**3. Mechanisms, aetiology, predisposing factors, and clinical features of dentine hypersensitivity**

#### **3.1 Mechanisms**

The currently accepted mechanism associated with DH as proposed by Brännström and Åström is hydrodynamic in nature [15]. This theory relies on minute fluid movements within the dentinal tubules in response to an external stimulus such as cold air or water to initiate pain in the dental pulp. There is also *in vitro* evidence that in areas of clinically identified sensitive dentine there are a greater number of open dentinal tubules compared to non-sensitive areas. Underpinning this theory is the presence of open dentinal tubules on the exposed root surface (cervical area), which in turn affects the degree of fluid flow through the tubule. This theory promotes two basic approaches for treatment: (1) by occluding the exposed open dentine tubules in the cervical region of the exposed root surface, which in turn reduces any stimulus-evoked fluid movements within the dentinal tubules and effectively prevents the transmission of the external stimulus (such as a cold stimulus) to the pulp, and (2) by potassium ion diffusion within the dentinal tubule to reduce intra-dental nerve excitability and prevent any nerve activation [16]. The question whether the hydrodynamic theory is also associated with root sensitivity has been questioned due to the presence of dental plaque on the root surface, which may encourage the ingress of bacteria within the dentinal tubules [12, 16].

#### **3.2 Etiology and predisposing factors**

The etiology of DH/RS is multifactorial in nature, and it is evident that the structure of dentine is altered because of a combination of the associated pre-disposing factors, which may include anatomical factors such as tooth position, quality of the buccal plate, and so on. For example, once the overlying hard and soft tissues have been removed exposing the underlying dentine surface through gingival recession, tooth surface loss through attrition, abrasion and possibly abfraction, over-zealous toothbrushing techniques, the effects of periodontal disease and its subsequent treatment, then these factors may play a part in widening the open dentinal tubules through erosion with the combination of over-zealous toothbrushing techniques. An epidemiological study in a population of young adults aged 18–35 years by West et al. [14] reported that there was a link between a healthy erosive diet and lifestyle and toothwear with DH.
