Tooth Eruption and Exfoliation

• Tooth eruption and exfoliation are key processes in dentistry
• Understanding these processes provides the foundation for oral health advice
• Being able to distinguish normal versus abnormal tooth eruption and exfoliation is crucial

Learning Outcomes

• To describe the three phases of tooth eruption
• To describe how the dento-gingival junction originates
• To describe the process of tooth exfoliation
• To outline the theories of tooth eruption
• To link developmental timelines of teeth to eruption and exfoliation ages for each tooth

Eruption Process

• The eruption process is a continuous process that occurs throughout life, beginning during early embryological development
• Tooth eruption involves three phases
• Eruption starts once tooth crown development is complete during the bell stage along with root development
• The first tooth becomes visible in the oral cavity at around 6 months
• Tooth eruption is a multi-factorial process, not entirely understood and there are various theories to explain it

Clinical Significance

• Knowledge of tooth eruption is important for oral health advice for 'teething', wobbly teeth, and erupting 'wisdom' teeth
• Recognizing abnormalities includes identifying supernumerary and missing teeth
• This knowledge is also needed when referring patients for orthodontics

Definitions

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

Embryological Links

• The process starts during the bell stage
• Ameloblasts, cells of the enamel organ, osteoclasts, odontoclasts, and oral epithelium are important
• Underpinning knowledge explains various theories on the mechanisms of the erupting tooth

Clinical Links

• The eruption timeline is essential for identifying the presence and position of teeth
• Abnormalities include missing teeth, supernumerary teeth, and crowding

Phases of Eruption

• The eruption process is divided into pre-eruptive, active, and post-eruptive phases outlined
• Histological perspectives of each phase can be viewed within dental tissue

The pre-eruptive phase

• It involves movement of the developing tooth within the alveolar bone until crown formation is complete
• This phase starts during the bell stage to remodel the bony crypt
• Remodeling the bony crypt allows more space for the developing tooth

Active eruption phase

• In the active eruption phase, teeth move through the alveolar bone (intraosseous) and then soft tissue (supraosseous) to the oral cavity
• The active phase begins around the same time as root formation
• The active phase continues until the tooth reaches occlusion
• Root formation continues after the tooth reaches occlusion

Post-eruptive phase

• Post-eruptive phase involves movements after active eruption to maintain occlusion
• It compensates for occlusal and proximal tooth wear as well as overall growth
• Compensation occurs throughout life, for example, compensation occurs when an opposing tooth is removed

Reduced Enamel Epithelium

• Once amelogenesis is complete, ameloblasts shrink and combine with outer enamel epithelium, stratum intermedium and stellate reticulum forming the reduced enamel epithelium
• Reduced enamel epithelium protects the developing tooth crown
• It fuses with oral epithelium creating an eruption pathway
• It forms the dento-gingival junction as an essential seal

Eruption Pathway

• The bone overlying the developing crown is resorbed by osteoclasts and odontoclasts of predecessor teeth

• This allows a pathway for movement and is initiated by complex signaling of the cells

• The reduced enamel epithelium protects the tooth crown from osteoclasts and odontoclasts

• The reduced enamel epithelium fuses with oral epithelium to create an eruption pathway

• At this stage, no blood vessels, or nerves are present

• Stimulation and trauma occurs to ectomesenchyme to enable fusion

• This fusion forms a seal that prevents exposure of underlying tissues

• As the tooth breaks through the oral epithelium, the reduced enamel epithelium and oral epithelium forms the dento-gingival junction

• The external oral cavity is sealed off from the rest of the body

• A shallow trough is created forming the gingival sulcus

• This junction has clinical significance for periodontal disease and gingiva

Rate of Eruptive Movement

• Movement through bone is slow, approximately 1-10um/day
• Movement through soft tissue is faster, approximately 75um/day until occlusion is achieved
• Muscular forces from the tongue, cheek, and lips guide the tooth into position
• A sustained force of 4-5g is required

Exfoliation

• Permanent successor teeth develop lingually, increasing in size and initiating eruptive phases
• Odontoclasts resorb the roots of primary teeth
• Masticatory forces also contribute to the exfoliation process by applying pressure
• Variations in exfoliation patterns can indicate clinical abnormalities

Theories of Eruption

• How a tooth erupts is not fully understood
• Theories attempt to explain the process
• Root formation theory suggests that the tooth crown is elevated by the thrust of root development
• Bone re-modelling may be caused by teeth erupting and is modulated by the dental follicle
• Dental follicle theory says signalling between the dental follicle and the reduced enamel epithelium induce bone re-modelling
• Periodontal Ligament Theory suggest that the formation of the periodontal ligament by fibroblasts contributes to the movement in tooth eruption

Molecular determininants

• Various molecules are thought to be involved in the complex process of tooth eruption and each plays a role
• More recent theories under investigation include bite forces on soft tissues and neuromuscular forces