GENERAL CONSIDERATIONS IN THE PHYSIOLOGY OF PERMANENT DENTITION www.indiandentalacademy.com
INDIAN DENTAL ACADEMY Leader in continuing dental education www.indiandentalacademy.com
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INTRODUCTION : Physiology is a study of the function of the organs and parts during life, as distinct from anatomy , which deals with there structure. The teeth are peculiar biologically in that their anatomic (macroscopic) and physiologic features are so closely merged that it is difficult to discuss anatomy without discussing function. Form and function are two expressions which imply one term when the clinical crown is discussed. Therefore dental anatomy and physiology are treated as one subject. www.indiandentalacademy.com
The relationship between the form of the teeth and function is usually discussed in terms of type of food in the diet of humans, jaw movements, and protection of the periodontium and stimulation of the gingiva. It is also recognized that the teeth not only contribute to the digestion of food but also are important in speech and personal appearance. The primary function of the teeth is to prepare food for swallowing and to facilitate digestion. The teeth have their respective forms to facilitate prehension, incision, and trituration of food. www.indiandentalacademy.com
The dentition, joints, and muscles in humans have the form and alignment to enable the mastication of both animal and vegetable foods. This
type
of
dentition
is
referred
to
as
omnivorous. The shapes of incisal and occlusal surfaces of the teeth are related not only to the function they perform but also to the movements of the mandible required to carry out chewing of a variety of foods. www.indiandentalacademy.com
In contrast to the facts regarding many animals, only up and down jaw closure is possible because of the interlocking conical form of the teeth, temporomandibular joint (TMJ) morphology, and lack of muscles to carry out lateral movements. In order to understand more completely the form and function of teeth, the protective aspects of form will be considered and the functional relationships will be considered in terms of what might be thought of as evolutionary teleology and comparative anatomy.. www.indiandentalacademy.com
Comparative Dental Anatomy To understand the human dentition, it is helpful to compare the dentitions of other vertebrates. In doing so, it should become clear that the dentition in humans is different in many ways from other vertebrates in form and function. However, it should be equally clear that the presence of related characteristics in a wide range of vertebrates suggests a plan common to all. Starting with the primordial form of tooth, the single cone or lobe that was the forerunner of combinations of lobes forming the more complicated teeth found in highly developed animals and in the human being today. www.indiandentalacademy.com
Man, like all mammals, has two sets of teeth. The first set, comprising 20 teeth in all, is called the primary or deciduous dentition. These teeth first begin to appear in the oral cavity at a mean age of 6 months and the last one erupts into the mouth at about 28 Âą4 months. For approximately the next 4 years (from 2 to 6 years of age) the child must function with his 20 primary teeth. Then, commencing at 6 years, the first of the permanent teeth appears. From 6 until 12 years of age the primary teeth are replaced, in rather definite succession, by the permanent teeth. www.indiandentalacademy.com
Additional teeth appear until, by the age of 18 (or even as late as 25), the full complement of 32 permanent teeth is present in the mouth. There are, three periods of dentition in man: (1)the primary dentition (6 months to 6 years) (2) the mixed dentition (6 to 12 years) (3) the permanent dentition (12 + years). If man is assigned an average life span of 70 years, it is obvious that he spends only 6 percent of this time chewing with his primary teeth but 91 percent, if he is fortunate, masticating with his permanent dentition www.indiandentalacademy.com
Basic Features of the Teeth of Vertebrates Vertebrate teeth always develop as a result of the interaction between two tissues. One of these is the epithelium of the mouth lining, which is of ectodermal origin; the other is the underlying mesenchyme, which is of neural crest origin. In reptiles and mammals these give rise respectively to the enamel and the dentin. True enamel does not develop in most fish; instead, the surface of the tooth is composed of "enameloid," the highly calcified outer layer of dentin. www.indiandentalacademy.com
However, even in fish the epithelium forms a cap-like structure around the developing tooth, corresponding to the enamel organ of mammals. Inside the cavity of the enamel organ is the mesenchymal papilla, which becomes converted into dentin, beginning at its outer surface. Dentin is one of the oldest of vertebrate tissues. It has been recognized in fossil remains as far back as more than 400 million years ago. These early examples of dentin do not come from teeth but from skeletal plates (denticles) that formed a head shield in the Agnatha. www.indiandentalacademy.com
These ossicles of dentin were initially not dental tissues, but rather sensory and protective organs, which later in evolution migrated to the mouth region to become attached to newly developed bone from the branchial arches (the jaws), as found in the placoderms. The modern elasmobranchs also have toothlike structures in their skin, known as placoid scales. These fishes have the capacity to form teeth not only in the mouth but over the surface of the body. Placoid scales are also found in some teleosts. www.indiandentalacademy.com
Within the mouth, teeth are frequently found not only along the edges of the jaws but also on the palate Palatal teeth are usually present in bony fishes, in salamanders, in lizards, and in snakes, being borne on such bones as the vomers, palatines, and pterygoids www.indiandentalacademy.com
Moreover, many bony fish have pharyngeal teeth on the gill arches. In the mammals,teeth have become restricted to the premaxillary, maxillary,and dentary bones.Teeth, then, are primarily structures that develop in the skin or the mouth lining,and only secondarily do they become attached to underlying bones. www.indiandentalacademy.com
In the cartilaginous group (e.g., sharks, rays), the teeth are all alike (homodont), simple cusped in shape (haplodont) and are constantly replaced with successive generations (polyphyodont). In some bony fishes these primitive dental characteristics evolve into more complex variably shaped teeth (heterodont) with fewer replacement generations and more rigid attachment to the jaws. In other vertebrates, teeth generally form some attachment to bone, but the attachment can take a number of different forms. www.indiandentalacademy.com
In many teleosts the tooth becomes fused to the surface of the bone (ankylosis); this occurs also in lizards and snakes. In some lizards the side of the tooth fuses to the lingual side of the vertical edge of the maxilla or dentary.In some fish the teeth do not fuse to the underlying bone, but are connected by elastic fibers, so arranged that the tooth can be bent over as by a hinge, allowing prey to pass into the mouth, and then flipping up to prevent escape. Among living reptiles, only the crocodiles and alligators bear their teeth in sockets. www.indiandentalacademy.com
In these, as in mammals, the tooth is not fused to the bone but connected to it by a layer of periodontal fibers. The crocodilians also resemble mammals in that the base of the tooth is covered with a thin layer of cementum, a tissue resembling bone but lacking in blood vessels, and thus it is possible in these forms to speak of the root of the tooth as distinct from the crown. Only in the mammals do we find teeth provided with more than one root. www.indiandentalacademy.com
Attachments of Teeth The attachment of teeth to the underlying jaws may be categorized into four types: (1) Fibrous, (2) Hinged, (3) Ankylosis, (4) Gomphosis. The first three types are confined to non-mammalian dentitions,and the complex gomphotic attachment Characterizes mammalian dentitions. www.indiandentalacademy.com
Fibrous attachments are generally associated with polyphyodont dentitions, which allow for rapid replacement of successive generations of teeth. Examples are found in sharks and rays. Hinged attachments allow for the inward bending of teeth to facilitate swallowing of prey that are prevented from escaping by the up righting of hinged teeth. Examples occur in fishes such as cod, hake, angler, and pike. Ankylosis forms the firmest attachment of teeth to bone, because there is no intervening soft tissue. www.indiandentalacademy.com
Dentin and bone are so intimately fused that an intermediate osteodentin blends impercepti-bly into the dentin of the tooth and the bone of the jaw. Examples are found in barracuda, haddock, frogs, eels, and pythons Gomphosis attachments require
a
socketed
characteristic
of
root
in
mammalian
alveolar dentitions.
bone, The
development of specialized tissues of attachment, cementum and a fibrous periodontal ligament, signifies most gomphosis, as seen in mammals. www.indiandentalacademy.com
However, gomphotic dental attachments exist in some reptiles (crocodiles) and in some fish (pristis or sawfish) in which succeeding teeth erupt into the same sockets. In most vertebrates the process
of
forming
new
teeth
continues
throughout life (polyphyodont dentitions) so that the
animal
is
constantly
provided
with
replacements as the old teeth become worn or broken. www.indiandentalacademy.com
When teeth form a row along the edges of the jaw, the new teeth develop from the oral epithelium or the lamina on the lingual side of the old ones, and when the old teeth are shed the new teeth move buccally to take their places in the functional row. In sharks there are several rows of functional teeth, and lingually several more rows are at various stages of development . New teeth appear at the deep edge of the lamina and move buccally as they develop. www.indiandentalacademy.com
Shark's jaw: the imbricated layers of successive generations (polyphyodont) of haplodont, homodont teeth. The layers move into function as escalator stairs. www.indiandentalacademy.com
Tooth shedding and replacement are not random, but are sequenced in alternate rows. In crocodiles and lizards the new tooth moves into a cavity that is resorbed out of the base of the old tooth, so that eventually the new tooth lies directly below the old one; when the old tooth is shed the new tooth is already in place and has only to erupt. The situation in mammals is rather similar to this. The fangs of snakes are highly specialized teeth fused to the maxilla. Poisonous snakes have a groove or canal in the fang through which poison is injected into the prey. New fangs lie horizontally in the roof of the mouth, and when the old fang is lost a new fang rotates into position and fuses to the maxilla to become functional.
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METHOD OF REPLACEMENT OF TEETH IN REPTILES www.indiandentalacademy.com
In some vertebrates, in which the teeth are large and durable, replacement takes place rarely or even not at all; this is true of the mammals, which typically replace their deciduous teeth only once with a permanent set constituting a diphyodont dentition. The permanent molars are never replaced at all. The simplest form of vertebrate tooth is a gently curved conical structure, a haplodont tooth. Such teeth are found in the majority of elasmobranchs and in many reptiles. They function mainly in holding prey.
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Sometimes the tip is blunt or rounded, and then the teeth are used for crushing food, as in fish that live on mollusks or crustaceans. In sharks the ridges are usually serrated to increase the cutting power, and some species possess accessory cusps developed on the ridges. Some lizards, too,possess mesial and distal accessory cusps, so that the tooth has three cusps in a mesio足 distal row. However, none of the other classes of vertebrates has such complex crown patterns as the mammals.
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In many vertebrates all the teeth in the jaw are alike (homodontism), differing from each other only in size. In the alligator some teeth are much larger than others, although they all have a similar shape. Some fish, however, have more than one sort of tooth, a condition described as heterodontism. Only in the mammals, however, is the dentition fully heterodont, consisting of a number of different sorts of teeth that perform different functions, so that
we
can
distinguish
incisors,
premolars, and molars. www.indiandentalacademy.com
canines,
Fishes and most reptiles have hinged jaws that can open wide and shut down on prey, which is processed little if at all before it is swallowed. The capability for anteroposterior jaw movement with the development of a sliding jaw joint with heterodont teeth marks a few advanced crocodilian reptiles and characterizes mammals. This chewing capability, associated with secondary palate development, has enabled exploitation of diets very different from those species with limited hinge movement. www.indiandentalacademy.com
Characteristics of the Mammalian Masticatory Apparatus Against the background of vertebrate jaws and teeth in general we can now point out some of the main characteristics of the mammalian masticatory apparatus: (1) Teeth are confined to the edges of the mouth and associated with the premaxillary, maxillary, and dentary bones; (2) They are provided with roots, usually more than one root to each tooth except on the simple teeth at the anterior end of the jaw. The roots are covered with cementum, and are connected by periodontal fibers to sockets in the alveolar bones (gomphosis). www.indiandentalacademy.com
(3) Teeth develop from a dental lamina and are
replaced at most only once (diphyodont), or, in the case of the permanent molars, or in rodents not at all (monophyodont); (4) They are heterodont, so that it is possible in each jaw to distinguish incisors, canines, premolars, and molars; (5) The molars, and often also the premolars, have a complex crown pattern, with more than one cusp; (6) There is a process of mastication, in which the cusps of opposing teeth relate together in a regular way www.indiandentalacademy.com
One of the main reasons for the complexity of the dentition of mammals is that this class of vertebrates has developed to a high degree the function of chewing food. Mammals have a high rate of metabolism. Unlike reptiles, which obtain heat by basking in the sun and become torpid when
the
external
temperature
falls
(poikilothermic), mammals keep warm by heat produced in their own bodies (homothermic). www.indiandentalacademy.com
Their hairy covering and subcutaneous fat enable them to retain heat, but the source of this heat is ultimately the energy taken in as food. Bearing this in mind, we can understand some of the characteristic features of the mammalian (hence human) mouth and jaws. The flexible lips and cheeks are provided with muscles, innervated by the facial nerve, which help in picking up food and manipulating it in the mouth. These muscles are absent in other vertebrates.
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The development of a secondary palate by the union of palatal folds resulted in separation of the respiratory passage from the mouth, enabling the mammal to go on breathing while its mouth is full of food, thus enabling respiration and mastication to take place concurrently. In reptiles the two passages are incompletely separated, and the food has to be swallowed quickly. Mammals retain this primitive inability to swallow and inspire at the same time. Any attempt to do so sets off coughing spells. Being able to retain food in its mouth, the mammal masticates, using its tongue and cheeks but especially its molar teeth, which grindwww.indiandentalacademy.com and break up the food.
The saliva differs from that of other vertebrates in containing a digestive enzyme, so that digestion begins even before the food is swallowed. The process of mastication involves a strong development of lip, cheek, tongue, and jaw musculature to control the food bolus over the molar teeth. The jaw muscles are differentiated into temporalis, masseter, medial, and lateral pterygoids and digastric muscles. These enable not only opening and closing, but also transverse and protrusive retrusive movements of the mandible. www.indiandentalacademy.com
Mastication is also associated with development of a peculiar type of jaw joint; in other vertebrates the lower jaw consists of several bones, and the joint is formed by two of them, the articular and the quadrate, whereas in mammals the mandible consists of the dentary alone, articulating with the squamosal (which equals part of the temporal bone in
man)
forming
the
dentary-squamosal
or
temporomandibular joint. Some of the bones that originally formed part of the reptilian jaw are preserved in the mammalian ear. www.indiandentalacademy.com
a) An early mammal like reptile
b) A more advanced mammal-like reptile
c) A primitive www.indiandentalacademy.com
mammal
The angular supports the ear drum and becomes the tympanic; the articular and quadrate become ear ossicles, respectively malleus and incus The stapes is retained as the original reptilian ear ossicle. Fetal mammals still develop the primitive jaw joint, and the earliest fetal jaw movements occur through the articular-quadrate or malleoincudal joint. It is retained after birth by marsupials, being replaced by the dentarysquamosal or temporomandibular joint in eutherian mammals. www.indiandentalacademy.com
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The newly established temporomandibular joint differs from other synovial joints that are cartilage derived. Both components of this joint are membrane bone derived, accounting for the lack of hyaline cartilage (present in other synovial joints)
at
the
site
of
articulation.
The
temporomandibular joint, in company with the only other membranous bone derived joints, the sternoclavicular and acromioclavicular joints, have fibrous
tissue
rather
than
covered ends of www.indiandentalacademy.com bones.
hyaline
cartilage
It is clear that the teeth cannot be understood by themselves;
they
are
part
of
a
complex
comprising jaws, muscles, cheeks, tongue, palate, and ear. All these characteristics of mammals did not arise in one step. There was a long process of evolution resulting in the attainment of the mammalian organization. www.indiandentalacademy.com
Evolution of the Mammalian Molar Tooth The Tritubercular Theory: The evolution of the homodont, haplodont reptilian dentition to the heterodont mammalian form involved reduction in the number of teeth in the jaws
and
reduction
in
the
number
of
replacements.Thus, polyphyodontism evolved into di- or monophyodontism. www.indiandentalacademy.com
The cheek teeth evolved to form molars consisting of a series of cusps, comprised of a single cone (protocone in the upper jaw, protoconid in the lower jaw) with mesial (paracone, upper, or paraconid, lower) and distal (metacone, upper, or metaconid, lower)and small cusps..
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The Reptilian or Haplodont stage is represented by the simplest form of tooth, the single cone.This type of dentition usually includes many teeth in both jaws that limit jaw movement. There is no occlusion of the teeth in this class, the teeth being used mainly for prehension or combat www.indiandentalacademy.com
Their main function is the procurement of food. The limitation of jaw relation is confined to that of a simple hinge movement. Jaw movements are related to and governed by tooth form in all cases
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The Early Mammalian or Triconodont stage exhibits three cusped cheek teeth in line in the development of posteriorteeth and were named Triconodonta, after their dentition, that had three cusps The largest, or anthropologically the original cusp, is centered with a smaller cusp located anteriorly and another posteriorly. www.indiandentalacademy.com
Today examples are not seen with a purely triconodont dentition. Some of the teeth in some breeds of dogs and other carnivores have some teeth that certainly indicate a triconodont background Nevertheless, the dog and other animals carnivorous by nature are considered to be in Tritubercular class. www.indiandentalacademy.com
As in reptiles, the upper and lower teeth alternated in the jaws, allowing the teeth to interdigitate on jaw closing. The Triconodont arrangement was modified in later orders by increasing the size of the accessory cusps, and rotating the cusps relative to the principal cusp to form triangular teeth,with the addition of sharp crests (lophs) between the cusps creating lophodont teeth. www.indiandentalacademy.com
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The relations of the upper and lower triangles to each other were reversed by the upper protocone becoming lingually displaced, while the lower protoconid
was
buccally
displaced,
allowing
interdigitation of the triangles. Teeth of this type characterize the order Symmetrodonta of the Jurassic period. These tritubercular or trigon teeth function not only by puncturing food between the cusps, but also by slicing food through a shearing action of the crests acting against each other. www.indiandentalacademy.com
The next evolutionary stage involved development of a distal "heel" or talonid in the lower molars, which formed a platform for crushing food against the protocone of the upper molar.
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These cuspal patterns form tribosphenic molars (Greek tribein: to rub; sphen: a wedge) that are ancestral to the cuspal patterns of primate molars. Later, an additional cusp developed on the distal aspect of the upper molar (the hypocone) to form a rectangular four-cusped tooth. Small additional cusps that form on the mesial and distal margins of the trigon are the protoconule and metaconule. Cusps developing along the buccal margins of the upper molar are styles: a parastyle at the mesial margin, a mesostyle in the middle, and a metastyle at the distal margin. www.indiandentalacademy.com
The lower molar talonid gave rise to three cuspsthe hypoconid (buccal), the entoconid (lingual), and the hypoconulid (distal)-producing a sixcusped lower molar.Such four-cusped upper molars and six-cusped lower molars are found in lower primates and occur in the living Tarsius. www.indiandentalacademy.com
It is important to realize that upper and lower molars evolved together as complementary parts of a single functional complex. Each cusp has definite occlusal relationships to cusps of the opposing dentition, revealed by the wearfacets. Like the cusps, corresponding wearfacets can be identified on teeth at different stages of evolution. www.indiandentalacademy.com
In humans, the protoconid in first lower molars is lost, producing a fivecusped tooth; additionally, the hypoconulid (distal cusp) is also lost in second lower molars, producing a four-cusped tooth. The five-cusped molar occurs in anthropoid apes and man www.indiandentalacademy.com
The Y-5 molar pattern, named after the Y-shaped pattern of fissures separating the five cusps, first appeared in a fossil pongid, Dryopithecus, that existed 24 million years ago and represents a very conservative genetically determined pattern still present in 90 percent of lower first permanent molars of modern man. The second and third molars are less conservative and more variable in their cusp and fissure patterns, departing from the archetypal Dryopithecus molar pattern. www.indiandentalacademy.com
The Achievement of upright Posture In order to assess the biological significance of the dentition in man it is important to review the mechanical and functional changes that took place in the terrestrial, quadrupedal, mammalian skeleton to achieve its human qualities. It is generally agreed that the Primate order originated among those small arboreal four-footed animals classified as insectivores or insect eaters. The descendants ofthese Early Paleocene animals still live today with relatively little modification. If we examine the skeleton of one of these survivors , some obvious characteristics are observed at once are: www.indiandentalacademy.com
(1) The head is long and is connected to the vertebral column at its most dorsal end (2)
The brain is located well in the back of the snout and
skull. The orbits are likewise placed far behind the snout. www.indiandentalacademy.com
(3) The fore and hind limbs bear the weight of the body, each approximately to an equal degree. (4) The weight of the body is transmitted to the limbs via the pectoral and pelvic girdles. (5) The vertebral column is like a suspension bridge, supporting the pendant weight of the thoracic and abdominal viscera. (6) The rib cage and the abdominal musculature support the weight only of the viscera immediately above. It is from this type of skeletal structure and organization that the primate (and peculiarly human) skeleton has developed over a period of www.indiandentalacademy.com about 70 million years.
It is not know why this effect (the centering of the head upon the vertebral column in man) had adaptive significance. It can only be said that it seems to have been a logical solution, in that the head became balanced upon the top of the body and could now rotate approximately 180 degrees, permitting a much wider field of vision In primates, it must be remembered, the orbits became directed forward and moved closer together than in other mammals.
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.
In man, the restriction of the simultaneous field of vision could be compensated for by rotation of the head. Although this reduced the total simultaneous field of vision, it did permit a greater overlap of vision and hence allowed better development of stereoscopic vision (depth perception). This evolutionary development is reflected in the relatively massive muscles of head rotation, the sternocleidomastoids, and the consequent increase in the size of the mastoid processes into which they insert.www.indiandentalacademy.com
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In all four-footed mammals the snout is more or less elongated, bearing long parallel rows of cheek teeth. The eyes are located to the side and behind the teeth, and the brain case is situated in the very back of the head. Since in these animals the head is generally the first part of the body to come into contact with the environment, the fighting and foodacquiring equipment (the teeth, particularly the incisors and canines) are strategically located in front, with the vital eyes and brain removed from the scene of action and possible damage. www.indiandentalacademy.com
The adaptive value of such an arrangement was nullified when the head was lifted far from the ground by the acquisition of semi and then fully erect posture and freed from the requirements of locomotion, the arms and hands of man became highly specialized for picking up, holding, and manipulating objects, including tools and weapons. The canines were no longer used for defense, and they were reduced in size. The teeth retained only the function of biting and chewing food already obtained and placed in the mouth by the hands. www.indiandentalacademy.com
Even this role was reduced in importance when stone tools were used to cut up the food, and fire was used to soften it by cooking. It is an interesting evolutionary fact that the assumption of erect posture freed both forelimbs and teeth from important duties the forelimbs from weight bearing and locomotion, the teeth from fighting, catching, and killing prey. A most important difference to remember is that the forelimbs found other functions to fulfill; the teeth did not www.indiandentalacademy.com
The centering of the head upon the vertebral column was accompanied by a great reduction in the size of the jaws and teeth, an increase in the size of the brain with a pronounced bulging of the frontal, parietal, and occipital portions, and a decrease in the nostrils and olfactory senses. The brain and eyes were now directly over the teeth, resulting in what
anthropologists
and
orthodontists
call
orthognathism. The canines became smaller in size so that all the teeth were now in the same occlusal plane.
www.indiandentalacademy.com
.
The mandible, now quite gracile as compared with the huge jaws of early hominoids, developed a chin a very recent acquisition of hominids (about 75,000 years). The lips became more or less everted, exposing the mucous portion that formerly formed part of the anterior wall of the oral cavity .The nasal bones, once arranged in the same flat plane as in the other primates, came to meet at an oblique angle to form the bridge of the nose a peculiarly human trait.. The heavy brow ridges, sagittal crests, temporal lines, and occipital protuberances have almost disappeared, although a sex difference can still www.indiandentalacademy.com be noted.
Tooth Form and Jaw Movements In general terms, the primates are bunodont and relatively isognathous and consequently have limited lateral movement. Bunodont refers to toothbearing conical cusps. Isognathous means equal jawed, and anisognathus refers to unequal jaws. Humans are not perfectly isognathus; i.e., the maxillary arch overlaps horizontally the mandibular arch. The shape of the glenoid cavity also is correlated with tooth form and jaw movements. www.indiandentalacademy.com
A number of living and extinct species demonstrate a chain of dental forms, gradually departing from the bunodont type and passing into the selenodont, accompanied by an increased mobility of the mandible in a lateral direction and increased anisognathism. When the incisal point is viewed from the front during mastication, the mandible moves up and down without lateral deviation in dogs, cats, pigs, and all other bunodonts. www.indiandentalacademy.com
Lateral movement increases in a number of animals to the extreme lateral excursion seen in the giraffe, camel, and ox. In relation to the latter type of movement, it has been proposed that where the condyle is greatly elongated transversely and very flat, there is great lateral movement during mastication with selenodont molars and a great degree of anisognathism. Also of interest is the general correlation between ridge and groove direction or curvature of the cross crests and intervening valleys of molars and a radius drawn to www.indiandentalacademy.com the center of the glenoid fossa
In contrast to such biaxial molars in some genera and in humans, no such correlation between ridge and groove direction and the center of rotation of the condyle exists because the center is midway between the condyles or glenoid cavities and thus uniaxial. It has been suggested that the earliest and simplest type of jaw movement is opening and closing without lateral excursion and coexistent with the simple bunodont molar. www.indiandentalacademy.com
With the increasing complexity of movement, there has been an apparent increase in the complexity of enamel folding, ridges, and crests. Such changes appear to follow correlatively with the shape of the mandibular articulation and the skull. Thus the forms of the teeth, joints, muscles, skull, bones, and jaw movement are all related in function www.indiandentalacademy.com
A Geometric Concept of Crown Outlines In a general way, all aspects of each tooth crown except the incisal or occlusal aspects may be outlined schematically within three geometric figures: a triangle, a trapezoid, and a rhomboid. To one unfamiliar with dental anatomy, it may seem an exaggeration to say that curved outlines of tooth crowns can be included within geometric figures. Nevertheless, to one who realizes the problems involved in crown design it seems very plausible to consider fundamental outlines schematically to www.indiandentalacademy.com assist in visualization.
Facial and Lingual Aspects of All Teeth The outlines of the facial and lingual aspects of all the teeth may be represented by trapezoids of various dimensions. The shortest of the uneven side represent the bases of the crowns at the cervices, and the longest of the uneven sides represent the working surfaces, or the incisal and occlusal surfaces, the line made through the points of contact of neighboring teeth in the same arch. Disregarding the overlap of anterior teeth and the cusp forms of the cusped teeth in the schematic drawing, we can easily see the fundamental plan governing the form and arrangement of the teeth from this aspect www.indiandentalacademy.com
The occlusal line that forms the longest uneven side of each of the trapezoids repre足sents the approximate point at which the opposing teeth come together when the jaw is www.indiandentalacademy.com closed.
This arrangement brings out the following fundamentals of form: 1
Interproximal spaces Interproximal tissue.
may
accommodate
2. Spacing between the roots of one tooth and those of another allows sufficient bone tissue for investment for the teeth and a supporting structure required to hold up gingival tissue to a normal level. Sufficient Circulation of blood to the parts would be impossible without this spacing. www.indiandentalacademy.com
3.Each tooth crown in the dental arches must be in contact at some point with its neighbor, or neighbors, to help protect the interproximal gingival tissue from trauma during mastication. The contact of one tooth with another in the arch tends to ensure their respective positions by mutual support. 4. Each tooth in each dental arch has two antagonists in the opposing arch excepting the mandibular central incisor and the maxillary third molar. In the event of loss of any tooth, this arrangement tends to prevent elongation of antagonists and helps stabilize the remaining teeth over a longer period than would be likely if each toothwww.indiandentalacademy.com had but a single antagonist
Mesial and Distal Aspects of the Anterior Teeth The mesial and distal aspects of the anterior teeth, central incisors, lateral incisors, and canines, maxillary and mandibular, may be included within triangles. The base of the triangle is represented by the cervical portion of the crown, and the apex by the incisal ridge. www.indiandentalacademy.com
The fundamentals portrayed here are: 1.A wide base to the crown for strength. 2.A tapered outline labially and lingually, narrowing down to a relatively thin ridge which facilitates the penetration of food material www.indiandentalacademy.com
Mesial and Distal Aspects of Maxillary Posterior Teeth The outlines of the mesial and distal aspects of all maxillary posterior teeth (premolars and molars) can be included within trapezoidal figures. Naturally, the uneven sides of the premolar figures are shorter than those of the molars
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The fundamental considerations to be observed when reviewing the mesial or distal aspects of maxillary posterior teeth are: 1. Because the occlusal surface is constricted, the tooth can be forced into food material more easily. 2. If the occlusal surface were as wide as the base of the crown, the additional chewing surface would multiply the forces of mastication; then, too, the tooth would be less "self-cleansing" during the process. www.indiandentalacademy.com
It has been found necessary to emphasize the fundamental outlines of these aspects through the medium of schematic drawings because the correct anatomy is overlooked so often. The tendency is to take for granted that the tooth crowns are narrowest at the cervix from all angles, which is not true. The measurement of the cervical portion of a posterior tooth is smaller than that of the occlusal portion when viewed from buccal or lingual aspects only; when it is observed from the mesial or distal aspects, the comparison is just the reverse; the occlusal surface tapers from the wide root base. www.indiandentalacademy.com
mesial or distal aspects,
buccal or lingual aspects
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Mesial and Distal Aspects of Mandibular Posterior Teeth The mandibular posterior teeth, when approached from the mesial or distal aspects, are somewhat rhomboidal in outline (The occlusal surfaces are con-stricted in comparison with the bases similar to the maxillary posterior teeth. The rhomboidal outline inclines the crowns lingual to the root bases, bringing the cusps into proper occlusion with the cusps of their maxillary opponents. www.indiandentalacademy.com
At the same time, the axes of crowns and roots of the teeth of both jaws are kept parallel. If the mandibular posterior crowns were to be set on their roots in the same relation of crown to root as that of the maxillary posterior teeth, the cusps would clash with one another. This would not allow the intercuspal relations necessary for proper function. www.indiandentalacademy.com
In proper alignment the teeth are arranged in arches in each jaw and placed in strong contact with their neighbors. If, in addition, each tooth in the arch is placed at its most advantageous angle to withstand forces brought to bear upon it, each tooth is more efficient, and the arches are stabilized by the collective action of the teeth in supporting each other.Present evidence suggests that tangential loading results in reduced chewing forces and that negative
feedback
from
receptors
periodontium mediate chewing forces. www.indiandentalacademy.com
in
the
Receptor thresholds for
axially
directed
forces appear to be higher than those for tangentially
directed
forces and suggest a positive control
feedback on
axially
directed tooth forces. www.indiandentalacademy.com
Contact of each tooth with its fellows in the archprotects the gingiva between them in the interproximal spaces. The gingiva is the soft tissue in the mouth that covers the alveolar bone and surrounds the teeth. The buccal and lingual contours of the teeth have an influence on the way in which food is directed to and away from the gingival tissues. www.indiandentalacademy.com
Food
impaction,
trauma
from
impinging
tough
foods,
accumulation of dental plaque may be the consequence of malposed teeth or over or undercontouring of restorations involving
buccolingual
surfaces. www.indiandentalacademy.com
Interproximal Form The interproximal space between the teeth is a triangular region, normally filled with gingival tissue, which is bounded by the two proximal surfaces of contacting teeth and the alveolar bone between the teeth, which acts as the base of the triangle. The gingiva within this space is called the gingival, or interdental papilla. www.indiandentalacademy.com
The gingival line follows the curvature not necessarily the level of the cervical line. The cervical line is defined as the "cemento enamel junction of crown and root." The gingival line and the cervical line must not be thought of as being identical; although they normally follow a similar curvature, they are seldom at the same level on the tooth. The cervical line is a stable anatomic demarcation, whereas the gingival line merely represents the gingival level on the tooth at anyone period in the individual's life, and this level is variable.
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Root Form The length arid shape of the root (or roots) of each tooth must be considered important: the canine, for instance, because of its position and the work required of it, would be torn out of its socket, or at least displaced, by forces brought to bear upon it, if the root were not of extra size and length. Fracture would be imminent if the root were not larger than that of other single-rooted teeth. The root form, therefore, is associated with the overall form of the tooth www.indiandentalacademy.com and the work it has to do.
The angle at which the incisal and occlusal surfaces of the tooth crowns are placed with respect to the root bases is also important. The mesial view of an anterior tooth will show that the incisal ridge or cusp is centered over the root. www.indiandentalacademy.com
The mesial view of an upper first molar, which is a multicusped tooth, demonstrates the same principle. The points on the occlusal surface that are contacted by opposing teeth will prove to be well within the confines of the root base of the crown. The measurement from cusp tip to cusp tip
buccolingually is much less
buccolingual diameter of the root base. www.indiandentalacademy.com
than
the
REVIEW OF LITERATURE: Complete crown form and the periodontium, J.Pros. Dent. 1961 Russell C. Wheelers concluded that the maximum portion of the tooth outline and tooth volume is intended for stabilization of the tooth in the dental arch. This leaves the minimum portion for a working surface or actual contact with the opposing teeth. The location of the crown margins should be decided upon by a logical approach to diagnosis and prognosis and not by taking a traditional design for granted. www.indiandentalacademy.com
Assuming good retention and repair as necessary the placement of the margin should depend mostly upon the tooth form and alignment and periodontal conditions. In the traditional approach , crown margins are always placed beneath the gum. In the biologic approach the margins are placed at the level which will best preserve the periodontium. www.indiandentalacademy.com
Facial and lingual contours of artificial complete crown restoration and their effect on periodontium.( J. Prosthet. Dent.; 1973) Ralph A. et al concluded that greater degree of facial and lingual bulge, the more plaque retained in the cervical region; the flatter the contour the less plaque retained . The explanation for this lies in the accessibility to oral hygiene measures. It is easier to keep the portion of the tooth occlusal to the height of contour plaque free than the cervical region gingival to the height of contour. Unless given special instructions in brushing and use of floss or devices as stim-u-dents. www.indiandentalacademy.com
They observed that the response of the gingiva to the posterior tooth that have been prepared for complete artificial crown and have lost their temporary crowns , in all instants the free gingival margins remained healthy and cervical region of such teeth demonstrated very little plaque retention compared to the approximating unprepared teeth. They have never observed teeth with little or no facial or lingual curvature that demonstrated a free gingival margin that ahs been stripped or pushed apically because of lack of protection and consequent over stimulation, www.indiandentalacademy.com
However they observed many teeth with excessively bulky contours which demonstrated disturbance attributable to over protection of free gingival margin and patient not able to reach the areas by routine oral hygiene measures. they advocated to flatten the facial and lingual contours of the restoration and have observed excellent response . Most probably because the cervical region is made more accessible for routine home care. www.indiandentalacademy.com
Periodontal consideration of crown contours. Morton L. Perel , J.Prosthet.Dent ;1971 Morton concluded that the alteration of the clinical crown to the gingival relationship does not necessarily require the placement of a deflective contour. www.indiandentalacademy.com
“If you are unable to carve the teeth up as you need to do in the restoration you have no moral right to carve them down” RUSSELL C. WHEELER
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LIST OF REFERENCES: • Wheeler’s Dental Anatomy, Physiology and Occlusion ; 7th edition. • Kraus Dental Anatomy And Occlusion; 2nd edition • Complete crown form and the periodontium,
Russell C. Wheelers J.Pros. Dent. 1961 • Facial and lingual contours of artificial complete crown restoration and their effect on periodontium. Ralph A. et al J. Prosthet. Dent.; 1973 • Periodontal consideration of crown contours. Morton L. Perel , J.Prosthet.Dent ;1971 www.indiandentalacademy.com
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