In a sequence describing dentine caries progression, which zone is first?

The initial phase in the progression of dentine caries is characterized by the zone of bacterial penetration. This phase occurs when bacteria invade the dentine layer beneath the enamel, where they begin to metabolize sugars and produce acids as byproducts. These acids play a crucial role in demineralizing the hard tissue of the tooth, leading to the initiation of caries. In this stage, the integrity of the dentine is compromised, allowing for the establishment of carious lesions.

Understanding this phase is critical because it sets the stage for the subsequent progression of caries, such as the development of demarcation lines and more advanced zones of carious destruction. The zone of bacterial penetration is a clear indicator of active disease, where proper intervention is necessary to prevent further deterioration of the tooth structure. In contrast, the other zones mentioned, such as reparative dentine and dentinal sclerosis, occur later in the caries process and involve responses of the tooth to damage rather than initial invasion by bacteria.