Tooth Eruption and Exfoliation Notes

Eruption And Exfoliation Of Teeth Introduction

Human beings are endowed with two sets of dentition, namely primary or deciduous dentition and secondary or permanent dentition. There are 20 primary teeth and 32 permanent teeth.

Each primary tooth has a permanent successor tooth replacing it in a particular sequence. The primary teeth are of a smaller size. They are suitable for the smaller arch size and lesser functional demands of children. As the child grows, the jaw size and corresponding arch sizes increase.

Read And Learn More: Paediatric Dentistry Notes

The functional demand also gradually increases in adulthood. To cope up with the demands, the smaller primary teeth are sequentially replaced with bigger and stronger permanent teeth. The permanent teeth are to serve in the mouth for the rest of life.

There is no biological replacement for the permanent teeth if lost. This chapter deals with the process of eruption of teeth and the physiological exfoliation of primary teeth.

Development Of Dentition

Teeth are derivatives of the ectomesenchyme. ‘U’-shaped, primary epithelial bands appear in the stomodeum at around 4–5 weeks of intrauterine life corresponding to the upper and lower jaws.

Each primary epithelial band gives rise to a dental lamina and a vestibular lamina. The dental lamina and vestibular lamina are responsible for the formation of teeth and the vestibule, respectively.

1. Dental Lamina The dental lamina gives rise to a series of epithelial proliferations that extend into the ectomesenchymal tissues. Each epithelial proliferation is a precursor of a deciduous tooth. Tooth formation begins at the tip of this proliferation. The development of the tooth can be described in three stages, namely bud stage, cap stage and bell stage.
   • Bud Stage The end of the epithelial proliferation tract is surrounded by dense ectomesenchymal cells. The end appears rounded, resembling a bud. This first stage of tooth development is hence called the ‘bud stage’.
   • Cap Stage Th rounded end of the bud stage then grows into the ectomesenchyme. There is a condensation or dense gathering of ectomesenchymal cells adjacent to the bud-shaped epithelial proliferation. Th bud-shaped proliferation starts covering the condensed ectomesenchyme. At this stage, the proliferation resembles a cap placed over a ball of ectomesenchyme. The ball of ectomesenchymal cells is called dental papilla and the cap is called a dental organ. The thick band of ectomesenchymal cells that surround the dental papilla and dental organ is called a dental follicle. The dental papilla, dental organ and dental follicle together are called the tooth germ.
   • Bell Stage As the tooth germ continues to grow, the cap-shaped dental organ deepens at its concave side and assumes the shape of a bell. This stage is characterised by his differentiation and morph differentiation, as cells differentiate to exhibit distinctly identifiable groups. The cells in the periphery form the external dental epithelium and are cuboidal in shape while the cells in the middle form the star-shaped stellate reticulum. The cells adjacent to the dental papilla differentiate into the intermediate layer called the stratum intermedium and short columnar cells called the internal dental epithelium. All the structures of the tooth are eventually formed from these differentiated component parts of the tooth germ.
2. Formation Of Permanent Dentition The permanent teeth are also derived from the dental lamina. They are formed from another new tooth germ on the lingual aspect of the primary teeth. The new tooth germ appears at the junction of the dental organ of the deciduous tooth germ and the dental lamina. Formation of the permanent teeth with no deciduous predecessors begins from the distal extension of the dental lamina. Similar in-growths and tooth germs are formed into the ectomesenchyme.

Eruption Of Teeth

The eruption is the movement of a tooth from the place of its formation within the alveolar bone to its destination, the functional occlusal plane as described by Massler. In this eruption journey, a tooth makes its way through bone and overlying mucosa.

The movement of a tooth does not stop upon reaching the occlusal plane but continues to compensate for the growth of jaws and occlusal wear. The process of this continuous change is described in three phases.

1. Phases Of Eruption The physiology of eruption movement is described in three phases:
   • Pre-eruptive phase/preparatory phase
   • Eruptive phase/functional phase
   • Post-eruptive phase/functional phase
     1. (A) Pre-eruptive phase: Movement of developing tooth germ inside the alveolus, before root formation.
     2. (B–D) Eruptive phase: Movement of tooth from root formation until it establishes occlusal contact.
     3. (E) Post-eruptive phase: Gradual movement of tooth to maintain occlusal contact and to compensate for growth changes in jaw (note the drift of tooth away from the vertical line between (D) and (E))
     • The pre-Eruptive Phase Pre-eruptive or the preparatory phase is a period prior to the beginning of root formation. At this stage, the alveolar bone is very dynamic with growth expressed as an increase in height, width and length, along with necessary remodelling. The tooth germs present within this growing bone move in order to maintain their spatial orientation. This is achieved in two ways—bodily movement and eccentric growth.
       1. Bodily movement: The body of a tooth germ moves as a whole by resorption of bone in the forefront and deposition of bone from behind.
       2. Eccentric growth: Any growing structure generally exhibits the same magnitude of growth in all its parts. On the contrary, a growing tooth germ at this stage exhibits growth only at the root end, while the crown remains the same.
          • The position of the permanent tooth bud in the beginning of the pre-eruptive phase is lingual and near the incisal or occlusal region of its predecessor primary tooth. Towards the end of this phase, it is seen near the root of the primary tooth. This happens not by the movement of the permanent tooth bud but by the eruption of the primary tooth and the increase in height of the supporting tissues. The maxillary molars are present with their occlusal surface facing the distal direction. The occlusal surface of the mandibular molars faces mesially.
     • Eruptive Phase The eruptive or functional phase is a period from the beginning of root formation of a tooth till the tooth reaches the occlusal plane. The outward movement of a tooth bud is initiated after an adequate length of the root has been formed. This happens due to some triggering factors that are so far poorly understood. The root of a deciduous predecessor tooth, if present, is gradually resorbed while guiding the eruption path of the permanent tooth. Four events take place during this period. They are as follows:
       • Root formation begins.
       • The reduced enamel epithelium covering the crown gets firmly attached to the oral epithelium. This forms a double epithelial cover over the erupting crown.
       • The tooth clinically erupts into the oral cavity when half to three-quarters of the root has formed. The tooth erupts by breaking open the double epithelial layer, which initiates the clinical eruption stage.
       • Active eruption of the tooth towards occlusion.
     • Post-Eruptive Phase The post-eruptive or functional eruptive phase extends from the time a tooth reaches its occlusal plane until it is lost or exfoliated from the oral cavity. The post-eruptive phase strives to maintain the occlusion of teeth, enabling them to perform their function. There is a continuous adjustment in the position of the teeth in order to keep pace with the growing alveolus and demanding functional changes. The occlusal movement continues even after the growth of the tooth is complete to compensate for the constant occlusal wear. Tooth material wear occurs not only on the occlusal aspect but also in the interproximal areas. th loss of tooth material by such interproximal wear is compensated by the mesial drifting of the tooth in its effort to maintain interproximal contact. Mesial drifting is caused by occlusal wear, contraction of the periodontal ligament and soft tissue pressure.
2. Theories Of Tooth Eruption The eruption is a physiological process that is not fully understood. Many theories have been put forth to explain the mechanism of eruption. The most popular theories are as follows:
   • Root growth theory
   • Hydrostatic/vascular pressure theory
   • Theory of selective deposition and resorption of bone (bone remodelling)
   • Periodontal ligament traction theory
     • Root Growth Theory Root growth theory proposes that the growing length of the root lifts the whole structure of the tooth to a more occlusal level, thus causing the eruption movement. The absence of a non-resilient structure against which a root can pit to translate or generate its force for eruption could not prove this theory beyond doubt.
     • Hydrostatic Theory Pressure systems in and around the tissues at the base of the tooth are said to cause eruption according to hydrostatic theory. Tissues can hold water and exert mild pressure. The fact that the excision of the growing root and tissues around the erupting tooth did not stop eruption does not favour this theory.
     • Theory Of Selective Deposition And Resorption Of Bone Selective deposition of bone in the apical region and resorption of bone in the coronal region occur around a growing tooth. The presence of the dental follicle seemed essential for such bone activity. The theory of selective deposition and resorption of bone mentions that coordination between dental follicles and bony changes causes the eruption tooth.
     • Periodontal Ligament Traction Theory Fibres of periodontal ligament and fibroblasts provide contractile force for eruption according to periodontal ligament traction theory. Disruption of periodontal fires stops or slows eruptive movement. The periodontal ligament, along with the dental follicle, is said to cause an eruption of the tooth. Other theories that do not have broad scientific data or acceptance are as follows:
       1. Papillary constriction theory: It states that the constriction of dental papilla due to dentin formation and reduction in the size of the pulp create a thrusting force causing an eruption of the tooth.
       2. Epithelial path theory: Structures like hair, nail and tooth are derivatives of ectoderm. According to epithelial path theory, all ectodermal derivatives have the tendency to come out to stay on the external surface of the body. This theory is so-called as they retrace their path to the epithelium. The tooth erupts out as it is also a derivative of the ectoderm.
       3. Thrust by follicle: The periodontal tissues and dental follicle proliferate to push the tooth and bone socket away from each other, thus causing an eruption of tooth. The dental follicle, through its rich vascularity, provides bone-forming cells – the cells for osteoclastic and osteoblastic activity causing bone remodelling.
       4. Hammock theory: It was earlier thought that a ligament called Hammock ligament existed at the apical region of the teeth. This resilient ligament was thought to play an important role in causing an eruption of teeth by bouncing back like a hammock when a tooth thrusts its growing roots into it. Histological studies have subsequently disproved the presence of Hammock ligament.
3. Factors Affecting Tooth Eruption Mechanisms associated with the initiation and progress of the tooth eruption phenomenon are yet to be understood fully. Experiments, research and observations describe some factors that influence the process of eruption. These factors can be classified as general and local factors.
   • General Factors
     • Inherited pattern: The inheritance or genetic factor plays a definite role in the pattern and timing of eruption.
     • Metabolic disease: Eruption of teeth is affected by the general metabolism and health status of an individual.
     • Endocrinopathies: Abnormal resorption is seen in pituitary, thyroid and parathyroid hormonal disturbances, which in turn affect the eruption process.
     • Dietary factor: Deficiency of vitamins causes disturbance in the eruption of teeth. Vitamin A is essential in the tooth formation stage. Vitaminosis A affects osteoblasts, osteoclasts and bone remodelling. Vitamin C is essential for collagen formation and vitamin D is essential for deposition of calcium and phosphorous in bones. Deficiencies of vitamins B, C and D cause beriberi, scurvy and rickets, respectively, and all these conditions affect the developmental timetable of the teeth.
     • Congenital defects: Delayed eruptions or impacted teeth are seen in congenital defects like cleidocranial dysostosis. Lack of muscle balance due to aberrant uncontrolled muscle function in cerebral palsy disturbs the establishment of occlusal contact.
   • Local Factors
     • The presence of hard- or soft-tissue barriers such as supernumerary tooth and mucosal thickening hinders the eruption of tooth.
     • Periapical infection of a primary tooth destroys the peri coronal tissues of the permanent successor. An erupting permanent tooth has to break through the physiological barrier provided by these pericoronal tissues. In some occasions, the eruption of the permanent tooth is observed before its scheduled time due to loss offer coronal tissues.
     • Ankylosis of the primary tooth affects the eruption process and eruption path of the successive permanent tooth.
     • Premature loss of the primary tooth may also lead to migration and impaction of the permanent tooth bud.

Exfoliation Of Primary Teeth

Exfoliation is the physiological process of shedding of the primary teeth. Thprimaryteetharenotfunctionally efficient after some years as the jaw size and masticatory requirements increase in volume.

The larger permanent teeth are introduced into the oral cavity gradually to replace the smaller-sized primary teeth when this higher demand arises. The physiological process of exfoliation is equally a grey area in our understanding like the tooth eruption mechanism.

1. Mechanism Of Exfoliation Of Teeth These factors can be attributed to the exfoliation of the teeth. They are as follows:
   • Pressure from erupting permanent tooth: Odontoclasts are osteoclasts involved in the removal of dental tissues. They are capable of removing all dental hard tissues including enamel. The scavenger cells are seen at places where the permanent tooth contacts the primary tooth and starts resorbing the latter structure. When a considerable amount of root is lost, the primary tooth gets exfoliated.
   • Weakening of support to the primary teeth: Modifiationofthealveolus and resorption of the root weaken support to the primary tooth. The supporting systems are also weakened by facial growth.
   • Muscle growth: Muscle growth and thereby the increased masticatory forces weaken the primary teeth and promote their resorption.
2. Disturbances In Exfoliation
   • Retained primary tooth: When the initiating stimulus or the permanent tooth is congenitally missing or retarded, the primary tooth is retained beyond its timetable.
   • Submerged tooth: Th absence of the successor tooth or trauma destroying supporting structures may cause ankylosis of the primary tooth. A submerged tooth is an ankylosed over-retained primary tooth. The adjacent permanent teeth erupt and the ankylosed tooth looks submerged, for example, the second primary molar becomes a submerged molar when it gets over-retained following the congenital absence of the second premolar.

Eruption And Exfoliation Of Teeth Summary

1. The eruption is the movement of teeth from its place of origin and formation within the alveolar bone to its destination, the functional occlusal plane.
2. Phases of eruption: Pre-eruptive phase/preparatory phase, eruptive phase/pre-functional phase and post-eruptive phase/functional phase.
3. Factors affecting tooth eruption