Arch oral Viol. Vo1.12, pp.4347, 1967. Pergamon Press Ltd. Printed in Gt. Britain.

VARIATIONS IN THE SENSITIVITY TO OSMOTIC STIMULA-

TION OF HUMAN DENTINE

D. J. ANDERSON, B. MATTHEWS and L. E. SHELTON*

Department of Physiology, Guy’s Hospital Medical School, London, S.E.l, England

Summary-When dentine is exposed by cutting a cavity in the crown of a tooth two variations in sensitivity to osmotic stimuli have been found. The hrst is a decline in sensitivity to a succession of ditferent solutions applied randomly, which has been investigated by cutting two cavities in the same tooth and applying the whole range of stimuli ftrst to one cavity then to the other. The second variation is a rise in sensitivity observed when dentine is stimulated a week after cavity preparation, the cavity having been tilled with gutta percha in the interim period. This was investigated by first cutting and stimulating one cavity, then filling it with gutta percha. After a week a second cavity was cut in the same tooth and both were stimulated with the same series of solutions as used previously.

It is shown that the variations in sensitivity were neither central in origin, nor could have been due to changes in the entire dental pulp. It is concluded that they must have resulted from a change in the dentine stimulated or in the immediately subjacent pulp tissue.

INTRODUCTION

THE CHARACTERISTICS of the mechanism of sensation in human teeth are being in- vestigated by a method in which various solutions are applied to dentine with the object of evoking a sensation of pain or modifying that sensation. It has been found that there appears to be a relationship between the osmotic pressure of solutions and their ability to cause pain (ANDERSON and RONNING, 1962). This relationship was established first with solutions of calcium chloride and later with other solutes MATTHEWS, 1966). It has also been reported (ANDERSON and RONNING, 1962)

that sensitivity appears to decrease during the course of an experiment, since subjects report sensations of pain less frequently towards the end of the experiment than at the beginning although the solutions are applied in random order. This decrease in sensitivity was first shown by arranging the results obtained from a number of subjects in four consecutive groups as shown in Table 1. The results suggest that some change is taking place either in the peripheral mechanisms concerned in the sensation of pain or in the mechanisms underlying the central appreciation of pain. The first part of this report covers experiments designed to investigate these alternatives.

It has also been found (ANDERSON, CURWEN and HOWARD, 1958) that 1 week after IUing a prepared cavity in dentine with gutta percha there is a greater number of reports of pain in response to stimulation than follows the same stimuli applied to

*Present address: University of Oregon Dental School, Portland 1, Oregon, U.S.A.

43

44 D.J. ANDERSON,B MATTHEWS AND L.E.!SHELTON

TABLE 1. THE DECLINE IN SENSITNITY DURING AN EXPERIMENT (FROM ANDERSON AND RONNING, 1962) (Reports of pain are expressed as % of stimulus applications)

1st quarter 2nd quarter 3rd quarter 4th quarter

Painful responses ( %) 82.5 55.0 40.0 25.0

Pooled results from two experiments in five subjects arranged in a sequence of four equal groups. Each group represents forty stimulations (eight per subject). The 1st quarter is from the beginning of the experiment and so on to 4th quarter at the end of the experiment. The stimuli were seven solutions of CaCl, and a Sugar Syrup exerting osmotic pressures in the range 200-2800 atmospheres. The solutions were applied in random order.

freshly cut dentine. This finding, like the previous one, could be the result of some change affecting either the peripheral mechanisms of pain or the mechanisms of central appreciation. The second part of this report covers investigations of this point.

General MATERIALS AND METHODS

The subjects were students in the age range 17-21 years and the experiments were conducted on lower premolar teeth showing clinical evidence of enamel caries. A cavity was cut just into the dentine, and then the solutions were applied in random order to the cavity. After each application, the subject was asked to report any sen- sation he experienced from the tooth.

The solutions applied to the dentine were Sugar Syrup, 2 Molal, 6 Molal, and a saturated solution of calcium chloride, all at 37°C. They were applied twice and in random order making a total of eight stimulations. Neither the subject nor the oper- ator who applied the solutions was aware of the order of application. The solutions were applied by soaking a pledget of cotton wool in the appropriate solution and gently placing the pledget in the cavity where it remained for 30 sec. At the end of this period the pledget was removed, the cavity was rinsed with warm water and the subject was asked for his comments.

After the results had been collected, the effectiveness of a solution in evoking pain was judged by the frequency with which its application resulted in a report of pain. No attempt was made to distinguish between sensations of different intensity. A change in the percentage response to a given series of solutions was taken to indicate a change in sensitivity.

Experiment 1 In the series of ten subjects in whom the progressive diminution in sensitivity

during an experimental session was investigated, the procedure was as follows: two small occlusal cavities were prepared in the same tooth, and the entire sequence of four solutions was applied twice and in random order to one of the cavities. Then the same random sequence was applied to the second cavity. The cavities were then filled with gutta percha. After the lapse of a week, the experiment was repeated as before but, on this occasion, the cavity which had been the first to be tested during the first session took second place. The object of this was to exclude the possibility that

VARIATIONS IN THE SENSlTNITv TO OSMOTIC STIMULATION OF HUMAN DENTINE 45

any difference in the response from the two cavities might be due to local differences such as depth of the cavity or nature of the dentine, rather than to the experimental procedure.

The results from both sessions were pooled and arranged in four consecutive quarters under the headings of Cavity 1 and Cavity 2, and are set out in Table 2.

TABLE 2. TH@ DECLINE IN SENSITIVITY DURING AN EXPERIMENT (PRESENT SERIES)

(Reports of pain are expressed as % of stimulus application)

Cavity 1 Cavity 2

Group 1 2 3 4 1 2 3 4

Painful responses (%) 80.0 50.0 35-o 22.5 _--___ 80.0 45.0 32.5 25.0---.

Pooled results from two experiments in ten subjects arranged in a sequence of four equal quarters. Each group represents forty stimulations (four per subject). Group I is from the beginning of the experiment and so on to group 4 at the end of the experi- ment. The stimuli were three solutions of CaCl, and a Sugar Syrup exerting osmotic pressures in the range 200-2800 atmospheres and the solutions were applied in random order.

Experiment 2 Eight subjects took part in the second experiment, designed to investigate the

apparent increase in sensitivity occurring from one experimental session to the next. The general procedures of cavity preparation and the application of stimulating

solutions were as previously described. One cavity was cut and tested on the first occasion with four solutions, each applied twice in random order; this cavity was then filled with gutta percha. A week later a second cavity was cut in the occlusal surface of the same tooth and both cavities were subjected to the same sequence of stimulations. Each stimulating solution was applied alternately first to one cavity then to the other throughout the sequence. The choice of the first cavity to be stimulated during this second session alternated between the freshly cut and the week-old cavity through the series of 8 subjects. The results are presented in Table 3.

TABLE 3. THE CL-LANGE IN SENSITIVITY FROM ONE EXPERIMENT TO THE NEXT A WEEK LATER

Cavity 1 Cavity 2 - -

Subject Reports of pain on Day 0 Reports of pain on Day 7 Reports of pain on Day 7 .____ __.-

1 2 :

8 3 8 1

3 3 5 4 5 8 ; 5 2 8 8 6 3 5 6 7 5 8 7 8 5 8 4

Mean 3.25 Mean 7.25 Mean 4.75

Within subject differences: Cavity 1 Day 7-Day 0 4OkO.7 p< 0.01 Cavity 2 Day 7-Cavity 1 Day 0 1.5&0.89p>O.l Cavity 1 Day 7-Cavity 2 Day 7 2.5 f 1.0 p < 0.05

Sugar syrup and three solutions of CaCl, were all applied twice, making a total of eight stimulations per day.

46 D. J. ANDERSON, B. MATTHTWS AND L. E. SIZLTON

RESULTS

Experiment 1 The grouped results presented in Table 2 show a decrease in the number of reports

of pain through the sequence of four groups, but this decline is not continuous from one cavity to the next for, had it been so, there would have been few, if any, responses from the second cavity. The total number of reports of pain from each of the two cavities is almost the same.

Experiment 2 Table 3 shows the data from the two cavities, e.g. cavity 1, freshly cut and investi-

gated on day 0 and again on day 7, and cavity 2, freshly cut and investigated for the first and only time on day 7. The results of statistical analysis show that (a) from cavity 1, the mean number of responses on day 7 is greater than on day 0 (p < O-01), thus confirming previous findings (ANDERSON and RONNING, 1962) and (b) the mean number of responses from cavity 2 on day 7 does not differ significantly from cavity 1 on day 0 (p> Oel), whereas it is significantly less than that from cavity 1 on day 7 (p < O-05).

DISCUSSION

Experiment 1. Decline in sensitivity If any part of the peripheral or central mechanism of dental pain is common to the

whole tooth, this cannot have been affected in our experiment. Had it been affected, a continuous decline in the number of reports of pain involving both cavities would have been expected, with the result that very few reports of pain would have been ob- tained from the second cavity tested in the latter part of the session. Furthermore, the change is not a permanent one because it has been common experience that there are more reports of pain from the dentine tested after the lapse of a week than from freshly cut dentine (see experiment 2). Two possible explanations remain: a change is occurring (1) in the dentine subjected to stimulation, or in the immediately subjacent pulp or (2) in some part of the central mechanism of appreciation of pain which is not common to both cavities. If the first possibility is correct, the change may be taking place in the receptor or in a transduction system peripheral to the receptor. Since it is common experience that localization of pain to the tooth being stimulated is difficult, and to a part of the tooth impossible, it would appear likely that the mechanism concerned in the central appreciation of pain is common to the whole tooth and so therefore, the second possibility can be discounted.

Experiment 2. Increased sensitivity after filling with gutta percha for a week The results do not support the hypothesis that the enhancement of sensitivity from

one session to the next is due to some change in the mechanism of central appreciation of pain. According to this hypothesis, both cavities 1 and 2 would be expected to show a similar number of responses on day 7. The results indicate that the change in sen- sitivity has a peripheral origin and, using the same argument as in the discussion of results from experiment 1, it is concluded that the site of this change must have been in the dentine exposed in cavity preparation, or in the immediately subjacent pulp.

VARIATIONS IN THE SENSITIVITY TO OSMOTIC STIMULATION OF HUMAN DENTINE 47

Acknowledgement-This investigation was supported by U.S.P.H.S. Research Grant D 1037 from the National Institute of Dental Research, National Institutes of Health, Bethesda, Maryland, U.S.A.

R~urn&-Lorsque la dentine est exposee par la preparation dune cavite dans la couronne dentaire, deux types de sensibilite a des stimuli osmotiques ont Ctt observes. Le premier est une diminution de sensibilite a une succession de differentes solutions appliquees au hasard, qui ont Ctt test&es en preparant deux cavitts dans la meme dent et en appliquant tous les stimuli a la premiere cavite puis a la seconde. Le deuxieme type est une augmentation de sensibilite, observ6e apres une stimulation dentinaire effect&e une semaine aprbs preparation dune cavite, qui a et& remplie entre-temps de gutta percha. Cela a 6% Btudie aprbs meulage puis stimulation d’une cavite, suivie d’obturation 1 la gutta percha. Aprbs une semaine, une seconde cavite est p&par& dans la m&me dent et les deux cavites sont stimu&s avec la m&me sbie de solutions utilis&s anterieurement.

11 apparait ainsi que les variations de sensibilite ne sont ni d’origine centrale ni ne semblent li&.s a des changements de la pulpe dentaire toute entiere. 11 semble qu’elles soient dues a un changement de la dentine stimulQ ou du tissu pulpaire immediatement sous-jacent.

Zusannnenfassung-Wenn Dentin durch Praparation einer Kavitiit in die Zahnkrone freigelegt wird, sind zwei Variationen der Empfhidlichkeit gegeniiber osmotischen Reizen gefunden worden. Die erste ist eine Abnahme der Empfindlichkeit auf eine Folge verschiedener, willkilrlich angewandter Liisungen, wie es durch Prlparation von zwei Kavitaten in demselben Zahn und durch Anwendung der Reize zuerst in der einen und dann in der anderen Kavitat untersucht wordenist. Die zweite Variation betrifft eine Steiaeruna der Sensibilitat. die beobachtet wird. wenn Dentin eine Woche nach der KavitaTenpriparation und n&h zwischenzeilicher’Fillung mit Guttapercha stimuliert wird. Dies wurde festgestellt, indem zuerst eine Kavitat prlpariert und stimu- liert, danach mit Guttapercha geftillt wurde. Eine Woche splter wurde eine zweite Kavitat in denselben Zahn prilpariert und beide wurden mit derselben Serie ver- schiedener Losungen, wie vorher benutzt, stimuliert.

Es wird gezeigt, dass die Variationen der Sensibilitlt weder eine zentrale Ursachen besassen noch auf Ver%nderungen in der gesamten Zahnpulpa zurtlckzuftihren waren. Es wird gefolgert, dass sie von einer Veranderung im stimulierten Dentin oder in dem unmittelbar darunterliegenden Pulpagewebe verursacht sein mtlssen.

REFERENCES

ANDERSON, D. J., CURIHBN, M. P. and HOWARD, L. V. 1958. The sensitivity of human dentine. J. dent. Res. 37,669-671.

ANDERSON. D. 3. and RONNMG, G. A. 1962. Osmotic excitants of pain in human dentine. At&s oral Biol. 7, 513-523.

MATTHEWS, B. 1966. The distribution of dental pain thresholds. International Association for Dental Research, British Division, p. 6.