INTRODUCTION

Porcelain veneer restorations were introduced in the early 1980s as a conservative and reliable esthetic restorative service.^1-3 These restorations can be used to modify tooth color and shape, close diastemata, and improve minor alignment problems. Significant changes in clinical procedures and porcelain materials have occurred during the past 25 years.^4-9 Low-fusing feldspathic porcelain was the first veneer restoration material used with a high degree of clinical success, and, more recently, leucite-reinforced and lithium-disilicate porcelains are now routinely used for porcelain veneer restorations. Clinically, the long-term success of porcelain veneer restorations is dependent on a preparation substrate in enamel, favorable occlusal relationships, and the ability of the porcelain to be etched and adhesively bonded to resin composite cements.^10,11 Although porcelain veneers are generally successful under the appropriate clinical conditions, it is recommended that informed consent be provided to patients, including the possibilities of postoperative sensitivity, marginal discoloration, fracture, debonding, and wear of opposing teeth.¹²

Fractures of porcelain veneers represent the most common mode of failure.^13-15 A static fracture occurs when a veneer segment fractures and the remainder of the veneer still remains intact on the tooth. A cohesive fracture is categorized by a loss of a segment of porcelain and typically occurs when there is excessive functional or parafunctional loading.^10,13 Cohesive fracture of a veneer restoration can potentially be repaired with resin composite as a short-term treatment option; however, a new veneer or full-coverage crown is often necessary.¹⁶ When a porcelain veneer completely debonds from the tooth in an adhesive failure mode and is still intact, an assessment is necessary to determine whether there is resin cement remaining on the tooth surface or on the internal surface of the veneer. If resin cement remains on the tooth, then the failure is likely due to a lack of adequate etching and silanation of the intaglio (internal) porcelain surface. On the other hand, if resin cement remains inside the veneer, then a poor-quality bond to the enamel substrate or perhaps a lack of adequate enamel with exposed dentin for adhesive bonding can be identified as the problem.^9,17,18

Contributing factors leading to the occurrence of fractures and/or debonding include exposure of dentin during tooth preparation, placing finish lines on large composites, bonding to endodontically treated teeth, and heavy parafunctional occlusal loading.⁵ A prospective, however relatively short-term clinical study published in 2009, evaluated the survival of 200 porcelain veneer restorations up to a period of 72 weeks.¹⁹ The most frequent failure type noted was debonding, occurring with 11 units , with nine veneer restorations being able to be rebonded, while two required modification of the preparation design and new veneer restorations. Porcelain veneers that are bonded to dentin at the margins or internally are more likely to debond than veneers bonded entirely within enamel.¹⁷ When confronted clinically with a significant amount of exposed dentin substrate, rebonding a porcelain veneer restoration will not be as successful as bonding to enamel entirely. Typically, when the porcelain veneer restoration fails and the tooth preparation has exposed dentin, the restoration will likely need to be changed to a full-coverage crown in order to gain the advantage of mechanical retention. The authors of a 16-year prospective study in 2007 reported that the overall survival rate of porcelain veneer restorations was 73%. Failed restorations were associated with unacceptable esthetics and mechanical complications that included occlusal trauma, veneer fracture, and loss of retention. Although loss of retention accounted for only 12.5% of failures, this clinical problem may not require restoration remakes and potentially can be managed by rebonding the veneer(s).²⁰

In cases where adhesive bond failures with porcelain veneer restorations occur, resin cement is usually retained on the internal surface of the porcelain veneer, and it is difficult to distinguish cement from the porcelain, especially when the resin cement is a similar shade. A predictable and noninvasive method for removing resin cement from the veneer is essential in order to rebond the restoration. References available in the literature to remove luting composite from inside a debonded intact veneer have recommended using a casting burnout oven or porcelain ovens.^12,21 The porcelain veneer with bonded resin cement is placed in the oven, and the temperature is increased to 600°C/1112°F and held at that temperature for 5 to 30 minutes. The porcelain veneer can then be removed, cooled to room temperature, cleaned with acetone, re-etched with 9.5% hydrofluoric acid, rinsed with water, dried, and coated with silane in preparation for rebonding. It is important to note that the type of porcelain (feldspathic, LR, or lithium disilicate) and the glass transition temperature (T_g) is not identified in these studies. The clinician needs to be fully aware of the type of porcelain and its T_g before subjecting the restoration to such high temperatures and risking deformation of the veneer restoration.

In addition to removing the resin composite cement from inside the porcelain veneer, successful rebonding of veneer restorations also requires attention to preparing the previously bonded tooth enamel substrate for subsequent bonding. Although resin cements can be bonded effectively to mineralized tooth surfaces, bonding to a previously resin-polymerized enamel surface requires additional consideration. Research investigating the repair of resin composite restorations suggests using a carbide or diamond bur with or without 50 μm aluminum oxide (Al₂O₃) air abrasion and/or 30 μm silica coating air abrasion for preparing a resin composite surface for rebonding.^23-25 Acid etching or the use of Al₂O₃ or silica-coating air abrasive techniques are not likely to cause significant changes in morphology of the prepared enamel surface; however, modification of the tooth preparation by roughening with a bur could alter the original enamel preparation, including finish lines, compromising a rebonding procedure. .

The purpose of this laboratory research was to investigate the efficacy of rebonding a debonded porcelain veneer restoration after determining a noninvasive procedure to remove resin composite cement from the internal surface of a leucite-reinforced (LR) porcelain veneer restoration.

METHODS AND MATERIALS

Thirty freshly extracted human molar teeth (UNMC-IRB#258-08NH) stored in 0.1% thymol were randomly divided into three groups of 10 and thoroughly rinsed with deionized water. The teeth were mounted in a 1 × ¾-inch phenolic ring form using Epoxide resin (Buehler, Lake Bluff, IL, USA) and finished to a flat surface using wet 600-grit silicon carbide (SiC) paper to produce a surface area of enamel 8.0 mm in diameter. .

Thirty OPC (Jeneric/Pentron, Wallingford, CT, USA) LR pressed porcelain veneer (5.0 × 0.75 mm) specimens were air abraded on the inner surface with 50 μm Al₂O₃ at 30 psi for 5 seconds (MicroEtcher, Danville Engineering Co, Danville, CA, USA), rinsed for 15 seconds with deionized water, etched with 9.5% hydrofluoric acid for 1 minute (Ultradent Porcelain Etch, Ultradent Products, Inc, South Jordan, UT, USA), rinsed with deionized water for 15 seconds, air-dried, and coated with silane (Kerr Silane Primer, Kerr Corp, Orange, CA, USA) for 2 minutes followed by air drying to remove excess silane. The control group (A) specimens (n=10) were prepared and bonded to an etched (37% phosphoric acid for 15 seconds, followed by a 15-second deionized water rinse, followed by air drying) enamel surface using Optibond Solo Plus adhesive resin and NX3 Nexus dual-cure resin composite cement according to the manufacturer's directions (Kerr Corp). A 200-kg perpendicular load using a dental surveyor was placed on the external surface of the veneer after cement application on the internal surface of the veneer for 30 seconds, excess resin cement was removed with a brush, and the external surface of the veneer was light activated for 40 seconds using a Model 101 SmartLite PS (Dentsply/Caulk, Milford, DE, USA) LED curing light with an 11-mm-diameter tip positioned 1.0 mm from the veneer surface.

According to the manufacturer, the LED curing light was fully compatible with the initiator chemistry of the bonding resin and resin composite cement used, and the light irradiance (intensity) of the LED curing light was confirmed with a Model 100 Curing Radiometer (Demetron Research Corp, Danbury, CT, USA) before light activating each specimen to be ≥800 mW/cm². Group A served as the control specimens and were stored in 37°C deionized water prior to further testing. Groups B (n=10) and C (n=10) were prepared similarly to group A with the exception that a 12-μm film thickness release agent (Rubber-Sep, George Taub Products, Inc, Jersey City, NJ, USA) was placed on the enamel surface immediately before the conditioned veneer with resin composite cement was seated on the enamel tooth substrate in order to simulate an adhesive debonding at the enamel–resin composite cement interface of the veneer specimen. After two trials with temperatures at 315°C/600°F and 398°C/750°F for 10 minutes each without successfully removing the resin composite, the debonded veneers from groups B and C were placed in a calibrated Radiance Multi-stage casting burnout oven (Radiance Co, St. Louis, MO, USA) and heated to 454°C/850°F for 10 minutes in a clean ceramic crucible in order to carbonize the resin composite cement well below the T_g of 590°C/1094°F²⁶ of the LR porcelain material. The internal surfaces of the veneers were air abraded with 50 μm Al₂O₃ for 5 seconds, which easily cleaned the residual carbonized resin composite cement, and were inspected using a 20× light microscope to verify removal of all visible residual resin cement, as depicted in Figure 1.

View Figure

• Download as Powerpoint
• Download Figure

The recovered veneer specimens and enamel surfaces were then prepared for rebonding. Preparation of the previously bonded enamel surfaces for group B specimens included air abrasion using 50 μm Al₂O₃ for 5 seconds followed by rinsing with deionized water, air drying, and an application of 37% phosphoric acid etching for 15 seconds followed by rinsing with deionized water and air drying. Group C previously bonded enamel surfaces were etched only with 37% phosphoric acid for 15 seconds, rinsed with deionized water, and air-dried. After the group B and C specimens were rebonded, all of the bonded veneer specimens were thermocycled between 5°C and 55°C for 2000 cycles using a 30-second dwell time. All specimens were then stored in 37°C deionized water for 2 weeks.

Group A, B, and C specimens were subsequently loaded in shear compression until fracture occurred using an Instron Universal testing machine Model 1123 (Instron Corp, Canton, MA, USA) with Bluehill software at a crosshead speed of 1.0 mm/min. The Instron testing configuration is illustrated in Figure 2. The bond strength at failure/fracture in megapascals (MPa) was recorded for all specimens. Observation of the mode of failure was also performed with a binocular light microscope at 20× magnification to categorize failure as adhesive or cohesive. Representative samples from the control group and each experimental group were sputter coated with a 20-μm layer of gold-palladium and examined with a JEOL JSM-6100 Scanning Electron Microscope (JEOL USA, Inc, Peabody, MA, USA) at 200× using an acceleration voltage of 25 kV to assist in visualization of the debonded or fractured specimens.

View Figure

• Download as Powerpoint
• Download Figure

Using a one-way analysis of variance (ANOVA), shear bond strength (SBS) values of the control and experimental groups were compared (α=0.05). Tukey-Kramer post hoc tests were conducted to determine pairwise differences between the groups (α=0.05).

The null hypothesis tested was that the SBS of veneer restorations after the experimental rebonding procedure would not be significantly different from the control bond strength.

RESULTS

SBS results in MPa (mean±SD) for the specimen groups were (A) 20.6 ± 5.1, (B) 18.1 ± 5.5, and (C) 17.2 ± 6.1, as depicted in Figure 3. One-way ANOVA indicated that there were no significant interactions. Tukey-Kramer post hoc comparisons detected no significant pairwise differences, and the null hypothesis was accepted. Group A (control) specimens had the highest mean SBS; however, this was not significantly different at α=0.05 from the experimental groups (B and C).

View Figure

• Download as Powerpoint
• Download Figure

The mode of shear bond failure observed was either adhesive or cohesive. Adhesively debonded specimens appeared to have minimal residual adhesive resin on the enamel surface. No adhesive failures were observed at the veneer–resin cement interface. Cohesively fractured specimens confirmed a mode of failure within the LR ceramic material itself. No cohesive failures were observed with the enamel tooth structure. An 80% incidence of cohesive fracture was observed in group A (control group), and rebonded groups B and C had 60% and 50% cohesive fracture, respectively. Adhesive debonding was observed to occur 20% of the time for control group A, 40% for experimental group B, and 50% for experimental group C, as illustrated in Figure 4. No damage to the prepared enamel surface was noted during the SBS testing since the 600-grit SiC surface striations from the enamel surface preparation were visible at 20× magnification and appeared undisturbed.

View Figure

• Download as Powerpoint
• Download Figure

Although significant differences were not present between the SBS mean values, the experimental groups (B and C) had lower SBS mean values and a higher incidence of adhesive failure compared with group A. Scanning electron micrographic (SEM) photomicrographs of the prepared enamel surface finished wet with 600-grit SiC paper, intact veneer interface, adhesive debonding interface, and cohesively fractured porcelain material at a magnification of 200× are depicted in Figures 5 (a-d) .

View Figure

• Download as Powerpoint
• Download Figure

DISCUSSION

In situations where sufficient enamel remains for successful adhesive bonding, an attempt to rebond an intact porcelain veneer restoration may be an appropriate treatment option. The results of this study suggest that conservative clinical procedures can be used to predictably remove resin composite cement from the internal surface of an adhesively debonded LR porcelain veneer restoration and to rebond the veneer restoration to the previously prepared and bonded enamel tooth surface. The null hypothesis was accepted whereby the mean SBS values of the control and experimental (rebonded) specimen groups were not significantly different.

The mean SBS value range of 17.2 to 20.6 MPa in our study is consistent with other research investigating the SBS of LR porcelain veneer materials bonded to enamel.^27,28 The OPC LR porcelain material (Jeneric/Pentron) used in this study has been shown to be similar to the Empress (Ivoclar/Vivadent AG, Schaan, Liechtenstein) LR porcelain material. Empress is more familiar to clinicians than OPC since it has been marketed for a longer period of time and continues to be available. Although Empress is “precerammed” and OPC is not, after heat processing, the two materials show no differences in mechanical properties.²⁹

A clinical procedure for rebonding an adhesively debonded and intact porcelain veneer recommends the removal of the resin composite cement from the inside of the veneer with carbide burs and/or diamond burs using magnification followed by sandblasting, re-etching with hydrofluoric acid, resilanating, and recementation.⁹ Although this technique seems reasonable, there is a risk of damaging the veneer, and a noninvasive means of resin cement removal from the porcelain veneer would more favorably minimize risk. In 1990 , it was reported that successfully burning out the residual resin cement from the internal surface of an adhesively debonded acid-etched fixed partial denture would require placing the prosthesis in a casting burnout oven at 700°C/1292°F for 10 to 15 minutes, followed by ultrasonic cleaning for 5 minutes.²² This protocol was developed for use with a base metal alloy, and if used with an LR porcelain, permanent deformation of the restoration would occur due to the burnout temperature exceeding the T_g (590°C/1094°F) of the LR porcelain.

Since it was known that previously published temperature recommendations for resin cement burnout were established for use with feldspathic porcelains (T_g range of 518°C/950°F to 945°C/1733°F), these high temperatures would have closely approached or exceeded the T_g for the OPC LR porcelain, and it was decided to initially expose the resin-bonded porcelain specimen to 315°C/600°F for 30 minutes.^12.21 This temperature did not result in any carbonization of the resin composite cement. It was subsequently decided to run a trial for 10 minutes at 398°C/750°F, and although the resin composite did exhibit a carbonized, or “charred,” surface, Al₂O₃ air abrasion was not able to easily remove the resin composite cement, and the use of rotary instrumentation to remove residual cement was considered potentially damaging.. We determined that the best results for complete carbonization of the resin composite cement were achieved at a temperature of 454°C/850°F held for 10 minutes, and the residual resin cement was very easily removed using 50 μm Al₂O₃ air abrasion. This temperature was below the OPC porcelain T_g of 590°C/1094°F, so deformational damage to the veneer was not anticipated. It is possible that an increase in the cement carbonization temperature we used for a shorter period of time might also be effective in removing resin cement from LR veneers, and future research is needed to identify the best temperature and time combination for both LR and newer lithium disilicate porcelain veneer materials. The T_g of a resin composite material 75% filled by weight is 107°C/225°F³⁰; however, the disintegration temperature values of resin composite materials used for cementation have not been reported in the literature. Newer resin composite cements typically have higher filler levels than previously available resin cements and may likely require higher burnout temperatures for adequate carbonization to occur.

It was preferable to use a casting burnout oven instead of a porcelain oven for this procedure since contamination of the porcelain oven “muffle” due to the carbonization of the resin composite material may cause discoloration of subsequently fired porcelain.³¹ Although 454°C/850°F for 10 minutes was optimally effective for the removal of the particular brand of resin composite cement used in our research and well below the T_g of the LR porcelain material, it is suggested that future research be done to quantify both the T_g and the carbonization temperatures of currently available resin composite materials.

The results of the SBS tests for the bonded control veneer specimens had higher mean values; however, as mentioned previously, these values were not significantly different from the rebonded (experimental) veneer specimens. The adhesively debonded enamel interface illustrated in Figure 5c shows the striations produced by the 600-grit SiC wet paper finishing of the enamel surface prior to bonding (Figure 5a). It is likely that this adhesively debonded enamel surface has residual resin tags in the tooth structure that would favor rebonding, as suggested in previous research.³² An intact veneer specimen top edge is illustrated in Figure 5b, providing a frame of reference for the cohesively fractured porcelain material illustrated in Figure 5d. The veneer specimen is still bonded to enamel, and it is evident that the edge of the veneer in Figure 5d has been damaged and that fragments of porcelain material have been lost.

Our research simulated a “clean” adhesive failure of a porcelain veneer between the acid-etched and bonding resin-coated enamel surface and the resin composite cement inside the veneer specimen. It is important to mention that the rebonded veneer specimens that were either acid-etched only (group C) or acid-etched after Al₂O₃ air abrasion (group B) had lower mean SBS values than the control veneer group, and although this was not significant, the rebonded veneer specimens did have a higher incidence of adhesive failure at the enamel interface compared to the bonded control veneer specimens. This may not be problematic clinically, however, since if the rebonded porcelain veneer would subsequently fail again in an adhesive mode, it could be rebonded a second time; however, at this point, it may be advisable to consider other, less conservative restorative options. These results are in agreement with previous research findings that when the SBS values are lower with a variety of porcelain veneer, cement, and tooth surface interfaces, there is a higher incidence of adhesive debonding.^5,17,33 Clinical research is needed to validate the significance of these laboratory research findings.

Suggested clinical procedures for repairing resin composite restorations have included the use of rotary instrumentation to remove partially bonded composite, air abrasion with Al₂O₃ or silica, phosphoric acid etching, and placement of a bonding resin.^23-25 Pit and fissure sealants often have to be reapplied during recall examination, and, similar to the recommendations for repairing composite resin, it is suggested that visibly remaining sealant be removed with a bur or Al₂O₃ air abrasion; however, cleaning of the enamel surface with nonfluoridated pumice is also recommended before phosphoric acid etching.³⁴ In our research, roughening the enamel surface with rotary instrumentation was not desirable since we did not want to alter the tooth preparation and increase the cement film thickness of the rebonded veneer. It has been reported that if the fit of the porcelain veneer has a resin cement thickness of ≤ 50 μm, there will be a lower incidence of adhesive bond failure.³⁵

A laboratory study published in 1980 revealed that when a composite restoration breaks cleanly at the enamel–composite interface, the best procedure for rebonding to that surface is by simply etching the debonded enamel surface with 37% phosphoric acid to avoid rotary instrumentation removal of rebondable resin tags remaining on the enamel surface.³² In addition, other research investigating Al₂O₃ air abrasion as an enamel surface treatment prior to acid etching for rebonding metallic orthodontic brackets did not result in significantly higher SBS compared to acid etching alone.³⁶ Although these research studies used different resin composite materials than our current study, acid-etching procedures, the use of bonding resins, and the enamel substrate can be considered similar, and the results of our current study are in agreement with the findings of these prior studies that acid etching alone is sufficient for surface preparation of the previously bonded enamel surface. It is realistic to assume, however, that if minor fragments of resin composite cement adhering to the enamel surface interfere with the complete seating of the porcelain veneer, judicious removal of the fragment with a finishing carbide or fine diamond bur may be required.

CONCLUSIONS

The long-term success of veneer restorations is dependent on an adequate enamel surface for bonding with a conservative preparation design, favorable occlusal considerations, and effective adhesive bonding systems. The results of this research suggest a conservative clinical technique when attempting to rebond a porcelain veneer restoration. The use of a casting burnout oven to selectively carbonize the resin composite cement at 454°C/850°F for 10 minutes, which was below the T_g of the LR porcelain veneer material, facilitated rebonding of an adhesively debonded veneer to a previously bonded and prepared tooth enamel surface.

No significant differences in SBS (MPa) were observed in the rebonded veneer groups regardless of whether the previously bonded enamel surface was air abraded with 50 μm Al₂O₃. The mean SBS values of the control group (A) were higher than the experimental groups (B and C), and 80% of these specimens fractured cohesively; however, these values were not significantly different from the mean SBS of the experimental groups. The incidence of adhesive shear bond failure at the resin composite cement–enamel surface interface was noted to be higher in the rebonded experimental groups than in the control group.

SEM photomicrographs at a magnification of 200× suggest that only bonding resin remained on the enamel substrate for the adhesively debonded veneer specimens. Also, failure within the ceramic material itself was evident for cohesively fractured veneer specimens. Future research is needed to evaluate the efficacy of rebonding veneer restorations fabricated from lithium disilicate porcelain bonded with the newly available adhesive resin composite cements.