Radiological anatomy of the oral cavity

The radiographs most frequently taken in general dental practice are of the teeth and their immidiate supporting tissues for detection of dental caries or assessment of bone loss in periodontal disease. Intraoral radiographs are taken by placing the X-ray-sensitive film or receptor in the mouth close to the teeth being investigated. Extraoral radiographs use larger films or receptors positioned externally and produce a view of the entire dentition and its supporting structures on a single film; they are used to ascertain the state of development of the dentitions prior to orthodontic treatment, for example. Dental panoramic tomographs (DPTs) are the most frequent extraoral radiographs. A radiograph is a negative photographic record. Dense structures such as bone are designated as radio-opaque; they absorb some X-rays and appear white on radiographs. More X-rays pass through less dense radiolucent structures such as air-filled cavities which show up as black areas. The contrast between different tissues of the structures which the X-ray beam passes through is determined by their radiodensity which, in turn, is largely due to their content of metallic elements. Calcium and iron are the prevalent heavy metals in the body. Calcium is combined with phosphate to form hydroxyapatite crystals in bones and mineralized tissues in teeth. Iron is present in haemoglobin in blood, but only large concentrations of blood, such as those found within the heart chambers, show up on X-rays. In sequence from densest to most lucent, the radiodensity of the dental and periodontal tissues are: enamel, dentine, cementum, compact bone, cancellous bone, demineralized carious enamel and dentine, dental soft tissues such as pulp and periodontal ligament, and air; gold and silver–mercury amalgam metallic restorative materials are even denser than enamel. A radiograph is a two-dimensional representation of a three-dimensional situation. The orientation of anatomical structures relative to the X-ray beam is a major factor determining their appearance on the film. For example, a beam travelling through the long axis of a radiodense structure will produce a whiter image on the film than one passing through its shorter axis because more X-rays are absorbed; the structure will also have a different shape.

The nasal cavity and paranasal sinuses

The nasal cavity is the entrance to the respiratory tract. Its functions are to clean, warm, and humidify air as it is inhaled. Respiratory mucosa covered by pseudostratified ciliated epithelium and goblet cells, as described in Chapter 5 and illustrated in Figure 5.2B, lines the majority of the nasal cavity. The cilia and mucus trap particles, thus cleaning the air; the mucus also humidifies the air and warming is achieved through heat exchange from blood in the very vascular mucosa. The efficiency of all these processes is increased by expanding the surface of the nasal cavity by folds of bone. The nasal cavity also houses the olfactory mucosa for the special sense of olfaction although the olfactory mucosa occupies a very small proportion of the surface of the nasal cavity. The nasal cavity extends from the nostrils on the lower aspect of the external nose to the two posterior nasal apertures between the medial pterygoid plates where it is in continuation with the nasopharynx. Bear in mind that in dried or model skulls, the nasal cavity is smaller from front to back and the anterior nasal apertures seem extremely large because the cartilaginous skeleton of the external nose is lost during preparation of dried skulls. As you can see in Figure 27.1 , the nasal cavity extends vertically from the cribriform plate of the ethmoid at about the level of the orbital roof above to the palate, separating it from the oral cavity below. Figure 27.1 also shows that the nasal cavity is relatively narrow from side to side, especially in its upper part between the two orbits and widens where it sits between the right and left sides of the upper jaw below the orbits. The nasal cavity is completely divided into right and left compartments by the nasal septum . From the anterior view seen in Figure 27.1 , you can see that the surface area of lateral walls of the nasal cavity are extended by the three folds of bone, the nasal conchae. The skeleton of the external nose shown in Figure 27.2 comprises the nasal bones, the upper and lower nasal cartilages, the septal cartilage, and the cartilaginous part of the nasal septum.

The temporomandibular joints, muscles of mastication, and the infratemporal and pterygopalatine fossae

It is essential that dental students and practitioners understand the structure and function of the temporomandibular joints and the muscles of mastication and other muscle groups that move them. The infratemporal fossa and pterygopalatine fossa are deep to the mandible and its related muscles; many of the nerves and blood vessels supplying the structures of the mouth run through or close to these areas, therefore, knowledge of the anatomy of these regions and their contents is essential for understanding the dental region. The temporomandibular joints (TMJ) are the only freely movable articulations in the skull together with the joints between the ossicles of the middle ear; they are all synovial joints. The muscles of mastication move the TMJ and the suprahyoid and infrahyoid muscles also play a significant role in jaw movements. The articular surfaces of the squamous temporal bone and of the condylar head (condyle) of the mandible form each temporomandibular joint. These surfaces have been briefly described in Chapter 22 on the skull and Figure 24.1A indicates their shape. The concave mandibular fossa is the posterior articulating surface of each squamous temporal bone and houses the mandibular condyle at rest. The condyle is translated forwards on to the convex articular eminence anterior to the mandibular fossa during jaw movements. The articular surfaces of temporomandibular joints are atypical; they covered by fibrocartilage (mostly collagen with some chondrocytes) instead of hyaline cartilage found in most other synovial joints. Figures 24.1B and 24.1C show the capsule and ligaments associated with the TMJ. The tough, fibrous capsule is attached above to the anterior lip of the squamotympanic fissure and to the squamous bone around the margin of the upper articular surface and below to the neck of the mandible a short distance below the limit of the lower articular surface. The capsule is slack between the articular disc and the squamous bone, but much tighter between the disc and the neck of the mandible. Part of the lateral pterygoid muscle is inserted into the anterior surface of the capsule. As in other synovial joints, the non-load-bearing internal surfaces of the joint are covered with synovial membrane.

The face and superficial neck

The surface anatomies of the face and neck and their supporting structures that can be palpated have been described in Chapter 20. It is now time to move to the structures that lie under the skin but which cannot be identified by touch starting with the neck and moving up on to the face and scalp. The cervical vertebral column comprises the seven cervical vertebrae and the intervening intervertebral discs. These have the same basic structure as the thoracic vertebrae described in Section 10.1.1. Examine the features of the cervical vertebra shown in Figure 23.1 and compare it with the thoracic vertebra shown in Figure 10.3. You will see that cervical vertebrae have a small body and a large vertebral foramen. They also have two distinguishing features, a bifid spinous process and a transverse foramen, piercing each transverse process; the vertebral vessels travel through these foramina. The first and second vertebrae are modified. The first vertebra, the atlas, has no body. Instead, it has two lateral masses connected by anterior and posterior arches. The lateral masses have concave superior facets which articulate with the occipital condyles where nodding movements of the head take place at the atlanto-occipital joints. The second cervical vertebra, the axis, has a strong odontoid process (or dens because of its supposed resemblance to a tooth) projecting upwards from its body. This process is, in fact, the body of the first vertebra which has fused with the body of the axis instead of being incorporated into the atlas. The front of the dens articulates with the back of the anterior arch of the atlas; rotary (shaking) movements of the head occur at this joint. The seventh cervical vertebra has a very long spinous process which is easily palpable. The primary curvature of the vertebral column is concave forwards and this persists in the thoracic and pelvic regions. In contrast, the cervical and lumbar parts of the vertebral column are convexly curved anteriorly. These anterior curvatures are secondary curvatures which appear in late fetal life. The cervical curvature becomes accentuated in early childhood as the child begins to support its own head and the lumbar curve develops as the child begins to sit up.

The skull

Dental students and practitioners require a sound knowledge of the structure, growth, and development of the skull as a whole. The structure of the skull can be examined and studied more efficiently if you have access to a dried skull or one of the very good plastic replica skulls which are now available; you can identify the structures on the diagrams accompanying the following descriptions and examine a skull at the same time to appreciate the size and relationships of individual components. This chapter outlines the basic principles of the development and structure of the skull and includes some reference to individual bones where this makes understanding easier. The more detailed aspects of particular regions of the skull will be covered in the appropriate chapter describing the whole anatomy of that region; it is much easier to learn the parts of the skull in context of overall structure and function rather than learning a long list of bones, foramina, and muscle attachments in isolation from the related soft tissue structures. Only the maxilla and mandible which are bones of significant clinical importance are described as separate bones. As already demonstrated in Chapter 20, the skull is the structural basis f or the anatomy of the head. The skull has many functions. • It encloses and protects the brain. • It provides protective capsules for the eyes and middle and inner ear. • It forms the skeleton of the entrances to the respiratory and gastrointestinal tracts (GIT) through the nose and mouth, respectively. Those skull components that form the entrance to the GIT also house and support the teeth and soft tissues of the oral region as part of this function. As already outlined in Chapter 20, the skull is made up of several bones joined together to form the cranium which articulates with the separate mandible forming the lower jaw at the temporomandibular joints. The cranium specifically refers to the skull without the mandible; the terms ‘skull’ and ‘cranium’ are not strictly synonymous but they are frequently used as though they are. The cranium can be subdivided into the braincase enclosing the brain and the facial skeleton.

Embryology of the head and neck

Embryology and development have been covered after the main anatomical descriptions in the previous sections, but it is going to precede them in this section. The reason for this departure is that the embryonic development of the head and neck explains much of the mature anatomy which can seem illogical without its developmental history. The development of the head, face, and neck is an area of embryology where significant strides in our understanding have been made in the last few years. The development of the head is intimately related to the development of the brain outlined in Chapter 19 and its effects on shaping the head will be described in Chapters 32 and 33. The major thrust of this chapter is the description of the formation of structures called the pharyngeal (or branchial) arches and the fate of the tissues that contribute to them. All four embryonic germ layers contribute to the pharyngeal arches and their derivatives, hence to further development of the head and neck. Figure 21.1 is a cross section through the neck region of a 3-week old embryo after neurulation and folding described in Chapter 8. It shows the structures and tissues that contribute to the formation of the head and neck: • The neural tube situated posteriorly and the ectomesenchymal neural crest cells that arise as the tube closes; • The paraxial mesoderm anterolateral to the neural tube; • The endodermal foregut tube anteriorly; • The investing layer of ectoderm. The development of all these tissues is intimately interrelated. The pharyngeal arches are very ancient structures in the evolutionary history of vertebrates. The arches and their individual components have undergone many modifications during their long history. In ancestral aquatic vertebrates, as in modern fishes, water was drawn in through the mouth and expelled through a series of gill slits (or branchiae, hence the term ‘branchial arch’) in the sides of the pharynx. Oxygen was extracted as the water was passed over a gill apparatus supported by a branchial arch skeleton moved by branchial muscles controlled by branchial nerves. Although ventilation and respiration is now a function of the lungs in land vertebrates, the pharyngeal arches persist during vertebrate development.

The heart, pericardium, and mediastinum

The heart, the arteries and veins leaving and entering the heart which are usually referred to as the great vessels, the trachea and bronchi, the oesophagus, and the vagus and phrenic nerves and sympathetic chains occupy the mediastinum , the area in the middle of the thoracic cavity between the two pleural sacs. The anteroposterior dimension of the thorax is narrowest in the mediastinum because of the presence of the thoracic vertebrae posteriorly. Laterally, the pleural sacs enclosing the lungs extend much further back alongside the vertebrae in the areas known as the paravertebral gutters. The great vessels enter and leave the superior aspect of the heart. The large veins draining the head, neck, and arms lie most superficially; they unite to form the superior vena cava that enters the right atrium of the heart. These veins overlie the two large arteries exiting the heart, the aorta, and pulmonary trunk. The aorta has a short ascending part, then forms the aortic arch passing backwards and to the left before continuing down the posterior wall of the thorax as the descending thoracic aorta. The subclavian and common carotid arteries, supplying blood to the arms and head and neck, respectively, arise from the aortic arch. The oesophagus is the deepest structure lying on the vertebrae and the trachea and main bronchi lie superficial to it. The sympathetic chains lie lateral to the vertebral bodies and the vagus and phrenic nerves are in intermediate positions. All these structures will be described in more detail in the rest of this chapter. The mediastinum is divided, for descriptive convenience, into the superior and inferior mediastinum. Figure 12.1 shows the imaginary line of division joining the sternal angle and the intervertebral disc below T4 that demarcates the boundaries of the superior and inferior of the mediastinum. The superior mediastinum occupies the space between the thoracic inlet above and the imaginary horizontal plane. The inferior mediastinum lies below that line and extends as far as the diaphragm. The lateral borders of both subdivisions of the mediastinum are the parietal pleura covering the medial aspect of the lungs, the mediastinal pleura.

The locomotor system

The locomotor system comprises the skeleton, composed principally of bone and cartilage, the joints between them, and the muscles which move bones at joints. The skeleton forms a supporting framework for the body and provides the levers to which the muscles are attached to produce movement of parts of the body in relation to each other or movement of the body as a whole in relation to its environment. The skeleton also plays a crucial role in the protection of internal organs. The skeleton is shown in outline in Figure 2.1A. The skull, vertebral column, and ribs together constitute the axial skeleton. This forms, as its name implies, the axis of the body. The skull houses and protects the brain and the eyes and ears; the anatomy of the skull is absolutely fundamental to the understanding of the structure of the head and is covered in detail in Section 4. The vertebral column surrounds and protects the spinal cord which is enclosed in the spinal canal formed by a large central canal in each vertebra. The vertebral column is formed from 33 individual bones although some of these become fused together. The vertebral column and its component bones are shown from the side in Figure 2.1B. There are seven cervical vertebrae in the neck, twelve thoracic vertebrae in the posterior wall of the thorax, five lumbar vertebrae in the small of the back, five fused sacral vertebrae in the pelvis, and four coccygeal vertebrae—the vestigial remnants of a tail. Intervertebral discs separate individual vertebrae from each other and act as a cushion between the adjacent bones; the discs are absent from the fused sacral vertebrae. The cervical vertebrae are small and very mobile, allowing an extensive range of neck movements and hence changes in head position. The first two cervical vertebrae, the atlas and axis, have unusual shapes and specialized joints that allow nodding and shaking movements of the head on the neck. The thoracic vertebrae are relatively immobile. combination of thoracic vertebral column, ribs, and sternum form the thoracic cage that protects the thoracic organs, the heart, and lungs and is intimately involved in ventilation (breathing).

Development and growth of the skull and age changes

The development of the facial bones is particularly important in the fields of paediatric dentistry and orthodontics. Dental students and dental practitioners who do not specialize in those subjects should have an appreciation of the subject to be aware of the changes to the face and jaws they are seeing in patients under continuous care as they grow, mature, and age. Human beings increase in both size and complexity during the growth period which lasts from conception until maturity at about 16 to 18 years of age. As we have seen in Chapters 8, 1, 19, 21, and 32, most of the increase in complexity occurs during the pre-embryonic and embryonic phases of prenatal development although changes still occur in many organs and tissues well into post-natal life. Size increase is also rapid prenatally and continues throughout the remainder of the growth period although the growth rate changes. Changes in overall size may occur in mature individuals due to obesity or other pathological conditions but this is not growth. Growth in overall size can be studied by examining the changes with age in easily measured parameters such as height and weight. There are two ways in which such data can be presented as shown in Figure 33.1. A distance curve is the simplest method illustrated in Figure 33.1A by plotting height against age on a graph. Changes in the rate of growth are demonstrated more clearly by plotting the increment in the measurement per unit of time such as the increase in height per year against age; this is a velocity curve shown in Figure 33.1B. You can see in Figure 33.1A that height increases more rapidly around the age of 14; the velocity curve in Figure 33.1B makes the rapid growth at this age much clearer. If distance curves are plotted for different body components, the curves show specific characteristics. The overall growth of the body is accurately indicated by measures of height and weight; these measurements plotted against age produce the somatic growth curve shown in Figure 33.2. Growth is rapid in the prenatal and early post-natal period then begins to slow down after about 4 years of age.

The oral cavity and related structures

Technically, the oral cavity consists of the vestibule between the lips and cheeks externally and the teeth and alveolar processes internally and the larger oral cavity proper located internal to the dental arches. In clinical practice, the whole mouth is simply referred to as the oral cavity, but ‘vestibule’ is used for the specific area defined above. For charting of teeth and similar dental procedures, the mouth is divided into quadrants—upper right and left and lower right and left with the midline and occlusal surfaces of the teeth forming the dividing lines. It is a crucial skill for dental students and practitioners to recognize the naked eye appearance of the structures in a normal healthy mouth and variations that occur; abnormal appearances can then be recognized, diagnosed, and treated successfully. Much of the macroscopical appearance is determined by the underlying gross anatomy so this must be understood too. The best way to examine the interior of the mouth is on a subject seated in a dental chair with clinical lighting and the use of a tongue spatula and a dental mirror where necessary. However, you will be able to see most of the important features by examining the inside of your own mouth in a well-lit household mirror. The following description and illustrations apply to an adult mouth with a full secondary (permanent) dentition of two incisors, one canine, two premolars, and three molars in each quadrant, making 32 teeth in total. Apart from size, the major differences in childrens’ mouths are in the dentition. The primary (deciduous) dentition erupts into the oral cavity between the age of 6 months and 2 years. It comprises two incisors, one canine, and two molars in each quadrant, giving a total of 20 teeth. Most of the teeth of the secondary dentition erupt between the ages of 6 to 12, replacing the primary teeth; a combination of primary and secondary teeth, a mixed dentition, is found between these ages. Primary incisors and canines are replaced by their permanent successors, but the deciduous molars are succeeded by the permanent premolars; the three permanent molars in each quadrant are additional teeth.