines may sometimes occur above the superior lines. The external occipital protuberance, nuchal lines and roughened external surface of the occipital bone between the nuchal lines, all afford attachment to muscles of the neck.

SUPERIOR VIEW

Seen from above, the contour of the cranial vault varies greatly but is usually ellipsoid, or more strictly, a modified ovoid with its greatest width lying nearer to the occipital pole (Fig. 26.3). Four bones constitute this view and articulate via three well-defined sutures. The squamous part of the frontal bone is anterior, the squamous part of the occipital bone is posterior and the two parietal bones meet in the midline and separate the frontal from the occipital bone. The maximal parietal convexity on each site is palpable at the parietal tuber or eminence: it is most conspicuous in the female who ostensibly retains a paedomorphic appearance. The superior and inferior temporal lines run close to the parietal eminence but are best seen in a lateral view.

The coronal suture marks the articulation between the posterior margin of the frontal bone and the anterior margins of the two parietal bones. It descends across the cranial vault and projects inferiorly until it meets the junction between the greater wing of the sphenoid and the squamous temporal bone at the pterion. The sagittal suture runs in the midline between the two parietal bones and extends from the bregma anteriorly to the lambda posteriorly. The lambdoid suture delineates the articulation between the posterior borders of the right and left parietal bones and the superior border of the occipital bone.

The bregma represents the position of the fetal anterior fontanelle and is the junction between the coronal and sagittal sutures. The anterior fontanelle is a diamond-shaped membrane-filled space located between the two frontal and two parietal bones of the developing fetal skull and persists until approximately 18 months after birth. The lambda, at the junction of the sagittal and lambdoid sutures, is the site of the posterior fontanelle, which persists for the first 2–3 months after birth.

A parietal foramen may pierce either or both parietal bones near the sagittal suture about 3.5 cm anterior to the lambda. It transmits a small emissary vein from the superior sagittal sinus. The vertex is the highest point on the skull and it usually occupies a position in the middle third of the sagittal suture.

LATERAL VIEW

The skull, viewed from the side, can be subdivided into three zones: face (anterior); temporal region (middle); occipital region (posterior) (Fig. 26.4). The face has been considered in the section on the anterior view of the skull.

The temporal region can be divided into an upper temporal fossa and a lower infratemporal fossa, separated by the zygomatic arch. The temporal fossa is bounded inferiorly by the zygomatic arch, superiorly and posteriorly by the temporal lines and anteriorly by the frontal process of the zygomatic bone, and is continuous inferiorly with the infratemporal fossa deep to the zygomatic arch. The temporal lines often present anteriorly as distinct ridges, but become much less prominent as they arch posteriorly across the parietal bone. Indeed, the superior line may become quite indistinct posteriorly. The inferior temporal line becomes more prominent as it curves down the posterior part of the squamous temporal bone, forming a supramastoid crest at the base of the mastoid process. The superior temporal line gives attachment to the temporal fascia while the inferior temporal line provides attachment for temporalis.

The floor of the temporal fossa is formed by the frontal and parietal bones superiorly and the greater wing of the sphenoid and squamous temporal inferiorly. All four bones of one side meet at an H-shaped sutural junction termed the pterion. This is an important anthropometric landmark as it overlies both the anterior branch of the middle meningeal artery and the lateral fissure of the cerebral hemisphere. The pterion corresponds to the site of the anterolateral (sphenoidal) fontanelle of the neonatal skull which closes in the third month after birth.

The vertical suture between the sphenoid and temporal bones, the sphenosquamosal suture, is formed by articulation between the posterior border of the greater wing of the sphenoid and the anterior border of the squamous part of the temporal bone.

The lateral surface of the ramus of the mandible will be described briefly here as its position lies within the middle region of this view of the skull (see Ch. 31). The ramus is a plate of bone projecting upwards from the back of the body of the mandible; its lateral surface gives attachment to masseter. The ramus bears two prominent processes superiorly, the coronoid process anteriorly and condylar process posteriorly, separated by the mandibular notch. The coronoid process is the site of insertion of temporalis and the condylar process articulates with the mandibular fossa of the temporal bone at the temporomandibular joint. The inferior and posterior borders of the mandible meet at the angle, which is often splayed in the male because of the larger site of muscle attachment for medial pterygoid on the internal surface.

The zygomatic arch stands clear of the rest of the skull, and the temporal and infratemporal fossae communicate via the gap thus created. In life, this space is largely filled by temporalis. The zygomatic bone is the principal bone of the cheek together with the zygomatic processes of the maxillae and temporal bones. The term ‘zygomatic arch’ is generally restricted to the temporal process of the zygomatic bone and the zygomatic process of the temporal bone, which articulate at the zygomaticotemporal suture. The suture between the zygomatic process of the frontal bone and the frontal process of the zygomatic bone is the frontozygomatic suture, that between the maxillary margin of the zygomatic bone and the zygomatic process of the maxillary bone is the zygomaticomaxillary suture, and between the sphenoid and zygomatic is the sphenozygomatic suture. As the zygomatic process of the temporal bone passes posteriorly, it becomes associated with the articular tubercle of the mandibular fossa anteriorly and the supramastoid crest posteriorly.

The temporal bone is a prominent structure on the lateral aspect of the skull. Its squamous part lies in the floor of the temporal fossa and its zygomatic process contributes to the structure of the cheek. Additional components visible in the lateral view of the skull are the mandibular fossa and its articular eminence (tubercle), the tympanic plate, the external acoustic meatus, and the mastoid and styloid processes.

The mandibular (glenoid) fossa is the part of the temporomandibular joint with which the condylar process of the mandible articulates. It is bounded in front by the articular eminence and behind by the tympanic plate. The articular eminence provides a surface over which the mandibular condyle glides during mandibular movements (see Ch. 31).

The tympanic plate of the temporal bone contributes most of the margin of the external acoustic (auditory) meatus, and the squamous part forms the posterosuperior region (see Ch. 36). The external margin is roughened to provide an attachment for the cartilaginous part of the meatus. A small depression, the suprameatal triangle, lies above and behind the meatus and is related to the lateral wall of the mastoid antrum.

The mastoid process is a small inferior projection of the temporal bone, lying posteroinferior to the external acoustic meatus and is the site of attachment of sternocleidomastoid. It is in contact behind with the posteroinferior angle of the parietal bone at the parietomastoid suture and with the squamous part of the occipital bone at the occipitomastoid suture. These two sutures meet the lateral end of the lambdoid suture at the asterion, which coincides with the site of the posterolateral fontanelle in the neonatal skull and which closes during the second year. A mastoid foramen may be found near or in the occipitomastoid suture and transmits an emissary vein from the sigmoid sinus. Sutural bones may appear in the parietomastoid suture.

The styloid process lies anterior and medial to the mastoid process and gives attachment to several muscles and ligaments. Its base is partly ensheathed by the tympanic plate and it descends anteromedially, its tip usually reaching a point medial to the posterior margin of the mandibular ramus. However, the styloid process is very variably developed, and ranges in length from a few millimetres to a few centimetres.

The infratemporal fossa is an irregular postmaxillary space located deep to the ramus of the mandible; it communicates with the temporal fossa deep to the zygomatic arch (see Ch. 31). It is best visualized, therefore, when the mandible is removed but, for completeness, is considered here. Its roof is the infratemporal surface of the greater wing of the sphenoid. The lateral pterygoid plate lies medial to the fossa and the ramus of the mandible and styloid process lie laterally and posteriorly respectively. The infratemporal fossa has no anatomical floor. Its anterior and medial walls are separated above by the pterygomaxillary fissure lying between the lateral pterygoid plate and the posterior wall of the maxilla. The infratemporal fossa communicates with the pterygopalatine fossa through the pterygomaxillary fissure.

INFERIOR (BASAL) VIEW

The inferior surface of the skull, the base of the cranium, is complex: it extends from the upper incisor teeth anteriorly to the superior nuchal lines of the occipital bone posteriorly (Fig. 26.5). The region contains many of the foramina through which structures enter and exit the cranial cavity. The inferior surface can be conveniently subdivided into anterior, middle, posterior and lateral parts. The anterior part contains the hard palate and the dentition of the upper jaw and lies at a lower level than the rest of the cranial base. The middle and posterior parts can be arbitrarily divided by a transverse plane passing through the anterior margin of the foramen magnum. The middle part is occupied mainly by the base of the sphenoid bone, the apices of the petrous processes of the temporal bones and the basilar part of the occipital bone. The lateral part contains the zygomatic arches, mandibular fossa, tympanic plate and the styloid and mastoid processes. The posterior part lies in the midline and is exclusively formed from the occipital bone. Whereas the middle and posterior parts are directly related to the cranial cavity (the middle and posterior cranial fossae), the anterior part (the palate) is some distance from the anterior cranial fossa, being separated from it by the nasal cavities.

ANTERIOR PART OF CRANIAL BASE

The bony palate within the superior alveolar arch is formed by the palatine processes of the maxillae anteriorly and the horizontal plates of the palatine bones posteriorly, all meeting at a cruciform system of sutures (Fig. 26.5). The median palatine suture runs anteroposteriorly and divides the palate into right and left halves. This suture is continuous with the intermaxillary suture between the maxillary central incisor teeth. The transverse palatine (palatomaxillary) sutures run transversely across the palate between the maxillae and the palatine bones. The palate is arched sagittally and transversely: its depth and breadth are variable but are always greatest in the molar region. The incisive fossa lies behind the central incisor teeth, and the lateral incisive foramina, through which incisive canals pass to the nasal cavity, lie in its lateral walls. Median incisive foramina are present in some skulls and open onto the anterior and posterior walls of the fossa. The incisive fossa transmits the nasopalatine nerve and the termination of the greater palatine vessels. When median incisive foramina occur, the left nasopalatine nerve usually traverses the anterior foramen and the right nerve traverses the posterior foramen. The greater palatine foramen lies near the lateral palatal border of the transverse palatine suture; a vascular groove, deeper behind and shallower in front, leads forwards from the foramen. The lesser palatine foramina, usually two, lie behind the greater palatine foramen and pierce the pyramidal process of the palatine bone which is wedged between the lower ends of the medial and lateral pterygoid plates. The palate is pierced by many other small foramina and is marked by pits for palatine glands. Variably prominent palatine crests extend medially from behind the greater palatine foramina. The posterior border projects back as the posterior nasal spine. In the adult, the alveolar arch bears a maximum of 16 sockets or alveoli for the teeth: the sockets vary in size and depth, some are single and some are subdivided by septa, according to the morphology of the dental roots.

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