Pain Transmission Theory

Tooth pain is not a recent phenomenon, as historical records show that sensitive teeth have plagued mankind for thousands of years.26 In the mid-19th century, a Philadelphia dentist conjectured that the hollow tubules of dentin were filled with a fluid secreted by the pulp, and that pressure on this fluid affected the pulp within. This information had been described earlier and was reiterated about 100 years later.26,27 For a long time it was unclear how the stimulus information was transmitted to the pulp tissue and recognized as pain. Initially, it was believed that the nerve fibers or the odontoblasts in teeth were responsible. More sophisticated microscopy techniques revealed that neither the nerve fibers nor the odontoblasts reached from the pulp all the way through the dentin.28 It wasn’t until the 1960s that Brännström developed a theory, based upon two decades of research, that became known as the hydrodynamic theory.29 Although his theory has not been unequivocally proven, it has not been disproved, and no other account has emerged to explain hypersensitivity. Brännström is credited with demonstrating the movement of fluid within the dentin tubules and the effects of various stimuli, including cold, heat, acids, pressure, chemicals, and high osmotic solutions in creating this movement. The theory suggests that a stimulus causes the fluid within the tubules to flow inward or outward, creating a mechanical disturbance or cellular perturbation.8 This excites the nerves in the tooth, thereby transmitting the signal to the pulp where the sensation of pain is registered by the same depolarization neural discharge mechanism that characterizes all nerve activity (Figure 2). Interestingly, and in support of the hydrodynamic theory, it has been shown that individuals with sensitive teeth exhibit open dentinal tubules and lack the presence of a smear layer (Figure 3).30,31

Current approaches to the treatment of hypersensitivity are designed to target the hydrodynamic process or to interfere with pain transmission. There are two main treatment approaches for the hydrodynamic theory: occlude the dentin tubule to prevent movement of the fluid, or decrease the lumen size of the tubules so that the movement of the fluid is reduced. Pain transmission is reduced by the action of potassium ions blocking repolarization of the nerves around the odontoblasts.32