Dental Materials 1997

Previous finite element modeling and physical testing at the Naval Dental School/NIST indicated that failure loads would not increase with ceramic thickness for all-ceramic crowns and inlays loaded in the occlusal fossa. In fact, this research suggested that ceramic restorations could support higher loads at 0.5 mm thick than at 2.0 mm thick, if they were well bonded and uniformly supported. The present investigation extended previous physical testing of simple ceramic bars to full ceramic crowns. Forty molar analogs were machined from epoxy-glass rods and forty matched crown patterns, ten each of 0.5 mm, 1.0 mm, 1.5 mm, and 2.0 mm occlusal thicknesses were machined from plexiglass rods, allowing an 80-100 micrometer precementation space. Crown patterns were sprued, invested, burned-out and pressed in a leucite reinforced feldspathic ceramic (OPC, Jeneric/Pentron). Pressing was accomplished in an Ivoclar Empress machine programmed for Jeneric Pentron OPC. Both crown analogs and patterns were prepared by acid etching, silage treatment, application of a low viscosity bonding agent, and cemented under a constant load of 100N. Specimens were loaded to failure at 0.5 mm per minute cross head speed using a blunt piston cushioned with a polyethylene sheet in a central fossa location. An acoustic transducer and transillumination were utilized for crack detection during loading. Significant differences among mean failure loads per thickness group were analyzed using ANOVA and a 95% multiple range test.

INFLUENCE OF CYCLIC FATIGUE ON INTERFACIAL FAILURE OF ALL-CERAMIC CROWNS
LCDR Michael R. Brenyo, DC, USN

Cracks responsible for clinical failure originate from the cementation (internal) surface, with little or no contact damage present. Unfortunately, traditional load-to-failure tests produce failure in a manner not seen clinically, involving cracks originating from the loaded outside surface. Naval Dental School/NIST research demonstrated that cementation surface crack systems can be introduced as long as contact pressures are clinically realistic. However, under Navy/NIST testing, failure loads were still higher than commonly reported from intraoral measurements. The purpose of this study was to determine whether cyclic loading would decrease failure loads for cementation surface cracks in bonded all-ceramic crowns. A second goal was to investigate whether low load cyclic conditions alone are sufficient to generate clinically-realistic failure. Thirty standardized molar crowns were fabricated from a leucite reinforced ceramic (OPC, Jeneric/Pentron). Crowns were cemented using clinical procedures to standardized prepared molar tooth analogs machined from a dentin substitute. During a pilot phase of this study, cemented crowns were cyclically loaded (20 Hz, 1.5 mm contact radius) in air at 200N, 400N, 600N, 800N, and 900N for up to 5 million cycles with little or no evidence of cementation surface crack formation. An additional set of specimens were cyclically loaded (20 Hz, 1 million cycles) at 200N, 400N, 600N, and 800N and subsequently loaded to failure. Significantly lower static failure loads, compared to control crowns that were not cyclically loaded, would be a clear indication of fatigue damage accumulation.

MICROLEAKAGE OF CONSOLIDATED SILVER WITH VARIOUS CAVITY LINERS
LCDR Janet Delorey-Lytle, DC, USN

The purpose of this study was to examine the microleakage of a hand-consolidated silver powder used with four different cavity lining materials. Class V cavity preparations were cut with occlusal margins in enamel and gingival margins in root dentin in the facial surfaces of 75 extracted human teeth. The cavities were randomly assigned one of the following liners (n=15 each): nothing (N), copal varnish (CV), polyamide varnish (PV), a filled polymer adhesive (FPA), and a polymer adhesive without filler (UPA). Precipitated silver powder was mixed with 10% w/w HBF4, decanted and rinsed in 2% w/w HBF4. The acid-powder slurry was then hand-consolidated into the cavities with a 5 mm serrated amalgam condenser. All samples were polished and thermocycled 3000 times between 5 ° and 55 °C and submersed in methylene blue dye. Samples were sectioned and penetration of the dye was evaluated and recorded; a 0-4 ordinal scale was used. Chi-square analysis of the occlusal enamel leakage indicated a significant dependence upon the liner type (p=0.017), with a leakage ranking of N < CV < PV < FPA < UPA. Gingival dentin leakage was also liner dependent (Chi-square, p<0.001), with a leakage rank order of PV < CV < FPA < N < UPA. Comparisons of enamel to dentin leakage for each material showed only N (Chi-square, p=0.01) and CV (Chi-square, p=0.009) to have significantly less enamel than dentin leakage. These results indicate that the use of a copal or polyamide varnish liner will result in the best combination of leakage reduction in both dentin and enamel. The use of an unfilled polymer adhesive will result in the highest leakage in both dentin and enamel. Without a liner a good seal on enamel but high leakage in dentin will result.

PROCESS-RELATED SURFACE TREATMENT TO CREATE MICRO-RETENTIVE FEATURES ON THE CEMENTATION SURFACE OF IN-CERAM CERAMIC
LCDR Kathleen S. Kenny, DC, USN

In-Ceram core ceramic (Vita Zahnfabrik, Germany) is composed of a porous network of alumina particles subsequently infiltrated with a glass and is reported to be three to four times stronger than other dental ceramics. One significant limitation of In-Ceram is that the ceramic cannot be selectively etched to develop micromechanically retentive features for bonding with dental cements. Bonded all-ceramic crowns can have much lower failure rates compared to non-bonded crowns. While two adhesive systems utilizing chemical bonds are available, there is still interest in the development of a mechanical bonding system for In-Ceram. Previous work at the Naval Dental School demonstrated that certain glass-modifying ions added to the porous alumina surface prior to glass infiltration allowed for subsequent selective etching, allowing moderate bond strengths to develop with resin cements. This present investigation added Ca, Na, La, Fl, Li, Si and combinations in one of two molecular weights of a carrier preceramic polymer (viscosity = 1000 and 60,000 cs). Treated discs were studied by light and scanning electron microscopy and line scan elemental analysis was performed by EDX for both pre- and post-infiltration states. Significant differences were found in treatment depths for the two polymers (p < 0.001, Student t), in the microstructure of treated material, and in the distribution of modifying ions. Certain post-etch surfaces (9.6 mass% HF) clearly indicated that selective etching had occurred. Further work is needed to optimize the effect, document bond strength improvements and to determine whether this treatment degrades or enhances ceramic strength.

EFFECTS OF DIFFERENT SHEAR TEST DESIGNS ON DENTIN BOND STRENGTHS
LCDR Michael F. Milos, DC, USN

This study compared six different shear test designs to evaluate whether parameters other than operator and laboratory differences could effect shear bond strength (SBS) data. Human molars were ground wet on SiC paper to expose the occlusal dentin, which was etched for 30 s with 10% H3PO4 gel, rinsed and kept moist. An acetone-based primer was applied to these surfaces with a brush in five consecutive coats and dried, followed by an unfilled resin that was air-thinned and light cured for 20 s. Composite resin was placed on the treated surfaces in a metal iris (groups 1 & 2), in a split mold, leaving a composite plug (groups 3-5) or in a Watanabe assembly (group 6), and light cured for 1 min. Twenty-four samples/group were prepared and tested by one of six different methods. Groups 1 used a knife-edged chisel pressing against the metal iris. Groups 2 was similar to 1, but used a device that added support to the tooth to resist tooth flexure. Group 3 used the chisel pressing directly onto the composite plug. Group 4 was tested the same way as group 3, but using the device that added support to the tooth. Group 5 used a wire loop that pulled against the composite plug on teeth mounted in half of a Watanabe test assembly. Group 6 used a slightly modified Watanabe assembly. Test samples were stored in distilled water at room temperature for 24 h before testing at a cross head speed of 0.5 mm/min. The SBS values ranged from 32 ± 7 (2) to 18 ± 3 MA (3). Statistical analysis indicated SB tests using the metal iris were significantly different (p < 0.05) from all other groups, but not from each other. The wire loop was significantly different from groups 3, 4, and 6. The results suggest that SBS values cannot be compared unless the actual shear test method used is identical.

WET STORAGE STRENGTH DEGRADATION OF INTERPENETRATING PHASE COMPOSITES AS A FUNCTION OF INFILTRATION POLYMER HYDROPHOBICITY
LCDR Craig M. Neitzke, DC , USN

Interpenetrating-phase composites (IPCs) consist of two or more phases, each of which is continuously connected three-dimensionally, unlike traditional composites having isolated filler particles. Fabrication of ceramic-based IPCs involves formation of a porous ceramic skeleton that is subsequently infiltrated with monomer and/or polymer. Although dry strengths exceed traditional composites, IPCs may be more sensitive to water storage due to their architecture providing a three-dimensional percolation (or reaction) path for hydrolytic degradation. In this study two hydrophobic, fluorinated monomers were investigated along with control poly(methylmethacrylate) (PMMA) as the infiltration phase for preformed alumina blocks (CELAY, Vita Zahnfabrik): (1) 73.25 mass% hexafluorobutylmethacrylate (HFBMA) in PMMA; (2) 100 mass% trifluoroethylmethacrylate (TFEMA). Porous blocks were silage treated (2 mass% A174 in acetone), infiltrated with chemically activated monomer, cured under ambient conditions and given a post cure (14 hr, 62 C, PMMA and HFBMA; 24 hr 90 C, TFEMA). Cured blocks were sectioned (0.6-2 mm), abraded (600 grit SiC paper) and tested in biaxial flexure (0.5 mm/min) following storage (3 weeks) at 37 C wet or dry. Significant differences were found based upon monomer type or storage condition (MANOVA, p = 0.0003), with TFEMA > PMMA > HFBMA and dry > wet (95% Duncan). For both PMMA and HFBMA wet storage significantly degraded strengths but not for TFEMA (ANOVA, 95% Duncan). Further investigations will compare TFEMA IPCs with traditional composite resins under accelerated aging.

MACHINABLE BLOCK PROCESSING OF INTERPENETRATING PHASE COMPOSITES
LCDR William N. Norman, DC, USN

Machining processes (including CAD/CAM) await development of tougher, more damage tolerant restorative materials for widespread application to fixed prosthodontics. Interpenetrating-phase composites (IPCs) hold promise as machinable materials (consisting of two or more phases each of which is continuously connected three-dimensionally). Fabrication of ceramic-based IPC’s involves two steps. First, a porous ceramic skeleton is created by neck formation between touching particles. Second, this porous compact is infiltrated with another material, i.e., a glass, a monomer or a polymer. In an advanced NIST/Navy process, the porous ceramic is formed by low temperature pyrolysis of preceramic polymers located between touching particles. This present project was devoted to investigating the processing of machinable blocks of IPC material using the NIST/Navy technology. Hypotheses tested include: (1) that the preceramic polymer used for interparticle neck formation would also serve as a binder allowing blocks to be successfully dry-pressed; and (2), that isostatic pressing (250 MA) of dry-pressed blocks would improve finished IPC strengths. A feldspathic dental ceramic powder (ave. particle size, 2 m) was treated with 5 mass%, 10 mass% and 15 mass% polydimethyl siloxane (PDMS) by rotary evaporation from cyclohexane. An amorphous calcium phosphate powder was treated with 10 mass% PDMS. Powders were dry-pressed into a special stainless steel die designed for production of blocks suitable for dental CAD/CAM machining. Half of the blocks were isostatically pressed. Blocks were heat treated, silanized and infiltrated with methylmethacrylate. Discs were cut from finished blocks, strength tested in biaxial flexure and the results analyzed by MANOVA and a 95% multiple range test.

BONDING TO DRY DENTIN WITH WATER-MODIFIED ACETONE-BASED PRIMERS
LCDR Richard L. Romney, DC, USN

Acetone-based dentin primers have stronger shear bond strengths (SBS) when applied to moist dentin than when applied to dry dentin. The goal of this in vitro study was to determine if a water-modified dentin primer applied to dry dentin could result in SBS as high as those obtained with water-free primer on moist dentin. One hundred thirty-five extracted human molars were cut horizontally to expose coronal dentin, wet sanded, treated with 10% H3PO4 for 30 seconds, and rinsed. Except for the control samples, which were kept visibly moist, each surface was thoroughly dried prior to applying 5 coats of a mixture of 20 ml of Primer A [Mg-bis-(N-tolylglycine-glycidyl methacrylate)] and 40 ml of Primer B (Pyromellitic glycerol dimethacrylate), each containing 0, 5, or 15 wt % H20, resulting in overall water concentration of 0 to 15 wt %. The teeth were assigned to one of 9 groups, and various combinations of the primers were used in each group: Group 1, moist control (0% H20); 2 (0% H20); 3 (1.5% H20); 4 (3.4% H20); 5 (5% H20); 6 (5% H20); 7 (10% H20); 8 (12% H20); 9 (15% H20). After priming, an unfilled resin was applied, air-thinned, and light-cured for 20 s, followed by placement and light-curing of a composite resin contained in a steel iris. The assemblies were immersed in water for 24 h and tested in shear mode by pressing on the steel iris. There was no statistically significant difference between the moist control and groups 3, 4, 5 and 6. Groups 7, 8 and 9 containing more than 10% H20 and group 2 (0% H20, dry dentin) had significantly lower SBS values. The results suggest that water added to the dentin primer system may remoisten the surface collagen of dried dentin leading to high SBS similar to those obtained on moist dentin, provided the amount of water does not adversely affect the inherent material properties leading to lower bond strength values.