Treatment of composite surfaces for indirect bonding

Dent Mater8:193-196, May, 1992

Treatment of composite surfaces for indirect bonding E.J. Swift, Jr., ~ C. Brodeur, E. Cvitko, 2 J~A.F. Pires 3 Department of Operative Dentistry, The University of Iowa, Iowa City, Iowa, USA eUniversidade Federal do Rio Grande Do Sul, Porto Alegre, Brasil 3Private practice, Porto Alegre, Brasil

Abstract. Compositecan be cured extraorallyto fabricate indirect or direct/indirect restorations. These restorations are bonded to the tooth with a dual-cure resin cement. This study evaluated various methods for treating the surface of an indirect glass-filled composite to improve its bond to enamel. The methods tested were air abrasion (sandblasting), hydrofluoric (HF) acid etching, silanation, and combinations of these techniques. Shear bond strengthswere measuredfor thermocycledand non-thermocycled specimens. Air abrasion, followed by silanation, provided the strongest and most consistent bonds. Etching with 9.6% HF gel decreasedthe bond strengths obtained by air abrasion. HF alone produced the weakest bonds, with most failures occurring adhesively between the indirect composite and the resin cement. INTRODUCTION Tooth-colored inlays and onlays are fabricated from porcelain, cast glass ceramic, or composite. Some composite inlay techniques require an initial appointment for tooth preparation and a subsequent appointment for try-in and cementation of the laboratory-fabricated restoration. Composite inlays can also be made in the dental office with a singleappointment "direct/indirect" technique. The dentist places and photo-polymerizes composite directly in the cavity preparation. The inlay is removed, contoured, and cured again extraorally (e.g., with heat). The internal surface of either type of restoration can be treated with hydrofluoric acid (HF) before bonding with a dual-cure resin cement (Hornbrook and Pai, 1990). HF, which is commonly used to etch porcelain for indirect restorations and repairs (Stangel et al., 1987; Lacy et al., 1988; A1 Edris et al., 1990), also etches the filler particles in glassfilled hybrid and small particle composites (Kula et al., 1983; 1986). Etching is intended to improve the bond of the luting resin to the indirect restoration. The recommended etching time for composites is 30 s or less (Miller, 1990; Hamilton, 1990). Research on the effectiveness of HF for composite bonding is rather limited. Crumpler et al. (1989) reported that HF did not significantly improve the shear strength of rebonded posterior resin composites. Mitchem et al. (1991) recently recommended that hybrid composites should not be etched because HF causes softening and porosity in the composite surface. Silane coupling agents are required to create a chemical bond between the etched inorganic fillers and the organic resin of a composite cement (Craig, 1989). Silanes are commonly used to enhance the bonding of composites to

etched porcelain by 25% or more (Stangel et al., 1987; Lacy et al., 1988; Nicholls, 1988). This study evaluated the bond of HF-etched composite to enamel with and without thermocycling. Mechanical roughening with an air-abrasion (sandblasting) technique was also evaluated, alone and in combination with HF etching. Finally, the effects of silanation on bonding of air-abraded and etched composite were tested.

MATERIALSAND METHODS Part 1. Thirty extracted human molars with sound proximal surfaces were selected and thoroughly cleaned. The teeth were mounted in phenolic rings (Buehler, Ltd., Lake Bluff, IL, USA) with cold-cure acrylic resin. A mounting jig was used to align the mesial and distal surfaces roughly perpendicular to the base of the mold. The specimens were then mounted on a level platform adjacent to a dental lathe. Flat bonding sites were made on the proximal enamel surfaces using a 918B-220 diamond disc (Brasseler, Inc., Savannah, GA, USA) mounted in the lathe. Two increments of a glass-filled hybrid composite (Herculite XRV, Kerr Manuf. Co., Romulus, MI, USA, shade A-3 "enamel", batch number 1149) were injected into rigid plastic molds (3 mm diameter, 4 mm deep) on a Mylarcovered glass slab. Each increment was photocured for 40 s. Sixty columns of composite were fabricated in this manner. The bottom surfaces of forty composite columns were airabraded (Microetcher, Danville Engineering, Danville, CA, USA) for 10 s with 50 pm aluminum oxide particles and rinsed with water for 30 s. Twenty of the air-abraded columns were also etched for 30 s with 9.6% HF gel (Porcelain Etch Gel, Pulpdent Corporation, Watertown, MA, USA) and rinsed for 2 min. The remaining columns were etched with HF for 30 s, without prior air abrasion. The teeth and surfaces (either mesial or distal) were assigned randomly to three groups for bonding. The flattened enamel surfaces were etched with 37% phosphoric acid for 20 s, rinsed with water for 20 s, and dried with compressed air. A phosphonate ester dentin/enamel bonding agent (Bondlite, Kerr) was applied to the etched enamel and treated composite surfaces and air-thinned. A resin cement (Porcelite Dual-Cure, Kerr) was mixed and applied to the surface of each composite column. The composite column was applied to the enamel and held with finger pressure while excess cement was removed with a brush. The resin cement was cured from three directions for 40 s, for a total exposure time of 120 s.

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Half of the specimens from each group were stored in distilled water at room temperature for 7 d before bond strength testing. The remaining specimens were also stored for 7 d, but then were thermocycled 500 times between water baths held at 5 ° and 55°C. The dwell time in each bath was 30 s. The failure loads for composite/enamel bonds were measured using a universal testing machine (Instron Model TT-D, Instron Corp., Canton, MA, USA) in the shear mode with a crosshead speed of 0.5 cm/min. Bond strengths were calculated using the cross-sectional areas of the debonded composite specimens. The enamel surfaces and composite columns were examined with an optical microscope (15x) to determine the nature of the bond failures. Additional composite columns were air-abraded and/or HF-etched for examination with scanning electron microscopy (SEM). Part 2. Twenty non-carious, unrestored human molars were obtained and cleaned. Forty composite columns were fabricated as described previously. The bottom surface of each composite column was air-abraded for 10 s with 50 pm aluminum oxide and rinsed with water for 30 s. Twenty of the air-abraded columns were also etched for 30 s with 9.6% HF gel and rinsed for 2 min. Ten sandblasted and ten etched composites were also silanated (Command Ultrafine Porcelain R e p a i r Bonding System, Kerr) according to manufacturer's directions. The composite columns were bonded to proximal enamel surfaces using the method described in Part 1. Specimens were stored in distilled water for 48 h before thermocycling 500 times between water baths held at 5 ° and 55°C, with a dwell time of 30 s in each bath. The shear failure loads for composite/enamel bonds were measured using a universal testing machine (Instron Model TM, Instron Corp.) with a crosshead speed of 0.05 cm/min. Bond strengths were calculated using the cross-sectional areas of the debonded composite specimens. The fractured surfaces were examined with an optical microscope to determine the nature and location of failures. RESULTS

The results of this study are summarized in Tables 1 and 2. Data were analyzed using the general linear models procedure of the SAS statistical software package (SAS Institute, Cary, NC, USA). ANOVAs revealed significant differences (p < 0.0001) in both parts of the experiment. Post hoc Duncan's multiple range tests (with alpha = 0.05) were used for pair-wise comparisons. In Part 1, air abrasion provided a significantly greater bond strength (19.4 + 2.9 MPa) than either HF etching (12.9 _+1.9 MPa) or air abrasion plus HF etching (12.5 _+2.3 MPa). Thermocycling resulted in a statistically significant (p < 0.05) decrease in bond strength only for air-abraded samples. Still, air abrasion produced higher mean bond strengths than either of the other t r e a t m e n t s , regardless of thermocycling. No purely adhesive failures occurred at the enamel surface of any specimen. Several air-abraded specimens fractured cohesively within the enamel, but most failed in a mixed cohesive/adhesive mode at the interface between the indirect composite and the resin cement. The specimens treated with both air abrasion and HF etching followed a similar failure pattern. In contrast, most of the HF-etched

194 Swift et aUIndirectcompositebonding

TABLE 1: SHEAR BOND STRENGTHS* OF INDIRECTCOMPOSITE SPECIMENS TREATED WITH AIR ABRASION OR HF ETCHING AND BONDED TO ENAMEL Surface ThermocyclingBondStrength** Duncan Treatment (MPa) Grouping# air abrasion none 19.4 (2.9) A air abrasion 500x 14.9 (2.5) B HF etch none 12.9 (1.9) BC air abrasion, HF 500x 12.9 (2.8) BC air abrasion, HF none 12.5 (2.3) BC HF etch 500x 11.5 (3.4) C crosshead speed was 0.5 cm/min, n = 10 **Standard deviations given in parentheses. #Groups with the same letter are not significantly different (alpha = 0.05). TABLE 2: SHEAR BOND STRENGTHS* OF INDIRECTHERCULITEXRV SPECIMENS TREATED WITH AIR ABRASION OR HF ETCHINGAND BONDED TO ENAMEL. ALL SPECIMENS WERE THERMOCYCLED 500 TIMES. Bond Strenath (MPal Duncan Surface Treatment Mean S.D C.V.** Grouping# air abrasion 26.0 9.9 38% A air abrasion, silane 28.4 3.7 13% A air abrasion, HF 18.1 8.3 46% B air abrasion, HF etch, silane 12.0 3.1 26% B *Note crosshead speed was 0.5 cm/min, n = 10 **Coefficient of variation. #Groups with the same letter are not significantly different (alpha = 0.05).

specimens failed adhesively at the interface between the composite materials. In Part 2, air abrasion provided a shear bond strength of indirect composite to enamel of 26.0 _+9.9 MPa. Silanation of air-abraded samples improved the bond strength to 28.4 _+ 3.7 MPa, but this improvement was not statistically significant. Etching with HF significantly reduced bond strengths. HF-etched, air-abraded composite had a mean shear bond strength of 18.1 _+8.3 MPa. Silanated etched composite had an even lower bond strength (12.0 _+ 3.1 MPa), but this difference was not statistically significant. Most failures of air-abraded specimens were cohesive in nature. However, the samples which were also etched with HF (and regardless of silanation) tended to fracture at the interface between the indirect composite and the resin cement, either adhesively or in a mixed adhesive/cohesive mode. SEM examination of the composite material showed that air abrasion caused extensive surface roughening (Figs. 1 and 2). HF etching of either untreated or air-abraded composite caused loss of filler particles, and hence porosity throughout the surface (Figs. 3 and 4). DISCUSSION

The bond strength required for long-term clinical retention and marginal seal of indirect composite restorations has not been determined. With direct composites, a shear bond strength of approximately 17 MPa is sufficient to counteract the forces of polymerization contraction (Munksgaard et al., 1985). However, contraction forces and bond strengths vary

Fig. 1. Glass-filled hybrid composite,average particle size of 0.6 ~m. Surface has not been treated in any way.

Fig. 3. Hybrid compositewhich has been etched for 30 secondswith 9.6% HF gel.

Fig.2. Surfaceof hybridcompositewhichhas beenair-abradedwith 50 i~maluminum oxide particlesfor 10 seconds.

Fig. 4. Hybridcompositewhich has been air-abradedand etched with HF.

with resin type and cavity geometry (Davidson et al., 1984). Feilzer et al. (1989) examined the shrinkage of thin bonded resin layers (i.e., cements) and concluded that shrinkage stresses are great enough to cause cohesive failures or loss of retention. Shear bond strengths of direct composite to enamel are generally in the range of 20 MPa (Barkmeier et al., 1986; Bastos et al., 1988; Gilpatrick et al., 1991), a value exceeded by two of the groups in our study. Air-abraded, silanated composite had the highest and most consistent enamel bond strengths. The bond strengths of the other groups were less than 20 MPa (Tables 1 and 2). Nevertheless, enamel bonding appeared to be adequate for all treatment groups, as no adhesive failures occurred at enamel surfaces. In fact, with air-abraded composite, cohesive failures of enamel were common. Overall, most bond failures occurred within the resin or at the composite/cement interface. Air abrasion of indirect composite with 50 pm aluminum oxide particles provided a mean enamel bond strength comparable to the values obtained with direct resin and etched porcelain veneers. HF-etched samples had lower bond strengths, even without thermocycling. These lower bond strengths, and the nature of the failures (primarily adhe-

sive), indicate that HF etching alone does not provide an ideal bonding surface on indirect composites. Even when combined with other treatments such as air abrasion and silanation, HF etching gave significantly weaker bonds than simple air abrasion. The failure of HF etching to improve adhesion has been shown in other studies as well (Crumpler et al., 1989; Mitchem et al., 1991). Air abrasion roughens the composite, removes some of the resin matrix, and leaves exposed filler particles on the surface. Theoretically, HF should etch these filler particles and create a more retentive surface than air abrasion alone. HF does etch glass particles, but a 30 s application of 9.6% HF may be excessive, possibly causing total dissolution of exposed glass particles (see Figs. 3 and 4). Silanation would not improve bonding if no glass particles were present on the surface. In addition, the acid might be absorbed into and cause softening of the resin matrix. One manufacturer instructs laboratories to neutralize acid (Stripit, National Keystone Products Co., Philadelphia, PA, USA) with sodium hydroxide to avoid potential absorption by the resin (N. Short, Kerr, personal communication). In summary, this study indicated that a glass-filled hybrid composite can be air-abraded to provide strong bonds to

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enamel using an indirect technique. Silanation may improve the bond provided by air abrasion alone. Etching with hydrofluoric acid is not recommended, as it significantly decreases bond strength. The results of any in vitro study, including this one, may not accurately predict clinical performance. Furthermore, it must be noted that only one composite was tested in this study. Thus, the results may not apply to other classes or brands of composite.

ACKNOWLEDGMENTS This project was supported by the Kerr Manufacturing Company, Romulus, MI. Received October 28, 1991/AcceptedJanuary 29,1992 Address correspondence and reprint requests to: E. J. Swift, Jr. Department of Operative Dentistry The University of Iowa Iowa City, Iowa 52242 USA

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