Based on the International Caries Detection and Assessment System (ICDAS), a new caries management process has been established, known as the International Caries Classification and Management System (ICCMS™) [1
]. This is a detection and assessment approach that classifies the stages of caries progression, and suggests that only the soft dentin should be removed in the case of deep cavities (ICDAS Code 5–6), and that the tooth structure should not be removed in the case of enamel decalcification (Code 0–2) [2
]. One material that is recommended in both scenarios is glass polyalkenoate cement [2
], as named by the International Organization for Standardization (ISO), and is also well known as glass ionomer cement (GIC).
Because GIC releases fluoride ions [4
] and exhibits good biocompatibility with pulp tissue [5
], strong chemical bonding with tooth structures and a low thermal expansion coefficient similar to that of human teeth [8
], it has been widely used as a dental material for some time now. Due to its antimicrobial [11
] and remineralization [4
] effects, GIC has been recommended for use in the Atraumatic Restoration Technique (ART) [13
]. However, there have been some concerns regarding the inferior physical strength of GIC [15
]. To overcome this disadvantage, many researchers have explored the addition of various reinforcing materials to GIC, such as Ag alloy powder, Ag-sintered glass and bioactive glass [16
]. Although these approaches have increased the strength of GIC, they also tend to reduce its ability to release fluoride ions [17
]. In our previous study, we found that the addition of hydroxyapatite (HAp) to GIC can increase its flexural strength while maintaining its compressive strength and simultaneously enhancing fluoride ion release [20
]. Porous, spherical HAp particles were found to be the most effective in this regard [23
]. This novel material, which we term apatite ionomer cement (AIC), has also exhibited high antibacterial activity [24
The mechanisms by which AIC releases more fluoride and is strengthened remain unclear. In the present work, in order to assess these theories, we chose two different types of microcrystalline cellulose that are widely used as pharmaceutical additives, are typically unreactive with other materials, and have similar characteristics and particle morphologies to porous, spherical HAp. Using different ratios of these materials, we compared the fluoride ion release concentration and compressive strengths of the resulting formulations to those of conventional GIC and to AIC.
The aim of this study was to elucidate the mechanisms by which porous, spherical HAp improves the mechanical strength and bioactive functioning of GIC. Our null hypotheses were that the addition of porous, spherical HAp has no effect on the mechanical strength and multi-element release ability of GIC, and that the mechanical strength and multi-element release ability of GIC is unaffected by the addition of cellulose.
The AIC specimens in our previous studies [20
] were made by reducing GIC powder and adding HAp powder instead. Because fluoride is contained in the GIC glass powder, it was not clear why the AIC had superior fluoride release properties in spite of a decrease in the amount of GIC powder. The fluoride release from GIC is normally due to an acid–base reaction, with the amount of fluoride released being proportional to the concentration of fluoride in the material [25
]. Therefore, the results obtained from these AICs appear contrary to the expected outcome. In Experiment I, we used constant masses of the Fuji III powder (the fluoro-aluminosilicate glass) and Fuji III liquid (the polyacrylic acid) in both the GIC I and in the experimental groups during the Condition A preparation to address this question. The fluoride release data obtained from the Condition A specimens showed that the amount of fluoride released from the AICS-A was significantly higher than from the GIC I, even though all groups had the same mass of the Fuji III powder, which was the only source of fluoride ions (Table 5
). These results suggest that HApS could play an important role in increasing the fluoride release based on a reaction between HApS and the GIC matrix or glass core. Moreover, in medical fields, porous HAp has also been studied as a drug delivery system, and it was reported that the microporosity of HAp allowed the slow release of drug [26
]. It was considered that HApS acted as the pathway of fluoride ion release due to its porosity. Namely, it is possible that the matrix included fluoride ion originated from GIC glass core infiltrated the pores of HApS in AICS, and the fluoride ions were released out of AICS.
We also looked at the results obtained when adding cellulose materials that were incapable of reacting with the GIC. The fluoride ion releases from the CPC-A and UFC-A were found to be similar to that obtained from the GIC I (Table 5
). However, the fluoride release from the CPC-A was not significantly different from that of the AICS-A. This might be explained by considering that the density of the UF-711 powder (0.22 g/cm3
) was lower than that of the HApS (0.33 g/cm3
), while the CP-203 was the densest (0.87 g/cm3
). For this reason, the volume of the CP-203 powder in the Condition A formulation was low compared to that of the UF-711 powder. The release of fluoride ions from the GIC powder is due to a reaction with the GIC liquid [27
], so it appears that a greater quantity of the Fuji III liquid was able to react with the Fuji III powder in the CPC specimens. To further verify the relationship between fluoride release and the P/L ratio, and to demonstrate that the fluoride release properties of AICs are not related to the volumes of additives, Condition B was designed. This fabrication process involved using volumes of the UF-711 and CP-203 powders equal to the volume of a 0.24 g quantity of the HApS powder, thereby removing the variations in the volume of additive powder. The fluoride release from the CPC-B was not significantly different from that of the AICS-B, which exhibited increased fluoride ion release properties. In addition, there was no significant difference between the UFC-B and AICS-B. The variation in fluoride release between the UFC and CPC groups may therefore be due to differences in their water absorption characteristics. It appears that the fluoride release properties of the formulations with UF-711 and CP-203 powder added to the GIC might not be related to chemical reactions between the GIC and the celluloses, but rather to physical factors such as the water absorption properties. Further research is still required to identify the regions within the GIC that deteriorate to release fluoride ions, but we can hypothesize that fluoride ion release is not the result of physical degradation of the material. These data suggest that the proportions of the GIC and HAp powders in AIC formulations should not be determined solely on considerations of volume.
Experiment II was designed to confirm the effects of different HAp materials on the mechanical strength and ion release properties of AICs. Arita et al. demonstrated that porous HAp made by grinding HAp with columnar crystal shapes using an automatic ball mill is suitable for use in dental restorative AIC formulations [21
]. In our study, a commercial porous HAp (HApS) was selected because of its low cost, and because it can produce an AIC with applications as a restorative and sealing material [23
]. However, an AIC including this spherical, porous HApS has not yet been compared with materials made with HAp powders with other morphologies. Accordingly, we selected HAp200, which has a typical hexagonal crystal shape. In addition, some researchers have reported that cellulosic fibers improve the mechanical properties of GIC, including its compressive or diametrical tensile strength [28
]. In our study, celluloses having either porous or spherical characteristics, UF-711 and CP-203, were also compared with HApS. Another control group, GIC II, was made for comparison purposes at the same P/L ratio as the experimental groups. The compressive strength data obtained in Experiment II showed no significant difference between the AICS and AIC200. The HAp200 particles are highly crystalline and exhibit high micro-compressive strength (1.54 MPa) [21
], while the strength of HApS is extremely low (0.06 MPa, Table 3
). Interestingly, the compressive strength of CPC was significantly lower than that of the GICs and AICs, even though the CP-203 particles had a higher micro-compressive strength (23.49 MPa). These data suggest that there is no benefit in adding reinforcing materials that do not undergo chemical reactions with the GIC. In addition, the HApS particles are breakable and can therefore disperse into the matrix layer [24
]. It has also been shown that HAp can act as a drug delivery carrier due to its superior adsorptive properties [30
]. Moreover, HApS showed the agglomeration of nano-HAp particles in the SEM image (Figure 1
(A-2)). Amorphous, not well-crystallized HAp primary particles typically exhibit higher solubility. It has been demonstrated that Ca enhances the formation of the GIC matrix, increasing the surface hardness [32
]. Therefore, the dispersion of HApS particle in the matrix leads to a chemical reinforcement effect and increases the compressive strength.
In terms of fluoride ion release properties, there was no significant difference in the fluoride ion release concentrations between AICS and AIC200, although the AICS tended to exhibit higher fluoride ion release compared to the AIC200. GIC glasses contain other elements, such as Al, Si, P and Sr, and these cations can produce a complex phosphate hydrogel matrix [33
]. In our previous study, it was observed that AIC samples had a GIC glass core within a polyacrylic acid matrix-gel layer [23
]. In the present work, the ICP-AES results demonstrated that HApS and HAp200 do not appear to increase the amounts of Al, Si and P ions released from the GIC, nor do they inhibit the original release concentrations. In Fuji III, Sr is added to the polyalkenoate glass instead of Ca, so the Sr detected by ICP-AES originated from the Fuji III powder. In contrast, the Ca came from the HApS or HAp200, so it was not detected in the control groups (GIC I and GIC II) or the two cellulose-added groups (UFC and CPC). The Sr and Ca release concentrations from the AICS were significantly higher than that from the AIC200. Due to its high specific surface area, HApS might be easier to expose to the Fuji III liquid compared to HAp200 which had well-crystalized primary particles. It was considered that HAp increase dissolution kinetics and might lead to an overall larger Ca ion release by the reason of its solubility. It has been reported that a combined Sr-F treatment for softened enamel promotes remineralization and prevents acid demineralization [34
]. GIC has been shown to exhibit remarkably high adhesion to tooth surfaces, and the intermediate layer between GIC and dentin contains Ca and P originating from the dentin material HAp [35
]. AICs are therefore expected to have superior tooth adhesion properties. Moreover, it is possible that the superior ion release properties of AICs could promote the remineralization of enamel and dentine and form a secondary dentin to prevent caries or secondary caries.