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| ORIGINAL RESEARCH |
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| Year : 2022 | Volume
: 13
| Issue : 2 | Page : 67-69 |
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Lack of Fluid Movement between Dentin Tubule and Pulp Tissue: An In Vitro Study
Chia-Yi Chen, Gunnar Hasselgren
Division of Endodontics, College of Dental Medicine, Columbia University, New York NY 10032, US
| Date of Submission | 10-Sep-2021 |
| Date of Decision | 04-May-2022 |
| Date of Acceptance | 05-May-2022 |
| Date of Web Publication | 12-Jul-2022 |
Correspondence Address:
DDS, PhD Gunnar Hasselgren
Professor of Dental Medicine, Division of Endodontics, College of Dental Medicine, Columbia University, 630 West 168 Street, New York NY 10032
US
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/denthyp.denthyp_128_21
Objective: In vitro studies supporting the hydrodynamic theory have reported that cavity provocation results in fluid flow from dentin tubules via pulp tissue to a capillary connected apically. Our preliminary findings did not corroborate this. Therefore, the aim was to perform experiments closely following the descriptions in the mentioned articles to find out if there is a direct fluid flow communication from a prepared cavity to pulp tissue detectable with this method. Material and Methods: Class V cavities were prepared and provocations (air blast, scraping, temperature change) initiated within 1 hour of extraction in 17 teeth and after 1 week in 3 teeth. Fluid flow was monitored during and after stimulation using a microscope. Results: No fluid flow was registered during stimulations performed within an hour of extraction. One week after extraction fluid movement was registered during the air blast. Conclusions: The finding of no direct continuum in freshly extracted teeth from cavity surface via dentin fluid to pulp tissue to an apically placed capillary does not per se disprove the hydrodynamic theory. As the in vitro experiment by Brännström et al. has been a major foundation for the hydrodynamic theory, it may be time to investigate the true mechanism(s) of dentin sensitivity.
Keywords: Dentin sensitivity, hydrodynamic theory, dentin tubules, dentin fluid, dental pulp, odontoblast
How to cite this article:
Chen CY, Hasselgren G. Lack of Fluid Movement between Dentin Tubule and Pulp Tissue: An In Vitro Study. Dent Hypotheses 2022;13:67-9 |
| Introduction | |  |
The hydrodynamic theory of dentin sensitivity postulates that tactile, thermal, or osmotic stimuli create a movement of fluid inside the dentin tubule activating intra-pulpal nerve endings.[1],[2] This theory has become widely accepted. Some of the studies supporting the hydrodynamic concept are in vitro experiments that have demonstrated a direct movement between dentin fluid and pulp tissue fluid.[3],[4] In these experiments, capillary tubes attached to extracted teeth were used to demonstrate the movement of pulp tissue fluid following the provocation of coronal cavities.
During preparation for a series of tests of base materials for deep cavity insulation, the in vitro model of Brännström et al.[3],[4] was used. Three freshly extracted, intact, young premolars were used; the apical root portions were removed; buccal class V cavities were prepared and 17% EDTA was used to clean the cavity walls; a glass capillary was attached to each exposed apical pulp; the capillary was filled with isotonic saline solution and a bubble was present to visualize movement. The cavities were exposed to air spray, but no movement of the fluid could be recorded. When the teeth were left without any provocation for 30 minutes, a slight inward movement of the capillary liquid was noted in two teeth.
As our findings did not corroborate the results of the mentioned studies[3],[4], it was decided to redo the in vitro experiments closely following the method described in these articles. Therefore, the aim of the study was to find out if there is direct fluid communication between the dental tubules and the pulp tissue using the in vitro model of Brännström.[3],[4]
| Material and Methods | | |
Twenty freshly extracted, intact, single-rooted teeth from patients under the age of 30 were used in the study. The study was declared a nonhuman study, as it dealt with extracted teeth, by the Institutional Review Board of the Columbia University Irving Medical Center. The teeth were extracted due to orthodontic reasons (n = 14) or juvenile periodontitis (n = 6). Following extraction, the teeth were stored in isotonic saline solution. In 17 teeth, the experiments were initiated within 1 hour of extraction. Three teeth were kept in a refrigerator (4°C) for 1 week prior to being subjected to the test procedures. The experiments followed the descriptions by Brännström et al.[3],[4] Buccal and lingual cavities measuring 2 mm in diameter and 1.5 mm in depth were prepared using a #4 (1.4 mm) round bur in a high-speed handpiece with water spray. To remove the smear layer, the cavity was filled with 17% EDTA for 2 min. The buccal cavity was then rehydrated with isotonic saline solution at room temperature for 15 minutes. Meanwhile, the lingual cavity was sealed with wax. The apical 2 mm of the root was sectioned off. The pulp was connected at the root canal opening to an isotonic saline-filled glass micro-capillary (inner diameter 0.2 mm; length 32 mm; capacity 1 μL; Drummond Scientific Co. Broomall, PA) with a polyether material (3M 3SPE, St. Paul, MN) or cyanoacrylate glue (Loctite 480, Henkel Adhesives, North America). An air bubble, or a droplet of methylene blue dye, was introduced to the saline-filled micro-capillary in order to visualize movement. The tooth-micro-capillary system was allowed to stabilize for 15 minutes.
During the first part of the experiment, air blast was applied to the buccal cavity for 10, 30, or 60 seconds. The second part tested the effects of scratching the bottom of the cavity with an explorer for 10 seconds. In the third part, water at 70 or 0°C was applied for 10 seconds. All cavities were subjected to all experiments. Between each procedure, the cavity was rinsed thoroughly with saline solution for 60 seconds. The position of the dye/bubble in the micro-capillary was recorded during stimulation and every 10 seconds for 10 minutes using a clinical microscope (Opmi Pico, Carl Zeiss, Jena, Germany). The same experiments were conducted again using the lingual cavity.
| Results | | |
In the 17 teeth tested within an hour of extraction, there was no immediate movement of the dye/bubble under mechanical, thermal, or osmotic stimulation in any of the teeth. The dye/bubble in the micro-capillary tube did not move when exposed to mechanical, thermal, or osmotic stimuli in 16 out of the 17 teeth. One sample showed outward movement 10 minutes after a cold water stimulus. Four samples showed inward movement without any stimulus. When the three teeth that had spent 1 week in a refrigerator were tested, the air blast resulted in a slight movement, or tremor, of the air bubble in the micro-capillary tube. Scratching of dentin in the bottom of the cavity or application of water at 70°C or 0°C did not result in any movement.
| Discussion | | |
The present study closely followed the methods described by Brännström et al.[3],[4] with two differences. The capillary, with an air bubble or methylene blue droplet, was watched by means of a clinical microscope to obtain better accuracy. Also, the cavities were treated with EDTA to remove the smear layer and improve access to tubules.
Seventeen of the teeth used in this study were freshly extracted and testing was started within an hour while the pulp tissue still is vital to avoid any changes due to tissue breakdown.[5],[6] These teeth showed no fluid movement to provocation. The three teeth that were stored in a refrigerator for a week revealed a tremor of the capillary fluid when subjected to air blast. Stored tissues will undergo autolysis and after that they will not have the same properties as that of a vital intact tissue. Odontoblasts are especially rich in lysosomal enzymes and will therefore undergo extensive autolysis when the blood supply is lost.[7],[8] This will, if the experiment is not performed soon after extraction, almost eliminate the odontoblasts and their processes, which would be a major obstacle for direct tubule to pulp fluid communication.
Also, the dentin tubule is far from being just a tube filled with liquid and even if the diameter of dentinal tubules at the dentin-enamel junction is around 0.5 to 0.9 μm, the tubules function as if they are markedly smaller with a diameter of 0.1 μm.[9] The tubule contains different structures: the odontoblastic process and its surrounding glycocalix and the lamina limitans covering the tubule wall, collagen fibrils, mineral deposits, and larger molecules.[10],[11],[12],[13] It has been suggested that human dentin could be considered a hydrogel.[14] Analysis of dentin fluid has revealed that it contains potassium, sodium, and chlorine[15] but even if it has not been possible to obtain a sufficient amount to properly analyze dentin liquid, it is likely that this tissue fluid contains an ion product of phosphate and calcium just above the solubility product constants for calcium phosphates.[10] Like all tissue fluids, dentin liquid is predominantly water and water exists in different molecular configurations depending on location. It has been found that biologic surfaces and tissues are covered with a special form of water.[16],[17] The molecular arrangement in this form of water is altered as regular water, with two hydrogen atoms attached to a single oxygen atom, forming clusters of four water molecules that can come together to form water bicyclo-octamers. Even large icosahedral (H2O)280 water clusters are often formed[16] (for a review, see www.lsbu.ac.uk/water). This part of the total tubule water content, known as clustered water, will not move inside the tubule following provocation. Considering the intratubular obstacles, including the comparatively large odontoblastic process, that are in place in the vital tooth, it is tempting to question the validity of the hydrodynamic theory.
The hydrodynamic theory has been challenged and other explanations for dentin sensitivity have been brought forward.[18],[19],[20],[21],[22],[23],[24] Still, the hydrodynamic theory is widely accepted and it is taught at dental schools and in textbooks.
When we were going to test base materials, as mentioned in the introduction, we took for granted that we were dealing with the “gold standard.” The finding in this study that a direct continuum from a coronal cavity surface via dentin fluid to pulp tissue and to an apically placed capillary could not be demonstrated in vital teeth using the method of Brännström et al.[3],[4] does not in itself disprove the hydrodynamic theory. However, as this in vitro experiment has been a major part of the foundation of the hydrodynamic theory, it may be time to investigate the true mechanism(s) of dentin sensitivity.
Acknowledgment
The authors want to thank Drs Gregory Warne, Linda Shin, and Brian Greenberg for help with the experiments.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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