Initiation of hard tissue development that is tooth eruption for all deciduous teeth occurs between 3.5 and 4.5 intrauterine months. The crowns have been seen to get mineralized about halfway by birth and become fully formed during first 12 months of postnatal life. Root formation continues and is completed after tooth eruption between the ages of 1.5 and 3 years.
Clinical eruption: Eruption is derived from the Latin word ‘erumpere’ i.e., to break through. It is defined as the movement of the tooth from its position within the jaws to its functional position in an occlusal plane, in vertical axial direction. Tooth eruption is intimately associated with normal dento-facial growth and occlusal development, and the knowledge of tooth eruption is essential for a clinician to diagnose a dental problem correctly to plan his treatment successfully.
Types of tooth eruption
Three distinct types of teeth are differentiated by their eruption pattern:
I) Continuously growing: Tooth formation and tooth eruption occur throughout the life. The dental tissues are formed from a proliferative base, and the anatomic crown and root are very similar morphologically. Typically, these teeth have extensive wear. The tooth eruption velocity which is relatively rapid under normal function, increases whenever the velocity of the wear increases or when the antagonist tooth is removed. Examples of these teeth are the incisors of rodents and mesomorphs.
ii) Continuously extruding: Teeth stop forming once root formation is complete. These teeth have a well-defined anatomic crowns and root and are usually associated with moderate occlusal wear. The height of the clinical crown is maintained by tooth eruption and apical migration of the surrounding epithelial attachment, without simultaneous deposition of the alveolar bone. As occlusal wear progresses, the tooth eventually loosens and exfoliates completely from its alveolar housing. Examples are the check teeth of cattle and sheep.
iii) Continuously invested teeth: Human teeth belong to this type of tooth eruption. These teeth also stop forming after a predictable amount of root development has occurred and have distinct anatomic crown and root structures, but the alveolar bone remodels in response to eruption. With normal attrition in the absence of periodontal disease, the clinical crown shortens as the tooth erupts to maintain vertical height and occlusal function and brings the alveolar bone with it.
Stages of human tooth eruption
Human tooth eruption occurs in two major stages which can be further divided into six phases.
1. Pre-emergence eruption:
During the early part of tooth formation, called the follicular phase of eruption, the follicle enlarges concentrically in even direction. It may slightly move facially within the alveolar bone, but there is little or no eruptive movement. The tooth starts to erupt, and the pre-emergent spurt phase of eruption begins, when crown formation is complete and root development starts.
Theories of eruption:
• Root growth theory
• Constriction of pulp
• Pulp growth
• Bone growth
• Tissue fluid pressure
• Shrinkage of collagen
For a tooth eruption intra-osseously, following two factors are necessary:
• The overlying tissue (bone, primary tooth root and gingiva) must resorb to provide an eruptive path. and
• A force must be generated to move the tooth vertically. Resorption over the tooth seems to be dependent on the presence of the coronal portion of the dental follicle.
The timing and development of the tooth eruption pathway is independent of eruption pressure or even the presence of a developing tooth.
Recent studies show that resorptive process may be regulated by local growth factors such as transforming growth factor beta 1 (TGF-B 1) and epidermal growth factor (EGF) produced within or around the dental follicle. These growth factors seem to chemo-attract monocytes from the peripheral blood vessels around the dental follicles. The resorptive process begins with the formation of ostecolasts from the coalescence of the monocytes..
How the eruptive force is generated in the pre-emergent stage remains unclear. Although metabolic activity within the PDL probably plays an important part, it is unlikely to be a source of the pre-emergent eruptive force, because the majority of the principal periodontal fibers are sparsely formed and incompletely linked with the alveolar bone at this stage.
Deposition of the bone beneath the erupting tooth is associated with eruption, but this seems to be a result of tooth eruption rather than its cause.
Vascular pressure has long been considered a leading possibility but the pre-emergent eruption mechanism remains an enigma.
2. Post emergent tooth eruption
• The post-emergent stage of tooth eruption begins with the pre-functional eruptive spurt, as the tooth emerges through the gingiva and moves into occlusal contact with its antimere. During this phase. a tooth typically erupts about 4 mm in 14 weeks.
• The juvenile occlusal equilibrium, circumpubertal eruptive spurt and adult occlusal equilibrium phases that follow are characterized by a slower velocity of eruption.
• After the teeth are in function, they continue to erupt at the same rate as vertical growth of the jaws unless there is occlusal wear or loss at the opposing tooth, in which case additional eruption occurs.
• Throughout the post-emergent stage of tooth eruption the PDL seems lobe the major contributor to the eruptive force. The principal fibers are now well oriented and attached firmly to the alveolar bone.
• The primary eruption mechanism is thought to be either contraction of collagen as it matures or (less likely) traction from contractile fibroblasts.
Vascular effects similar to pre-emergent eruption may also contribute.
It seems reasonable to presume that forces opposing the eruption mechanism control post-emergent eruption. analogues to the control of pre emergent eruption by the resorption mechanism. There are two points of evidence to support this view:
i) in continuously erupting rodent incisors, the rate of eruption slows as an erupting tooth comes into function and
ii) in teeth of all types, if the opposing tooth is removed, eruption occurs at a faster rate. Human teeth wilt erupt beyond their normal position if the antagonist is removed.
The circadian rhythm in tooth eruption is potentially significant in clinical practice. Studies indicate that circadian rhythm exists during the pre-functional stage of eruption of human teeth. The teeth intrude transiently in conjunction with masticatory activity and then erupt significantly overnight. The mean daily eruption velocity was seen to be 71 um/day.
It is also believed that the effect on eruption of a supine position versus an upright position during the night could be due to a change in intra-oral pressure against the erupting tooth from either resting soft tissue pressure (cheek and tongue) or functional intraoral pressures (e.g. swallowing).
Clinical Significance of Eruption Rhythm
i) Timing of eruption – controlling intervention. Excessive eruption of the posterior teeth is a major characteristic of the long face, and control of tooth eruption during growth seems to be the key to successful treatment. The amount of force, its direction, and the total hours of wear of the appliance often are considered the most important clinical factors that affect treatment outcome. If teeth erupt primarily during the night with little or no net eruption during the day, it is quite possible that wearing an appliance (functional appliance, bite blocks, high pull head gear etc) is effective during the night and early morning period when the tooth eruption is most active is sufficient.
ii) Daily rhythm in skeletal growth: Ample evidence demonstrates the skeletal growth requires an adequate level of HGH which increases in the night. The rhythm in tooth eruption also reflects this increase soon after the child goes to sleep. The clinician should be aware that there is a rhythm in skeletal growth and modification of treatment may be more effective at the night than during the day.
iii) Effect of tooth movement more generally: Because tooth eruption depends on metabolic activity within the PDL, the rhythm associated with post-emergent eruption almost surely reflects a in PDL activity.
The cellular activities of periodontal fibroblasts have a periodicity similar to tooth eruption and orthodontic movements are also related to cellular activity in PDL. Therefore, at the time of treatment planning, clinician should explore fully this periodicity of periodontal cellular activity for the control of pain and hyalinization or undermined resorption.