The two extremes of physiological tooth resorption in primary tooth with or without the permanent successor tooth

ABSTRACT Introduction: Assessment of two radiographic images reveals two distinct, extreme situations of physiological tooth resorption, characteristic of primary teeth with or without permanent successor, due to partial anodontia. Discussion: In all primary teeth, rhizolysis begins after the completion of formation, thanks to the apoptosis of their cells. When apoptosis induced by cementoblasts has denuded the root of these cells, the process of rhizolysis inevitably begins: This will be accelerated by mediators arising from the pericoronal follicle. When there is no permanent successor due to partial anodontia, rhizolysis occurs extremely slowly, and months later, without the epithelial rests of Malassez that were dead due to apoptosis, alveolodental ankylosis becomes established, and the tooth will gradually be replaced by bone, still within a physiological context. Conclusion: Rhizolysis and physiological tooth resorption may occur rapidly or slowly, early or late, and this depends on the presence of the permanent tooth, or its absence due to partial anodontia.

Palavras-chave: Dente decíduo. Rizólise. Anquilose alveolodentária. Reabsorção por substituição. When mineralized structures, such as bone, cement, dentin and enamel are exposed to connective tissues, they tend to attract the clasts biochemically. These are juxtaposed on the surfaces and initiate a slow process of resorption.
When there is a source of stimulatory mediators of mineralized tissue resorption in the proximities of the denuded mineralized surface areas, the process is significantly accelerated. 7 In primary teeth with exposed root surfaces, the sources of these mediators may be the following: a) The pericoronal follicle of the permanent tooth, which has an important epithelial part in its structure, consisting of the reduced epithelium of the enamel organ adhered to the crown, and of the epithelial remnants of the dental lamina that formed part of the Gubernacular cord. 4 The epithelial cells secrete a mediator denominated EGF, or epithelial or epidermal growth factor, and they biochemically command the other mediators of the pericoronal follicle. This mediator stimulates pericoronal bone resorption of teeth for the purpose of opening the way for tooth eruption to occur, moreover, it is the structure responsible for this function. 4,5,6 b) A pulp, periapical or periodontal inflammatory process due to caries or periodontal disease in the same primary tooth in the process of rhizolysis or in a neighboring tooth. 4 As they approach a primary tooth, the mediators of the pericoronal follicle of the permanent tooth that stimulate bone resorption, these equally stimulate the clasts that are positioned on the denuded surface of the root. 7 Thus rhizolysis is accelerated, especially on the surfaces facing the permanent tooth. 7 The primary tooth gradually loses all of its periodontal and bone support, and naturally exfoliates, as this is about to occur in Figure 1A Without the proximity of the absent permanent tooth, due to partial anodontia, there will be no acceleration of the resorption process initiated by apoptosis of the cementoblasts, odontoblasts, fibroblasts and epithelial rests of Malassez. 4 Apoptosis affects these cells in a random manner as a biological trigger of rhizolysis, leaving areas of cement and dentin exposed in the connective tissue, which attracts the clasts to juxtapose themselves and initiate the physiological primary tooth resorption.
Without the presence of a source of mediators that induce mineralized tissue resorption, which arise from the pericoronal follicle of the permanent tooth or an inflammatory process of pulp, periapical and/or periodontal origin, rhizolysis is extremely slow and occurs in a random and irregular manner, throughout the entire deciduous root. 4 For many years, the primary tooth can remain with a root structure that provides periodontal support for its permanence, nevertheless, it must always be considered a tooth in a state of rhizolysis to be exfoliated. If this tooth is in a position of occlusion in an adolescent or adult patient, the masticatory load will be an occlusal trauma and will be a source of mediators that will accelerate mineralized tissue resorption through an inflammatory process induced by this cause.  There are two types of deciduous rhizolysis: one early and fast, as in A, and one late and slow, as in B. We emphasize that both are physiological and inevitable within the context of the exfoliation of human primary teeth.  There will be a mixture between dental and bone tissues that can initially be identified in radiographic and macroscopic images.
However, after a certain degree of development, these dental tissues will no longer be identifiable, not even macroscopically, as bone tissue will completely replace them. This may be observed, in an exemplary, didactic and elegant manner in Figure 1B. This case is self-explanatory in the image because this process that slowly follows alveolodental ankylosis is denominated "replacement" root resorption.

FINAL CONSIDERATIONS
The two radiographic images revealed the two different extreme situations, which form part of physiological tooth resorption, in an exemplary manner: when there is a subjacent permanent successor, and when this is not present due to partial anodontia. In all primary teeth, rhizolysis begins after its formation is complete, thanks to the apoptosis of their cells, which represents their biological trigger. Because of this apoptosis, the teeth may exfoliate more rapidly or more slowly in the following ways: a) When apoptosis induced by cementoblasts leaves the root denuded of these cells, the process of early rhizolysis inevitably begins. This will be accelerated by mediators arising from the pericoronal follicle. Therefore, an early and rapid rhizolysis will occur. b) Without the mediators of the pericoronal follicle of the permanent successor tooth, rhizolysis will occur extremely slowly. Months later, without the epithelial rests of Malassez, dead due to apoptosis, alveolodental ankylosis will become established and the tooth will gradually be substituted by bone, still within a physiological context. Ankylosis and resorption by substitution constitute late and slow primary tooth rhizolysis, however, physiologically, as part of the human species, primary teeth exfoliate.
The origin of the word deciduous (primary) is a word that means "something that falls. This may occur sooner or later.