Eruption of primary and permanent dentitions

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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 3 phases, the pre-eruptive, eruptive, and post eruptive 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-eruptive phase.

In this phase, the tooth germs are growing within the jawbones.

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 jawbones 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 buccally, 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.

Summary

Primary dentition - also known as baby teeth - erupt during the first 6 years of life. Permanent dentition - also known as adult teeth - begins to erupt around age 6 and is complete by age 12 to 13. The primary dentition is eventually lost and replaced by the permanent dentition. The eruption of primary and permanent teeth occurs in a predictable pattern, beginning with the front teeth and proceeding towards the back of the mouth. For most people, the first tooth to erupt is the lower central incisor (front tooth). The last tooth to erupt is typically the second molar, located at the back of the mouth.