Transcript for:
Eruption of Teeth

Eruption of teeth is the process by which developing teeth move through the jaw bones and overlying mucosa to emerge in their respective place within the oral cavity eventually. Now, humans are diphyodonts, which means we have two sets of teeth during our lifetime. The first set, the primary teeth, also called deciduous teeth or baby teeth, start erupting at around six months of age. And, by about six years of age, the primary teeth fall out and are replaced by a set of permanent teeth that stays with you for the rest of your life. Tooth eruption occurs in three phases, the pre-ereptive, eruptive, and post-ereptive phase.

The driving force behind the eruption of teeth is still unknown, but is thought to be due to a combination of a few factors, which include signals originating from the dental follicle, root formation and elongation, which could drive the tooth in order to acquire space within the jaw, and formation of PDL, which provides an occlusal force, aiding in eruption. Okay, let's look at all the stages one by one, starting with the pre-ereptive phase. In this phase, the tooth germs are growing within the jaw bones.

As the bones grow in length, width, and height, more space becomes available for the tooth germs to spread out and grow without affecting each other. Lengthening of the jaw bones allows the anterior tooth germs to move mesially, or forward, and the molar tooth germs to move distally, or backward. At the same time as the bone widens, the tooth germs move buckly, or outward, and as the height increases, the tooth germs move closer to the surface, that is, the mandibular tooth germs move upwards, and the maxillary tooth germs move downwards.

In this phase, movement of the tooth germ is achieved by two methods. Bodily movements and eccentric growth. In bodily movement, there is a remodeling of the bony crypts through resorption of bony crypt wall in the direction that the tooth is moving.

This is followed by depositing new bone on the opposite crypt wall, hence creating a path for the tooth germ. Eccentric growth is where a part of the tooth grows relatively faster than the rest of the tooth. Eccentric growth helps the crown of the tooth reorient itself as the jawbone keeps growing. Now, the position of permanent tooth germs is also changing in relation to the primary tooth germs. At the beginning of the pre-eruptive phase, the successional tooth germs lie lingual to their primary predecessors, with their occlusal surfaces on the same plane.

And, by the end of the pre-eruptive phase, the permanent anterior teeth ends up lingual and apical to the primary anterior teeth. And the permanent premolars come to rest underneath the divergent roots of the primary molars. However, this change in position is more due to the movement of the primary tooth and growth of the jaws rather than the movement of the permanent tooth itself. Now, you'd think that without a primary predecessor, the permanent molars would be developing unhindered. But their location right near the ends of the jaw is a very cramped up space.

And so, the permanent molar tooth germs in the maxilla grow with their crowns tilted distally, and those in the mandible have their crowns tilted mesially. Only when the jaw grows to a sufficient size can the permanent molars move into a more vertical position. Next, the tooth germs enter the eruptive phase, or the pre-functional eruptive phase, which begins simultaneously with root formation. The tooth ascends through the bony crypt and the connective tissue and finally comes into contact with the oral epithelium.

The reduced enamel epithelium over the tooth crown then proliferates along with the overlying oral epithelium to form a single membrane. and this epithelium is pierced by the tooth to finally enter the oral cavity. The fused epithelium lining the sides of the tooth is now called the functional, or attachment epithelium. Through the rest of the eruptive phase, the tooth gradually rises into the oral cavity till it reaches the plane of functional occlusion.

Now, several changes are happening in tissue surrounding the teeth that help in the eruptive phase. In the region above each tooth, a path called the eruptive pathway, is cleared through the bone and connective tissue. The reduced enamel epithelium covering the dental follicles secretes colony-stimulating factor I, or CSF1, and transforming growth factor beta-1, or TGF-beta-1, into the surrounding tissue. The CSF1 and TGF-beta-1 attract tons of monocytes that then differentiate into osteoclasts.

These osteoclasts eat away or resorb the part of the bony crypt overlying the tooth. Without the bony crypt, the dental follicular cells become continuous with the cells of the lamina propria of the overlying oral mucosa. The reduced enamel epithelium then goes on to secrete some enzymes that break down overlying the connective tissue, nerves, and blood vessels, forming an eruption pathway for the tooth.

The lack of blood vessels and nerves helps in a relatively bloodless and painless eruption. Now, as the tooth erupts, the epithelial root sheath begins to proliferate and starts to form the root. At the same time, bundles of collagen fibers start appearing in the surrounding dental follicle.

The collagen bundles get invaded by fibroblasts that lay down fibrous tissue and thickens the bundles to form the periodontal ligament, or PDL. The first PDL fibers appear at the cervical area of the tooth root and radiate coronally to the alveolar process. As the tooth erupts, more PDL fibers appear along the length of the root and existing ones become more prominent.

In addition to fibroblasts, PDL fibers contain a special type of fibroblast called the myofibroblast. Myofibroblasts are cells with contractile properties and are thought to speed along the eruption. Moreover, researchers have found that PDL fibers continuously attach and detach from the tooth as it erupts.

It is thought that these dynamically changing PDL fibers play a role in stabilizing the tooth and help in the eruption process. Some changes also happen to the tissue below the teeth. Eruption of the tooth leaves behind some space at the bottom of the bony crypt.

This space gets filled up by fine bony trabeculae, or bone with tiny gaps in its structure. The bony trabecule, also sometimes known as the bony ladder, provides support to the apical tissues. At the end of the eruptive phase, this bony ladder is resorbed to give way for the developing root tip. Ultimately, bundles of PDL fibers are laid down at the apex, extending from the root tip to the bony floor.

Once the tooth reaches a plane of functional occlusion, the post-irruptive phase begins. It is a phase for checking to see if the teeth remain in the optimal position and correcting any abnormalities due to jaw growth and occlusal or interproximal wear. As the jaw height keeps increasing, new bone is deposited along the socket floor to maintain the height of the erupted tooth in the arch. Occlusal wear is compensated by cemental deposition around the apex. Whereas interproximal wear, which is the wearing of the contact surface between the teeth, is compensated by a process called mesial or aproximal shift.

The mesial shift is brought upon by a combination of forces acting on the teeth. These include the anteriorly directed force generated by the teeth when they're in occlusion, the pressure exerted by transseptal fibers of the PDL to keep neighboring teeth in contact, and finally, the pressures exerted by the surrounding soft tissue like the tongue and cheeks. The post-ereptive phase continues as long as the tooth remains in the oral cavity. Although most teeth have a similar mechanism of eruption, they vary in their eruption and exfoliation times.

The eruption of primary teeth is commonly known as teething, and it usually begins at around 6 months of age and goes on until about 24 months of age. The first to come are the mandibular, lower central incisors, which erupt at around 6 to 10 months of age. Next come the maxillary, upper central incisors at around 8 to 12 months, and then the upper lateral incisors at around 9 to 13 months. The lower lateral incisors erupt at around 10 to 16 months, the upper first molar at 13 to 19 months, lower first molar at 14 to 18 months, upper canine at 16 to 22 months, lower canines at 17 to 23, lower second molar at 23 to 31 months, and finally the upper second molar at 25 to 33 months.

Now, both the central incisors are exfoliated by 6 to 7 years. both lateral incisors by 7-8 years, both first molars fall by 9-11 years, both canines and second molars fall off by 10-12 years. Now, the first permanent teeth to come in are the permanent molars, which usually appear around 6-7 years of age. The permanent first molars erupt distal to the primary second molars, thus no primary tooth exfoliates to make room for them. The next to erupt are the lower central incisors at around 6-7 years.

The eruption of these and all subsequent permanent teeth, except permanent molars, is preceded by the exfoliation of an overlying primary tooth. Then come the upper central incisors and upper and lower lateral incisors at around 7-8 years. The lower canines erupt at around 9-10 years, the first and second premolars at around 10-12 years, and the upper canines at around 11-12 years. The second permanent molars erupt around 12-13 years. Finally, the third permanent molars, or wisdom teeth, usually don't erupt until around the ages of 17-25, and sometimes never erupt, so they remain impacted in the jaw or simply aren't present at all.

Alright, as a quick recap, eruption of teeth is the process by which developing teeth move through the jawbones and overlying mucosa to emerge in their respective place within the oral cavity eventually. The first set, the primary teeth, also called the deciduous teeth, start erupting at around six months of age. And, by about six years of age, the primary teeth start to get replaced by a set of permanent ones that pretty much stick around for the rest of your life with proper care.

The eruption process itself can be divided into three phases, pre-eruptive, eruptive, and post-eruptive phases. The pre-eruptive phase is a preparatory phase that occurs before root formation and positions the tooth germ for eruption within the jawbone, kind of like how an airplane is wheeled onto the runway before takeoff. The eruptive phase, also called the pre-functional eruptive phase, begins at the time of root formation and consists of movements that raise the tooth upwards. from within the bones to a functional occlusal position within the oral cavity.

The post-eruptive, also called the functional eruptive phase, begins once the teeth are in occlusion and consists of movements that maintain the position of erupted teeth as the jaw grows and compensates for any wearing of teeth.