Pain and the rate of dentinal fluid flow produced by hydrostatic pressure stimulation of exposed dentine in man
Introduction
The sensory transduction mechanism by which impulses are generated in intradental nerves when potentially painful stimuli are applied to enamel or dentine appears to involve displacement of the contents of dentinal tubules; the tubules acting as hydraulic links between the enamel or an exposed dentine surface and the nerve endings located either in the pulpal ends of the tubules or in the underlying pulp. This is the so-called hydrodynamic mechanism (for reviews see Refs. 1, 2, 3, 4). Much of the evidence for this is based on data obtained by recording from intradental afferents in experimental animals.5, 6, 7, 8, 9 Vongsavan and Matthews10 developed a method for recording fluid flow through dentine in vivo in cats and this method has been used to relate dentinal flow to the impulse discharge evoked in intradental afferents during the application to exposed dentine of hydrostatic pressure stimuli between 500 mmHg above, and 500 mmHg below atmospheric pressure.11, 12, 13 In those studies it was found that the afferents that responded to dentinal flow (hydrodynamic afferents) were much more sensitive to the outward flow through the dentine produced by negative (subatmospheric) pressure stimuli than to the inward flow produced by positive pressure stimuli.11, 12, 13
The aim of the present experiments was to determine the relationship between pain intensity and the rate of fluid flow through the dentine in human subjects, and to compare this relationship with the properties of hydrodynamic receptors determined earlier. Positive and negative pressure stimuli over a range similar to those used in the cat were applied to exposed dentine in man and the intensity of any pain evoked was recorded. For technical reasons, it was not possible to record simultaneously the flow through dentine produced by the stimuli. This was recorded later, in vitro. The number and diameters of the tubules through which the fluid flowed were estimated from SEM images of the cavity floor.
Section snippets
Materials and methods
The experiments were carried out on 16 premolar teeth in 13 subjects (mean age 20 years, range 15–25). All the teeth were scheduled to be extracted for orthodontic reasons. They were fully erupted, vital, and free of caries and restorations. The study was approved by The Committee on Human Rights Related to Human Experimentation of Mahidol University.
Without local anaesthesia, dentine was exposed at the tip of the buccal cusp of each tooth by cutting a cavity (diameter approximately 2 mm, depth
Results
Fig. 3 shows the VAS scores recorded at each stimulus intensity by two subjects and Fig. 4 shows the pooled data for all 16 teeth. The pain thresholds for negative pressure stimuli ranged from −50 to −200 mmHg (median −125 mmHg) and, for positive pressure stimuli, from 150 to 400 mmHg (median 200 mmHg). Considering only the absolute values of the thresholds, irrespective of whether they were above or below atmospheric, the median threshold for negative pressure stimuli was significantly lower than
Discussion
Hydrostatic pressure stimulation was used in this experiment because it could be well controlled and, unlike thermal stimuli for example, will selectively excite hydrodynamic receptors as a result of the flow it produces in the contents of the dentinal tubules. In an attempt to minimise damage to the odontoblasts and underlying pulp, the stimuli were kept brief and negative and positive pressure pulses were applied in pairs of matched intensity, as in the cat in previous studies.10, 11, 12
This
Acknowledgements
This work was supported by The Thailand Research Fund (TRF) and a research grant from the Faculty of Dentistry, Mahidol University.
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