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JADA Specialty Scan - Cosmetic/Esthetic
Cosmetic/Esthetic - A Quarterly Newsletter on Dental SpecialtiesJADA

Strong translucent ceramics for dental restorations

Zirconia ceramics with a slight variation in composition have substantially different translucencies, mechanical properties and hydrothermal stability.

That’s the key finding of a study published in the December issue of Dental Materials.

In the study, a team of researchers set out to measure the optical properties, mechanical properties, and aging stability of yttria-stabilized zirconia with different compositions. Researchers compared different commercially available and experimental starting powder compositions in an effort to achieve the ideal balance of translucency, hydrothermal stability, and mechanical properties of sintered 3 mol% (mole percent) yttria-stabilized tetragonal zirconia procrystalline (3Y-TZP).

Researchers evaluated 5 zirconia compositions; 3 were commercially available, and 2 were made in a laboratory. The powders made in the laboratory were based on co-precipitated alumina-free 3Y-TZP powder, which was also used as a reference material for the translucency measurement. All starting powders contained a fraction of monoclinic phase zirconia.

Researchers prepared disk-shaped zirconia specimens for translucency measurements. They controlled the final thickness during the polishing process by using a digital micrometer and measured surface roughness with a surface profiler to ensure an identical roughness level for all ceramic grades. They calculated the translucency parameter by measuring the color difference between the same specimen against white and black backgrounds and the contrast ratio from the spectral reflectance of light of the specimen over a black and white background.

No monoclinic phase was found in any of the zirconia ceramics after sintering. The hardness of the 5 zirconia ceramics was statistically the same, while the fracture toughness and bending strength were significantly different. Significant differences in translucency and contrast ratio were noted among the 5 zirconia ceramics of similar thickness and roughness.

Reducing the amount of alumina, increasing yttria content, and adding 0.2% lanthanum oxide all improved the translucency of 3Y-TZP ceramics. The biggest factors limiting the translucency of 3Y-TZP were light scattering at the grain boundaries, pores, and secondary phases.

“In order to avoid light scattering from distinct alumina particles,” the authors wrote, “the alumina content had to be reduced below 0.25 wt.%, but full elimination of alumina was not necessary,” Reducing alumina content significantly lowered the hydrothermal stability of 3Y-TZP ceramics. Introducing optically isotropic cubic phase zirconia by increasing the yttria content had the best effect at enhancing the translucency as well as aging resistance over 3Y-TZP ceramics.

Read the original article.


Consulting Editor: Luiz Meirelles DDS, MS, PhD
Director, Professional Products and Standards
ADA Science Institute

The influence of restorative materials on mucosa discoloration

The use of reconstructive materials can result in a noticeable discoloration of the mucosal tissue that tends to decrease with increased mucosal thickness. Using fluorescent zirconia or gold alloy can lead to the least amount discoloration.

Those findings are from a study published online July 25 in Clinical Oral Implants Research.

In the study, researchers sought to measure the discoloration of mucosa caused by different ceramic and metal-based materials depending on the soft-tissue thickness.

Researchers used maxillae from 6 pigs within 4 hours of sacrifice. All animals were roughly 6 months old. Researchers chose the in vitro animal model because of its anatomy with partially edentulous sites and mucosa that resembles human keratinized mucosa in color and texture.

Researchers used ceramic and metal-based reconstructive materials for the study. They produced test specimens of 4- x 4 x 1.5-millimeters for each material.

Researchers made incisions at each side of the pig maxillae in the region distopalatal to the last molar, and a 1-mm partial-thickness trap-door–type flap. They obtained spectrophotometric measurements at these sites to serve as controls for the native mucosa. Researchers placed the specimens underneath the trap-door–type partial-thickness flap before obtaining another spectrophotometric measurement.

Researchers obtained 36 measurements per group. They chose a circular area 2 mm in diameter for shade determination and obtained colorimetric measurements by using a spectrophotometer. They obtained 3 consecutive pictures perpendicular to the mucosa and pooled them together and averaged them. They calculated differences in color by subtracting the baseline measurements from the consecutive measurements with the specimens.

For the group of ceramic abutment materials, researchers found that the use of fluorescent zirconia produced the most favorable esthetic results in terms of discoloration. For the group of metal-based materials, researchers found that use of gold alloy or pink-shaded titanium alloy as a modification of the implant shoulder produced the most promising results in cases of a thin peri-implant mucosal tissue.

A key limitation of the study was the translation of in vitro data into clinical settings. This related specifically to the use of animals and the lack of blood flow. Researchers recommended further research to obtain the same data in a clinical environment applying the same technologies.

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Premolar axial wall height effect on CAD/CAM crown retention

Premolars restored with adhesively luted, computer-aided design/computer-aided manufacturing– (CAD/CAM) fabricated lithium disilicate crowns based on a 10% total occlusal convergence (TOC) showed similar failure stress resistance with occlusocervical axial wall heights of 1, 2, and 3 millimeters.

That finding is from a study published in the November/December issue of Operative Dentistry.

The authors conducted the study to measure whether adhesion technology could compensate for reduced axial wall height in premolar all-ceramic crowns luted to preparations with a 10% TOC.

Researchers assigned 48 freshly extracted premolars to 1 of 4 groups of 12 each. Occlusal surfaces were removed to 1 millimeter below the marginal ridge with a slow-speed, water-cooled diamond saw. They mounted the sectioned teeth in autopolymerizing denture base methyl-methacrylate resin.

Teeth in the 4 groups received occlusal reduction to occlusocervical (OC) heights of 0, 1, 2, and 3 mm, respectively. The 0-mm axial wall height group received a buccal lingual groove preparation featuring the approximate width and one-half depth of a no. 8 round bur across the total occlusal surface. Researchers then measured the prepared tooth surface areas by using a digital recording microscope that showed the bonding surface area.

One operator restored the specimens by using a CAD/CAM device. Restorations were milled from a lithium disilicate ceramic material followed by crystallization and glaze in a dental laboratory ceramic furnace.

The operator then prepared the restoration’s intaglio surface with a 5% hydrofluoric acid-etch solution for 20 seconds, rinsed it with water spray, and dried it with oil-free compressed air. The operator applied a coat of silane agent to the etched surface using a monobrush. After 60 seconds of reaction time, the operator air-dried the silicone agent by using oil-free compressed air.

The operator placed a self-adhesive resin cement onto the intaglio surface of the ceramic restoration and seated it on the preparation using finger pressure. After excess cement was removed, the operator light cured the restorations at the buccal, lingual, and proximal marginal areas for 80 seconds.

Researchers found that preparations with 3-mm axial height had much greater failure resistance than the preparations with 0- and 1-mm axial wall heights. There was no difference in failure load between the 2- and 3-mm axial wall height groups. Mode of failure, they concluded, depended largely on the preparation wall height.

Researchers cautioned that the study’s preparation TOC of 10% “may be considered very conservative and may not be routinely achieved in the clinical environment.” They further noted that all OC axial wall height preparations showed failure loads greater than those reported for normal human bite strength.

Read the original article.


Opalescence of human teeth and dental esthetic restorative materials

Composite and ceramic materials that can reproduce the opalescence of natural teeth should be designed by adjusting the composition of the filler and matrix phase.
That key finding is part of new research published in the December issue of Dental Materials Journal.

Researchers set out to review the opalescent property of teeth and the application and mimetic reproduction of this property in dental esthetic restorations based on previously published articles.

The study, which included 29 articles and 24 related references, used 2 key indexes to evaluate the opalescence of teeth. The OP-RT is calculated as the difference in the yellow-blue and red-green coordinates between the reflected and transmitted colors measured by spectrophotometers. OP-BW is the difference in the quantity of red-green and yellow-blue over black and white backgrounds in the reflected color.

Researchers noted that the opalescence varied based on the configuration of the spectrophotometer used for measurement and the origin of the enamel. They also noted that OP-RT values varied by the illuminant used to measure color.

In 1 study, researchers observed that OP-RT values increased as the thickness of resin composite increased from 1 millimeter to 2 mm. In resin composites with a thickness greater than 1 mm, the opalescence was influenced by the thickness, and the translucency greatly decreased with a significant increase in opalescence.

When considering changes after polymerization or aging, researchers cited a study that noted a mean OP-RT value for indirect resin composites of 24.3 before polymerization and 19.9 after polymerization. Mean OP-RT values for direct resin composites were 25.6 before polymerization and 12.4 afterward. Opalescence of the resin materials varied depending on the material, shade group, and polymerization.

In studying the influence of resin composites on opalescence, researchers cited a study that noted that such composites with 0.1 to 0.25% TiO2 nanoparticles could simulate the opalescence of human enamel. “Since the scattering and absorption characteristics influence the color of the resin composites,” they wrote, “the size and volume fraction of the fillers should be optimized for best color reproduction, considering the refractive indices of filler and resin matrix.”

In studying the influence of ceramics on opalescence, researchers cited a study that compared the microstructures of teeth, natural opals, and opal dental enamel ceramics. That study found that the presence of dispersed particles or phase-separated glass was found to cause opalescence in dental porcelains. The phase-separated glass in 1 of the enamel porcelains was found to best resemble the microstructure of natural opal mineral and teeth and was found to be the most opalescent material.

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Share the smile-enhancing benefits of veneers

The ADA offers a brochure, “Dental Veneers,” to help patients visualize how veneers can improve their smiles. The brochure outlines the reasons to choose veneers, with before-and-after photos and illustrations of veneer placement.
“Dental Veneers” (item W288) also lists the benefits of porcelain versus composite veneers, as well as placement steps and factors to consider before opting for veneers. The 6-panel brochure is sold in packs of 50. Brochure contents can be viewed on this page. To order, call 1-800-947-4746 or go to Readers who use the code 16406E before December 30 can save 15 percent on all ADA Catalog products.


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Any statements of opinion or fact are those of the authors and do not necessarily reflect the views of the American Dental Association. Neither the ADA nor any of its subsidiaries have any financial interest in any products mentioned in this publication. Any reference to a product or service, whether in advertisements or otherwise, is not intended as an endorsement or as approval by the ADA or any of its affiliated organizations unless accompanied by an authorized statement that such approval or endorsement has been granted.

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