Eruption Of Teeth

Head of oral biology

By Hesham Dameer

Al-Azhar dental college

Tooth eruption is the process by which developing teeth emerge through the soft tissue of the jaws and the overlying mucosa toenter the oral cavity, contact the teeth of the opposing arch, and function in mastication .These movement are too complex and do not cease after reaching the teeth of the opposing arch, but continue through the whole life span of the normally functioning teeth. Tooth eruption begin as far as the crown of the developing tooth has matured and the root commences its growth .

Physiologic tooth movement ( eruption ) occur in order to attain and maintain the functional alignment of teeth in relation to each other in the same jaw and with those found in the opposing jaw .Terminology of eruption :

Active eruption : it is the actual movement of the tooth from its developmental site to its position in the dental arch.Passive eruption : does not involve tooth movement but occurs due to apical recession of gingival tissue exposing more tooth structure into the oral cavity.

Anatomical crown : it is that part of the tooth which is covered by enamel.Clinical crown : it is that part of the tooth exposed to the oral cavity

Phases of tooth eruption1. Preeruptive phase: which begins in the early bell stage and ends at the beginning of root formation.2. Prefunctional eruptive phase: Starts with initiation of root formation and completed when the teeth reached its functional position in occlusion. This phase has an intraosseous and extraosseous compartments. 4 stages: root formation, movement, penetration and occlusal contact .

3. Functional (Posteruptive) phase: Takes place after the teeth are functioning to maintain the position of the erupted tooth in occlusion while the jaws are continuing to grow and compensate for occlusal and proximal tooth wear and ends at the end of life span of the tooth.

Relative position of primary and permanent incisorteeth.A, Preeruptive period. B, Prefunctional eruptive period.

1-PATTERN OF TOOTH MOVEMENT IN THE PRE-ERUPTIVE PHASE:a- DECIDUOUS TEETH:

At the beginning , the differentiated deciduous tooth germs have enough space between them. Rapid growth of deciduous tooth germs utilizes the available space between them leading to crowding of the developing deciduous germs particularly in the incisors and canine region. This crowding is relieved by:

1-growth of the jaws in length, which provides room for the deciduous molars to move backward and for incisors and canine to move forward toward the midline by bodily or drifting movement.

2-Increase in width of the jaws ,which leads to outward movement of the tooth germs labially or bucally by bodily movement.3-Increase in height of the jaws, which leads to the occlusal movement by eccentric growth.

b-PERMENANT TEETH WITH DECIDUOUS PREDECESSOR:

A-Permanent incisor and canine tooth germs develop lingual to their predecessors at the level of their incisal surfaces and in the same bony crypt. Then the anterior permanent tooth germs occupy a more apical position as their predecessor erupt.

They lie in their own bony crypt which is attached to the bone of the lingual plate of the jaws through a canal called GUBERNACULAR CANAL which is filled with the gubernacular cord. This cord consists of C.T. and remnants of the dental lamina. It is a link between the crypt of the permanent tooth germ and the oral mucous membrane.IT HAS BEEN PROPOSED THAT GUBERNACULAR CANAL PROVIDE THE DIRECTIONAL PATH FOR ERUPTION.

B- The premolars also begin their development lingual to their predecessors at the level of their occlusal surface and in the same bony crypt. At the end of this phase , they shift to a position beneath the divergent roots of their predecessors. The opening of the gubernacular canal of the premolars crypts are found within the socket of the corresponding deciduous molars.C-Permenant molars have no predecessors show different movements.The maxillary molars develop in the tuberosity of the maxilla,their occlusal surfaces facing downward and

distally. Then move round when sufficient space is provided by growth of maxilla at eruptive phase.The mandibular molars develop with their occlusal surfaces inclined mesially.Then become upright when available space is obtained.

TYPES OF MOVEMENT DURING THE PRE-ERUPTIVE MOVEMENT:

1- Bodily [drifting] movement of the tooth germ during which osteoclastic activity with bone resorption on the surface of the crypt wall in advance of the moving tooth[i.e. the surface toward which tooth moves],while bone deposition occurs on the crypt wall behind. 2-Excentric growth : it means that one part of the developing tooth germ remains stationary and the remainder continues to grow leading to a shift in its center e.g. the deciduous incisor maintain their superfacial position as the jaw grow in height.During excentric movement only bone resorption occurs and is found on the surface of the crypt faces the growing tooth germ.

II -PATTERN OF TOOTH MOVEMENT IN THE ERUPTIVE PHASE :During this phase the tooth moves from its position within the bone of the growing jaw to its functional position in the occlusal plane. The principle direction of movement is axial or occlusal. However, movement in other plane also occurs.So, during the eruptive phase tooth moves in different directions which are :a-Axial [occlusal] movement in the direction of the long axis of the tooth. 

b-Drifting [bodily] movement in distal,mesial,lingual or buccal direction. c-Tilting or tipping movement: around the transverse axis of the tooth[ as in case of permanent molars ].d-Rotating movement : around the longitudinal axis of the tooth [in case of lower central incisors, and lower canines.THE DEVELOPMENTAL CHANGES OCURRING DURING THE ERUPTIVE PHASE INCLUDE :1-ROOT FORMATION2-FORMATION OF THE ATTACHMENT APPARATUS [P.L.,BONE& CEMENTUM].

3 -DENTOGINGIVAL JUNCTION.

III- PATTERN OF TOOTH MOVEMENT IN POST-ERUPTIVE PHASE : During this phase the tooth moves to :1) Maintain its position while the jaw increase in height . This movement occurs most actively between the age of 14 -18 years and is associated with condylar growth, then it ceases when jaw growth is completed . The principle movement is axial during this period .Histologically , bone deposition occurs at the alveolar crest and in the socket floor. This bone is not responsible for tooth movement because it occurs latter to tooth movement.2) Compensate for occlusal and proximal wear of the teeth which continue throughout the life span of the

tooth. so 

a. The occlusal wear is compensated by axial movement of the tooth. It is assumed that the continuous deposition of cementum around the apices of the roots is sufficient to compensate for occlusal wear, but there is no evidence that deposition of cementum actively move the tooth. It is only infilling phenomenon.b. Proximal wear also takes place at the contact points between teeth. To compensate this wear & maintain tooth contact ,mesial drift occurs. Histologically , this mesial drift is seen as selective deposition and resorption of bone on the socket wall by osteoblasts & osteoclasts respectively. With electron microscope collagen remodeling in the periodontal ligament is seen.

MECHANISM OF TOOTH MOVEMENTThe nature of the intrinsic forces involved in active tooth eruption is not fully understood . Available experimental evidence seems to support factors related to tissue tension theories. Experiments where tooth is wired to the lower border of the mandible show that inspite of immobilizing the tooth , an eruptive path is formed by resorption of the overlying bone. However ,if the dental follicle associated with erupting tooth is removed ,no such pathway in bone is formed this finding lead to the fact that human teeth according to

a sepcific chronology imply the presence of a programmed mechanism that leads to tooth eruption. Such mechanism is probably a multifactorial one that includes control by specific gene[s],hormones as well as several growth factors. The mechanism that brings about tooth movement is not properly understood.

Epithelial/connective tissue interactions and molecular signaling during tooth eruption:

During active eruption particularly the prefunctional stage, epithelial/connective tissue interactions have been demonstrated between the reduced dental epithelium covering the enamel of erupting teeth and the adjacent connective tissue; of the dental follicle. Signals from the reduced dental epithelium lead to attraction of monocytes to the dental follicle where CSFI (Colony stimulating factor 1) promotes their differentiation into osteoclasts. EGF (Epidermal Growth Factor) which upregulates the expression of TGF B ( Transforming Growth Factor beta) have been localized in the dental follicle.

The reduced dental epithelium also secretes proteolytic enzymes which break down the connective tissue in the pathway of the erupting teeth to facilitate their movement, path of least resistance to their place in the dental arch. Another product of the reduced dental epithelium is IL-I A (Inerleukin 1 alpha) which promotes bone resorption. At the apical region of the alveolar bone a transcription factor is expressed in the dental follicle, namely Cbfa-I (Core binding factor a-I). This factor is involved in osteoblast differentiation and these cells form bone at sites of appositin needed for the remodeling that accompany tooth eruption. At sites of bone resorption TGF B downregulates the expression of Cbfa-l.

Tooth eruptionultra-low power section of developing jaw

Key: 1 = milk teeth; 2 = developing adult teeth; 3 = jaw bone; 4 = gum1 mm

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Tooth eruption

Osteoclasts erode jaw bone to make way for erupting adult tooth

ERUPTION PATH :

GUBERNACULAR CORD AND CANAL:The fibrocellular follicle surrounding the permanent tooth retains its connection with the lamina propria of the oral mucous membrane by means of a strand of fibrous tissue containing remnants of the dental lamina, known as the GUBERNACULAR CORD. It is contained in a bony canal known as GUBERNACULAR CANAL. In the anterior teeth ,these canals are seen in the lingual plate of bone ,while in premolars the canals are present in the corresponding deciduous molars.

At a given time, bone resorption by osteoclasts will occur in this GUBRNACULAR CANAL even if the tooth is stationary. Consequently, formation of the tooth eruption pathway is a localized, genetically programmed event that does not require pressure from the erupting tooth.The rate of tooth eruption depends on the phase of movementIntraosseous phase: 1 to 10 µm/dayExtraosseous phase: 75 μm/day

Environmental factors affecting the final position of the tooth:Muscular forcesThumb-sucking

Diagram of a developing eruption pathway. A, Earlydeveloping eruption pathway. B, Resorption of bone in eruptionpathway.

Erupting deciduous tooth

Mechanisms of eruptive Tooth MovementA. MECHANISM OF AXIAL TOOTH MOVEMENT Several theories have been advocated in regard to

the probable mechanism which may determine and bring about the axial tooth movements :

1. Root formation theory : Contention : the growing apical portion of the root

including ( the root sheath, pulp tissue, and root sheath ) is believed to be concerned with the axial movements of the tooth .

The growth of these components either collectively or separately is believed to provide a force . Such force permits for the axial tooth movements, by pushing against an apical structure termed cushioned hammock ligament .

This ligament is considered as suspensory structure found at the base of tooth crypt and serves in transforming the generated pressure of the growing root at the bottom of the crypt into tension so the tooth moves axially .

Fallacies : a. Some teeth are evidenced to move a

distance greater than the length of the developing root can ultimately attain for example , the canine .

b. tooth eruption is a process not limited to a certain period of time .

c. If the root formation is regarded to be responsible

for tooth eruption, it would be expected that the onset of root formation must coincide with the commencement of the eruptive tooth movement .

d. The concept regarding the cushioned hammock ligament to act as a fixed base is explained to be not valid . This is because the hammock ligament is evidenced to turns around the apical root end , but does not posses any insertion to the wall of the alveolar socket e. When the growing components of the apical end of the root are experimentally removed, the remaining portion of the tooth is still progressively erupting .f. When a tooth is experimentally prevented from eruption by root pinning, the apical root portion is still growing and create a pressure on the base of the alveolar socket . Such finding indicate that the root growth generate a force, however, this force is not exactly directed to cause tooth eruption .

2. Bone remodeling theory :Contention : this theory presumes that eruption of the teeth occurred through selective bone remodeling . The end result of bone remodeling is a considerable bone deposition at the bottom of the socket .Fallacies : 1. It has been evidenced that the alveolar bone remodeling which occurs around the root, that end by bone deposition, is the outcome and not the cause of axial tooth movement because as the tooth commences its eruptive movement, bone resorption is initiated at the base of the socket and so bone mounting is not a cause for eruption but a filling process occurring after the tooth has been axially moved .

3. Vascular pressure theory :Contention : this theory supposes that a local increase in tissue fluid pressure in the periapical region is sufficient to move the tooth .Fallacies : the experimental removal or transection of the growing apical root portion and the associated tissue does not interfere with the tooth eruption . 4. Periodontal ligament traction theory :This theory denotes that the tooth eruption occurs through the generation of a traction force to bring the tooth into occlusion . The traction force is presumed to be established by either the fibroblasts or collagen fibers found in the periodontal ligament , or by both .Drugs that interrupt the proper formation of collagen in the periodontal ligament ( like vit .C deficiency ) lead to either slowing or ceasation of the eruptive tooth movement.

Furthermore, experimental transection or removal of the actively growing apical root portion do not cease the eruption activity of the distal root portion . Therefore, it is concluded that : Firstly : the root growth cannot be considered as the prime mover of tooth to erupt . Secondly : the only viable tissue in the distal root portion is the periodontal ligament , thus the eruptive forces of a tooth is presumed to reside in this ligament . In concern to the collagen fibers of the periodontal ligament it has been suggested that they bring the tooth into the oral cavity through their elaborated contraction while they are formed . In regard to the role played by the fibroblasts, it is presumed that they provided with contractile elements .

These fibroblasts has both extracellular and intracellular characteristics :1) Their cytoplasm contain large number of actin

contractile filaments , which when contract translate its action into forces outside the cell .

2) The cells establishes a firm connection to the adjacent cells by special cellular junctions .

3) The cell establishes firm connection to the adjacent collagen fibers by a special structure called fibronexus which is formed of :

a. intracellular microfilament . b. Thickening of the fibroblastic cell membrane . c. Extracellular filaments . d. A sticky glycoprotein called fibronectin that bind

the cell to the fiber.So the fibroblasts should contract collectively to

insure

massive contraction . This contraction can be secured through the harmonious play-making of the whole fibroblasts results in summating their contractile forces which collectively cause tooth eruption .

Several factors control mesial drift :

(a)Contraction of the transseptal (INTERDENTAL) fibers: These fibers connect the cervical portion of the adjacent teeth together and as the proximal tooth surfaces of adjacent teeth become worn from functional tooth movement, the transseptal fibers of the periodontal ligament become shorter (due to contraction) and thereby maintain tooth contact .

(b) Pressure of the oral and para-oral tissues : buccal mucosa and tongue push teeth mesially

B. MECHANISM OF MESIAL TOOTH MOVEMENT

(c) Anterior component of masticatory forces : As the masticatory forces work, the second and third permanent molars act as a braces to the teeth anterior to them and this prevent any tendency for their distal displacement . Therefore, during mastication, the more pronounced mesial inclination of the long axes of the mandibular permanent molars leads to their striking with their corresponding maxillary teeth through mesially inclined stresses .

The summation of the anterior component of the occlusal forces would be transmitted mesially thus the teeth are forced in the same direction .

Thank you

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Dr. Hesham Dameer