Clin Oral Invest DOI 10.1007/s00784-014-1272-8

ORIGINAL ARTICLE

Color stability of adhesive resin cements after immersion in coffee Maho Shiozawa & Hidekazu Takahashi & Yuya Asakawa & Naohiko Iwasaki

Received: 27 January 2014 / Accepted: 9 June 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract Objectives Marginal discoloration of luting cement may affect the appearance of esthetic restorations. This study evaluated the color stability of current adhesive resin cements after immersion in coffee. Materials and methods Four dual-cured resin cements (Clearfil SA cement Automix Universal, Maxcem Elite Clear, Maxcem Elite Yellow, and RelyX Unicem2 Automix A2) and two chemical-cured resin cements (Super-Bond C&B Clear and Super-Bond C&B Esthetic) were examined. The CIE L*a*b* of 2.0-mm-thick disc-shaped specimens was measured using a spectrophotometer on a white background (n=6). The color differences (ΔE) after 1-day and 1-week immersion in 37 °C water or coffee were analyzed by twoway ANOVA by selecting immersion solution and product as main factors, followed by Tukey’s HSD test (α=0.05). Water sorption and solubility were also evaluated. Results The two-way ANOVA of the ΔEs suggested that the two main factors and their interaction were significant. The ΔEs after coffee immersion were significantly greater than those after water immersion, except for Super-Bond C&B Esthetic. The ΔEs after water immersion were not significantly different among the products; those of Maxcem Elite Clear and Maxcem Elite Yellow after coffee immersion were

M. Shiozawa Removable Partial Prosthodontics, Department of Masticatory Function Rehabilitation, Division of Oral Health Sciences, Graduate School, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan H. Takahashi (*) : Y. Asakawa : N. Iwasaki Oral Biomaterials Engineering, Course for Oral Health Engineering, School of Oral Health Care Sciences, Faculty of Dentistry, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan e-mail: [email protected]

significantly greater than the others. The water sorption and solubility significantly correlated with the ΔEs. Conclusions The ΔEs of the adhesive resin cements examined after 1-week coffee immersion were significantly different among the products. The product showing greater water sorption and solubility displayed greater color change. Clinical relevance Adhesive resin cements should be carefully selected when the marginal appearance of the ceramic restoration is important. Keywords Adhesive resin cements . Color stability . Water sorption . Water solubility

Introduction Ceramic restorations such as all-ceramic crowns, porcelain laminate veneers, and ceramic inlays and onlays are popular due to their esthetic appearance. Adhesive resin cements are the material of choice for the luting of ceramic restorations because of their excellent esthetics and good mechanical properties such as flexural strength, compressive strength, and shear and tensile bond strength [1–3]. The adhesive nature of the resin cements promotes superior retention and fracture resistance of the restorations [4–6]. The adhesive resin cements are categorized based on bonding procedure and/or use: total etch, bond, plus resin; one-step etch-bond, plus resin; self-adhesive resin; and dual-affinity adhesive resin [2]. These cements are also categorized by polymerization type: chemical-cured, light-cured, and dual-cured [2, 4, 5]. Recently, dual-cured adhesive resin cements have often been selected for luting because of their ability to self-cure under restorations [7]. Dual-cured adhesive resin cements can be hardened in deep areas or beneath restorations where the curing light cannot penetrate, while they can also be

Clin Oral Invest

polymerized by the curing light at the restoration margins for easy removal of excess cement [5]. Because adhesive resin cements at the restoration margins are exposed to the oral environment, discoloration of resin cements is caused by not only intrinsic but also extrinsic factors. Intrinsic factors are material-related factors such as the composition of the resin matrix [8] and filler [9], the loading and particle size distribution of the filler, the type of photoinitiator [10] and polymerization systems [11], and the conversion ratio of the carbon-carbon double bonds [12]; these intrinsic factors are accelerated by UV irradiation [13] or thermal change [12]. Extrinsic factors are stains caused by food components, beverages, and smoking; these factors are modulated by the conversion ratio and physicochemical characteristics of the resin cements, such as water sorption rate [14] and surface roughness [15]. Discoloration of the adhesive resin cements affects the esthetic appearance of ceramic restorations. Marginal discoloration of ceramic inlays and onlays increases with time due to the color changes in adhesive resin cements [16]. Therefore, these cements need to have sufficient color stability to maintain good esthetics. Adhesive resin cements usually contain an adhesive functional monomer consisting of hydrophilic and hydrophobic functional groups. Hydrophilic functional groups are considered to possess higher water sorption [7, 17, 18], and thus, they undergo greater color change. However, the color stability of current adhesive resin cements after immersion in discoloring media has not been clearly elucidated. Many investigations evaluated the color stability of resinbased composite materials in a variety of beverages such as coffee [8, 9, 13, 14, 19–25], black tea [8, 11, 13, 14, 19, 22–25], red wine [8, 9, 11, 13, 22–25], and cola [8, 19, 20, 22–25]. These beverages are common in people’s daily diet and are reported to have the potential to stain restorative materials. The results of these experiments showed that the color changes were generally greater in red wine, coffee, and tea compared to cola [19, 8, 23, 24]. Coffee is considered to be one of the beverages with a higher potential to stain [8]. The objective of the present study was to evaluate the color stability of current adhesive resin cements after immersion in coffee. The null hypothesis was that the immersion solution and the type of adhesive resin cements had no influence on the color change of adhesive resin cements.

listed in Table 2. They were prepared at 23±2 °C according to each manufacturer’s instructions. The dual-cured type was prepared using the light-curing mode. The chemical-cured type was prepared using the bulk-mix technique. Color stability measurement Disc-shaped specimens (10 mm in diameter and 2.1 mm thick) were prepared in an acrylic mold (n=6). The dual-cured specimens were polymerized with a light-emitting diode (LED) light-curing unit (Elipar S10, 3 M ESPE, Seefeld, Germany) with light intensity of 1,200 mW/cm2 for 40 s on each side. The chemical-cured-type specimens were polymerized in an incubator at 37±2 °C for 1 h. The specimens were ground sequentially using a series of SiC grinding sheets (#600, 1,000, and 1,500). The grinding and polishing procedures were performed on one side of the specimens to make 2.0-mm-thick specimens. The final thickness was checked with a digital micrometer (MDC-25 M, Mitsutoyo, Tokyo, Japan) having an accuracy of ±0.01 mm. Before each measurement, the specimens were ultrasonically cleaned in distilled water for 10 min and dried with compressed air. Each specimen was immersed in distilled water at 37±2 °C for 24 h in a 100-mL brown glass vial. Before performing the colorimetric measurements, the specimens were removed from the vials and washed with distilled water, gently wiped with filter paper, and dried by shaking in air for 30 s. The CIE L*a*b* values of the polished surfaces of each specimen were measured on a white glossy photo paper (KA420PG, Seiko Epson Corp., Nagano, Japan): L*=91.57, a* = −1.02, and b* = 1.56 using a spectrophotometer (Crystaleye, Olympus Corp., Tokyo, Japan) [26]. Prior to data acquisition, the spectrophotometer was calibrated using a calibration plate and positioned at a consistent distance of 16 mm from the specimen surface using the special contact cap for object measurement. The light source illumination of seven-band LED lights corresponding to average daylight (D65) was irradiated at a 45° angulation, and the diffusion refraction light was measured. The color difference (ΔE) was obtained by calculating the color difference between the specimen before and after immersion in one of the solutions using the following equation: h i1=2 ΔE ¼ ðL1 −L2 Þ2 þ ða1 −a2 Þ2 þ ðb1 −b2 Þ2

Materials and methods Materials and immersion solution The four dual-cured adhesive resin cements and two chemical-cured adhesive resin cements evaluated in the present study are listed in Table 1; four cements were toothcolored and two were transparent. Their compositions are

where L* refers to the brightness, a* for redness to greenness, and b* for yellowness to blueness. The subscripts 1 and 2 refer to the color coordinates before and after immersion, respectively. A high ΔE value indicates great color difference. Three measurements were taken for each specimen, and the average value was recorded.

Clin Oral Invest Table 1 Adhesive resin cements investigated Product

Code

Shade

Polymerization type

Filler content (wt%)a

Lot no.

Manufacturer

Clearfil SA Cement Automix

CSA

Universal

Dual-cured

66

056AAA

Maxcem Elite Maxcem Elite RelyX Unicem 2 Automix

MEC MEY RXU

Clear Yellow A2

Dual-cured Dual-cured Dual-cured

69 69 70

4438232 4772310 463439

Kuraray Noritake Dental Inc. Kerr Corp. Kerr Corp. 3 M ESPE

Super-Bond C&B

SBC

Clear

Chemical-cured

0

Super-Bond C&B

SBE

Esthetic

Chemical-cured

0

Monomer: FX1, catalyst: FM43F, powder: FX31 Monomer: FX1, catalyst: FM43F, powder: FW1

a

Sun Medical Co. Sun Medical Co.

Claimed by the manufacturer

Two types of immersion solution were prepared: distilled water and coffee solution. The coffee solution was prepared by dissolving 0.51 g of instant coffee powder (Nescafe Gold Blend, Nestle Japan, Kobe, Japan) in 50 mL of distilled water [9]. Six specimens of each resin cement were immersed in one of the immersion solutions in a shaking incubator (37±2 °C, 1 Hz frequency). The immersion solution was changed every day. The color measurements were taken after 1-day and 1week immersion. Water sorption and solubility The water sorption and solubility of each resin cement were determined according to ISO 4049:2009. Disc-shaped specimens (15 mm in diameter and 1 mm thick) were prepared using a metal mold (n=5). The dual-cured-type specimen was exposed to overlapping irradiation with the

LED light-curing unit for 40 s on each side and then stored in the incubator at 37±2 °C for 15 min. The chemical-cured-type specimen was polymerized in the incubator at 37±2 °C for 1 h. The specimens were removed from the mold and kept in a desiccator maintained at 37± 2 °C. After 22 h, the specimens were removed and stored in another desiccator kept at 23±2 °C for 2 h and then weighed using a precision digital balance (AUW120D, Shimadzu, Kyoto, Japan; the minimum reading is 0.01 mg). This cycle was repeated until a constant mass was obtained, i.e., until the mass loss of each specimen was not more than 0.1 mg in any 24-h period. The diameter and height of each specimen were measured to calculate the volume. The specimens were immersed in distilled water at 37 °C for 1 week, then removed, gently dried with filter paper, and weighed again. The specimens were returned to the desiccators using the above-

Table 2 Compositions of adhesive resin cements Code CSA

Base resin

10-Methacryloyloxydecyl dihydrogen phosphate (MDP), Bis-GMA, TEGDMA, hydrophobic aromatic dimethacrylate Paste B Bis-GMA, hydrophobic aromatic dimethacrylate, hydrophobic aliphatic dimethacrylate MEY Base paste Urethane dimethacrylate MEC Catalyst paste Bis-GMA, glycerol dimethacrylate, glycerol phosphate dimethacrylate RXU Base paste Methacrylate monomers containing phosphoric acid groups, methacrylate monomers Catalyst paste Methacrylate monomers SBE SBC

Paste A

Catalyst Monomer Polymer

Filler

Other

Silanated barium glass filler, silanated colloidal silica

dl-Camphorquinone, benzoyl peroxide, initiator

Silanated barium glass filler, silanated colloidal silica Fluoroalminosilicate glass Barium aluminoborosilicate glass

Accelerators, pigments, surfacetreated sodium fluoride Camphorquinone

Silanated fillers

Initiator components, stabilizers, rheological additives Alkaline (basic) fillers, silanated fillers Initiator components, stabilizers, pigments, rheological additives Tri-n-butylborane (TBB), acetone

Methyl-methacrylate, 4-methacryloxyethyl trimellitate anhyderide (4-META) Polymethyl-methacrylate

Bis-GMA bis-phenol A diglycidylmethacrylate, TEGDMA triethyleneglycol dimethacrylate

Pigments (esthetic)

Clin Oral Invest

mentioned procedure until a constant mass was obtained. The water sorption and solubility were calculated using the following equations:

carbon. Three randomly selected areas of the specimen surface were observed using a scanning electron microscope (S-4500, Hitachi High-Technologies Corp., Tokyo, Japan).

Water sorption ¼ ðm2 −m3 Þ=V Water solubility ¼ ðm1 −m3 Þ=V

Statistical analysis

where m1 is the mass obtained after the specimen was initially dried (μg); m2 is the mass after the period of immersion in water (μg); m3 is the final mass after the specimen was dried (μg); and V is the initial volume of each specimen (mm3).

The color differences (ΔE) after immersion were analyzed by two-way analysis of variance (ANOVA) by selecting the immersion solution and product as the main factors using JMP software (version 9.0, SAS Institute, Cary, NC, USA). Tukey’s honestly significant difference test was used to compare the individual data. The level of significance was set at α=0.05.

Filler content The inorganic filler content of each specimen was measured using the standard ash method according to ISO 1172:1996. An alumina crucible (PC2, Nikkato, Osaka, Japan) was held in an electric furnace (Auto Furnace QF1, GC Corp., Tokyo, Japan) set at 625 °C for 30 min. After cooling to ambient temperature in the desiccator, the weight of the crucible was measured until a constant mass was obtained. A droplet of the resin cement (approximately 0.1 g, n=3) was inserted in the crucible, and the weight of each specimen including the crucible was measured with the precision digital balance. The specimen and crucible were then heated in the electric furnace at 625 °C for 30 min to burn out the organic matrix. After cooling to ambient temperature in the desiccator, the residue and crucible were reweighed until a constant mass was obtained. The inorganic filler content (wt%) was determined using the following equation: Filler content ¼ ðm3 −m1 Þ=ðm2 −m1 Þ  100

where m1 is the initial mass of the dry crucible (g); m2 is the initial mass of the dry crucible plus the dried specimen (g); and m3 is the final mass of the crucible plus the residue after calcination (g). Morphological observation The polished surfaces of the specimens were observed using a scanning electron microscope (SEM) to evaluate the filler morphology. Disc-shaped specimens (4 mm in diameter and 2 mm thick) were prepared using a Teflon mold according to the same procedure described for the color measurement experiment. The specimens were embedded in an acrylic resin (Palapress Vario, Heraeus Kulzer, Hanau, Germany). The surface of the specimens was polished with sheets of SiC paper (1,000, 1,500, and 2,000 grit) and 0.06-μm Al2O3 suspensions. The specimens were ultrasonically cleaned, dried, mounted on aluminum stubs, and then coated with

Results The specimens after immersion in coffee became darker in color as the immersion period increased; the MEC and MEY specimens in particular showed greater color changes. However, the color changes of the specimens after water immersion were not perceptible by visual observation. Table 3 shows the L*, a*, and b* values and the means and standard deviations of the color differences after immersion for 1 day and 1 week. The ΔE values after coffee immersion greatly increased as the immersion period increased, but those after water immersion only slightly increased. Two-way ANOVA revealed that the two main factors, immersion solution and product and their interaction, were significant. The ΔE values after coffee immersion for 1 day for MEC, MEY, and RXU were significantly greater than those after water immersion; the ΔE values after coffee immersion for 1 week were significantly greater than those after water immersion except for SBE. The ΔE values after water immersion were not significantly different among the products, while the ΔE values after coffee immersion of MEY and MEC were significantly greater than those of the others; the ΔE of MEC was significantly greater than that of MEY. Regarding the coffee solution immersion, the L* value decreased with an increase of the immersion period, but the a* and b* values increased regardless of the product. Regarding SBC and SBE after water immersion, the b* value slightly increased with an increase of the immersion period, but the L* and a* values did not change. The means and standard deviations for the water sorption and solubility are shown in Table 4. The water sorption and solubility displayed a similar tendency. MEC and MEY showed greater water sorption and solubility values compared to the other products. The water solubility value of RXU was negative. The filler contents of the adhesive resin cements are summarized in Table 5. The dual-cured-type cements contained

Clin Oral Invest Table 3 Color difference (ΔE) after immersion in distilled water and coffee solution for 1 day and 1 week Polymerization type

Dual-cured

Code

CSA

MEC

MEY

RXU

Chemical-cured

SBC

SBE

Water

Coffee

Start

1 day

1 week

Start

1 day

1 week

L* a* b* ΔE L* a* b* ΔE L* a* b* ΔE L* a*

82.9 1.1 25.4

82.6 1.3 25.6 0.5 (0.1)d,e,f 86.8 −1.9 2.7 0.3 (0.2)e,f 78.9 2.0 25.5 0.2 (0.1)f 80.1 1.9

81.8 1.6 25.6 1.3 (0.2)b,c 87.0 −1.8 2.6 0.5 (0.3)d,e,f 79.4 1.7 25.1 0.7 (0.2)d,e,f 80.3 1.4

82.8 1.2 25.9

80.9 1.8 26.4 2.4 (1.2)D,E 77.0 0.7 19.7 20.1 (3.1)B 76.7 2.7 28.4 4.5 (0.8)D 78.7 2.6

78.8 2.3 27.3 4.8 (2.1)D 69.7 4.1 25.8 29.7 (2.4)A 73.7 4.1 32.4 9.6 (1.4)C 77.7 2.8

b* ΔE L* a* b* ΔE L* a* b* ΔE

21.2

21.5 0.4 (0.1)e,f 90.1 −2.3 1.9 0.9 (0.4)c,d 79.2 6.1 22.6 0.9 (0.2)c,d

21.7 0.8 (0.3)d,e 90.0 −2.7 3.1 2.3 (0.4)a 79.3 5.9 23.3 1.5 (0.3)b

21.4

23.2 2.6 (0.2)D,E 89.4 −2.3 3.4 2.4 (0.3)D,E 78.3 6.4 23.3 1.6 (0.1)E

24.8 4.5 (0.8)D 88.5 −2.6 5.4 4.7 (0.3)D 77.9 6.5 24.4 2.7 (0.2)D,E

86.8 −1.9 2.5 78.8 1.9 25.3 80.2 1.9

90.3 −2.0 1.0 79.2 6.3 21.8

86.9 −2.1 2.5 79.0 1.7 24.8 80.4 1.7

90.1 −2.1 1.1 79.3 6.4 22.1

Standard deviations are in parentheses, and the same superscript letter denotes homogenous subsets (p>0.05)

63.6–68.2 wt% of inorganic filler; however, a trace of inorganic filler was found in the chemical-cured-type cements. These results corresponded with the filler content claimed by the manufacturer except for RXU, which was smaller than manufacturer claimed value. This discrepancy might be due to using the standard ash method. SEM images of representative areas of the specimens are displayed in Fig. 1. The size of inorganic filler particles was largest in CSA, approximately 70 μm,

followed by MEC and MEY. Pre-polymerized filler particles were found in CSA. No inorganic filler particles were observed in SBC and SBE, but partially dissolved crushed poly(methyl methacrylate) (PMMA) particles were seen. Figures 2 and 3 showed the relationships among the water sorption and solubility and the color differences after 1-week coffee and water immersion, respectively.

Table 4 Water sorption and solubility after water immersion for 1 week (μg/mm3)

Table 5 Filler content of adhesive resin cements (wt%)

Polymerization type

Code

Water sorption

Water solubility

Polymerization type

Code

Filler content

Dual-cured

CSA MEC MEY RXU SBC SBE

6.4 (0.1) 16.0 (0.2) 12.0 (0.1) 7.9 (0.2) 7.1 (0.2) 7.2 (0.1)

0.3 (0.0) 2.5 (0.3) 1.1 (0.1) −0.8 (0.1) 0.2 (0.2) 0.1 (0.0)

Dual-cured

CSA MEC MEY RXU SBC SBE

64.6 (0.3) 68.2 (0.1) 67.9 (0.1) 63.6 (0.1) 0.1 (0.1) 0.1 (0.2)

Chemical-cured

Chemical-cured

Clin Oral Invest Fig. 1 SEM images of adhesive resin cement surface

The ΔE after coffee immersion significantly correlated with both the water sorption and solubility (p=0.006 and 0.01), but those after water immersion did not (p=0.12 and 0.39).

Discussion The dual-cured adhesive resin cements evaluated in the present study were self-adhesive resin cements, which have been advertised as bonding to the tooth structure without any pretreatment. Several in vitro studies reported promising results with respect to dentin bond strength compared to resinmodified glass ionomer cements and conventional resin cements [27–30]. Luting cements at the restoration margins are usually exposed to the oral environment. In the present study, marginal discoloration of the dual-cured and chemical-cured resin cements caused by intrinsic and extrinsic factors was simulated. The physical properties of dual-cured adhesive resin cement

Fig. 2 Relationship between water sorption and color difference after 1week immersion. r2 with * was statistically significant (p

Color stability of adhesive resin cements after immersion in coffee.

Marginal discoloration of luting cement may affect the appearance of esthetic restorations. This study evaluated the color stability of current adhesi...
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