Book - Oral Histology and Embryology (1944) 11
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Orban B. Oral Histology and Embryology (1944) The C.V. Mosby Company, St. Louis.
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Chapter XI - Eruption of the Teeth
The human teeth develop in the jaws and do not enter the oral cavity until the crown has matured. In the past, the term eruption was generally applied only to the appearance of the teeth in the oral cavity. It is, however, known that the movements of the teeth do not cease when the teeth meet their antagonists.“ 7 Movements of eruption begin at the time of root formation and continue throughout the life span of a tooth. The emergence through the gingiva is merely an incident in the process of eruption. The eruption of deciduous as well as permanent teeth can be divided into the prefunctional and functional phases. At the end of the prefunctional phase the teeth come into occlusion. In the functional phase the teeth continue to move to maintain a proper relation to the jaw and to each other.
Eruption is preceded by a period in which the developing and growing teeth move to adjust their position in the growing jaw.’ A knowledge of the movements of the teeth during this preeruptive phase is necessary for a complete understanding of eruption. Thus, the movements of the! teeth can be divided into three phases: (1) preeruptive phase; (2) prefunctional phase; (3) functional phase.
During these phases the teeth move in diﬁerent directions.” These movements can be termed: ( 1) axial: occlusal movement in the direction of the long axis of the teeth; (2) drifting: bodily movement in a distal, mesial, lingual or buccal direction; (3) tilting: movement around a transverse axis; (4) rotating: movement around a longitudinal axis.
2. Histology of Eruption
During the ﬁrst, preeruptive phase the enamel organ develops to its Preemptive full size, and formation of the hard substances of the crown takes place.
First draft submitted by Joseph P. Weinmann.
At this time the tooth germs are surrounded by the loose connective tissue of the dental sac and by the bone of the tooth crypt. The development of the teeth and the growth of the jaw are simultaneous and interdependent processes. The microscopic picture of the growing jaw indicates that extensive growth takes place in that area of the jaws where the alveolar crest ultimately develops (Fig. 227). The tooth germs maintain their relationship to the growing alveolar margin by moving occlusally and buccally.‘
Fig. 227. Cross section through lower jaw and deciduous molar or human fetus (4th month). Tooth germ moves bucoally by excentric growth indicated by resorption at inner surface of the buccal alveolar plate and lack of apposition at the inner surface of the lingual plate.
Two processes are responsible for the developing teeth attaining and maintaining their position in the growing jaw, especially in relation to the alveolar ridge: the bodily movement of the teeth, and the excentric growth of the tooth germs. Bodily movement is characterized by a shift of the entire tooth germ. It is recognized by apposition of bone behind the moving tooth, and by resorption of bone in front of it. In excentric growth one part of the tooth germ remains ﬁxed: the growth gives rise to a. shift of the center of the tooth germ. Excentric growth is characterized solely by resorption of the bone at the surface toward which the tooth germ grows.
During most of the time when the deciduous teeth develop and grow, upper and lower jaws grow in length by apposition in the midline and at their posterior ends. Accordingly, the growing germs of the deciduous teeth shift in vestibular direction; at the same time the anterior teeth move mesially, the posterior distally, into the expanding alveolar arches." These movements of the deciduous teeth are partly bodily movements, in part caused by excentric growth. The deciduous tooth germ grows in length at about the same rate as the jaws grow in height. The deciduous teeth maintain, therefore, their superﬁcial position throughout the preeruptive phase.
The permanent teeth which have temporary predecessors undergo an intricate movement before they reach the position from which they emerge. Each permanent incisor (Fig. 228) and cuspid develops at first lingually to the deciduous tooth germ at the level of its occlusal surface.“ At the close of the preemptive phase they are found lingual to the apical region of their deciduous predecessors. The permanent bicuspids (Fig. 229) begin their development lingually to and at the level of the occlusal plane of the deciduous molars.“ Later, they are found between the divergent roots and, at the end of the preeruptive phase, below the roots of the deciduous molars (see chapter on Shedding). The changes in axial relationship, between deciduous and permanent teeth, are due to the occlusal movement of the deciduous teeth and the growth of the jaw in height. The germs of the bicuspids move by their buccally directed excentric growth into the interradicular space of the deciduous molars.
The second phase of tooth movement, the prefunctional phase of erup-. pmucﬂom tion, begins with the formation of the root (page 42) and is completed! P1350 01 when the teeth reach the occlusal plane. In the beginning of this phase nmptm the crown is covered by enamel epithelium. While the crown moves toward the surface, the connective tissue between the reduced enamel epithelium and the oral epithelium disappears, probably by the desmolytic action of the enamel epithelium. When the cusp of the crown approach the oral mucosa, the oral epithelium and reduced enamel epithelium fuse. In the center of the area of fusion the epithelium degenerates and the tip of the cusp emerges into the oral cavity. The gradual emergence of the crown is due to the occlusal movement of the tooth (active eruption), and also to the separation of the epithelium from the enamel (passive eruption).
The reduced enamel epithelium remains in organic connection with that part of the crown which has not yet emerged. (See section on Epithelial Attachment.) The growth of the root or roots of a tooth occurs by simultaneous and correlated proliferation of Hertwig’s epithelial root sheath and the connective tissue of the dental papilla. The proliferation of the epithelium takes place by mitotic division of the cells of the epithelial diaphragm. The proliferation of the connective tissue cells is concentrated in the area above the diaphragm.
During the prefunctional phase of eruption the primitive periodontal membrane, derived from the dental sac, is adapted to the relatively rapid movement of the teeth. Three layers of the periodontal membrane can be distinguished around the surface of the developing root: one, adjacent to the surface of the root (dental ﬁbers); another attached to the
Fig: 228. BuccoIingua1 sections through lower central incisors of seven consecutive stages, from newborn infant to 9 years of age.
primitive alveolus (alveolar ﬁbers) ; and a third, the intermediate plexus (Fig. 137). The intermediate plexus’ consists mainly of precollagenous ﬁbers, whereas the alveolar and dental ﬁbers are mainly collagenous. The collagenous ﬁbers can be traced into the intermediate plexus for a short distance. The intermediate plexus permits continuous rebuilding and rearranging of the periodontal membrane during the phase of rapid eruption.
In the region of the fundus, the dental sac differentiates into two layers: one, close to the bone, consists of loose connective tissue, whereas the other, adjacent to the growing end of the tooth, consists of a network of rather thick ﬁbers and contains a large amount of ﬂuid in the tissue spaces between the ﬁbers (Fig. 230). Strong strands of ﬁbers from the periodontal area at the side of the root curve as a strong ligament around the edge of the root, and then divide into a network forming spaces that are ﬁlled with ﬂuid. This entire structure is designated as “cushioned hammock ligament.”
In the prefunctional phase of eruption the alveolar ridge of the jaws grows rapidly. To emerge from the growing jaws the deciduous teeth must move more rapidly than the ridge increases in height. Growth of the root is not always suﬂicient to meet these requirements. A rapid formation of bone begins at the alveolar fundus Where it is laid down in trabeculae, parallel to the surface of the alveolar fundus” (Fig. 231). The number of trabeculae increases markedly during the prefunctional 292 om. phase, and varies in diﬁerent teeth: the smallest number of trabeculae is found at the fundus of the molars. This variance in the number of trabeculae seems to depend upon the distance which the teeth have to cover during this phase of tooth eruption.
The germs of most permanent teeth develop in a crowded position. They occupy, therefore, a position which diﬁers markedly from their ultimateposition after emergence. The molars are tilted; the occlusal surface of the upper molars, which develop in the maxillary tuberosity, is directed distally and downward. The occlusal surface of the lower molars, which develop in the base of the mandibular ramus, is directed mesially and upward. The long axis of the upper cuspids deviates mesially. The lower incisors are frequently rotated around their long axis. In the later stages of the prefunctional phase of eruption these teeth undergo intricate movements to rectify their primary position. During these tilting and rotating movements, bone apposition takes place in those areas of the tooth crypt from which the tooth moves away, and resorption occurs in the areas toward which the tooth moves. In all other details, the his tologic changes correlated to eruption are identical in permanent and deciduous teeth.
Fig. 229. Buccolingus sections through lower deciduous ﬂrst molar and ﬂrst bicuspid or eight consecutive stages, from newborn infant to 14 years.
The histologic ﬁndings in erupting multirootecl teeth present a picture quite diﬂerent from that in single-rooted teeth. The epithelial root sheath does not form an epithelial diaphragm, a cushioned hammock ligament is absent, and the proliferating pulp protrudes beyond the root end. The bone at the crest of the interradicular septum shows all signs of rapid growth. Apposition of cementum is also evident at the bifurcation.
After the erupting teeth have met their antagonists their movements are not easily ascertained For a long time it was believed that functioning adult teeth do not erupt any longer. However, clinical observations and his- mph” tologic ﬁndings show that the teeth continue to move throughout their life span. The movements are in an occlusal as well as in a mesial direction.
Clinically, the continued active movement of teeth can be proved by an analysis of the so-called shortened and submerged teeth (see page 322). Histologically, the changes in the alveolar bone furnish concrete evidence for the movements of the teeth in their functional period (see page 205).
During the period of growth, the occlusal movement of the teeth is. fairly rapid. The bodies of the jaws grow in height almost exclusively at‘ the alveolar crests and the teeth have to move occlusally as fast as the jaws grow, in order to maintain their functional position. The eruptive movement in this period is masked by the simultaneous growth of the jaws.
Fig. 230.—Cushioned hammock ligament Root end or an erupting 10WeI' cuspid. Proliferation zone of the pulp above the epithelial diaphI'a€m- Note the numerous tissue spaces in the ligament. (slcher-.3‘)
F13-. 231. Erupting upper deciduous cuspicl (A) and lower permanent cuspid (B), Note formation of numerous parallel bone trebeculae at alveolar fundus. Formation of bone trabeculae at the alveolar crest of deciduous cuspids (A) is 9. sign or rapid growth or the maxilla in height. (Kronteldﬁ) Bundle bone at fundus
Fig. 232 [esia._l drift and vertical eruption. Meslodistal section through upper first. and second bxcuspids. Arrow indicates direction of drifting movement. Apposition or bundle bone at the distal, resorption of bone on the meslal surfaces or the alveoli. Ap Dosition of bundle bone at the tundus and alveolar crest. (Weinma.nn.")
Fig. 233. Higher magniﬁcation of crest of interdental septum between ﬁrst and second upper bicuspids of Fig. 232. Arrows indicate direction of movement. Apposition or bundle bone on surface of septum facing the ﬂrst blcuspid and at alveolar crest, resorption of bone on surface of septum facing the second bicuspid. (Wein1na.nn.")
The continued vertical eruption also compensates for occlusal. or mcisal attrition. Only in this way can the occlusal plane and the distance between the jaws during mastication be maintaincd—a condition which is essential for the normal function of the masticatory muscles.
Fig. 234. A. Bundle bone at tundus of alveolus and in wall of canal leading blood vessels and nerves to apical roramina. Epitl-ielial rests some distance from apex along blood vessels and nerves. B. High magniﬁcation of epithelial rests of A.
The mobility of the individual teeth leads to friction at the contact points and to increasing wear in these areas. Sharp contact of the teeth is maintained despite the loss of substance at the proximal surfaces only because of the continuous movement of the teeth toward the midline. This movement is termed physiologic mesial drift.
Apposition of cementum continues along the entire surface of the root, but the apposition of bone is restricted principally to the fundus, alveolar crest, and distal Wall of the socket (Fig. 233). The mesial wall of the socket shows resorption in wide areas. However, even on the mesial surface of the alveolus, zones of reparative bone apposition can always be found. The tissue changes in the diiferent phases of tooth movements are summarized in Table VII.
Table Vii Tissue Changes During the Phases of Tooth Movements
”"‘E°"-'I°N cmuens or OF EPITIIELIUM ————e———— 1>,1),M_ MOVEMENT TOOTH BONE - 0ccluso-ax- Enamel organ Eccentric Growth of jaw Dental sa, Preﬁhrzgrve ial; buccal growth of G tooth germ Prefunc- 0ccluso-ax- Fusion of re- Root growth Apposition Intermeditional ial; duced enamel (trabecular ate plexus; Phase _straighten- organ with _ bone) at fun- cushioned of Eruption mg oral ep1thel1- due and a.1veo- hammock tuirn; ffonnagh lar ridge ligament on o 1 elial attac ment. Hert wig ’s sheath; epithelial rests
Functional 0ccIuso-ax- Down growth Attrition. Root Apposition Functional Phase ial; mesial of epithelial resorption and (bundle bone) arrange of Eruption attachment shedding of at fundus and ment of (passive erup- deciduous alveolar ridge suspensory ' tion) teeth. Ce- and distal al- apparatus
mentum appo- veolar wall; sition of per- resorption at manent teeth mesial wall
3. Mechanism of Eruption
Many theories have been advanced on the causes of tooth eruption} The following factors have been eonsidered:"”' 1‘ growth of the root;? growth of dentin; proliferation of the dental tissues; pressure from mus-p cular action; pressure from the vascular bed in the pulp and periapieal tissue; apposition and resorption of bone.
The eruptive movements of a tooth are the effect of differential growth. # One speaks of diiferential growth if two topographically related organs, or parts of an organ, grow at different rates of speed. Changes in the spatial relations of such organs, or of the parts of an organ, are the inevitable consequence of differential growth. The ontogenesis of almost any organ and of the whole embryo proves that diiferential growth is one of the most important factors of morphogenesis. In the jaws, it is the differential growth between tooth and bone which leads to the movement of a tooth.
The most obvious eruptive “force” is generated by the longitudinal growth of the root of the tooth. However, the different movements of an erupting tooth cannot be explained by the development of its root? alone. Some teeth, even while their roots develop, travel a. distance which is longer than the fully developed root. An auxiliary factor must account for the additional distance. Most teeth move in different directions, for instance by tilting, rotating, drifting; the growth of the root can only account for the axial or vertical movement. The “force” that can explain the variety of eruptive movements is generated by the growth of bone tissue in the neighborhood of the tooth germ.
- 1 It is also a fact that the teeth move extensively after their roots have been fully formed. The continued growth of the cementum covering the root and of the surrounding bone causes the movements of the tooth in this period.
Before the development of the root starts, the outer and inner enamel epithelium continue from the region of the future cemento-enamel junction as a double epithelial layer, the epithelial diaphragm, which is bent into the plane of the dental cervix. It forms a deﬁnite boundary between the coronal pulp of the tooth germ and the underlying connective tissue which intervenes between tooth germ and bony wall of the crypt. Thus, growth and development of the root is possible only under active proliferation of the pulpal tissue.
The importance of this single fact for the eruption of the tooth can best be realized by comparing our knowledge of root development with the long disproved, old concept of the function of Her-twig’s epithelial root sheath. It was thought that this double layer of epithelium g_rew into the underlying mesenchyme, punching out, as it were, part of this tissue, isolating it and transforming it into pulpal tissue. If this were true, the growth of the pulp would be by incorporation of new tissue and therefore passive rather than active. The presence of the epithelial diaphragm makes an inward growth of the epithelial sheath impossible and the pulp is “forced” to grow by multiplication of its cells and new formation of intercellular substance ;, in other words, the pulp enlarges by active growth, which creates the tissue pressure which can be seen - as the primary “force” of eruption. 1 The pressure generated by the increase in volume of the pulp in the restricted space of the dental crypt would act against the bone in the _bottom of the crypt and cause resorption of this bone and could not cause an eruptive movement of the tooth germ if there were no auxiliary structure. The auxiliary structure, which protects the bone at the bot'; tom of the crypt from pressure, prevents resorption of the bone, and causes the tooth to grow or move away from the bottom of the crypt, is the “hammock ligament.” If, by the proliferation of the growing pulp, tissue pressure increases, this ligament is tensed, the pressure is transmitted as traction to the bone to which the ligament is anchored, and no pressure is directed against the bone at the bottom of the crypt. Thus, the hammock ligament is the ﬁxed base or plane from which the tooth erupts because elongation of the tooth can only result in growth toward the surface of the jaws.
It has been mentioned before that the growth of the root alone cannot move a crown as far as is necessary to reach the occlusal plane. Some teeth, for instance the cuspids, develop far from the surface of the jaws. While all the teeth are erupting, the jaws continue to grow at their alveolar borders. The vertical erupting movement of these teeth is aided by growth of bone at the bottom of the crypt, lifting the growing tooth with the hammock ligament toward the surface. The formation of bone at the bottom of the crypt occurs in diiferent teeth at a different rate of speed. Where the production of new bone is slow, new layers of bone are laid down upon the old bone and a more or less compact bone results. Where growth of bone is rapid, spongy bone is formed in the shape of a, framework of trabeculae. These trabeculae develop by the growth of small projections of bone from the old surface, which then, at a given distance, seem to mushroom and to form new trabeculae parallel to the old surface (Fig. 231). In this way, tier after tier of bone tissue develops in the deep part of the socket.
The increased tissue pressure which is inevitably linked with the proliferation of bone in the crypt would tend to compress the hammock ligament, thereby destroying the ﬁxed base which is essential for the normal eruption of a tooth; ﬁnally, the bone would encroach upon tooth andj pulp, bringing the eruption to a standstill. These consequences are prevented by a peculiar structural differentiation of the hammock ligament. Teleologically speaking, the hammock ligament is rendered incompressible by the accumulation of a ﬂuid or a semiﬂuid substance between the ﬁbers and thus transformed into the “cushioned hammock ligament.” The ﬂuid is distributed throughout the ligament in small round drop- , lets. The presence of ﬂuid in conﬁned spaces is, of course, the cause of the incompressibility of this ligament. The incompressibility is relative but gives entirely sufficient protection if one considers the low intensity of the pressure forces generated during tooth eruption. That this pressure normally never reaches any higher intensity is explained by the simple fact that reactive tissue changes immediately follow the increase of tissue pressure and relieve it.
While the hammock ligament and tooth are lifted toward the surface, the anchoring ﬁbers of the hammock ligament have to be continually reconstructed. In other words, the hammock ligament has to shift its anchoring plane toward the surface of the jaws. Details of the mechanism of this shift are, as yet, not known.
Enlargement of the root does not cease when the root is fully formed. By continuous apposition of cementum, the root grows slightly in its transverse diameters and more rapidly in length. Cementum apposition is not only increased in the apical area of roots but the bifurcation of two or three-rooted teeth is also a site of fairly intensive cementum apposition. It is also well known that there is continuous apposition of bone at the fundus of the socket and at the crests of the alveolar process.
The bone apposition at the fundus and at the free border of the alveolar process is very rapid in youth, slows down in the thirties, but normally never ceases. Apposition at the alveolar crest, however, is found only ‘when the tissues are entirely normal. The frequency of inﬂammatory changes at the gingivodental junction accounts for the fact that this site of bone growth has been overlooked for a long time. There is also constant apposition of bone on the distal wall of each socket while the mesial wall shows resorption of the bone alternating with reparative apposition.
Though the correlation of bone changes and movement of the teeth is self-evident, the question still must be raised whether the bone changes are primary and thus the cause of the movement of the teeth, or not. The impossibility of ﬁnding any internal or external “forces” which would account for the continuous vertical eruption and mesial drift is an indication that the apposition of bone in the functional period plays the same role which one can ascribe to it in the preeruptive and prefunctional eruptive movements.
The apposition of bone can lead to a movement of a tooth only if the root surface is protected against resorption. This protection is actually given by the surface layers of uncalciﬁed cementum, the cementoid tissue, which regularly covers the surface of the cementum. The resistance of cementoid tissues to resorption has been demonstrated repeatedly.
The mechanism of tooth movement in the functional period can therefore be described in the following way: The entire surface of the root is protected against resorption by the growth pattern of the cementum which shows continuous, though not even, apposition throughout the life of the tooth. If apposition of bone occurs at the bottom of the crypt, the" slight increase of tissue pressure can lead to a movement of the tooth in occlusal direction only. This is because a relief of the pressure is not possible by resorption of the root. For the normal occlusal movement of a tooth in the functional period, the normal general growth of the cementum and the patterned growth of the bone are of equal importance. It is necessary to point out that only simultaneous growth of the opposing surfaces of cementum and bone can lead to a movement of a itooth. It is therefore clear that the apposition of cementum at the apex - can compensate only in part for the loss of tooth substance at the occlusal surface, that is, for the shortening of the tooth by attrition. A con sequence of this behavior is the fact that the teeth do shorten during the functional period.
In the life of every tooth there comes a time in which the “forces” of eruption change abruptly. It is, of course, the time when the pulp is fully grown and the root is fully formed. From now on, it is the differential growth of bone and cementum, and not that of pulp and bone, which causes the continued vertical movement of the tooth. The eruptive mechanism of multirooted teeth differs in that the shift from one to the other mechanism of eruption occurs much earlier, namely, as soon as the bifurcation is fully formed, though the roots are still growing.
The mesial drift is caused, in principle, by similar changes of bone and tooth which seem to be an adaptive, genetically determined process. However, this movement is greatly complicated by the fact that extensive bone resorption at the mesial alveolar walls has to open the space into which the teeth move, while the vertical movement is not opposed by bone.
Apposition of bone on the distal surface of the socket leads to an increase of the interalveolar pressure. This can be relieved only by resorption of bone at the mesial wall of the socket since the growing surface of the bone and the entire surface of the root are protected by their own growth, that is, by the presence of a thin layer of uncalciﬁed ground substance on their surface.
With both vertical and mesial movement of the functioning teeth, a continual rearrangement of the principal ﬁbers of the periodontal membrane has to be postulated. Details of this process, however, are al most entirely unknown. The changes which prevent a destruction of the ligamentous anchorage of the tooth on its mesial surface during the con tinuous mesial drift are explained by the peculiar reaction of bone to. pressure (or during modeling resorption) which could be called “thel law of excessive resorption.” Resorption of bone under pressure is, as '~ a rule, more extensive than necessary to relieve the pressure. The sur-' face layers of a bone are structurally adapted to the functional needs of the particular area. If they are destroyed during resorption, the newly exposed surface lacks this adaptation. Therefore, the resorption con tinues until room is provided for a reconstruction of a functionally , adequate new surface. This is the reason that, under normal circumstances, resorption is almost never a continuous process but instead occurs in waves, periods of resorption alternating with periods of reparative or reconstructive apposition.
This sequence of events can also be observed during the mesial drift of a tooth. Some principal ﬁbers lose their attachment during the period of bone resorption and are then reattached, or replaced by new ﬁbers, which are anchored in the bone apposed during the period of repair. Furthermore, it can be observed that bone resorption does not occur at the same time on the entire extent of the mesial alveolar surface. Instead, at a given moment, areas of resorption alternate with areas of reparative apposition. It seems that the tooth moves mesially in a complicated manner. Thus, resorption occurs only in restricted areas in one period and reconstruction occurs in the same area, while the tooth, minutely tilting or rotating, causes resorption in another area. Only this can account for the fact that the functional integrity of the tooth is maintained in spite of its continued movements.
4. Clinical Considerations
The eruption of teeth is a part of general development and growth, and therefore the progress of tooth eruption may serve as an indicator of the physical condition of a growing individual. The time of emergence of a tooth is readily observed by clinical examination. Considerable work has been done in compiling data regarding this particular stage of eruption. Table VIII illustrates that the time of emergence of all teeth varies widely.” 5' 13 Only those cases which are not within the range of variation may be considered abnormal. Retarded eruption is by far more frequent than accelerated eruption and may have a local or systemic etiology.
Local causes, such as premature loss of deciduous teeth and closure of the space by a shift of the neighboring teeth, may retard the eruption of some permanent teeth. Severe acute trauma may result in an arrest of active tooth eruption during the functional phase if the periodontal mem - brane of the tooth has been injured. Resorption of the root may ensue in which event deposition of bone in the spaces opened by resorption may lead to an ankylosis by fusion of alveolar bone and root.‘‘» 29 The movement of such a tooth is then arrested whﬂe the other teeth continue to erupt. If this disturbance takes place in the permanent dentition, a so-called “shortened” tooth results. An ankylosed deciduous tooth may, eventually, be covered by the rapidly growing alveolar bone. Such teeth are called submerged teeth (see chapter on Shedding).
If the eruption of the entire deciduous or permanent dentition is delayed, hereditary or systemic factors may be responsible. Among the systemic causes are: disturbances of the endocrine system and nutritional deﬁciencies. Hypothyroidism is of the former, and vitamin D deﬁciency of the latter group. The eifects of hypothyroidism and vitamin D deﬁciency on tooth eruption can be explained by a retardation of the growth of teeth and bone. Delayed eruption of the teeth usually accompanies cleidocranial dysostosis, a hereditary disease aifecting membrane bones.
The movements of the teeth, during eruption, are intricate and are accomplished by minute coordination of growth of tooth, growth of the alveolar bone, and growth of the jaws. Any break in this correlation may affect the direction of the movements; this, in turn, may lead to an impaction or embedding of a tooth. At the time the third molars develop, the jaw has not reached its full length. Normally, the oeclusal surface of a third lower molar turns anteriorly and upward. It is frequently prevented from straightening out because of a lack of correlation between growth in length of the lower jaw and tooth development. In such cases, the eruption of the lower third molar is arrested because its crown comes in contact with the roots of the second molar. If, at this time, the roots of the third molar are not as yet fully developed, they will grow into the bone and may become deformed. Cuspids, sometimes found in an oblique or horizontal position, due to crowding of the teeth, may also fail to correct this malposition and remain embedded.
Cnnoxonoov or THE HUMAN’ Dr.N'rI'rIoN Logan and Kronfeld (slightly modified by McCall and Schour)
FORM.A'.l‘rON ENAMEL MATRIX AMOUNT OF ENAMEL ENAMEL EMER/GENOE ROOT
TOOTH AND DEN,-MN MATRIX FORMED COMPLETED INTO ORAL COMPLETED
BEGINS AT BIRTH CAVITY
Central incisor 4 mo. in utero Five-sixths 1'} m0- 7% 1110- 1} )7!‘ Lateral incisor 4} mo. in utero Two-thirds 21} mo. 9 mo. 2 yr.
Maxillary Cuspid 5 mo. in utero One-third 9 mo. 18 mo. 31 yr.
First molar 5 mo. in utero Cusps united 6 mo. 14 mo. 2} yr.
Deciduous Second molar 6 mo. in utero Cusp tips still isolated 11 mo. 24 mo. 3 yr.
5931310“ Central incisor 4} mo. in utero Three-ﬂftlis 2-} mo. 6 mo. 1'} yr. Lateral incisor 4} mo. in utero Three-ﬁfths 3 ma. 7 mo. 1-} yr.
Mandibular Ouspid 5 mo. in utero One-third 9 mo. 16 mo. 3% yr.
First molar 5 mo. in utero Cnsps united 5-} mo. 12 mo. 2} yr.
Second molar 6 mo. in utero Cusp tips still isolated 10 mo. 20 mo. 3 yr.
7- 8 yr. 10 yr. 8- 9 yr. 11 yr. 11-12 yr. 13-15 yr. 10-11 yr. 12-13 yr. 10-12 yr. 12-14 yr. 6- 7 yr. 9-10 yr. 512-13 yr. 14-16 yr. 17-21 yr. 18-25 yr. 6- 7 yr. 9 yr. 7- 8 yr. 10 yr. 9-10 yr. 12-14 yr. 10-12 yr. 12-13 yr. 11-12 yr. 13-14 yr. 6- 7 yr. 9-10 yr. 11-13 yr. 14-15 yr. 17-21 yr. 18-25 yr.
Central incisor 3 - 4 mo _-_..___.._-_..___.. Lateral incisor 10 -12 mo. .___-___.._..__-..Cuspid 4 - 5 mo. ________ __---___ First bicuspid 1§- 15 yr. ______________ __ Second bicuspid 2 - 2% yr. ______________ __ xFirst molar At birth Sometimes a. trace Second molar 24- 3 yr. _--_--.._..__.._-..Permanent Third molar 7 - 9 yr. ___-- ____ -__..__
dentition Central incisor 3 - 4 mo. _______ _______-_ Lateral incisor 3 - 4 mo. ........... ---- Cuspid 4 - 5 mo. ...... ..--..----....
. First bicus id 15- 2 . .----_-_----.._.. M‘“‘d‘b“1‘“' Second bicgspid 21- 2; $1. __--______-_____
First molar At birth Sometimes a trace
Second molar 21- 3 yr. _________ ..----_..
Third molar 8 -10 yr. .......... --_--- 1
Resorptlon ** —
Enamel -'— Resorption ‘ 4" .
B. . D.
Fig. 235. Root resorption on distal surface or second lower molar caused by pressure or erupting third molar. and repair.
4. Relation of germ or third molar to second molar at the beginning of pretunctional phase of eruption. Note oblique position of the crown ot third molar.
B. Area. of contact between tooth erm oi third molar and root of second molar in high magniﬁcation Resorption rea into dentin.
0. Relation or lower second and third molars when third molar has a.tta.ined its up~ right position.
D. High magniﬁcation of alveolar crest. Resorption on distal root surface is partly repaired by apposition or cementum. (Orba.n.1')
Erupting teeth may cause resorption on the roots of neighboring teeth.“ This has been observed very frequently on the lower second molars, due to the oblique position of the erupting third molar (Fig. 235). This tooth turns its occlusal surface mesially and upward, and attains its upright position only in the last stages of eruption. Therefore, its crown comes into closest relation to the distal surface of the distal root of the second molar, and exerts pressure leading to resorption of cementum and dentin to a varying depth; it can be so extensive that the pulp may be exposed. When the pressure is relieved during the normal movement of the wisdom tooth, repair by apposition of cementum follows. Such resorption was observed in about two-thirds of investigated jaws. A horizontal position of the lower third molar might later lead to impaction. In such cases the destruction on the root of the second molar may be severe.
Impacted or embedded upper third molars may cause similar resorption of the root of the second molar. Embedded upper cuspids may exert pressure upon the root of the lateral incisor. During the time of eruption of teeth, the reduced or united enamel epithelium may undergo changes which result in cyst formation. Such a cyst forms around the crown of the developing tooth and is known as a dentigerous cyst. Those which arise late may cause a noticeable swelling on the surface and are sometimes known as eruptive cysts, although they are simply forms of dentigerous cysts.
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(The Correlation of Active and Passive Eruption,
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