mechanisms of tooth eruption

MECHANISM OF TOOTH ERUPTION

MECHANISM OF TOOTH ERUPTION

Phases:

1. Pre-eruptive phase

- the enamel organ lying within the developing jaw reaches full size along with completion of tooth crown calcification.

2. Eruptive or Pre-functional phase

- begins with root formation, periodontal ligament & dento-gingival junction.

- it is completed when the tooth reaches the occlusal plane.

*The crown penetrates the oral mucosa to appear in the oral cavity.

3. Post-eruptive or Functional phase

- small tooth movements that occur after the tooth has reached the occlusal plane.

*Post-eruptive tooth movement maintain the position of the erupted tooth while the jaw continues to grow & it compensates occlusal

& proximal wear.

Mammalian Teeth Can Be Subdivided into 3 Broad Categories On The Basis Of Their Eruptive Characteristics:

1. Continuously Growing Tooth- no gross separation between the anatomical crown & anatomical root.

- continuous growth of the tooth at the apex & continuous eruption occur throughout the life of the animal.

- clinical crown is constantly replaced by root covered with enamel in the progressive stages of development.

- with the loss of tooth substance due to occlusal attrition, more tooth substance is extruded from the socket to maintain the clinical crown

- this form of tooth is characteristic of animals with rapid occlusal wear & eruption.

(ex. Rodent incisor teeth)

2. Continuously Extruding Tooth

- has a defined crown & anatomical root.

- as the tooth is worn, more of the anatomical crown extrudes & epithelial attachment migrates apically.

- since no new tooth structure is being formed, it results to gradual loosening & final exfoliation of the tooth.

- this tooth form is characteristic of the lower incisors of sheep & cattle.

3. Continuously Erupting Tooth

- eruption does not occur by enlargement of the clinical crown but rather by addition to the alveolar process.

- this is characteristic of human teeth.

Stages of Tooth Eruption:(Classification)

1. Follicular growth (Steedle & Proffit)Stage 1 – Preparatory Stage

(Noyes & Schour)**INITIATION OF CALCIFICATION**

- The permanent tooth germs that bud from the deciduous teeth generally assume a lingual position relative to their precursors.

- As the deciduous teeth develop & erupt, the permanent tooth follicles undergo somewhat complex migrations from their initial to their pre-eruptive positions.

- Once the crown formation has begun, the follicles of the posterior teeth move bucally.

- Eruptive movement of the tooth per se begins as soon as the root begins to form.

2. Pre-emergent eruptive spurtStage 2 – Migration toward the

oral epithelium**INITIATION OF ROOT

DEVELOPMENT**

- Root formation continues as the tooth begins a period of rapid eruption in the occlusal direction.

- Two processes necessary for eruptive phase:

a) There must be resorption of the overlying bone & of the overlying deciduous roots in the case of permanent tooth.

b) The eruption mechanism itself must move the tooth in the direction where the path has been cleared.

3. Post-emergent eruptive spurtStage 3 – Emergence of crown tip

into the oral cavity**GINGIVAL EMERGENCE**

- At the time of gingival emergence, the rate of human tooth eruption is at its greatest.

- Eruption rate slows as the tooth approaches the occusal plane.

4. Juvenile occlusal equilibriumStage 4 – First occlusal contact**FIRST OCCLUSAL CONTACT**

- Once the permanent tooth reaches occlusion, occlusal movement stops or incredibly slows down for several years.

- This period of relative quiescence ends at the beginning of puberty, when the second active phase of eruption begins.

5. Circumpubertal occlusal eruptive spurtStage 5 – Full occlusal contact**PERIOD OF QUIESCENCE &

PUBERTAL GROWTH**

- Occurs between 11 & 16 years of age, the teeth in occlusion begins a second active eruption phase lasting for 2-3 years.

- This period is characterized by increase in lower facial height.

- Facial tissues undergo a period of accelerated growth with:

>lengthening of the facial & masticatory muscle

>lowering of the mandible & the associated soft tissues

- This eruptive spurt slows as the face reaches maturity & a state of relative equilibrium re-established by 18 years of age.

6. Adult occlusal equilibriumStage 6 – Continuous eruption**PERIOD OF ADULTHOOD**

- Once physical maturity is reached, vertical tooth movement does not stop abruptly.

- Throughout life, small increase in lower facial height & continued eruption occur.

- Lower facial height increases by:

* 0.3 mm/year in the second decade* 0.1 mm/year in the third decade* 0.07 mm/year in the seventh decade

- Mesial & occlusal eruption continuous in life:

> interproximal & occlusal attrition> tooth loss> marked occlusal abrasion>periodontal breakdown

Theories of tooth eruption:

2 MAIN CLASSES:

a) The tooth is pushed out of the bone by forces generated beneath & around the tooth.

b) The tooth is pulled out as a result of tension developed in the periodontal ligament.

Factors that influence tooth eruption:

1. Gubernacular Cord

- remnants of dental lamina & connective tissues contained in the gubernacular canal, which are found in successional teeth.

- connects the follicles of permanent teeth to the oral mucosa & has an important role in controlling the movement of the developing teeth through the growing jaws.

- provides a path of least resistance & is actively engaged in pulling the tooth out from the underlying tissues.

2. Alveolar bone growth

- continued alveolar bone deposition results in the eruption of teeth to their occlusal location.

3. Root growth

- pressure generated from the growing root-end may result in the resorption at the base of the alveolar socket.

4. Intrapulpal pressure

- once a tooth has penetrated into the oral cavity, there must be some form of pressure change favoring eruption; & such change of pressure may be derived from the vascular pressure within the pulp cavity.

5. Periodontal ligament

- Three theories:

a) The cross-linking & shrinkage of collagen fibers

– tractional forces are generated within the oblique fiber system of the periodontal ligament due to the cross-linking and aggregation that occurs during the collagen maturation.

b) The contractility & mobility of fibroblasts

– fibroblasts within the periodontal ligament may generate an eruptive force either by their contractility or locomotor activity.

c) The thixotropic gel-like nature of the periodontal membrane

Thixotropic- property of certain gels of becoming liguid when shaken or stirred.

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