10-methacryloyloxydecyldihydrogenphosphate (MDP) containing primers improve bonding of yttria-stabilised tetragonal zirconia (Y-TZP) to methacrylate resins. The present study investigated the role played by water in the deterioration of MDP-mediated zirconia-resin bonds.
Grit-blasted Y-TZP plates were conditioned with two MDP primers and bonded with resin for shear bond strength (SBS) testing. Additional bonded plates were aged hydrothermally and compared with unaged Y-TZP after 24 h of water-storage or 6 months of water/acid/alkali-storage. The monoclinic phase ( m –ZrO 2 ) in different groups was determined by X-ray diffraction. Hydrolytic stability of the coordinate bond between MDP and zirconia in neutral/acid/alkaline environment was analysed using thermodynamic calculations. Microleakage and release of the element phosphorus from MDP-mediated Y-TZP/resin-bonded interfaces were evaluated via methylene blue dye infiltration and inductively coupled plasma-mass spectrometry (ICP-MS).
Hydrothermal ageing did not significantly alter SBS. Ageing in acidic or neutral medium led to significant decline in SBS. The m –ZrO 2 phase increased after hydrothermal ageing but no m –ZrO 2 was detected in the water/acid/alkali-aged specimens. A higher equilibrium constant was identified in the MDP– t –ZrO 2 complex when compared with the MDP– m –ZrO 2 complex. MDP-conditioning failed to prevent infiltration of the methylene blue dye. Phosphorus was detected by ICP-MS from the solutions used for soaking the resin-bonded specimens.
Hydrolysis of the coordinate bond between MDP and ZrO 2 , rather than t → m phase transformation, weakens the bond integrity between MDP-conditioned Y-TZP and methacrylate resin.
Hydrolysis of the coordinate bond between MDP and zirconia is responsible for deterioration of the integrity of the bond between MDP-conditioned Y-TZP and methacrylate resin.
Yttrium-stabilised tetragonal zirconia (Y-TZP) is widely used in medicinal sciences because of its strength, toughness and fatigue resistance. Because Y-TZP is chemically inert and lacks hydroxyl groups, conventional strategies employed for silica-based ceramics are not adept at improving the bonding of methacrylate resins to Y-TZP . Over the past two decades, tribochemical silica coating followed by silanisation has been used almost exclusively for bonding Y-TZP, and has been considered as the gold standard for the evaluation of novel techniques to improve the bonding of Y-TZP . The tribochemical silica coating technique abrades the Y-TZP surface with silica-coated alumina particles, thereby embedding and/or coating the Y-TZP surface with silica. This process not only provides a silica-rich surface for silanisation, but also provides micromechanical retention. Because the technique involves multiple steps and is device-dependence, it is not ideal for clinical use. In addition, the stresses and local high temperature produced during the blasting procedure induce transformation of the metastable tetragonal (t) zirconia phase into the more stable monoclinic (m) structure . This transformation, in turn, results in deterioration of long-term mechanical properties. These factors provide the motivation to develop alternative methods to improve resin bonding to Y-TZP.
The phosphate ester resin monomer, 10-methacryloyloxydecyldihydrogenphosphate (MDP) has been used to condition Y-TZP surfaces for improving bonding of methacrylate resins . The MDP molecule possesses a functional phosphoric acid end that can bond chemically with zirconia. The chemical reaction between MDP and zirconia has been well established via Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS) and thermodynamic methods . The chemical affinity of MDP for zirconia is effective at normal pressure and temperature, and may be achieved via a brief contact period. Clinical studies have demonstrated long-term durable bonding of grit-blasted zirconia restorations when bonded to enamel with MDP-containing resins . Many in vitro studies reported that the presence of MDP in the resin luting cements creates a stable and strong bond to grit-blasted Y-TZP, before and after thermocycling . However, in vitro studies also showed that mechanical roughening should be performed prior to MDP conditioning to provide stronger and more durable bonding . Both water storage and thermocycling have been commonly used to examine the ageing resistance of zirconia-resin bonds. According to a meta-analysis, water-induced degradation of the zirconia-resin bond contributes more to bond degradation than temperature-induced changes . Hence, it is speculated that water plays a key role in decreasing the bond durability of MDP-mediated zirconia-resin bonds. Although water sorption caused degradation of the resin-based cements employed for luting restorations , it is not clear why MDP conditioning resulted in reduction of ageing resistance in the absence of mechanical pre-treatment, even when same resin cements were used .
To date, “incised” and “emboss” techniques such as grit-blasting, laser irradiation, hot acid etching, selective infiltration-etching, argon-ion bombardment, piranha solution etching , and sintered nano-alumina coating have been developed to create a roughened Y-TZP surface for bonding. Among these techniques, grit-blasting is still the most commonly used in the clinical setting. Unfortunately, “incised” techniques could lead to the formation of microcracks, loss of substance, or t → m zirconia phase transformation during the invasive procedure, while “emboss” techniques might influence the marginal fitness. To circumvent these issues, there is continuous quest for “no damage” bonding of methacrylate resins to zirconia to replace pre-roughening protocols.
Because water is ubiquitous in the oral cavity, it is important to determine whether water weakens the zirconia-resin bond. The effect of water on the stability of zirconia-resin bonds may be considered from different perspectives. First, if the ability of MDP to improve resin bonding of Y-TZP based on its chemical affinity to zirconia, hydrolytic degradation of this chemical bond would decrease the long-term effectiveness of MDP-mediated Y-TZP resin bonds. Second, directly exposure of Y-TZP restorations to the moist oral environment and mechanical stimuli would precipitate t → m phase transformation . This phenomenon has been attributed to the leaching of yttrium via water sorption, which, in turn, destabilise the tetragonal grain structure . Water expedites the t → m phase transformation of zirconia crystals. If the coordinate bond MDP– m –ZrO 2 is not as strong as MDP– t –ZrO 2 , resin bonding of MDP-conditioned Y-TZP will be weakened by t → m phase transformation. Likewise, if the hydrolytic stability of MDP– m –ZrO 2 is weaker than MDP– t –ZrO 2 , t → m phase transformation after water ageing will decrease the durability of MDP-mediated Y-TZP-resin bonds. To date, it is not known whether it is the water-induced t → m phase transformation, or hydrolysis of the Zr-O-P coordinate bond, that contributes to the long-term degradation of the zirconia-resin bond in the oral environment. Accordingly, the present study was designed to understand the role played by water in weakening the bond integrity between of MDP-conditioned Y-TZP and methacrylate resin. Three alternative hypotheses were tested: 1) t → m phase transformation produces a weaker MDP–ZrO 2 coordinate bond that is responsible for the degradation of MDP-mediated Y-TZP-resin bonds; 2) t → m phase transformation does not weaken the MDP–ZrO 2 bond but adversely affects its hydrolytic stability; 3) hydrolysis of the Zr-O-P bond, rather than t → m phase transformation, is responsible for bond deterioration in resin-bonded MDP-conditioned Y-TZP.
Materials and methods
Leakage and inductively coupled plasma-mass spectrometry
Thirty-six plates (10 × 10 × 1 mm 3 ) sectioned from machinable Y-TZP blocks (KaVo Everest, ZS-Ronde; Kaltenbach and Voigt GmbH, Biberach, Germany) were sintered according to the manufacturer’s instructions. The bonding surface of Y-TZP plate was blasted with 50 μm alumina particles from a distance of 10 mm for 20 s, at 0.2 MPa, using a blasting device (JNBP-2, Jianian Futong Medical Equipment Co., Ltd., Tianjin, China). Each half of Y-TZP plates was conditioned with one of the two MDP-containing primers according to the recommendations of the respective manufacturer: Clearfil Ceramic Primer (Kuraray Noritake Dental Inc., Tokyo, Japan) or Z-Prime Plus (Bisco Inc., Schaumburg, IL, USA). Two identically pre-treated Y-TZP plates in each group were bonded to each other (N = 9) under a constant load with a layer of resin cement (RelyX™ Veneer Cement, 3 M ESPE, USA). After removal of the excess cement, each specimen was protected with an oxygen-blocking gel (De-Ox, Ultradent Products Inc., South Jordan, UT, USA). Light-curing was performed from both sides of the cemented blocks for 60 s each using a light-emitting diode polymerisation unit (Elipar FreeLight 2, 3 M ESPE, St. Paul, MN, USA) to ensure complete polymerisation of the resin cement. For each bonded specimen, one side of the bonded interface was wet-polished with silicon carbide papers of increasing fineness (800, 1000, 1200, 1500, 2000, 2500, 3000 and 4000 grit). After polishing and exposing the bonded interface, 8 bonded specimens for each group were immersed in 2% methylene blue solution for 48 h, followed by rinsing in running water for 2 h. The deepest depth of penetration of the methylene blue dye along the interface was quantified at 80 × magnification with a stereomicroscope (SMZ1000, Nikon, Japan). Comparison of the leakage status between the two groups was performed using two-tailed Student t -test, after ascertaining the normality and homogeneity of variance of the data sets. Statistical significance was pre-set at α = 0.05.
The remaining bonded specimen for each group was immersed in 3 mL of deionised water for 2 weeks. The soaking solutions were analysed with inductively coupled plasma-mass spectrometry (ICP-MS; Model 7500ce, Agilent Corp., Santa Clara, CA, USA) to determine their phosphorus contents, using a pre-determined calibration curve that correlates spectrometry readings with known phosphorus concentrations.
Shear bond strength
An additional batch of alumina-grit-blasted Y-TZP plates was assigned to 10 groups to create resin-bonded Y-TZP specimens (N = 10). Each resin-bonded specimen consisted of a pre-polymerised resin composite cylinder (6 mm inner diameter, 3 mm high; Venus, 3 M ESPE) cemented to a MDP-conditioned Y-TZP plate using the aforementioned resin cement. Zirconia conditioning was performed using Clearfil Ceramic Primer or Z-Prime Plus. Similar to the leakage test, each specimen was protected with an oxygen-blocking gel, and light-cured at four different locations for 60 s each.
Prior to bonding, 20 grit-blasted Y-TZP plates received additional hydrothermal ageing in an autoclave (Vacuklav 24B; Melag, Berlin, Germany) at 134 °C and 0.2 MPa for 20 h to accelerate t → m phase transformation. Except for these hydrothermally-aged bonded Y-TZP specimens and the groups designated as control, the other groups were stored at ambient temperature in deionised water (pH 6.8), acidic solution (pH 1–2) or alkaline solution (pH 10–12) for 6 months. The hydrothermally-aged and the control groups were tested after storage in deionised water for 24 h. Details of the materials employed are included in Table 1 .
|Material/Trade name||Major composition||Manufacturer|
|Y-TZP/Everest ZS-Ronde||ZrO 2 + HfO 2 (94.4 wt%),||KAVO, Kaltenbach|
|Y 2 O 3 (5.2 wt%),||& Voigt GmbH & Co.|
|Al 2 O 3 (0.2–0.5 wt%)||KG, Germany|
|Zirconia primer/Clearfil Ceramic Primer||Ethanol (˃80%),||Kuraray Noritake|
|3-trimethoxysilylpropyl methacrylate||Dental Co.|
|Zirconia primer/Z-Prime Plus||Ethanol (<9%),||Bisco Inc.|
|biphenyl dimethacrylate (<10%);||Schaumburg, IL, USA|
|HEMA (<20%); MDP|
|Light-polymerised resin cement/RelyX™ Veneer Cement||Filler: zirconia, silica||3M ESPE|
|Base resin: TEGDMA, bis-GMA||St. Paul, MN, USA|
|Light-polymerised resin composite/Valux Plus||Filler: Barium aluminum fluoride glass + highly dispersive silica (80–90%)||3M ESPE|
|Base resin: bis-GMA (5–10%)|
Shear bond strength evaluation was performed using a universal testing machine (Model 3365, Instron Corp., Canton, MA, USA) at a crosshead speed of 1 mm/min. Bond strength data for the groups were analysed by two-way analysis of variance and Fisher least significant difference multiple comparisons to examine the influence of two different categorical independents, primers (Clearfil Ceramic Primer or Z-Prime Plus) and ageing conditions (deionised water/acid/alkali storage and hydrothermal ageing), and the influence of their interactions. The use of parametric statistical methods was performed after satisfying the normality and homogeneity of variance assumptions of the data sets. Statistical significance was pre-set at α = 0.05.
The percentages of m -ZrO 2 phase present on Y-TZP surfaces after alumina grit-blasting, post-blasting hydrothermal ageing, or post-blasting 6-month ageing in deionised water, aqueous acidic medium or aqueous alkaline medium were determined by X-ray diffraction (X’PERT PRO, PANalytical, Holland). The test parameters were: Ni-filtered Cu K (λ = 1.5418 Å) radiation at ambient temperature; step size = 0.02° 2θ; start angle = 25°; end angle = 80°; scan speed = 2° 2θ/min. The quantification of monoclinic phase was calculated with Eqs. (1) and (2) .
X m = I m ( − 111 ) + I m ( 111 ) I m ( 111 + I m ( − 111 ) + I t ( 101 )
where X m : monoclinic fraction; Im(−111)
I m ( − 111 )
: intensity of monoclinic (−1 1 1) (2θ = 28.1°); Im(111)
I m ( 111 )
: intensity of monoclinic (1 1 1) (2θ = 31.17°); It(101)
I t ( 101 )
: intensity of tetragonal (1 0 1) (2θ = 29.9°); and V m = volume fraction of monoclinic phase.
Thermodynamic data on the hydrolytic stability of the coordinate bonds present in MDP– t/m –ZrO 2 were calculated using frequency analysis (Gaussian, Wallingford, CT, USA). Analysis was conducted based on the optimised MDP– t/m –ZrO 2 models reported previously . All the data was calculated using standard conditions of 1 atmosphere and 298 K.
To determine whether water ageing results in the hydrolysis of MDP–zirconia coordinate bonds, the Gibbs free energies of the hydrolysis of MDP– t/m –ZrO 2 complexes were calculated. For the thermodynamic calculations, hydrolysis of the MDP– t/m –ZrO 2 cluster in an acidic or neutral environment may be expressed as :
R − O P O 2 − Z r 4 O 8 2 − ( s ) + 2 H 3 O + ( a q ) → h y d r o l y s i s R − O P ( O H ) 2 ( a q ) + Z r 4 O 8 ( s ) + 2 H 2 O ( a q )
Hydrolysis of the MDP– t/m –ZrO 2 cluster in an alkaline environment may be expressed as:
R − O P O 2 − Z r 4 O 8 2 − ( s ) + 2 H 2 O ( a q ) → h y d r o l y s i s R − O P ( O H ) 2 ( a q ) + Z r 4 O 8 ( s ) + 2 O H − ( a q )