In dentistry, the hydrodynamic or fluid movement theory is one of three main theories developed to explain dentine hypersensitivity, which is a sharp, transient pain arising from stimuli exposure. It states that different types of stimuli act on exposed dentine, causing increased fluid flow through the dentinal tubules. In response to this movement, mechanoreceptors on the pulp nerves trigger the acute, temporary pain of dentine hypersensitivity. The fluid flow mechanism behind hydrodynamic theory was first introduced by Alfred Gysi in 1900, and subsequently developed by Martin Brännström in the 1960s through a series of experimental studies. Further supporting evidence has since been collected from epidemiological surveys and experimental data comparing dentinal tubule numbers in hypersensitive and non-hypersensitive teeth. Alternate theories include the “dentine innervation” and “odontoblast transduction” theories, both of which lack substantial supporting evidence. The hydrodynamic theory is currently the accepted explanation for dentine hypersensitivity, upon which several treatment and diagnostic strategies have been built by dental practitioners.
Overview of theory
The hydrodynamic theory proposes that when dentinal tubules are exposed at the pulp and dentine surface, external stimuli cause changes in fluid flow. Dentinal tubules may become exposed due to various reasons: e.g. dental erosion, enamel loss and periodontal diseases. When exposed dentinal tubules are then subject to stimuli, the direction and rate of fluid flow movement within the tubules change. This causes shifts in pressure within the dentine, which stimulate the myelinated nerves located in the pulp, causing the sensation of pain. The main nerve fibres responsible for responding to the stimuli are mostly type A-beta (Aβ), which carry tactile information, and some A-delta (Aδ) nerves, which relay pain and temperature information to the brain.
Origin and development
Alfred Gysi - 1900
Dr. Gysi (1865-1957) was a Swiss dentist who specialised in jaw movement and structure, as well as prosthodontics. He studied dentistry in Geneva, Switzerland and the Philadelphia Dental College, eventually becoming a dentistry professor at the University of Zurich. At the time, it was still believed that dentine hypersensitivity was caused by either nerve fibre stimulation within dentine or odontoblast processes acting as pain receptors. In 1900, Gysi first introduced the concept of fluid-flow movement as an explanation for dentine hypersensitivity in his publication for the British Journal of Dental Science: An attempt to explain the sensitiveness of dentine. In his clinical practice, Gysi observed that the removal of fluid from the cavity floor of his patients' teeth produced the sharp, short pain of dentine hypersensitivity. He subsequently concluded that drying the cavity fluid caused an interruption in the fluid flow movement within the dentinal tubules. This led to Gysi's hypothesis that within the dentinal tubules, there was a natural outward flow of fluid. Physical and thermal forms of stimuli would cause an increase change in the direction of fluid flow, which activated the pulpal nerve endings.
Martin Brännström - 1960s Dr.Brännström (1922-2001) was a Swedish dentist, specialising in Oral Pathology and the mechanism of tooth sensitivity. In the 1960s, Brännström provided evidence to support Gysi's hydrodynamic theory through a series of experimental studies in vitro to show that various stimuli caused shifts in fluid movement across dentine, producing pain. Brännström's 1965 experimental study indicated that hypersensitive dentine exhibited a higher number of exposed, patent tubules than non-sensitive dentine. This hypersensitivity was noticeably lessened when the tubules were then deliberately occluded to restrict fluid flow. Brännström conducted trials using different stimuli. It was shown that various types of cold thermal, evaporative cooling, osmotic stimuli and hypertonic chemical substances could cause an increase in outward fluid flow along the dentinal tubules. Contrastingly, thermal hot stimuli caused an inward fluid flow. In vitro studies indicated that stimuli causing an outward flow of fluid from the dentine-pulp complex led to a greater intensity of dentine hypersensitivity than stimuli triggering inward flow. Brännström's multiple publications of both experimental and observational data provided significance evidence to support the theory of hydrodynamic mechanism causing dentine hypersensitivity.
Supporting experiments
Epidemiological surveys have shown that dentine hypersensitivity arises when the dentinal tubules are both exposed and patent. It was proposed that if the hydrodynamic fluid flow was responsible for hypersensitivity, then there must be higher numbers of dentinal tubules exposed at the surface of the root and patent to the dental pulp. A strong positive correlation has been identified between the levels of hypersensitivity in exposed dentine and the number and density of dentinal tubules.
The patency of dentinal tubules 1987 In 1987, A study of the patency of dentinal tubules in sensitive and non-sensitive cervical dentine, was published in the Journal of Clinical Periodontology. This experiment was conducted by E.G. Absi, M. Addy and D.Adams from the Dental School of Cardiff University. It compared the patency of sensitive and non-sensitive cervical dentine and provided significant evidence to support the hydrodynamic theory. The aim of the experiment was to investigate whether patients with dentine hypersensitivity would have higher numbers of dentinal tubules exposed at the surface of the root. There had been little evidence to support the theory prior to this experiment, with the exception of Brännström's in vitro studies. With the use of a scanning electron microscope and dye penetration technology, the findings concluded that the dentinal tubules were eight times greater in number and 2 times wider in diameter in dentine of “hypersensitive” classified patients than in non-hypersensitive dentin. The results supported the theory in that hypersensitive dentin had wider, more exposed dentinal tubules and provided evidence of stimuli transmission via a hydrodynamic movement along dentinal tubules.
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