This study evaluated contact angle and shear bond strength of three commercial zirconia primers and compared them to a recently developed fluorination pre-treatment. Earlier investigations reported that plasma fluorinated zirconia modifies the chemical bonding structure creating a more reactive surface.
Materials and methods
Yttria-stabilized zirconia (LAVA, 3M ESPE) plates were highly polished using 3 μm diamond paste ( R a ∼200 nm) prior to pretreatments. After primer and fluorination treatment, contact angles were measured to quantify surface hydrophobicity before and after ethanol clean. Additionally, simple shear bond tests were performed to measure the adhesion strength to a composite resin.
Plasma fluorination produced the lowest contact angle (7.8°) and the highest shear bond strength (37.3 MPa) suggesting this pretreatment facilitates a more “chemically” active surface for adhesive bonding.
It is hypothesized that plasma fluorination increase hydroxylation at the surface, making it more reactive, thus allowing for covalent bonding between zirconia surface and resin cement. A strong correlation was observed between contact angle and adhesion strength for all specimens; a relationship which may help understand the frequency and modes of failures, clinically. It is also believed that this surface treatment can increase long-term viability of zirconia restorations over other adhesive techniques.
The use of zirconia for dental applications has increased substantially over the past decade. This is evident from the wide variety of commercial products available on today’s market. Its uses range from single-unit crowns and fixed-partial dentures to entire dental implant systems and nanoparticle fillers in composite resins . Zirconia, sometimes described as “ceramic steel,” possesses the ideal properties for dental use: superior strength, toughness, and fatigue resistance, excellent wear properties, and biologically inert. However; the nonreactive surface of zirconia presents a consistent issue of poor adhesion strength to other substrates (synthetic or tissue).
Adhesion between two substrates depends on both chemical adhesion and micromechanical interlocking. For silica-based substrates (e.g., feldspathic porcelain), surface roughening coupled with silane chemistry is well understood and bond strengths are considered reliable. However, these techniques are not applicable to zirconia. Traditional methods such as surface roughening (airborne-particle abrasion or diamond rotary instrument) combined with glass ionomer cements have been shown to increase bond strength . Yet, others reported that aggressive roughening could lead to the creation of critical-size surface flaws, drastically reducing long-term viability . Tribochemical techniques, which impart non-bonded silica layer on the surface and allow traditional silanation techniques to be employed, are commercially marketed, but still incorporate surface roughening and display relatively low bond strengths . Others have evaluated varying processes to coat zirconia surfaces with silica; such as thermal spray, fusion of glass bead, or chloro-silane chemical vapor treatment . Interestingly, the chloro-silane treatment, described as thin Si x O y surface layer, yielded significantly higher bond strengths than tribochemical functionalization and was statistically similar to porcelain. Aboushelib et al. introduced a process called “selective infiltration etching,” which creates inter-granular porosity at the surface allowing for resin cements to flow and interlock between the grains . Bond strengths were reported higher than clinically accepted techniques; however, removal of surface material may lead to premature failures.
More recently, published research has focused on surface functionalization and investigating novel approaches; such as complex phosphate primers and resin cements . The chemical bond between zirconia surfaces and phosphate monomers is often reported as a reaction with surface OH groups. However, zirconia itself is described as hydrophobic and having very low surface concentrations of OH groups . Lohbauer et al. reported that native surfaces had approximately 5.4% OH coverage suggesting very little reactive groups for chemical bonding. The authors of this manuscript have reported on fluorination techniques that enhance zirconia surface wettability and chemical bonding to various dental materials . The prior data showed improvements in bond strength with treatment time for fluorinated specimens with and without the use of organo-silane primers. Initial mechanical and surface analysis data confirm the potential performance improvements attainable by this fluorination treatment. Lastly, contact angle measurements demonstrated that the wetting properties were significantly improved for the fluorinated specimens compared to untreated controls. The significant reduction in contact angle may correlate to a commensurate increase in hydroxylation of the oxyfluoride surface. Additionally, identification of surface stoichiometries and a model of adhesion were reported earlier based on the chemical alteration of zirconia surfaces . The enhanced surface reactivity is attributed to the reactivity of the oxyfluoride phase(s), specifically their susceptibility to hydroxylation. Fig. 1 is a schematic representation of the proposed bonding mechanism between a fluorinated YSZ surface and a typical phosphate monomer used in acrylate adhesives. Following fluorine plasma treatment, we hypothesize that the surface becomes hydrolyzed upon exposure to atmosphere. Pantano and Brow demonstrated the auto-hydroxylation of fluoride and oxyfluoride phases in the presence of water . After some period of time, the surface becomes saturated with hydroxyl groups. When reacted with the phosphate ester – containing monomer in the adhesive, hydroxyl groups on the ester may form hydrogen bonds with surface hydroxyl groups. Hydrogen bonding weakens the P O bonds, eventually breaking them to produce water and a reactive oxygen site on the surface. The phosphor group may then bond directly to the oxygen in the oxyfluoride.
The present study evaluates the adhesive properties of this fluorination pre-treatment and compares them to other commercially available primers on non-roughened zirconia surfaces. The rationale for using non-roughened specimens centers on the need to differentiate between chemical and physical bonding between zirconia and other dental materials, and ultimately how the types of bonding might correlate to clinical failure. Chemical bonding between surfaces is critical for long-term viability of dental constructs and, there are limited clinical data evaluating different adhesive techniques and their correlation to clinical failure of restorations. The present study focuses on the role of enhanced chemical-bonding achieved through various primers, without surface roughening, in hopes that further improvements in chemically-active pretreatments may lead to increased restoration lifetimes and lower incidences of catastrophic failure. The fluorination pre-treatment is compared to three different commercially available YSZ primers and simple shear bond tests were used to measure the resulting adhesion strengths.
Materials and methods
Pre-sintered YSZ plates (LAVA, 3M ESPE AG; Seefeld, Germany) were obtained for the testing substrates. Surfaces were polished using 3 μm diamond paste and ultrasonically cleaned in ethanol then deionized water for 5 min and were not altered in any way before bonding procedures. Surface roughness was measure by stylus profilometry reveals R a ∼200 nm and R MS ∼250 nm (±10%).
Specimen preparation and characterization
YSZ surfaces were modified using the following for each group tested:
No modification (control)
ClearFil™ Ceramic Primer (Kuraray Dental, Kurashiki, Japan [ contents include 3-trimethoxysilylpropyl methacrylate and 10-Methacryloyloxydecyl dihydrogen phosphate ]) – surfaces were coated with one application using a micro-brush and air thinned for 5 s. ( Note : all shear bond test specimens had a diameter of 2.38 mm and were prepared the same as described above .) A thin coat ProBond resin bonding agent (Dentsply/Caulk, Milford, DE) was brushed on, air thinned for 5 s, then light-cured for 10 s. Composite (Filtek Ultra Supreme, 3M ESPE AG, Seefeld, Germany) was packed into specialized jig (Ultradent shear bond specimen fabrication device) and light cured for 20 s (Demi Plus LED, Kerr Corp., Orange, CA).
MonoBond™ Plus (Ivoclar-Vivadent AG, Schaan, Liechtenstein [contents include 3-trimethoxysilylpropyl methacrylate and Methacrylated phosphoric acid ester] ) – surfaces were coated with one application using a microbrush, left undisturbed for 60 s, then air thinned for 5 s.
Z-Prime™ (Bisco Inc., Schaumburg, IL [ contents include biphenyl dimethacrylate and hydroxyethyl methacrylate ]) – surfaces were coated with two applications using a microbrush and air thinned for 5 s.
Fluorination gas phase treatment – surfaces were exposed to the fluorination treatment previously described by Piascik et al. for 5 min .
Contact angle goniometer (DROPimage, Rame-hart Instrument Co., Netcong, NJ) was used to evaluate differences in contact angle between surfaces. Shear bond test specimens were stored in DI water at 37 °C for a period of 24 h prior to testing, then fixed to a custom mechanical fixture to ensure vertical compliance. Specimens ( n = 10/group) were subjected to a force with a notched (semicircular) piston at a crosshead speed of 0.5 mm/min in an electromechanical testing system (Instron Corp., Norwood, MA). Shear bond strengths were calculated by dividing peak load by the cross-sectional area of the composite cylinder. Single-factor analysis of variance (ANOVA) at a 5% confidence level was performed for the bonding strength data for statistical similarities.