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1. Root formation theory :
The root formation theory proposes that the tooth movement in the axial or occlusal direction is mainly because of the formation of the root.
The simultaneous occurrence of these two activities primarily gives the impression that root formation is an important prerequisite for the eruption of teeth.
Some points contradicting this theory are as follows:
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If root formation were to cause the pushing force for the eruption of the tooth,the root has to form on a fixed base.
Only then can it exert a force in the opposite direction. The histology of root formation does not suggest any such fixed base. To counterargue this, some investigators suggested that a structure called the cushion–hammock ligament may be acting as a fixed base at the apex of the forming tooth. This concept is not accepted as the cushion–hammock ligament is only a soft tissue separating the pulpal and periapical tissues and it cannot act as a fixed base.
- In certain developmental disturbances, the teeth are rootless, but they erupt into the oral cavity. This suggests that root formation although occurring simultaneously with the eruption might not be the only factor influencing eruption.
- There was no direct relationship observed between the length of the root and the distance travelled by the tooth during eruption. Some teeth move a greater distance than their root length.
- Experimental intervention leading to stoppage of root formation did not cause failure of eruption.
- Experiments also demonstrated that a tooth still erupts even after the completion of root formation.
2. Bone-remodelling theory:
- Bone-remodelling theory explains that the eruption takes place in a manner similar to the pre-eruptive movements.
- The resorption of the bone at the site of pressure and the deposition of the bone at the site of tension may cause eruption.
- The pressure exerted by the growing tooth germ in the axial direction initiates the remodelling of the bone to create a pathway for the erupting tooth.
- Placement of silica replica as a substitute to tooth germ with a dental follicle eruptive pathway was evident.
- On experimental removal of the dental follicle, there was no eruptive pathway. This shows that bone remodelling alone is not solely responsible for tooth movement, and the dental follicle plays a vital role in the eruption of the tooth.
- Through subsequent studies, it was established that dental follicle conduits osteoclasts which are responsible for bone resorption and also supplies bone forming cells.
3. Vascular pressure theory:
- The vascular pressure theory suggests that the change in the arterial pressure of the surrounding vessels could be responsible for the eruption.
- Experiments conducted in dogs revealed that the ground substance can swell by 50% on addition of water which would cause a differential pressure. This pressure difference was proved to be sufficient enough to cause tooth movement.
- Hypotensive drugs can cause vasodilatation.
- This vasodilatation can increase the hydrostatic pressure. Experimentally, this pressure was shown an increased rate of tooth eruption.
- Stimulation of sympathetic nerves can cause vasoconstriction. This vasoconstriction can decrease the hydrostatic pressure.
4. Ligament traction theory:
- This theory states that the dental follicle–periodontal ligament complex pulls the tooth into the oral cavity, similar to traction.
- Many histologic changes are observed in the developing tooth to support this theory.
- Strong evidence in support of this theory is that when a silica replica was planted in place of a tooth with the follicle tissue intact and all other tissues removed, the replica erupted into the oral cavity.
- This experiment proved that the dental follicle plays a major role in the eruption of the teeth.
- A disc of root tissue placed on ligament fibroblasts, on further experimental induction, exhibited contractile movement.
- Further, it was shown that the drugs that interfere with collagen formation in the periodontal ligament interrupted tooth eruption. This theory is the most accepted one.
