In humans, there are two sets of teeth that erupt. They are the primary teeth or baby teeth and the permanent teeth or the adult teeth.
Most children have around 20 primary teeth, ten teeth in the upper jaw and ten teeth in the lower jaw. These teeth erupt till about the age of 6 years. The process of eruption of primary teeth is called teething.
By the age of thirteen years, most children have 28 permanent teeth, which include the incisors, canines, premolars and molars. Between the ages of 17 to 21 years, a third set of molars called the wisdom teeth begin to erupt. These do not play a major role in chewing and sometimes are difficult to keep clean.
Theories of Teeth Eruption
Some of the theories that have attempted at answering the question as to why teeth erupt include the Growth displacement theory and the Continued bone formation theory. But the latest and current theory that gives a satisfactory answer to this is the role of the periodontal ligament. A force is generated in this ligament, below the unerupted teeth, making the teeth drive out through the bone.