Bond strength of glass ionomers as

 
Glass lonomer cement is considered to be a recent material used for restoring teeth. It was launched on the market since 1971. Studies show that 86.4% of dentists use this material and also 71 % of dentists use it only for cementing restorations and crowns for its chemicaly bonding properties. In this paper, the adhe­ sive properties were tested in two kinds of extracted human teeth. First group, recently extracted teeth well preserved in saline and the second group, teeth extracted and preserved in formaline for many years. There was no significant difference in the adhesion between the two groups. The Japanese Fuji showed more adhesion to the recently extracted teeth.

 
The use of glass ionomer restorative materials has significantly increased since its introduction in 19711 A recent study reported that 86.4% of the dentists surveyed have used glass ionomer cements.2 Another study3 found that glass ionom-ers were used by 71 % of the respondents as a luting cement, 50% as a base and core material, 61 % as a liner, and 57% as a restorative material. The development of adhesive restorative mate-rials has created new ways of restoring teeth and, in some cases has lessened the dependence on mechanical retention. Chemically bonded restora­ tions can preserve tooth structure and simplify treatment procedures. These restorations are espe­ cially useful in Class V erosion and abrasion lesions.
The longevity of chemically bonded restorations depends, in part, on the bond strength between the restoration and the tooth. Therefore, it is important for the clinician to know the bond strength of the various available materials. A number of studies have determined the bond strength of glass ionom-
ers to dentin and enamel.4-6 These laboratory studies have used extracted teeth, and do not specify the age of the extracted teeth or storage media. The relationship of bond strenghts obtained in these laboratory tests to that in clinical situations is unclear.
The purpose of this paper is to examine the bond strength of glass ionomers when used as restorative materials to recently extracted teeth stored in saline, and extracted teeth stored in formalin for various period of time.

Two glass ionomers were evaluated for their ability to bond to dentin of recently extracted teeth stored in saline, and aged teeth stored in formalin. The materials investigated were Ketac-Fil (ESPE, Seefeld, Germany) and Fuji II (G-C International Corp., Tokyo, Japan).
A group of 10 recently extracted teeth and 10 aged extracted teeth were collected and prepared for each glass ionomer. The recently extracted teeth were stored in saline solution and varies in age up to several years. The aged teeth were stored in formaline for many years.
Immediately prior to bonding, the enamel on the buccal or lingual surface of the teeth was removed and the dentin ground flat to a 500 grit on water-cooled abrasive belts and strips (Buehler Ltd., Evanston, lL 60204 USA).
Each bond site was conditioned with polyacrylic acid (GC Dentin Conditioner, GC International, Tokyo, Japan) for 10 seconds, washed with distil­ led water and air dried. The two materials were mixed according to the manufacturers' instructions and applied to the treated dentin surfaces in a cylin­ drical shaped plastic matrix, which produced a glass ionomer cylinder 2.99 mm in diameter and approximately 3 mm in length. Each specimen was coated with a varnish (ESPE/Premier, Norriaton, PA 19401) and allowed to set undisturbed for 15 minutes. The test specimens were stored in 37°C saline for 24 hours before testing.
After the designated storage time, the teeth were mounted vertically in a 15 mm diameter PVC ring with self curing polymethyl-methacrylate resin to provide a base for testing. The mounted specimens were placed in a Universal Testing Machine (In-stron Engineering Co., Canton, MA 02021 USA) so that the force from the crosshead was parallel to the prepared tooth surface. The bonded cylinders were placed under continuous loading at a cross-head speed of 0.5 mm/minute until fracture occur­ red. The shear bond strength was then calculated and recorded in megapascals (MPa).

 
The results are shown in Figure 1. The mean bond strength of Ketac-Fil to both groups of teeth was practically identical. The bond strength to aged teeth was 5.2 MPa (± 1.83) whereas for the recently extracted teeth it was 5.3 MPa (± 1.15). The bond strength of Fuji II showed greater vari­ ation between the two groups . The bond strength to the aged teeth was 3.9 MPa (± 1.17) while to recently extracted teeth was 4.8 MPa (± 0.77). All samples separated adhesively at the glass ionomer/ dentin interface. When this data was subjected to a t-Test the following was observed:
  -   there was no significant difference between the bond strength of Ketac-Fil to aged and recently extracted teeth (p > 0.5).
  -   Fuji II had a significantly stronger bond to recently extracted teeth than to aged teeth (p < 0.5).
  -   Ketac-Fil and Fuji II showed no significant differences in their bond strengths to aged teeth (p > .05), or to recently extracted teeth (p > .05).

 
Glass ionomers are very useful for restoration of Class V erosion and abrasion lesions. Often, these restorations can be placed without removing any additional tooth structure. Glass ionomers also release fluoride which can be beneficial in cases where caries is a problem.
When the bond strength of Ketac-Fil to aged extracted teeth was compared to its bond strength to recently extracted teeth, it was practically the same. While the bond strength of Fuji II to recently extracted teeth was statistically greater than that to the aged extracted teeth, the two values were very close. While the bond strengths obtained on recently extracted teeth may not be similar to those obtained in the clinical situation, its proximity to that obtained on aged teeth may add validity to laboratory tests involving glass ionomer bonding using aged teeth. This observation may enhance the practitioner's ability to assess these materials.
The bond strengths obtained in this study are similar, but somewhat higher, than those obtained in previous studies. Aboush & Jenkins7 found that ChemFil II had a bond strength of 3.88 MPa at 15 minutes, which increased to 4.60 MPa at 24 hours. When the dentin surface was treated with a
Prophy-Jet, the bond strength increased to 7.21 MPa. Kakaboura et al8 evaluated the effects of the Prophy-Jet treatment of dentin surfaces on bond strength. Both Ketac-Fil and Fuji had an increased bond strength when the dentin surface was treated with Prophy-Jet. Their results on untreated dentin are similar to those obtained in this study, but slightly lower.
Powis et al9 studied the effects of various dentin treatments on glass ionomer bond strengths. Sev­ eral of the dental treatments increased the bond strength of ASPA. Polyacrylic acid produced one of the largest increases and the bend was slightly greater than the ones obtained in this study. Vou-giouklakis et al10 studied the effects of surface con­ dition on the bond strenghts of two glass ionomers (ASPA and Fuji II) as well as a composite and a polycarboxylate cement. The bond strenghts obtained for Fuji were similar to the ones in this study. Vougiouklakis et al10 found that the Fuji
bond strength decreased slightly when the dentin was treated with acrylic acid.
As can be seen from the previous discussion, a number of studies have evaluated the effects of various dentin surface treatments on glass ionomer bond strength. Generally, those treatments that remove the smear layer increased the bond strength. Duke et al11 performed an SEM (scanning electron microscope) study of various agents' abil­ ity to clean dentin. They found that polyacrylic acid (Durelon liquid) gave the best result and that flour of pumice or a prophylaxis paste used with a rub­ ber cup were also effective. The implication was that improved adaptation of restorative materials and improved adhesive bonding would occur with removal of the smear layer. All of the samples in the present study were treated with polyacrylic acid (GC Dentin Conditioner) and may account for the slightly higher bond strengths obtained.
The bond strength of glass ionomers is rather modest when compared to that of composite resin bonded to etched enamel. The enamel-composite bond is reported to be in the 16-20 MPa range, and has clinically been shown to be a durable bond.12 However, this decreased bond strength of glass ionomers may be compensated for by the similarity of coefficient of thermal expansion to that of the tooth.

 
The bond strength of two glass ionomer restora­ tive materials (Ketac-Fil and Fuji II) to recently extracted teeth and extracted teeth stored in forma­ lin was studied. The bond strengths of Ketac-Fil to aged extracted and recently extracted teeth was essentially the same. The bond strength of Fuji II to recently extracted teeth was statistically stronger than to aged extracted teeth, but this difference was not thought to be of practical significance. It may be extrapolated that since there was no difference in bond strength between the aged and recently extracted teeth, the current in vitro studies (on aged teeth) are not irrelevant to the clinical situation.