Pain and the rate of dentinal fluid flow produced by hydrostatic pressure stimulation of exposed dentine in man

https://doi.org/10.1016/j.archoralbio.2006.12.014Get rights and content

Abstract

Objective

To determine the relationship between pain intensity and the rate of fluid flow through dentine in human subjects.

Design

The experiments were carried out on 16 premolars in 13 human subjects (aged 15–25 years). Dentine was exposed at the tip of the buccal cusp, etched with acid and covered with saline. A series of 5 s hydrostatic pressure stimuli between 400 mmHg above and 400 mmHg below atmospheric were applied to the dentine, in steps of 50 mmHg. The subject indicated the intensity of any pain produced on a visual analogue scale (VAS). The fluid flow through dentine during application of the same stimuli was measured in vitro within 3 h after tooth extraction.

Results

The median pain threshold with negative (subatmospheric) stimuli was −125 mmHg and, with positive pressure stimuli, 200 mmHg, which corresponded to dentinal fluid flow rates of 3.29 nL/(s mm2) exposed dentine and 5.75 nL/(s mm2), respectively. Both the median pressure and the mean rate of flow at threshold with negative pressures were significantly lower than with positive pressures. The curves relating VAS score to stimulus intensity were similar with both negative and positive pressures.

Conclusion

The sensory transduction mechanism for pain in human teeth is more sensitive to outward than inward flow through dentinal tubules. The difference in sensitivity was however much less than that of the hydrodynamic receptors in the cat, which respond very much more strongly to negative than positive pressure stimuli.

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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