Bond Strength of Novel CAD/CAM Restorative Materials to Self-Adhesive Resin Cement: The Effect of Surface Treatments Shaymaa E. Elsaka

Purpose: To evaluate the effect of different surface treatments on the microtensile bond strength (μTBS) of novel CAD/CAM restorative materials to self-adhesive resin cement. Materials and Methods: Two types of CAD/CAM restorative materials (Vita Enamic [VE] and Lava Ultimate [LU]) were used. The specimens were divided into five groups in each test according to the surface treatment performed; Gr 1 (control; no treatment), Gr 2 (sandblasted [SB]), Gr 3 (SB + silane [S]), Gr 4 (hydrofluoric acid [HF]), and Gr 5 (HF + S). A dual-curing self-adhesive resin cement (Bifix SE [BF]) was applied to each group for testing the adhesion after 24 h of storage in distilled water or after 30 days using the μTBS test. Following fracture testing, specimens were examined with a stereomicroscope and SEM. Surface roughness and morphology of the CAD/CAM restorative materials were characterized after treatment. Data were analyzed using ANOVA and Tukey’s test. Results: The surface treatment, type of CAD/CAM restorative material, and water storage periods showed a significant effect on the μTBS (p < 0.001). For the LU/BF system, there was no significant difference in the bond strength values between different surface treatments (p > 0.05). On the other hand, for the VE/BF system, surface treatment with HF + S showed higher bond strength values compared with SB and HF surface treatments (p < 0.05). Surface roughness and SEM analyses showed that the surface topography of CAD/CAM restorative materials was modified after treatments. Conclusion: The effect of surface treatments on the bond strength of novel CAD/CAM restorative materials to resin cement is material dependent. The VE/BF CAD/CAM material provided higher bond strength values compared with the LU/BF CAD/CAM material. Keywords: adhesion, microtensile, polymer-infiltrated ceramic network, resin cement, resin nanoceramic, surface roughness, surface treatments. J Adhes Dent 2014; 16: 531–540. doi: 10.3290/j.jad.a33198

T

he use of dental CAD/CAM restorative materials for indirect restorations has recently increased considerably. Ceramics and composite resins are the two main groups of CAD/CAM restorative materials.10 Ceramic restorations have several advantages, including highly esthetic appearance, wear resistance, biocompatibility, and color stability.26,40 However, ceramics are brittle, causing excessive wear to opposing dentition, and susceptible to fracture due to the formation of flaws or defects in the intaglio surfaces.3,10,26 On the other hand, indirect composite restorations are more compliant and

Assistant Professor, Department of Dental Biomaterials, Faculty of Dentistry, Mansoura University, Mansoura, Egypt. Correspondence: Shaymaa E. Elsaka, Department of Dental Biomaterials, Faculty of Dentistry, Mansoura University, Mansoura PC 35516, Egypt. Tel: +20-100-410-1558. e-mail: [email protected]

Vol 16, No 6, 2014

Submitted for publication: 30.01.14; accepted for publication: 21.08.14

soft, easier to finish and polish, create less wear on opposing dentition, and are conducive to making add-on adjustments, although they experience high wear.10,24 Consequently, the development of restorative materials that combine the advantages of ceramics and composites will enhance the properties and longevity of indirect esthetic restorations.10 Recently, a novel CAD/CAM restorative material for indirect restorations has been developed, based on a polymer-infiltrated ceramic network material (PICN).10 The PICN restorative material consists of a dominant ceramic network (86 wt%) which is reinforced by an acrylate polymer network (14 wt%) with both networks fully penetrating one another.46 It was manufactured by introducing a polymeric second phase with a lower modulus of elasticiy into ceramic networks.9 Accordingly, the PICN restorative material combines the positive characteristics of ceramics and resin-based com531

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Table 1

Materials used in this study

Material

Product and composition*

Lot

Manufacturer

CAD/CAM restorative material

Vita Enamic (86 wt% feldspar ceramic, 14 wt% polymer)

37990

Vita Zahnfabrik; Bad Säckingen, Germany

Lava Ultimate (80 wt% nanoceramic, 20 wt% resin)

N392136

3M ESPE; St Paul, MN, USA

Dual-curing resin cement

Bifix SE – Base: bis-GMA, UDMA, acidic phosphate monomers, glycerindimethacrylate, benzoyl peroxide, aerosol silica, hydroxypropylmethacrylate, catalysts, initiators, stabilizers, glass fillers (70 wt%); catalyst: UDMA, glycerindimethacrylate, catalysts, initiators

1322421

VOCO; Cuxhaven, Germany

Composite resin

Filtek Z250 Universal Restorative, A2 Shade – organic matrix: TEGDMA < 1–5%, bis-GMA < 1–5%, bis-EMA 5–10%, UDMA 5–10%; fillers: 0.01−3.5 μm silica/zirconia particles 82 wt%

8LXJ

3M ESPE

*Manufacturers’ data. Bis-GMA: bisphenol-A-glycidylmethacrylate; UDMA: urethane dimethacrylate; TEG-DMA: triethyleneglycol dimethacrylate; bis-EMA: bisphenol-A polyethyleneglycol dietherdimethacrylate.

posites.46 It has been reported that the PICN CAD/CAM is characterized by reduced brittleness, rigidity, and hardness, together with enhanced flexibility, fracture toughness, and improved machinability compared with ceramics.10 In addition, it was found that the mechanical properties of the PICN were close to those of human enamel and dentin.10,22,23 Another CAD/CAM-system-machinable dental material has been developed to meet the esthetic demands of prosthetic restorations. Termed resin nanoceramic (RNC) and based on nanotechnology, it is composed of 80 wt% zirconia/silica nanoceramic particles embedded in a highly cross-linked resin matrix (20 wt%).28 The RNC restorative materials contains silica nanomers (20 nm), zirconia nanomers (4 to 11 nm), nanocluster particles derived from the nanomers (0.6 to 10 μm), silane coupling agent, and resin matrix.29 In terms of materials science, the RNC CAD/CAM restorative material belongs in the resin composite category.8 Bonding of indirect esthetic restorations to the tooth structure remains a challenging matter, as the bonding interfaces are increased with the indirect restorative procedure. The bonding interfaces include the tooth structure and the fitting surface of the restoration. Consequently, to establish a strong, durable bond, appropriate treatment of the respective surfaces is crucial.14 Non-destructive, simple, and applicable methods for pre-treating indirect restoration surfaces would be clinically beneficial.35 In addition, using adhesive cements enhances the fracture resistance of indirect esthetic restorations.14 It is essential to improve the bond strength between indirect restorative materials and cements, since they are the principal factors in the success of resin-bonded fixed dental prostheses.16,33 To date, no study has determined the effect of surface treatments to enhance the adhesion of the recently introduced CAD/CAM PICN and RNC restorative materials to resin cement. Accordingly, the present study aimed to evaluate the effect of different surface treatments on the microtensile bond strength of CAD/ CAM PICN and RNC restorative materials to self-adhesive resin cement. 532

MATERIALS AND METHODS Two types of CAD/CAM restorative materials were used in this study: Vita Enamic (VE) and Lava Ultimate (LU). The manufacturers and the compositions of the materials are presented in Table 1. Grouping of specimens VE and LU specimens were cut from their respective blocks using a water-cooled diamond blade (Diamond Wafering Blade, Buehler; Lake Bluff, IL, USA) with a low-speed cutting saw (Isomet, Buehler) to produce specimens with the required dimensions for each test. The specimens from each CAD/CAM restorative material block were wet ground on only one surface using 600-grit silicon carbide (SiC) paper (Leco; St Joseph, MI, USA) and then ultrasonically cleaned (Sonorex, Bandelin; Berlin, Germany) in distilled water for 5  min. Then, the specimens in each test were divided into 5 groups according to the surface treatment performed. y Group 1: Control (C; no treatment). The specimens were exposed only to grinding and ultrasonic cleaning as mentioned above. y Group 2: Sandblasted (SB). The specimens were abraded with 110-μm aluminum-oxide (Al 2O3) particles with a dental airborne-particle abrasion unit (Micro-Blaster, Daedong Industrial; Daegu, Korea) was applied for 20 s at a pressure of 2 bar with a distance of 15 mm between the nozzle and the surface. Then, the specimens were ultrasonically cleaned for 5 min in distilled water and air dried. y Group 3: Sandblasted + silane (SB + S). The specimens were sandblasted as in group 2, and then a silane coupling agent (Ultradent silane, Ultradent Products; South Jordan, UT, USA) was applied on the surface of the specimen with a brush and air dried for 1 min. y Group 4: Hydrofluoric acid (HF). The specimens were etched with hydrofluoric acid (Ultradent Porcelain Etch 9% Buffered, Ultradent) for 1 min, then rinsed for 1 min and air dried. The Journal of Adhesive Dentistry

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y Group 5: Hydrofluoric acid + silane (HF + S). The specimens were etched with hydrofluoric acid as in group 4, and then a silane coupling agent was applied on the surface of the specimen with a brush and air dried for 1 min. Microtensile Bond Strength (μTBS) Test A total of fifteen blocks from each CAD/CAM restorative material (10 mm × 10 mm × 10 mm) were prepared and divided into 5 test groups according to the surface treatments applied (n = 3/group). Each block was duplicated in composite resin (Filtek Z250; Table 1) with the same dimensions using a mold made of a silicone impression material (Express, 3M ESPE; St Paul, MN, USA). Composite resin layers were incrementally (2 mm) condensed into the mold until it was full, and each layer was cured for 40 s with an LED light-curing unit (Elipar S10, 3M ESPE; light output: 1200 mW/cm2). A glass microscope slide was used to compress the last increment to obtain a flat surface. After that, the specimen was removed from the mold and an additional 40 s of irradiation was performed on the areas that were previously in contact with the silicone mold. The bonding surfaces of these blocks were wet ground using 600-grit SiC paper. One composite resin block was constructed for each CAD/CAM restorative material block.11,21 Luting procedure The composite resin blocks were bonded to the treated CAD/CAM restorative material block surfaces using a dual-curing self-adhesive resin cement (Bifix SE; BF) (Table 1) according to the manufacturer’s instructions. A standardized constant pressure of 1 kg (0.098  MPa) was applied to lute CAD/CAM restorative material/composite blocks using a customized metallic tool. The compressive force was applied for the first 5  min, leaving the material to set in the self-curing modality. After that, the specimen was light cured for 40 s from each side to ensure optimal polymerization. The bonded specimens were stored in distilled water for 24 h at 37°C prior to μTBS testing.11,21 After that, the specimens were vertically sectioned into serial slabs and further into beams with cross-sectional areas of approximately 1 mm2 using a water-cooled diamond blade with a low-speed cutting saw. Twenty beams were obtained from each block. The peripheral slices were excluded, as the results could be influenced by either an excess or a deficient amount of resin cement at the interface.36 Ten of them were tested after 24 h storage in distilled water at 37°C, and the other ten beams were tested after 30 days storage in distilled water at 37°C. The specimens were attached to a Bencor Multi-T testing device (Danville Engineering; San Ramon, CA, USA) using a cyanocrylate adhesive (Zapit, Dental Ventures of America; Anaheim, CA, USA) and stressed to failure in tension using a universal testing machine (Model TT-B, Instron; Canton, MA, USA) at a crosshead speed of 0.5 mm/min. The load at failure (N) and the surface area (mm2) for each specimen was used to calculate the μTBS in MPa. A digital Vol 16, No 6, 2014

caliper (Mitutoyo; Kawasaki, Japan) was used to measure the cross-sectional area at the site of fracture. Statistical analysis (SPSS 15.0; Chicago, IL, USA) of the μTBS (MPa) values was performed using a three-way ANOVA considering three factors (surface treatment, type of CAD/CAM restorative material, and water storage periods) and their interaction. Multiple comparisons were made by Tukey’s test. Statistical significance was set at the 0.05 probability level. Debonded specimens were examined under a stereomicroscope (Olympus SZX-ILLB100-Olympus Optical; Tokyo, Japan) at 50X magnification to determine the fracture pattern. Representative fractured specimens from each group were rinsed with 96% ethanol and air dried, mounted on metallic stubs, sputtered with a gold layer (SPI-Module Sputter Coater, Structure Probe; West Chester, PA, USA), and then examined under a scanning electron microscopy (SEM) (JSM-6510LV, JEOL; Tokyo, Japan) at 500X magnification. The modes of failure were classified according to one of four types: type 1, adhesive failure (complete CAD/CAM restorative material surface was visible); type  2, mixed failure in CAD/ CAM restorative material surface and luting resin (partial CAD/CAM restorative material surface and a partial luting resin cover were visible); type 3, cohesive failure within the luting resin layer (almost all of the fracture surface was covered with luting resin); type  4, mixed failure in CAD/CAM restorative material surface, luting resin, and resin composite (both luting resin and resin composite were detected on the CAD/CAM restorative material surface).7 Surface Roughness Measurement The surface roughness of the treated VE and LU plates (10 mm × 10 mm × 2 mm) (n = 10/group) was measured using a portable surface 2D texture-measuring instrument (Surftest SJ-201 P, Mitutoyo) to measure R a (average roughness height) in micrometers (μm). A diamond stylus with a tip radius of 5 μm was used for the measurements. The detector moves over the specimen with a driving speed of 0.25 mm/s for a measured length of 1.25 mm. Ra for each specimen was measured at five different sites and the mean roughness average was then calculated. Mean surface roughness values were analyzed using two-way ANOVA considering two factors (surface treatment and type of CAD/CAM restorative material) and their interaction. Multiple comparisons were made by Tukey’s test. Statistical significance was set at the 0.05 probability level. SEM Evaluation Three representative specimens from each group of the VE and LU plates (10 mm × 10 mm × 2 mm) were rinsed with 96% ethanol and air dried, mounted on metallic stubs, sputtered with a gold layer, and then examined under a SEM at magnification of 1000X to observe the features of the treated surfaces.

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Table 2 Mean (standard deviation) of the μTBS (MPa) of CAD/CAM restorative material/luting resin combinations with different treatments Surface treatment

VE/BF 24 h

C

18.67

(3.1)aA

LU/BF 30 days 12.67

(2.13)aB

24 h 11.99

30 days

(2.52)aB

8.28 (1.99)aC

SB

21.87 (3.75)bA

16.71 (3.42)bB

16.93 (3.66)bB

13.49 (3.4)bC

SB + S

24.95 (3.79)cdA

19.48 (3.18)bcB

18.82 (3.69)bB

13.88 (3.47)bC

HF

23.86 (3.19)bdA

18.86 (3.31)bB

18.35 (2.84)bB

13.45 (3.24)bC

HF + S

27.47 (4.28)cA

22.21 (3.04)cB

19.21 (3.87)bC

14.35 (2.56)bD

Mean values represented with same superscript uppercase letters (row) or lowercase letters (column) are not significantly different according to Tukey’s test (p > 0.05).

RESULTS Three-way ANOVA of the μTBS testing data (surface treatment, type of CAD/CAM restorative material, and water storage periods) revealed that the bond strength was significantly affected by the surface treatment, type of CAD/CAM restorative material, and water storage periods (p < 0.001). There were significant interactions between surface treatment and type of CAD/ CAM restorative material (p < 0.001). However, there were no significant interactions between type of CAD/ CAM restorative material and water storage periods (p = 0.06), the surface treatment and water storage periods (p = 0.854), or the type of CAD/CAM restorative material, water storage periods, and surface treatment (p = 0.644). The mean μTBS values (MPa) and standard deviations are presented in Table 2. The bond strength values achieved with HF + S, SB + S, HF, and SB groups were significantly higher compared with the control groups for both types of CAD/CAM restorative materials under different aging periods (p < 0.05). Regarding the type of CAD/ CAM restorative material, there was a significant difference in the bond strength values between VE and LU after different aging periods (p < 0.05). For the LU/BF system, there was no significant difference in the bond strength values between different surface treatments under different aging periods (p > 0.05). On the other hand, for the VE/BF system, surface treatment with HF + S showed higher bond strength values compared with SB and HF surface treatments under different aging periods (p < 0.05). Additionally, for the VE/BF system, no significant difference in bond strengths was found between either HF + S and SB + S (p = 0.239) or HF and SB surface treatments (p = 0.699). After 30 days of water storage, the μTBS of BF to VE and LU significantly decreased in all groups (p < 0.05) (Table 2). Regarding the failure modes, mixed failures of type 2 (both a part of the CAD/CAM restorative material surface and partial luting resin coverage were visible, 46.34%) were observed in all tested groups. In addition, cohesive 534

failure in the luting resin (type 3, 21.99%), mixed failures of type 4 (both luting resin and resin composite were detected on the CAD/CAM restorative material surface, 16.17%), and adhesive failures (15.5%) (type 1, complete CAD/CAM restorative material surface was visible) were also observed. After 30 days of water storage, the adhesive failure modes increased in the control groups (Fig 1). Representative SEM images of fractured beams are presented in Figs 2 and 3. Two-way ANOVA of the surface roughness testing data (surface treatment and type of CAD/CAM restorative material) revealed that the surface roughness was significantly affected by the surface treatment and type of CAD/ CAM restorative material (p < 0.001). There were significant interactions between surface treatment and type of CAD/CAM restorative material (p = 0.006). Means and standard deviations of the average surface roughness (Ra, μm) of treated CAD/CAM restorative material with their significant differences are presented in Table 3. In general, VE treated with SB showed the significantly highest Ra values (4.29 ± 0.34 μm) compared with the other groups (p < 0.05). Regarding the type of CAD/CAM restorative material, LU showed significantly lower Ra values compared with VE among the groups (p < 0.05) (Table 3). Representative SEM images of the treated VE and LU CAD/CAM restorative materials are presented in Figs 4 and 5, respectively. The analysis showed a variation in the surface microstructures of the VE and LU CAD/CAM restorative materials with the surface treatments. The surface topography of untreated VE revealed two continuous interpenetrating networks: the polymer (dark gray areas) and the ceramic (light gray areas) with micropores (Fig 4A). However, the untreated LU showed a more homogeneous surface with tiny micropores (Fig 5A). The surfaces of specimens treated by SB showed welldefined micro-sized elevated and depressed areas with crevices and pits which possibly resulted from the high impact of blasting particles (Figs 4B and 5B). VE specimens treated with HF showed a change in surface texture with formation of numerous irregular and randomly distributed gaps and micropores (Fig 4D). HF acid treatment appeared The Journal of Adhesive Dentistry

A

C

LU/BF 24 h 30 days VE/BF

H+S HF SB +S SB C H+S HF SB +S SB C H+S HF SB +S SB C H+S HF SB +S SB C

Type 1 Type 2 Type 3 Type 4

0

10

20

50 μm

40

50

60

SB

C

80

C

HF

G

90

100

50 μm

SB

Fig 2 Representative SEM micrographs (500X) of the debonded surfaces of the VE/BF system. A−E: after 24 h storage in distilled water; F−J: after 30 days storage in distilled water. White arrows: retained luting agent; black arrows: VE restorative material; gray arrow: retained composite. All specimens showed mixed mode of failures except for E and J, which exhibited cohesive failure within the luting agent, and A and F with adhesive failure at the interface.

50 μm

H

50 μm

I

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70

D

SB + S

50 μm

F

50 μm

30

Failure modes (%)

B

E

HF + S

24 h

Fig 1 Failure pattern distribution of different groups tested. Type 1, adhesive failure: complete CAD/CAM restorative material surface was visible. Type 2: mixed failure in CAD/CAM restorative material surface and luting resin, both a part of the CAD/CAM restorative material surface and partial luting resin coverage were visible. Type 3: cohesive failure within the luting resin layer, almost all of the fracture surface was covered with luting resin. Type 4: mixed failure in CAD/CAM restorative material surface, luting resin and resin composite, both luting resin and resin composite were detected on the CAD/CAM restorative material surface.

30 days

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SB + S

50 μm

J

HF 50 μm

HF + S

50 μm

535

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A

C

B

50 μm

E

SB

C

SB + S 50 μm

F

HF + S I

HF

50 μm

G

C 50 μm

D

50 μm

50 μm

H

SB

SB + S

50 μm

50 μm

J

HF 50 μm

HF + S

50 μm

Fig 3 Representative SEM micrographs (500X) of the debonded surfaces of the LU/BF system. A−E: after 24 h storage in distilled water. F−J: after 30 days storage in distilled water. White arrows: retained luting agent; black arrows: LU restorative material; gray arrow: retained composite. All specimens showed mixed mode of failures except for C, which showed cohesive failure within the luting agent, and F and I, with adhesive failure at the interface.

Table 3 Mean (standard deviation) of the average surface roughness values (Ra, μm) of CAD/CAM restorative materials with different surface treatments CAD/CAM restorative material

Surface treatments C

SB

SB + S

HF

HF + S

VE

2.21 (0.16)aA

4.29 (0.34)aB

3.91 (0.26)aC

3.77 (0.11)aC

3.29 (0.29)aD

LU

1.12 (0.1)bA

2.73 (0.2)bB

2.57 (0.19)bBC

2.34 (0.22)bCD

2.12 (0.23)bD

Mean values represented with same superscript uppercase letters (row) or lowercase letters (column) are not significantly different according to Tukey’s test (p > 0.05).

to partially dissolve the polymer and glassy phases of VE, which possibly served for micromechanical retention of resin bonding. However, LU specimens revealed a slight resistance to HF treatment. Tiny micropores and pits appeared on the LU surface without extensive dissolution of the glassy phase (Fig 5D). Application of S after SB and HF surface treatments covered the surface irregularities created by these treatments and the surface appeared smoother for both VE (Figs 4C and 4E) and LU (Figs 5C and 5E). 536

DISCUSSION In recent years, improvements in the adhesion of indirect esthetic restorative materials to resin cement have progressed substantially. Successful adhesion of indirect restorations to tooth structure can be achieved by creating a reliable bond between the internal surface of the restoration and the luting agent.37 In this study, the adhesion of recently introduced VE and LU CAD/CAM restorative materials to dual-curing self-adThe Journal of Adhesive Dentistry

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A

C

B

10 μm

SB

C

10 μm

D

Fig 4 Representative SEM micrographs (1000X) of VE specimens after different surface treatments. White arrows: polymer phase; black arrow: ceramic phase.

A

C

HF

SB

10 μm

hesive resin cement after different surface treatments was determined using the μTBS test. Clinically, indirect restorations are commonly bonded either to dental tissues or to a core material. In the present study, CAD/ CAM restorative materials were bonded to composite resin substrates instead of dentin disks. The rationale was to avoid the weak link located in the tooth structure/luting system interface. The strong bond developed between composite resin and the luting system allowed the weak link to be at the CAD/CAM Vol 16, No 6, 2014

HF

HF + S

10 μm

C

10 μm

D

Fig 5 Representative SEM micrographs (1000X) of LU specimens after different surface treatments.

10 μm

E

B

10 μm

SB + S

SB + S

10 μm

E

10 μm

HF + S

10 μm

restorative material/luting system interface. Otherwise, failures might happen at other sites rather than at the restorative material surface, thus masking the effects of the surface treatments.13 In addition, variations in the tooth microstructure could lead to misinterpreting the findings.21 Thus, in this study, the CAD/CAM restorative material specimens were luted to composite disks rather to dental substrates. The μTBS method was chosen as it provides a more accurate estimation of bond strength compared with 537

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the conventional shear bond test. The fracture pattern in shear testing often results in cohesive bulk fracture within the substrate rather than the interface, due to the generation of inhomogeneous stress distribution during testing, which may also lead to invalid interpretation of the data.1,4,12,14 On the other hand, the μTBS test allows a more uniform and homogeneous stress distribution during loading, and failure predominantly occurs at the adhesive interface due to the small bonded interfaces (approximately 1 mm2) of the specimens used.1,4,12,14 In addition, the μTBS method enables recognition of the weakest part in the adhesive system based on the location of failure, allowing enhancements to be made to the relevant part of the adhesive complex.25,36 Selection of the luting agent assumes to be a significant factor while bonding to indirect restorations.11,15-17 In the present study, BF dual-curing self-adhesive resin cement was used as the luting agent to evaluate the adhesion with VE and LU CAD/CAM restorative materials. The bond strength of a dual-curing self-adhesive resin cement with indirect restorations was found to be more effective than that of a self-curing self-adhesive resin cement.15,16 This finding was attributed to the acidity of the adhesive system utilized with the self-curing resin cement, explained by the presence of non-polymerized residual acidic monomers in the resinous cement layer. These residual acidic monomers react with tertiary amines found in the self-curing resinous cement, neutralizing them; consequently, the tertiary amines are unable to react with the benzoyl peroxide, which is responsible for the setting reaction of the polymerization process of this type of cement.18 Additionally, the self-curing resin cements bond poorly at the beginning of the luting procedure, meaning that to avoid dislodgement, the restoration cannot be subjected to masticatory forces within the first hour.39 Accordingly, microleakage and caries formation could happen if the restoration is dislodged.6 Hence, dual-curing resin-based cements are favored as luting materials for indirect restorations.15,16,39 In this study, the application of HF + S on the bonding surface of VE CAD/CAM restorative material evidently provided higher bond strength compared with SB and HF surface treatments (Table 2). The HF + S treatment causes the specimens to fail more in a mixed mode, often including cohesive failure within the resin cement (Fig 1). HF surface treatment modifies the microstructure of the treated surface by partial dissolution of the glassy or crystalline phases of the restorative material,47 as shown for both VE and LU (Figs 4D and 5D). HF acid forms microporosities on the restorative material surface, increases the surface area, and enhances the establishment of mechanical interlocking with luting resin.34 Roughening the indirect esthetic restorative material surface followed by silanization was anticipated as the preferred technique to achieve a high bond strength between indirect restorations and resin-based luting agents.19,30,42 Surface treatment of VE with SB + S showed comparable bond strength to HF + S. These findings suggest that surface treatments of VE restorative material either with HF or SB followed 538

by S application are appropriate treatments. Silane acts as a coupling agent in the indirect esthetic restorative material/resin bond, which adsorbs onto the surface of the restorative material, thus facilitating chemical interaction.1 On the other hand, for the LU/BF system, all surface treatments showed comparable enhancement of the bond strength values compared with the control group under different aging periods (Table 2). These findings suggest that the effect of surface treatment on the bond strength of resin cement to CAD/CAM restorative is material dependent. Failure mode analysis supported the results of μTBS test, as the experimental groups treated with HF  +  S, SB + S, HF, and SB showed a combination of cohesive failure in the luting resin and mixed failures, independent of the type of CAD/CAM restorative material, which presented with higher bond strength values compared with the control groups (Fig 1, Table 2). This finding is in agreement with previous studies.7,31 Cohesive failure of the luting resin exhibits the perfect bonding status that can be obtained, as the principal source of failure arises from flaws within the resin and not at the interface.25,36 In addition, it is noteworthy that the control group, which had no surface treatment, exhibited more adhesive failures between the CAD/CAM restorative material and the luting resin compared with the other groups and showed significantly lower bond strength values (Fig 1, Table 2). Mixed and cohesive failure modes are clinically preferable to total adhesive failure, as the adhesive type of failure is typically associated with low bond strength values.31,44 This study reveals that the unmodified (untreated) surface may cause inadequate bonding between the restorative material and luting agent. Similar findings were reported previously.14,16 The present study verified the effect of SB and HF surface treatments on the surface topography of VE and LU CAD/CAM restorative materials, as shown by surface roughness and SEM evaluation (Table 3, Figs 4 and 5). Surface roughness improves mechanical interlocking with the luting agent, considered as an important factor that could influence bond strength. SB is anticipated to enhance the bond strength by improving mechanical interlocking, increasing wettability and surface area.15,20 However, it was reported in previous studies that SB contaminates the indirect esthetic restorative material surface.1,32 In this study, all surface treatments increased the roughness of VE and LU surface compared with the control groups (Table 3). Surface roughness should increase surface area and, consequently, may improve the CAD/CAM restorative material/luting agent bond. For the groups etched with HF, the surface roughness was lower than SB treatment; however, the bond strength values of these groups were comparable to each other (Table 2). It has been reported that the stress concentration at the indirect restorative material/luting agent interface could be increased by airborne-particle abrasion, which creates sharp angles that could impede complete wetting and produce voids at the interface.1,32,36 Consequently, higher surface roughness will not ensure a higher bond strength.2,7,11,27 The Journal of Adhesive Dentistry

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In this study, water storage was used to artificially age the resin bond for adhesion testing to determine the hydrolytic stability of resin cement to CAD/CAM restorative materials. After 30 days of water storage, the specimens exhibited significantly lower resin bond strength. This finding could be due to the vulnerability of resin cement to hydrolytic degradation, as over time, water absorption at the interface could reduce the bond strength.41 It has been reported that the resin cement layer becomes less stiff over time because of water sorption, which could negatively affect stress distribution across the luted interfaces to indirect restorations, probably causing debonding even under weak occlusal loads.45 This finding is in agreement with previous studies.5,31,36,41 It is noteworthy that after aging, the VE/BF system showed higher bond strength values compared with the LU/BF system (Table 2). This finding indicates that the bond strength of the VE/BF system appears to be more hydrolytically stable and durable than the LU/BF system, even with a significant decrease in bond strength values after aging. In addition, the bond strength for the VE/BF system was statistically significantly higher than for the LU/BF system in all groups, regardless of the surface treatment (Table 2). This finding could be attributed to the different composition and microstructure between VE and LU. The microstructure of VE is comprised of a feldspar ceramic network that is fully integrated with a polymer network. VE presented with a low polymer content (14 wt%). This microstructure could have a significant influence on the mechanical properties, such as increased chemical stability, reduced monomer absorption, and greater biocompatibility. It has been claimed that the strength and elasticity of the PICN restorative material could provide a highly fracture-resistant material.10,23,38,43,46 It has also been reported that VE could withstand high levels of elongation at high stress before fracturing, due to the integrated polymer network that facilitates high levels of stability, in spite of a lower modulus of elasticity.43 It has been supposed that entirely merging the networks with each other like this also provides integrated crack prevention as well as creating a structure that offers greater fault tolerance.10,23,43,46 On the other hand, regarding the microstructure of LU, it is a polymer network (20 wt%) reinforced by 80 wt% zirconia/ silica nanoceramic particles.28 It could be postulated that LU showed lower adhesion values with BF luting agent compared with VE due to the differences in the microstructures, composition, filler type, filler concentration, and mechanical properties of the two tested CAD/CAM restorative materials. It is noteworthy that the surface treatments performed also enhanced the adhesion of LU with BF luting resin compared with the control group (Table 2).

CONCLUSIONS Based on the results presented and within the limitations of this study, it can be concluded that the effect of surface treatments on the bond strength of novel CAD/ CAM restorative materials to resin cement is material dependent. The VE/BF system revealed higher bond Vol 16, No 6, 2014

strength than the LU/BF system. Water aging showed a considerable influence on restorative material/luting resin bond degradation. The VE/BF system appears to be more hydrolytically stable and durable than the LU/BF system. Further research is needed to evaluate the physical properties of the novel CAD/CAM restorative materials VE and LU to clarify their behavior. Such evaluations might lead to a better understanding of the behavior of these materials. Future studies are also required on the participation of the tooth substrates in the test complex under standardized conditions. Only one brand of resin cement was tested in this study; the findings related to this product may not be valid for other, similar materials. Additionally, evaluation of the clinical performance is required to provide reliable recommendations for dental practitioners.

ACKNOWLEDGMENTS The author would like to thank VOCO, Vita Zahnfabrik, and Ultradent for supplying the materials for this research.

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Clinical relevance: The VE/BF system provided higher bond strength than the LU/BF system under different aging periods. The influence of surface treatments on the bond strength was more effective on the VE CAD/CAM restorative material than on the LU material.

The Journal of Adhesive Dentistry

CAM restorative materials to self-adhesive resin cement: the effect of surface treatments.

To evaluate the effect of different surface treatments on the microtensile bond strength (μTBS) of novel CAD/CAM restorative materials to self-adhesiv...
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