Jeffrey Y. Thompson

Adhesion/cementation to zirconia and other non-silicate ceramics: Where are we now?

Non-silicate ceramics, especially zirconia, have become a topic of great interest in the field of prosthetic and implant dentistry. A clinical problem with use of zirconia-based components is the difficulty in achieving suitable adhesion with intended synthetic substrates or natural tissues. Traditional adhesive techniques used with silica-based ceramics do not work effectively with zirconia. Currently, several technologies are being utilized clinically to address this problem, and other approaches are under investigation. Most focus on surface modification of the inert surfaces of high strength ceramics. The ability to chemically functionalize the surface of zirconia appears to be critical in achieving adhesive bonding. This review will focus on currently available approaches as well as new advanced technologies to address this problem.

Fracture resistance of prepared teeth restored with bonded inlay restorations

STATEMENT OF PROBLEM intact, prepared, and restored human maxillary premolars. MATERIAL AND METHODS Fifty intact, noncarious human maxillary premolars were divided into 5 groups of 10 and were mounted with their roots imbedded in autopolymerized acrylic. In the first group, the teeth were intact with no preparation. In the other 4 groups, Class II MOD preparations were made with a water-cooled high-speed hand piece. In 1 group, the cavity preparations were restored with bonded CAD/CAM ceramic inlays. In 2 groups, the preparations were restored with bonded CAD/CAM composite inlays (acid etched or air particle abraded). In the final group, the teeth were prepared but unrestored. Specimens were tested individually in a universal testing machine, in which a 4.82-mm-diameter steel sphere plunger was mounted in the crosshead moving at 0.5 mm/min. The plunger contacted the facial and lingual triangular ridges beyond the margins of the restorations. Peak load to fracture (N) was measured for each specimen. Means were calculated and analyzed with analysis of variance (P</=.05). RESULTS MOD preparations weakened the teeth by approximately 59%. Restoring the teeth with ceramic or composite inlays did not significantly strengthen the teeth under this testing system. Of the restored teeth, those restored with indirect composite inlays cemented following manufacturers recommendations had the highest fracture resistance. CONCLUSION Within the limitations of this study, under static compression load testing, bonded inlay restorations did not strengthen maxillary premolars with large MOD preparations.

Effect of acid etching and collagen removal on dentin wettability and roughness

It was recently reported that removal of the collagen network from etched dentin does not affect dentin bond strengths. The aim of this study was to determine if the removal of the collagen fibers results in changes in dentin roughness and wettability. Twenty cary-free extracted human third molars were sectioned parallel to the occlusal surface to expose either superficial or deep dentin. Dentin was ground flat through 600-grit SiC abrasive paper under water to provide uniform surfaces. Observed contact angle measurements were performed to assess wettability by using the axisymmetric drop shape analysis technique using water and a water-based primer. Average roughness was determined with a profilometer. The specimens were analyzed just after being ground, after etching with 35% H(3)PO(4) gel for 15 s, and after etching and deproteinization with 5% NaOCl for 2 min. Data were analyzed with two-way ANOVA and Newman-Keuls multiple comparison t test procedure. Etching resulted in an increase in surface roughness and dentin wettability. For deep dentin, collagen removal did not influence the average roughness, but it resulted in a significantly greater degree of wettability. The degree of wettability for deep dentin was greater than for superficial dentin.

Irradiance effects on the mechanical properties of universal hybrid and flowable hybrid resin composites

OBJECTIVES A potential problem with high-intensity lights might be failure of polymer chains to grow and cross-link in a desired fashion, thereby affecting the structure and properties of the polymers formed. The purpose of this study was to evaluate mechanical properties of resin composites polymerized using four different light-curing units. METHODS A conventional quartz-tungsten-halogen (QTH) light, a soft-start light, an argon-ion laser, and a plasma-arc curing light were used to polymerize disk-shaped (9.0mm diameter x 1.0 mm high) and cylinder-shaped (4mm diameter x 8 mm high) specimens of a universal hybrid and a flowable hybrid composite. Biaxial flexure strength, fracture toughness, hardness, compressive strength, and diametral tensile strength were determined for each composite. RESULTS The use of the plasma-arc curing light, a high-intensity light, resulted in significantly lower hardness for the universal hybrid composite compared with the hardness obtained using the conventional QTH and the soft-start units. Hardness was the only mechanical property that was adversely affected by the use of a high-intensity light. SIGNIFICANCE High-intensity lights might affect some resin composite mechanical properties, but this effect cannot be generalized to all resin composites and all properties.

Long-term microtensile bond strength of surface modified zirconia:

OBJECTIVE To compare long-term microtensile bond strength of zirconia, surface-modified via a novel treatment, to current surface conditioning methods for zirconia, when resin bonded to dental composite. METHODS Two ProCAD (porcelain) and 10 sintered ZirCAD (ZrO(2)) blocks (18 mm × 14 mm × 12 mm) were obtained from manufacturers. Twelve Herculite XRV composite blocks were fabricated (18 mm × 14 mm × 12 mm). Bonding surface of blocks was polished through 1200-grit SiC and air-abraded (50 μm alumina, 0.28MPa, 20s). Blocks were then separated into six groups: (1) porcelain (control), HF-etched/silane-treated, (2) ZrO(2), tribochemical-coated/silane-treated, (3) ZrO(2), primer-treated, (4) ZrO(2), modified via novel 3.2 nm silica layer/silane-treated, (5) ZrO(2), modified via novel 5.8nm silica layer/silane-treated, and (6) ZrO(2), modified via novel 30.4 nm silica layer/silane-treated. Blocks were bonded to composite using Clearfil Esthetic cement. Blocks were stored in distilled water (37°C, 24h), then cut into microtensile bars (n=8/gp), then bond strengths were measured using a universal testing machine at 0, 1, 3, and 6 months. All groups were statistically analyzed (ANOVA, Tukeys, p<0.05). RESULTS At 6 months (aging), all silica seed layer specimens displayed microtensile bond strength similar to CoJet specimens but less than that of silane-modified dental porcelain. CONCLUSION The deposition of a silica layer on zirconia resulted in similar or superior long-term resin bond strength when compared to traditional silanation and bonding techniques for zirconia but lower than that for silane-treated dental porcelain.

Mechanical properties of a dental ceramic coated by RF magnetron sputtering

Metal and ceramic thin film coatings were deposited onto a dental ceramic via radio frequency (RF) magnetron sputtering. The objectives of the study were to determine if a coherent interface could be produced between the coating and the substrate and if the coating significantly would improve the mechanical properties of the ceramic. Thin films of Au, Al, and AlN were deposited in this study. Mechanical testing results indicated that a significant improvement in flexural strength was observed with both Au and Al coatings while significant improvements in the flexural modulus were observed with all three materials. SEM analysis indicated that the interfaces were coherent and also suggested two mechanisms (crack bridging and crack blunting) that could be responsible for the enhanced mechanical properties.

Free-electron laser etching of dental enamel

OBJECTIVE The purpose of this study was to evaluate the Mark-III free-electron laser as a means of etching enamel surfaces, with potential application to resin bonding. METHODS The FEL was tuned to wavelengths ranging from 3.0 to 9.2 microm. Specific wavelengths that are resonantly absorbed by phosphates, proteins, and water were used. First, bovine enamel was polished and exposed to static FEL exposures. Lased enamel was examined using scanning electron microscopy (SEM). Additional bovine enamel specimens were exposed to FEL at similar wavelengths, but with rastering to create treated rectangular areas on each specimen. Surface roughness was evaluated using profilometry and atomic force microscopy (AFM). Composite was bonded to the lased enamel, and shear bond strengths were determined using an Instron universal testing machine. As a control, the surface roughness of, and shear bond strengths to, acid-etched enamel were determined. RESULTS Static FEL exposures caused changes in the enamel ranging from an etched appearance to pits, cracks, and frank cratering. The surface roughness of lased enamel was much greater than that of acid-etched enamel, and was qualitatively different as well. Shear bond strengths of resin to acid-etched enamel were significantly higher than bond strengths to lased enamel. CONCLUSIONS Under the conditions used in this study, the FEL did not offer a practical and effective method of etching enamel for resin bonding. However, the ability of the FEL to deliver many specific wavelengths makes it an interesting tool for further research of laser effects on tooth structure.

Effect of deposition interruption and substrate bias on the structure of sputter-deposited yttria-stabilized zirconia thin films

The possibility of altering the morphological structure of radio-frequency magnetron sputter- deposited yttria-stabilized zirconia thin films was studied. A columnar grain structure is often observed in sputter deposited thin films; however, this morphology may not be desirable. Two potential methods of disrupting the columnar grain structure were investigated; deposition interruption and periodic application of a substrate bias. Simple interruption of the deposition process was not effective in altering the columnar grain structure of the sputter-deposited films. The use of a periodic substrate bias produced a laminate structure with alternating layers of distinctly different microstructure. X-ray diffraction showed that as the thickness of the layers decreased (with increasing number of layers), the monoclinic phase was eliminated from the deposited thin films, and the films showed a preference for the (200)t orientation.

Evaluation of crystallinity and film stress in yttria-stabilized zirconia thin films

Yttria (3 mol %)-stabilized zirconia (YSZ) thin films were deposited using radio frequency (rf) magnetron sputtering. The YSZ thin films were deposited over a range of temperatures (22–300 °C), pressures (5–25 mTorr), and gas compositions (Ar∕O ratio). Initial studies characterized a select set of properties in relation to deposition parameters including: refractive index, structure, and film stress. X-ray diffraction (XRD) showed that the films are comprised of mainly monoclinic and tetragonal crystal phases. The film refractive index determined by prism coupling, depends strongly on deposition conditions and ranged from 1.959 to 2.223. Wafer bow measurements indicate that the sputtered YSZ films can have initial stress ranging from 86 MPa tensile to 192 MPa compressive, depending on the deposition parameters. Exposure to ambient conditions (25  °C, 75% relative humidity) led to large increase (∼100MPa) in the compressive stress of the films. Environmental aging suggests the change in compressive stress ...

Stress evolution as a function of substrate bias in rf magnetron sputtered yttria-stabilized zirconia films

An increase in compressive stress was observed in rf magnetron sputtered yttria-stabilized zirconia thin films upon exposure to ambient conditions (25°C and 75% relative humidity). This increase was attributed to absorption of water molecules into intergranular pores. It was shown that increasing substrate bias power disrupted columnar grain growth and reduced the percent change in compressive stress when exposed to ambient environments. Transmission electron microscopy confirmed a reduction in intergranular porosity for substrate bias depositions but an increase in lateral defects. These defects are hypothesized to be stress-induced microcracks caused by a tetragonal to monoclinic phase transformation.