MA bioadhesive copolymer in a silica base

journal of dentistry 39 (2011) 293–301

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Effects of dentin tubule occlusion by dentifrice containing a PVM/MA bioadhesive copolymer in a silica base Xuejun Liu a,b, Virginia Barnes c, William DeVizio c, Hong Yang d, Hans Malmstrom a, Yanfang Ren a,* a

Division of General Dentistry, University of Rochester Eastman Institute for Oral Health, Rochester, NY, USA Department of Restorative Dentistry and Endodontics, Zhengzhou University School of Stomatology, Zhengzhou, China c Colgate Palmolive Technology Center, Piscataway, NJ, USA d Department of Chemical Engineering, University of Rochester School of Engineering and Applied Sciences, Rochester, NY, USA b

article info

abstract

Article history:

Objective: To study the effectiveness of a dentifrice containing polymethyl vinyl ether-

Received 20 August 2010Received in

maleic acid (PVM/MA) copolymer in a silica base in occluding dentin tubules for treatment

revised form

of dentin sensitivity.

26 October 2010

Methods: Thirty-two human dentin discs were divided into two groups and brushed in the

Accepted 26 October 2010

morning for 30 s each to study the dentifrices with and without PVM/MA copolymer. Dentin tubule occlusion and dentin permeability were evaluated with a focus variation three dimensional vertical scanning microscope (IFM) and electrochemical impedance spectros-

Keywords:

copy (EIS). After second brushing for 30 s in the afternoon the dentin discs were immersed in

Dentin sensitivity

saliva for 16 h and then subjected to erosion using orange juice for 10 min. The effects of

Dental erosion

saliva and orange juice on tubule occlusion used in the study of dentifrices were further

Dentifrice

evaluated with IFM.

Dentin

Results: On average 97.7% of the dentin tubules were occluded after brushing in the PVM/MA

Dentin tubules

group, as compared to 13.3% in the control group ( p < 0.0001). EIS showed that the impedance of the dentin disc increased after treatment with PVM/MA but not in the control group ( p < 0.05). After 16 h of storage in saliva and 10 min of erosion by orange juice, 86% of the dentin tubules remained occluded in the PVM/MA treated dentifrice. The sizes of the tubule openings were increased after orange juice erosion in the control group but not in the PVM/ MA group. Conclusion: Dentifrice containing PVM/MA copolymer in a silica base effectively occluded dentin tubules. The intra-tubular plugs were resistant to saliva and orange juice challenges. # 2010 Elsevier Ltd. All rights reserved.

1.

Introduction

Dentin sensitivity is associated with patent dentin tubules that are open to dental pulp and to environmental stimuli. Dentin sensitivity is characterised as transient acute pain associated with thermal, mechanical, chemical and osmotic

stimuli that cannot be attributed to any other dental diseases. The prevalence of dentin sensitivity has been reported to be in the range of 15–25% in the general population,1–3 53–57% in dental patients4,5 and 60–68% in patients attending periodontal clinics.6,7 The hydrodynamic theory, proposed by Bra¨nnstro¨m,8,9 has been widely accepted by dental professionals to

* Corresponding author at: University of Rochester Eastman Institute for Oral Health, 625 Elmwood Avenue, Rochester, NY 14620, USA. Tel.: +1 585 273 5588. E-mail address: [email protected] (Y. Ren). 0300-5712/$ – see front matter # 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jdent.2010.10.016

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journal of dentistry 39 (2011) 293–301

explain dentin sensitivity.10 The theory suggests that external stimuli, such as a puff of air blast or a sudden change in temperature, will cause fluid movement in the dentin tubules, which in turn activates the sensory receptors located in the pulp side of the tubules, and causes acute pain. Fluid movements in dentin tubules were shown to induce action potentials of sensory nerves associated with the exposed dentin and cause transient pain.11–13 For fluid movement to occur inside the dentin tubules, the latter must be patent, with one end open to the dentin surface and the other to the pulp. Imaging studies of human dentin showed that the tubules are larger in size and greater in number in sensitive areas than in non-sensitive areas of dentin,14,15 further substantiating the role of open dentin tubules in the aetiology of dentin sensitivity. Based on the hydrodynamic theory, two strategies have been developed for the treatments of sensitive dentin. One is to physically occlude the dentin tubules to isolate the tubule contents from the oral environment and prevent the fluid flow, and the other is to chemically desensitise the sensory nerves to block the transmission of the noxious stimuli from the dentin tubules to the central nervous system. Though both strategies have been shown to be effective in reducing dentin sensitivity, recent evidence suggested that occlusion of dentin tubules is more likely to be successful than chemical desensitisation in clinical applications.10 Potassium salts in dentifrices are the most common agents used for desensitisation of sensory nerves. As potassium ions diffuse into the dentin tubules and reach the nerve endings, they may disrupt the normal potassium ion concentration gradient across the nerve cell membrane, prevent the re-polarisation of the nerve fibres and thus interrupt the transmission of sensory impulses.16 A recent study showed that filtering 500 mmol/ L potassium chloride across the dentin under 150 mmHg hydrostatic pressure might temporarily decrease the pain response to air blasts and probing.17 However, there is no evidence that potassium ions delivered through dentifrices could diffuse into dentin tubules in sufficient amount to affect the extracellular ion concentrations. Mathematical modelling predicted that the increase in potassium levels was transient after tooth brushing and it was not possible to maintain nerve blockage between brushings.18 A recent review summarised the limited number of clinical trials and concluded that there is insufficient evidence to support the application of potassium salts for treatment of dentin sensitivity.19 Dentin desensitisation based on occlusion of dentin tubules can be applied at-home or in-office. Repeated desensitisation treatments are usually necessary to maintain the treatment effects, because exposed dentin continues to sustain erosive challenges in the oral cavity. Remedies that can be applied at-home by patients are preferable over frequent visits to dental offices for their convenience and low cost. Brushing with dentifrices containing mineral salts and mild abrasives has been shown to form intra-tubular plugs and occlude the dentin tubules.20,21 The dentifrice plugs so formed however are not stable and cannot withstand the erosion occurring during daily dietary activities, such as drinking orange juice.20 For dentifrices to be effective and consistent in occluding the dentin tubules, the dentifrice plugs should adequately adhere to the dentin tubule wall, prevent

the outward fluid flow, and resist erosive challenges of oral and dietary fluids. Polymethyl vinyl ether-maleic acid (PVM/MA) copolymer is a well recognised bioadhesive. Its tissue adhesion properties have been studied when used as a denture adhesive and as controlled-release drug carriers.22–24 Dentifrices containing such copolymer and triclosan are able to retain the antibacterial agents in oral tissues for a sustained period of time and maintain a prolonged effect after a single use.25,26 Combined with submicron particles of silica, the bioadhesives in this type of dentifrices may improve the stability of dentifrice plugs and exhibit superior occlusion of dentin tubules. The purpose of this study is to compare the effectiveness of a dentifrice containing PVM/MA copolymer in a silica base in occluding dentin tubules with that of a control dentifrice, and to examine the stability of the dentifrice plugs in the dentin tubules following erosive challenges in vitro. The null hypothesis for the present study was that the two study dentifrices have no difference in their effectiveness in occluding dentin tubules.

2.

Methods

2.1.

Sample collection and preparation

A total of 32 dentin discs, approximately 1.0 mm thick, were prepared from coronal sections of freshly extracted human third molars as follows: the occlusal part of the enamel was removed with a slow speed diamond saw to expose the dentin. A parallel cut was then made above the cementoenamel junction to produce a dentin disc that was approximately 2 mm in thickness. The dentin discs were ground on a 320 grit carbide plate to remove any remnants of the enamel on the occlusal side and the pulp horn on the pulp side of the disc, and polished with 320, 600 and 1200 grit carbide polishing papers. The prepared dentin discs were cleaned for 3 min in an ultrasonic cleaner using 1% Micro-901 cleaner (International Product Corp., Burlington, NJ). The dentin discs were inspected under a focus-variation 3D scanning microscope (InfiniteFocus1 G4, Alicona Imaging, Graz, Austria) to confirm that the dentin tubules were patent. Impacted third molars with intact crowns were stored in 1% Thymol solution before use within one month after the extraction. Collection of discarded teeth after extraction was conducted following the guidelines for using discarded tissues for research at the authors’ institution. No patient identifiers were collected. Teeth were sterilised in ethylene oxide for 12 h before sample preparation.

2.2.

Baseline sample measurements

2.2.1.

Focus-variation 3D vertical scanning microscopy

Two diamond shaped indentations (one at the centre and one at the peripheral) were made on the dentin discs at 25 g of load and 5 s of dwell time using a microhardness tester equipped with a Vickers indenter. These indentation marks served as orientation landmarks for repeated imaging measurements of the samples before and after dentifrice treatments. 3D images of the dentin discs were obtained adjacent to the indentation

journal of dentistry 39 (2011) 293–301

marks using the focus-variation 3D scanning microscope at a magnification of 2000. The images were imported into a public domain imaging program (ImageJ1, developed at the U.S. National Institutes of Health) for analysis of open and occluded tubules. The Cell Counter function of the ImageJ1 software was utilised to mark the open and occluded tubules, and the degree of dentin tubule occlusion was calculated as percentage of tubules that were totally occluded in the examination fields.

2.2.2.

Electrochemical impedance spectroscopy

The electrochemical impedance of the dentin discs were measured before and after brushing treatments following a method modified from Pradelle-Plasse et al.27 and Sword et al.28 The dentin discs were placed between a custom-made split chamber diffusion cell device with a silicon rubber O-ring (d = 6 mm) on each side. A pair of platinum electrodes were used as the working and counter electrodes and inserted into each half-cell for impedance measurements. A Ag/AgCl electrode was used as the reference electrode. The chambers of each halfcell were filled with electrolyte (0.1 M KCl). The impedance of the dentin disc was measured with an electrochemical analyzer (CHI604D, CH Instruments, Austin, TX) connected to a PC and the data was analysed with the Electrochemical Impedance Spectroscopy (EIS) software package. The dentin disc impedance R(d) in kilo-ohm (kV) were calculated as the total impedance R(t) minus the impedance of the electrolyte R(e). The impedance of the dentin discs was measured at frequencies from 0.1 Hz to 65,000 Hz to derive an impedance spectrum in the Nyquist Plane. At the frequency where the Z00 value is the lowest, the Z value approaches that of R(t). R(e) were obtained by removing the dentin disc and measuring the impedance of the electrolyte alone. R(d) were calculated as R(t)  R(e).

2.3.

after dentifrice brushing treatments. After completion of the impedance measurements, the dentin discs were rinsed thoroughly in deionised water and cleaned for 3 min in an ultrasonic cleaner. The dentin discs in both groups were brushed for the second time at 5:00 pm in the afternoon for 45 strokes in 30 s, rinsed for 2 min in the rocking incubator, washed for 5 s and placed in 150 ml of clarified fresh human saliva overnight for 16 h till 9:00 am the following day, when the dentin discs were removed from the saliva and rinsed for 15 s in deionised water to prepare for the imaging analysis for the third time. Un-stimulated human whole saliva was collected at the same day of the experiment and centrifuged for 20 min at 2000 rpm using a centrifuge to produce clarified saliva. Dentin discs were placed in 150 ml of new saliva after the imaging measurements for a minimum of 15 min before erosive challenges by orange juice.

2.5.

Erosive challenges by orange juice

The dentin discs were removed from saliva, immersed in orange juice (Minute MaidTM Premium, pH of 3.8) in 6-well cell culture dishes for 10 min in an incubator gently rocking at 60 rpm to keep the solution well-mixed and simulate the fluid movement during the sipping of juice.

2.6.

Post-erosion measurements

The dentin discs were washed in deionised water for 15 s after orange juice erosion. Imaging analysis of the dentin discs was then performed for the fourth and final time.

2.7. Scanning electron microscopy/energy dispersive spectroscopy

Dentifrice treatments

Sixteen of the experimental dentin discs were treated with the study dentifrice containing PVM/MA copolymer (Colgate Total Sensitive1, Colgate Palmolive Co., Piscataway, NJ) and sixteen of the control dentin discs were treated with a control dentifrice without the copolymer from the same company (Colgate Cavity Protection1), respectively. Though both dentifrices contained a hydrate silica base, they differed in the sizes of the silica particles, which range from sub-micron to 3 mm in the PVM/MA group and 5–10 mm in the control group. A soft-bristle toothbrush (ADA standard) was used to hand brush the dentin disc using a circular motion with the dentifrice for 45 strokes in 30 s in the morning (9:00 am) at a force of approximately 200 g. Concentrations of the silica particles were also higher in the PVM/MA group (35%) than in the control group (25%). After brushing, the discs were placed in deionised water and shaken in a rocking incubator at 37 8C for two minutes at a frequency of 120 rpm to simulate rinsing, and washed afterward for 5 s. The samples were then placed in deionised water for post-treatment imaging and impedance measurements.

2.4.

295

Post-treatment measurements

Imaging analysis and electrochemical impedance spectroscopy as described in the baseline measurements were repeated

The dentin samples were dried in room temperature, fractured into two equal halves to expose the long axes of the dentin tubules and coated with gold. The dentin tubule plugs were then observed using a field emission scanning electron microscope (FESEM, Zeiss Supra 40VP, Carl Zeiss NTS GmbH, Germany) and its chemical composition was measured with energy dispersive spectroscopy (EDS) using an EDAX Xray spectrometer (EDAX Inc, Mahwah, NJ) on board the FESEM. The relative density of the elements Si, Ca, P, C, and O were examined.

2.8.

Statistical analyses

Percentage of occluded dentin tubules and dentin impedance were compared between the two groups before and after treatments. Unpaired and paired t-tests were used in each group. The non-parametric Mann–Whitney test and Wilcoxon Signed Rank test were also used to determine if the data distribution affected the results of the parametric t-tests. Sample size estimation was based on previous studies using dentifrice for dentin tubule occlusion and on our pilot test using a single dentin disc for each group. Dentifrice containing the PVM/MA copolymer could effectively occlude at least a mean of 80% of dentin tubules (SD 15%). A sample size of 16 had 85% power to detect a 20% difference in the mean percentage of occluded tubules between the two groups. Thus,

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Table 1 – Proportions of occluded dentin tubules after brushing with the study dentifrices, immersion in saliva for 16 h, or erosion by orange juice for 10 min. PVM/MA

After brushing After 16 h After OJ erosion

Control

Mean

SD

Mean

SD

97.7 88.5 86.1

5.0 17.3 15.0

13.3 6.8 2.4

9.4 7.1 4.8

t

p

31.781 17.521 21.312

<0.0001 <0.0001 <0.0001

power analysis was used to determine that the sample should be 16 discs per group.

3.

Results

3.1.

Focus variation 3D vertical scanning microscopy

Brushing with dentifrice containing PVM/MA copolymer resulted in near complete occlusion of dentin tubules whilst brushing with the control dentifrice did not have such an effect. As shown in Table 1, on average 97.7% of the dentin tubules were occluded after brushing in the PVM/MA group, as compared to 13.3% in the control group ( p < 0.0001). Storage in saliva for 16 h and subsequent rinsing reduced the proportion of occluded tubules to 88.5% in the PVM/MA group and 6.8% in the control group ( p < 0.0001). Acidic erosion challenge for 10 min with orange juice did not cause significant reduction in the proportion of occluded dentin tubules in the PVM/MA group (Table 1). After 16 h of storage in saliva and 10 min of erosion by orange juice, 86% of the dentin tubules remained occluded. As shown in Fig. 1, dentin tubules were uniformly occluded by dentifrice constituents after brushing and rinsing in the PVM/MA group. Some dentifrice constituents appeared to adhere to the dentin disc surfaces and were resistant to the action of rinsing for 2 min. The dentifrice substances adhered on the dentin surfaces were largely removed after storage in saliva for 16 h and subsequent rinsing, but those in the dentin tubules were mostly retained. Those dentifrice constituents that remained in the dentin tubules appeared to be resistant to acidic erosion by orange juice because the erosive challenge did not significantly affect the appearance of the occluded dentin tubules (Fig. 1D). In contrast, most dentin tubules remained patent after brushing and rinsing in the control group (Fig. 1A0 –D0 ). Only a limited number of dentin tubules were occluded by the control dentifrices. Storage in saliva for 16 h and subsequent rinsing further reduced the number of occluded dentin tubules. Acidic erosion by orange juice for 10 min effectively opened almost all the dentin tubules in the dentin discs.

Besides the difference in patency of dentin tubules between the two study groups, it was noticed that the sizes of the tubule openings were increased after orange juice erosion in the control group but not in the PVM/MA group. As shown in Fig. 2, the diameters of the dentin tubule openings in the control group increased from a mean of 2.81 um (SD 0.36) at baseline to a mean of 3.89 um (SD 0.41) after orange juice erosion (t = 12.196, p < 0.0001), whilst those in the PVM/MA group remained unchanged after the erosive challenges (2.89  0.28 vs. 2.96  0.22 um, p > 0.05).

3.2.

Electrochemical impedance spectroscopy

The mean and standard deviation of dentin disc impedance before and after treatment are presented in Table 2. Impedance of the dentin discs increased significantly in the PVM/MA group but not in the control group (Fig. 3).

3.3. Scanning electron microscopy and energy dispersive X-ray spectroscopy SEM images showed that dentin tubule openings were occluded with plugs 1–2 mm in length after a single treatment (Fig. 4). EDS confirmed that the substances that occluded the dentin tubules in the PVM/MA group were consistent with the components of the study dentifrice (Fig. 5).

4.

Discussion

The findings of the present study indicate that dentifrices containing the PVM/MA bioadhesive copolymer and small silica particles can form more stable intra-tubular plugs and occlude the dentin tubules more effectively when compared to a control dentifrice. The null hypothesis that the two study dentifrices have no difference in their effectiveness in occluding dentin tubules was thus rejected. The dentifrice plugs in the PVM/MA group were mostly retained in the dentin tubules after 16 h immersion in fresh human saliva and subsequent rinsing, and following an aggressive erosive challenge by orange juice, signifying that the intra-tubular plugs adhered well to the dentin tubule walls and were resistant to salivary enzymatic and acidic erosions. Such properties of the dentifrice plugs are very important for reducing the permeability of the dentin tubules for the intervals that span the two brushing periods and maintain a sustained effect on dentin sensitivity. PVM/MA copolymer is widely used as a bioadhesive for controlled release of medical and dental therapeutic agents. The bioadhesive properties of PVM/MA may be related with

Table 2 – Dentin disc impedance (R(d), kV) before and after brushing with the study dentifrices. PVM/MA Mean Baseline After brushing

1.782 2.810 t = 3.258

Control SD 0.676 0.876 p < 0.01

Mean 1.908 2.023 t = 0.931

t

p

0.396 2.469

>0.05 <0.05

SD 0.750 0.740 p > 0.05

journal of dentistry 39 (2011) 293–301

[()TD$FIG]

297

Fig. 1 – Dentin discs at baseline (A), after brushing with dentifrices (B), after storage in saliva for 12 h and subsequent rinsing (C), and after erosive challenges for 10 min by orange juice (D). Images were taken with focus-variation 3D microscope. Bar length = 10 mm.

the formation of carboxylic groups from the polyanhydride residues of the copolymer.29 The carboxylic groups may enhance the ability of the polymers to form hydrogen bonds with components of tissue surfaces.24 Electrostatic attractions

between negatively charged PVM/MA copolymer and positively charged dentin surfaces may also play an important role.30,31 A thin layer of dentifrice substances remained on the dentin surfaces after rinsing and washing only when PVM/MA

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[()TD$FIG]

Fig. 2 – Dentin discs at baseline (A) and after immersion in orange juice for 10 min (B) in the control group. The sizes of dentin tubule openings increased from a mean of 2.81 um (SD 0.36) to a mean of 3.89 um (SD 0.41), indicating occurrence of significant erosion and peri-tubular tissue loss. Images were taken with focus-variation 3D microscope. Bar length = 10 mm.

was present in the dentifrice (Fig. 1B), suggesting that the bioadhesive property of the copolymer contributed to the retention of the dentifrice on the dentin disc surfaces. Though rinsing after immersion in saliva overnight for 16 h removed most of the dentifrice substances on the dentin surfaces, 86% of the dentifrice plugs in the dentin tubules remained intact. EDS examination indicated that silica were the major component detected in the intra-tubular plugs besides phosphate and calcium. Electrochemical impedance spectroscopy is a useful tool in evaluating the patency of the dentin tubules. Because the electrochemical impedance of the dentin disc at various alternating current frequencies is dependent on the differences in potentials between the two half-cells on each side of the dentin disc and on the current flow through the dentin tubules, it will be low when the tubules are patent and high when the tubules are occluded. In other words, the impedance values of the dentin discs depend on the ability of the bulk movements of fluid and ions through the dentin tubules.27 The electrochemical impedance method has been used in assessing the dentin permeability,27,32 bonding and microleakage of

[()TD$FIG]

resin composites,28,33,34 and sealing of temporary restorations after root canal therapy.35 The set-up of the diffusion cell device used in the present study is similar to that of the hydraulic conductance cell described and perfected by Pashley and coworkers.36,37 Instead of measuring the movement of an air bubble caused by fluid shift through the dentin tubules under certain hydraulic pressure (15 cm H2O), electrochemical impedance spectroscopy measures the resistance to movements of charged ions through the dentin tubules under a low alternating potential (10 mV). Occlusion of dentin tubules causes a decrease in hydraulic conductance (less fluid shift) and an increase in resistance to electrical currents (less ionic movement). Therefore, electrochemical impedance spectroscopy may be a useful alternative to the hydraulic conductance test models. However, it was observed that brushing with the dentifrice containing the PVM/MA copolymer left a smear layer of dentifrice substances on the dentin disc surfaces (Fig. 1B). This smear layer was composed of a mixture of the copolymer and the silica abrasives, which were outside the dentin tubules and might also affect the electrochemical impedance of the dentin discs. We did not

Fig. 3 – Electrochemical Impedance Spectroscopy showing an increase in total impedance (kV) of dentin discs after brushing with dentifrice containing PVM/MA copolymer (A) but not after brushing with the control dentifrice (B).

journal of dentistry 39 (2011) 293–301

[()TD$FIG]

299

Fig. 4 – SEM images of dentin discs showing tubule opening occluded by dentifrice plug (arrows) after storage in saliva for 16 h and subsequent rinsing and after erosive challenges for 10 min by orange juice. Left: view of dentin surface showing details of an occluded tubule. Right: view of fractured surface showing a dentifrice plug.

measure the impedance values of the dentin discs after storage in saliva or orange juice erosion because the process used in such measurement may affect the integrity of the intra-tubular dentifrice plugs and consequently alter the effects of the treatment. Future experiments will compare the electrochemical impedance model with the hydraulic conductance model under the same experimental conditions and clarify the relationship between fluid shift and ionic movement through the dentin tubules as indicators of dentin permeability. The PVM/MA dentifrice is based on the commercial product Colgate Total1, which contains 2% PVM/MA copolymer and 0.3% Triclosan, and has been shown to have significant antiplaque and anti-inflammatory effects for the prevention of dental caries and gingivitis.26 The particles in the silica base of Colgate Total1 were reduced from 5 mm or larger to 3 mm or smaller in the formulation of this study to facilitate the occluding efficacy of the dentifrice when applied through tooth brushing. In comparison to the control dentifrice, the PVM/MA dentifrice had a hydrated silica material with high concentration (35% vs. 25%) and small particle size (3 mm vs.

[()TD$FIG]

Fig. 5 – Energy dispersive X-ray spectroscopy shows chemical composition of dentin tubule plugs.

5 mm). As the openings of human dentin tubules are about 3 mm in diameter (Fig. 2), this formulation facilitated the entrance of smaller silica particles and the formation of a silica-based plug inside the dentin tubules. Owing to its strong bioadhesive properties, PVM/MA co-polymer significantly improved the stability of the intra-tubular plugs. Most of the intra-tubular plugs remained intact after 16 h overnight storage in human saliva followed by 10 min of erosive challenges by orange juice (Figs. 1 and 4). Besides the effect of tubule occlusion, the dentifrice prepared in this study appeared to have a significant role in the prevention of dentin erosion by orange juice. The sizes of dentin tubule openings increased significantly in the control group but not in the PVM/MA group, where the dentin tubule walls were adequately covered by the intra-tubular dentifrice plugs (Fig. 2). This finding indicated that the dentifrice substances adhered on the dentin surfaces and in the dentin tubules had a protective effect against acidic challenges. Acidic erosion of dental hard tissues and subsequent exposure of dentin tubules are widely accepted as one of the most common causes of dentin sensitivity.38,39 Currently available treatments for dentin sensitivity have largely focused on either occluding the tubules or desensitising the nerve endings, but seldom addressed the etiological factors causing the sensitivity. The findings of the present study provided encouraging evidence that self-application of dentifrices containing PVM/MA copolymer in a hydrated silica base could not only reduce dentin sensitivity through efficient dentin tubule occlusions, but potentially also prevent dentin erosion that caused the sensitive symptoms. Though the dentin disc model is very well established and has proven to be an effective tool for evaluating potential desensitising agents in vitro,40,41 the results of the present study must be interpreted with caution. Because the experimental dentifrice contained more silica (35% vs. 25%) of a smaller size (3 mm vs. 5 mm) than the control group, there is some uncertainty as to the relative contribution of PVM/MA vs. the silica composition, on tubule occlusion. It is not known if the presence of internal pressure will affect the effectiveness of dentin tubule occlusion using dentifrices. Clinical studies in situ/in vivo are necessary to verify the findings of this study in vitro.

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Acknowledgements This study is supported in part by a stipend to Dr. Xuejun Liu from Bureau of Science and Technology, Henan Province, China, and a grant from Colgate Palmolive Company.

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