Clinical Significance of Dental Anatomy, Histology, Physiology ..., Assignments of Dental Anatomy

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1 Clinical Significance of Dental Anatomy,

Histology, Physiology, and Occlusion LEE W. BOUSHELL, JOHN R. STURDEVANT

structures of teeth (enamel, dentin, cementum, and pulp) and their relationships to each other and to the supporting structures is necessary, especially when treating dental caries. The protective^ Athorough understanding of the histology, physiology, and^ occlusal interactions of the dentition and supporting tissues^ is essential for the restorative dentist. Knowledge of the

function of the tooth form is revealed by its impact on masticatory muscle activity, the supporting tissues (osseous and mucosal), and the pulp. Proper tooth form contributes to healthy supporting tissues. The contour and contact relationships of teeth with adjacent and opposing teeth are major determinants of muscle function in mastication, esthetics, speech, and protection. The relationships of form to function are especially noteworthy when considering the shape of the dental arch, proximal contacts, occlusal contacts,

and mandibular movement. Teeth and Supporting Tissues

Dentitions Humans have primary and permanent dentitions. The primary dentition consists of 10 maxillary and 10 mandibular teeth. Primary teeth exfoliate and are replaced by the permanent dentition, which

consists of 16 maxillary and 16 mandibular teeth. Classes of Human Teeth: Form and Function

Human teeth are divided into classes on the basis of form and function. The primary and permanent dentitions include the incisor, canine, and molar classes. The fourth class, the premolar, is found only in the permanent dentition (Fig. 1.1). Tooth form predicts the function; class traits are the characteristics that place teeth into functional categories. Because the diet of humans consists of animal and plant foods, the human dentition is called omnivorous.

Incisors Incisors are located near the entrance of the oral cavity and function as cutting or shearing instruments for food (see Fig. 1.1). From a proximal view, the crowns of these teeth have a relatively triangular

shape, with a narrow incisal surface and a broad cervical base. During mastication, incisors are used to shear (cut through) food.

Incisors are essential for proper esthetics of the smile, facial soft tissue contours (e.g., lip support), and speech (phonetics). Canines

Canines possess the longest roots of all teeth and are located at the corners of the dental arches. They function in the seizing, piercing, tearing, and cutting of food. From a proximal view, the crown also has a triangular shape, with a thick incisal ridge. The anatomic form of the crown and the length of the root make canine teeth strong, stable abutments for fixed or removable prostheses. Canines not only serve as important guides in occlusion, because of their anchorage and position in the dental arches, but

also play a crucial role (along with the incisors) in the esthetics of the smile and lip support. Premolars

Premolars serve a dual role: (1) They are similar to canines in the tearing of food, and (2) they are similar to molars in the grinding of food. Although first premolars are angular, with their facial cusps resembling canines, the lingual cusps of the maxillary premolars and molars have a more rounded anatomic form (see Fig. 1.1). The occlusal surfaces present a series of curves in the form of concavities and convexities that should be maintained throughout life for correct occlusal contacts and function. Although less visible than incisors

and canines, premolars still play an important role in esthetics. Molars Molars are large, multicusped, strongly anchored teeth located

nearest the temporomandibular joint (TMJ), which serves as the fulcrum during function. These teeth have a major role in the crushing, grinding, and chewing of food to dimensions suitable for swallowing. They are well suited for this task because they have broad occlusal surfaces and anchorage (Figs. 1.2 and 1.3). Premolars and molars are important in maintaining the vertical dimension of the face (see Fig. 1.1).

Structures of Teeth Teeth are composed of enamel, the pulp–dentin complex, and cementum (see Fig. 1.3). Each of these structures is discussed individually.

completion. The strategic placement of the grooves and fossae complements the position of the opposing cusps so as to allow movement of food to the facial and lingual surfaces during mastica tion. A functional cusp that opposes a groove (or fossa) occludes- on enamel inclines on each side of the groove and not in the depth of the groove. This arrangement leaves a V-shaped escape path between the cusp and its opposing groove for the movement of food during chewing (Fig. 1.4). features and may approach zero depending on the effectiveness of adjacent cusp coalescence. Failure or compromised coalescence of the enamel of the developmental lobes results in a deep invagination^ Enamel thickness varies in the area of these developmental in the groove area of the enamel surface and is termed Noncoalesced enamel at the deepest point of a fossa is termed fissure pit ..

Incisors

Incisors

Canine

Canine

Premolars

Premolars

Molars

- tion. The classes of teeth are incisors, canines, premolars, and molars. Cusps of mandibular teeth are one half cusp anterior of corresponding Fig. 1.1 Maxillary and mandibular teeth in maximum intercuspal posiMolars - cusps of teeth in the maxillary arch. (From Logan BM, Reynolds P, Hutch ings RT: 2010, Mosby.) McMinn’s color atlas of head and neck anatomy, ed 4, Edinburgh,- - molars after several years of use, showing rounded curved surfaces and minimal wear. Fig. 1.2 Occlusal surfaces of maxillary and mandibular first and second

3c (^11109) 12

5

(^86) 4 7

3a 1a^1413

3b 2

- tures. Fig. 1.3 1 , Enamel; Cross section of the maxillary molar and its supporting struc 1a, gnarled enamel;^1 2 , dentin; 3a, pulp chamber; 3b-, pulp horn; fibers in periodontal ligament; mucosa; Retzius; 14, dentinoenamel junction (DEJ). 10 3c, submucosa;, pulp canal; 4 , apical foramen; 11 , blood vessels; 7 , alveolar bone; 5 , cementum; 12 , gingiva; 8 , maxillary sinus; 6 , periodontal 13 , lines of 9 ,

Enamel Enamel formation, ameloblasts known as. These cells originate from the embryonic germ layer ectoderm . Enamel covers the anatomic crown of the amelogenesis , is accomplished by cells called^ • contact. Note the grooves for escape of food.^ Fig. 1.4^ Maxillary and mandibular first molars in maximum intercuspal

tooth, varies in thickness in different areas, and is securely attached to the dentin by the dentinoenamel junction (DEJ) (see Fig. 1.3). It is thicker at the incisal and occlusal areas of the crown and becomes progressively thinner until it terminates at the cemen- toenamel junction (CEJ). The thickness also varies from one class of tooth to another, averaging 2 2.3 to 2.5 cusps of molars. mm at the cusps of premolars, and 2.5 to 3 mm at the incisal ridges of incisors, mm at the ossification centers, which form into developmental lobes. Adjacent developmental lobes increase in size until they begin to coalesce. Grooves and fossae result in the areas of coalescence (at the junction^ Cusps on the occlusal surfaces of posterior teeth begin as separate of the developmental lobes of enamel) as cusp formation nears

arrangement for each group or layer of rods as they progress radially from the dentin toward the enamel surface. They initially follow a curving path through one third of the enamel next to the DEJ. After that, the rods usually follow a more direct path through the remaining two thirds of the enamel to the enamel surface. Groups of enamel rods may entwine with adjacent groups of rods and follow a curving irregular path toward the tooth surface. These constitute gnarled enamel , which occurs near the cervical regions and also in incisal and occlusal areas (Fig. 1.6). Gnarled enamel is not subject to fracture as much as is regular enamel. This type of enamel formation does not yield readily to the pressure of bladed, hand-cutting instruments in tooth preparation. The orienta- tion of the enamel rod heads and tails and the gnarling of enamel rods provide strength by resisting, distributing, and dissipating impact forces. Changes in the direction of enamel rods, which minimize the potential for fracture in the axial direction, produce an optical appearance called appear to be composed of alternate light and dark zones of varying widths that have slightly different permeability and organic content. Hunter-Schreger bands (Fig. 1.7). These bands These bands are found in different areas of each class of teeth. Because the enamel rod orientation varies in each tooth, Hunter- Schreger bands also have a variation in the number present in each tooth. In anterior teeth, they are located near the incisal surfaces. They increase in numbers and areas of teeth, from canines to premolars. In molars, the bands occur from near the cervical region to the cusp tips. In the primary dentition, the enamel rods in the cervical and central parts of the crown are nearly perpendicular to the long axis of the tooth and are similar in their direction to permanent teeth in the occlusal two thirds of the crown. and about 8 Enamel rod diameter near the dentinal borders is about 4 μm near the surface. This diameter difference accom μm- modates the larger outer surface of the enamel crown compared with the dentinal surface at the DEJ. Enamel rods, in transverse section, have a rounded head or body section and a tail section, forming a repetitive series of interlocking rods. Microscopic (~5000 rounded head portion of each rod lies between the narrow tail portions of two adjacent prisms (Fig. 1.8). Generally, the rounded×) cross-sectional evaluation of enamel reveals that the

Fissures and/or pits represent non–self-cleansing areas where acidogenic biofilm accumulation may predispose the tooth to dental caries (Fig. 1.5). Chemically, enamel is a highly mineralized crystalline structure. Hydroxyapatite, in the form of a crystalline lattice, is the largest mineral constituent (90%–92% by volume). Other minerals and trace elements are present in smaller amounts. The remaining constituents of tooth enamel include organic matrix proteins (1%–2%) and water (4%–12%) by volume. (or “prisms”), rod sheaths, and a cementing interrod substance. Enamel rods, which are the largest structural components, are Structurally, enamel is composed of millions of enamel rods formed linearly by successive apposition of enamel in discrete increments. The resulting variations in structure and mineralization are called rings that form during amelogenesis (see Fig. 1.3). The striae of incremental striae of Retzius and may be considered growth Retzius appear as concentric circles in horizontal sections of a tooth. In vertical sections, the striae are positioned transversely at the cuspal and incisal areas in a symmetric arc pattern, descending obliquely to the cervical region and terminating at the DEJ. When these circles are incomplete at the enamel surface, a series of alternating grooves, called Elevations between the grooves are called continuous around a tooth and usually lie parallel to the CEJ and imbrication lines of Pickerill perikymata , are formed.; they are each other. Rods vary in number from approximately 5 million for a mandibular incisor to about 12 million for a maxillary molar. In general, the rods are aligned perpendicularly to the DEJ and the tooth surface in the primary and permanent dentitions except in the cervical region of permanent teeth, where they are oriented outward in a slightly apical direction. Microscopically, the enamel surface initially has circular depressions indicating where the enamel rods end. These concavities vary in depth and shape, and gradually wear smooth with age. Additionally, a structureless outer layer of enamel about 30 the cervical area of the tooth crown and less commonly on cusp tips. There are no visible rod (prism) outlines in this area and all μm thick may be commonly identified toward of the apatite crystals are parallel to one another and perpendicular to the striae of Retzius. This layer, referred to as may be more heavily mineralized. Each ameloblast forms an individual enamel rod with a specific prismless enamel , length based on the specific type of tooth and the specific coronal location within that tooth. Enamel rods follow a wavy, spiraling course, producing an alternating clockwise and counterclockwise

e d

fc td dc ec

- and bacteria predisposing the tooth to dental caries (d), (td); Fig. 1.5 enamel caries lesion early enamel demineralization Fissure (f) at junction of lobes allows accumulation of food (ec), dentin caries lesion (arrow). (dc), (c). transparent dentinEnamel (e), dentin - BJ: Mosby.) Fig. 1.6 Oral anatomy, histology and embryology Gnarled enamel. (From Berkovitz BKB, Holland GR, Moxham, ed 4, Edinburgh, 2009,

certain ions and molecules. The route of passage may be through structural units such as rod sheaths, enamel cracks, and other defects that are hypomineralized and rich in organic content. Water^ Although enamel is a hard, dense structure, it is permeable to plays an important role as a transporting medium through the small intercrystalline spaces. Enamel tufts are hypomineralized structures of interrod substance between adjacent groups of enamel rods that project from the DEJ (Fig. 1.10). These projections arise in dentin, extend into enamel in the direction of the long axis of the crown, and may play a role in the spread of dental caries. Enamel lamellae are thin, leaflike faults between the enamel rod groups that extend from the enamel surface toward the DEJ, sometimes extending into dentin (see Fig. 1.10). They contain mostly organic material and may predispose the tooth to the entry of bacteria and subsequent development of dental caries. Enamel permeability decreases with age because of changes in the enamel matrix, a decrease referred to as dissolution is not uniform. Solubility of enamel increases from the enamel surface to the DEJ. When fluoride ions are present Enamel is soluble when exposed to acidic conditions, but the enamel maturation. during enamel formation or are topically applied to the enamel surface, the solubility of surface enamel is decreased. Fluoride

head portion is oriented in the incisal or occlusal direction; the tail section is oriented cervically. The final act of the ameloblasts, upon the completion of enamel rod formation, is the secretion of a membrane layer that covers the ends of the enamel rods. This layer is referred to as Ameloblasts degenerate upon completion of Nasmyth membrane, which covers the newly erupted tooth and is worn away by mastica tion and cleaning. The membrane is replaced by an organic deposit Nasmyth membrane , or primary enamel cuticle .- called the Microorganisms may attach to the pellicle to form a biofilm (bacterial plaque), which, if acidogenic in nature, may become a precursor to dental disease. pellicle , which is a precipitate of salivary proteins. crystallites that vary in size and shape. The crystallites are tightly packed in a distinct pattern of orientation that gives strength and structural identity to the enamel rod. The long axis of the apatite^ Each enamel rod contains millions of small, elongated apatite crystallites within the central region of the head (body) is aligned almost parallel to the rod long axis, and the crystallites incline with increasing angles (65 degrees) to the rod axis in the tail region. The susceptibility of these crystallites to acidic conditions, from the caries process or as a result of an etching procedure, may be correlated with their orientation. Acid-induced mineral dissolution (demineralization) occurs more in the head region of each rod. The tail region and the periphery of the head region are relatively resistant to acidic demineralization. The crystallites are irregular in shape, with an average length of 160 of 20 to 40 of unit cells that have a highly ordered arrangement of atoms. A nm. Each apatite crystallite is composed of thousands nm and an average width crystallite may be 300 unit cells long, 40 cells wide, and 20 cells thick in a hexagonal configuration (Fig. 1.9). An organic matrix surrounds individual crystals.

Hunter-Schreger^ AlternatingEnamelDentinbands

- graphed obtained by reflected light. (Modified from Chiego DJ Jr: tials of oral histology and embryology: A clinical approach 2014, Mosby.) Fig. 1.7 Photomicrograph of enamel Hunter-Schreger bands. Photo, ed 4, St Louis, Essen--

B

T

- enamel. Crystal orientation is different in “bodies” Approximate level of magnification WJ, Neal RJ: Structure of mature human dental enamel as observed by Fig. 1.8 Electron micrograph of cross section of rods in mature human ×5000. (From Meckel AH, Griebstein (B) than in “tails” (T). electron microscopy, Arch Oral Biol 10(5):775–783, 1965.)

elastic modulus, high compressive strength, and low tensile strength). The ability of the enamel to withstand masticatory forces depends on a stable attachment to the dentin by means of the DEJ. Dentin is a more flexible substance that is strong and resilient (low elastic modulus, high compressive strength, and high tensile strength), which essentially increases the fracture toughness of the more superficial enamel. The junction of enamel and dentin (DEJ) is scalloped or wavy in outline, with the crest of the waves penetrating toward enamel (Fig. 1.11). The rounded projections of enamel fit into the shallow depressions of dentin. This interdigitation may contribute to the durable connection of enamel to dentin. The DEJ is approximately 2 μm wide and is comprised of a mineralized complex of interwoven dentin and enamel matrix proteins. In addition to the physical, scalloped relationship between the enamel and dentin, an interphase matrix layer (made primarily of a fibrillary collagen network) extends 100 to 400 μm from the DEJ into the enamel. This matrix-modified interphase layer is considered to provide fracture propagation limiting properties to the interface between the enamel and the DEJ and thus overall structural stability of the enamel attachment to dentin. (^1) Enamel rods that lack a dentin base because of caries or improper preparation design are easily fractured away from neighboring rods. For optimal strength in tooth preparation, all enamel rods should be supported by dentin (Fig. 1.12).

concentration decreases toward the DEJ. Fluoride is able to affect the chemical and physical properties of the apatite mineral and influence the hardness, chemical reactivity, and stability of enamel, while preserving the apatite structures. Trace amounts of fluoride stabilize enamel by lowering acid solubility, decreasing the rate of demineralization, and enhancing the rate of remineralization. may vary over the external tooth surface according to the location; Enamel is the hardest substance of the human body. Hardness also, it decreases inward, with hardness lowest at the DEJ. The density of enamel also decreases from the surface to the DEJ. Enamel is a rigid structure that is both strong and brittle (high

- tallites. (From Nanci A: Fig. 1.9 Electron micrograph of mature, hexagon-shaped enamel crys Ten Cate’s oral histology: development, structure,^ 20 nm - and function, ed 7, St Louis, 2008, Mosby.) Lamella^ Enamel DentinoenamelDentinal partof lamellajunctionDentin - surface into dentin. Note the enamel tufts Popowics T: St. Louis, 2016, Saunders. Courtesy James McIntosh, PhD, Assistant Fig. 1.10 (^) Illustrated dental embryology, histology, and anatomyMicroscopic view through lamella that goes from enamel (arrow). (From Fehrenbach MJ,, ed 4, Professor Emeritus, Department of Biomedical Sciences, Baylor College of Dentistry, Dallas, TX.)

E D

- arrow trated dental embryology, histology, and anatomy Saunders. Courtesy James McIntosh, PhD, Assistant Professor Emeri Fig. 1.11 ). E, Enamel; Microscopic view of scalloped dentoenamel junction (DEJ; D, dentin. (From Fehrenbach MJ, Popowics T:, ed 4, St. Louis, 2016, Illus-- tus, Department of Biomedical Sciences, Baylor College of Dentistry, Dallas, TX.) - away readily by pressure from hand instrument. B, Cervical preparation showing enamel rods supported by dentin base. Fig. 1.12 A A, Enamel rods unsupported by dentin base are fractured^ B

objective during operative procedures must be the preservation of the health of the pulp. odontoblasts Dentin formation,. Odontoblasts are considered part of pulp and dentin dentinogenesis , is accomplished by cells called tissues because their cell bodies are in the pulp cavity, but their long, slender cytoplasmic cell processes (Tomes fibers) extend well (100–200 1.14). μm) into the tubules in the mineralized dentin (Fig. a living tissue, with the capability of reacting to physiologic and pathologic stimuli. Odontoblastic processes occasionally cross the DEJ into enamel; these are termed^ Because of these odontoblastic cell processes, dentin is considered enamel spindles when their ends are thickened (Fig. 1.15). Enamel spindles may serve as pain receptors, explaining the sensitivity experienced by some patients during tooth preparation that is limited to enamel only. Dentin forms the largest portion of the tooth structure, extending almost the full length of the tooth. Externally, dentin is covered by enamel on the anatomic crown and cementum on the anatomic root. Internally, dentin forms the walls of the pulp cavity (pulp chamber and pulp canals) (Fig. 1.16). Dentin formation begins immediately before enamel formation. Odontoblasts generate an extracellular collagen matrix as they begin to move away from adjacent ameloblasts. Mineralization of the collagen matrix, facilitated by modification of the collagen matrix by various noncollagenous proteins, gradually follows its secretion. The most recently formed layer of dentin is always on the pulpal surface.

Pulp–Dentin Complex Pulp and dentin tissues are specialized connective tissues of mesodermal origin, formed from the dental papilla of the tooth bud. Many investigators consider these two tissues as a single

tissue, which forms the pulp–dentin complex, with mineralized dentin constituting the mature end product of cell differentiation and maturation. Dental pulp occupies the pulp cavity in the tooth and is a unique, specialized organ of the human body that serves four functions: (1) formative (developmental), (2) nutritive, (3) sensory (protective), and (4) defensive/reparative. The formative function is the production of primary and secondary dentin by odontoblasts. The nutritive function supplies mineral ions, proteins, and water to dentin through the blood supply to odontoblasts and their processes. The sensory function is provided by nerve fibers within the pulp that mediate the sensation of pain. Dentin nervous nociceptors are unique because various stimuli elicit only pain as a response. The pulp usually does not differentiate between heat, touch, pressure, or chemicals. Motor nerve fibers initiate reflexes in the muscles of the blood vessel walls for the control of circulation in the pulp. The defensive/reparative function is discussed in the subsequent section on Pathologic Challenge The pulp is circumscribed by dentin and is lined peripherally. The Pulp-Dentin Complex: Response to by a cellular layer of odontoblasts adjacent to dentin. Anatomically, the pulp is divided into (1) coronal pulp located in the pulp chamber in the crown portion of the tooth, including the pulp horns that are located beneath the incisal ridges and cusp tips, and (2) radicular pulp located in the pulp canals in the root portion of the tooth. The radicular pulp is continuous with the periapical tissues through the apical foramen or foramina of the root. Accessory canals may extend from the pulp canals laterally through the root dentin to periodontal tissue. The shape of each pulp conforms generally to the shape of each tooth (see Fig. 1.3). channels, connective tissue cells, intercellular substance, odonto The pulp contains nerves, arterioles, venules, capillaries, lymph- blasts, fibroblasts, macrophages, collagen, and fine fibers. pulp is circumscribed peripherally by a specialized odontogenic area composed of the odontoblasts, the cell-free zone, and the cell-rich zone.^2 The during tooth preparation. In general, the pulp cavity is a miniature contour of the external surface of the tooth. Pulp cavity size varies with tooth size in the same person and among individuals. With^ Knowledge of the contour and size of the pulp cavity is essential advancing age, the pulp cavity usually decreases in size. Radiographs are an invaluable aid in determining the size of the pulp cavity and any existing pathologic condition (Fig. 1.13). A primary

- person. Note the difference in the size of the pulp cavity Fig. 1.13^ A^ Pulp cavity size. A, Premolar radiograph of young person. B, Premolar radiograph of olderB (arrows). d pd

tf o mf 10 m

- extend through the predentin Fig. 1.14 Odontoblasts (o) (pd) have cell processes (Tomes fibers [tf]) that into dentin (d). mf, Mineralization front.

odontoblast and is lined with a layer of peritubular dentin, which is much more mineralized than the surrounding intertubular dentin (see Fig. 1.18). The surface area of dentin is much larger at the DEJ and dentinocemental junction than it is on the pulp cavity side. Because odontoblasts form dentin while progressing inward toward the pulp, the tubules are forced closer together. The number of tubules increases from 15,000 to 20,000/mm (^2) at the DEJ to 45,000 to 65,000/mm from the DEJ to the pulp surface. In coronal dentin, the average diameter of tubules at the DEJ is 0.5 to 0.9 to 2 to 3 μm near the pulp (Fig. 1.19).^2 at the pulp.^3 The lumen of the tubules also varies μm, but this increases

This unmineralized zone of dentin is immediately next to the cell bodies of odontoblasts and is called formation begins at areas subjacent to the cusp tip or incisal ridge and gradually spreads, at the rate of ~4 predentin μ (^) m/day, to the apex of(see Fig. 1.14). Dentin the root (see Fig. 1.16). In contrast to enamel formation, dentin formation continues after tooth eruption and throughout the life of the pulp. The dentin forming the initial shape of the tooth is called primary dentin and is usually completed 3 years after tooth eruption (in the case of permanent teeth). process of dentinogenesis and extend through the entire width of dentin, from the pulp to the DEJ (Figs. 1.17 and 1.18). Each The dentinal tubules are small canals that remain from the tubule contains the cytoplasmic cell process (Tomes fiber) of an

A A

- extend into enamel as enamel spindles GR, Moxham BJ: burgh, 2009, Mosby. Courtesy of Dr. R. Sprinz.) Fig. 1.15 Longitudinal section of enamel. Odontoblastic processes Oral anatomy, histology and embryology (A). (From Berkovitz BKB, Holland, ed 4, Edin-

e c

- enamel covering the anatomic root. Fig. 1.16 (e) (^) covering the anatomic crown of the tooth and cementumPattern of formation of primary dentin. This figure also shows (c)

T

- acid. The artificial crack shows part of the dentinal tubules apertures are opened and widened by acid application. (From Brännström M: Fig. 1.17 Dentin and pulp in restorative dentistry Ground dentinal surface, acid-etched with 37% phosphoric, London, 1982, Wolfe Medical.) (T). The tubule

I P

- tubular dentin Brännström M: Wolfe Medical.) Fig. 1.18 Dentinal tubules in cross section, 1.2 (P) Dentin and pulp in restorative dentistry is more mineralized than intertubular dentin mm from pulp. Peri, London, 1982, (I). (From-

are seen in enamel, indicating minute fractures of that structure. The craze lines usually are not clinically significant unless associated with cracks in the underlying dentin. The ultimate tensile strength of dentin is approximately 98 megapascals (MPa), whereas the ultimate tensile strength of enamel is approximately 10 compressive strength of dentin and enamel are approximately 297 and 384 During tooth preparation, dentin usually is distinguished from MPa, respectively. 5 MPa. The enamel by (1) color and opacity, (2) reflectance, (3) hardness, and (4) sound. Dentin is normally yellow-white and slightly darker

tooth crown, but the tubules are straighter in the incisal ridges, cusps, and root areas (Fig. 1.20). Tubules are generally oriented perpendicular to the DEJ. Along the tubule walls are small lateral^ The course of the dentinal tubules is a slight S-curve in the openings called formed as a result of the presence of secondary (lateral) branches of adjacent odontoblastic processes during dentinogenesis. Near the DEJ, the tubules are divided into several branches, forming canaliculi or lateral canals. The lateral canals are an intercommunicating and anastomosing network (Fig. 1.21). at a reduced rate (~0.4 stimuli, although the rate and amount of this physiologic secondary After the primary dentin is formed, dentin deposition continues μm/day) even without obvious external dentin vary considerably among individuals. In secondary dentin, the tubules take a slightly different directional pattern in contrast to the primary dentin (Fig. 1.22). The secondary dentin forms on all internal aspects of the pulp cavity, but in the pulp chamber, in multirooted teeth, it tends to be thicker on the roof and floor than on the side walls. primary dentin gradually thicken, through ongoing mineral deposi The walls of the dentinal tubules (peritubular dentin) in the (^4) - tion, with age. The dentin therefore becomes harder, denser, and, because tubular fluid flow becomes more restricted as the lumen spaces become smaller, less sensitive. The increased amount of mineral in the primary dentin is defined as dentin sclerosis. Dentin sclerosis resulting from aging is called material and 30% organic material by volume. The organic material is approximately 90% type I collagen and 10% noncollagenous Human dentin is composed of approximately 50% inorganic physiologic dentin sclerosis. proteins. Dentin is less mineralized than enamel but more mineral ized than cementum or bone. The mineral content of dentin increases with age. The mineral phase is composed primarily of hydroxyapatite crystallites, which are arranged in a less systematic- manner than are enamel crystallites. Dentinal crystallites are smaller than enamel crystallites, having a length of 20 to 100 width of about 3 and cementum. (^4) Dentin is significantly softer than enamel butnm, which is similar to the size seen in bone nm and a harder than bone or cementum. The hardness of dentin averages one fifth that of enamel, and its hardness near the DEJ is about three times greater than near the pulp. Although dentin is a hard, mineralized tissue, it is flexible, with a modulus of elasticity of approximately 18 gigapascals (GPa). the more brittle, less resilient enamel. Dentin is not as prone to fracture as is the enamel rod structure. Often small “craze lines”^5 This flexibility helps support

A

C B

- Fig. 1.19 D Tubules in superficial dentin close to the dentoenamel junc- tion (DEJ) deep dentin dentin of coronal dentin. (D) are smaller and less numerous than those in comparable depths(A) (B). are smaller and more sparsely distributed compared with The tubules in superficial root dentin (C) and deep root - is in a slight S-curve in the crown, but straight at the incisal tip and in the root. (From Young B, Lowe JS, Stevens A, Heath JW: histology: a text and colour atlas Fig. 1.20 Ground section of human incisor. Course of dentinal tubules, ed 5, Edinburgh, 2006, Churchill Wheater’s functional Livingstone.)

to occur. These components include water, matrix proteins, matrix- modifying proteins, and mineral ions. The vital dental pulp has a slight positive pressure that results in continual dentinal fluid flow toward the external surface of the tooth. Enamel and cementum, though semipermeable, provide an effective layer serving to protect the underlying dentin and limit tubular fluid flow. When enamel or cementum is removed during tooth preparation, the protective layer is lost, allowing increased tubular fluid movement toward the cut surface. Permeability studies of dentin indicate that tubules are functionally much smaller than would be indicated by their measured microscopic dimensions as a result of numerous constric tions along their paths (see Fig. 1.18). (^7) Dentin permeability is not- uniform throughout the tooth. Coronal dentin is much more permeable than root dentin. There also are differences within coronal dentin (Fig. 1.24). remaining dentin thickness (i.e., length of the tubules) and the (^8) Dentin permeability primarily depends on the diameter of the tubules. Because the tubules are shorter, more

than enamel. In older patients, dentin is darker, and it can become brown or black when it has been exposed to oral fluids, old restorative materials, or slowly advancing caries. Dentin surfaces are more opaque and dull, being less reflective to light than similar enamel surfaces, which appear shiny. Dentin is softer than enamel and provides greater yield to the pressure of a sharp explorer tine, which tends to catch and hold in dentin. Dentin sensitivity is perceived whenever nociceptor afferent nerve endings, in close proximity to odontoblastic processes within the dental tubules, are depolarized. The nerve transduction is most often interpreted by the central nervous system as pain. Physical, thermal, chemical, bacterial, and traumatic stimuli are remote from the nerve fibers and are detected through the fluid-filled dentinal tubules, although the precise mechanism of detection has not been conclusively established. The most accepted theory of stimulus detection is the hydrodynamic theory , which suggests that stimulus-initiated rapid tubular fluid movement within the dentinal tubules accounts for nerve depolarization. that involve cutting, drying, pressure changes, osmotic shifts, or changes in temperature result in rapid tubular fluid movement, (^6) Operative procedures which is perceived as pain (Fig. 1.23). plasma that contains all components necessary for mineralization Dentinal tubules are filled with dentinal fluid, a transudate of

- branching close to the dentoenamel junction (DEJ). (From Berkovitz BKB, Holland GR, Moxham BJ: Edinburgh, 2010, Mosby.) Fig. 1.21 Ground section showing dentinal tubules and their lateral Oral anatomy, histology, and embryology, ed 4,

Primary Secondary Pulp

Primary Secondary

- tubules curve sharply as they move from primary to secondary dentin. Fig. 1.22Pulp Ground section of dentin with pulpal surface at right. Dentinal Dentinal tubules are more irregular in shape in secondary dentin. (From Nanci A: ed 8, St. Louis, 2013, Mosby.) Ten Cate’s oral histology: development, structure, and function,

Enamel or exposed dentin Dentin Predentin afferentnervePulp

- distort odontoblasts and afferent nerves pain. Many operative procedures such as cutting or air-drying induce rapid fluid movement. Fig. 1.23 Stimuli that induce rapid fluid movements in dentinal tubules (arrow), leading to a sensation of

D D

- ration of a third molar. Dark blue dye was placed in the pulp chamber under pressure after tooth preparation. Dark areas of dye penetration show that the dentinal tubules of axial walls are much more permeable Fig. 1.24 Ground section of MOD (mesio-occluso-distal) tooth prepa (D)- than those of the pulpal floor of preparation.

remineralization of the intertubular dentin, in addition to the mineral occlusion of the dentinal tubules, such that the final hardness of the dentin in this affected area is greater than normal primary dentin. The increased overall mineralization of this caries- affected primary dentin is referred to as the pulpal inflammatory response and result in the generation of tertiary dentin Deep dentin formation processes occur simultaneously with at the pulp–dentin interface. The net effect of these reactive dentin sclerosis. processes is to increase the thickness/effectiveness of the dentin as a protective barrier for the pulp tissue. Two types of tertiary dentin form in response to lesion formation. In the case of mild injury (e.g., a shallow caries lesion), primary odontoblasts initiate increased formation of dentin along the internal aspect of the dentin beneath the affected area through secretion of (or “reactionary dentin”). Reactionary dentin is tubular in nature and is continuous with primary and secondary dentin. reactionary tertiary dentin of the primary odontoblasts. When therapeutic steps successfully resolve the injury, replacement cells (variously referred to as odontoblasts, odontoblast-like cells,^ More severe injury (e.g., a deep caries lesion) causes the death or odontoblastoid cells ) differentiate secondary from pulpal mesenchymal cells. The secondary odontoblasts subsequently generate dentin”) as a part of the ongoing host defense. Reparative dentin usually appears as a localized dentin deposit on the wall of the reparative tertiary dentin (or “reparative pulp cavity immediately subjacent to the area on the tooth that had received the injury (Fig. 1.27). Reparative dentin is generally atubular and therefore structurally different from the primary and secondary dentin.

Cementum Cementum is a thin layer of hard dental tissue covering the anatomic roots of teeth. It is formed by cells known as develop from undifferentiated mesenchymal cells in the connective cementoblasts , which

tissue of the dental follicle. Cementum is slightly softer than dentin and consists of about 45% to 50% inorganic material (hydroxy apatite) by weight and 50% to 55% organic matter and water by weight. The organic portion is composed primarily of collagen- and protein polysaccharides. principal collagen fibers of the periodontal ligament embedded in cementum and alveolar bone to attach the tooth to the alveolus (Fig. 1.28). Cementum is avascular. Sharpey fibers are portions of the It is formed continuously throughout life because, as the superficial^ Cementum is yellow and slightly lighter in color than dentin.

- Dark blue dye was placed in the pulp chamber under pressure. Deep dentin areas (over pulp horns) are much more permeable than superficial dentin. (From Pashley DH, Andringa HJ, Derkson GD, Derkson ME, Kal Fig. 1.25 Horizontal section in the occlusal third of a molar crown.- athoor SR: Regional variability in the permeability of human dentin, Oral Biol UK.) 32:519–523, 1987, with permission from Pergamon, Oxford, Arch

c

- Fig. 1.26 Transparent dentin (arrow) beneath a caries lesion (c).

numerous, and larger in diameter closer to the pulp, deep dentin is a less effective pulpal barrier compared with superficial dentin (Fig. 1.25).

The Pulp–Dentin Complex: Response to Pathologic Challenge The pulp–dentin complex responds to tooth pathology through pulpal immune-inflammation defense systems and dentin repair/

formation. The defensive and reparative functions of the pulp are mediated by an extremely complex host-defense response to bacterial, chemical, mechanical, and/or thermal irritation. toblasts are the first to respond to lesion formation and communicate (^9) Primary odon- with the deeper pulp tissue (via cytokines and chemokines) such that an adaptive and innate inflammatory reaction begins. Mild to moderate injury normally causes a reversible inflammatory response in the pulp, referred to as reversible pulpitis, which resolves when the pathology is removed. Moderate to severe injury (e.g., deep caries) may cause the degeneration of the affected odontoblastic processes and death of the corresponding primary odontoblasts. Toxic bacterial products, molecules released from the demineralized dentin matrix, and/or high concentrations of inflammatory response mediators may signal death of the primary odontoblasts. In cases of severe injury, an irreversible inflammatory response of the pulp (irreversible pulpitis) will ultimately result in capillary dilation, local edema, stagnation of blood flow, anoxia, and ultimately pulpal necrosis (see Chapter 2). block the advancement of a caries lesion by means of the precipita Very early host-defense processes in primary dentin seek to- tion of mineral in the lumens of the dentinal tubules of the affected area. The physical occlusion of the tubular lumens increases the ability of light to pass through this localized region (i.e., increases its transparency). This dentin is referred to as transparent dentin (Fig. 1.26). dentin because of mineral loss in the intertubular dentin (see Chapter 2). Successful host-defense repair processes result in the^10 Dentin in this area is not as hard as normal primary

Cementum is capable of repairing itself to a limited degree and is not resorbed under normal conditions. Some resorption of the apical portion of the root cementum and dentin may occur, however, if orthodontic pressures are excessive and movement is too fast

(Fig. 1.29). Physiology of Tooth Form

Function Teeth serve four main functions: (1) mastication, (2) esthetics, (3) speech, and (4) protection of supporting tissues. Normal tooth form and proper alignment ensure efficiency in the incising and

reduction of food. The various tooth classes—incisors, canines, premolars, and molars—perform specific functions in the mastica tory process and in the coordination of the various muscles of mastication. The form and alignment of anterior teeth contribute- to the esthetics of personal physical appearance. The form and alignment of anterior and posterior teeth assist in the articulation of certain sounds so as to effect proper speech. Finally, the form and alignment of teeth assist in the development and protection

of supporting gingival tissue and alveolar bone. Contours Facial and lingual surfaces possess a degree of convexity that affords

protection and stimulation of supporting tissues during mastication. The convexity generally is located at the cervical third of the crown on the facial surfaces of all teeth and the lingual surfaces of incisors and canines. Lingual surfaces of posterior teeth usually have their height of contour in the middle third of the crown. Normal tooth contours act in deflecting food only to the extent that the passing food stimulates (by gentle massage) and does not irritate (abrade) supporting soft tissues. If these curvatures are too great, tissues usually receive inadequate stimulation by the passage of food. Too little contour may result in trauma to the attachment apparatus. Normal tooth contours must be recreated in the performance of operative dental procedures. Improper location and degree of facial or lingual convexities may result in iatrogenic injury, as illustrated in Fig. 1.30, in which the proper facial contour is disregarded in the design of the cervical area of a mandibular molar restoration. Overcontouring is the worst offender, usually resulting in increased plaque retention that leads to a chronic inflammatory state of the gingiva. to the maintenance of periodontal tissue health as is the proper Proper form of the proximal surfaces of teeth is just as important form of facial and lingual surfaces. The proximal height of contour serves to provide (1) contacts with the proximal surfaces of adjacent teeth, thus preventing food impaction, and (2) adequate embrasure

layer of cementum ages, a new layer of cementum is deposited to keep the attachment intact. cementoblasts) is predominately associated with the coronal half of the root. Cellular cementum Acellular cementum is more frequently associated with (i.e., there are no the apical half of the root. Cementum on the root end surrounds the apical foramen and may extend slightly onto the inner wall of the pulp canal. Cementum thickness may increase on the root end to compensate for attritional wear of the occlusal or incisal surface and passive eruption of the tooth. permanent tooth. The attachment of cementum to dentin, although not completely understood, is very durable. Cementum joins enamel The cementodentinal junction is relatively smooth in the to form the CEJ. In about 10% of teeth, enamel and cementum do not meet, and this can result in a sensitive area as the openings of the dentinal tubules are not covered. Abrasion, erosion, caries, scaling, and restoration finishing/polishing procedures may denude dentin of its cementum covering. This may lead to sensitivity to various stimuli (e.g., heat, cold, sweet substances, sour substances).

d rd

- Fig. 1.27 Reparative dentin in response to a caries lesion.p d, Dentin; rd, gress during the past 25 years, reparative dentin; p, pulp. (From Trowbridge HO: Pulp biology: Pro Aust Endo J 29(1):5–12, 2003.) -

Fibersperforatingthe alveolarboneRadicular dentin Fibersperforatingthe cementum

- layer of cementum as Sharpey fibers. (Modified from Chiego DJ Jr: tials of oral histology and embryology: A clinical approach 2014, Mosby.) Fig. 1.28 Principal fibers of periodontal ligament continue into surface, ed 4, St Louis, Essen- - orthodontic tooth movement. Fig. 1.29 Radiograph showing root resorption on lateral incisor after

12 CHAPTER 1 Clinical Significance of Dental Anatomy, Histology, Physiology, and Occlusio from the incisor region through all the remaining teeth, the contact n area is located near the junction of the incisal (or occlusal) and middle thirds or in the middle third. Proximal contact areas typically are larger in the molar region, which helps prevent gingival food impaction during mastication. Adjacent surfaces near the proximal

contacts (embrasures) usually have remarkable symmetry. Embrasures Embrasures are V-shaped spaces that originate at the proximal

contact areas between adjacent teeth and are named for the direction toward which they radiate. These embrasures are (1) facial, (2) lingual, (3) incisal or occlusal, and (4) gingival (see Figs. 1.32 and 1.33). When the form and function of teeth are ideal and optimal oral^ Initially, the interdental papilla fills the gingival embrasure.

A B C

- surface of mandibular molar during mastication. A, Overcontour deflects food from gingiva and results in understimulation of supporting tissues. B, Undercontour of tooth may result in irritation of soft tissue. C, Correct Fig. 1.30 Contours. Arrows show pathways of food passing over facial contour permits adequate stimulation and protection of supporting tissue.^ •^ proximal contact areas. These spaces are occupied by soft tissue and bone for the support of teeth.^ Fig. 1.31^ Portion of the skull, showing triangular spaces beneath

A B

Facial embrasureLingual embrasure

- teeth. B, Mandibular teeth. Facial and lingual embrasures are indicated. Fig. 1.32 Proximal contact areas. Black lines show positions of contact faciolingually. A, Maxillary

space (immediately apical to the contacts) for gingival tissue, supporting bone, blood vessels, and nerves that serve the supporting structures (Fig. 1.31).

Proximal Contact Area When teeth initially erupt to make proximal contact with previously erupted teeth, a contact in size to become a proximal contact point is present. The contact point increases area as the two adjacent

tooth surfaces abrade each other during physiologic tooth movement (Figs. 1.32 and 1.33). proximal contacts cannot be overemphasized; they promote normal The physiologic significance of properly formed and located healthy interdental papillae filling the interproximal spaces. Improper contacts may result in food impaction between teeth, potentially increasing the risk of periodontal disease, caries, and tooth move ment. In addition, retention of food is objectionable because of- its physical presence and the halitosis that results from food decomposition. Proximal contacts and interdigitation of maxillary and mandibular teeth, through occlusal contact areas, stabilize and maintain the integrity of the dental arches. approximating surfaces of maxillary and mandibular central incisors (see Fig. 1.33). It is positioned slightly facial to the center of the proximal surface faciolingually (see Fig. 1.32). Proceeding posteriorly^ The proximal contact area is located in the incisal third of the

health is maintained, the interdental papilla may continue in this position throughout life. When the gingival embrasure is filled by the papilla, trapping of food in this region is prevented. In a facio lingual vertical section, the papilla is seen to have a triangular- shape between anterior teeth, whereas in posterior teeth, the papilla may be shaped like a mountain range, with facial and lingual peaks and the col (“valley”) lying beneath the contact area (Fig. 1.34). This col, a central faciolingual concave area beneath the contact, is more vulnerable to periodontal disease from incorrect contact and embrasure form because it is covered by nonkeratinized epithelium. The correct relationships of embrasures, cusps to sulci, marginal ridges, and grooves of adjacent and opposing teeth provide for the escape of food from the occlusal surfaces during mastication. When an embrasure is decreased in size or absent, additional stress is created on teeth and the supporting structures during mastication. Embrasures that are too large provide little protection to the supporting structures as food is forced into the interproximal space by an opposing cusp (Fig. 1.35). A prime example is the failure to restore the distal cusp of a mandibular first molar when placing a restoration (Fig. 1.36). Lingual embrasures are usually larger than facial embrasures; and this allows more food to be displaced lingually because the tongue can return the food to the occlusal surface more easily than if the food is displaced facially into the buccal vestibule (see Fig. 1.32). The marginal ridges of adjacent posterior teeth should be at the same height to have proper contact and embrasure forms. When this relationship is absent, it may

A

B

Incisal embrasureOcclusal embrasureGingival embrasure

- gingivally. Incisal, occlusal, and gingival embrasures are indicated. A, Maxillary teeth. B, Mandibular teeth. Fig. 1.33 Proximal contact areas. Black lines show positions of contact incisogingivally and occluso- Contact area Soft tissue outline^ Col - Fig. 1.34 Relationship of ideal interdental papilla to molar contact area.

w y w y x z

- Fig. 1.35 Embrasure form.x w, Improper embrasure form caused byz overcontouring of restoration resulting in unhealthy gingiva from lack of stimulation. has resulted in decrease of embrasure dimension. form is good, supporting tissues receive adequate stimulation from foods x, Good embrasure form. y, Frictional wear of contact area z, When the embrasure during mastication. x y - underlying supporting tissue during mastication. establish adequate contour for good embrasure form. Fig. 1.36 Embrasure form. x, Portion of tooth that offers protection to y, Restoration fails to

and the condyle make up the superior border of each ramus. The mandible initially contains 10 mandibular primary teeth and later 16 mandibular permanent teeth in the alveolar process. Maxillary and mandibular bones comprise approximately 38% to 43% inorganic material and 34% organic material by volume. The inorganic material is hydroxyapatite, and the organic material is primarily type I collagen, which is surrounded by a ground substance of glycoproteins and proteoglycans.

Oral Mucosa The oral mucosa is the mucous membrane that covers all oral structures except the clinical crowns of teeth. It is composed of two layers: (1) the stratified squamous epithelium and (2) the

supporting connective tissue, called propria of the gingiva in Fig. 1.38, may be keratinized, parakeratinized, or nonkeratinized, depending on its location. The lamina propria varies in thickness and supports lamina propria indicator 8 .) The epithelium. (See the lamina the epithelium. It may be attached to the periosteum of alveolar bone, or it may be interposed over the submucosa, which may vary in different regions of the mouth (e.g., the floor of the mouth, the soft palate). The submucosa, consisting of connective tissues varying in density and thickness, attaches the mucous membrane to the underlying bony structures. The submucosa contains glands, blood vessels, nerves, and adipose tissue. Oral mucosa is classified into three major functional types: (1) masticatory mucosa, (2) lining or reflective mucosa, and (3) special ized mucosa. The masticatory mucosa comprises the free and attached gingiva (see Fig. 1.38, indicators 6 and 9 ) and the mucosa-

cause an increase in the problems associated with inadequate proximal contacts and faulty embrasure forms. in function maintains masticatory efficiency throughout life (see Preservation of the curvatures of opposing cusps and surfaces Fig. 1.2). Correct anatomic form renders teeth more self-cleansing because of the smoothly rounded contours that are more exposed to the cleansing action of foods and fluids and the frictional movement of the tongue, lips, and cheeks. Failure to understand

and adhere to correct anatomic form may contribute to the breakdown of the restored system (Fig. 1.37). Maxilla and Mandible

The human maxilla is formed by two bones, the maxilla proper and the premaxilla. These two bones form the bulk of the upper jaw and the major portion of the hard palate and help form the floor of the orbit and the sides and base of the nasal cavity. They contain 10 maxillary primary teeth initially and later contain 16 maxillary permanent teeth in the alveolar process (see Figs. 1.1 and 1.3, label 7). The mandible, or the lower jaw, is horseshoe shaped and relates to the skull on either side via the TMJs. The mandible is composed of a body of two horizontal portions joined at the midline symphysis mandibulae and the rami, the vertical parts. The coronoid process

A

B

- contact/amalgam gingival excess and resultant vertical osseous loss. B, Fig. 1.37 PoorC anatomic restorative form. A, Radiograph of flat Radiograph of restoration with amalgam gingival excess and absence of contact resulting in osseous loss, adjacent root caries. C, Poor embrasure form and restoration margins.

(^12) (^345) (^678) 10119 (^121314)

- structures: margin; 9 (^) , attached gingiva; Fig. 1.38 6 , free gingiva; 1 (^) , enamel;Vertical section of a maxillary incisor illustrating supporting 10 2 7 , dentin;, mucogingival junction;, free gingival groove; 3 , pulp; 4 , gingival sulcus; 8 , lamina propria of gingiva; 11 , periodontal ligament; 5 , free gingival 12 , alveolar bone; 13 , cementum; 14 , alveolar mucosa.

CHAPTER 1 Clinical Significance of Dental Anatomy, Histology, Physiology, and Occlusio unless dictated by caries, previous restoration, esthetics, or other n 15

preparation requirements. Attachment Apparatus The tooth root is attached to the alveolus (bony socket) by the

periodontal ligament (see Fig. 1.38, connective tissue containing numerous cells, blood vessels, nerves, and an extracellular substance consisting of fibers and ground substance. Most of the fibers are collagen, and the ground substance indicator 11 ), which is a complex is composed of a variety of proteins and polysaccharides. The periodontal ligament serves the following functions: (1) attachment and support, (2) sensory, (3) nutritive, and (4) homeostatic. Bundles of collagen fibers, known as principal fibers of the ligament , serve to connect between cementum and alveolar bone so as to suspend and support the tooth. Coordination of masticatory muscle function is achieved, through an efficient proprioceptive mechanism, by the sensory nerves located in the periodontal ligament. Blood vessels supply the attachment apparatus with nutritive substances. Specialized cells of the ligament function to resorb and replace cementum, the periodontal ligament, and alveolar bone. The alveolar process—a part of the maxilla and the mandible— forms, supports, and lines the sockets into which the roots of teeth fit. Anatomically, no distinct boundary exists between the body of the maxilla or the mandible and the alveolar process. The alveolar process comprises thin, compact bone with many small openings through which blood vessels, lymphatics, and nerves pass. The inner wall of the bony socket consists of the thin lamella of bone that surrounds the root of the tooth and is termed proper. The second part of the bone is called supporting alveolar alveolar bone bone Supporting bone is composed of two parts: (1) the cortical plate, consisting of compact bone and forming the inner (lingual) and outer (facial) plates of the alveolar process, and (2) the spongy, which surrounds and supports the alveolar bone proper.

base that fills the area between the plates and the alveolar bone proper. Occlusion

Occlusion the contact of teeth in opposing dental arches when the jaws are closed (static occlusal relationships) and during various jaw move ments (dynamic occlusal relationships). The size of the jaw and literally means “closing”; in dentistry, the term means- the arrangement of teeth within the jaw are subject to a wide range of variation. The locations of contacts between opposing teeth (occlusal contacts) vary as a result of differences in the sizes and shapes of teeth and jaws and the relative position of the jaws. A wide variety of occlusal schemes are found in healthy individuals. Consequently, definition of an ideal occlusal scheme is fraught with difficulty. an ideal occlusal scheme, but these descriptions are so restrictive (^11) Repeated attempts have been made to describe that few individuals can be found to fit the criteria. Failing to find a single adequate definition of an ideal occlusal scheme has resulted in the conclusion that “in the final analysis, optimal function and the absence of disease is the principal characteristic of a good occlusion.” conform to the concepts of normal, or usual, occlusal schemes and include common variations of tooth-and-jaw relationships. The masticatory system (muscles, TMJs, and teeth) is highly^11 The dental relationships described in this section adaptable and usually able to successfully function over a wide range of differences in jaw size and tooth alignment. Despite this great adaptability, however, some patients are highly sensitive to changes in tooth contacts (which influence the masticatory muscles

of the hard palate. The epithelium of these tissues is keratinized, and the lamina propria is a dense, thick, firm connective tissue containing collagen fibers. The hard palate has a distinct submucosa except for a few narrow specific zones. The dense lamina propria of the attached gingiva is connected to the cementum and peri osteum of the bony alveolar process (see Fig. 1.38, cheek, and vestibule, the lateral surfaces of the alveolar process The lining or reflective mucosa covers the inside of the lips, indicator 8 ).- (except the mucosa of the hard palate), the floor of the mouth, the soft palate, and the ventral surface of the tongue. The lining mucosa is a thin, movable tissue with a relatively thick, nonkera tinized epithelium and a thin lamina propria. The submucosa- comprises mostly thin, loose connective tissue with muscle and collagenous and elastic fibers, with different areas varying from one another in their structures. The junction of the lining mucosa and the masticatory mucosa is the mucogingival junction, located at the apical border of the attached gingiva facially and lingually in the mandibular arch and facially in the maxillary arch (see Fig. 1.38, the tongue and the taste buds. The epithelium is nonkeratinized indicator 10 ). The specialized mucosa covers the dorsum of

except for the covering of the dermal filiform papillae. Periodontium

The periodontium consists of the oral hard and soft tissues that invest and support teeth. It may be divided into (1) the gingival unit, consisting of free and attached gingiva and the alveolar mucosa, and (2) the attachment apparatus, consisting of cementum, the

periodontal ligament, and the alveolar process (see Fig. 1.38). Gingival Unit As mentioned, the free gingiva and the attached gingiva together

form the masticatory mucosa. The free gingiva is the gingiva from the marginal crest to the level of the base of the gingival sulcus (see Fig. 1.38, between the tooth and the free gingiva. The outer wall of the indicators 4 and 6 ). The gingival sulcus is the space sulcus (inner wall of the free gingiva) is lined with a thin, nonke ratinized epithelium. The outer aspect of the free gingiva in each gingival embrasure is called gingival groove is a shallow groove that runs parallel to the marginal gingival or interdental papilla. The free- crest of the free gingiva and usually indicates the level of the base of the gingival sulcus (see Fig. 1.38, stratified, squamous epithelium, extends from the depth of the The attached gingiva, a dense connective tissue with keratinized, indicator 7 ). gingival sulcus to the mucogingival junction. A dense network of collagen fibers connects the attached gingiva firmly to cementum and the periosteum of the alveolar process (bone). The alveolar mucosa is a thin, soft tissue that is loosely attached to the underlying alveolar bone (see Fig. 1.38, (^14) underlying submucosa contains loosely arranged collagen fibers, elastic tissue, fat, and muscle tissue. The alveolar mucosa is delineated). It is covered by a thin, nonkeratinized epithelial layer. The indicators 12 and from the attached gingiva by the mucogingival junction and continues apically to the vestibular fornix and the inside of the cheek. Clinically, the level of the gingival attachment and gingival sulcus is an important factor in restorative dentistry. Soft tissue health must be maintained by teeth having the correct anatomic form and position to prevent recession of the gingiva and possible abrasion and erosion of the root surfaces. The margin of a tooth preparation should not be positioned subgingivally (at levels between the marginal crest of the free gingiva and the base of the sulcus)

16 CHAPTER 1 Clinical Significance of Dental Anatomy, Histology, Physiology, and Occlusio contact , maximum closure , and n maximum habitual intercuspation (MHI) line and the maxillary central fossa occlusal line coincide exactly. The maxillary lingual occlusal line and the mandibular central In Fig. 1.39C (proximal view), the mandibular facial occlusal. fossa occlusal line identified in Fig. 1.39A also are coincidental. The cusps that contact opposing teeth along the central fossa occlusal line are termed holding, or stamp cusps); the cusps that overlap opposing teeth functional cusps (synonyms include supporting, are termed or nonholding cusps). The mandibular facial occlusal line identifies the mandibular functional cusps, whereas the maxillary facial cusps are nonfunctional cusps. These terms are usually applied only to nonfunctional cusps (synonyms include nonsupporting posterior teeth to distinguish the functions of the two rows of cusps. In some circumstances, the functional role of the cusps may be reversed, as illustrated in Fig. 1.40C.2. Posterior teeth are well suited to crushing food because of the mutual cusp–fossa contacts (Fig. 1.41D). ship in MI, but they also show the characteristic maxillary overlap. Incisors are best suited to shearing food because of their overlap In Fig. 1.39D, anterior teeth are seen to have a different relation- and the sliding contact on the lingual surface of maxillary teeth. In MI, mandibular incisors and canines contact the respective lingual surfaces of their maxillary opponents. The amount of horizontal (overjet) and vertical (overbite) overlap (see Fig. 1.40A.2) can significantly influence mandibular movement and the cusp design of restorations of posterior teeth. Variations in the growth and development of the jaws and in the positions of anterior teeth may result in open bite, in which vertical or horizontal discrepancies

prevent teeth from contacting (see Fig. 1.40A.3). Anteroposterior Interarch Relationships In Fig. 1.39E, the cusp interdigitation pattern of the first molar

teeth is used to classify anteroposterior arch relationships using a system developed by Angle. tooth cusps and fossae guide the teeth into maximal contact. Three interdigitated relationships of the first molars are commonly (^13) During the eruption of teeth, the observed. See Fig. 1.39F for an illustration of the occlusal contacts that result from different molar positions. The location of the mesiofacial cusp of the maxillary first molar in relation to the mandibular first molar is used as an indicator in Angle classification. The most common molar relationship finds the maxillary mesiofacial cusp located in the mesiofacial developmental groove of the mandibular first molar. This relationship is termed Angle Class I. Slight posterior positioning of the mandibular first molar results in the mesiofacial cusp of the maxillary molar settling into the facial embrasure between the mandibular first molar and the mandibular second premolar. This is termed Class II and occurs in approximately 15% of the U.S. population. Anterior positioning of the mandibular first molar relative to the maxillary first molar is termed Class III and is the least common. In Class III relation ships, the mesiofacial cusp of the maxillary first molar fits into the distofacial groove of the mandibular first molar; this occurs in- approximately 3% of the U.S. population. Significant differences in these percentages occur in people in other countries and in different ethnic groups. Although Angle classification is based on the relationship of the cusps, Fig. 1.39G illustrates that the location of tooth roots in alveolar bone determines the relative positions of the crowns and cusps of teeth. When the mandible is proportionally similar in size to the maxilla, a Class I molar relationship is formed; when the mandible is proportionally smaller than the maxilla, a Class

and TMJs), which may be brought about by orthodontic and restorative dental procedures. Static occlusion is defined further by the use of reference positions Occlusal contact patterns vary with the position of the mandible. that include fully closed, terminal hinge (TH) closure, retruded, protruded, and right and left lateral extremes. The number and location of occlusal contacts between opposing teeth have important effects on the amount and direction of muscle force applied during mastication and other parafunctional activities such as mandibular clenching, tooth grinding, or a combination of both (bruxism). In extreme cases, these forces damage the teeth and/or their sup porting tissues. Forceful tooth contact occurs routinely near the- limits or borders of mandibular movement, showing the relevance of these reference positions. occlusal relationship Tooth contact during mandibular movement is termed. Gliding or sliding contacts occur during (^12) dynamic mastication and other mandibular movements. Gliding contacts may be advantageous or disadvantageous, depending on the teeth involved, the position of the contacts, and the resultant masticatory muscle response. The design of the restored tooth surface will have important effects on the number and location of occlusal contacts, and both static and dynamic relationships must be taken into consideration. The following sections discuss common arrangements and variations of teeth and the masticatory system. Mastication

and the contacting relationships of anterior and posterior teeth are described with reference to the potential restorative needs of teeth. General Description

Tooth Alignment and Dental Arches In Fig. 1.39A, the cusps have been drawn as blunt, rounded, or pointed projections of the crowns of teeth. Posterior teeth have one, two, or three cusps near the facial and lingual surfaces of

each tooth. Cusps are separated by distinct developmental grooves and sometimes have additional supplemental grooves on cusp inclines. Facial cusps are separated from the lingual cusps by a deep groove, termed central groove. If a tooth has multiple facial cusps or multiple lingual cusps, the cusps are separated by facial or lingual developmental grooves. The depressions between the cusps are termed aligned in a smooth curve. Usually, the maxillary arch is larger fossae (singular, fossa ). Cusps in both arches are than the mandibular arch, which results in maxillary cusps overlap ping mandibular cusps when the arches are in maximal occlusal contact (see Fig. 1.39B). In Fig. 1.39A, two curved lines have been drawn over the teeth to aid in the visualization of the arch form.- These curved lines identify the alignment of similarly functioning cusps or fossae. On the left side of the arches, an imaginary arc connecting the row of facial cusps in the mandibular arch have been drawn and labeled facial occlusal line. Above that, an imaginary line connecting the maxillary central fossae is labeled occlusal line central fossa occlusal line coincide exactly when the mandibular arch is fully closed into the maxillary arch. On the right side of. The mandibular facial occlusal line and the maxillary central fossa the dental arches, the maxillary lingual occlusal line and mandibular central fossa occlusal line have been drawn and labeled. These lines also coincide when the mandible is fully closed. In Fig. 1.39B, the dental arches are fully interdigitated, with maxillary teeth overlapping mandibular teeth. The overlap of the maxillary cusps may be observed directly when the jaws are closed. Maximum intercuspation (MI) when teeth are brought into full interdigitation with the maximal refers to the position of the mandible number of teeth contacting. Synonyms for MI include intercuspal

Central fossa line

Right side

F. Molar Classes I , II , andFacial occlusal line III relationships

C. Molar view

A. Dental arch cusp and fossa alignment B.in opposing arches are in maximal contact^ Maximum intercuspation (MI): the teeth

D. Incisor view

E. Facial view of anterior-posterior variations

1. The maxillary lingual occlusal line and themandibular central fossa line are coincident.2. The mandibular facial occlusal line and themaxillary central fossa line are coincident.

G. Skeletal Classes I , II , and III relationships

Lingualocclusal line

Maxilla Left

Mandible

Central fossa line Central fossa line

Lingual occlusal line Facial occlusal line

Right

Centralfossa line Class I Class II Class III

Class I

Class I

Class II

Class III

Class II Class III

- Fig. 1.39 Dental arch relationships.

The overlap is characterized in two dimensions: (1) horizontal overlap (overjet) and (2) vertical overlap (overbite). Differences in the sizes of the mandible and the maxilla can result in clinically significant variations in incisor relationships, including open bite as a result of mandibular deficiency or excessive eruption of posterior teeth, and crossbite as a result of mandibular growth excess (see

II relationship is formed; and when the mandible is relatively greater than the maxilla, a Class III relationship is formed. Interarch Tooth Relationships

Fig. 1.40 illustrates the occlusal contact relationships of individual teeth in more detail. In Fig. 1.40A.2, incisor overlap is illustrated.

Horizontal(overjet)overlap Open bite (mandibulardeficiency)

A.2 Incisor relationships Vertical overlap(overbite)

A.3 Variations in incisor relationships

B.2 Variations in premolar relationships

C.2 Variations in molar relationships

Tooth-to-toothcusp marginalridge

Facial-lingual Normal^ crossbiteFacial longitudinalsection

Mesial-distallongitudinalsection

C.1 Molar relationships

B.1 Premolar relationships

Transverse archrelationships

Proximal view crossbiteLingual

Tooth-to-two-toothcusp marginalridge Tooth-to-toothcusp fossa

posterior teeth)(excessiveeruption ofOpen bite (mandibularCrossbiteexcess)growth

A.

- Fig. 1.40 Tooth relationships.

Cusp ridge Outer inclinesInner inclines Marginal ridgeEach cusp has four ridges:1. Outer incline (facial or lingual ridge)2. Inner incline (triangular ridge)3. Mesial cusp ridge4. Distal cusp ridge

Outer inclinesFacial cusp ridges

Mesial and distal triangular fossae

(2) Inner inclinesTriangular ridges (3)Cusp ridgesMesial

Major developmental grooves separate cusps

Supplemental grooves on inner inclines

Drawing conventions: the height of themarginal ridges and cusp ridges aremarked with a circumferential line thatoutlines the occlusal table.

Cusp ridge names:1. Outer inclines are named for their surface.2. Inner inclines are triangular ridges named3. Cusp ridges are named for their direction.for cusp.

Pattern of cusps and grooves aresimilar to mortar and pestle forcrushing food. Mesial and distal triangular fossaedefine marginal ridges and sharpenocclusal contacts. Supplemental grooves widenpathways for opposing cuspmovement.

B

A

C

D

E

F

(1) (1) (2) (3)

- Fig. 1.41 Common features of all posterior teeth.

20 CHAPTER 1 Clinical Significance of Dental Anatomy, Histology, Physiology, and Occlusio characteristic facial and lingual profiles of the cusps as viewed from n the facial or lingual aspect. At the base of the cusp, the mesial or distal cusp ridge abuts to another cusp ridge, forming a develop mental groove, or the cusp ridge turns toward the center line of the tooth and fuses with the marginal ridge. Marginal ridges are- elevated, the rounded ridges being located on the mesial and distal edges of the tooth’s occlusal surface (see Fig. 1.41A). The occlusal table of posterior teeth is the area contained within the mesial and distal cusp ridges and the marginal ridges of the tooth. The occlusal table limits are indicated in the drawings by a circumferential line connecting the highest points of the curvatures of the cusp ridges and marginal ridges. The unique shape of cusps produces the characteristic form of individual posterior teeth. The mandibular first molars have longer triangular ridges on the distofacial cusps, causing a deviation of the central groove (see Fig. 1.41B.2). The mesiolingual cusp of a maxillary molar is much larger than the mesiofacial cusp. The distal cusp ridge of the maxillary first molar mesiolingual cusp curves facially to fuse with the triangular ridge of the distofacial cusp (see Fig. 1.41C.2). This junction forms the oblique ridge, which is characteristic of maxillary molars. The transverse groove

crosses the oblique ridge where the distal cusp ridge of the mesio lingual cusp meets the triangular ridge of the distofacial cusp. Functional Cusps -

In Fig. 1.42, the lingual occlusal line of maxillary teeth and the facial occlusal line of mandibular teeth mark the locations of the functional cusps. These cusps contact opposing teeth in their corresponding faciolingual center on a marginal ridge or a fossa. Functional cusp–central fossa contact has been compared to a mortar and pestle because the functional cusp cuts, crushes, and grinds fibrous food against the ridges forming the concavity of the fossa (see Fig. 1.41D). Natural tooth form has multiple ridges and grooves ideally suited to aid in the reduction of the food bolus during chewing. During chewing, the highest forces and the longest duration of contact occur at MI. Functional cusps also serve to prevent drifting and passive eruption of teeth—hence the term holding cusp five characteristic features: 1. 2. They contact the opposing tooth in MI.They maintain the vertical dimension of the face.. The functional cusps (see Fig. 1.42) are identified by 14

3. 4. 5. They are nearer the faciolingual center of the tooth than nonfunctional cusps.Their outer (facial) incline has the potential for contact.They have broader, more rounded cusp ridges with greater dentin the functional cusps are located on the maxillary lingual occlusal line (see Fig. 1.42D), whereas the mandibular functional cusps^ support than nonfunctional cusps.^ Because the maxillary arch is larger than the mandibular arch, are located on the mandibular facial occlusal line (see Fig. 1.42A and B). Functional cusps of both arches are more robust and better suited to crushing food than are the nonfunctional cusps. The lingual tilt of posterior teeth increases the relative height of the functional cusps with respect to the nonfunctional cusps (see Fig. 1.42C), and the central fossa contacts of the functional cusps are obscured by the overlapping nonfunctional cusps (see Fig. 1.42E and F). A schematic showing removal of the nonfunctional cusps allows the functional cusp–central fossa contacts to be studied (see Fig. 1.42G and H). During fabrication of restorations, it is important that functional cusps are not contacting opposing teeth in a manner that results in lateral deflection. Rather, restorations should provide contacts on plateaus or smoothly concave fossae

Fig. 1.40A.3). These variations have significant clinical effects on the contacting relationships of posterior teeth and resultant mastica tory activity during various jaw movements because the anterior teeth are not contributing to mandibular guidance. - each mandibular premolar is located one half of a tooth width anterior to its maxillary antagonist. This relationship results in the mandibular facial cusp contacting the maxillary premolar mesial^ Fig. 1.40B.1 illustrates a normal Class I occlusion, in which marginal ridge and the maxillary premolar lingual cusp contacting the mandibular distal marginal ridge. Because only one antagonist is contacted, this is termed stable maxillary/mandibular tooth relationship results from the tooth-to-tooth relationship. The most contact of the functional cusp tips against the two marginal ridges, termed position of teeth produce different relationships (see Fig. 1.40B.2). When the mandible is slightly distal to the maxilla (termed Class tooth-to-two-tooth contact. Variations in the mesiodistal root II tendency), each functional cusp tip occludes in a stable relation ship with the opposing mesial or distal fossa; this relationship is a cusp–fossa contact. Fig. 1.40C illustrates Class I molar relationships in more detail.- Fig. 1.40C.1 shows the mandibular facial cusp tips contacting the maxillary marginal ridges and the central fossa triangular ridges. A faciolingual longitudinal section reveals how the functional cusps contact the opposing fossae and shows the effect of the develop- mental grooves on reducing the height of the nonfunctional cusps opposite the functional cusp tips. During lateral movements, the functional cusp is able to move through the facial and lingual developmental groove spaces without contact. Faciolingual position variations are possible in molar relationships because of differences in the growth of the width of the maxilla or the mandible. crossbite, and lingual crossbite relationships. Facial crossbite in Fig. 1.40C.2 illustrates the normal molar contact position, facial posterior teeth is characterized by the contact of the maxillary facial cusps in the opposing mandibular central fossae and the mandibular lingual cusps in the opposing maxillary central fossae. Facial crossbite (also termed buccal crossbite ) results in the reversal of roles of the cusps of the involved teeth. In this reversal example, the mandibular lingual cusps and maxillary facial cusps become functional cusps, and the maxillary lingual cusps and mandibular facial cusps become nonfunctional cusps. Lingual crossbite results

in a poor molar relationship that provides little functional contact. Posterior Cusp Characteristics Four cusp ridges may be identified as common features of all cusps.

The outer incline of a cusp faces the facial (or the lingual) surface of the tooth and is named for its respective surface. In the example using a mandibular second premolar (see Fig. 1.41A), the facial cusp ridge of the facial cusp is indicated by the line that points to the outer incline of the cusp. The inner inclines of the posterior cusps face the central fossa or the central groove of the tooth. The inner incline cusp ridges are widest at the base and become narrower as they approach the cusp tip. For this reason, they are termed triangular ridges mandibular premolar is indicated by the arrow to the inner incline. Triangular ridges are usually set off from the other cusp ridges by one or more supplemental grooves. In Fig. 1.41B.1 and C.1, the. The triangular ridge of the facial cusp of the outer inclines of the facial cusps of the mandibular and maxillary first molars are highlighted. In Fig. 1.41B.2 and C.2, the triangular ridges of the facial and lingual cusps are highlighted. Mesial and distal cusp ridges extend from the cusp tip mesially and distally and are named for their directions. Mesial and distal cusp ridges extend downward from the cusp tips, forming the

20°

Synonyms forfunctionalcusps include:1. Centric cusps2. Holding cusps

3. Stamp cusps

The mandibular arch is smaller thanthe maxillary arch, so the functionalcusps are located on the facial occlusalline. The mandibular lingual cusps thatoverlap the maxillary teeth are nonsupporting cusps.

Facialocclusal line Facialocclusal line

Mandibularfunctionalcusp inopposingmaxillary fossa Lingual occlusal line

Maxillary functional cusps occluding inopposing fossae and on marginal ridges Mandibular functional cusps occluding inopposing fossae and on marginal ridges

Mandibular functional cusps are locatedon the facial occlusal line.

Maxillaryfunctional cuspin opposingmandibularfossa Functional cusps are located on thelingual occlusal line in maxillary arch.

A. Mandibular arch (^) B. Mandibular right quadrant

C.teeth in occlusion Proximal view of molar D.^ Maxillary right quadrant

E.occlusion Lingual view of left dental arches in F.occlusion^ Facial view of left dental arches in G.removed Mandibular nonfunctional cusps H.^ Maxillary nonfunctional cusps removed

Functional cusp features:1. Contact opposing tooth in MI2. Support vertical dimension3. Nearer faciolingual center oftooth than nonsupporting

4. Outer incline has potential for5. More rounded thancuspscontactnonsupporting cusps - Fig. 1.42 Functional cusps.

22 CHAPTER 1 Clinical Significance of Dental Anatomy, Histology, Physiology, and Occlusio anteroposterior, providing sliding movement between the disc and n the glenoid fossa. One condyle may move anteriorly, while the other remains in the fossa. Anterior movement of only one condyle produces reciprocal lateral rotation in the opposite TMJ. The TMJ does not behave like a rigid joint as those on articulators (mechanical devices used by dentists to simulate jaw movement and reference positions [see the subsequent section on and Mandibular Movement articulating bones and an intervening disc composed of soft tissues]). Because soft tissues cover the two Articulators is present, some resilience is to be expected in the TMJs. In addition to resilience, normal, healthy TMJs have flexibility, allowing small posterolateral movements of the condyles. In healthy TMJs, the movements are restricted to slightly less than 1 mm laterally and a few tenths of a millimeter posteriorly. of a TMJ because of disease, the disc–condyle relationship is possibly altered in many ways, including distortion, perforation, or tearing When morphologic changes occur in the hard and soft tissues of the disc, and remodeling of the soft tissue articular surface coverings or their bony support. Diseased TMJs have unusual disc–condyle relationships, different geometry, and altered jaw movements and reference positions. Textbooks on TMJ disorders and occlusion should be consulted for information concerning the evaluation of diseased joints. of the movement and position of the mandible is based on normal, healthy TMJs and does not apply to diseased joints. (^15) The remainder of this discussion

Review of Normal Masticatory Muscle Function and Mandibular Movement Masticatory muscles work together to allow controlled, subtle movements of the mandible. The relative amount of muscle activity

depends on the interarch relationships of maxillary and mandibular teeth as well as the amount of resistance to movement. muscles involved in mandibular movements include the anterior temporalis, middle temporalis, posterior temporalis, superficial16-19 (^) Primary masseter, deep masseter, superior lateral pterygoid, inferior lateral pterygoid, medial pterygoid, and digastric muscles. suprahyoid, infrahyoid, mylohyoid, and geniohyoid muscles also are involved in mandibular movements but not usually included17,18,20 (^) The in routine clinical examinations. activity of the various muscles has been identified through the use of electromyographic technology, in which electrodes were placed in the evaluated muscles,17,18,22 (^) as well as on the skin immediately18,21^ The relative amount of muscle adjacent to the muscles of interest. dimensional arrangement of the muscles and the corresponding force vectors allow for the complete range of finely controlled mandibular movements. The reader should consult an appropriate12,17,18,20,21-30^ The strategic three- human anatomy textbook to identify the location, size, shape, three-dimensional orientation, and bony insertion of the various muscles discussed in this section. Simple jaw opening requires the activation of digastric and inferior lateral pterygoid muscles. accomplished by simultaneous mild antagonistic activity of the medial pterygoid. mild masseter activation allows further stabilization and fine17,18 (^) When resistance is applied to jaw opening,17,18,22^ Fine control of opening is control. pterygoid. middle, and posterior) muscles activate as well. Jaw closure requires the activation of the masseter and medial17,18 (^18) Once teeth come into contact, the temporalis (anterior,17,18 (^) The masseter, medial pterygoid, and temporalis muscles act to elevate the mandible and are generally referred to as maximum activation of the masseter and temporalis, moderate activation of the medial pterygoid and superior lateral pterygoid, elevator muscles. Clenching involves

so that masticatory forces are directed approximately parallel to the long axes of teeth (i.e., approximately perpendicular to the occlusal plane).

Nonfunctional Cusps Fig. 1.43 illustrates that the nonfunctional cusps form a lingual occlusal line in the mandibular arch (see Fig. 1.43D) and a facial occlusal line in the maxillary arch (see Fig. 1.43B). Nonfunctional

cusps overlap the opposing tooth without contacting the tooth. Nonfunctional cusps are located, when viewed in the anteroposterior plane, in facial (lingual) embrasures or in the developmental groove of opposing teeth, creating an alternating arrangement when teeth are in MI (see Fig. 1.43E and F). The maxillary premolar non functional cusps also play an essential role in esthetics. In the occlusal view, the nonfunctional cusps are farther from the facio lingual center of the tooth than are the functional cusps and have-- less dentinal support. Nonfunctional cusps have sharper cusp ridges that may serve to shear food as they pass close to the functional cusp ridges during chewing strokes. The overlap of the maxillary nonfunctional cusps helps keep the soft tissue of the cheek out and away from potential trauma from the occlusal table. Likewise, the overlap of the mandibular nonfunctional cusps helps keep the tongue out from the occlusal table. Therefore, the position of the maxillary and mandibular nonfunctional cusps help to prevent

self-injury during chewing. Mechanics of Mandibular Motion

Mandible and Temporomandibular Joints The mandible articulates with a depression in each temporal bone called (TMJs) glenoid fossa because they are named for the two bones (temporal and. The joints are termed temporomandibular joints

mandible) forming the articulation. The TMJs allow the mandible to move in all three planes (Fig. 1.44A). a true mechanical ball-and-socket joint in some very important A TMJ is similar to a ball-and-socket joint, but it differs from aspects. The ball part (the mandibular condyle) is smaller than the socket (the glenoid fossa) (see Fig. 1.44B). The space resulting from the size difference is filled by a tough, pliable, and movable stabilizer termed the articular disc. The disc separates the TMJ into two articulating surfaces lubricated by synovial fluid in the superior and inferior joint spaces. Rotational opening of the mandible occurs as the condyles rotate under the discs (see Fig. 1.44C). Rotational movement occurs between the inferior surface of the discs and the condyle. During wide opening or protrusion of the mandible, the condyles move or slide anteriorly in addition to the rotational opening (see Fig. 1.44D and E). The TMJ is referred to as a ginglymoarthrodial joint because it has hinge (ginglymus) capability as well as sliding/gliding/translating (arthro dial) capability. and produce a sliding movement in the superior joint space between The discs move anteriorly with the condyles during opening- the superior surface of the discs and the articular eminences (see Fig. 1.44B). TMJs allow free movement of the condyles in the anteroposterior direction but resist lateral displacement. The discs are attached firmly to the medial and lateral poles of the condyles in normal, healthy TMJs (see Fig. 1.45B). The disc–condyle arrangement of the TMJ allows simultaneous sliding and rotational movement in the same joint. Because the mandible is a semirigid, U-shaped bone with joints on both ends, movement of one joint produces a reciprocal move ment in the other joint. The disc–condyle complex is free to move-

The maxillary arch is larger than themandibular arch causing the maxillaryfacial line (nonfunctional cusps) tooverlap the mandibular teeth.

Facialocclusal line

cusp overlappingmaxillary toothnonfunctionalMandibular 20° E.showing interdigitation of nonfunctional cusps Views of left dental arches in occlusion

Nonfunctional cusp location:1. Opposing embrasure2. Opposing developmental groove

D. Mandibular left quadrant Maxillarynonfunctionalcusp overlappingmandibular tooth (^) Nonfunctional cusp features:

1. Do not contact opposing tooth2. Keep soft tissue of tongue or3. Farther from faciolingual centerin MIcheek off occlusal table
2. Outer incline has no potential5. Have sharper cusp ridges thanof tooth than supporting cuspsfor contactsupporting cusps F. (^) showing facial and lingual occlusal linesViews of left dental arches in occlusion

Mandibular nonfunctional cuspsare located on the lingual occlusal line.Lingual occlusal line

Maxillary nonfunctional cusps arelocated on the facial occlusal line.

1

1 2 2

A. Maxillary arch

C. Molar teeth in occlusion

B. Maxillary left quadrant

- Fig. 1.43 Nonfunctional cusps.

Lateral movement isapproximately 10 mm.

C Rotation about an axis

D Translation

E Complex

FLeft lateral movement

Maximum opening isapproximately 50 mm.

The mandible can protrudeapproximately 10 mm.

Hinge opening produces about 25 mmof separation of the anterior teeth.

Mandibular opening: Hingeopening

Maximumopening

Protrusion

Translatingcondyle Rotatingcondyle Translatingcondyle Rotatingcondyle NW W NW W

WNW   working side nonworking side

A ParasagittalTransverse(horizontal) Temporomandibular joint sagittal section Coronal(frontal)

Superior joint spaceArticular discB Midsagittal Articular eminenceInferior joint space Glenoid fossaCondyle Externalauditorymeatus Lateral pterygoid muscle:Superior headInferior head

- Fig. 1.44 Types and directions of mandibular movements.

(CRO)^ MI

b

B. Frontal view

Determination of sagittal borders:Superior - tooth contactPosterior - joint ligamentsInferior - muscle lengtheningAnterior - joint ligaments

c 10 mm, limit of protrusion Posselt’sdiagram

CRO Limits of condyle motion:10-12 mm anterior to CR0.2 mm posterior to CR5-6 mm vertical displacementdue to curvature of eminence

Normal TMJ flexibilityallows up to 1.5 mm oflateral shifting (Bennett shift).

10-12 mm5-6 mm TH, rotationalmotion of condylesa 25 mm, limit of rotational opening Advancing condyles b 50 mm, limit of opening

Left TMJ, sagittal section

Medial .75 mm^ .75 mm pole Lateralpole d e

Left TMJ, horizontal view0.75 mm 10 mm d (^) e Condyle motion:0.75 mm left/right10-12 mm, limit of protrusionanterior/posterior c

MI (CRO)

Right Left

C. Horizontal view Borders are arcs of circlesbased on rotation of thecondyles in retruded andprotruded positions.

d 10 mm rightlateral jaw movement

Superior borderdetermined by toothcontact (canine guidance). e 10 mm leftlateral jaw movement

Articular eminence

Right Left Left TMJ, frontal section

A. Sagittal view

- grams.) CO Fig. 1.45 (^) =MI (i.e., there is no functional shift and, therefore, is termed centric relation occlusion [CRO]).Capacity of mandibular movement. (Mandible drawings are not to scale with border dia-