Tooth Eruption and Exfoliation embryology

• Embryology of tooth eruption and exfoliation is the area of focus

Learning Outcomes

• Describe the three phases of tooth eruption
• Describe how the dento-gingival junction originates
• Describe the process of tooth exfoliation
• Outline the theories of tooth eruption
• Link the processes to the developmental timeline of teeth and to eruption and exfoliation ages for each tooth

Eruption Process of Teeth

• The eruption process begins before birth and continues throughout life
• Beginning in early childhood around 6 months of age is part of the active eruption phase, but eruptive processes start much earlier
• Eruption continues even after all teeth are present in the mouth
• The permanent dentition erupts after the primary dentition

Eruption of Teeth

• The eruption process of teeth is a continuous process that occurs throughout life, beginning during early embryological development
• It involves three phases
• It begins once tooth crown development completes during the bell stage and alongside root development
• The process begins much earlier than when the first tooth is seen in the oral cavity around 6 months of age
• It is a multi-factorial process which is not entirely understood

Clinical Significance of Tooth Eruption and Exfoliation

• Providing the physiological foundation of oral health advice for teething for parents
• Providing the physiological foundation of oral health advice for wobbly teeth in children and parents
• Providing the physiological foundation of oral health advice for wisdom teeth erupting
• Clinically distinguish normal and abnormal tooth eruption and exfoliation,
  identify a supernumerary tooth, a missing tooth and know when to refer for orthodontics

Definitions

• Eruption is the physiological movement of teeth from their developmental position in the alveolar bone through the soft tissues to their position of function in the oral cavity
• The eruption process has three phases
• Exfoliation is the physiological resorption of primary teeth until they are lost

Embryological Links to Eruption and Exfoliation

• Begins during the bell stage
• Ameloblasts, cells of the enamel organ, osteoclasts, odontoclasts and oral epithelium play a role
• Underpinning knowledge explains the various theories on the mechanisms of the erupting tooth

Clinical Links to Eruption and Exfoliation

• The developmental timeline during embryology follows into life with the eruption timeline that is essential to identify the presence and position of missing teeth
• The developmental timeline during embryology follows into life with the eruption timeline that is essential to identify the presence and position of supernumerary teeth
• The developmental timeline during embryology follows into life with the eruption timeline that is essential to identify the presence and position of crowding of teeth and when orthodontic referral is indicated

Phases of the Eruption Process

• Pre-eruptive phase
• Eruptive phase
• Post-eruptive phase

1. The Pre-Eruptive Phase

• The developing tooth moves within the alveolar bone until crown formation is complete
• This movement begins during the bell stage, which remodels the bony crypt to allow more space for the developing tooth

2. Active Eruption Phase

• Teeth move through the alveolar bone, the intraosseous phase, and then soft tissue, the supraosseous phase, to the oral cavity
• This phase starts around the time of root formation and continues until the tooth reaches occlusion
• Root formation continues after this phase

3. Post-Eruptive Phase

• Teeth move after active eruption to maintain occlusion and compensate for occlusal and proximal tooth wear, as well as growth
• This movement occurs throughout life, like when an opposing tooth is removed

Tooth Eruption Bell Stage

• Tooth eruption starts during the bell stage of tooth development

Active Eruption Phase: Role of the Reduced Enamel Epithelium

• Amelogenesis completes, ameloblasts shrink and combine with the outer enamel epithelium, stratum intermedium and any residual stellate reticulum to form the reduced enamel epithelium

Functions of the Reduced Enamel Epithelium

• Protects the developing tooth crown
• Fuses with oral epithelium, creating an eruption pathway
• Forms the dento-gingival junction as an essential seal

Tooth Eruption Pathway

• The bone overlying the developing crown is resorbed by osteoclasts, including odontoclasts of predecessor teeth
• This allows a pathway for movement that is initiated by complex signaling of the cells
• The reduced enamel epithelium protects the tooth crown from the osteoclasts and odontoclasts

Tooth Eruption Pathway

• The reduced enamel epithelium fuses with the oral epithelium to create an eruption pathway
• There are no blood vessels or nerves present yet
• Stimulation and trauma occur to the ectomesenchyme to enable fusion
• This fusion forms a seal that prevents exposure of the underlying ectomesenchyme and haemorrhage

Formation of the Dento-Gingival Junction and Sulcus

• As the tooth breaks through the oral epithelium, the reduced enamel epithelium and oral epithelium combine to form the dento-gingival junction
• The dento-gingival junction seals off the external oral cavity from the rest of the body
• A shallow trough forms, creating the gingival sulcus
• The junction has clinical significance for periodontal disease and the long junctional epithelium of the gingiva

Rate of Eruptive Movement

• Movement through bone is slow = 1-10um per day
• Movement through soft tissue is faster = 75um per day until occlusion is reached
• Muscular forces of the tongue, cheek, and lips guide the tooth into position
• Sustained force of 4-5g is required
• This can be caused by thumb sucking and using a dummy

Reduced Enamel Epithelium

• The reduced enamel epithelium surrounds the developing tooth crown during eruption

Exfoliation: Shedding of Primary Teeth

• Permanent successor teeth develop and increase in size, starting eruptive phases in a lingual position
• Exfoliation of primary teeth signals where odontoclasts slowly resorb the roots of primary teeth up to its crown, leaving it largely intact
• Masticatory forces contribute to exfoliation
• Patterns should be similar
• Variations are key indicators of abnormalities

Root Formation Theory

• The tooth crown is elevated by the thrust of root development
• Refuted since eruption occurs throughout life and root development doesn't

Bone Re-Modelling

• It is unsure whether bone resorption and deposition causes teeth to erupt or whether this is an effect
• It is not the only mechanism; it is believed that it is modulated by the dental follicle

Dental Follicle Theory

• Signals between the dental follicle and the reduced enamel epithelium induce bone re-modelling
• This explains consistency of eruption times linked to the lifecycle of ameloblasts
• A = enamel organ
• B = dental papilla
• C = dental follicle

Periodontal Ligament Theory

• The formation of the periodontal ligament by fibroblasts contributes to the movement in tooth eruption
• This is refuted similar to root formation theory

Molecular Determinants of Tooth Eruption

• Various molecules are involved in the complex process of tooth eruption playing a different role
• Recent theories include bite forces on the soft tissues and neuromuscular forces

Summary of Tooth Eruption and Exfoliation

• Eruption and exfoliation are complex and multi-factorial processes
• The three phases of eruption
• The active phase in detail
• How the phases link to embryological development
• Outline of the theories of eruption