Evaluation of the bonding strength of orthodontic bands cemented by glass ionomer modified with amorphous calcium phosphate (ACP), laboratory study (In-vitro)

dissertation to receive a general doctorate

Abstract

Introduction and purpose

One of the problems of banding in orthodontics is the creation of decalcified areas around the bands.  On the other hand, during the treatment, the bands are prone to loosening and the cement used must have sufficient strength. Recently, new compounds containing amorphous calcium phosphate (ACP) have been introduced to deal with demineralization on enamel surfaces. The purpose of this study was to investigate the strength of orthodontic bands cemented with glass ionomer cement containing ACP. Materials and methods: 120 healthy mandibular third molars mounted in an acrylic block were randomly divided into 4 groups of 30. Group 1 and 3 were sealed with ordinary glass ionomer cement (GC Corporation, Gold). Label) and groups 2 and 4 were prepared for bonding with glass ionomer cement containing ACP. Then the samples were kept in distilled water and in an incubator at 37 degrees for 48 hours. For groups 1 and 2 after this time and for groups 3 and 4 after thermocycling (5000 cycles between 5°C and 55°C), the strength of the strap was measured using a Universal testing machine with a crosshead speed of 1mm/min, and the findings were statistically analyzed using multi-factor analysis of variance (ANOVA) and Tukey's test.

Results

The highest average strength (1.5140 Mpa) was related to group 1 (ordinary GI cement without thermocycling conditions) and the lowest (1.1695 Mpa) was related to group 2 (GI cement containing ACP without thermocycling conditions). Based on the results of Tukey's test, the difference between group 1, groups 2 and group 3 (GI in thermocycling conditions) was statistically significant (P<0.05). The difference between group 2 and groups 1 and 4 (GI-ACP under thermocycling conditions) was statistically significant (P<0.05).

Conclusion

Although the strength of the glass ionomer was reduced by adding ACP, after thermocycling the strength of the samples sealed with glass ionomer containing ACP was much higher. There was no significant difference with the glass ionomer cement group in conditions without thermocycling. It seems that the glass ionomer cement containing ACP in the oral environment has sufficient resistance against the forces applied to the posterior teeth. Key words: retaining strength - glass ionomer - amorphous calcium phosphate In order to achieve treatment goals in orthodontic patients, it is necessary to change the position of the teeth. In comprehensive orthodontic treatment, this is done by fixed attachments. The use of metal bands on molar teeth during orthodontic treatment is used as a common method to stabilize the archwire position (1). Placing braces in the posterior part of the mouth exposes them to tensile and shearing forces such as chewing forces and physical traumas, and makes them prone to loosening and failure in banding (2), therefore, braces are very important during treatment (3). Ideally, the strength of the cement bond should be such that it keeps the band in place during the period of orthodontic treatment and does not cause damage to the tooth surface when removing the bands. In addition, it should be easy to use, create a suitable flood, prevent decay and have a reasonable price. The unfavorable characteristics of many cements, such as high solubility in oral fluids and weak bond strength, can create a suitable substrate for the penetration of plaque and debris under the bond, followed by the start of demineralization on the tooth surface (2).

Zinc phosphate was one of the first compounds used as cement for bonding in orthodontics. This cement was introduced in 1878 (5) and is considered as the golden standard and other cements are compared with it (6-9).. The primary bond with this cement is established mechanically between the enamel and the cement and on the other hand between the cement and the stainless steel band. This cement does not establish a chemical bond with tooth enamel (5).

In 1960, fluoride was added to the composition of this cement to reduce the solubility of the cement and on the other hand to strengthen remineralization in the tooth structure (8). The properties of zinc phosphate include high compressive strength, low tensile strength and high brittleness, short working time and high solubility in oral fluids, resulting in microleakage and increased enamel demineralization (5 and 10).

Unlike zinc phosphate, polycarboxylate cement has the ability to establish a chemical bond with tooth enamel and stainless steel braces, but its undesirable characteristics such as High viscosity, very short hardening time and high solubility in the oral environment led to its less use as cement for banding in orthodontics (5 and 11).

Another common cement in orthodontic treatment is glass ionomer (GICs), which was introduced as a restorative material in 1971 by Wilson and Kent (5). This cement has significant properties and advantages in physical properties compared to the cements that were used before. Among the favorable characteristics of this cement, we can mention low solubility in saliva, higher compressive and tensile strength compared to zinc phosphate, and participating in an acid-base reaction with enamel and dentin and creating an ionic bond with stainless steel (2), which ultimately causes a reduction in bond failure (10 and 12). Also, due to the fact that in most cases, the type of failure in this cement occurs at the border between the cement and the band, and due to its low solubility, microleakage is reduced (10 and 13).  It has been said that the release of fluoride from this cement in the long term has no adverse effect on its strength (5, 14, 15).

The next generation of cements used in banding were resin modified glass ionomers (RMGI) and are hardened by dual cure (light curing and acid-base reaction). This cement has the desirable properties of glass ionomer along with the greater strength of its resin component (13 and 16). Among its desirable properties, it can be mentioned that it is easier to use and has a longer working time due to the way it hardens and has a higher resistance to moisture (17). It has been reported that the bond strength of this cement is higher than that of glass ionomer (17), however, in Fricker's study in 1997, there was no clinically significant difference in bonding failure between GICs and RMGI (12). This cement has the ability to release fluoride (although less than RMGI), and among its physical properties, we can mention low solubility in the oral environment, high resistance to cracking and breaking, relatively higher shear and compressive strength compared to zinc phosphate (2). Unlike GICs, this cement tends to create a failure at the border between the cement and the tooth, so the risk of microleakage and then demineralization increases(12). Straps and brackets and other components used during treatment such as elastics, pleats, springs, etc. It causes problems for the patients in terms of health control, and certainly the accumulation of plaque around these devices will be more (18) so that one of the biggest problems of banding after the end of the treatment is the creation of decalcified areas around the occlusal and especially gingival surface of the bands. This decalcification is visible after 4 weeks of placing the band and brackets (19). Demineralization occurs when certain bacteria remain on the enamel surface for a long time. Bacteria metabolize carbohydrates and create organic acids, and these acids lead to the removal of calcium and phosphate from enamel and dentin (18). Demineralization starts at a pH below 5.5 (20 and 21).