INTRODUCTION AND TERMINOLOGY
There are three basic tooth forms in both dentitions: incisiform, caniniform and molariform. Incisiform teeth (incisors) are cutting teeth, and have thin, blade-like crowns. Caniniform teeth (canines) are piercing or tearing teeth, and have a single, stout, pointed, cone-shaped crown. Molariform teeth (molars and premolars) are grinding teeth and possess a number of cusps on an otherwise flattened biting surface. Premolars are bicuspid teeth that are restricted to the permanent dentition and replace the deciduous molars.
The tooth-bearing region of the jaws can be divided into four quadrants, the right and left maxillary and mandibular quadrants. A tooth may thus be identified according to the quadrant in which it is located (e.g. a right maxillary tooth or a left mandibular tooth). In both the deciduous and permanent dentitions, the incisors may be distinguished according to their relationship to the midline. Thus, the incisor nearest the midline is the central (first) incisor and the incisor that is more laterally positioned is termed the lateral (second) incisor. The permanent premolars and the permanent and deciduous molars can also be distinguished according to their mesiodistal relationships. The molar most mesially positioned is designated the first molar, and the one behind it is the second molar. In the permanent dentition, the tooth most distally positioned is the third molar. The mesial premolar is the first premolar, and the premolar behind it is the second premolar.
The terminology used to indicate tooth surfaces is shown in Fig. 33.13. The aspect of teeth adjacent to the lips or cheeks is termed labial or buccal, that adjacent to the tongue being lingual (or palatal in the maxilla). Labial and lingual surfaces of an incisor meet medially at a mesial surface and laterally at a distal surface, terms which are also used to describe the equivalent surfaces of premolar and molar (postcanine) teeth. On account of the curvature of the dental arch, mesial surfaces of postcanine teeth are directed anteriorly and distal surfaces are directed posteriorly. Thus, the point of contact between the central incisors is the datum point for mesial and distal. The biting or occlusal surfaces of postcanine teeth are tuberculated by cusps which are separated by fissures forming a pattern characteristic of each tooth. The biting surface of an incisor is the incisal edge.
TOOTH MORPHOLOGY (Figs 33.13, 33.14)
Behind each lateral incisor is a canine tooth with a single cusp (hence the American term cuspid) instead of an incisal edge. The maxillary canine is stouter and more pointed than the mandibular canine. The canine root, which is the longest of any tooth, produces a bulge (canine eminence) on the alveolar bone externally, particularly in the upper jaw. Although canines usually have single roots, that of the lower may sometimes be bifid.
Distal to the canines are two premolars, each with a buccal and lingual cusp (hence the term bicuspid). The occlusal surfaces of the maxillary premolars are oval (the long axis is buccopalatal) and a mesiodistal fissure separates the two cusps. In buccal view, premolars resemble the canines but are smaller. The maxillary first premolar usually has two roots (one buccal, one palatal) but may have one, and very rarely three, roots (two buccal and one palatal). The maxillary second premolar usually has one root. The occlusal surfaces of the mandibular premolars are more circular or more square than those of the upper premolars. The buccal cusp of the mandibular first premolar towers above the lingual cusp to which it is connected by a ridge separating the mesial and distal occlusal pits. In the mandibular second premolar a mesiodistal fissure usually separates a buccal from two smaller lingual cusps. Each lower premolar has one root, but very rarely the root of the first is bifid. Lower second premolars fail to develop in about 2% of individuals.
Figure 33.13 A, The permanent teeth of the upper dental arch: occlusal aspect. B, The permanent teeth of the lower dental arch: occlusal aspect. C, Terminology employed for the identification of teeth according to their location in the lower jaw. The same terminology is employed for the teeth in the upper jaw
Figure 33.14 The permanent upper and lower teeth of the right side: labial and buccal surfaces.
Posterior to the premolars are three molars whose size decreases distally. Each has a large rhomboid (upper jaw) or rectangular (lower jaw) occlusal surface with four or five cusps. The maxillary first molar has a cusp at each corner of its occlusal surface and the mesiopalatal cusp is connected to the distobuccal by an oblique ridge. A smaller cusplet or tubercle (cusplet of Carabelli) usually appears on the mesiopalatal cusp (most commonly in Caucasian races). The tooth has three widely separated roots, two buccal and one palatal. The smaller maxillary second molar has a reduced or occasionally absent distopalatal cusp. Its three roots show varying degrees of fusion. The maxillary third molar, the smallest, is very variable in form. It usually has three cusps (the distopalatal being absent) and commonly the three roots are fused.
The mandibular first molar has three buccal and two lingual cusps on its rectangular occlusal surface, the smallest cusp being distal. The cusps of this tooth are all separated by fissures. It has two widely separated roots, one mesial and one distal. The smaller mandibular second molar is like the first, but has only four cusps (it lacks the distal cusp of the first molar) and its two roots are closer together. The mandibular third molar is smaller still and, like the upper third molar, is variable in form. Its crown may resemble that of the lower first or second molar and its roots are frequently fused. As it erupts anterosuperiorly, the third molar is often impacted against the second molar, which produces food packing and inflammation, both indications for surgical removal. The maxillary third molar erupts posteroinferiorly and is rarely impacted. One or more third molars (upper or lower) fail to develop in up to 30% of individuals.
Impacted mandibular third molars
In many subjects there is a disproportion between the size of the teeth and the size of the jaws such that there is insufficient space for all the teeth to erupt. As the third mandibular molar teeth (the wisdom teeth) are the last to erupt they are often impeded in their eruption and either become impacted against the distal aspect of the second molar or remain unerupted deeply within the jaw bone. If the tooth is completely covered by bone and mucosa it is very unlikely to cause any symptoms, and the subject remains unaware of their presence unless the teeth are seen on a routine dental radiograph. Very rarely the surrounding dental follicle may undergo cystic degeneration which can 'hollow out' the jaw, usually the mandible, to a considerable degree. The developing cyst may displace the tooth as it expands and the tooth may end up as far away as the condylar neck or coronoid process.
More commonly, the erupting wisdom tooth erupts partially before impacting against the distal aspect of the second molar. When this occurs, symptoms are common due to recurrent soft tissue infection around the partially erupted tooth. This condition is known as pericoronitis and if the infecting organism is virulent, the infection may rapidly spread into the adjacent tissue spaces as described elsewhere. It is for this reason that so many wisdom teeth are removed in adolescents and young adults. The surgery itself requires considerable skill as the lingual nerve passes across the surface of the periosteum lingually, separated from the tooth only by a cortical plate of bone no thicker than an egg shell. Damage to this nerve results in altered sensation to the ipsilateral side of the tongue. The root apices of the impacted tooth often lie immediately above the inferior alveolar canal, and removal of the tooth can result in damage to the underlying nerve and artery. Maxillary third molars are only rarely impacted.
Deciduous teeth (Figs 33.15, 33.16)
Figure 33.15 The deciduous upper and lower teeth of the right side: labial and buccal surfaces.
Figure 33.16 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.)
The incisors, canine and premolars of the permanent dentition replace two deciduous incisors, a deciduous canine and two deciduous molars in each jaw quadrant. 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 permanent molars rather than their successors, the premolars. Each second deciduous molar has a crown which is almost identical to that of the posteriorly adjacent first permanent molar. 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, and its two buccal cusps are separated from its two lingual cusps by a zigzagging mesiodistal fissure. Like permanent molars, upper deciduous molars have three roots 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.
Eruption of teeth (Fig. 33.17)
Information on the sequence of development and eruption of teeth into the oral cavity is important in clinical practice and also in forensic medicine and archaeology. The tabulated data provided in Table 33.2 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 three years for the root to be completed. For deciduous teeth, root completion is more rapid (Table 33.2). The developmental stages of initial calcification and crown completion are less affected by environmental influences than eruption, the timing of which may be modified by several factors such as early tooth loss and severe malnutrition.
Figure 33.18 shows the panoramic appearance of the dentition seen with orthopantomograms at the time of birth, 3, 6½, 10 and 16 years of age.
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 (Fig. 33.19). 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 free-way 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 for they are more commonly found in the population: c.75% of the population in the USA have some degree of occlusal 'disharmony'). However, the majority of malocclusions should be regarded as anatomical variations rather than abnormalities for they are rarely involved in masticatory dysfunction or pain, although they may be aesthetically displeasing.
Variations in tooth number, size and form
Figure 33.17 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 33-2. Chronology of the human dentition
Dentition Tooth First evidence of calcification (weeks in utero for deciduous teeth) Crown completed (months) Eruptiom (months) Root completed (years)
Deciduous Upper i1 14 1 ½ 10 (8-12) 1½
i2 16 2½ 11 (9-13) 2
C 17 9 19 (16-22) 3¼
m1 15½ 6 16 (13-19) 2½
m2 19 11 29 (25-33) 3
Deciduous lower i1 14 2½ 8 (6-10) 1½
i2 16 3 13 (10-16) 1½
C 17 9 20 (17-23) 3¼
m1 15½ 5½ 16 (14-18) 2¼
m2 18 10 27 (23-31) 3
Permanent upper l1 3-4 months 4-5 yrs 7-8 yrs 10
l2 10-12 months 4-5 yrs 8-9 yrs 11
C 4-5 months 6-7 yrs 11-12 yrs 13-15
P1 1½-1¾yrs 5-6 yrs 10-11 yrs 12-13
P2 2-2¼yrs 6-7 yrs 10-12 yrs 12-14
M1 at birth 2½-3 yrs 6-7 yrs 9-10
M2 2½-3 yrs 7-8 yrs 12-13 yrs 14-16
M3 7-9 yrs 12-16 yrs 17-21 yrs 18-25
Permanent lower l1 3-4 months 4-5 yrs 6-7 yrs 9
l2 3-4 months 4-5 yrs 7-8 yrs 10
C 4-5 months 6-7 yrs 9-10 yrs 12-14
P1 1¾-2 yrs 5-6 yrs 10-12 yrs 12-13
P2 2¼-2½yrs 6-7 yrs 1-12 yrs 13-14
M1 at birth 2½-3 yrs 6-7 yrs 9-10
M2 2½-3 yrs 7-8 yrs 11-13 yrs 14-15
M3 8-10 yrs 12-16 yrs 17-21 yrs 18-25
(Modified with permission from Ash MM 1993 Dental Anatomy, Physiology and Occlusion. Philadelphia: WB Saunders.)
Figure 33.18 Orthopantomogram of the dentition at birth. B, Orthopantomogram of the dentition at 2½years. C, Orthopantomogram of the dentition at 6½years. D, Orthopantomogram of the dentition at 10 years. E, Orthopantomogram of the dentition at 16 years. (B-F, by permission from Berkovitz BKB, Holland GR, Moxham BJ 2002 Oral Anatomy, Embryology and Histology, 3rd edn. Edinburgh: Mosby; D-F, also by kind permission from Eric Whaites.)
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. Although supernumerary teeth in the incisor region are often small with simple conical crowns, they may impede the eruption of the permanent incisors. 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 (Figs 33.20, 33.21, 33.22)
Figure 33.19 Lateral view of the dentition in centric occlusion.
Figure 33.20 An extracted upper right canine tooth viewed from its mesial aspect, showing its principal parts. Note the root covered by cement (partially removed), and the curved cervical margin, convex towards the cusp of the tooth.
The root is surrounded by alveolar bone, its cementum separated from the osseous socket (alveolus) by the connective tissue of the periodontal ligament, c.0.2 mm thick (Fig. 33.23). Coarse bundles of collagen fibres, embedded at one end in cementum, cross the periodontal ligament to enter the osseous alveolar wall. 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.
Enamel is an extremely hard and rigid material which covers the crowns of teeth. It is a heavily mineralized 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.
Figure 33.21 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. 33.31). Within the dentine the lines of the dentinal tubules are visible, forming S-shaped curves in the apical region but straighter in the root.
Figure 33.22 Longitudinal section of a tooth and its environs.
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 c.20 μm of the surface, where they are generally replaced by prismless (non-prismatic, aprismatic) enamel (Fig. 33.24). In cross-section the prisms are mainly horse-shoe 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. 33.25) and the tails are directed rootwards. The appearance of prism boundaries results from sudden changes in crystallite orientation. Prisms have a diameter of c.5 μm, and are packed with flattened hexagonal hydroxyapatite crystals, far larger than those found in the other collagenous-based mineralized tissues.
Figure 33.23 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, peridontal space. (By kind permission from Dr D Lunt.)
Figure 33.24 SEM of acid-etched outer enamel (A) showing enamel prisms, each (c.5 μm wide. A layer of prismless enamel (B) is evident on the surface. (By kind)permission from Professor D Whittaker.)
Two types of incremental lines are visible in enamel, short-term and long-term. At intervals of c.4 μm 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. 33.26). 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. Each line, known as an enamel stria, represents a period of 7-8 days enamel growth (Fig. 33.27). A prominent striation, the neonatal line, is formed in teeth whose mineralization spans birth (see above). 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. 33.13) 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.
Figure 33.25 Ground cross-section of enamel showing cross-sectional keyhole (fish scale) appearance of enamel prisms (pattern 3). (By permission from Berkovitz BKB, Holland GR, Moxham BJ 2002 Oral Anatomy, Embryology and Histology, 3rd edn. Edinburgh: Mosby.)
Figure 33.26 Ground longitudinal section of enamel viewed with phase contrast showing prisms (vertical lines) and cross-striations (horizontal lines). (By permission from Berkovitz BKB, Holland GR, Moxham BJ 2002 Oral Anatomy, Embryology and Histology, 3rd edn. Edinburgh: Mosby.)
Figure 33.27 Ground longitudinal section of enamel showing enamel striae (arrows). Viewed between crossed polarizing filters. (By kind permission from Dr AD Beynon.)
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 c.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, c.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. 33.28) 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. 33.29). 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.
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 predentine 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 are c.20 μm apart: they represent increments of about 6-10 days (Fig. 33.30). Daily incremental lines (von Ebner lines) are c.4 μm apart. 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.
Primary dentine formation proceeds at a steady but declining rate as first the crown and then the root is completed. This slow and intermittent deposition of dentine (regular secondary dentine) continues throughout life and further reduces the size of the pulp chamber. 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. 33.31).
Figure 33.28 Ground longitudinal section of dentine showing branching of dentine tubules near the enamel-dentine junction (arrow). (By permission from Berkovitz BKB, Holland GR, Moxham BJ 2002 Oral Anatomy, Embryology and Histology, 3rd edn. Edinburgh: Mosby.)
Figure 33.29 Microradiograph of transversely sectioned dentinal tubules surrounded by a more radiopaque and therefore more mineralized zone of peritubular dentine. (By permission from Berkovitz BKB, Holland GR, Moxham BJ 2002 Oral Anatomy, Embryology and Histology, 3rd edn. Edinburgh: Mosby.)
Figure 33.30 Ground longitudinal section of dentine 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.)
Figure 33.31 Longitudinal ground section of an incisor tooth.
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 (Fig. 33.22). 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.
Cementum is a bone-like tissue which covers the dental roots, and is c.50% by weight mineralized (mainly hydroxyapatite crystals). However, unlike bone, cementum is avascular and lacks nerves. The cementum generally overlaps the enamel slightly, although it may meet it end on. Occasionally, the two tissues may fail to meet, in which case dentine is exposed in the mouth. If the exposed dentinal tubules remain patent then the teeth may be sensitive to stimuli such as cold water. In older teeth, the root may become exposed in the mouth as a consequence of occlusal drift and gingival recession, and cementum is often abraded away by incorrect tooth brushing and dentine exposed.
Like bone, cementum is perforated by Sharpey's fibres, which represent the attachment bundles of collagen fibres in the periodontal ligament (extrinsic fibres). New layers of cementum, which are deposited incrementally throughout life to compensate for tooth movements, incorporate new Sharpey's fibres. Incremental lines are irregularly spaced. The first cement to be formed is thin (up to 200 μm), acellular and contains only extrinsic fibres. Cementum formed later towards the root apex is produced more rapidly and contains cementocytes lying in lacunae joined by canaliculi. The latter are mainly directed towards their source of nutrients from the periodontal ligament. This cementum contains both extrinsic fibres derived from the periodontal ligament and intrinsic fibres of cementoblastic origin which lie parallel to the surface. Varying arrangements of layering between cellular and acellular cement occur. With increasing age, cellular cement may reach a thickness of a millimetre or more around the apices and at the branching of the roots, where it compensates for the loss of enamel by attrition. Cementum is not remodelled but small areas of resorption with evidence of repair may be seen.
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; 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 an 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. 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.
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 c.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) 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. 33.32). 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.
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 (p. 62). 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.
Figure 33.32 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.)
Figure 33.33 Anterior part of the left side of the mandible, with the superficial bone removed on the buccal side to show the 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. 33.35), and the flat table of bone surmounting the interdental bone septa. In this specimen the inferior alveolar canal is widely separated from the roots of the teeth, a variable condition.
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. 33.33). 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.
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 septas. 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. 33.34).
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. 33.35). 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 radio-opacity 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).
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 teeth by fracturing the buccal than the palatal plate. 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 mechanically easier to remove this tooth, when impacted, via the lingual plate, although it is important to realize that the lingual nerve is here exposed to damage.
VASCULAR SUPPLY AND LYMPHATIC DRAINAGE OF THE TEETH AND SUPPORTING STRUCTURESArterial supply of the teeth
Figure 33.34 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.)
Figure 33.35 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 kind permission from Ms Nadine White.)
The main arteries to the teeth and their supporting structures are derived from the maxillary artery, a terminal branch of the external carotid artery. The upper teeth are supplied by branches from the superior alveolar arteries and the lower teeth by branches from the inferior alveolar arteries.
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 (Fig. 30.7). 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; 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.
INNERVATION OF THE TEETH (Fig. 33.36)
Superior alveolar nerves
Figure 33.36 Longitudinal demineralized section of a tooth stained with a silver impregnation technique. Note the horizontal nerve trunk (top) within the pulp, with fine nerve fibres, one of which (A) passes between the odontoblasts (B) lining the surface of the predentine (C).
The teeth in the upper jaw are supplied by the three superior alveolar (dental) nerves (Fig. 30.6). These 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, descending 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 (Figs 30.4-30.7, 30.6)
The course of the inferior alveolar nerve in the infratemoral fossa is described on page 523. 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 (Fig. 30.6). 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. It 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, where they communicate with branches of the inferior alveolar nerve. Foramina occur in c.10% of retromolar fossae and infiltration in this region can abolish 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).
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 (p. 523). 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. 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 Bell's palsy due to loss of conduction from the facial nerve in the region of the parotid gland. Finally, if the needle is not sterile, infection of the pterygomandibular space may ensue, which could spread to other important tissue spaces (p. 525). Diffusion of anaesthetic solution through the inferior orbital fissure could give temporary orbital symptoms such as paralysis of lateral rectus due to anaesthesia of the abducens nerve.