TOOTH ERUPTION – PRE-ERUPTIVE, ERUPTIVE AND POST-ERUPTIVE TOOTH MOVEMENT
Author: Sanketh DS, MDS
Eruption is the process of tooth movement from its developmental position in the jaws to a functional position in the oral cavity and involves movement of the tooth through the bone and oral mucosa to reach the occlusal plane. These movements can be divided into three phases called, pre-eruptive, eruptive and post-eruptive phases.
Pre-eruptive phase: Is that phase when the tooth germ moves within the jaws before root formation. The developing tooth germs move in relation to each other and the growing jaws and position themselves for eruption.
Eruptive phase: Also called “pre-functional eruptive phase” is when the tooth moves in an occlusal direction from its position in the crypt to appear in the oral cavity in functional occlusion position.
Post-eruptive phase: Also called “functional eruptive phase” begins when teeth are in occlusion. The movements in this phase maintain positions of erupted teeth by accommodating for continued jaw growth and compensating for any proximal or occlusal wear of teeth.
PHYSIOLOGIC TOOTH MOVEMENT
Movements to the tooth germs in this phase are preparatory for the eruptive phase and place them in position for eruption. The tooth germs grow and move in relation to the growth of the jaws in length, width and height. Lengthening of jaws help in anterior tooth germs to move forward (mesial) and molar tooth germs to move backward (distal). Simultaneously, the growth in width and height move the tooth germs outward (buccal/facial) and upward in case of mandible or downward in case of maxilla (occlusal).
Permanent tooth germs also move in relation to the primary tooth germs. Initially, permanent tooth germs lie lingual to their primary predecessors and are on the same occlusal plane. But at the end of this phase, pre-eruptive movements finally position the permanent anterior teeth lingual and apical to the primary anterior teeth. The permanent pre-molars are positioned under the divergent roots of the primary molars. The permanent molars however do not have a primary predecessor. The upper permanent molars are positioned with their occlusal surfaces tilted distally and lower molars are tilted mesially. Permanent molars are positioned this way for want of space. When the jaws grow sufficiently permanent molars are positioned vertically.
Pre-eruptive tooth movements are accomplished by “Bodily movement” and “eccentric growth”
Bodily movement is that movement of the entire tooth germ. Bony remodelling of the crypts helps this process. For example, if a tooth moves mesially, bone resorption occurs on the mesial surface of the bony crypt and deposition of bone occurs on the distal surface facilitating movement of the entire tooth germ.
Eccentric growth is the process of growth in one part of the tooth while the other remains constant. Again, it is bony remodelling of the crypt that helps in accommodating the growth of the tooth.
This phase starts with the initiation of root formation. It involves the occlusal movement of teeth through the bone and oral mucosa into the oral cavity where they attain functional occlusion. There are several events happening during this phase.
1. Firstly, this phase begins with initiation of root formation when the epithelial root sheath begins to proliferate.
2. The tooth erupts through the bony crypt and enters the connective tissue. It moves through the connective tissue and makes contact with the oral epithelium. When this happens, the reduced enamel epithelium covering the tooth crown fuses with the overlying epithelium.
3. The tooth crown pierces the epithelium and enters the oral cavity. As it erupts further, the lateral borders of the oral epithelium which fused with the reduced enamel epithelium form the attachment epithelium or the junctional epithelium.
4. The tooth continues to move occlusally, and finally reaches functional occlusion. That part of the crown exposed to the oral cavity is called “clinical crown” and that portion of the crown covered by enamel but not exposed to the oral cavity is called “anatomic crown”.
Now, it has to also be understood that, several changes to tissues happening above, around and below the teeth facilitate movements of the eruptive phase.
Changes to tissues above the teeth
For teeth to successfully erupt, they have to move through the overlying bone and connective tissue; an eruptive pathway must be established. Only then can they erupt into the oral cavity. The reduced enamel epithelium attracts monocytes, which differentiate into osteoclasts and resorption of the bony crypt is accomplished by these differentiated osteoclasts. Once this happens, the dental follicle surrounding the tooth becomes continuous with the lamina propria of the overlying oral mucosa. This portion of the fibrous dental follicle (connective tissue) also contains remnants of the dental lamina and is called the gubernacular cord. The canal housing this is called the gubernacular canal. After bone resorption, the reduced enamel epithelium also secretes proteolytic enzymes that degrade the connective tissue in the gubernacular canal as well as lamina propria of the oral mucosa. This makes for an eruption pathway for the tooth thus facilitating eruption. The degradation of blood and nerve vessels along with the connective tissue accounts for the eruption of tooth without bleeding and pain.
In case of the permanent pre-molar situated below the primary molar, root resorption of the primary molar happens similar to bone resorption. This facilitates the shedding of the primary molar, paving way for the permanent premolar to erupt.
Changes to tissues around the teeth
As the tooth erupts, the epithelial root sheath begins to proliferate and the root starts to form simultaneously. Initially, a bundle of periodontal ligament fibres appear at the cervical area of the tooth root and extend at an angle coronal to the bone. As the tooth erupts and root starts to elongate, periodontal ligament fibres become more prominent and attach with the root on one side and the bone on the other. It is speculated that the fibres detach from the tooth as the tooth erupts, and then re-attach with the tooth to stabilize it. This is hypothesized to happen throughout the eruption process facilitating it. Apart from fibroblasts, periodontal ligament fibres have been reported to harbour a special type of fibroblast called myofibroblasts. These cells have contractile properties and have been speculated to aid in tooth eruption.
Changes to tissues below the teeth
The fundic bone or the bone below the tooth germ gives way for root lengthening during this phase. As the tooth erupts, bone is deposited here in the form of a ladder (referred to as “bony ladder”) to give support to the erupting tooth. After the eruptive phase, the bony ladder is resorbed to give way for the rest of the root to form. Finally, periodontal ligament fibres are established at the apex between the socket floor and the root tip.
This phase starts after teeth reach functional occlusion. This phase is at its peak during 14-18 years of age, though tooth movements in this phase continue to occur till teeth remain in the oral cavity. These movements are slight occlusal and proximal movements that help in keeping the teeth in position despite growth in jaws and wearing away of tooth surfaces be it, occlusal or proximal. When there is too much occlusal wear, there is extra cemental deposition in the apex, accompanying the occlusal movement to compensate for the attrition. However, it is not established that cemental deposition accounts for the occlusal movement. It is in fact, not yet known clearly how the tooth moves occlusally post-eruption.
Also, when the teeth are brought together in occlusion, an anteriorly directed force is generated. This accounts for mesial drifting of the teeth. Together with this force, it is also speculated that trans-septal fibres of the periodontal ligament and soft tissue pressures exerted by the tongue and cheeks may generate a force pushing the teeth mesially.
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