The preparation of a tooth has to be done very carefully by using rotating (burs). To avoid any warming up, an intensive water-cooling is used. However, there is still a risk of damage to the vitality of the pulp inside of the tooth.
The prepared surface of the dentin in connected to the pulp by more than ten thousand small tubules, called dentinal tubules. The dentinal tubules have a diameter of some 1 to 2 µm (1 µm is one thousandth of a mm). It is well known that the shorter the tubules and the greater their diameter, the greater the risk of damage to the dental pulp. Each therapy step in the crowning process has a traumatising effect on the pulp: The preparation, taking an impression, the placing and replacing of a removable crown and the cementing of the fixed crown. Therefore, the minimal thickness of the remaining dentinal layer is 0.7 mm for teeth with narrow dentinal canals and 1.4 mm for teeth with wide dentinal canals.
Dentists can not see the extension of the dental pulp during crown preparation. So they can only use his experience to avoid going too deep during preparation. But it is well known, that the extension of the pulp chamber can vary. So it happens sometimes that a dentist opens the pulp chamber unintentionally. Then in most cases an endodontic treatment is needed.
The picture shows an opened pulp
But also an insufficient remaining dentinal layer is a risk for the vitality of the pulp. Five years after crowning Kerschbaum found that 5 % of crowned teeth had apical ostitis, which was shown by X-ray pictures.