Skull

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Chapter 9 Skull

The skull is the bony skeleton of the head and is the most complex osseous structure in the body. It is protective, shielding the brain, the organs of special sense and the cranial parts of the respiratory and digestive systems. It also provides attachments for many of the muscles of the head and neck, thus allowing for movement. Of particular importance is movement of the lower jaw (mandible), which occurs at the temporomandibular joint. The marrow within the skull bones is a site of haemopoiesis, at least in the young skull.

The skull is composed of 28 separate bones, most of which are paired; however, some bones in the median plane are single. Many of the bones are flat bones, consisting of two thin plates of compact bone enclosing a narrow layer of cancellous bone containing bone marrow. In terms of shape, however, the bones are far from flat and can exhibit pronounced curvatures. The term diploë is used to describe the cancellous bone within the flat bones of the skull. The inner table is thinner and more brittle; the outer table is generally very resilient. Many bones are so thin that the tables are fused, for example, the vomer and pterygoid plates. The skull bones vary in thickness in different regions but tend to be thinner where they are covered by muscles, such as in the temporal and posterior cranial fossae. The skull is thicker in some races, but there is no relationship between skull thickness and cranial capacity, which averages approximately 1400 ml. In all races the bone is thinner in women and children than in adult males.

The majority of bones in the skull are held firmly together by fibrous joints termed sutures. In the developing skull, sutures allow for growth. There are three main arrangements: the margins of adjacent bones of a suture may be smooth and meet end to end, resulting in a simple (butt-end) suture (e.g. median palatine suture); the margins of adjacent bones may be bevelled, so that the border of one bone overlaps the other (e.g. zygomaticomaxillary suture); or the margins of adjacent bones may 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) commences at approximately 30 years of age, but the variability of this process precludes its use to determine the age of skulls. Fusion commences on the internal surface of the cranium, and the sagittal suture is one of the first affected. At approximately 40 years of age the sphenofrontal, lambdoid and occipitomastoid sutures close. In the facial region the posterior part of the median palatine suture starts to close at about 30 years, followed by the sutures around the nose. The squamosal, zygomaticofrontal and anterior parts of the intermaxillary suture rarely exhibit synostosis. Premature fusion of sutures during the early growth phase of the skull results in various cranial abnormalities.

The bones forming the base of the skull develop endochondrally and play an important part in growth. In this region, therefore, primary cartilaginous joints are encountered during growth; one of the most important is the spheno-occipital synchondrosis, which disappears at approximately 14 to 16 years of age. The skull articulates with the first cervical vertebra at the synovial atlanto-occipital joints. These joints allow flexion and extension of the skull. 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 termed foramina. The skull is a prime site for fractures resulting from trauma, and these structures can be damaged as a result of head injury. Detailed clinical examination should reveal signs and symptoms that, together with radiological examination, provide information regarding the extent and seriousness of a traumatic incident. In addition to the main foramina, irregular emissary foramina allow veins situated externally on the face and scalp to communicate with those lying intracranially. Spread of infection along these routes can have serious clinical consequences.

For ease of navigation, the skull can be divided into the cranium and the mandible, based on the fact that the mandible is easily detached, whereas most of the bones of the skull articulate by relatively fixed joints. The cranium can then be subdivided into a number of regions: the cranial vault, which is the upper, dome-like part of the skull and includes the skullcap or calvaria; the cranial base, which consists of the inferior surface of the skull extracranially and the floor of the cranial cavity intracranially; the facial skeleton, which includes the orbital cavities and the nasal fossae; the tooth-bearing bones or jaws; the acoustic cavities, which contain the middle and inner ears; and the cranial cavity, which houses the brain. Alternatively, the skull can be divided into the neurocranium and viscerocranium. The neurocranium is defined as that part of the skull that houses and protects the brain and the organs of special sense, whereas the viscerocranium is associated with the cranial parts of the respiratory and digestive tracts.

Inferior (Basal) Surface

The inferior surface of the skull, the base of the cranium, is complex and extends from the upper incisor teeth in front to the superior nuchal lines of the occipital bone behind (Fig. 9.1). The region contains many of the foramina through which structures enter and exit the cranial cavity. The inferior surface can be conveniently divided into anterior, middle, posterior and lateral parts. The anterior part contains the hard palate and the dentition of the upper jaw, and it 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 petrous processes of the temporal bones and the basilar part of the occipital bone. The lateral part contains the zygomatic arches and the mastoid and styloid processes. 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.

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Fig. 9.1 Inferior view of the skull.

(By permission from Berkovitz, B.K.B., Moxham, B.J., 1994. Color Atlas of the Skull. Mosby-Wolfe, London.)

Anterior Part of the Cranial Base

The bony palate within the superior alveolar arch is formed by the palatine processes of the maxillae and the horizontal plates of the palatine bones, which meet at a cruciform system of sutures. 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 maxillary 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, with the average width between the maxillary first molars being approximately 50 mm. 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, present in some skulls, open on 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 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, and a vascular groove that is deep posteriorly leads forward from it. 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 a median posterior nasal spine. The alveolar arch has 16 sockets or alveoli for teeth, varying in size and depth; some are single, and some are divided by septa in adaptation to tooth roots.

The nasal fossae, separated in the midline by the nasal septum, lie above the hard palate. The two posterior nasal apertures (choanae) are located where the nasal fossae end. The posterior part of the septum is formed by the vomer. The upper border of the vomer is applied to the inferior aspect of the body of the sphenoid, where it expands into an ala on each side. The lateral border of each ala reaches a thin vaginal process that projects medially from the medial pterygoid plate. The two may touch, or the vaginal process may overlap the ala of the vomer inferiorly. The inferior surface of the vaginal process bears an anteroposterior groove, which is converted into a canal anteriorly by the superior aspect of the sphenoidal process of the palatine bone. This palatovaginal canal opens anteriorly into the pterygopalatine fossa and transmits a pharyngeal branch of the pterygopalatine ganglion and a pharyngeal branch from the third part of the maxillary artery. An inconstant vomerovaginal canal may lie between the ala of the vomer and the vaginal process of the sphenoid bone, medial to the palatovaginal canal, and lead into the anterior end of the palatovaginal canal. It transmits the pharyngeal branch of the third part of the maxillary artery.

Middle Part of the Cranial Base

The middle part of the cranial base is made up of the occipital, sphenoid and temporal bones. The body of the sphenoid bone lies anteriorly, and the basilar part of the occipital bone lies posteriorly, just in front of the foramen magnum. Where these two bones meet in the growing skull, the junction between them is a primary cartilaginous joint, the spheno-occipital synchondrosis. This joint is important for growth of the skull in an anteroposterior direction and ossifies at approximately 14 to 16 years of age. The basilar part of the occipital bone bears a small midline pharyngeal tubercle, which provides an attachment to the pharyngeal raphe and the highest attachment of the superior pharyngeal constrictor.

The middle part of the cranial base is completed by the petrous processes of the two temporal bones, which pass from the lateral sides of the base of the skull toward the site of union of the sphenoid and occipital bones. Each petrous process meets the basilar part of the occipital bone at a petro-occipital suture, which is deficient posteriorly at the jugular foramen. The petrosphenoidal suture and the groove for the pharyngotympanic tube lie between the petrous process and the infratemporal surface of the greater wing of the sphenoid. The apex of the petrous process does not meet the spheno-occipital suture, and the deficit produced is called the foramen lacerum.

Each pterygoid process of the sphenoid bone bears medial and lateral pterygoid plates separated by a pterygoid fossa. Anteriorly, the plates are fused, except below, where they are separated by the pyramidal process of the palatine bone. Sutures are usually discernible at this site. Laterally, the pterygoid plates are separated from the posterior maxillary surface by the pterygomaxillary fissure, which leads into the pterygopalatine fossa. The posterior border of the medial pterygoid plate is sharp and bears a small projection near the midpoint, above which it is curved and attached to the pharyngeal end of the pharyngotympanic tube. Above, the medial pterygoid plate divides to enclose the scaphoid fossa; below, it projects as a slender pterygoid hamulus, which curves laterally and is grooved anteriorly by the tendon of tensor veli palatini. The pterygoid hamulus gives origin to the pterygomandibular raphe. The lateral pterygoid plate projects posterolaterally, and its lateral surface forms the medial wall of the infratemporal fossa. Superiorly and laterally, the pterygoid process is continuous with the infratemporal surface of the greater wing of the sphenoid bone, which forms part of the roof of the infratemporal fossa. This surface forms the posterolateral border of the inferior orbital fissure and bears an infratemporal crest associated with the origin of the upper part of the lateral pterygoid. The infraorbital and zygomatic branches of the maxillary nerve and accompanying vessels pass through the inferior orbital fissure. Laterally, the greater wing of the sphenoid bone articulates with the squamous part of the temporal bone. Features associated with the pterygoid plate region can be assessed radiographically (Fig. 9.2).

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Fig. 9.2 Horizontal computed tomography scan at the level of the upper part of the ramus of the mandible shows the relationships of the pterygoid plates.

(By permission from Berkovitz, B.K.B., Moxham, B.J., 1994. Color Atlas of the Skull. Mosby-Wolfe, London.)

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