Tensile bond strength of dentin adhesives: a comparison of materials and methods G. Bilo S. Olsson NIOM Scandinavian Institute of Dental Materials Kirkeveien 71B POB 70 N-1344 Haslum, Norway Received August 16, 1989 Accepted March 5, 1990 Dent Mater 6:138-144, April, 1990
Abstract-The influence of two different storage conditions on the bond strengths of four dentin adhesives [Gluma, Scotchbond (dual), Scotchbond 2, and Tenure] was studied. 5000 cycles from 7°C to 60°C reduced the bond strength significantly for all materials except Gluma, as compared with that found after 24 h of storage in water at 37°C. The influence of two different tensile test methods and two different locations, Le., occlusal and buccal, of the dentin substrate on the bond strength of Gluma and Scotchbond 2 was also studied. The type of dentin as well as the choice of tensile test method changed the bond strength values significantly. One method gave as much as 3 x higher bond strength values for one material when used on the same type of dentin. Bond strength values obtained on the dentin from the buccal side of the tooth were from 20% to 50% higher than those obtained on occlusally located dentin with the same method.
ith the increasing use of denrtal composites in larger cavities, there is a special interest in bonding to dentin as well as to enamel. Although bonding between dentin and composite is weaker than bonding to etched enamel, it is still clinically important. Dentin bonding may eliminate or limit gaps along the dentinal walls of the cavity and thereby reduce microleakage, secondary caries, and post-operative sensitivity. Various systems have been developed to establish dentin bonding. The treatment of the smear layer is an important factor, and a bond can be achieved, either by incorporating the smear layer in the system (Bowen et al., 1982) or by removal of the smear layer (Causton, 1987; White et al., 1989). Both the phosphate ester and the oxalate bonding systems rely on the smear layer for their bonding effect. Among the products marketed in the "phosphate group" are Scotchbond (dual) (3M) and in the "oxalate group" Tenure (Den-Mat). The newly marketed Scotchbond 2 (3M) is also said to be dependent upon a pretreatment or "priming" of the smear layer. A removal of the smear layer by t r e a t m e n t of the dentin with EDTA is used in the Gluma system (Bayer). An important parameter for the efficacy and lifetime of a bond is the bond strength. Bond strength can be measured in various ways, e.g., by application of tensile, shear, or torsion stress on the bonding material. A tensile test seems to be the least complex method (Oilo, 1987). A wide variation of tensile bond strength values has been obtained, depending on the materials and test methods used (ADA, 1987). Several factors influence the measured bond strength value, in addition to variations in materials and methods. One set of factors is the storage conditions, i.e., time and
138 OILO & OLSSON/BOND STRENGTH MEASUREMENTS
temperature in water after bonding. Storage in water for 24 h is normally sufficient to discriminate between those materials which can and those which cannot withstand a wet environment. Long-term storage in water is considered necessary for a better simulation of clinical conditions. Thermal cycling is suggested to provide even more information than longterm storage alone, since it combines the effect of water with the stresses developed by differences in thermal expansion of the bonded materials (Chohayeb, 1988). Another factor which influences the measured bond strength is the quality of the dentin to which the bonding material is applied. It has been shown that Scotchbond has a higher bond strength to dentin just beneath the dentino-enamel junction than to dentin near the pulp (Causton, 1984). The aim of this study was to compare the tensile bond strengths of four dentin bonding agents under various storage conditions and to compare two different tensile test methods used on adhesives applied to dentin surfaces at two different locations in the tooth. MATERIALS AND METHODS
Teeth. - O n e hundred and sixty newly extracted human third molars were used. A fresh tooth was used for each experiment. Immediately after extraction, the teeth were put in a saline solution mixed with 0.5 wt% chloramine to p r e v e n t bacterial growth. After a few days, the storage solution was replaced by distilled water, and the teeth were stored no longer than six weeks in a container which was kept refrigerated. The extracted teeth were embedded in epoxy resin. Dentin exposed occlusally and buccally was used for bonding. The occlusal dentin specimen was made by turning the crown
part of the molar on a lathe to form a dentin cylinder with a diameter of either 3 mm or 5 mm. All enamel was removed on the occlusal part of the cylinder, and the dentin was thereafter ground wet on carborundum paper no. 1000 at a right angle to the mid-axis of the dentin cylinder. The buccal dentin specimen was obtained by grinding down the buccal surface of the tooth t h r o u g h enamel on wet carborundum paper no. 1000 until a similar dentin area (rain. 3 mm or 5 mm diameter) was exposed. The tooth protruded from the resin so that contamination during grinding could be avoided.
Method / . - T h e first tensile t e s t method e m p l o y e d has b e e n described by ~ilo (1979) and Odin and ~ilo (1986). Knife-edged, separable stainless steel cups, 5 mm in diameter, were mounted on the exposed dentin surface in a special jig to ensure alignment between the long axis of the dentin substrate and the material holder during bonding. When bond s t r e n g t h t e s t i n g was performed, the load was applied by means of a flexible connection to the cross-head of the universal testing machine (Instron 1121, High Wycombe, UK). Cross-head speed during loading was 1 mm/min. Both the occlusal and buccal types of dentin specimens were used for this method. Method 2 . - T h e second method was originally described by Kemper and Kilian (1976), and was later proposed as a standard ISO method. In this s y s t e m , t h e s p e c i m e n s are mounted in two brass holders, one for the tooth and the other for the composite material. The holders are mounted on the two stainless steel rods. The diameter of the material holder was 3 ram. Two alignment blocks, one Vshaped to be used during bonding and a rectangular frame to be used during tensile testing, will ensure proper alignment of the bonded assembly during mounting as well as during tensile loading of the bond. The tensile stress is obtained by connecting the two solid rods gliding through holes in the measurement alignment frame with the testing machine. The load is applied to the upper rod by
TABLE MATERIALSUSEDIN THE PRESENTSTUDY Materials
Dentin adhesives Gluma Dentin Bond Scotchbond (dual) Scotchbond II Tenure
Composites Silar Universal Lumifor Universal P 50 Universal Visarfil
Gluma Bond 5940 Gluma Resin L 5935 Gluma Cleanser5939 Adhesive Resin7533R Adhesive Liquid 7533L Dental AdhesiveNo. 7502A Dentin Primer No. 7502P Visar Seal No. 3806 Tenure PowderA No. 1142 Tenure PowderB No. 1143 Tenure Liquid No. 1144 Tenure ConditionerNo. 1141 8601 A + B 75936 935ou 59 38502
means of a flexible string. Cross-head speed during loading was the same as for Method 1. Both the occlusal and buccal types of dentin specimens were used for this method.
Materials.- The materials used in this study are shown in the Table. Gluma bond is an aqueous solution of glutaraldehyde and the surface-active H E M A (hydroxyethyl-methacrylate). According to the manufacturer's recommendation, the dentin was cleaned with Gluma Cleanser (EDTA, 0.5 M), subsequently rinsed with water, and blown dry before the application of Gluma Bond. After 60 s, the dentin surface was carefully dried and coated with a light-flowing resin (light-cm-ed Bayer Resin L) before the application of the composite. Gluma was used for a comparison of storage conditions as well as for test methods and dentin variations. The adhesive Scotchbond (dual) is a two-component system. The resin contains phosphate esters of BISGMA, diluent resin, and benzoyl peroxide. The liquid is, an alcoholic solution of a tertiary aromatic amine and sodium benzene sulphinate. The manufacturer recommends leaving the smear layer undisturbed, and the dentin should be pre-treated only with water. Scotchbond (dual) was used for a comparison of storage conditions. Scotchbond 2 is a light-cured adhesive. The resin component of the product is composed primarily of hy-
Bayer Dental, Leverkusen,FRG 3M Dental Products, St. Paul, MN, USA 3M Dental Products, St. Paul, MN, USA Den-Mat Corp., Santa Maria, CA, USA
3M Dental Products, St. Paul, MN, USA Bayer Dental, Leverkusen,FRG 3M Dental Products, St. Paul, MN, USA Den-Mat Corp., SantaMaria, CA, USA
drophilic methacrylate monomer, BIS-GMA, and a photo-initiator. Prior to application of the adhesive, the dentin was rinsed with water, blown dry, and pre-treated with a primer. The primer is composed of an aqueous solution of a hydrophilic methacrylate monomer and an organic acid. Scotchbond 2 was used for comparison of storage conditions, test methods, and dentin variations. Tenure belongs to the oxalatedentin-bonding system. The smear layer is transformed into a rigid surface structurally joined to the dentin by aluminum oxalate together with two acetone solutions of NTG-GMA [adduct of N(p-tolyl)glycine and glycidyl methacrylate] and PMDM (adduct of pyromellitic acid dianhydride and 2-hydroxyethyl methacrylate). The layer is treated with two adhesion-promoting resins t h a t bond chemically to the sealed dentin surface. Tenure was used only for comparison of storage conditions. The materials were used according to the manufacturers' instructions and in combination with the m a n u f a c t u r e r s ' own light-curing composites and light-curing lamps. The dentin adhesives were applied in a thin layer on the occlusal part of the dentin cylinder or on a masked area of the buccal surface. The buccal area was determined by application of an electric insulating tape on the dentin surface. A hole was thereafter punched in the tape with
Dental Materials/April 1990 139
tween 70°C and 600C and was started one h after immersion in water. The exposure to each bath was 20 s, and the transfer time between the two containers was three s. All four types of adhesives were used for this test.
Comparison of Test Methods and Dentin Variations.-For comparison of Methods 1 and 2, a second test set was
:::::::: i!!!!!!! ,.....-. ,.....-. ...-.-.
\\ \\ %',
Fig. 1. The effects of two different storage conditions on the tensile bond strengths of four dentin adhesives. MPa
l ~ = SCOTCHBOND 2
25 ]= 20
O =OCCLUSAL B = BUCCAL
performed, with only two dentin adhesives used, Gluma and Scotchbond 2. Forty third molars were used for each method tested. Twenty teeth were turned into occlusal cylinders, and 20 teeth were tested on the buccal surface. All specimens had been stored for 24 h in 37°C water before being mounted in a universal testing machine and loaded with a cross-head speed of 1 mm/min until fracture occurred. SEM Inspection.- After the break of bond, the two fractured surfaces were inspected under a scanning electron microscope (JSM 840, Jeol Ltd., Tokyo, Japan).
Statistics.- Differences between the mean bond strength values were tested for significance by Tukey-HSD procedure at a 0.05 significance level (SPSS/PC+, SPSS, Inc., Chicago, IL). RESULTS
Comparison of Storage Conditions.-The
Fig. 2. Tensile bond strength values obtained for two dentin adhesives with two different methods on occlusal and buccal dentin specimens.
the sharp edge of a material cup when tooth and cup were mounted in the bonding alignment block. After the adhesive was cured, a layer of corresponding composite was applied and cured. All material extending beyond the test areas of 3-mm or 5-mm diameter was trimmed away before the adhesive was cured. Silar (3M) was used for the f'Lxation of the material holder to the previously lightcured composite (Odin and Oilo, 1986). I m m e d i a t e l y a f t e r b e i n g bonded, the specimens w e r e removed from the bonding alignment
apparatus and immersed in distilled water at 37°C.
Comparison of Storage Conditions.-
Twenty teeth were used to compare the effects of different storage conditions for each dentin adhesive tested. Specimens were made according to Method 1 on the occlusal type of dentin specimen. Ten specimens were stored in 37°C distilled water for 24 h prior to being tested, and 10 specimens were thermocycled. The latter storage condition consisted of 5000 cycles in water be-
14(} OILO & OLSSON/BOND STRENGTH MEASUREMENTS
bond strengths obtained with Method 1 in occlusally located dentin after 24 h in distilled water and after thermocycling are shown in Fig. 1. The bond strength values observed after 24 h of storage were approximately 3.5 MPa for Gluma, Scotchbond 2, and Tenure and 1.4 MPa for Scotchbond (dual). The values for Gluma and Scotchbond 2 were significantly larger than those for Scotchbond (dual) (p < 0.05). Thermocycling reduced the bond strength values significantly (p < 0.05) for Scotchbond 2 and Tenure. Gluma showed a 20% decrease in strength, but the reduction was not statistically significant (Fig. 1).
Comparisonof the TestMethodsand Dentin Variations.-The bond strength v a l u e s o b s e r v e d for Gluma and Scotchbond 2 with Methods 1 and 2, either at the occlusal or the buccal
surfaces, are shown in Fig. 2. Method 2 showed values significantly (p < 0.05) higher than those of Method 1 for corresponding surfaces and for both materials. The highest bond strength values for both materials were found when Method 2 was used on the buccal surface, with a mean of 12.0 MPa and a range from 6 MPa to 18 MPa for Gluma and a mean of 15.2 MPa and a range between 6.4 MPa and 26.6 MPa for Scotchbond 2. The difference between the mean values of the two materials was not significant. The bond strength value obtained for Gluma with Method 1 was significantly (p < 0.05) higher on the buccal surface than on the occlusal surface. The bond s t r e n g t h obtained for Scotchbond 2 with Method 2 was significantly (p