Occlusion of Dentinal Tubules by Nano-Hydroxyapatite

1759

K.OHTA, H.KAWAMATA, T.ISHIZAKI and R.HAYMANSANGI Co., Ltd. Tokyo, Japan

1. Introduction

2. Materials and Methods

Roughly 97% of tooth enamel and 70% of dentin comprises
hydroxyapatite. Nanoparticle hydroxyapatite, applied in toothpastes andgels, is known to restore mineral density to demineralized surface andsubsurface enamel, improving its smoothness and protecting against decay.
Occlusion of exposed dentinal tubules by nano-hydroxyapatite/protein
complexes, helping to reduce hypersensitivity, has also been reported.
We examined the ability of pure nano-hydroxyapatite (nano-HAP),
applied in a commercial toothpaste (Apagard Renamel, SANGI Co., Ltd.),
to provide surface coating to areas of exposed dentin and to occlude
exposed dentinal tubules. In an in vitro study, we compared the nano-HAP
product with three commercial toothpastes each containing ingredientsdesigned to alleviate hypersensitivity.

for 9 minutes daily for five days, washed with DW, and preserved in air having a relative
humidity of 100% at 37 ℃ between immersions.

2.1 Preparation of Dentin Specimens
Specimens of extracted human dentin were cut longitudinal to the tooth axis with a handengine (EX, Royal) and polished on one side, from the cementum surface in the direction of thepulp cavity, using a micro grinding machine (MG-400CS, EXAKT) (Fig. 1) with P1200 (grainsize: 15.3 µm), P2400 (8.4 µm) and P4000 (5.0 µm) abrasive papers, respectively, under
running water. The specimens were sonicated to eliminate contaminants, but conventional acidetching of the dentin surface was not performed. All specimens were masked on the unpolishedside with carbon tape covered with nail enamel to prevent penetration from that side by theagents used in this study.

2.3 Treatment of Dentin Specimens by Brushing
A second group of specimens was treated by brushing with the same test slurries as describedin 2.2. The specimens were mechanically brushed with the slurries (at 100 g weight and 60
oscillations per minute using felt) for 9 minutes daily for five days, washed with DW, andpreserved in air between brushings under the same conditions as in 2.2.
2.4 Characterization of specimens

2.2 Treatment of Dentin Specimens by Immersion
One group of specimens was treated by immersion in test slurries. These comprised 25 g oftoothpaste mixed with distilled water (DW) to a level of 40 ml, and respectively contained purenano-HAP (Product A), nano-HAP/protein complexes (Product B), aluminum lactate (ProductC) and strontium chloride (Product D). The specimens were immersed in the respective slurries

Figure 1 Micro grinding machineused in this study

To observe specimens in cross-section, a portion were cut vertically to the polished surfaceusing a saw microtome (SP1600, Leica). For microscopic observation, specimens were thendehydrated in a series of 70, 80, 90 and 100% ethanol, freeze-dried by t-butyl alcohol, and ioncoated with platinum palladium. The surfaces and cross-sections of the specimens wereobserved by FE-SEM (S-4500, Hitachi).

3. Results and Discussion

Figure 2. FE-SEM photograph of dentin surface
after mechanical polishing,showing cleanly exposed
tubules, intertubular dentin(ID) and peritubular dentin(PD).

Product B(nano-HAP/protein)

Product C(aluminum lactate)

Product D(strontium chloride)

Fig. 3 (lower row) shows FE-SEM observation of specimens treated by brushing with the test agents. Control specimens(brushed with water only) showed increased surface roughness after brushing, believed to be due to abrasive loss of dentinal
apatite particles. Specimens brushed with Products A and B, each containing nano-HAP, showed relatively even coating of thedentin surface and tubular occlusion. In
Table Surface coating and occlusion abilities of various slurries obtained by immersion
contrast, specimens treated with Productsor brushing
C and D showed only partial occlusion of
the tubules, and a uniform coating layer
MethodProduct (ingredient)
Surface coating
Occlusion
was not observed. (Cross-sectional data ofbrushed specimens not shown).
A (nano-HAP)
3.4 Comparison of occlusion and coating
abilities between immersion and brushingmethods

3.1 Observation of dentinal surface

Figure 3. FE-SEM surface photographs of dentin specimens treated by immersion (upper row) and brushing (lower row) using the respective
test toothpastes: Left, untreated control. Product A (nano-HAP), Product B (nano-HAP/protein complexes), Product C (aluminum lactate),
Product D (strontium chloride).

ImmersionSpecimens obtained bymechanical polishing, as seen inFig. 2, showed cleanly exposed
dentinal tubules and intertubularand peritubular dentin, free fromthe exposure of collagen fibrils on
the dentin surface and the spongelike surface erosion or 'smear layer'that commonly result from acidetching. This allowed the degree of
surface coating and occlusion ofdentinal tubules after treatment
with the test agents to be moreclearly observed.

FE-SEM observation of specimens treated by immersion in the test agents (Fig. 3, upper row, and Fig. 4) showed mild tomoderate surface coating and partial occlusion of dentinal tubules in the case of Products A and B, each containing nano-HAP.
But in the case of Products C and D, though aggregated particles were seen adhering to the orifice of dentinal tubules, tubularocclusion and a uniform coating layer were not observed.
3.3 Dentin Surface Treated by Brushing MethodBrushingPDID

Product A(nano-HAP)

Immersion

Control

3.2 Dentin Surface Treated by Immersion Method

Almost no difference in the coating
ability of each slurry was observed
between immersion and brushingmethods; however, the occlusion ability
was much improved by brushing asindicated in Table.

◎○B (nano-HAP/protein complexes)◎○C (aluminum lactate)△○

D (strontium chloride)△△

A (nano-HAP)◎◎

B (nano-HAP/protein complexes)◎◎

C (aluminum lactate)△○

D (strontium chloride)○○Immersion

Brushing◎ excellent (100-80 %), ○ good (80-20%), △ poor (20-0 %)4. Conclusion

Figure 4. FE-SEM cross-sectional photographs of dentin specimens treated by immersion in the respective test toothpastes (same order asabove).

These results demonstrated that nano-HAP, when applied in a toothpaste, with or without the addition of protein, has theability to occlude exposed dentinal tubules and form a surface coating over exposed dentin. Nano-HAP, made up of the same
main component as the teeth, may thus be useful in the treatment of dentin hypersensitivity in toothpaste formulations.
85th General Session & Exhibition of the IADR /March, 2007
E-mail: ohta@sangi-j.co.jp