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Influence of temporary cement contamination on the surface free energy and dentine bond strength of self-adhesive cements
journal of dentistry 40 (2012) 131–138
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Influence of temporary cement contamination on the surface free energy and dentine bond strength of self-adhesive cements Masayuki Takimoto, Ryo Ishii, Masayoshi Iino, Yusuke Shimizu, Akimasa Tsujimoto, Toshiki Takamizawa, Susumu Ando, Masashi Miyazaki * Department of Operative Dentistry, Nihon University School of Dentistry, 1-8-13, Kanda-Surugadai, Chiyoda-Ku, Tokyo 101-8310, Japan
article info
abstract
Article history:
Objectives: The surface free energy and dentine bond strength of self-adhesive cements
Received 21 August 2011
were examined after the removal of temporary cements.
Received in revised form
Methods: The labial dentine surfaces of bovine mandibular incisors were wet ground with
11 November 2011
#600-grit SiC paper. Acrylic resin blocks were luted to the prepared dentine surfaces using
Accepted 14 November 2011
HY Bond Temporary Cement Hard (HY), IP Temp Cement (IP), Fuji TEMP (FT) or Freegenol Temporary Cement (TC), and stored for 1 week. After removal of the temporary cements with an ultrasonic tip, the contact angle values of five specimens per test group were
Keywords:
determined for the three test liquids, and the surface-energy parameters of the dentine
Bond strength
surfaces were calculated. The dentine bond strengths of the self-adhesive cements were
Self-adhesive cement
measured after removal of the temporary cements in a shear mode at a crosshead speed of
Surface free energy
1.0 mm/min. The data were subjected to one-way analysis of variance (ANOVA) followed by
Temporary cement
Tukey’s HSD test. Results: For all surfaces, the value of the estimated surface tension component g dS (dispersion) was relatively constant at 41.7–43.3 mJ m2. After removal of the temporary cements, the value of the g hS (hydrogen-bonding) component decreased, particularly with FT and TC. The dentine bond strength of the self-adhesive cements was significantly higher for those without temporary cement contamination (8.2–10.6 MPa) than for those with temporary cement contamination (4.3–7.1 MPa). Conclusions: The gS values decreased due to the decrease of g hS values for the temporary cement-contaminated dentine. Contamination with temporary cements led to lower dentine bond strength. Clinical significance: The presence of temporary cement interferes with the bonding performance of self-adhesive cements to dentine. Care should be taken in the methods of removal of temporary cement when using self-adhesive cements. # 2011 Elsevier Ltd. All rights reserved.
1.
Introduction
There is increasing demand for tooth-coloured indirect restorations for anterior and posterior lesions, which require
the formation of durable bonds between resin-based luting cements and dentine tissues to achieve clinical success.1,2 Recent attempts to simplify the pretreatment procedures for tooth surfaces have led to the development of self-adhesive resin cements.3,4 These are based on acidic functional
* Corresponding author. Tel.: +81 3 3219 8141; fax: +81 3 3219 8347. E-mail address: [email protected] (M. Miyazaki). 0300-5712/$ – see front matter # 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jdent.2011.11.012
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journal of dentistry 40 (2012) 131–138
monomers, fillers and initiator systems. Self-adhesive cements can bond to smear layer-covered dentine without pretreatment, thereby simplifying the clinical procedures. However, limited etching potential and superficial interactions with dentine have been reported for some self-adhesive cements.5–7 During the fabrication of indirect restorations, it is necessary to employ provisional restorations that are attached with temporary cement in order to avoid infection, tooth sensitivity and tooth movement. These temporary cements must be removed from the dentine surface prior to definitive cementation; however, it is difficult to eliminate all of the materials from the dentine surface.8 The presence of residual temporary cements and debris on prepared abutment teeth might have a negative effect on the performance of definitive self-adhesive cements by interfering with the penetration of adhesive monomers into the tooth substrate. Previous studies have examined the effects of the application of temporary cements on the bond strength of subsequent tooth-coloured restorations to dentine.9,10 Although bond strength data are available for dentine contaminated with temporary cements, little is known about the influence of the chemical characteristics of the adherend surface. The strength of the bond formed between the dentine and the luting cement depends on several factors, including the characteristics and wetting of the adherend surface.11,12 The wetting of the adherend surface by the luting cement can be evaluated using the contact angle. Measurements of the contact angle on the adherent surfaces provide information about the surface free energy in relation to the bonding characteristics of the solids.13 The surface free energy of a solid (gS) is defined as the sum of the dispersion, hydrogenbonding and polar forces.14 The dispersion force (g dS ) represents the London interactions between apolar molecules. The p polar (non-dispersion) force (g S ) represents the electric and metallic interactions, in addition to the dipolar interactions. The hydrogen-bonding force (g hS ) relates to the water and hydroxyl components. The hydration of the adherend surface is central to the wetting behaviour. It is therefore important to determine the polar interactions, including the dipole-bonding and hydrogen-bonding characteristics,15 for the interaction with water.
The current study examined the influence of temporary cement contamination on the surface free energy of dentine surfaces. The shear bond strength to bovine dentine was also examined for self-adhesive resin cements after removal of the temporary cements. The null hypothesis was that the surface free energy and dentine bond strength of self-adhesive resin cements were not affected by temporary cement contamination.
2.
Materials and methods
2.1.
Materials tested
The temporary cements used were HY Bond Temporary Cement Hard (HY) and IP Temp Cement (IP) from Shofu Inc. (Kyoto, Japan), and Fuji TEMP (FT) and Freegenol Temporary Cement (TC) from GC Corp. (Tokyo, Japan), as shown in Table 1. The self-adhesive cements used were Clearfil SA Luting (SA) from Kuraray Medical Inc. (Tokyo, Japan), and G-Luting (GL) and G-CEM (GC) from GC Corp., as shown in Table 2.
2.2.
Surface free-energy measurement
Mandibular incisors that had been extracted from cattle aged 2–3 years were used as a substitute for human teeth. After removing the roots using a slow-speed saw with a diamondimpregnated disk (Isomet; Buehler Ltd., Lake Bluff, USA), the pulp was removed, and the pulp chamber of each tooth was filled with cotton to avoid penetration of the embedding media. Each tooth was then mounted in self-curing acrylic resin (Tray Resin II; Shofu Inc.) to expose the labial surface of the dentine, and placed in tap water. The final finish was accomplished by grinding the surface with wet #600-grit silicon carbide paper. For the experimental groups, acrylic resin blocks simulating the provisional restorations were luted to the dentine surfaces using temporary cements. After immersion in distilled water at 37 8C for 1 week, the blocks were removed, and the surfaces were cleaned with an ultrasonic scaler (Piezon Master 600; Electro Medical Systems, Switzerland) and
Table 1 – Temporary cements used. Code
Resin cement
Main components
Manufacturer
HY
HY Bond Temporary Cement Hard (P: 100814, L: 110353)
Powder: zinc oxide, magnesia, silicon dioxide, HY agent, pigment Liquid: poly (acrylic acid-tricarboxylic acid) sodium salt, water
Shofu Inc. (Kyoto, Japan)
IP
IP Temp Cement (P: 110309, L: 110353)
Powder: zinc oxide, magnesia, silicon dioxide, S-PRG filler Liquid: poly (acrylic acid-tricarboxylic acid) sodium salt, water, phosphoric acid
Shofu Inc. (Kyoto, Japan)
FT
Fuji TEMP (1008191)
A: fluoro-alumino-silicate glass, water, glycerine, silicon dioxide, paraben B: barium sulphate, water, silicon dioxide, glycerine, polyacrylic acid, paraben
GC Corp. (Tokyo, Japan)
TC
Freegenol Temporary Cement (A: 0086, B: 0086)
Base: zinc oxide, vegetable oil, petrolatum Accelerator: ortho-ethoxybenzoic acid, carnauba wax, octanoic acid
GC Corp. (Tokyo, Japan)
S-PRG, surface pre-reacted glass-ionomer.
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Table 2 – Self-adhesive cements tested. Code
Resin cement
Main components
Manufacturer
SA
Clearfil SA Luting (0042AA)
Paste A: Bis-GMA, TEGDMA, MDP, other methacrylate monomers, silanated barium glass filler, silanated colloidal silica, dl-camphorquinone, benzoyl peroxide, others Paste B: Bis-GMA, other methacrylate monomers, silanated barium glass filler, silanated colloidal silica, surface treated sodium fluoride, accelerators, pigments
Kuraray Medical Inc. (Tokyo, Japan)
GL
G-Luting (1004221)
A: fluoro-alumino-silicate glass, UDMA, dimethacrylate, silicon dioxide, initiator B: silicon dioxide, UDMA, dimethacrylate, phosphoric ester monomer, initiator
GC Corp. (Tokyo, Japan)
GC
G-Cem (1008271)
Powder: fluoroaluminosilicate glass, initiators, pigments, others Liquid: methacrylate ester, 4-MET, phosphate ester monomer, purified water, silica nano filler, initiators, others
GC Corp. (Tokyo, Japan)
Bis-GMA, 2,2bis[4-(2-hydrogen-3-methacryloyloxypropoxy)phenyl]propane; TEGDMA, triethylene glycol dimethacrylate; MDP, 10-methacryloyloxydecyl dihydrogen phosphate; CQ, dl-camphorquinone; UDMA, di(methacryloxyethyl)trimethylhexamethylene diurethane; 4-MET, 4methacryloxyethyl trimellitic acid.
Table 3 – Surface free-energy values for test liquids (mN mS1). Liquid 1-Bromonaphthalene Diiodomethane Distilled water
p
g dL
gL
g hL
gL
Manufacturer
44.4 46.8 29.1
0.2 4.0 1.3
0.0 0.0 42.4
44.6 50.8 72.8
Wako Pure Chemical Industries Wako Pure Chemical Industries –
p
g dL , Dispersion force; g L , polar force; g hL , hydrogen-bonding force; gL, total free energy of liquid.
rinsed with water until they were free of materials visible to the naked eye. Specimens without temporary cement were employed as controls. Sample size for each group was five. The surface free energy of the dentine surface was determined by measuring the contact angle on the surface for each of the three test liquids, 1-bromonaphthalen, diiodomethane and distilled water, with known surfaceenergy parameters (Table 3). The surface free energy of the set cements was also determined. Automatic measurements of the contact angles were made using a Drop Master DM500 apparatus (Kyowa Interface Science, Saitama, Japan) fitted with a charge-coupled device (CCD) camera (Fig. 1). Sessile drops of each test liquid (1.0 ml) were dispensed with an 18-gauge Kateran needle to control the droplets at a certain size, and placed on the tooth surface. Drop images were acquired at 0.5 s after the test liquids were dispenced. For each test liquid, the equilibrium contact angle (u) was measured using the sessile-drop method for five specimens. All measurements were performed in a temperature controlled room at 23 1 8C with relative humidity at 50 5%. The surface-energy parameters of the solids were then determined based on the fundamental concepts of wetting.14 The Young– Dupre´ equation describes the work of adhesion for the solid (S) and the liquid (L) that are in contact (WSL), the interfacial free energy between the solid and the liquid (gSL), and the surface free energy of the liquid and solid (gL and gS, respectively), as follows: WSL ¼ g L þ g S g SL ¼ g L ð1 þ cos uÞ:
By extending the Fowkes equation, the gSL is expressed as 1=2
p p 1=2
g SL ¼ g L þ g S 2ðg dL g dS Þ 2ðg L g S Þ p p g L ¼ g dL þ g L þ g hL ; g S ¼ g dS þ g S þ g hS ;
1=2
2ðg hL g hS Þ
where gd, gp and gh are components of the surface free energy (g) arising from the dispersion force, the polar (permanent and induced) force and the hydrogen-bonding force, respectively. The u values were determined for the three test liquids, and the surface-energy parameters of the dentine surfaces were calculated based on the equations using add-on software and the interface measurement and analysis system (FAMAS; Kyowa Interface Science).
2.3.
Bond-strength test
The mandibular incisors were prepared as described for the surface free-energy measurement. A piece of double-sided adhesive tape, with a 4-mm-diameter hole, was firmly attached to define the bonded area of the dentine. A Teflon (Sanplatec Corp., Osaka, Japan) mould, 2.0 mm in height and 4.0 mm in diameter, was used to shape the cement and to hold it in place on the dentine surface. The mixed cement was condensed into the mould and light-cured for 30 s. The finished specimens were transferred to distilled water and stored at 37 8C for 24 h. Ten specimens per test group were tested in a shear mode using a shear knife-edge testing apparatus in an Instron testing machine (Type 5500R; Instron Corp., Canton, MA, USA) at a crosshead speed of 1.0 mm/min. The shear bond-strength values (in MPa) were calculated from the peak load at failure divided by the specimen surface area. After testing, the
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journal of dentistry 40 (2012) 131–138
Fig. 1 – TH was present across the entire dentine surfaces, IP was present only on parts of the surfaces, and TP and FT were largely absent, with the smear layer and smear plug visible.
specimens were examined under an optical microscope (SZH131; Olympus Ltd., Tokyo, Japan) at a magnification of 10 to define the location of the bond failure. The type of failure was determined based on the percentage of substrate-free material as adhesive failure, cohesive failure in composite or cohesive failure in dentine.
then transferred to a critical-point dryer. The surfaces were coated in a vacuum evaporator (Quick Coater Type SC-701; Sanyu Denshi Inc., Tokyo, Japan) with a thin film of gold (Au). The specimens were observed using a field-emission electronprobe SEM (ERA-8800FE; Elionix Ltd., Tokyo, Japan) at an accelerating voltage of 10 kV.
2.4.
2.5.
Scanning electron microscopy (SEM) observation
Temporary cement-contaminated dentine surfaces were observed under SEM. All of the SEM specimens were dehydrated in ascending concentrations of tert-butanol, and
Statistical analysis
The results were analysed by calculating the mean and standard deviation for each date obtained. A statistical analysis was performed to determine how the data were
Table 5 – Surface free energy and its components for set self-adhesive cements (mJ mS2).
Self-adhesive cement
γSd
γSp
γSh
SA
42.4 (0.5)
2.6 (1.2)
9.0 (2.5)
54.0 (1.6)
GL
41.7 (0.3)
2.9 (1.2)
12.5 (1.5)
57.1 (1.9)
GC
42.0 (0.5)
2.5 (1.1)
24.5 (1.4)
69.0 (2.2)
γS
N = 5, values in parenthesis indicate standard deviations. Values connected by horizontal lines showed no significant differences (P > 0.05). g dS , p Dispersion force; g S , polarity; g hS , hydrogen bonding; gS, total surface free energy.
journal of dentistry 40 (2012) 131–138
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Table 4 – Influence of temporary cement contamination on surface free energy and its components for dentine surfaces (mJ mS2).
Temporary cement
γSd
γSp
γSh
γS
Control
43.3 (0.7)
0.0 (0.0)
28.0 (2.5)
70.7 (3.1)
HY
42.3 (0.4)
1.9 (0.5)
20.5 (1.5)
64.7 (1.6)
IP
42.2 (0.5)
2.2 (0.4)
22.5 (1.4)
66.9 (1.6)
FT
42.1 (0.3)
2.0 (0.6)
9.4 (0.6)
54.5 (1.4)
TC
41.7 (0.3)
1.0 (0.5)
7.1 (1.1)
49.8 (1.0)
N = 5, values in parenthesis indicate standard deviations. Values connected by horizontal lines showed no significant differences (P > 0.05). g dS , p Dispersion force; g S , polarity; g hS , hydrogen bonding; gS, total surface free energy.
influenced by the temporary cement contamination. Also, differences in the surface free energy for the set self-adhesive cements were analysed. The data for each group were tested for homogeneity of variance using Bartlett’s test, and then subjected to one-way analysis of variance (ANOVA) followed by Tukey’s honestly significant difference (HSD) test at a = 0.05. The statistical analysis was carried out with the Sigma Stat software system (Ver. 3.1; SPSS Inc., Chicago, IL, USA).
3.
Results
Table 4 shows the surface free energy and its components after removal of the temporary cements. For all of the dentine surfaces, the g dS value remained relatively constant
(41.7–43.3 mJ m2). The gS value decreased due to the substantial decrease of the g hS value for the surfaces contaminated with temporary cements compared with the controls (without contamination). The gS value showed different responses depending upon the temporary cement used: dentine surfaces contaminated by HY and IP had significantly higher values than those contaminated with FT and TC. Table 5 shows the surface free energy and its components for the set cements. There were no significant differences amongst p the g dS and g S values for the cements tested. However, the gS value differed significantly amongst the cements (54.0 mJ m2 for SA, 57.1 mJ m2 for GL, and 69.0 mJ m2 for GC, respectively), due to the substantial differences in the g hS values. Table 6 shows the shear bond strength of the self-adhesive cements after removal of the temporary cements. The shear
Table 6 – Influence of temporary cement contamination on shear bond strength of self-adhesive cements to dentine.
Temporary cement
Self-adhesive cement SA
GL
GC
Control
10.6 (1.7) [8/2/0]
9.3 (2.2) [8/2/0]
7.3 (1.8) [10/0/0]
HY
7.1 (1.9) [8/2/0]
6.8 (1.3) [10/0/0]
5.8 (2.1) [10/0/0]
IP
7.4 (1.6) [10/0/0]
7.0 (1.4) [10/0/0]
6.1 (1.9) [10/0/0]
FT
6.5 (1.3) [10/0/0]
6.0 (2.1) [10/0/0]
4.7 (1.4) [10/0/0]
TC
6.1 (2.3) [10/0/0]
5.8 (2.0) [10/0/0]
4.3 (1.6) [10/0/0]
N = 10, values in parenthesis indicate standard deviations. Failure mode is given as [adhesive failure/cohesive failure in resin/cohesive failure in dentine]. Values connected by horizontal lines showed no significant differences (P > 0.05).
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journal of dentistry 40 (2012) 131–138
bond strength of the self-adhesive cements was significantly lower in specimens with temporary cement contamination (4.3–7.4 MPa) than in control specimens (8.2–7.4 MPa). SEM observation of the dentine surfaces revealed remnants of the temporary cements (Fig. 1). The amount of contamination observed differed amongst the temporary cements tested: TH was present across the entire dentine surfaces, IP was present only on parts of the surfaces, and TP and FT were largely absent, with the smear layer and smear plug visible.
4.
Discussion
The etching effect of self-adhesive cements is related to the acidic functional monomers; these interact with the mineral components of the tooth substrate, creating a bond via dentine demineralisation. The self-adhesive cements are heavily filled and highly viscous compared with the bonding agents, which limits their ability to infiltrate the demineralised dentine. Optimal wettability is important to enable self-adhesive cements to spread across the entire adherend surface and to establish adhesion. The strength of the bond between the dentine and the self-adhesive cement depends on several factors, including the characteristics and wettability of the adherend surface.12 The wetting of the adherend surface by self-adhesive cements can be evaluated using the contact angle.16 On the other hand, one must consider that the polymerised surface of the self-adhesive cements on which the surface energy characteristics were determined may be different from the polymer that forms the interface between adherend and polymerised self-adhesive cements. In the past several decades, numerous techniques have been used to measure the contact angle which were inspired by the idea of using Young’s equation.14 The derivation of Young’s equation assumes that the solid surface is smooth, homogeneous and rigid; it should also be chemically and physically inert with respect to the liquids to be employed. Ideally, according to Young’s equation, a unique contact angle is expected for a given system. In a real system, however, a range of contact angles is usually obtained instead. The upper limit of the range is the advancing contact angle, which is the contact angle found at the advancing edge of a liquid drop. The lower limit is the receding contact angle, which is the contact angle found at the receding edge. The difference between the advancing and receding contact angles is known as the contact angle hysteresis. Practically, all solid surfaces exhibit contact angle hysteresis and because of this hysteresis, the contact angle interpretation in terms of Young’s equation is contentious.17 Accordingly, it should be taken into account that not all the experimentally measured or observed contact angles are reliable and appropriate. Measurements of the contact angle on adherent surfaces provide information about the surface free energy that relate to the bonding characteristics of the solids.18–20 According to Hata et al.,14 the Fowkes equation for interfacial free energy can be extended to an interface that includes intermolecular interactions of polar and hydrogen bonding, as well as dispersion bonding. Contact angle data for three types of liquid, purely nonpolar (1-bromonaphthalene), polar (diiodomethane) and hydrogen bonded (water), were used in calculating the surface
free energy.21 The surface free energy of a solid (gS) is defined as the sum of the dispersion, hydrogen-bonding and polar forces. The dispersion force (g dS ) represents the London interactions p between apolar molecules. The polar (non-dispersion) force (g S ) represents the electric and metallic interactions, in addition to the dipolar interactions. The hydrogen-bonding force (g hS ), which relates to the water and hydroxyl components, was also calculated in the current study. Since the hydration of the adherend surface is central to the wetting behaviour in relation to dentine bonding, the polar interactions, including the dipolebonding and hydrogen-bonding characteristics, should be accurately estimated for the interaction with water. The current study revealed significant decreases in the gS value for all of the temporary cement-contaminated dentine surfaces tested, which were due to substantial decreases of the g hS value. The application of temporary cements has been reported as a potentially confounding variable that could adversely affect the bond strengths of resin luting cements. Some previous studies reported that the use of temporary cements, particularly those containing eugenol, negatively affected the bond strength between resin cements and dentine.22,23 However, other studies failed to observe a reduction in dentine bond strength.24 The current study demonstrated that the application of both eugenol-containing and eugenol-free temporary cements reduced the gS value. One reason for this finding was the difficulty of completely removing the temporary cements from the dentine surfaces. Residues of temporary materials might change the wettability of the dentine surface, leading to changes in the gS value. Various approaches have been tried for removing temporary materials from tooth surfaces, including dentine conditioners, air-borne particle abrasion, excavators and ultrasonic scalers.25 The present study employed a routine clinical procedure using an ultrasonic scaler to remove the temporary cement mechanically from the dentine surface. SEM observation of the dentine surfaces demonstrated that the procedure was unable to remove all traces of the temporary cements, especially TH and IP (Fig. 1). The residues of TH and IP were much thicker than those of TP and FT, and the latter showed significantly lower gS values. Components such as glycerine and vegetable oil might have been responsible for the decreased g hS values, reflected in the lower gS values, observed for TP and FT (Table 1). The reliability and validity of tensile and shear bond strength determinations of dentine-bonded interface have been questioned. Much of the research related to dentine bonding has been done in an attempt to assess the integrity and strength of the interfacial bond. Experimental approaches for measurement of adhesive bond strengths in dentistry have consisted primarily of tensile or shear bond strength determinations performed within a defined area in vitro. Although the testing procedures used are apparently similar, the results presented in different studies may differ tremendously, as discussed in detail later. Although it is well known that the coefficient of variation associated with such bond strength figures is rather high and commonly greater than 30%, the wide differences are slightly, but statistically significant. However, large variations in bond strength determinations and the lack of standardised laboratory test procedures have contributed to ambiguities in data interpretation.26
journal of dentistry 40 (2012) 131–138
The current in vitro study investigated the influence of temporary cements on the dentine bonding of self-adhesive cements. The bond strength of the self-adhesive cements was significantly lower in specimens with temporary cement contamination (4.3–7.4 MPa) than in control specimens (8.2– 7.4 MPa). The predominant mode was adhesive failure for all of the specimens tested. The results showed that temporary cement contamination resulted in significant decreases of the g hS value, leading to significant decreases of the gS value. A previous report found no correlation between bond strength and the calculated thermodynamic work of adhesion. This might have been due to the fact that the work of adhesion is a measure of energy, whereas the bond strength is a measure of stress. However, the relationship between surface free energy and bond strength allows for other explanations.19 After the application of self-adhesive cements to dentine surfaces, etching involving resin infiltration can occur along with removal of the smear layer. According to the adhesion– decalcification concept,27 the functional group ionically interacts with calcium in hydroxyapatite. Depending on the stability of the resulting calcium–monomer complex in the adhesive suspension, chemical bonding might lead to either decomposition or demineralisation of the tooth surface, and chemical bonding with calcium ions could form a functional monomer–calcium salt-layered structure on the hydroxyapatite surface.28 In general, temporary cements are applied over the entire smear layer and their ingredients leach into the dentinal tubules, contaminating the dentine surface. Temporary cement remnants might act as a barrier that inhibits the interactions between acidic functional monomers and inorganic components of dentine. Despite the significant decrease in the gS value, the dentine bond strength for GC contaminated by HY or IP did not significantly differ from that of the control. GC contains 4MET and phosphoric ester functional monomers, which ionically interact with enamel and dentine.29 It also contains water, fluoro-alumino-silicate glass and acidic monomers (Table 2). A setting reaction similar to that of glass-ionomer cements might therefore occur. Water in the cement composition is expected to aid the conditioning reaction, increasing the g hS value of the dentine surfaces. This might lead to dentine bond strength for HY and IP contaminated dentine surface convertible as bond strength for the control. Amongst the set cements, the gS value of GC was relatively high, as was the g hS value, which reflected the increasing surface density of the water-interactive Lewis (polar) sites on the dentine surfaces. The surface energy parameters were measured for set self-adhesive cements, whereas the unset material was applied to the dentine surface. Although the differences in the surface energy parameters of set and unset cements are not known, the components of the set cements were assumed to have similar surface energy characteristics to the unset cements from which they were formed.12 Theoretically, the phosphoric-acid esters of self-adhesive cements behave similarly, and need water to become ionised, and to acid-etch and interact with dentine.30 Water was not listed as a main component of SA or GL, and might have mainly derived from the interactions of phosphoric-acid groups and alkaline glass particles or tooth apatite. Intrinsic dentinal wetness might have optimised these acid–base reactions allowing better setting. However, concerns remain
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regarding the ability of these high-viscosity materials to etch through clinically relevant smear layers into the underlying dentine.31 The relatively weak bonding potential and the high viscosity of the mixed cements were expected to have a negative influence on the chemical reactions, resulting in relatively low bond strengths for the self-adhesive cements used in the current study. From the results of the present study, decreases in the gS value due to the substantial decrease of the g hS value for the temporary cement-contaminated dentine were observed. Bond strengths of the self-adhesive cements to temporary cement-contaminated dentine were lower than those of the control. Further research is needed to determine whether these findings are consistent with clinical performance.
Acknowledgements This work was supported, in part, by Grants-in-Aid for Scientific Research (C) 23592808 and 23592810 from the Japan Society for the Promotion of Science. This project was also supported, in part, by the Sato Fund and by a grant from the Dental Research Centre of Nihon University School of Dentistry, Japan.
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11. Rosales-Leal JI, Osorio R, Toledano M, Cabrerizo-Vı´lchez MA, Millstein PL. Influence of eugenol contamination on the wetting of ground and etched dentin. Operative Dentistry 2003;28:695–9. 12. Asmussen E, Peutzfeldt A, Sahafi A. Bonding of resin cements to post materials: influence of surface energy characteristics. Journal of Adhesive Dentistry 2005;7:231–4. 13. Comyn J. Contact angles and adhesive bonding. International Journal of Adhesion and Adhesives 1992;12:145–9. 14. Hata T, Kitazaki Y, Saito T. Estimation of the surface energy of polymer solids. Journal of Adhesion 1987;21:177–94. 15. Aguilar-Mendoza JA, Rosales-Leal JI, Rodrı´guez-Valverde MA, Gonza´lez-Lo´pez S, Cabrerizo-Vı´lchez MA. Wettability and bonding of self-etching dental adhesives. Influence of the smear layer. Dental Materials 2008;24:994–1000. ´ . Toledano M, 16. Wege HA, Aguilar JA, Rodrı´guez-Valverde MA ´ .. Dynamic contact angle Osorio R, Cabrerizo-Vı´lchez M.A and spreading rate measurements for the characterization of the effect of dentin surface treatments. Journal of Colloid and Interface Science 2003;263:162–9. 17. Chibowski E. On some relations between advancing, receding and Young’s contact angles. Advances in Colloid and Interface Science 2007;133:51–9. 18. Asmussen E, Peutzfeldt A. Resin composites: strength of the bond to dentin versus surface energy parameters. Dental Materials 2005;21:1039–43. 19. Bortolotto T, Ferrari M, Krejci I. Wetting ability of singlecomponent self-etching adhesives on enamel and dentin. American Journal of Dentistry 2008;21:59–63. 20. Tsujimoto A, Iwasa M, Shimamura Y, Murayama R, Takamizawa T, Miyazaki M. Enamel bonding of single-step self-etch adhesives: influence of surface energy characteristics. Journal of Dentistry 2010;38:123–30. 21. Combe EC, Owen BA, Hodges JS. A protocol for determining the surface free energy of dental materials. Dental Materials 2004;20:262–8.
22. Silva JPL, Queiroz DM, Azevedo LH, Leal LC, Rodrigues JL, Lima AF, et al. Effect of eugenol exposure time and post-removal delay on the bond strength of a self-etching adhesive to dentin. Operative Dentistry 2011;36:66–71. 23. Carvalho CN, de Oliveira Bauer JR, Loguercio AD, Reis A. Effect of ZOE temporary restoration on resin-dentin bond strength using different adhesive strategies. Journal of Esthetic and Restorative Dentistry 2007;19:144–52. 24. Peutzfeldt A, Asmussen E. Influence of eugenol-containing temporary cement on bonding of self-etching adhesives to dentin. Journal of Adhesive Dentistry 2006;8:31–4. 25. Yap AU, Shah KC, Loh ET, Sim SS, Tan CC. Influence of ZOE temporary restorations on microleakage in composite restorations. Operative Dentistry 2002;27:142–6. 26. Scherrer SS, Cesar PF, Swain MV. Direct comparison of the bond strength results of the different test methods: a critical literature review. Dental Materials 2010;26:e78–93. 27. Yoshioka M, Yoshida Y, Inoue S, Lambrechts P, Vanherle G, Nomura Y, et al. Adhesion/decalcification mechanisms of acid interactions with human hard tissues. Journal of Biomedical Materials Research 2002;59:56–62. 28. Yoshihara K, Yoshida Y, Hayakawa S, Nagaoka N, Torii Y, Osaka A, et al. Self-etch monomer-calcium salt deposition on dentin. Journal of Dental Research 2011;90:602–6. 29. Nagakane K, Yoshida Y, Hirata I, Fukuda R, Nakayama Y, Shirai K, et al. Analysis of chemical interaction of 4-MET with hydroxyapatite using XPS. Dental Materials Journal 2006;25:645–9. 30. Moszner N, Salz U, Zimmermann J. Chemical aspects of selfetching enamel-dentin adhesives: a systematic review. Dental Materials 2005;21:895–910. 31. Gerth HU, Dammaschke T, Zu¨chner H, Scha¨fer E. Chemical analysis and bonding reaction of RelyX Unicem and Bifix composites—a comparative study. Dental Materials 2006;22:934–41.
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Influence of temporary cement contamination on the surface free energy and dentine bond strength of self-adhesive cements Masayuki Takimoto, Ryo Ishii, Masayoshi Iino, Yusuke Shimizu, Akimasa Tsujimoto, Toshiki Takamizawa, Susumu Ando, Masashi Miyazaki * Department of Operative Dentistry, Nihon University School of Dentistry, 1-8-13, Kanda-Surugadai, Chiyoda-Ku, Tokyo 101-8310, Japan
article info
abstract
Article history:
Objectives: The surface free energy and dentine bond strength of self-adhesive cements
Received 21 August 2011
were examined after the removal of temporary cements.
Received in revised form
Methods: The labial dentine surfaces of bovine mandibular incisors were wet ground with
11 November 2011
#600-grit SiC paper. Acrylic resin blocks were luted to the prepared dentine surfaces using
Accepted 14 November 2011
HY Bond Temporary Cement Hard (HY), IP Temp Cement (IP), Fuji TEMP (FT) or Freegenol Temporary Cement (TC), and stored for 1 week. After removal of the temporary cements with an ultrasonic tip, the contact angle values of five specimens per test group were
Keywords:
determined for the three test liquids, and the surface-energy parameters of the dentine
Bond strength
surfaces were calculated. The dentine bond strengths of the self-adhesive cements were
Self-adhesive cement
measured after removal of the temporary cements in a shear mode at a crosshead speed of
Surface free energy
1.0 mm/min. The data were subjected to one-way analysis of variance (ANOVA) followed by
Temporary cement
Tukey’s HSD test. Results: For all surfaces, the value of the estimated surface tension component g dS (dispersion) was relatively constant at 41.7–43.3 mJ m2. After removal of the temporary cements, the value of the g hS (hydrogen-bonding) component decreased, particularly with FT and TC. The dentine bond strength of the self-adhesive cements was significantly higher for those without temporary cement contamination (8.2–10.6 MPa) than for those with temporary cement contamination (4.3–7.1 MPa). Conclusions: The gS values decreased due to the decrease of g hS values for the temporary cement-contaminated dentine. Contamination with temporary cements led to lower dentine bond strength. Clinical significance: The presence of temporary cement interferes with the bonding performance of self-adhesive cements to dentine. Care should be taken in the methods of removal of temporary cement when using self-adhesive cements. # 2011 Elsevier Ltd. All rights reserved.
1.
Introduction
There is increasing demand for tooth-coloured indirect restorations for anterior and posterior lesions, which require
the formation of durable bonds between resin-based luting cements and dentine tissues to achieve clinical success.1,2 Recent attempts to simplify the pretreatment procedures for tooth surfaces have led to the development of self-adhesive resin cements.3,4 These are based on acidic functional
* Corresponding author. Tel.: +81 3 3219 8141; fax: +81 3 3219 8347. E-mail address: [email protected] (M. Miyazaki). 0300-5712/$ – see front matter # 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jdent.2011.11.012
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journal of dentistry 40 (2012) 131–138
monomers, fillers and initiator systems. Self-adhesive cements can bond to smear layer-covered dentine without pretreatment, thereby simplifying the clinical procedures. However, limited etching potential and superficial interactions with dentine have been reported for some self-adhesive cements.5–7 During the fabrication of indirect restorations, it is necessary to employ provisional restorations that are attached with temporary cement in order to avoid infection, tooth sensitivity and tooth movement. These temporary cements must be removed from the dentine surface prior to definitive cementation; however, it is difficult to eliminate all of the materials from the dentine surface.8 The presence of residual temporary cements and debris on prepared abutment teeth might have a negative effect on the performance of definitive self-adhesive cements by interfering with the penetration of adhesive monomers into the tooth substrate. Previous studies have examined the effects of the application of temporary cements on the bond strength of subsequent tooth-coloured restorations to dentine.9,10 Although bond strength data are available for dentine contaminated with temporary cements, little is known about the influence of the chemical characteristics of the adherend surface. The strength of the bond formed between the dentine and the luting cement depends on several factors, including the characteristics and wetting of the adherend surface.11,12 The wetting of the adherend surface by the luting cement can be evaluated using the contact angle. Measurements of the contact angle on the adherent surfaces provide information about the surface free energy in relation to the bonding characteristics of the solids.13 The surface free energy of a solid (gS) is defined as the sum of the dispersion, hydrogenbonding and polar forces.14 The dispersion force (g dS ) represents the London interactions between apolar molecules. The p polar (non-dispersion) force (g S ) represents the electric and metallic interactions, in addition to the dipolar interactions. The hydrogen-bonding force (g hS ) relates to the water and hydroxyl components. The hydration of the adherend surface is central to the wetting behaviour. It is therefore important to determine the polar interactions, including the dipole-bonding and hydrogen-bonding characteristics,15 for the interaction with water.
The current study examined the influence of temporary cement contamination on the surface free energy of dentine surfaces. The shear bond strength to bovine dentine was also examined for self-adhesive resin cements after removal of the temporary cements. The null hypothesis was that the surface free energy and dentine bond strength of self-adhesive resin cements were not affected by temporary cement contamination.
2.
Materials and methods
2.1.
Materials tested
The temporary cements used were HY Bond Temporary Cement Hard (HY) and IP Temp Cement (IP) from Shofu Inc. (Kyoto, Japan), and Fuji TEMP (FT) and Freegenol Temporary Cement (TC) from GC Corp. (Tokyo, Japan), as shown in Table 1. The self-adhesive cements used were Clearfil SA Luting (SA) from Kuraray Medical Inc. (Tokyo, Japan), and G-Luting (GL) and G-CEM (GC) from GC Corp., as shown in Table 2.
2.2.
Surface free-energy measurement
Mandibular incisors that had been extracted from cattle aged 2–3 years were used as a substitute for human teeth. After removing the roots using a slow-speed saw with a diamondimpregnated disk (Isomet; Buehler Ltd., Lake Bluff, USA), the pulp was removed, and the pulp chamber of each tooth was filled with cotton to avoid penetration of the embedding media. Each tooth was then mounted in self-curing acrylic resin (Tray Resin II; Shofu Inc.) to expose the labial surface of the dentine, and placed in tap water. The final finish was accomplished by grinding the surface with wet #600-grit silicon carbide paper. For the experimental groups, acrylic resin blocks simulating the provisional restorations were luted to the dentine surfaces using temporary cements. After immersion in distilled water at 37 8C for 1 week, the blocks were removed, and the surfaces were cleaned with an ultrasonic scaler (Piezon Master 600; Electro Medical Systems, Switzerland) and
Table 1 – Temporary cements used. Code
Resin cement
Main components
Manufacturer
HY
HY Bond Temporary Cement Hard (P: 100814, L: 110353)
Powder: zinc oxide, magnesia, silicon dioxide, HY agent, pigment Liquid: poly (acrylic acid-tricarboxylic acid) sodium salt, water
Shofu Inc. (Kyoto, Japan)
IP
IP Temp Cement (P: 110309, L: 110353)
Powder: zinc oxide, magnesia, silicon dioxide, S-PRG filler Liquid: poly (acrylic acid-tricarboxylic acid) sodium salt, water, phosphoric acid
Shofu Inc. (Kyoto, Japan)
FT
Fuji TEMP (1008191)
A: fluoro-alumino-silicate glass, water, glycerine, silicon dioxide, paraben B: barium sulphate, water, silicon dioxide, glycerine, polyacrylic acid, paraben
GC Corp. (Tokyo, Japan)
TC
Freegenol Temporary Cement (A: 0086, B: 0086)
Base: zinc oxide, vegetable oil, petrolatum Accelerator: ortho-ethoxybenzoic acid, carnauba wax, octanoic acid
GC Corp. (Tokyo, Japan)
S-PRG, surface pre-reacted glass-ionomer.
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journal of dentistry 40 (2012) 131–138
Table 2 – Self-adhesive cements tested. Code
Resin cement
Main components
Manufacturer
SA
Clearfil SA Luting (0042AA)
Paste A: Bis-GMA, TEGDMA, MDP, other methacrylate monomers, silanated barium glass filler, silanated colloidal silica, dl-camphorquinone, benzoyl peroxide, others Paste B: Bis-GMA, other methacrylate monomers, silanated barium glass filler, silanated colloidal silica, surface treated sodium fluoride, accelerators, pigments
Kuraray Medical Inc. (Tokyo, Japan)
GL
G-Luting (1004221)
A: fluoro-alumino-silicate glass, UDMA, dimethacrylate, silicon dioxide, initiator B: silicon dioxide, UDMA, dimethacrylate, phosphoric ester monomer, initiator
GC Corp. (Tokyo, Japan)
GC
G-Cem (1008271)
Powder: fluoroaluminosilicate glass, initiators, pigments, others Liquid: methacrylate ester, 4-MET, phosphate ester monomer, purified water, silica nano filler, initiators, others
GC Corp. (Tokyo, Japan)
Bis-GMA, 2,2bis[4-(2-hydrogen-3-methacryloyloxypropoxy)phenyl]propane; TEGDMA, triethylene glycol dimethacrylate; MDP, 10-methacryloyloxydecyl dihydrogen phosphate; CQ, dl-camphorquinone; UDMA, di(methacryloxyethyl)trimethylhexamethylene diurethane; 4-MET, 4methacryloxyethyl trimellitic acid.
Table 3 – Surface free-energy values for test liquids (mN mS1). Liquid 1-Bromonaphthalene Diiodomethane Distilled water
p
g dL
gL
g hL
gL
Manufacturer
44.4 46.8 29.1
0.2 4.0 1.3
0.0 0.0 42.4
44.6 50.8 72.8
Wako Pure Chemical Industries Wako Pure Chemical Industries –
p
g dL , Dispersion force; g L , polar force; g hL , hydrogen-bonding force; gL, total free energy of liquid.
rinsed with water until they were free of materials visible to the naked eye. Specimens without temporary cement were employed as controls. Sample size for each group was five. The surface free energy of the dentine surface was determined by measuring the contact angle on the surface for each of the three test liquids, 1-bromonaphthalen, diiodomethane and distilled water, with known surfaceenergy parameters (Table 3). The surface free energy of the set cements was also determined. Automatic measurements of the contact angles were made using a Drop Master DM500 apparatus (Kyowa Interface Science, Saitama, Japan) fitted with a charge-coupled device (CCD) camera (Fig. 1). Sessile drops of each test liquid (1.0 ml) were dispensed with an 18-gauge Kateran needle to control the droplets at a certain size, and placed on the tooth surface. Drop images were acquired at 0.5 s after the test liquids were dispenced. For each test liquid, the equilibrium contact angle (u) was measured using the sessile-drop method for five specimens. All measurements were performed in a temperature controlled room at 23 1 8C with relative humidity at 50 5%. The surface-energy parameters of the solids were then determined based on the fundamental concepts of wetting.14 The Young– Dupre´ equation describes the work of adhesion for the solid (S) and the liquid (L) that are in contact (WSL), the interfacial free energy between the solid and the liquid (gSL), and the surface free energy of the liquid and solid (gL and gS, respectively), as follows: WSL ¼ g L þ g S g SL ¼ g L ð1 þ cos uÞ:
By extending the Fowkes equation, the gSL is expressed as 1=2
p p 1=2
g SL ¼ g L þ g S 2ðg dL g dS Þ 2ðg L g S Þ p p g L ¼ g dL þ g L þ g hL ; g S ¼ g dS þ g S þ g hS ;
1=2
2ðg hL g hS Þ
where gd, gp and gh are components of the surface free energy (g) arising from the dispersion force, the polar (permanent and induced) force and the hydrogen-bonding force, respectively. The u values were determined for the three test liquids, and the surface-energy parameters of the dentine surfaces were calculated based on the equations using add-on software and the interface measurement and analysis system (FAMAS; Kyowa Interface Science).
2.3.
Bond-strength test
The mandibular incisors were prepared as described for the surface free-energy measurement. A piece of double-sided adhesive tape, with a 4-mm-diameter hole, was firmly attached to define the bonded area of the dentine. A Teflon (Sanplatec Corp., Osaka, Japan) mould, 2.0 mm in height and 4.0 mm in diameter, was used to shape the cement and to hold it in place on the dentine surface. The mixed cement was condensed into the mould and light-cured for 30 s. The finished specimens were transferred to distilled water and stored at 37 8C for 24 h. Ten specimens per test group were tested in a shear mode using a shear knife-edge testing apparatus in an Instron testing machine (Type 5500R; Instron Corp., Canton, MA, USA) at a crosshead speed of 1.0 mm/min. The shear bond-strength values (in MPa) were calculated from the peak load at failure divided by the specimen surface area. After testing, the
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Fig. 1 – TH was present across the entire dentine surfaces, IP was present only on parts of the surfaces, and TP and FT were largely absent, with the smear layer and smear plug visible.
specimens were examined under an optical microscope (SZH131; Olympus Ltd., Tokyo, Japan) at a magnification of 10 to define the location of the bond failure. The type of failure was determined based on the percentage of substrate-free material as adhesive failure, cohesive failure in composite or cohesive failure in dentine.
then transferred to a critical-point dryer. The surfaces were coated in a vacuum evaporator (Quick Coater Type SC-701; Sanyu Denshi Inc., Tokyo, Japan) with a thin film of gold (Au). The specimens were observed using a field-emission electronprobe SEM (ERA-8800FE; Elionix Ltd., Tokyo, Japan) at an accelerating voltage of 10 kV.
2.4.
2.5.
Scanning electron microscopy (SEM) observation
Temporary cement-contaminated dentine surfaces were observed under SEM. All of the SEM specimens were dehydrated in ascending concentrations of tert-butanol, and
Statistical analysis
The results were analysed by calculating the mean and standard deviation for each date obtained. A statistical analysis was performed to determine how the data were
Table 5 – Surface free energy and its components for set self-adhesive cements (mJ mS2).
Self-adhesive cement
γSd
γSp
γSh
SA
42.4 (0.5)
2.6 (1.2)
9.0 (2.5)
54.0 (1.6)
GL
41.7 (0.3)
2.9 (1.2)
12.5 (1.5)
57.1 (1.9)
GC
42.0 (0.5)
2.5 (1.1)
24.5 (1.4)
69.0 (2.2)
γS
N = 5, values in parenthesis indicate standard deviations. Values connected by horizontal lines showed no significant differences (P > 0.05). g dS , p Dispersion force; g S , polarity; g hS , hydrogen bonding; gS, total surface free energy.
journal of dentistry 40 (2012) 131–138
135
Table 4 – Influence of temporary cement contamination on surface free energy and its components for dentine surfaces (mJ mS2).
Temporary cement
γSd
γSp
γSh
γS
Control
43.3 (0.7)
0.0 (0.0)
28.0 (2.5)
70.7 (3.1)
HY
42.3 (0.4)
1.9 (0.5)
20.5 (1.5)
64.7 (1.6)
IP
42.2 (0.5)
2.2 (0.4)
22.5 (1.4)
66.9 (1.6)
FT
42.1 (0.3)
2.0 (0.6)
9.4 (0.6)
54.5 (1.4)
TC
41.7 (0.3)
1.0 (0.5)
7.1 (1.1)
49.8 (1.0)
N = 5, values in parenthesis indicate standard deviations. Values connected by horizontal lines showed no significant differences (P > 0.05). g dS , p Dispersion force; g S , polarity; g hS , hydrogen bonding; gS, total surface free energy.
influenced by the temporary cement contamination. Also, differences in the surface free energy for the set self-adhesive cements were analysed. The data for each group were tested for homogeneity of variance using Bartlett’s test, and then subjected to one-way analysis of variance (ANOVA) followed by Tukey’s honestly significant difference (HSD) test at a = 0.05. The statistical analysis was carried out with the Sigma Stat software system (Ver. 3.1; SPSS Inc., Chicago, IL, USA).
3.
Results
Table 4 shows the surface free energy and its components after removal of the temporary cements. For all of the dentine surfaces, the g dS value remained relatively constant
(41.7–43.3 mJ m2). The gS value decreased due to the substantial decrease of the g hS value for the surfaces contaminated with temporary cements compared with the controls (without contamination). The gS value showed different responses depending upon the temporary cement used: dentine surfaces contaminated by HY and IP had significantly higher values than those contaminated with FT and TC. Table 5 shows the surface free energy and its components for the set cements. There were no significant differences amongst p the g dS and g S values for the cements tested. However, the gS value differed significantly amongst the cements (54.0 mJ m2 for SA, 57.1 mJ m2 for GL, and 69.0 mJ m2 for GC, respectively), due to the substantial differences in the g hS values. Table 6 shows the shear bond strength of the self-adhesive cements after removal of the temporary cements. The shear
Table 6 – Influence of temporary cement contamination on shear bond strength of self-adhesive cements to dentine.
Temporary cement
Self-adhesive cement SA
GL
GC
Control
10.6 (1.7) [8/2/0]
9.3 (2.2) [8/2/0]
7.3 (1.8) [10/0/0]
HY
7.1 (1.9) [8/2/0]
6.8 (1.3) [10/0/0]
5.8 (2.1) [10/0/0]
IP
7.4 (1.6) [10/0/0]
7.0 (1.4) [10/0/0]
6.1 (1.9) [10/0/0]
FT
6.5 (1.3) [10/0/0]
6.0 (2.1) [10/0/0]
4.7 (1.4) [10/0/0]
TC
6.1 (2.3) [10/0/0]
5.8 (2.0) [10/0/0]
4.3 (1.6) [10/0/0]
N = 10, values in parenthesis indicate standard deviations. Failure mode is given as [adhesive failure/cohesive failure in resin/cohesive failure in dentine]. Values connected by horizontal lines showed no significant differences (P > 0.05).
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journal of dentistry 40 (2012) 131–138
bond strength of the self-adhesive cements was significantly lower in specimens with temporary cement contamination (4.3–7.4 MPa) than in control specimens (8.2–7.4 MPa). SEM observation of the dentine surfaces revealed remnants of the temporary cements (Fig. 1). The amount of contamination observed differed amongst the temporary cements tested: TH was present across the entire dentine surfaces, IP was present only on parts of the surfaces, and TP and FT were largely absent, with the smear layer and smear plug visible.
4.
Discussion
The etching effect of self-adhesive cements is related to the acidic functional monomers; these interact with the mineral components of the tooth substrate, creating a bond via dentine demineralisation. The self-adhesive cements are heavily filled and highly viscous compared with the bonding agents, which limits their ability to infiltrate the demineralised dentine. Optimal wettability is important to enable self-adhesive cements to spread across the entire adherend surface and to establish adhesion. The strength of the bond between the dentine and the self-adhesive cement depends on several factors, including the characteristics and wettability of the adherend surface.12 The wetting of the adherend surface by self-adhesive cements can be evaluated using the contact angle.16 On the other hand, one must consider that the polymerised surface of the self-adhesive cements on which the surface energy characteristics were determined may be different from the polymer that forms the interface between adherend and polymerised self-adhesive cements. In the past several decades, numerous techniques have been used to measure the contact angle which were inspired by the idea of using Young’s equation.14 The derivation of Young’s equation assumes that the solid surface is smooth, homogeneous and rigid; it should also be chemically and physically inert with respect to the liquids to be employed. Ideally, according to Young’s equation, a unique contact angle is expected for a given system. In a real system, however, a range of contact angles is usually obtained instead. The upper limit of the range is the advancing contact angle, which is the contact angle found at the advancing edge of a liquid drop. The lower limit is the receding contact angle, which is the contact angle found at the receding edge. The difference between the advancing and receding contact angles is known as the contact angle hysteresis. Practically, all solid surfaces exhibit contact angle hysteresis and because of this hysteresis, the contact angle interpretation in terms of Young’s equation is contentious.17 Accordingly, it should be taken into account that not all the experimentally measured or observed contact angles are reliable and appropriate. Measurements of the contact angle on adherent surfaces provide information about the surface free energy that relate to the bonding characteristics of the solids.18–20 According to Hata et al.,14 the Fowkes equation for interfacial free energy can be extended to an interface that includes intermolecular interactions of polar and hydrogen bonding, as well as dispersion bonding. Contact angle data for three types of liquid, purely nonpolar (1-bromonaphthalene), polar (diiodomethane) and hydrogen bonded (water), were used in calculating the surface
free energy.21 The surface free energy of a solid (gS) is defined as the sum of the dispersion, hydrogen-bonding and polar forces. The dispersion force (g dS ) represents the London interactions p between apolar molecules. The polar (non-dispersion) force (g S ) represents the electric and metallic interactions, in addition to the dipolar interactions. The hydrogen-bonding force (g hS ), which relates to the water and hydroxyl components, was also calculated in the current study. Since the hydration of the adherend surface is central to the wetting behaviour in relation to dentine bonding, the polar interactions, including the dipolebonding and hydrogen-bonding characteristics, should be accurately estimated for the interaction with water. The current study revealed significant decreases in the gS value for all of the temporary cement-contaminated dentine surfaces tested, which were due to substantial decreases of the g hS value. The application of temporary cements has been reported as a potentially confounding variable that could adversely affect the bond strengths of resin luting cements. Some previous studies reported that the use of temporary cements, particularly those containing eugenol, negatively affected the bond strength between resin cements and dentine.22,23 However, other studies failed to observe a reduction in dentine bond strength.24 The current study demonstrated that the application of both eugenol-containing and eugenol-free temporary cements reduced the gS value. One reason for this finding was the difficulty of completely removing the temporary cements from the dentine surfaces. Residues of temporary materials might change the wettability of the dentine surface, leading to changes in the gS value. Various approaches have been tried for removing temporary materials from tooth surfaces, including dentine conditioners, air-borne particle abrasion, excavators and ultrasonic scalers.25 The present study employed a routine clinical procedure using an ultrasonic scaler to remove the temporary cement mechanically from the dentine surface. SEM observation of the dentine surfaces demonstrated that the procedure was unable to remove all traces of the temporary cements, especially TH and IP (Fig. 1). The residues of TH and IP were much thicker than those of TP and FT, and the latter showed significantly lower gS values. Components such as glycerine and vegetable oil might have been responsible for the decreased g hS values, reflected in the lower gS values, observed for TP and FT (Table 1). The reliability and validity of tensile and shear bond strength determinations of dentine-bonded interface have been questioned. Much of the research related to dentine bonding has been done in an attempt to assess the integrity and strength of the interfacial bond. Experimental approaches for measurement of adhesive bond strengths in dentistry have consisted primarily of tensile or shear bond strength determinations performed within a defined area in vitro. Although the testing procedures used are apparently similar, the results presented in different studies may differ tremendously, as discussed in detail later. Although it is well known that the coefficient of variation associated with such bond strength figures is rather high and commonly greater than 30%, the wide differences are slightly, but statistically significant. However, large variations in bond strength determinations and the lack of standardised laboratory test procedures have contributed to ambiguities in data interpretation.26
journal of dentistry 40 (2012) 131–138
The current in vitro study investigated the influence of temporary cements on the dentine bonding of self-adhesive cements. The bond strength of the self-adhesive cements was significantly lower in specimens with temporary cement contamination (4.3–7.4 MPa) than in control specimens (8.2– 7.4 MPa). The predominant mode was adhesive failure for all of the specimens tested. The results showed that temporary cement contamination resulted in significant decreases of the g hS value, leading to significant decreases of the gS value. A previous report found no correlation between bond strength and the calculated thermodynamic work of adhesion. This might have been due to the fact that the work of adhesion is a measure of energy, whereas the bond strength is a measure of stress. However, the relationship between surface free energy and bond strength allows for other explanations.19 After the application of self-adhesive cements to dentine surfaces, etching involving resin infiltration can occur along with removal of the smear layer. According to the adhesion– decalcification concept,27 the functional group ionically interacts with calcium in hydroxyapatite. Depending on the stability of the resulting calcium–monomer complex in the adhesive suspension, chemical bonding might lead to either decomposition or demineralisation of the tooth surface, and chemical bonding with calcium ions could form a functional monomer–calcium salt-layered structure on the hydroxyapatite surface.28 In general, temporary cements are applied over the entire smear layer and their ingredients leach into the dentinal tubules, contaminating the dentine surface. Temporary cement remnants might act as a barrier that inhibits the interactions between acidic functional monomers and inorganic components of dentine. Despite the significant decrease in the gS value, the dentine bond strength for GC contaminated by HY or IP did not significantly differ from that of the control. GC contains 4MET and phosphoric ester functional monomers, which ionically interact with enamel and dentine.29 It also contains water, fluoro-alumino-silicate glass and acidic monomers (Table 2). A setting reaction similar to that of glass-ionomer cements might therefore occur. Water in the cement composition is expected to aid the conditioning reaction, increasing the g hS value of the dentine surfaces. This might lead to dentine bond strength for HY and IP contaminated dentine surface convertible as bond strength for the control. Amongst the set cements, the gS value of GC was relatively high, as was the g hS value, which reflected the increasing surface density of the water-interactive Lewis (polar) sites on the dentine surfaces. The surface energy parameters were measured for set self-adhesive cements, whereas the unset material was applied to the dentine surface. Although the differences in the surface energy parameters of set and unset cements are not known, the components of the set cements were assumed to have similar surface energy characteristics to the unset cements from which they were formed.12 Theoretically, the phosphoric-acid esters of self-adhesive cements behave similarly, and need water to become ionised, and to acid-etch and interact with dentine.30 Water was not listed as a main component of SA or GL, and might have mainly derived from the interactions of phosphoric-acid groups and alkaline glass particles or tooth apatite. Intrinsic dentinal wetness might have optimised these acid–base reactions allowing better setting. However, concerns remain
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regarding the ability of these high-viscosity materials to etch through clinically relevant smear layers into the underlying dentine.31 The relatively weak bonding potential and the high viscosity of the mixed cements were expected to have a negative influence on the chemical reactions, resulting in relatively low bond strengths for the self-adhesive cements used in the current study. From the results of the present study, decreases in the gS value due to the substantial decrease of the g hS value for the temporary cement-contaminated dentine were observed. Bond strengths of the self-adhesive cements to temporary cement-contaminated dentine were lower than those of the control. Further research is needed to determine whether these findings are consistent with clinical performance.
Acknowledgements This work was supported, in part, by Grants-in-Aid for Scientific Research (C) 23592808 and 23592810 from the Japan Society for the Promotion of Science. This project was also supported, in part, by the Sato Fund and by a grant from the Dental Research Centre of Nihon University School of Dentistry, Japan.
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