Oral cavity

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CHAPTER 30 Oral cavity

The mouth or oral cavity extends from the lips and cheeks externally to the anterior pillars of the fauces internally, where it continues into the oropharynx (Fig. 30.1). The mouth can be subdivided into the vestibule external to the teeth and the oral cavity proper internal to the teeth. The palate forms the roof of the mouth and separates the oral and nasal cavities. The floor of the mouth is formed by the mylohyoid muscles and is occupied mainly by the tongue. The lateral walls of the mouth are defined by the cheeks and retromolar regions. Three pairs of major salivary glands (parotid, submandibular and sublingual) and numerous minor salivary glands (labial, buccal, palatal, lingual) open into the mouth. The muscles in the oral cavity are associated with the lips, cheeks, floor of the mouth and tongue. The muscles of the lips and cheeks are described with the face in Chapter 29. The muscles of the soft palate are described with the pharynx in Chapter 33.

The mouth is concerned primarily with the ingestion and mastication of food, which is mainly the function of the teeth. The mouth is also associated with phonation and ventilation.

ORAL VESTIBULE

The oral vestibule is a slit-like space between the lips or cheeks on one side and the teeth on the other. When the teeth occlude, the vestibule is a closed space that only communicates with the oral cavity proper in the retromolar regions behind the last molar tooth on each side. Where the mucosa that covers the alveolus of the jaw is reflected onto the lips and cheeks, a trough or sulcus is formed which is called the fornix vestibuli. A variable number of sickle-shaped folds containing loose connective tissue run across the fornix vestibuli. In the midline these are the upper and lower labial frena (or frenula). Other folds may traverse the fornix near the canines or premolars. The folds in the lower fornix are said to be more pronounced than those in the upper fornix (Fig. 30.2).

The upper labial frenulum is normally attached well below the alveolar crest. A large frenulum with an attachment near or on the crest may be associated with a midline gap (diastema) between the maxillary first incisors. This can be corrected by simple surgical removal of the frenulum (frenulectomy), because it contains no structures of clinical importance. Prominent frena may compromise the stability of dentures.

ORAL MUCOSA

The oral mucosa is continuous with the skin at the labial margins (vermilion border) and with the pharyngeal mucosa at the oropharyngeal isthmus. It varies in structure, function and appearance in different regions of the oral cavity and is traditionally divided into lining, masticatory and specialized mucosae.

MASTICATORY MUCOSA AND THE GINGIVAE

Masticatory mucosa, i.e. mucosa that is subjected to masticatory stress, is bound firmly to underlying bone or to the necks of the teeth, and forms a mucoperiosteum in the gingivae and palatine raphe. Gingival, palatal and dorsal lingual mucosae are keratinized or parakeratinized.

The gingivae may be further subdivided into the attached gingivae and the free gingivae. Attached gingivae are firmly bound to the periosteum of the alveolus and to the teeth, whereas free gingivae, which constitute approximately a 1 mm margin of the gingivae, lie unattached around the cervical region of each tooth. The free gingival groove between the free and attached gingivae corresponds roughly to the floor of the gingival sulcus which separates the inner surface of the attached gingivae from the enamel. The interdental papilla is that part of the gingivae which fills the space between adjacent teeth. The surface of the attached gingivae is characteristically stippled, although there is considerable interindividual variation in the degree of stippling, and variation according to age, sex and the health of the gingivae. The free gingivae are not stippled. A mucogingival line delineates the attached gingivae on the lingual surface of the lower jaw from the alveolar mucosa towards the floor of the mouth. There is no corresponding obvious division between the attached gingivae and the remainder of the palatal mucosa because this whole surface is orthokeratinized masticatory mucosa, which is pink.

A submucosa is absent from the gingivae and the midline palatine raphe, but is present over the rest of the hard palate. Posterolaterally it is thick where it contains mucous salivary glands and the greater palatine nerves and vessels, and it is anchored to the periosteum of the maxillae and palatine bones by collagenous septa.

Vascular supply and lymphatic drainage

The gingival tissues derive their blood supply from the maxillary and lingual arteries. The buccal gingivae around the maxillary cheek teeth are supplied by gingival and perforating branches from the posterior superior alveolar artery and by the buccal branch of the maxillary artery. The labial gingivae of anterior teeth are supplied by labial branches of the infraorbital artery and by perforating branches of the anterior superior alveolar artery. The palatal gingivae are supplied primarily by branches of the greater palatine artery.

The buccal gingivae associated with the mandibular cheek teeth are supplied by the buccal branch of the maxillary artery and by perforating branches from the inferior alveolar artery. The labial gingivae around the anterior teeth are supplied by the mental artery and by perforating branches of the incisive artery. The lingual gingivae are supplied by perforating branches from the inferior alveolar artery and by its lingual branch, and by the main lingual artery, a branch of the external carotid artery.

No accurate description is available concerning the venous drainage of the gingivae, although it may be assumed that buccal, lingual, greater palatine and nasopalatine veins are involved. These veins run into the pterygoid plexuses (apart from the lingual veins, which may pass directly into the internal jugular veins).

The lymph vessels of the labial and buccal gingivae of the maxillary and mandibular teeth unite to drain into the submandibular nodes, though in the labial region of the mandibular incisors they may drain into the submental lymph nodes. The lingual and palatal gingivae drain into the jugulodigastric group of nodes, either directly or indirectly through the submandibular nodes.

Innervation

The nerves supplying the gingivae in the upper jaw come from the maxillary nerve via its greater palatine, nasopalatine, and anterior, middle and posterior superior alveolar branches (see Table 30.2). Surgical division of the nasopalatine nerve, for example during the removal of an ectopic canine tooth, causes no obvious sensory deficit in the anterior part of the palate, which suggests that the territory of the greater palatine nerve reaches as far forwards as the gingivae lingual to the incisor teeth or the nerve has large regenerative potential. The mandibular nerve innervates the gingivae in the lower jaw by its inferior alveolar, lingual and buccal branches.

FLOOR OF THE MOUTH

The floor of the mouth is a small horseshoe-shaped region situated beneath the movable part of the tongue and above the muscular diaphragm formed by the mylohyoid muscles (Fig. 30.4, see Fig. 30.7). A fold of tissue, the lingual frenulum, extends onto the inferior surface of the tongue from near the base of the tongue. It occasionally extends across the floor of the mouth to be attached onto the mandibular alveolus, known colloquially as a ‘tongue tie’; historically, this has been removed to aid speech, but the evidence for this is scanty. The submandibular salivary ducts open into the mouth at the sublingual papilla (caruncle), which is a large centrally positioned protuberance at the base of the tongue.

The sublingual folds lie on either side of the sublingual papilla and cover the underlying submandibular ducts and sublingual salivary glands. The blood supply of the floor of the mouth is described with the blood supply of the tongue (see p. 505). The main muscle forming the floor of the mouth is mylohyoid, with geniohyoid lying immediately above it.

Mylohyoid

Mylohyoid lies superior to the anterior belly of digastric and, with its contralateral fellow, forms a muscular floor for the oral cavity. It is a flat, triangular sheet attached to the whole length of the mylohyoid line of the mandible (Fig. 30.4A, see Fig. 30.7). The mylohyoid line is of variable length, sometimes ending before the lower third molar (wisdom) tooth. The posterior fibres of mylohyoid pass medially and slightly downwards to the front of the body of the hyoid bone near its lower border. The middle and anterior fibres from each side decussate in a median fibrous raphe that stretches from the symphysis menti to the hyoid bone. The median raphe is sometimes absent, in which case the two muscles form a continuous sheet, or it may be fused with the anterior belly of digastric. In about one-third of subjects there is a hiatus in the muscle through which a process of the sublingual gland protrudes.

PALATE

The palate forms the roof of the mouth and is divisible into two regions, namely the hard palate in front and soft palate behind.

HARD PALATE

The hard palate is formed by the palatine processes of the maxillae and the horizontal plates of the palatine bones (see Fig. 30.14A). The hard palate is bounded in front and at the sides by the tooth-bearing alveolus of the upper jaw and is continuous posteriorly with the soft palate. It is covered by a thick mucosa bound tightly to the underlying periosteum. In its more lateral regions it also possesses a submucosa containing the main neurovascular bundle. The mucosa is covered by keratinized stratified squamous epithelium which shows regional variations and may be ortho- or parakeratinized.

The periphery of the hard palate consists of gingivae. A narrow ridge, the palatine raphe, devoid of submucosa, runs anteroposteriorly in the midline. An oval prominence, the incisive papilla, lies at the anterior extremity of the raphe. It covers the incisive fossa at the oral opening of the incisive canal and also marks the position of the fetal nasopalatine canal. Irregular transverse ridges or rugae, each containing a core of dense connective tissue, radiate outwards from the palatine raphe in the anterior half of the hard palate: their pattern is unique.

The submucosa in the posterior half of the hard palate contains minor mucous-type salivary glands (Fig. 30.3). They secrete via numerous small ducts which often drain into a larger duct that opens bilaterally at the paired palatine foveae. These depressions, sometimes a few millimetres deep, flank the midline raphe at the posterior border of the hard palate. They provide a useful landmark for the extent of an upper denture; if not observed during construction of the denture they cause the denture to become unstable when the soft palate moves during deglutition and mastication. The upper surface of the hard palate is the floor of the nasal cavity and is covered by ciliated respiratory epithelium.

Innervation of the hard palate

The sensory nerves of the hard palate are the greater palatine and nasopalatine branches of the maxillary nerve, which all pass through the pterygopalatine ganglion. The greater palatine nerve descends through the greater palatine canal, emerges on the hard palate from the greater palatine foramen, runs forwards in a groove on the inferior surface of the bony palate almost to the incisor teeth and supplies the gums and the mucosa and glands of the hard palate (Fig. 30.3). It also communicates with the terminal filaments of the nasopalatine nerve. As it leaves the greater palatine canal, it supplies palatine branches to both surfaces of the soft palate. The lesser (middle and posterior) palatine nerves, which are much smaller, descend through the greater palatine canal and emerge through the lesser palatine foramina in the tubercle of the palatine bone to supply the uvula, tonsil and soft palate. The nasopalatine nerves enter the palate at the incisive foramen and are branches of the maxillary nerve which pass through the pterygopalatine ganglion to supply the anterior part of the hard palate behind the incisor teeth.

Fibres conveying taste impulses from the palate probably pass via the palatine nerves to the pterygopalatine ganglion, and travel through it without synapsing to join the nerve of the pterygoid canal and the greater petrosal nerve to the facial ganglion, where their cell bodies are situated. The central processes of these neurones traverse the sensory root of the facial nerve (nervus intermedius) to pass to the gustatory nucleus in the nucleus of the tractus solitarius. Parasympathetic postganglionic secretomotor fibres from the pterygopalatine ganglion run with the nerves to supply the palatine mucous glands.

TONGUE

The tongue is a highly muscular organ of deglutition, taste and speech. It is partly oral and partly pharyngeal in position, and is attached by its muscles to the hyoid bone, mandible, styloid processes, soft palate and the pharyngeal wall. It has a root, an apex, a curved dorsum and an inferior surface. Its mucosa is normally pink and moist, and is attached closely to the underlying muscles. The dorsal mucosa is covered by numerous papillae, some of which bear taste buds. Intrinsic muscle fibres are arranged in a complex interlacing pattern of longitudinal, transverse, vertical and horizontal fasciculi and this allows great mobility. Fasciculi are separated by a variable amount of adipose tissue which increases posteriorly. The root of the tongue is attached to the hyoid bone and mandible, and between them it is in contact inferiorly with geniohyoid and mylohyoid. The dorsum (posterosuperior surface) is generally convex in all directions at rest. It is divided by a V-shaped sulcus terminalis into an anterior, oral (presulcal) part which faces upwards, and a posterior, pharyngeal (postsulcal) part which faces posteriorly. The anterior part forms about two-thirds of the length of the tongue. The two limbs of the sulcus terminalis run anterolaterally to the palatoglossal arches from a median depression, the foramen caecum, which marks the site of the upper end of the embryonic thyroid diverticulum (thyroglossal duct). The oral and pharyngeal parts of the tongue differ in their mucosa, innervation and developmental origins.

ORAL (PRESULCAL) PART

The presulcal part of the tongue is located in the floor of the oral cavity. It has an apex touching the incisor teeth, a margin in contact with the gums and teeth, and a superior surface (dorsum) related to the hard and soft palates. On each side, in front of the palatoglossal arch, there are four or five vertical folds, the foliate papillae, which represent vestiges of larger papillae found in many other mammals. The dorsal mucosa has a longitudinal median sulcus and is covered by filiform, fungiform and circumvallate papillae (Fig. 30.5). The mucosa on the inferior (ventral) surface is smooth, purplish and reflected onto the oral floor and gums: it is connected to the oral floor anteriorly by the lingual frenulum. The deep lingual vein, which is visible, lies lateral to the frenulum on either side. The plica fimbriata (fimbriated fold), a fringed mucosal ridge directed anteromedially towards the apex of the tongue, lies lateral to the vein. This part of the tongue develops from the lingual swellings of the mandibular arch and from the tuberculum impar, and this embryological derivation explains its sensory innervation.

MUSCLES OF THE TONGUE

The tongue is divided by a median fibrous septum, attached to the body of the hyoid bone. There are extrinsic and intrinsic muscles in each half, the former extending outside the tongue and moving it bodily, the latter wholly within it and altering its shape. The extrinsic musculature consists of four pairs of muscles, namely genioglossus, hyoglossus, styloglossus (and chondroglossus) and palatoglossus.

Genioglossus

Genioglossus is triangular in sagittal section, lying near and parallel to the midline. It arises from a short tendon attached to the superior genial tubercle behind the mandibular symphysis, above the origin of geniohyoid. From this point it fans out backwards and upwards (Fig. 30.6). The inferior fibres of genioglossus are attached by a thin aponeurosis to the upper anterior surface of the hyoid body near the midline (a few fasciculi passing between hyoglossus and chondroglossus to blend with the middle constrictor of the pharynx). Intermediate fibres pass backwards into the posterior part of the tongue, and superior fibres ascend forwards to enter the whole length of the ventral surface of the tongue from root to apex, intermingling with the intrinsic muscles. The muscles of opposite sides are separated posteriorly by the lingual septum. Anteriorly they are variably blended by decussation of fasciculi across the midline. The attachment of the genioglossi to the genial tubercles prevents the tongue from sinking back and obstructing respiration, therefore anaesthetists pull the mandible forward to obtain the full benefit of this connection.

Styloglossus

Styloglossus is the shortest and smallest of the three styloid muscles (Fig. 30.6). It arises from the anterolateral aspect of the styloid process near its apex, and from the styloid end of the stylomandibular ligament. Passing downwards and forwards, it divides at the side of the tongue into a longitudinal part, which enters the tongue dorsolaterally to blend with the inferior longitudinal muscle in front of hyoglossus, and an oblique part, overlapping hyoglossus and decussating with it.

Stylohyoid ligament

The stylohyoid ligament is a fibrous cord which extends from the tip of the styloid process to the lesser cornu of the hyoid bone (Fig. 30.6). It gives attachment to some fibres of styloglossus and the middle constrictor of the pharynx and is closely related to the lateral wall of the oropharynx. Below it is overlapped by hyoglossus. The ligament is derived embryologically from the second branchial arch. It may be partially calcified.

Intrinsic muscles

The intrinsic muscles are the bilateral superior and inferior longitudinal, the transverse and the vertical (Fig. 30.7).

VASCULAR SUPPLY AND LYMPHATIC DRAINAGE OF THE TONGUE

Lingual artery

The tongue and the floor of the mouth are supplied chiefly by the lingual artery, which arises from the anterior surface of the external carotid artery. It passes between hyoglossus and the middle constrictor of the pharynx to reach the floor of the mouth accompanied by the lingual veins and the glossopharyngeal nerve. At the anterior border of hyoglossus, the lingual artery bends sharply upwards (Fig. 30.8). It is covered by the mucosa of the tongue and lies between genioglossus medially and the inferior longitudinal muscle laterally. Near the tip of the tongue, it anastomoses with its contralateral fellow; this contribution is important in maintaining the blood supply to the tongue in any surgical resection of the tongue. The branches of the lingual artery form a rich anastomotic network, which supplies the musculature of the tongue, and a very dense submucosal plexus. Named branches of the lingual artery in the floor of the mouth are the dorsal lingual, sublingual and deep lingual arteries.

Lymphatic drainage

The mucosa of the pharyngeal part of the dorsal surface of the tongue contains many lymphoid follicles aggregated into dome-shaped groups, the lingual tonsils. Each group is arranged around a central deep crypt, or invagination, which opens onto the surface epithelium. The ducts of mucous glands open into the bases of the crypts. Small isolated follicles also occur beneath the lingual mucosa. The lymphatic drainage of the tongue can be divided into three main regions, marginal, central and dorsal. The anterior region of the tongue drains into marginal and central vessels, and the posterior part of the tongue behind the circumvallate papillae drains into the dorsal lymph vessels. The more central regions may drain bilaterally, and this must be borne in mind when planning to remove malignant tumours of the tongue that are approaching the midline. If the tumour has a propensity for lymphatic spread, both cervical chains may be involved.

INNERVATION OF THE TONGUE

The muscles of the tongue, with the exception of palatoglossus, are supplied by the hypoglossal nerve. Palatoglossus is supplied via the pharyngeal plexus (see Ch. 33). The pathways for proprioception associated with the tongue musculature are unknown, but presumably may involve the lingual, glossopharyngeal or hypoglossal nerves, and the cervical spinal nerves which communicate with the hypoglossal nerve.

The sensory innervation of the tongue reflects its embryological development: the anterior two-thirds is derived from first-arch mesenchyme and the posterior one-third from third-arch mesenchyme. The nerve of general sensation to the anterior two-thirds is the lingual nerve, which also carries taste sensation derived from the chorda tympani branch of the facial nerve. The nerve supplying both general and taste sensation to the posterior one-third is the glossopharyngeal nerve. An additional area in the region of the valleculae is supplied by the internal laryngeal branch of the vagus nerve.

Lingual nerve

The lingual nerve is sensory to the mucosa of the floor of the mouth, mandibular lingual gingivae and mucosa of the presulcal part of the tongue (excluding the circumvallate papillae). It also carries postganglionic parasympathetic fibres from the submandibular ganglion to the sublingual and anterior lingual glands.

The lingual nerve arises from the posterior trunk of the mandibular nerve in the infratemporal fossa (see Fig. 31.13A) where it is joined by the chorda tympani branch of the facial nerve and often by a branch of the inferior alveolar nerve. It then passes below the mandibular attachment of the superior pharyngeal constrictor and pterygomandibular raphe, closely applied to the periosteum of the medial surface of the mandible, until it lies opposite the distal (posterior) root of the third molar tooth, where it is covered only by the gingival mucoperiosteum. At this point it can contact the lingual cortical plate and may be at the level of, or higher than, the alveolar bone crest. It next passes medial to the mandibular attachment of mylohyoid, which carries it progressively away from the mandible, and separates it from the alveolar bone covering the mesial root of the third molar tooth, and then passes downward and forward on the deep surface of mylohyoid (i.e. the surface nearer the mucosa covering the floor of the mouth), crossing the lingual sulcus beneath the mucosa. In this position it lies on the deep portion of the submandibular gland which bulges over the top of the posterior border of mylohyoid. It passes below the submandibular duct which crosses it from medial to lateral, and curves upward, forward and medially to enter the tongue (Figs 30.8, 30.10). Within the tongue the lingual nerve lies first on styloglossus and then the lateral surface of hyoglossus and genioglossus, before dividing into terminal branches that supply the overlying lingual mucosa. The lingual nerve is connected to the submandibular ganglion (see Fig. 31.15) by two or three branches, and also forms connecting loops with twigs of the hypoglossal nerve at the anterior margin of hyoglossus.

The lingual nerve is at risk during surgical removal of (impacted) lower third molars, and, after such operations, up to 0.8% of patients may develop lingual sensory disturbance, which may persist in 0.3% (Robinson & Smith 1996). The nerve is also at risk during operations to remove the submandibular salivary gland, because the duct must be dissected from the lingual nerve, and because its connection to the submandibular ganglion pulls it into the operating field.