Vascular supply and innervation

The cheek receives its arterial blood supply principally from the buccal branch of the maxillary artery, and is innervated by cutaneous branches of the maxillary division of the trigeminal nerve, via the zygomaticofacial and infraorbital nerves, and by the buccal branch of the mandibular division of the trigeminal nerve.

Vascular supply and innervation

The lips are mainly supplied by the superior and inferior labial branches of the facial artery. The upper lip is innervated by superior labial branches of the infraorbital nerve and the lower lip is innervated by the mental branch of the mandibular division of the trigeminal.

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.

Table 30.2 Nerve supply to the teeth and gingivae.

Vascular supply and lymphatic drainage of the hard palate

The palate derives its blood supply principally from the greater palatine artery, a branch of the third part of the maxillary artery. The greater palatine artery descends with its accompanying nerve in the palatine canal, where it gives off two or three lesser palatine arteries which are transmitted through lesser palatine canals to supply the soft palate and tonsil, and anastomose with the ascending palatine branch of the facial artery. The greater palatine artery emerges onto the oral surface of the palate at the greater palatine foramen adjacent to the second maxillary molar and runs in a curved groove near the alveolar border of the hard palate to the incisive canal. It ascends this canal and anastomoses with septal branches of the nasopalatine artery to supply the gingivae, palatine glands and mucous membrane.

The veins of the hard palate accompany the arteries and drain largely to the pterygoid plexus.

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.

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.

Fig. 30.8 Left half of the tongue, viewed from the medial side, showing the lingual artery and ramifications of the hypoglossal and lingual nerves.

Lingual veins

The lingual veins are formed from the union of the dorsal lingual and deep lingual veins and the vena comitans of the hypoglossal nerve. The veins draining the tongue follow two routes. Dorsal lingual veins drain the dorsum and sides of the tongue, join the lingual veins accompanying the lingual artery between hyoglossus and genioglossus, and empty into the internal jugular vein near the greater cornu of the hyoid bone. The deep lingual vein begins near the tip of the tongue and runs back just beneath the mucous membrane on the inferior surface of the tongue. It joins a sublingual vein from the sublingual salivary gland near the anterior border of hyoglossus and forms the vena comitans nervi hypoglossi, which run back with the hypoglossal nerve between mylohyoid and hyoglossus to join the facial, internal jugular or lingual vein. The lingual veins usually join the facial and retromandibular veins (anterior division) to form the common facial vein, which drains into the internal jugular vein.

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.

Marginal vessels

Marginal vessels from the apex of the tongue and the lingual frenulum area descend under the mucosa to widely distributed nodes. Some vessels pierce mylohyoid as it contacts the mandibular periosteum to enter either the submental or anterior or middle submandibular nodes, or else to pass anterior to the hyoid bone to the jugulo-omohyoid node. Vessels arising in the plexus on one side may cross under the frenulum to end in contralateral nodes. Efferent vessels of median submental nodes pass bilaterally. Some vessels pass inferior to the sublingual gland and accompany the companion vein of the hypoglossal nerve to end in jugulodigastric nodes. One vessel often descends further to reach the jugulo-omohyoid node, and passes either superficial or deep to the intermediate tendon of digastric.

Vessels from the lateral margin of the tongue cross the sublingual gland, pierce mylohyoid and end in the submandibular nodes. Others end in the jugulodigastric or jugulo-omohyoid nodes. Vessels from the posterior part of the lingual margin traverse the pharyngeal wall to the jugulodigastric lymph nodes (Fig. 30.9).

Fig. 30.9 Lymphatic drainage of the tongue. Removal of sternocleidomastoid has exposed the whole chain of deep cervical lymph nodes.

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.

Glossopharyngeal nerve

The glossopharyngeal nerve is distributed to the posterior one-third of the tongue and the circumvallate papillae. It communicates with the lingual nerve. The course of the glossopharyngeal nerve in the neck is described in Chapter 28.

Hypoglossal nerve

The course of the hypoglossal nerve in the neck is described in Chapter 28. After crossing the loop of the lingual artery a little above the tip of the greater cornu of the hyoid, it inclines upwards and forwards on hyoglossus, passing deep to stylohyoid, the tendon of digastric and the posterior border of mylohyoid. Between mylohyoid and hyoglossus the hypoglossal nerve lies below the deep part of the submandibular gland, the submandibular duct and the lingual nerve, with which it communicates. It then passes onto the lateral aspect of genioglossus, continuing forwards in its substance as far as the tip of the tongue (Fig. 30.8). It distributes fibres to styloglossus, hyoglossus and genioglossus and to the intrinsic muscles of the tongue. If the nerve suffers either iatrogenic or pathological damage, the tongue, on protrusion, will deviate towards the unaffected side and there may be wasting of the affected side.

Special sensory innervation of the tongue

The sense of taste is dependent on scattered groups of sensory cells, the taste buds, which occur in the oral cavity and pharynx and are particularly plentiful on the lingual papillae of the dorsal lingual mucosa.

Dorsal lingual mucosa

The dorsal mucosa is somewhat thicker than the ventral and lateral mucosae, is directly adherent to underlying muscular tissue with no discernible submucosa, and is covered by numerous papillae. The dorsal epithelium consists of a superficial stratified squamous epithelium, which varies from non-keratinized stratified squamous epithelium posteriorly, to fully keratinized epithelium overlying the filiform papillae more anteriorly. These features probably reflect the fact that the apex of the tongue is subject to greater dehydration than the posterior and ventral parts and is subject to more abrasion during mastication. The underlying lamina propria is a dense fibrous connective tissue, with numerous elastic fibres, and is continuous with similar tissue extending between the lingual muscle fasciculi. It contains numerous vessels and nerves from which the papillae are supplied, and also large lymph plexuses and lingual glands.

Lingual papillae

Lingual papillae are projections of the mucosa covering the dorsal surface of the tongue (Fig. 30.5). They are limited to the presulcal part of the tongue, produce its characteristic roughness and increase the area of contact between the tongue and the contents of the mouth. There are four principal types, named filiform, fungiform, foliate and circumvallate papillae, and all except the filiform papillae bear taste buds. Papillae are more visible in the living when the tongue is dry.

Filiform papillae

Filiform papillae are minute, conical or cylindrical projections which cover most of the presulcal dorsal area, and are arranged in diagonal rows that extend anterolaterally, parallel with the sulcus terminalis, except at the lingual apex, where they are transverse. They have irregular cores of connective tissue and their epithelium, which is keratinized, may split into whitish fine secondary processes (Fig. 30.11). They appear to function to increase the friction between the tongue and food, and facilitate the movement of particles by the tongue within the oral cavity.

Fig. 30.11 Dorsal surface of the anterior tongue showing non-keratinized fungiform (left) and two keratinized filiform papillae (centre and right) with non-keratinized regions between.

(By permission from Young B, Heath JW 2000 Wheater’s Functional Histology. Edinburgh: Churchill Livingstone.)

Fungiform papillae

Fungiform papillae occur mainly on the lingual margin but also irregularly on the dorsal surface, where they may occasionally be numerous (Fig. 30.11). They differ from filiform papillae because they are larger, rounded and deep red in colour, this last reflecting their thin, non-keratinized epithelium and highly vascular connective tissue core. Each usually bears one or more taste buds on its apical surface.

Foliate papillae

Foliate papillae lie bilaterally in two zones at the sides of the tongue near the sulcus terminalis, each formed by a series of red, leaf-like mucosal ridges, covered by a non-keratinized epithelium. They bear numerous taste buds.

Circumvallate papillae

Circumvallate papillae are large cylindrical structures, varying in number from 8 to 12, which form a V-shaped row immediately in front of the sulcus terminalis on the dorsal surface of the tongue. Each papilla, 1–2 mm in diameter, is surrounded by a slight circular mucosal elevation (vallum or wall) which is separated from the papilla by a circular sulcus (Figs 30.12, 30.13). The papilla is narrower at its base than its apex and the entire structure is generally covered with non-keratinized stratified squamous epithelium. Numerous taste buds are scattered in both walls of the sulcus, and small serous glands (of von Ebner) open into the sulcal base.

Fig. 30.12 Section through a circumvallate papilla. Serous glands (of von Ebner) empty via ducts into the base of the trench and numerous taste buds are contained within the stratified epithelium of the papillary wall (pale structures on the inner wall of the cleft, left side).

(By permission from Young B, Heath JW 2000 Wheater’s Functional Histology. Edinburgh: Churchill Livingstone.)

Fig. 30.13 Circumvallate papilla. A, Scanning electron micrograph showing a circumvallate papilla surrounded by a trench. B, Section of circumvallate papilla showing pale barrel-shaped taste buds (B) in its walls. P, apical pore.

(A, By courtesy of S Franey and by permission from Berkovitz BKB, Holland GR, Moxham BJ 2002 Oral Anatomy, Embryology and Histology, 3rd edn. Edinburgh: Mosby; B, by permission from Dr JB Kerr, Monash University, from Kerr JB 1999 Atlas of Functional Histology. London: Mosby.)

Autonomic innervation of the tongue

The parasympathetic innervation of the various glands of the tongue is from the chorda tympani branch of the facial nerve which synapses in the submandibular ganglion: postganglionic branches are distributed to the lingual mucosa via the lingual nerve. The postganglionic sympathetic supply to lingual glands and vessels arises from the carotid plexus and enters the tongue through plexuses around the lingual arteries. Isolated nerve cells, perhaps postganglionic parasympathetic neurones, have been reported in the postsulcal region: presumably they innervate glandular tissue and vascular smooth muscle.

Deciduous teeth

The incisors, canine and premolars of the permanent dentition replace two deciduous incisors, a deciduous canine and two deciduous molars in each jaw quadrant (Figs 30.16, 30.17). The deciduous incisors and canine are shaped like their successors but are smaller and whiter and become extremely worn in older children. The deciduous second molars resemble the first permanent molars rather than their successors, the premolars. The upper first deciduous molar has a triangular occlusal surface (its rounded ‘apex’ is palatal) and a fissure separates a double buccal cusp from the palatal cusp. The lower first deciduous molar is long and narrow; its two buccal cusps are separated from its two lingual cusps by a zigzagging mesiodistal fissure. Both deciduous molars have large buccal protuberances on their mesial aspect. Upper deciduous molars have three roots (fusion of the palatal and distobuccal root is commonplace), and lower deciduous molars have two roots. These roots diverge more than those of permanent teeth because each developing premolar tooth crown is accommodated directly under the crown of its deciduous predecessor. The roots of deciduous teeth are progressively resorbed by osteoclast-like cells (odontoclasts) prior to being shed.

Fig. 30.16 The deciduous upper and lower teeth of the right side: labial and buccal surfaces.

Fig. 30.17 A, The upper deciduous dentition. Note the channels (arrows) leading to the developing permanent teeth. B, The lower deciduous dentition.

(By permission from Berkovitz BKB, Moxham BJ 1994 Color Atlas of the Skull. London: Mosby.)

Eruption of teeth

Information on the sequence of development and eruption of teeth into the oral cavity (Fig. 30.18) is important in clinical practice and also in forensic medicine and archaeology. The tabulated data provided in Table 30.2 are largely based on European-derived populations and there is evidence of ethnic variation. When a permanent tooth erupts, about two-thirds of the root is formed and it takes about another 3 years for the root to be completed. For deciduous teeth, root completion is more rapid (Table 30.1). The developmental stages of initial calcification and crown completion are less affected by environmental influences than is eruption, the timing of which may be modified by several factors such as early tooth loss and severe malnutrition.

Fig. 30.18 Development of the deciduous (blue) and permanent (yellow) teeth.

(Modified with permission from Schour I, Massler M 1941 The development of the human dentition. J Am Dent Assoc 28: 1153–1160.)

Table 30.1 Chronology of the human dentition.

Fig. 30.19 shows the panoramic appearance of the dentition seen with orthopantomograms at the time of birth, 2½, 6½, 10 and 16 years of age.

Fig. 30.19 Orthopantomograms of the dentition at birth (A), 2½ years (B), 6½ years (C), 10 years (D) and 16 years (E).

(B–E, By permission from Berkovitz BKB, Holland GR, Moxham BJ 2002 Oral Anatomy, Embryology and Histology, 3rd edn. Edinburgh: Mosby; D and E, also by courtesy of Dr Eric Whaites.)

Dental alignment and occlusion

It is possible to bring the jaws together so that the teeth meet or occlude in many positions. When opposing occlusal surfaces meet with maximal ‘intercuspation’ (i.e. maximum contact), the teeth are said to be in centric occlusion. In this position the lower teeth are normally opposed symmetrically and lingually with respect to the upper. Some important features of centric occlusion in a normal (idealized) dentition may be noted. Each lower postcanine tooth is slightly in front of its upper equivalent and the lower canine occludes in front of the upper. Buccal cusps of the lower postcanine teeth lie between the buccal and palatal cusps of the upper teeth. Thus, the lower postcanine teeth are slightly lingual and mesial to their upper equivalents. Lower incisors bite against the palatal surfaces of upper incisors, the latter normally obscuring about one-third of the crowns of the lower. This vertical overlap of incisors in centric occlusion is the overbite. The extent to which upper incisors are anterior to lowers is termed the overjet. In the most habitual jaw position, the resting posture, the teeth are slightly apart, the gap between them being the freeway space or interocclusal clearance. During mastication, especially with lateral jaw movements, the food is comminuted, which facilitates the early stages of digestion.

The ideal occlusion is a rather subjective concept. If there is an ideal occlusion, it can only presently be defined in broad functional terms. Therefore, the occlusion can be considered ‘ideal’ when the teeth are aligned such that the masticatory loads are within physiological range and act through the long axes of as many teeth in the arch as possible; mastication involves alternating bilateral jaw movements (and not habitual, unilateral biting preferences as a result of adaptation to occlusal interference); lateral jaw movements occur without undue mechanical interference; in the rest position of the jaw, the gap between teeth (the freeway space) is correct for the individual concerned; the tooth alignment is aesthetically pleasing to its possessor.

Variations from the ideal occlusion may be termed malocclusions (although these could be regarded as normal, since they are more commonly found in the population: 75% of the population in the USA has some degree of occlusal ‘disharmony’). However, the majority of malocclusions should be regarded as anatomical variations rather than abnormalities, as they are rarely involved in masticatory dysfunction or pain, although they may be aesthetically displeasing.

Variations in tooth number, size and form

The incidence of variation in number and form, which is often related to race, is rare in deciduous teeth but not uncommon in the permanent dentition. One or more teeth may fail to develop, a condition known as hypodontia. Conversely, additional or supernumerary teeth may form, producing hyperdontia. The third permanent molar is the most frequently missing tooth: in one study, one or more third molars failed to form in 32% of Chinese, 24% of English Caucasians and 2.5% of West Africans. In declining order of incidence, other missing teeth are maxillary lateral incisors, maxillary or mandibular second premolars, mandibular central incisors and maxillary first premolars.

Hyperdontia affects the maxillary arch much more commonly than the mandibular dentition. The extra teeth are usually situated on the palatal aspect of the permanent incisors or distal to the molars. More rarely, additional premolars develop. Supernumerary teeth in the incisor region can often impede the eruption of the permanent teeth and indeed this is often the first indication of their presence. A supernumerary tooth situated between the central incisors is known as a mesiodens. Teeth may be unusually large (macrodontia) or small (microdontia). For example, the crowns of maxillary central incisors may be abnormally wide mesiodistally; in contrast, a common variant of the maxillary lateral incisor has a small, peg-shaped crown. Epidemiological studies reveal that hyperdontia tends to be associated with macrodontia and hypodontia with microdontia, the most severely affected individuals representing the extremes of a continuum of variation. Together with family studies, this indicates that the causation is multifactorial, combining polygenic and environmental influences.

Some variations in the form of teeth, being characteristic of race, are of anthropological and forensic interest. Mongoloid dentitions tend to have shovel-shaped maxillary incisors with enlarged palatal marginal ridges. The additional cusp of Carabelli is commonly found on the mesiopalatal aspect of maxillary first permanent or second deciduous molars in Caucasian but rarely in Mongoloid dentitions. In African races, the mandibular second permanent molar often has five rather than four cusps.

General arrangement of dental tissues

A tooth consists of a crown covered by very hard translucent enamel and a root covered by yellowish bone-like cementum (Fig. 30.20A). These meet at the neck or cervical margin. A longitudinal ground section (Fig. 30.20B) reveals that the body of a tooth is mostly dentine (ivory) with an enamel covering up to about 2 mm thick, while the cementum is much thinner. The dentine surrounds a central pulp cavity, expanded at its coronal end into a pulp chamber and narrowed in the root as a pulp canal, opening at or near its tip by an apical foramen, occasionally multiple. The pulp is a connective tissue, continuous with the periodontal ligament via the apical foramen. It contains vessels for the support of the dentine and sensory nerves.

Fig. 30.20 The principal parts of a tooth. A, An extracted upper right canine tooth viewed from its mesial aspect. The root is covered by cement (partially removed), and the curved cervical margin is convex towards the cusp of the tooth. B, A ground section of a young (permanent) lower first premolar tooth sectioned in the buccolingual longitudinal plane, photographed with transmitted light. The enamel striae are incremental lines of enamel growth (compare with Fig. 30.24). Within the dentine the lines of the dentinal tubules are visible, forming S-shaped curves in the apical region but straighter in the root.

The root is surrounded by alveolar bone, its cementum separated from the osseous socket (alveolus) by the connective tissue of the periodontal ligament, approximately 0.2 mm thick (Fig. 30.21). Coarse bundles of collagen fibres, embedded at one end in cementum, cross the periodontal ligament to enter the osseous alveolar wall, these bundles of collagen being termed Sharpey’s fibres. Near the cervical margin, the tooth, periodontal ligament and adjacent bone are covered by the gingiva. On its internal surface the gingiva is attached to the tooth surface by the junctional epithelium, a zone of profound clinical importance because just above it is a slight recess, the gingival sulcus. As the sulcus is not necessarily self-cleansing, dental plaque may accumulate in it and this predisposes to periodontal disease.

Fig. 30.21 Demineralized section of a tooth with its root attached to the surrounding bone by the periodontal ligament. A, alveolar bone; C, root of tooth lined by cementum; arrow indicates periodontal space.

(By courtesy of Dr D Lunt.)

Enamel

Enamel is an extremely hard and rigid material which covers the crowns of teeth. It is a heavily mineralized epithelial cell secretion, containing 95–96% by weight crystalline apatites (88% by volume) and less than 1% organic matrix. The organic matrix comprises mainly unique enamel proteins, amelogenins and non-amelogenins such as enamelins, tuftelins. Although comprising a very small percentage of the weight and volume of enamel, the organic matrix permeates the whole of enamel. As its formative cells are lost from the surface during tooth eruption, enamel is incapable of further growth. Repair is limited to the remineralization of minute carious lesions.

Enamel reaches a maximum thickness of 2.5 mm over cusps and thins at the cervical margins. It is composed of closely packed enamel prisms or rods. In longitudinal section, enamel prisms extend from close to the enamel–dentine junction to within 20 μm of the surface, where they are generally replaced by prismless (non-prismatic, aprismatic) enamel (Fig. 30.22A). In cross-section the prisms are mainly horseshoe-shaped and are arranged in rows that are staggered such that the tails of the prisms in one row lie between the heads of the prism in the row above (prism pattern 3) (Fig. 30.22B) and the tails are directed rootwards. The appearance of prism boundaries results from sudden changes in crystallite orientation. Prisms have a diameter of approximately 5 μm, and are packed with flattened hexagonal hydroxyapatite crystals, far larger than those found in collagen-based mineralized tissues.

Fig. 30.22 Enamel. A, Scanning electron micrograph of acid-etched outer enamel (O) showing enamel prisms, approximately 5 μm wide. A layer of prismless enamel (E) is evident on the surface. B, Ground cross-section showing cross-sectional keyhole (fish scale) appearance of enamel prisms (pattern 3). C, Ground longitudinal section viewed with phase contrast showing prisms (vertical lines) and cross-striations (horizontal lines). D, Low-power scanning electron micrograph illustrating the relationship of enamel prism direction (long arrow), striae of Retzius (SR) and surface perikymata (SP). E, Ground longitudinal section showing enamel striae (arrows). Viewed between crossed polarizing filters.

(A, by courtesy of Professor D Whittaker; B, by permission from Berkovitz BKB, Holland GR, Moxham BJ 2002 Oral Anatomy, Embryology and Histology, 3rd edn. Edinburgh: Mosby; C, by permission from Berkovitz BKB, Holland GR, Moxham BJ 2002 Oral Anatomy, Embryology and Histology, 3rd edn. Edinburgh: Mosby; D, by permission from: Kelley J, Smith TM. Age at first molar emergence in early Miocene Afropithecus turkanensis and life-history evolution in the Hominoidea. Journal of Human Evolution 2003; 44:307–329; E, by courtesy of Dr AD Beynon)

Two types of incremental lines are visible in enamel, short-term and long-term. At intervals that average 4 μm – with a range of 2–3 μm at the enamel dentine junction and 4–6 μm in outer enamel – along its length, each prism is crossed by a line, probably reflecting diurnal swelling and shrinking in diameter during its growth. This short-term daily growth line is known as a cross-striation (Fig. 30.22C). The longer-term incremental lines pass from the enamel–dentine junction obliquely to the surface, where they end in shallow furrows, perikymata, visible on newly erupted teeth (Fig. 30.22D). Each line, known as an enamel stria of Retzius, represents a period that ranges between 6 and 12 days of enamel growth with a modal value of 9 days. This is known as an individual’s periodicity and it is constant in all teeth from the same individual but varies considerably between individuals (Reid & Dean 2006) (Fig. 30.22E). A prominent striation, the neonatal line, is formed in teeth whose mineralization spans birth. Neonatal lines are present in the enamel and dentine of teeth mineralizing at the time of birth (all the deciduous teeth and the first permanent molars; see Fig. 30.14) and are therefore of forensic importance, indicating that an infant has survived for a few days after birth. They reflect a disturbance in mineralization during the first few days after birth. Other accentuated lines can be observed throughout enamel growth; these are generally reflections in disturbances in ameloblast secretion caused by illness or nutritional deficiencies. In serious cases these disturbances will manifest as hypoplasia (enamel thinning on the tooth surface).

Dentine

Dentine is a yellowish avascular tissue which forms the bulk of a tooth. It is a tough and compliant composite material, with a mineral content of 70% dry weight (largely crystalline hydroxyapatite with some calcium carbonate) and 20% organic matrix (type I collagen, glycosaminoglycans and phosphoproteins). Its conspicuous feature is the regular pattern of microscopic dentinal tubules, 3 μm in diameter, which extend from the pulpal surface to the enamel–dentine junction. The tubules show lateral and terminal branching near the enamel–dentine junction (Fig. 30.23A) and may project a short distance into the enamel (enamel spindles). Each tubule encloses a single cytoplasmic process of an odontoblast whose cell body lies in a pseudostratified layer which lines the pulpal surface. Processes are believed to extend the full thickness of dentine in newly erupted teeth, but in older teeth they may be partly withdrawn and occupy only the pulpal third, while the outer regions contain probably only extracellular fluid. The diameter of the dentine tubule is narrowed by deposition of peritubular dentine. This is different from normal dentine (intertubular dentine) because it is more highly mineralized and lacks a collagenous matrix. Peritubular dentine can therefore be identified by microradiography (Fig. 30.23B). In time, it may completely fill the tubule, a process which gives rise to translucent dentine and which commences in the apical region of the root.

Fig. 30.23 Dentine. A, Ground longitudinal section showing branching of dentine tubules near the enamel–dentine junction (arrow). B, Microradiograph of transversely sectioned dentinal tubules surrounded by a more radiopaque and therefore more mineralized zone of peritubular dentine. C, Ground longitudinal section viewed in polarized light showing alternate light and dark bands representing long-period incremental lines (Andresen lines). The bands are approximately orientated at right angles to the direction of the dentinal tubules (arrows).

(By permission from Berkovitz BKB, Holland GR, Moxham BJ 2002 Oral Anatomy, Embryology and Histology, 3rd edn. Edinburgh: Mosby.)

The outermost zone (10–20 μm) of dentine differs in the crown and the root. In the crown it is referred to as mantle dentine and differs in the orientation of its collagen fibres. In the root, the peripheral zone presents a granular layer – with less overall mineral – beyond which is a hyaline layer which lacks a tubular structure and may function to produce a good bond between the cementum and dentine.

Dentine is formed slowly throughout life, and so there is always an unmineralized zone of predentine at the surface of the mineralized dentine, adjacent to the odontoblast layer at the periphery of the pulp. Biochemical changes within the mineralizing matrix mean that preentine stains differently to the matrix of the mineralized dentine. The predentine–dentine border is generally scalloped, because dentine mineralizes both linearly and as microscopic spherical aggregates of crystals (calcospherites). Dentine, like enamel, is deposited incrementally, and exhibits both short- and long-term incremental lines. Long-term lines are known as Andresen lines and in the mid-axial region are approximately 20 μm apart: they represent increments of about 6–12 days (Fig. 30.23C). Daily incremental lines (von Ebner lines) in the mid-axial plane are typically 4 μm apart; they are considerably closer together (1–2 μm apart) at the enamel–dentine junction and in root dentine. Where mineralization spans birth (i.e. all deciduous teeth and usually the first permanent molars) a neonatal line is formed in dentine similar to that which is seen in enamel, and it signals the abrupt change in both environment and nutrition which occurs at birth. As with enamel, disturbances in growth of dentine will exhibit as accentuated lines, commonly called contour lines of Owen.

Primary dentine formation proceeds at a steady but declining rate as first the crown and then the root is completed. A slow and intermittent deposition of dentine (regular secondary dentine) continues throughout life and further reduces the size of the pulp chamber; it is distinguished from primary dentine by the reduction and haphazard arrangement of dentinal tubules within it. The presence of the odontoblast process means that dentine is a vital tissue. It responds to adverse external stimuli – such as rapidly advancing caries, excessive wear or tooth breakage – by forming poorly mineralized dead tracts, in which the odontoblasts of the affected region die and the tubules remain empty (tertiary dentine). A dead tract may be sealed from the pulp by a thin zone of sclerosed dentine and the deposition of irregular (tertiary) dentine by newly differentiated pulp cells (Fig. 30.24).

Fig. 30.24 Longitudinal ground section of an incisor tooth.

Dental pulp

Dental pulp provides the nutritive support for the synthetic activity of the odontoblast layer. It is a well-vascularized, loose connective tissue, enclosed by dentine and continuous with the periodontal ligament via apical and accessory foramina. Several thin-walled arterioles enter by the apical foramen and run longitudinally within the pulp to an extensive subodontoblastic plexus. Blood flow rate per unit volume of tissue is greater in the pulp than in other oral tissues, and tissue fluid pressures within the pulp appear to be unusually high.

As well as typical connective tissue cells, pulp uniquely contains the cell bodies of odontoblasts whose long processes occupy the dentinal tubules. Pulp also has dendritic antigen-presenting cells. Approximately 60% of pulpal collagen is type I, and the bulk of the remainder is type III. As dentine deposition increases with age, the pulp recedes until the whole of the crown may be removed without accessing the pulp.

Dental pulp is extensively innervated by unmyelinated postganglionic vasoconstrictor sympathetic nerve fibres from the superior cervical ganglion, which enter with the arterioles, and by myelinated (Aδ) and unmyelinated (C) sensory nerve fibres from the trigeminal ganglion, which traverse the pulp longitudinally and ramify as a plexus (Raschkow’s plexus) beneath the odontoblast layer (Figs 30.25, 30.26). Here, any myelinated nerve fibres lose their myelin sheaths and continue into the odontoblast layer, and some enter the dentinal tubules, especially the region beneath the cusps. Stimulation of dentine, whether by thermal, mechanical or osmotic means, evokes a pain response. Pulp nerves release numerous neuropeptides.

Fig. 30.25 Longitudinal section of a tooth and its surrounding tissues.

Fig. 30.26 Longitudinal demineralized section of a tooth stained with a silver impregnation technique. Note the small bundle of nerves within the pulp (top); one of the fine axons from the bundle (A) passes between the odontoblasts (B) lining the surface of the predentine (C).

Forensic anatomy of teeth

In forensic medicine, dental evidence is valuable in identification of individuals, especially following mass disasters; estimation of age at death of skeletonized remains; and establishing guilt in cases of criminal injury by biting.

If teeth have been restored, extracted or replaced by a denture, an individual will have a virtually unique dentition which may have been recorded by the dentist in the form of charts, radiographs or plaster casts. Teeth are the most indestructible bodily structures and can provide identification when trauma or fire has rendered the face unrecognizable. Moreover, the chronology of crown development, eruption and root formation can be used to estimate age until the third molar is completed at about 21 years. The method is even applicable to the fetus because the weight of mineralized tissue in teeth is closely related to age from about 22 weeks’ gestation until birth. The time taken for a crown to form can be calculated from ground sections with considerable accuracy by counting the number of daily cross-striations from the neonatal line. For permanent teeth, the time taken for the crown to form can be calculated by counting the number of the enamel striae and multiplying by the individual’s periodicity. The age of adult teeth can be estimated from factors such as wear of the crown, reduction in size of the pulp and increase in thickness of cement in the apical half of the root. However, probably the most useful single characteristic is the amount of translucent dentine in the root, which is proportional to age. Such estimations are within 5–7 years of the chronological age and likely to be closer to the true age than those derived from skeletal changes.

Periodontal ligament

The principal functions of the periodontal ligament are to support the teeth, generate the force of tooth eruption and provide sensory information about tooth position and forces to facilitate reflex jaw activity. The periodontal ligament is a dense fibrous connective tissue 0.2 mm wide which contains cells associated with the development and maintenance of alveolar bone (osteoblasts and osteoclasts) and of cementum (cementoblasts and odontoclasts). It also contains a network of epithelial cells (epithelial cell rests of Malassez) which are embryological remnants of an epithelial root sheath. They have no evident function but may give rise to dental cysts.

The majority of collagen fibres of the periodontal ligament are arranged as variously oriented dense fibre bundles that connect alveolar bone and cementum and which may help to resist movement in specific directions (Fig. 30.27). About 80% of the collagen in the periodontal ligament is type I, most of the remainder is type III. The rate of turnover of collagen is probably the highest of any site in the body, for reasons which are as yet unclear. A very small volume of fibres are oxytalan fibres.

Fig. 30.27 Decalcified longitudinal section of a tooth showing groups of periodontal ligament fibres (the alveolar crest fibres and the horizontal fibres) in the region of the alveolar crest. van Gieson stain.

(By permission from Berkovitz BKB, Holland GR, Moxham BJ 2002 Oral Anatomy, Embryology and Histology, 3rd edn. Edinburgh: Mosby.)

The periodontal ligament has a rich nerve and blood supply. The nerves are both autonomic (for the vasculature) and sensory (for pain and proprioception). The majority of proprioceptive nerve endings appear to be Ruffini-like endings. The blood vessels tend to lie towards the bone side of the periodontal ligament and the capillaries are fenestrated. Tissue fluid pressures appear to be high.

Alveolar bone

That part of the maxilla or mandible which supports and protects the teeth is known as alveolar bone. An arbitrary boundary at the level of the root apices of the teeth separates the alveolar processes from the body of the mandible or the maxilla (Fig. 30.28). Like bone in other sites, alveolar bone functions as a mineralized supporting tissue, gives attachment to muscles, provides a framework for bone marrow and acts as a reservoir for ions, especially calcium. It is dependent on the presence of teeth for its development and maintenance, and requires functional stimuli to maintain bone mass. Where teeth are congenitally absent, as for example in anodontia, it is poorly developed, and it atrophies after tooth extraction, which can cause difficulties in the prosthodontic rehabilitation of patients.

Fig. 30.28 Anterior part of the left side of the mandible, with superficial bone removed on the buccal side to show roots of a number of teeth, some of which have also been sectioned vertically. Note the cortical plate of compact bone lining the sockets of the teeth (the lamina dura of radiographs: see Fig. 30.30), and the flat table of bone surmounting the interdental bone septa. In this specimen the mandibular canal is widely separated from the roots of the teeth, a variable condition.

The alveolar tooth-bearing portion of the jaws consists of outer and inner alveolar plates. The individual sockets are separated by plates of bone termed the interdental septa, while the roots of multi-rooted teeth are divided by interradicular septa. The compact layer of bone which lines the tooth socket has been called either the cribriform plate, on account of its content of vascular (Volkmann’s) canals which pass from the alveolar bone into the periodontal ligament, or bundle bone, because numerous bundles of Sharpey’s fibres pass into it from the periodontal ligament (Fig. 30.29).

Fig. 30.29 Decalcified section of a root of a tooth showing Sharpey’s fibres from the periodontal ligament entering alveolar bone (A). The Sharpey’s fibres in bone are seen to be thicker, but less numerous, than those entering the cementum (B) on the tooth surface. van Gieson stain.

(By permission from Berkovitz BKB, Holland GR, Moxham BJ 2002 Oral Anatomy, Embryology and Histology, 3rd edn. Edinburgh: Mosby.)

In clinical radiographs, the bone lining the alveolus commonly appears as a continuous dense white line about 0.5–1 mm thick, the lamina dura (Fig. 30.30). However, this appearance gives a misleading impression of the density of alveolar bone: the X-ray beam passes tangentially through the socket wall, and so the radiopacity of the lamina dura is an indication of the quantity of bone the beam has passed through, rather than the degree of mineralization of the bone. Superimposition also obscures the Volkmann’s canals. Chronic infections of the dental pulp spread into the periodontal ligament, which leads to resorption of the lamina dura around the root apex. The presence of a continuous lamina dura around the apex of a tooth therefore usually indicates a healthy apical region (except in acute infections where resorption of bone has not yet begun).

Fig. 30.30 Bite-wing radiograph of teeth and surrounding bone. Note the different radiopacities of enamel and dentine. In a healthy tooth, such as the first molar illustrated here, the lamina dura is complete and appears as a radiopaque line. In the case of the adjacent second molar tooth in which the bulk of the crown has been lost due to dental caries, an abscess has formed at the base of the tooth and as a result the lamina dura has lost its continuity.

(By courtesy of Ms Nadine White.)

On the labial and buccal aspects of upper teeth, the two cortical plates usually fuse, and there is very little trabecular bone between them, except where the buccal bone thickens over the molar teeth near the root of the zygomatic arch. It is easier and more convenient to extract upper and lower teeth by widening the tooth socket in a buccal direction due to its thinness. Anteriorly in the lower jaw, labial and lingual plates are thin, but in the molar region the buccal plate is thickened as the external oblique line. Near the lower third molar, the lingual bone is much thinner than the buccal and it is important to realize that the lingual nerve is easily damaged by poor technique.

Superior alveolar arteries

The upper jaw is supplied by posterior, middle and anterior superior alveolar (dental) arteries. The posterior superior alveolar artery usually arises from the third part of the maxillary artery in the pterygopalatine fossa. It descends on the infratemporal surface of the maxilla, and divides to give branches that enter the alveolar canals to supply molar and premolar teeth, adjacent bone and the maxillary sinus, and other branches that continue over the alveolar process to supply the gingivae. The middle and anterior superior alveolar arteries are branches from the infraorbital artery.

The infraorbital artery often arises with the posterior superior alveolar artery. It enters the orbit posteriorly through the inferior orbital fissure and runs in the infraorbital groove and canal with the infraorbital nerve. When the small middle superior alveolar artery is present, it runs down the lateral wall of the maxillary sinus and forms anastomotic arcades with the anterior and posterior vessels, terminating near the canine tooth. The anterior superior alveolar artery curves through the canalis sinuosus to supply the upper incisor and canine teeth and the mucous membrane in the maxillary sinus. The canalis sinuosus swerves laterally from the infraorbital canal and inferomedially below it in the wall of the maxillary sinus, following the rim of the anterior nasal aperture, between the alveoli of canine and incisor teeth and the nasal cavity. It ends near the nasal septum where its terminal branch emerges. The canal may be up to 55 mm long.

Inferior alveolar artery

The inferior alveolar (dental) artery, a branch of the maxillary artery, descends in the infratemporal fossa posterior to the inferior alveolar nerve. Here, it lies between bone laterally and the sphenomandibular ligament medially. Before entering the mandibular foramen it gives off a mylohyoid branch, which pierces the sphenomandibular ligament to descend with the mylohyoid nerve in its groove on the inner surface of the ramus of the mandible. The mylohyoid artery ramifies superficially on the muscle and anastomoses with the submental branch of the facial artery. The inferior alveolar artery then traverses the mandibular canal with the inferior alveolar nerve to supply the mandibular molars and premolars and divides into the incisive and mental branches near the first premolar.

The incisive branch continues below the incisor teeth (which it supplies) to the midline, where it anastomoses with its fellow, although few anastomotic vessels cross the midline. In the canal the arteries supply the mandible, tooth sockets and teeth via branches which enter the minute hole at the apex of each root to supply the pulp. The mental artery leaves the mental foramen and supplies the chin and anastomoses with the submental and inferior labial arteries. Near its origin the inferior alveolar artery has a lingual branch, which descends with the lingual nerve to supply the lingual mucous membrane. The pattern of branching of the inferior alveolar artery reflects that of the nerves of the same name.

Arterial supply of periodontal ligaments

The periodontal ligaments supporting the teeth are supplied by dental branches of alveolar arteries. One branch enters the alveolus apically and sends two or three small rami into the dental pulp through the apical foramen, and other rami into the periodontal ligament. Interdental arteries ascend in the interdental septa, sending branches at right angles into the periodontal ligament, and terminate by communicating with gingival vessels that also supply the cervical part of the ligament. The periodontal ligament therefore receives its blood from three sources: from the apical region; ascending interdental arteries; and descending vessels from the gingivae. These vessels anastomose with each other, which means that when the pulp of a tooth is removed during endodontic treatment, the attachment tissues of the tooth remain vital.

Venous drainage of the teeth

Veins accompanying the superior alveolar arteries drain the upper jaw and teeth anteriorly into the facial vein, or posteriorly into the pterygoid venous plexus. Veins from the lower jaw and teeth collect either into larger vessels in the interdental septa or into plexuses around the root apices and thence into several inferior alveolar veins. Some of these veins drain through the mental foramen to the facial vein; others drain via the mandibular foramen to the pterygoid venous plexus.

Lymphatic drainage of the teeth

The lymph vessels from the teeth usually run directly into the ipsilateral submandibular lymph nodes. Lymph from the mandibular incisors, however, drains into the submental lymph nodes. Occasionally, lymph from the molars may pass directly into the jugulodigastric group of nodes.

Superior alveolar nerves

The teeth in the upper jaw are supplied by the three superior alveolar (dental) nerves which arise from the maxillary nerve in the pterygopalatine fossa or in the infraorbital groove and canal. The posterior superior alveolar (dental) nerve leaves the maxillary nerve in the pterygopalatine fossa and runs anteroinferiorly to pierce the infratemporal surface of the maxilla, at which point it is possible to perform a local anaesthetic block which will anaesthetize the pulps of the premolar and molar ipsilateral teeth, and then descends under the mucosa of the maxillary sinus. After supplying the lining of the sinus the nerve divides into small branches which link up as the molar part of the superior alveolar plexus, supplying twigs to the molar teeth. It also supplies a branch to the upper gingivae and the adjoining part of the cheek.

The middle superior alveolar (dental) nerve arises from the infraorbital nerve as it runs in the infraorbital groove, and runs downwards and forwards in the lateral wall of the maxillary sinus. It ends in small branches which link up with the superior dental plexus, supplying small rami to the upper premolar teeth. This nerve is variable, and it may be duplicated or triplicated or absent.

The anterior superior alveolar (dental) nerve leaves the lateral side of the infraorbital nerve near the midpoint of its canal and traverses the canalis sinuosus in the anterior wall of the maxillary sinus. It curves first under the infraorbital foramen, then passes medially towards the nose and finally turns downwards and divides into branches to supply the incisor and canine teeth. It assists in the formation of the superior dental plexus and it gives off a nasal branch, which passes through a minute canal in the lateral wall of the inferior meatus to supply the mucous membrane of the anterior area of the lateral wall as high as the opening of the maxillary sinus, and the floor of the nasal cavity. It communicates with the nasal branches of the pterygopalatine ganglion and finally emerges near the root of the anterior nasal spine to supply the adjoining part of the nasal septum.

Inferior alveolar (dental) nerve

The course of the inferior alveolar nerve in the infratemporal fossa is described in Chapter 31. Just before entering the mandibular canal the inferior alveolar nerve gives off a small mylohyoid branch which pierces the sphenomandibular ligament and enters a shallow groove on the medial surface of the mandible, following a course roughly parallel to its parent nerve. It passes below the origin of mylohyoid to lie on the superficial surface of the muscle, between it and the anterior belly of digastric, both of which it supplies. It gives a few filaments to supply the skin over the point of the chin.

In the mandibular canal, the inferior alveolar nerve runs downward and forward, generally below the apices of the teeth until below the first and second premolars where it divides into terminal incisive and mental branches. The incisive branch continues forward in a bony canal or in a plexiform arrangement, giving off branches to the first premolar, canine and incisor teeth, and the associated labial gingivae. The lower central incisor teeth receive a bilateral innervation, fibres probably crossing the midline within the periosteum to re-enter the bone via numerous canals in the labial cortical plate.

The mental nerve passes upward, backward and outward to emerge from the mandible via the mental foramen between and just below the apices of the premolar teeth. It immediately divides into three branches, two of which pass upward and forward to form an incisor plexus labial to the teeth, supplying the gingiva (and probably the periosteum). From this plexus and the dental branches, fibres turn downwards and then lingually to emerge on the lingual surface of the mandible on the posterior aspect of the symphysis or opposite the premolar teeth, probably communicating with the lingual or mylohyoid nerve. The third branch of the mental nerve passes through the intermingled fibres of depressor anguli oris and platysma to supply the skin of the lower lip and chin. Branches of the mental nerve also communicate with terminal filaments of the mandibular branch of the facial nerve.

Variations in the fascicular organization of the inferior alveolar nerve are clinically important when extracting impacted third molars. The nerve may appear as a single bundle lying a few millimetres below the roots of the teeth, or it may lie much lower and almost reach the lower border of the bone, so that it gives off a variable number of large rami which pass anterosuperiorly towards the roots before dividing to supply the teeth and interdental septa. Only rarely is it plexiform. The nerve may lie on the lingual or buccal side of the mandible (slightly more commonly on the buccal side). Even when the third molar tooth is in a normal position, the nerve may be so intimately related to it that it grooves its root. Exceptionally the nerve may be similarly related to the second molar.

Nerves may pass from the substance of temporalis to enter the mandible through the retromolar fossa or the condylar head, where they communicate with branches of the inferior alveolar nerve. Foramina occur in some 10% of retromolar fossae and infiltration in this region can abolish the sensation which occasionally remains after an inferior alveolar nerve block. Similarly, branches from the buccal, mylohyoid and lingual nerves may enter the mandible and provide additional routes of sensory transmission from the teeth. Thus, even when the inferior alveolar nerve has been anaesthetized correctly, pain may still be experienced by a patient when undergoing dental cavity preparation.

Pain sensation in teeth

The teeth are supplied by nociceptors that generate pain sensation of a very high order. The mechanism underlying this sensitivity is of considerable clinical significance and is controversial. Currently, the most widely accepted view is that fluid movements through the dentine tubules stimulate nerve endings at the periphery of the dental pulp (hydrodynamic hypothesis).

Local analgesia

It is technically possible to achieve profound regional anaesthesia by depositing local anaesthetic solution adjacent to the trigeminal nerve trunks or their branches within the infratemporal fossa. These injections can either be performed transorally – posterior superior alveolar nerve block, maxillary nerve block, inferior alveolar nerve block, lingual nerve block and mandibular nerve block – or more rarely by an external route through the skin of the face – maxillary nerve block, inferior alveolar nerve block and mandibular nerve block.

In the case of the mandible, the anterior teeth can be anaesthetized by simple diffusion techniques as the bone is relatively thin. However, this is not adequate for the cheek teeth due to the increased thickness of the bone. In this case, the inferior alveolar nerve has to be anaesthetized before it enters the inferior alveolar canal. The needle has to be placed within the pterygomandibular space to achieve a successful inferior alveolar nerve block. The lingual nerve is also usually blocked, as it lies close to the inferior alveolar nerve. Because of the other structures within the infratemporal fossa, it is vitally important that the operator has a detailed knowledge of the anatomy in this region to understand, and therefore try to avoid, the complications that may arise. Any damage to blood vessels in the infratemporal fossa, generally the pterygoid venous plexus, can lead to haematoma formation. In extreme cases, bleeding can track through the inferior orbital fissure resulting in a retrobulbar haematoma, which can result in loss of visual acuity or blindness. Intravascular injection of local anaesthetic solution (which usually contains adrenaline [epinephrine]) can have profound systemic effects and for this reason an aspirating syringe is always used to check that the needle has not entered a vessel prior to injection (vessels in this area that theoretically may be entered include the maxillary and internal carotid arteries). If the needle is placed too medially, it may enter medial pterygoid, while if directed too laterally, it may penetrate temporalis. In either case, there will be lack of anaesthesia followed later by trismus. If the needle is placed too deeply, anaesthetic solution may cause a temporary facial nerve palsy due to loss of conduction from the facial nerve in the region of the parotid gland.

Superficial part of the submandibular gland

The superficial part of the gland is situated in the digastric triangle where it reaches forward to the anterior belly of digastric and back to the stylomandibular ligament, by which it is separated from the parotid gland. Above, it extends medial to the body of the mandible. Below, it usually overlaps the intermediate tendon of digastric and the insertion of stylohyoid. This part of the submandibular gland presents inferior, lateral and medial surfaces, and is partially enclosed between two layers of deep cervical fascia that extend from the greater cornu of the hyoid bone. The superficial layer is attached to the lower border of the mandible and covers the inferior surface of the gland. The deep layer is attached to the mylohyoid line on the medial surface of the mandible and covers the medial surface of the gland.

The inferior surface, covered by skin, platysma and deep fascia, is crossed by the facial vein and the cervical branch of the facial nerve. Near the mandible the submandibular lymph nodes are in contact with the gland and some may be embedded within it.

The lateral surface is related to the submandibular fossa on the medial surface of the body of the mandible and the mandibular attachment of medial pterygoid. The facial artery grooves its posterosuperior part, lies at first deep to the gland and then emerges between its lateral surface and the mandibular attachment of the medial pterygoid to reach the lower border of the mandible.

The medial surface is related anteriorly to mylohyoid, from which it is separated by the mylohyoid nerve and vessels and branches of the submental vessels. More posteriorly, it is related to styloglossus, the stylohyoid ligament and the glossopharyngeal nerve, which separate it from the pharynx. In its intermediate part the medial surface is related to hyoglossus, from which it is separated by styloglossus, the lingual nerve, submandibular ganglion, hypoglossal nerve and deep lingual vein (sequentially from above down). Below, the medial surface is related to the stylohyoid muscle and the posterior belly of digastric.

Deep part of the submandibular gland

The deep part of the gland extends forwards to the posterior end of the sublingual gland. It lies between mylohyoid inferolaterally, hyoglossus and styloglossus medially, the lingual nerve superiorly, and the hypoglossal nerve and deep lingual vein inferiorly (Fig. 30.10).

Vascular supply and lymphatic drainage

The arteries supplying the gland are branches of the facial and lingual arteries. The lymph vessels drain into the deep cervical group of lymph nodes (particularly the jugulo-omohyoid node), interrupted by the submandibular nodes.

Innervation

The secretomotor supply to the submandibular gland is derived from the submandibular ganglion. This is a small, fusiform body which lies on the upper part of hyoglossus. There are additional ganglion cells in the hilum of the gland. Like the ciliary, pterygopalatine and otic ganglia, the submandibular is a peripheral parasympathetic ganglion. It is superior to the deep part of the submandibular gland and inferior to the lingual nerve, and is suspended from the latter by anterior and posterior filaments. Though related to the lingual nerve, the ganglion is connected functionally with the facial nerve and its chorda tympani branch.

As with the other cranial parasympathetic ganglia, there are three roots associated with the submandibular ganglion. The motor, parasympathetic root is the posterior filament which connects it to the lingual nerve. This conveys preganglionic fibres from the superior salivatory nucleus which travel in the facial, chorda tympani and lingual nerves to the ganglion, where they synapse. The postganglionic fibres are secretomotor to the submandibular and sublingual salivary glands. Some fibres may also reach the parotid gland. The sympathetic root is derived from the plexus on the facial artery. It consists of postganglionic fibres from the superior cervical ganglion which traverse the submandibular ganglion without synapsing. They are vasomotor to the blood vessels of the submandibular and sublingual glands. Five or six branches from the ganglion supply the submandibular gland and its duct. Other fibres pass through the anterior filament that connects the submandibular gland to the lingual nerve and are carried to the sublingual and anterior lingual glands. Sensory fibres are derived from the lingual nerve.

Submandibular duct

The submandibular duct is usually 5 cm long and has a thinner wall than the parotid duct. It begins from numerous tributaries in the superficial part of the gland and emerges from the medial surface of this part of the gland behind the posterior border of mylohyoid. It traverses the deep part of the gland, and then passes at first up and slightly back for approximately 5 mm, this sharp bend over the free edge of mylohyoid being known as the genu of the duct. It then runs forwards between mylohyoid and hyoglossus passing between the sublingual gland and genioglossus to open in the floor of the mouth on the summit of the sublingual papilla at the side of the frenulum of the tongue (Figs 30.4, 30.10). It lies between the lingual and hypoglossal nerves on hyoglossus, but, at the anterior border of the muscle, it is crossed laterally by the lingual nerve, terminal branches of which ascend on its medial side. As the duct traverses the deep part of the gland it receives small tributaries draining this part of the gland. It has been suggested previously that the genu of the duct predisposes to the stasis of saliva and thereby encourages salivary stone (sialolith) formation, but this is somewhat controversial and largely unproven.

Like the parotid gland, the duct system of the submandibular gland can be visualized by sialography (Fig. 30.32).

Fig. 30.32 Sialogram showing a normal submandibular duct (long arrow). Unusually, a sublingual duct is also evident (short arrow).

(By courtesy of Dr N. Drage.)

Vascular supply, innervation and lymphatic drainage

The arterial supply is from the sublingual branch of the lingual artery and the submental branch of the facial artery. Innervation is via the submandibular ganglion. Lymphatic drainage is to the submental nodes.

Sublingual ducts

The sublingual gland has 8–20 excretory ducts (Fig. 30.10). Smaller sublingual ducts open, usually separately, from the posterior part of the gland onto the summit of the sublingual fold (a few sometimes open into the submandibular duct). Small rami from the anterior part of the gland sometimes form a major sublingual duct (Bartholin’s duct), which opens with, or near to, the orifice of the submandibular duct. This duct may be visualized occasionally in a submandibular sialogram (Fig. 30.32).

Ranula

If the ducts draining any salivary gland become obstructed, the gland itself is at risk of developing a retention cyst where the retained secretions dilate the gland itself rather like a balloon. This phenomenon is seen mostly in the minor salivary glands which line the lips and oral cavity, where it is known as a mucocele. Trauma such as persistent lip biting results in scarring of the overlying oral mucosa and obstruction of the small drainage duct. When trauma occurs in the floor of the mouth and obstructs the drainage duct/s of the sublingual gland, the resulting retention cyst is known as a ranula. (Ranula is the Latin name for a frog and is used here because the tense cystic swelling is said to resemble the throat of a croaking frog.)

A ranula usually presents as a large tense bluish swelling anteriorly in the floor of the mouth just to one side of the midline, which often displaces the tongue. Occasionally the developing retention cyst herniates through a midline dehiscence where the two mylohyoid muscles fail to meet in the midline anteriorly, and then the ranula may present as a submental swelling or as a combined submental and floor of mouth swelling, a ‘plunging’ ranula. The treatment of a ranula is excision of the sublingual gland responsible.

Ducts

Intercalated, striated (both intralobular) and extralobular collecting ducts lead consecutively from the secretory endpieces. The lining cells of intercalated ducts are flat nearest the secretory endpiece, but become cuboidal. Intercalated ducts function primarily as a conduit for saliva but, together with the striated ducts, may also modify its content of electrolytes and secrete IgA. Striated ducts are lined by a low columnar epithelium and are so called because their lining cells have characteristic basal striations. The latter are regions of highly infolded basal plasma membrane, between which lie columns of vertically aligned mitochondria. The nuclei are consequently displaced by the basal striations from a typical ductal basal position to a central or even apical location (Fig. 30.34). Infolding of the basal plasma membrane, and local abundance of mitochondria, are typical features of epithelial cells which actively transport electrolytes. Here, the cells transport potassium and bicarbonate into saliva: they produce a hypotonic saliva by reabsorbing sodium and chloride ions in excess of water. Striated ducts modify electrolyte composition and secrete IgA, lysozyme and kallikrein. IgA is produced by subepithelial plasma cells and transported transcytotically across the epithelial barrier to be secreted, once it has been dimerized by epithelial secretory component, into the saliva. This is also a function of serous acinar cells and other secretory epithelia, notably the lactating breast (see Ch. 54). The intralobular ductal system of the sublingual gland is less well developed than that of the parotid and submandibular glands.

Collecting ducts are metabolically relatively inert conduits which run within interlobular connective tissue septa in the glands. They transport saliva to the main duct which opens onto the mucosal surface of the buccal cavity. The lining epithelium of collecting ducts varies. It may be pseudostratified columnar, stratified cuboidal or columnar in the larger ducts, and has a distinct basal layer. It becomes a stratified squamous epithelium near the buccal orifice.

Myoepithelial cells

Myoepithelial cells (Fig. 30.33) are contractile cells (see Ch. 2) associated with secretory endpieces and with much of the ductal system. They lie between the basal lamina and the epithelial cells proper. They extend numerous cytoplasmic processes around serous acini and are often termed basket cells. Myoepithelial cells associated with ducts are more fusiform in shape, and are aligned along the length of the duct. Their cytoplasm contains abundant actin microfilaments which mediate contraction under the control of both sympathetic and parasympathetic stimulation. The outflow of saliva is thus accelerated through reduction in the luminal volume of secretory endpieces and ducts, contributing to the secretory pressure.