Effect of two desensitizing agents on dentin permeability in vitro

Abstract Objective The aim of this in vitro study was to investigate the effect of two desensitizing agents and water on hydraulic conductance in human dentin. Material and Methods GLUMA Desensitizer PowerGel (GLU) contains glutaraldehyde (GA) and 2-hydroxyethyl methacrylate (HEMA), and Teethmate Desensitizer (TD) is a powder comprising tetracalcium phosphate (TTCP) and dicalcium phosphate anhydrous (DCPA) that is mixed with water. Deionized water was used as a negative control (CTR). Thirty discs with a thickness of 1.2 mm were cut from the coronal dentin of the third molars and cleaned with 0.5 M EDTA (pH 7.4). After being mounted in a split-chamber device, the discs were pressurized with water at 1 kPa and 3 kPa in order to measure flow rates with a highly sensitive micro-flow sensor and to calculate hydraulic conductance as a baseline value (BL). Following the application of GLU, TD, and CTR (n=10), hydraulic conductance was remeasured with intermittent storage in water after 15 min, 1 d, 1 w, and 1 m. Reduction in permeability (PR%) was calculated from hydraulic conductance. Data were statistically analyzed using nonparametric methods (α<0.05). Representative discs were inspected by SEM. Results PR% for GLU and TD were 30-50% 15 min and 1 m after their application. Post hoc tests indicated that PR% of CTR was significantly greater than those of GLU and TD at all time points tested. The PR% of GLU and TD were not significantly different. SEM examinations showed noncollapsed collagen meshes at the tubular entrances after GLU, and crystalline precipitates occluding the tubular orifices after TD, whereas CTR specimens showed typical patterns of etched dentin. Conclusions The present study on hydraulic conductance in dentin discs treated with two chemically different desensitizing agents and water as a control demonstrated that both products may be characterized as effective.


Introduction
Dentin hypersensitivity (DH) is a frequently reported pain condition. Depending on whether examination, its prevalence varies widely, ranging between 3 -98% 4 . This variation may be attributed to study samples, the types of practices in which data are collected, and regional variations. Therefore, a careful differential diagnosis is essential because other conditions may produce similar pain. DH is commonly considered a diagnosis of exclusion. The internationally Advisory Board, describes DH as "short, sharp pain arising from exposed dentin in response to stimuli typically thermal, evaporative, tactile, osmotic or chemical and which cannot be ascribed to any other form of dental defect or disease" 3 . globally accepted explanation for DH 1 . Hypersensitive dentin is mostly found in buccal tooth areas, in which enamel is missing because of abrasion, attrition, or erosion. A precondition for DH is that the dentinal tubules should be open at both ends. In short, external stimuli on exposed dentin lead to inward or outward odontoblasts. Based on this theory, a reasonable and logical treatment approach is the occlusion/sealing of peripheral dentin tubules. This concept is widely used to treat DH with an array of different types of desensitizing agents 20 .
Evaluations of dentin desensitizing agents are mostly performed in laboratory studies that predominantly use a human dentin disc model to assess hydraulic conductance, a measure for the ease with desensitizing agents 7,9,10,15,21,23 . Dentin discs may also be used to study morphological details, such as tubular occlusion on the treated surface as well as the partial or total obturation of tubules after fracturing the discs, thereby allowing inspections inside the tubules by SEM or light microscopy 11,18  Germany) under water-cooling from mid-coronal dentin perpendicular to the long axis of the teeth. The apical sides of the discs were cut as close as possible to the pulp horns, and the coronal aspects were free from enamel ( Figure 1). The discs were rinsed with water, immersed in 0.5 M EDTA solution (pH 7.4), and ultrasonicated for two minutes in order to remove the cutting smear and open the dentinal tubules before a Specimens were randomly allocated to three groups (n=10): GLU application, TD treatment, and water as control agent (CTR). Compositions and applications are listed in Figure 2.
The schematic drawing in Figure 3 shows the setup and function of the measuring device. Dentin discs cleaned with EDTA were placed between two O-rings inside a split-chamber device in order to measure hydraulic conductance. Each specimen was mounted on a ring style retainer in which the circumference of resin. The split chamber always accepted the retainer to reproduce an identical position for the measurement area of the specimen. Water was pressurized through the discs from the pulpal side at simulated pulpal (LG16-0150, Sensirion AG, Staefa ZH, Switzerland).
was measured at intervals of 0.1 seconds. The system mean value of the last four minutes from the cycle and registered on a personal computer. After a three minutes interruption, each of the two subsequent measurement cycles were performed as above.
of the specimen were then obtained at the alternative pressure.
After baseline (BL) permeability measurement, specimens were stored in deionized water, mounted in exactly the same position, and remeasured 15 treatment with the respective desensitizing agent or water control using the same procedure described    Table 1 shows the means and standard deviations of hydraulic conductance by material, pressure, and time. BL variations in the three groups were very TD group than in the two other groups.  show aspects of free and fractured surfaces after the GLU treatment. Under the EDTA cleaning condition, demineralized, the exposed collagen mesh was not collapsed. Collagen strings were supposedly crosscollagen mesh was detected to a depth of several micrometers. SEMs C and D show the free surface and a view of the fractured specimen of dentin treated with TD. All tubules and some intertubular dentin were closed and covered with a crystalline grainy substance.
considered to be dehydration gaps that had occurred under the high vacuum during sputtering and/or observed to a depth of several micrometers (white arrow), and were presumably precipitates from the dissolution process of the primary TD phosphates.
The corresponding surfaces of the control specimen after 1-month storage in water are shown in E and F.

Discussion
The present in vitro investigation has provided evidence to support GLU and TD being effective agents for immediate and lasting reductions in dentin disc permeability. Hydraulic conductance after the application of the two commercial products was times tested from BL until 1 month. Therefore, the null difference among the three agents investigated was rejected.
The active components in the plasma protein precipitant GLU are GA and HEMA. In a spectroscopic investigation, the reaction mechanism between GA and HEMA was described as a two-step reaction. GA TTCP and DCPA powder that upon mixing with water or an aqueous solution is readily transformed into hydroxyapatite (HA). This transformation is based on a dissolution-precipitation reaction mechanism 6,14,28 .
In an aqueous environment, TTCP and DCPA dissolve and supply Ca 2+ and PO 4 3-. Since this solution is supersaturated concerning apatite, the less soluble compound HA is precipitated. In the oral cavity, new continuously formed HA crystals may be precipitated because of the supersaturation of human saliva with calcium phosphate salts 16 .
The results of the present study showed wide variations in hydraulic conductance values, particularly at BL. Possible reasons for this relatively large scattering may be the age of the donors teeth, the location of the slice cut from coronal dentin, regional variability, and tubule density and diameter. The as substrates for in vitro experiments represents one testing of dentin.
We selected a very low pressure in order to simulated human pulpal tissue pressure, as reported by Ciucchi, et al. 5 (1995) andPashley, et al. 22 (1981).