Edward A. Monaco

The effect of zirconia surface treatment on flexural strength and shear bond strength to a resin cement.

STATEMENT OF PROBLEM Despite the expanded applications for zirconia in restorative dentistry, there is no clear recommendation in the literature regarding surface treatment before bonding. PURPOSE The purpose of this study was to evaluate the effect of mechanical surface treatment of yttria-partially stabilized zirconia on its flexural strength and the effect of mechanical and chemical surface treatments on its bond strength to a resin cement. MATERIAL AND METHODS For flexural strength evaluation, zirconia bars (4 x 5 x 40 mm) were prepared from zirconia blocks, finished using a diamond rotary cutting instrument, sintered, then assigned into 4 groups: (1) control (no treatment), (2) airborne-particle abrasion, (3) silicoating, and (4) wet hand grinding. After storage for 24 hours at 37 degrees C, flexural strength was determined using a 3-point bending test, and the results were analyzed using 1-way ANOVA (alpha=.05). For shear bond strength evaluation, zirconia rods (2.5 x 3 mm) were prepared from zirconia blocks, sintered, and assigned into 16 groups. Each group underwent a combination of the following mechanical and chemical treatments. Mechanical treatment included: (1) control (no treatment), (2) airborne-particle abrasion, (3) silicoating, or (4) wet hand grinding. Chemical treatment included: (1) control (no treatment), (2) acid etching followed by silanation, (3) silanation only, or (4) application of zirconia primer. Dentin specimens were prepared from extracted molars stored in 0.5% chloramine-T. Zirconia rods were bonded to dentin using a resin cement (Multilink Automix), then light polymerized. After storage, the specimens were loaded to failure with the notched shear bond test method in a universal loading apparatus. For artificial aging analysis, the groups that achieved the highest bond strength values were duplicated, stored at 37 degrees C and 100% humidity for 90 days, and thermal cycled before being loaded to failure. Results were analyzed using 2-way ANOVA (alpha=.05). RESULTS Airborne-particle abrasion and hand grinding significantly increased flexural strength. The highest shear bond strength values were achieved for the following groups: silicoated + silanated > hand ground + zirconia primer > airborne-particle abraded + silanated > zirconia primer > airborne-particle abraded + zirconia primer. Artificial aging resulted in significantly lower shear bond strength for the silicoated/silanated and the zirconia primer groups. CONCLUSIONS Mechanical modification of the surface increased the flexural strength of Y-TZP. The resin bond to Y-TZP was improved by surface treatment. A combination of mechanical and chemical conditioning of the zirconia surface was essential to develop a durable resin bond to zirconia.

Sensitivity of Candida Albicans Biofilm Cells Grown on Denture Acrylic to Antifungal Proteins and Chlorhexidine

OBJECTIVES Candida albicans cells form biofilms on polymeric surfaces of dentures and other prostheses introduced into the oral cavity. Many biofilm microorganisms exhibit resistance to antimicrobial agents; C. albicans cells may also develop resistance to naturally occurring antifungal peptides in human saliva including histatins (Hsts) and defensins (hBDs). Therefore, we evaluated Hst 5 activity on C. albicans biofilm cells compared to planktonic cells and measured whether surface treatment of denture acrylic with Hst 5, hBD-3, or chlorhexidine gluconate could inhibit in vitro biofilm development. METHODS Acrylic disks were preconditioned with 500 microl saliva for 30 min, and inoculated with C. albicans cells (10(6)cells/ml) for 1h, at 37 degrees C. Non-adherent cells were removed by washing and disks and were incubated in YPD growth medium for 24, 48, and 72 h at 37 degrees C. Candidacidal assays were performed on 48-h-biofilms and on planktonically grown cells using Hst 5 (15.5, 31.25, and 62 microM). Cell adhesion was compared on disks pre-coated with 0.12% chlorhexidine gluconate, 50 microM Hst 5, or 0.6 microM hBD-3 after 24, 48, and 72 h growth. RESULTS No significant difference was observed in sensitivity to Hst 5 of biofilm cells compared to planktonic cells (p>0.05). Pre-coating disks with hBD-3 did not inhibit biofilm development; however, Hst 5 significantly inhibited biofilm development at 72 h, while 0.12% chlorhexidine significantly inhibited biofilm development at all time intervals (p<0.05). CONCLUSIONS C. albicans biofilm cells grown on denture acrylic are sensitive to killing by Hst 5. Surface coating acrylic with chlorhexidine or Hst 5 effectively inhibits biofilm growth and has potential therapeutic application.

Defining a natural tooth color space based on a 3-dimensional shade system

STATEMENT OF PROBLEM The natural tooth color space reported by a manufacturer may not represent the comprehensive spectrum of natural teeth for all population groups. PURPOSE The purpose of this study was to define a natural tooth color space within the Greater Buffalo, New York population and to compare that to the color space determined by a manufacturer. MATERIAL AND METHODS Nine hundred and thirty-three maxillary central incisors (501 patients) were measured with a shade-taking device (Vita Easyshade). For each tooth, L*, a*, b* values, chroma, hue, and the closest shade (Vita 3D-Master) were recorded. A linear regression analysis was performed to determine how well the manufacturers values predict actual values for L*, a*, and b*. Color differences (DeltaE*) between the Buffalo population and the closest shade were also calculated. A 1-sample t test was used to determine whether the color differences seen in the sample were statistically different from the perceptibility threshold, DeltaE*=3.7 (alpha=.05). RESULTS All 3 attributes of the Buffalo population displayed a broader range than those from the shade guide. However, the regression analysis revealed a significantly positive relationship between the L*, a*, and b* values of the 2 methods (P<.001). The 1-sample t test revealed a significant DeltaE* (mean DeltaE*=6.15) difference from the perceptibility threshold of DeltaE*=3.7 (P<.001). CONCLUSIONS Color differences between the Buffalo population and the shade guide were frequently above published perceptibility thresholds, but within the range of acceptability. The Buffalo population tooth color space encompassed the manufacturers color space.

Comparison of fracture resistance of pressable metal ceramic custom implant abutments with CAD/CAM commercially fabricated zirconia implant abutments

STATEMENT OF PROBLEM The adequacy of the strength of dental zirconia abutments under heavy occlusal load conditions is questionable, with degradation of the mechanical properties of zirconia having been reported. Therefore, some alternatives must be considered. PURPOSE The purpose of this study was to determine the efficacy of fabricating pressable metal ceramic custom implant (Pr) abutments, and to evaluate the fracture resistance of these abutments. MATERIAL AND METHODS Two groups of implant abutment specimens were fabricated (n=10). The experimental group consisted of Pr abutments, and the control group consisted of CAD/CAM-designed zirconia-based ceramic (Zr) abutments. For the experimental group, custom metal abutments were cast using a compatible metal alloy (Lodestar); this was followed by injection molding with lithium disilicate pressable ceramic (IPS e.max Press) around the metal column of the custom implant abutment. For the control group, 10 specimens were fabricated from CAD/CAM-designed zirconia abutments (Procera Zirconia). Following scanning, all-ceramic crowns with the average dimensions of a human central incisor were fabricated for the experimental and control abutments (n=20) using lithium disilicate pressable ceramic (IPS e.max Press). Each crown was cemented to the implant abutments with a resin luting agent (Variolink II). The crown-abutment test specimens were fixed to titanium implant analogs and placed in a test stand at 30 degrees from the vertical axis of the specimens in a computer-controlled universal testing device. The independent t test was used to detect if the mean values of the fracture load differed significantly (alpha=.05) between the 2 groups. RESULTS The mean (SD) fracture load was significantly higher in the Pr group (901.67 (102.05) N) than in the Zr group (480.01 (174.46) N, P<.005). CONCLUSIONS This study found that Pr abutments are stronger than Zr abutments.

Effect of two connector designs on the fracture resistance of all-ceramic core materials for fixed dental prostheses

STATEMENT OF PROBLEM Most all-ceramic fixed dental prostheses (FDPs) fail at the connectors. PURPOSE The purpose of this study was to determine the effect of 2 connector designs on the fracture resistance of core materials used for all-ceramic FDPs. MATERIAL AND METHODS Three materials were tested: (1) heat-pressed lithium disilicate glass ceramic (IPS e.max Press (Press)), (2) milled lithium disilicate glass ceramic (IPS e.max CAD (CAD)), and (3) milled yttrium-stabilized tetragonal zirconia polycrystals (Y-TZP) (IPS e.max ZirCAD (ZirCAD)). Specimens were made into 30 x 4 x 4-mm bars to represent 3-unit FDPs. Two connector designs, round (0.60 +/-0.01-mm radius of curvature) and sharp (0.06 +/-0.001-mm radius of curvature), with a 3.00 +/-0.05-mm cross-section for each connector, were studied (n=5). Each specimen was loaded to fracture in a universal testing machine with a crosshead speed of 0.1 mm/min. Data were analyzed with a 2-way univariate ANOVA and Tukey HSD test (alpha=.05). RESULTS Mean (SD) failure loads for round connector designs were 684.2 (70.1) N for ZirCAD, 260 (7.8) N for CAD, and 172.9 (35.5) N for Press. Mean (SD) failure loads for sharp connector designs were 386.3 (51.5) N for ZirCAD, 87.9 (7.0) N for CAD, and 125.1 (15.1) N for Press. The 2-way univariate ANOVA indicated statistically significant differences (P<.005) for material and connector design, and, also, a significant interaction between material and connector design. Higher maximum failure loads were found for the round connector design when compared to the sharp connector design, for ZirCAD and CAD. However, this difference was not statistically significant for the Press groups. SEM subjective assessment of the fractured specimens revealed that the fracture initiated from the gingival surface (tensile) of the connector toward the pontic (central loading point). CONCLUSIONS Fracture resistance of ceramic core materials is affected by fabrication technique and connector design. Connector design affected fracture resistance of the milled ceramic, but not the pressed ceramic.

Altering occlusal vertical dimension provisionally with base metal onlays: A clinical report

This article presents a method for altering occlusal vertical dimension (OVD) to restore dentitions with limited restorative space due to loss of tooth structure. A provisional increase in OVD is achieved using predominately base metal onlays, which offer advantages over conventional techniques. The onlays are resistant to wear, may be bonded well to nonrestored and restored tooth surfaces with resin cements, and provide a fairly reversible method of increasing OVD. A patient situation is presented which demonstrates the use of provisional base metal onlays in complete mouth rehabilitation.

Applying microwave technology to sintering dental zirconia

STATEMENT OF PROBLEM When sintering zirconia, conventional processing may not provide uniform heating and consumes more energy than an alternative method using microwave energy. PURPOSE The purpose of this study was to compare the surface quality, mechanical and physical properties, and dimensional stability obtained by sintering yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) in a conventional furnace versus a microwave furnace. MATERIAL AND METHODS Twenty bars of Y-TZP were prepared from Zircad blocks. Ten specimens were used for sintering in a conventional furnace. The remaining 10 specimens were sintered in a microwave furnace. The sintering temperature used for both techniques was 1500°C. The flexural strength of all specimens was measured with the 3-point bend test with a universal testing machine with a cross head speed of 1.0 mm/min. Density was measured by applying the Archimedes method, and specimen length, width, and thickness were measured with a digital micrometer. The phase composition and average grain size of these ceramics were examined by using X-ray diffraction, and microstructure characteristics were studied with scanning electron microscopy. Data obtained were analyzed by using independent t tests (α=.05). RESULTS No significant difference between conventional and microwave sintering for either flexural strength, t18=0.49 (P=.63) or density, t18=0.07 (P=.95) was found. Specimens in both groups exhibited a uniform firing shrinkage of approximately 24.6% in all dimensions. The surface of selected specimens examined with a scanning electron microscope showed no visible difference in grain shape or porosity size between the 2 sintering methods. CONCLUSIONS Under the conditions of this study, it appears that either microwave or conventional zirconia sintering may be used for processing zirconia for dental use. However, microwave energy provides uniformity of heating, allowing the use of higher heating rates, which can increase productivity and save energy.

Comparison of porcelain surface and flexural strength obtained by microwave and conventional oven glazing.

STATEMENT OF PROBLEM Although the superior qualities of microwave technology are common knowledge in the industry, effects of microwave glazing of dental ceramics have not been investigated. PURPOSE The purpose of this study was to investigate the surface roughness and flexural strength achieved by glazing porcelain specimens in a conventional and microwave oven. MATERIAL AND METHODS Thirty specimens of each type of porcelain (Omega 900 and IPS d.Sign) were fabricated and sintered in a conventional oven. The specimens were further divided into 3 groups (n=10): hand polished (using diamond rotary ceramic polishers), microwave glazed, and conventional oven glazed. Each specimen was evaluated for surface roughness using a profilometer. The flexural strength of each specimen was measured using a universal testing machine. A 2-way ANOVA and Tukey HSD post hoc analysis were used to determine significant intergroup differences in surface roughness (alpha=.05). Flexural strength results were also analyzed using 2-way ANOVA, and the Weibull modulus was determined for each of the 6 groups. The surfaces of the specimens were subjectively evaluated for cracks and porosities using a scanning electron microscope (SEM). RESULTS A significant difference in surface roughness was found among the surface treatments (P=.02). Follow-up tests showed a significant difference in surface roughness between oven-glazed and microwave-glazed treatments (P=.02). There was a significant difference in flexural strength between the 2 porcelains (P<.005), but no significant difference in flexural strength by surface treatment (P=.48). The Weibull modulus value for the Omega 900 microwave-glazed group was the highest (1.9) as compared to the other groups. CONCLUSIONS The surface character of microwave-glazed porcelain was superior to oven-glazed porcelain. Omega 900 had an overall higher flexural strength than IPS d.Sign. Weibull distributions of flexural strengths for Omega 900 oven-glazed and microwave-glazed specimens were similar. SEM analysis demonstrated a greater number of surface voids and imperfections in IPS d. Sign as compared to Omega 900.

Repairing an implant titanium milled framework using laser welding technology: A clinical report

The application of laser welding technology allows titanium to be welded predictably and precisely to achieve accurate fit of a milled framework. Laser energy results in localized heat production, thereby reducing thermal expansion. Unlike soldering, laser energy can be directed to a small area, making it possible to laser weld close to acrylic resin or ceramic. This article describes the use of laser welding to repair an implant titanium milled fixed denture. A quick, cost-effective, accurate repair was accomplished, and the repaired framework possessed adequate strength and the same precise fit as the original framework.

Comparison of fracture resistance of pressable metal ceramic custom implant abutment with a commercially fabricated CAD/CAM zirconia implant abutment.

STATEMENT OF PROBLEM The predictable nature of the hot pressing ceramic technique has several applications, but no study was identified that evaluated its application to the fabrication of custom implant abutments. PURPOSE The purpose of this study was to compare the fracture resistance of an experimentally designed pressable metal ceramic custom implant abutment (PR) with that of a duplicate zirconia abutment (ZR). MATERIALS AND METHODS Two groups of narrow platform (NP) (Nobel Replace) implant abutment specimens were fabricated (n=10). The experimental abutment (PR) had a metal substructure cast with ceramic alloy (Lodestar) and veneered with leucite pressable glass ceramic (InLine PoM). Each PR abutment was individually scanned and 10 duplicate CAD/CAM ZR abutments were fabricated for the control group. Ceramic crowns (n=20) with the average dimensions of a human lateral incisor were pressed with lithium disilicate glass ceramic (IPS e.max Press) and bonded on the abutments with a resin luting agent (Multilink Automix). The specimens were subjected to thermocycling, cyclic loading, and finally static loading to failure with a computer-controlled Universal Testing Machine. An independent t test (1 sided) determined whether the mean values of the fracture load differed significantly (α=.05) between the 2 groups. RESULTS No specimen failed during cyclic loading. Upon static loading, the mean (SD) load to failure was significantly higher for the PR group (525.89 [143.547] N) than for the ZR group (413.70 [35.515] N) for internal connection narrow platform bone-level implants (P=.025). Failure was initiated at the screw and internal connection level for both groups. CONCLUSIONS It is possible to fabricate PR abutments that are stronger than ZR abutments for Nobel Biocare internal connection NP bone-level implants. The screw and the internal connection are the weak links for both groups.