The Formation of Secondary Dentin and Related Problems

Abstract

It is generally believed that the formation of secondary dentin under fillings, caries and other forms of exposure of dentin is caused by stimuli which reach the pulp through the dentinal fibrils. It is known that the odontoblasts do not form dentin. Accordingly these stimuli must be transmitted from the odontoblasts to the surrounding connective tissue which is the source of dentin formation. Since, at least the first formed layer of secondary dentin, in most cases, has either fewer tubules than primary dentin or no tubules at all it is difficult to explain further reaction through this new dentin. If the stimulation of the fibers is the activator for the formation of secondary dentin it can not be formed in considerable thickness once the first atubular layer is deposited. Fig. 1, a photomicrograph of a ground section through a tooth which was markedly abraded, shows tubules that were present and became opaque by filling in, probably with calcium salts from outside. The light area (B) is secondary dentin without tubules. There was no possibility of conducting stimuli through odontoblastic processes to the pulp in such a case, once the first layer of atubular dentin has been deposited. Nevertheless formation of secondary dentin went on to a considerable degree. In fig. 2, a decalcified section, the primary dentin shows regular arrangement of dentinal tubules. Beneath, is a thick layer of practically atubular dentin. If a stimulus to the pulp is necessary for formation of secondary dentin no way can be found for the continuous transmission of stimuli in formation of thick layers. Wherever secondary dentin is deposited in a limited area due to some peripheral irritation the corresponding odontoblasts appear damaged. They are either smaller than the neighboring undamaged odontoblasts or hardly existing at all. Recently (1) this was reported from dog experiments and the same observation can be made in human beings. The odontoblastic processes keep open drainage canals for the flow of tissue fluid to the dentino-enamel junction. The odontoblatsts must be arranged at the inside of the dentin in order to be able to continue into the dentinal fibrils. This arrangement results in them forming a separation wall between the dentin