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Eruption of Deciduous TeethEruptionEruption is essentially the process whereby a tooth moves from its developmentalposition in the jaw into its functional position in the mouth. There is no evidence tosuggest that eruption entirely ceases once a tooth meets its antagonist in the mouth, andoutward axial movements occurring during the functional phase may also be eruptivemovements. (viz. overeruption following removal of the antagonist tooth in the oppositejaw).While the main direction of the eruptive force is axial (i.e. related to the long axis ofthe tooth), movement also occurs in other planes, accounting for tilting and drifting.Eruption rates of teeth are greatest at the time of crown emergence. Rates also differaccording to tooth type. Permanent maxillary central incisors are reported to erupt atabout 1mm/month; the rates for mandibular second premolars have been determined to beas great as 4.5mm in 14 weeks. For permanent third molars, where space is available,eruption rates of 1mm in 3 months have been recorded. In crowded dentitions, however,the rates are less than 1mm in 6 months.As a tooth approaches the oral cavity, the overlying bone is resorbed and there aremarked changes in the overlying soft tissues. The enamel surface is covered by thereduced enamel epithelium, which is a vestige of the enamel organ.Pre-eruptive tooth movement: Pre-eruptive phase, which starts with the initiationof tooth developmentn.2) Eruptive: Tooth eruption, which begins once the roots begin to form.3) Post-eruptive tooth movement: After the teeth have emerged into the oral cavity,there is a protracted phase concerned with the development and maintenance of occlusion(the functional phase).Eruption of TEETH(I)Patten of tooth movement1. Pre-eruptive tooth movementWhen deciduous tooth germs first differentiate, there is a good deal of spacebetween them. Because of their rapid growth, this available space is utilized and thedeveloping teeth become crowded together, especially in the incisor and canineregion.This crowding is relieved by growth in the length of the infant jaws,which provides room for the second deciduous molars to drift backward and anteriorteeth to drift forward.
At the same time the tooth germs also move outward as the jaws increasein width, and upward( downward in the upper jaw) as the jaws increase in height.2. Eruptive tooth movementDuring the phase of eruptive tooth movement the tooth moves from itsposition within the bone of the jaw to its functional position in occlusion and theprincipal direction of movement is occlual or axial .It is important to recognize that jaw growth is normally occurring while mostteeth are erupting, so that movement in planes other than axial is superimposed oneruptive movement.3. Post-eruptive tooth movement(1) maintain the position of theee erupted tooth while the jaw continues to grow(2) compensate for occlusal and proximal wear.The former movement, like eruptive movement, occurs principally in an axialdirection so as to keep pace with the increase in height of the jaws. It involves both thetooth its socket and ceases when jaw growth is completed. The movements compensatingfor occlusal and proximal wear continue throughout life and consist of axialand mesial migration, respectively.Histology of Tooth MovementPre eruptive phase:1. Total bodily movement of the germ2. There is its excentric growth:excentric growth: one part of the developing tooth germ remains stationary while theremainder continues to grow, leading to a shift in its center.Eruptive phase:During the eruptive phase of physiologic tooth movement, significant developmentalchanges occur:1. Formation of root2. Formation of periodontal ligament,3. Formation of dentogingival junction4. Another specialized feature associated with the erupting permanent tooth is thepresence of a gubernacular canal.or cord.Post-eruptive phase:(III) Mechanism of tooth movement1. Root growth2. Vascular pressure Root growth3. Bone growth4. Ligament traction
Shedding of deciduous teeth(I) Patten of SheddingFor a deciduous incisor or canine, root resorption initially occurs on the lingualsurface adjacent to the developing permanent tooth. With subsequent movement andrelocation of the teeth in the growing jaws, the developing permanent tooth comes to liedirectly beneath the deciduous tooth and further resorption occurs from the apex.For a deciduous molar, root resorption often commences on the inner surfaceswhere the permanent premolars initially develop.The premolars later come to lie beneath the roots of the deciduous molar andfurther resorption occurs from the root apices. The shift in position of the deciduous toothrelative to the permanent successor may account for the intermittent nature ofroot resorption.(II) Mechanism of resorption and histology of sheddingThe initiation of root resorption may be an inherent developmental process or it maybe related to pressure from the permanent successor against the overlying bone or tooth.Mechanism of TOOTH EruptionTooth eruption is traditionally considered to be a developmental process whereby thetooth moves in an axial direction from its position within the alveolar crypt of the jawinto a functional position within the oral cavity. However, eruption can be regarded as alifelong process since a tooth will often move axially in response to changing functionalsituations (e.g. overeruption resulting from the removal of an antagonist, andcompensatory eruption related to attrition).The rate of eruption represents a balance between forces tending to move the tooth intothe mouth (eruptive force) and forces tending to prevent this movement (resistive force).Resistance may be produced by overlying soft tissues and alveolar bone, the viscosityof the surrounding periodontal ligament and occlusal forces. Thus, changes in the rate oftooth movement may be brought about by changes in either the eruptive forces and/or theresistive forces.At present, little is known about the nature, source and magnitude of either the eruptiveor resistive forces. Furthermore, it is not known whether the forces are of the same natureand value at various stages of the eruptive cycle. By and large, this situation results fromdifficulties encountered in producing experimental systems which isolate for study singlepossible agents associated with the eruptive process.All tissues within the vicinity of the tooth thought capable of generating a force have,at one time or another, been implicated in the eruptive process.The tooth is pushed out as a result of forces generated beneath and around it, eitherby alveolar bone growth, root growth, blood-pressure/tissue fluid pressure, or cellproliferation. Alternatively, the tooth may be pulled out as a result of tension within theconnective tissue of the periodontal ligament. Although no one theory is yet supported bysufficient experimental evidence.
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