Eruption-shedding-1.pdf

6 CHAPTER Eruption and shedding...

6 CHAPTER Eruption and shedding of the teeth Overview 77 Key Terms—cont’d Preeruptive phase 77 Intracellular phase Preeruptive phase Prefunctional eruptive phase 78 Intraoral occlusal/incisal Prefunctional eruptive Changes in tissues 80 movement phase Overlying the teeth 80 Mixed dentition period Root formation Surrounding the teeth 83 Movement Ruffled border Osteoblasts Shedding Underlying the teeth 84 Osteoclasts Tissues: overlying, Functional eruptive phase 86 Penetration surrounding, underlying Possible causes of tooth eruption 87 Sequence and chronology of tooth eruption 87 Shedding of primary teeth 88 Comparisons of the primary and permanent OVERVIEW dentitions 89 Tooth number and size 89 Tooth eruption is the process by which developing teeth Roots 89 emerge through the soft tissue of the jaws and the overlying mucosa to enter the oral cavity, contact the teeth of the Tooth structure 90 opposing arch, and function in mastication. The movements Pulp shape and size 90 related to tooth eruption begin during crown formation and Arch shape 90 require adjustments relative to the forming bony crypt. This is Root resorption and pulp degeneration 90 the preeruptive phase. Tooth eruption is also involved in the Self-evaluation questions 91 initiation of root development and continues until the tooth’s emergence into the oral cavity, which is the prefunctional Consider the patient discussion 91 eruptive phase. The teeth continue to erupt until they reach Suggested reading 91 incisal or occlusal contact. Then, they undergo functional eruptive movements, which include compensation for jaw growth and occlusal wear of the enamel. This stage is the functional eruptive phase. Eruption is actually a continuous process that ends only with the loss of the tooth. Each denti- Learning Objectives tion, primary and permanent, has various problems during Describe the three phases of tooth eruption: preeruption, eruption and in the sequencing of eruption in the oral cavity. prefunctional, and functional. Teeth differ extensively in their eruptive schedules as well. Describe the initial growth of the tooth and the compen- This chapter describes these events. Finally, the process sational changes that occur in the surrounding overlying of tooth shedding or exfoliation of the primary dentition is and underlying tissues. discussed (Boxes 6-1 and 6-2 ). Primary tooth loss results from three fundamental causes: root resorption, bone resorp- tion, and size of crown too small to withstand mastication. Key Terms PREERUPTIVE PHASE Diphyodont Functional eruptive The preeruptive phase includes all movements of primary and Eruption pathway phase permanent tooth crowns from the time of their early initiation Extracellular phase Gubernaculum dentis or and formation to the time of crown completion. Therefore Fibroblast gubernacular cord this phase is finished with early initiation of root formation. 77 The developing crowns move constantly in the jaws during the 6 ESSENTIALS OF ORAL HISTOLOGY AND EMBRYOLOGY Box 6-1 Theories of Tooth Eruption 1. Root growth 2. Proliferation of pulp tissue 3. Increased vascularity of the pulp 4. The gubernaculum dentis 5. Development of the “hammock ligament” 6. Development of apical bone (boney ladder) A B 7. Occurrence of an eruptive pathway 8. Organization and increased vascularity of the periodontal Fig. 6.2 Relative position of primary and permanent molar ligament teeth. A, Preeruptive period. B, Prefunctional eruptive period. Box 6-2 Clinical Areas Where Root Resorption Is Important 1. Normal shedding of deciduous teeth 2. Orthodontic tooth movement 3. Transplantation and implantation 4. Idiopathic resorption (internal and external) preeruptive phase. They respond to positional changes of the neighboring crowns and to changes in the mandible and Fig. 6.3 Human jaws at 8 to 9 years of age, during the maxilla as the face develops outward, forward, and downward mixed dentition period. Permanent teeth are replacing primary away from the brain in its maturing growth path. During the teeth, and positions of each are shown. The permanent man- dibular molar has not emerged from the coronoid process. lengthening of the jaws, primary and permanent teeth make mesial and distal movements. Eventually the permanent tooth crowns move within the jaws, adjusting their position to the resorptive roots of the primary dentition and the remodel- surfaces slanted distally. Mandibular molars develop in the ing alveolar processes, especially during the mixed dentition mandibular rami with their occlusal surfaces slanting mesially period from 8 to 12 years of age. (see Fig. 6-3). This slant is the result of the angle of eruption Early in the preeruptive period, the permanent anterior as the molars arise from the curvature of the condyle of the teeth begin developing lingual to the incisal level of the pri- posterior mandible. All movements in the preeruptive phase mary teeth (Figs. 6-1 and 6-2 ). Later, however, as the primary occur within the crypts of the developing and growing crown teeth erupt, the permanent successors are positioned lingual to before root formation begins. the apical third of their roots. The permanent premolars shift from a location near the occlusal area of the primary molars to a location enclosed within the roots of the primary molars PREFUNCTIONAL ERUPTIVE PHASE (see Fig. 6-2). This change in relative position is the result of the eruption of the primary teeth and an increase in height The prefunctional eruptive phase starts with the initiation of of the supporting structures. On the other hand, the permanent root formation and ends when the teeth reach occlusal con- molars, which have no primary predecessors, develop without tact. Four major events occur during this phase: this type of relationship (Fig. 6-3 ). Maxillary molars develop within the tuberosities of the maxilla with their occlusal 1. Root formation requires space for the elongation of the roots. The first step in root formation is proliferation of the epithelial root sheath, which in time causes initiation of root dentin and formation of the pulp tissues of the forming root. Root formation also causes an increase in the fibrous tissue of the surrounding dental follicle (Fig. 6-4 ). 2. Movement occurs incisally or occlusally through the bony crypt of the jaws to reach the oral mucosa. The movement is the result of a need for space in which the enlarging roots can form. The reduced enamel epithelium next con- tacts and fuses with the oral epithelium (Fig. 6-5 ). Both of A B these epithelial layers proliferate toward each other, their Fig. 6.1 Relative position of primary and permanent incisor cells intermingle, and fusion occurs. A reduced epithelial teeth. A, Preeruptive period. B, Prefunctional eruptive period. layer overlying the erupting crown arises from the reduced 78 enamel epithelium (Fig. 6-6).  ERUPTION AND SHEDDING OF THE TEETH 6 Oral epithelium Site of proliferation Fused of reduced enamel oral and epithelium enamel epithelium Enamel space Reduced enamel epithelium Epithelial diaphragm Enamel space Oral epithelium Fig. 6.4 Histology of the prefunctional eruptive phase. Fig. 6.6 Fused reduced enamel epithelium and oral The root develops, and reduced epithelium overlying the crown epithelium overlie the enamel of crown. Enamel space approaches oral mucosa. Reduced enamel epithelium prolifer- occurs as enamel is dissolved in preparation of slide. ates, anticipating fusion. Clinical crown Oral epithelium Dentinogingival Enamel junction space Junctional or epithelial attachment Developing tooth Root Epithelial diaphragm Fig. 6.7 An erupting primary tooth appears in the oral cavity. The permanent tooth’s position is shown on the left. The dashed line indicates cuticle overlying the enamel surface of the erupting tooth. Fig. 6.5 Histology of an erupting cuspid tooth. The crown tip is in contact with oral epithelium. (see Fig. 6-7). The exposed crown is the clinical crown, extending from the cusp tip to the area of the gingival 3. Penetration of the tooth’s crown tip through the fused attachment. In contrast, the anatomic crown is the entire epithelial layers allows entrance of the crown enamel crown, extending from the cusp tip to the cementoenamel into the oral cavity. Only the organic developmental junction. cuticle (primary), secreted earlier by the ameloblasts, covers the enamel (Fig. 6-7 ). CLINICAL COMMENT 4. Intraoral occlusal or incisal movement of the erupting tooth continues until clinical contact with the opposing Hypereruption occurs with the loss of an opposing tooth. This crown occurs. The crown continues to move through the condition allows the tooth or teeth to erupt farther than normal mucosa, causing gradual exposure of the crown surface, into the space provided. 79 with an increasingly apical shift of the gingival attachment 6 ESSENTIALS OF ORAL HISTOLOGY AND EMBRYOLOGY Primary tooth Erupting permanent tooth crown Eruption Enamel pathway space Fig. 6.8 Histology of a prefunctional erupting tooth. Observe the appearance of the eruption pathway developed overlying the crown. Fig. 6.9 Observe the relation of the functional primary tooth root on the right to the permanent prefunctional erupting crown Changes in Tissues on the left. The prefunctional eruptive phase is characterized by signifi- cant changes in the tissues overlying, surrounding, and underlying the erupting teeth. Overlying the Teeth permanent teeth (Fig. 6-11). As the roots resorb, the primary The dental follicle changes and forms a pathway for the erupt- crowns are lost or shed (Fig. 6-12 ). Dentin resorption is ing teeth. A zone of degenerating connective tissue fibers and similar to bone resorption (see Fig. 6-10). cells immediately overlying the teeth appears first (Figs. 6-8 The resorptive process of primary and permanent teeth re- and 6-9 ). During the process, the blood vessels decrease in sults from action of osteoclasts that arise from monocytes of number, and nerve fibers break up into pieces and degenerate. the circulating bloodstream. These monocytes appear and fuse The altered tissue area overlying the teeth becomes visible as with others to form the multinucleated osteoclasts. Their an inverted triangular area known as the eruption pathway. function is to resorb the hard tissue. They do so by first sepa- In the periphery of this zone, the follicular fibers, regarded as rating the mineral from the collagen matrix through the action the gubernaculum dentis or gubernacular cord (Fig. 6-10 ), of the hydrolytic enzymes secreted by the osteoclasts. This are directed toward the mucosa. Some scientists believe that enzymatic action is believed to occur within lacunae, which these fibers guide the teeth in their movements to ensure are developed by the osteoclasts. The osteoclast’s cell mem- complete tooth eruption. brane is in contact with the bone and becomes modified by an Macrophages appear in the eruption pathway tissue. enfolding process termed the ruffled border (Figs. 6-13 and These cells cause the release of hydrolytic enzymes that aid 6-14 ). This border greatly increases the surface area of the in the destruction of the cells and fibers in this area with osteoclast and allows the cell to function maximally in bone the loss of blood vessels and nerves. Osteoclasts are found resorption (Fig. 6-15 ). along the borders of the resorptive bone overlying the teeth. Hard tissue resorption is believed to occur in two phases: This bone loss adjacent to the teeth keeps pace with the the extracellular phase, in which the mineral is separated eruptive movements of the teeth (see Fig. 6-9). Osteoclasts from the collagen and is broken into small fragments (see and osteoblasts constantly remodel the alveolar bone as Fig. 6-15), and the intracellular phase, in which the osteoclast the teeth enlarge and move forward in the direction of the ingests these mineral fragments and continues the dissolution growing face. of this mineral. Crystals appear in cytoplasmic vacuoles of the Although the process of eruption for permanent teeth is osteoclast and are gradually digested within them. Resorption similar to that of the primary teeth, the presence of roots of mineral occurs at the ruffled border interface outside the from primary teeth poses a problem. The resorption of their cell, and the mineral is then taken within the cell (Fig. 6-16). roots is similar to the process of bone resorption for the Special fibroblast cells are believed to destroy the remaining emergence of primary teeth. Permanent teeth establish an collagen fibers secondarily by ingesting them in an intracellular eruptive path lingual to the primary anterior teeth and the phagolysosome system (Fig. 6-17 ). Amino acids resulting premolars under the primary molars. Permanent molars from this breakdown are used in the formation of collagen erupt into the alveolar free space behind primary teeth (see within this same cell and can be used in this same area Fig. 6-9). Small foramina just posterior to the primary for bone formation. Only the posterior permanent molars, 80 tooth row are evidence of the eruption sites of the anterior which have no primary predeciduous teeth, erupt through  ERUPTION AND SHEDDING OF THE TEETH 6 Gubernacular cord Resorbing bone, Dental enlarging canal follicle for eruption Dental follicle Fig. 6.10 Diagram of a developing eruption pathway. A, Early developing eruption pathway. B, Resorption of bone in eruption pathway. A B Foramina Fig. 6.11 Foramina palatal to maxillary primary incisors. These are sites of eruption for permanent incisors. Permanent teeth Functional primary tooth Permanent tooth Fig. 6.12 Histology of maxilla in the mixed dentition period. Roots of erupted primary teeth are undergoing resorption. Crowns of developing permanent teeth appear below primary teeth. 81 6 ESSENTIALS OF ORAL HISTOLOGY AND EMBRYOLOGY Fig. 6.13 Histology of active resorption sites on primary tooth Osteoclasts Dentin roots. Osteoclasts appear in lacunae in root cementum and dentin. of root Root resorption Multinucleated osteoclast Fig. 6.14 Histology of osteoclasts in advancing resorption lacunae. Observe the large multinucleated cells shown within the lacunae. Ruffled border of osteoclast (brush border) Osteoclasts Release of crystals Disrupted E collagen A D Osteoclast with multiple nuclei Fig. 6.15 Osteoclast activity. A, Osteoclasts in lacunae on bone surface. B, Large multinucleated osteoclasts with brush border in contact with bone spicule. C, High magnification of ruffled border of osteoclast showing mineral crystals passing into spaces between cell extensions. Unmasked collagen fibers are nearby. D, Clear zone on Bone osteoclast surface. E, Ruffled border of osteoclast in constant motion B spicule 82 or change. C Changes in ruffled borders  ERUPTION AND SHEDDING OF THE TEETH 6 Crystal uptake by vacuoles Fig. 6.16 Diagram of ruffled border of an osteoclast. A, High magnification of unmasked collagen fibers. Mineral crystals are near the osteoclast surface. B, Diagram of uptake of crystals Breakdown of bone into Crystals visible within into osteoclast vacuoles. A collagen fibers and crystals B ruffled border A AA AA AA A A A A A A AA A A Fig. 6.17 Fibroblasts are capable of synthesis of collagen as well as its breakdown. Collagen fibers are phagocytosed into cells and are broken down to release amino acids (AA). These amino acids are then used to form new collagen molecules. alveolar bone (Fig. 6-18 ). Fig. 6-19 summarizes what happens Fig. 6.18 The relationship between primary and permanent in the tissues overlying the teeth during their prefunctional teeth during the mixed dentition period. (From Berkovitz BKB, eruptive phase. Bone loss occurs as the tooth approaches the Holland GR, Moxham BJ: Oral anatomy, histology, and embry- oral epithelium and forms an eruption pathway while the re- ology, ed 4, St. Louis, 2009, Mosby.) duced enamel epithelium fuses with the oral ectoderm to form the junctional epithelium, which attaches to the develop- mental cuticle by hemidesmosomes formed by the gingival keratinocytes and helps prevent oral bacterial and other sub- Surrounding the Teeth stances present in the oral cavity from entering the body (see The tissues around the teeth change from delicately fine fibers Fig. 6-19, A). The tooth organ epithelium makes contact with lying parallel to the surface of the tooth to bundles of fibers the oral mucosa (see Fig. 6-19, B and C). This contact causes attached to the tooth surface and extending toward the peri- stretching and thinning of the oral membrane and finally its odontium. The first fibers to appear are those in the cervical rupture and penetration by the tooth (see Fig. 6-19, D and E). area as root formation begins (Fig. 6-20 , A). As the root elon- Only a thin developmental cuticle then covers the tooth (see gates, bundles of fibers appear on the root surface (see Fig. 6-19, E and F). As the tooth emerges farther into the Fig. 6-20, B and C). Fibroblasts are the active cells in both the mouth, more crown is exposed, and as clinical contact with formation and the degradation of the collagen fibers. With the opposing tooth is made, the epithelial attachment shifts to tooth eruption, the alveolar bone crypt increases in height to the cervical area (see Fig. 6-19, G). Clinically, tooth eruption accommodate the forming root. After the teeth attain func- is seen as a blanching of the mucosa, and this condition may tional occlusion, the fibers gain their mature orientation (see persist for several days because the eruptive process is neither Fig. 6-20, C). Special fibroblasts have been found in the peri- rapid nor continuous. Each eruptive movement, however, re- odontium around the erupting teeth. These fibroblasts have sults in greater exposure of the crown. With successive eruptive contractile properties. During eruption, collagen fiber forma- movements, the area of attached epithelium becomes lower on tion and fiber turnover are rapid, occurring within 24 hours. 83 the clinical crown. This mechanism enables fibers to attach and release and