The ultimate test of dental adhesives is not their performance in the laboratory, but their performance in the clinical environment. The purpose of this study, therefore, was to evaluate the 36-month clinical performance of filled and unfilled one-bottle dentin adhesives on Class V restorations placed without retentive grooves or enamel bevels. The adhesives tested were a filled, ethanol-based adhesive (OptiBond Solo, SDS Kerr) and an unfilled acetone-based adhesive (Prime & Bond 2.1, Dentsply Caulk). (Slightly different versions of these products are now marketed.) The ethanol-based adhesive is filled with barium glass and silica (approximately 25 percent by weight). It also contains bisphenol A glycidyl dimethacrylate, glycerophosphoric acid dimethacrylate, hydroxyethyl methacrylate, and sodium hexafluorosilicate. The unfilled acetone-based adhesive contains urethane dimethacrylate, dipentaerythritol pentaacrylate phosphoric acid ester, R5-62-1 resin, bisphenol A dimethacrylate, butylated hydroxytoluene, 4-ethyl dimethyl aminobenzoate, camphorquinone, and cetylamine hydrofluoride.

	MATERIALS AND METHODS

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Selection criteria. Thirty-three subjects ranging in age from 27 to 77 years were recruited from the Department of Operative Dentistry, University of North Carolina at Chapel Hill, and enrolled in the study. We excluded patients with fewer than 20 teeth. The dental health status of the patients was considered normal in all respects. Before participating in the study, all patients signed a consent form that had been reviewed and approved by an institutional review board.

Teeth treated in the study had noncarious cervical lesions (caused by abrasion, erosion or abfraction) with no undercuts. Although caries was an exclusion criterion, we removed small carious areas from two teeth in one patient. These restorations were examined during the course of the study, but were not included in any data analysis. In general, no more than 50 percent of the cavosurface margin involved enamel, and at least 75 percent of the surface area of the restoration was in contact with dentin. All restored teeth contacted the opposing teeth in a normal occlusal relationship, and patients had healthy periodontia.

To minimize any patient-related effects that might distort the outcome of the study, we allowed no more than three restorations per patient for each restorative system. Nearly all patients received restorations of both types, which were distributed on a random basis. The distribution of restorations was approximately equal between maxillary and mandibular arches, and approximately 70 percent of the 101 restorations were placed in premolars. Ten percent were placed in molars, with the rest placed in anterior teeth.

We found only minor differences in lesion size and other characteristics between the two restorative groups. Mean lesion volumes were not significantly different (Student t-test, P = .49). Approximately 65 percent of the lesions were relatively nonsclerotic (box, "Dentin Sclerosis Scoring System"), and these lesions were distributed almost equally between the two groups. We described a similar percentage of teeth as having nontraumatic occlusion (with traumatic occlusion defined by the presence of fremitus or wear facets).

Ethanol-based adhesive. We applied the two adhesives to randomly selected lesions according to directions given by the manufacturers. For the ethanol-based adhesive, enamel and dentin were etched for 15 seconds with 37.5 percent phosphoric acid. The etchant was rinsed off, and the tooth surface was lightly dried with compressed air, but was not desiccated. The ethanol-based adhesive was applied with a light brushing motion for 15 seconds. In accordance with the manufacturer’s instructions, the material was not air-dried, but was immediately light-cured for 20 seconds. We applied a second coat of adhesive and cured it in the same manner.

Acetone-based adhesive. For the acetone-based adhesive, we etched the enamel and dentin for 15 seconds using 34 percent phosphoric acid. After rinsing, we blotted excess moisture with a moistened cotton pellet. The adhesive then was applied to the tooth surface, and was replenished as needed during a 20-second period. The treated surface was gently air-dried for 5 seconds. If the surface did not appear glossy, we applied more adhesive. The material was light-cured for 10 seconds. A second coat was applied, but following the manufacturer’s directions, we did not light cure it.

Resin-based composite. We restored the teeth with a light-cured hybrid resin-based composite material (Prodigy [SDS Kerr] for the ethanol-based adhesive and TPH Spectrum [Dentsply Caulk] for the acetone-based adhesive). We inserted the composite in increments of 2 millimeters or smaller. Each increment was polymerized for 40 seconds with a visible light-curing device (Optilux 401, Demetron Kerr) that had an output of more than 500 milliwatts/square centimeter throughout the study. After curing, we finished the restorations using tapered or flame-shaped 12-fluted carbide finishing burs or microfine diamond burs. We polished the restorations with slow-speed polishing cups and points (Enhance, Dentsply Caulk).

Direct evaluation criteria and procedures. We examined the restorations at baseline and at six months, 18 months and 36 months after placement. The following characteristics were recorded according to modified Cvar/Ryge direct evaluation criteria⁶ (box, "Modified Cvar/Ryge Direct Evaluation Criteria"):

– retention;

– color match;

– marginal discoloration (interfacial staining);

– recurrent caries;

– wear (loss of anatomical form or contour);

– marginal adaptation (integrity);

– postoperative sensitivity;

– other failure.

Postoperative sensitivity was recorded as present (Charlie) or absent (Alfa) after we questioned the patient or if the tooth was sensitive to a three-second stream of compressed air 2 to 3 cm from the restoration. We recorded retention as completely retained (Alfa), partially retained (Bravo) or completely lost (Charlie).

Intraoral color photographs were taken immediately after the restorations were inserted and at each recall appointment for evaluation by a second investigator. The clinical photographs were 35-mm transparencies taken at an original magnification of x 1.5. Whenever the clinical evaluation and the evaluation based on the transparencies did not agree, the principal investigator made the final scoring determination.

We used the following equation to calculate restoration failure rates⁷:

where PF equals the number of previous failures before the current recall appointment; NF equals the number of new failures during the current recall appointment; and RR equals the number of restorations recalled during the current recall appointment.

	RESULTS

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All but one of the subjects had a recall visit three years after placement (97 percent recall rate). Two restorations from the ethanol-based adhesive group were not available for evaluation at any recall appointment because the subject had moved out of state shortly after the restorations were placed. One other restoration could not be evaluated because the tooth received a crown after the six-month recall visit. In addition, two of the 51 restorations in the acetone-based adhesive group were not evaluated at the final recall appointment because the teeth had received crowns after the 18-month recall appointment.

Tables 1 and 2 and the Figure (page 1122) summarize the clinical evaluations (18-month evaluations were also reported elsewhere⁸). Retention rates at six months were 97.9 percent for restorations in the ethanol-based adhesive group and 100 percent for restorations in the acetone-based adhesive group. The first failure in the acetone-based adhesive group occurred almost immediately after the six-month recall visit, so the six-month retention rates were virtually identical for the two adhesive groups. Using the ADA guidelines formula,⁷ we calculated the 18-month retention rates to be 95.7 percent for the ethanol-based adhesive group and 98.0 percent for the acetone-based adhesive group. These retention rates are not significantly different (Cramer’s V, P = .51). At three years, the cumulative retention rates were 93.3 percent for restorations in the ethanol-based adhesive group and 89.4 percent for restorations in the acetone-based adhesive group. The difference was not statistically significant (Cramer’s V, P = .51).

View larger version (47K): [in this window] [in a new window]   Figure. Cumulative retention of Class V restorations bonded with a filled ethanol-based adhesive (OptiBond Solo, SDS Kerr) or an unfilled acetone-based adhesive (Prime & Bond 2.1, Dentsply Caulk).

The only problem worthy of note at three years was interfacial staining.

The only problem worthy of note at three years was interfacial staining, which was present surrounding 12 percent of the retained restorations in both groups. We detected no recurrent caries in either group.

	DISCUSSION

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We should note that the adhesives tested in this study have differences other than the presence or absence of filler. For example, one contains ethanol as a solvent, while the other contains acetone. In addition, different composites (both hybrids) were used with the two adhesives. Differences in the composition and properties of the adhesive or resin-based composite may or may not have contributed to the results of the study.

Retention. At this final 36-month recall visit, the cumulative retention rates were 93.3 percent for the ethanol-based adhesive and 89.4 percent for the acetone-based adhesive. Therefore, the retention rate for each adhesive was approximately at the 90 percent level required by the ADA guidelines for full acceptance after 18 months of clinical service.⁷

Interfacial staining. Of the retained restorations, 7 percent that received the ethanol-based adhesive and Prodigy resin-based composite had interfacial staining that resulted in less-than-ideal marginal adaptation, and 6 percent that received the acetone-based adhesive and TPH Spectrum resin-based composite had interfacial staining. ADA criteria for full acceptance call for less than 10 percent Charlie microleakage at 18 months.⁷ Although Kidd¹⁰ defined microleakage as being clinically undetectable, we believe that marginal integrity is important. The marginal adaptation of teeth in both restoration groups remained excellent at three years, but interfacial staining was noted surrounding 12 percent of retained restorations.

Fifth-generation adhesives. Little information is available regarding the clinical performance of fifth-generation one-bottle adhesives like the ones evaluated in this study. However, many clinical trials on fourth-generation etch, prime and bond systems have been published.^11–19 Retention rates have ranged from 69 to 100 percent in trials running up to three years. We conducted a Med-line search and found that the only publications on fifth-generation systems showed retention rates for one product (ONE-STEP, Bisco) of 95 percent at one year²⁰ and 32 percent (bonded to nonsclerotic dentin) and 65 percent (bonded to sclerotic dentin) at three years.¹⁹

In our study, both of the restorative systems performed well during the course of the study, with retention rates and all other evaluation criteria rated Alfa for 88 to 100 percent of the restorations. The concerns noted in this report are relatively minor.

This study evaluated the clinical performance of a filled ethanol-based (OptiBond Solo) and an unfilled acetone-based (Prime & Bond 2.1) adhesive system. Filled adhesives or intermediate resins are designed to act as so-called "elastic buffers" to relieve stresses within resin-based composite restorations caused by polymerization contraction, thermal changes and occlusal loading.^2,4,21 Some systems that include a filled adhesive or a filled intermediate low-viscosity resin, or LVR, have been shown to significantly reduce marginal gaps, leakage around resin-based composite restorations, or both in vitro.^3,5,22–29

The resin-dentin interdiffusion zone, or hybrid layer, also has a relatively low Young’s modulus (or modulus of elasticity) and may also relieve polymerization contraction stresses.²¹ Uno and Finger³⁰ showed that removal of surface collagen from demineralized dentin before primer application resulted in somewhat higher shear bond strengths, but also larger marginal gaps in three-dimensional cavities. However, removal of collagen prevented formation of this interdiffusion zone and resulted in wider marginal gaps, suggesting that the hybrid layer does function as a stress-absorbing layer between resin-based composite and dentin. The resin-dentin interdiffusion zone is not as wide as the adhesive layer (or combination of adhesive and LVR) that lies above it, so it is expected to play a smaller role than the LVR in relieving stresses and maintaining marginal integrity.⁴

It is possible that filled adhesives provide a clinical benefit by acting as shock absorbers between relatively stiff resin-based composites and tooth substrates. For example, the original OptiBond adhesive has been shown to maintain its marginal seal well when tested under simulated occlusal loading in vitro.^24,25,29 In our study, the unfilled acetone-based adhesive performed slightly better (although not significantly better) than the filled ethanol-based adhesive 18 months after restoration placement. At three years, however, the retention rates were reversed (Figure). Although the difference was not statistically significant, this finding may indicate that the filled adhesive is more resistant to fatigue forces and could provide more durable retention than the unfilled adhesive. It would be interesting to evaluate these restorations at a later date, perhaps five years from baseline, to determine whether the retention rates of the filled and unfilled adhesives continue to diverge.

	CONCLUSION

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This study showed that a filled ethanol-based adhesive and an unfilled acetone-based adhesive provided excellent retention of Class V restorations during three years of clinical service. Despite the differences in their chemical composition and filler content, the performance of the two materials was remarkably similar.