Innovations in Ceramics
Milled options are becoming stronger, more translucent
The art and science of dental materials are constantly evolving to create more efficient methods of producing stronger and more esthetic restorations. Digital technology appears to be playing a large role in this evolution for technicians and dentists. According to a recent survey,1 70% of full-service laboratories offer digitally manufactured restorations. Although a much smaller portion of dentists do their own milling, interest is slowly rising. A number of new restorative material options allow dental professionals to be more time-efficient in fabricating beautiful, natural-appearing restorations.
Veneered Zirconia Restorations
Zirconia is one of the only restorative materials that must be fabricated in the laboratory or the dental office by means of digital technology. Today this material is used as a full-contour monolithic single crown and as multiple-unit monolithic fixed partial dentures. Initially, partially sintered zirconia blocks were milled and stained prior to the sintering process, which was completed in 9 to 12 hours. For individual posterior single units, where esthetics are not as critical, a stained monolithic zirconia restoration has been acceptable.
In the anterior region, zirconia was first used as a substructure that was fully veneered with a presumably compatible porcelain. Problems began to surface with the delamination of the veneering porcelains. Chipping and bulk fractures of the veneering porcelain without exposure of the zirconia core were found to be occurring.2,3 Causes included a disparity between the coefficients of thermal expansion between the veneering porcelain and the zirconia substructure, low fracture strength of the porcelains, framework design flaws, and sliding contact fatigue.4-6 The bond strength of a feldspathic veneering porcelain can also be affected by the sintering process and the surface treatment of the zirconia substructure.7
For single and multiple anterior units (fixed partial dentures), advances include changes to framework design and fabrication methods. For example, the lingual aspect can be left exposed and milled to full contour while the facial aspect is milled, leaving space for veneering porcelains. For fabrication, hand-layering of the veneering porcelain has been replaced with a heat-pressed method similar to the press-to-metal technique.3,8 In theory, this technique–called "overpressing"9–would seem to have an increase in fracture resistance due to a reduction in air voids and fewer firing cycles.3
Another modification to the fabrication of a veneered zirconia bridge restoration is the CAD-on-restoration technique, where the veneering portion of the restoration is milled and then secured to the fully sintered zirconia framework. For example, a feldspathic material (VITABLOCS® TriLuxe Forte, VITA Zahnfabrik, www.vita-zahnfabrik.com) can be milled and then cemented to the framework with a resin cement, or a lithium disilicate (IPS e.max® CAD) can be milled and then attached to the fully sintered zirconia framework using glass fusion ceramics10 before the CAD material is crystalized or sintered.
Polychromatic Full-Contour Zirconia Restorations
Despite improvements, the susceptibility of porcelain-to-zirconia to chipping and delamination still exists. Lazari and colleagues11 reported that incomplete bonding between the veneering porcelain and the zirconia framework can affect the biomechanical behavior of the ceramic veneer, increasing chipping or fractures.
In recent years, there has been an increasing trend away from a bi-layered to a monolithic full-contour zirconia restoration, eliminating the veneering porcelain and all the problems associated with that combination. Full-contour zirconia single units were first used in the posterior regions mainly because of their inferior esthetic qualities, stemming from their large grain size and resulting high level of opacity.12,13 In the laboratory, the milled monolithic opaque crown would then require a time-consuming manual dip-coloring and drying process before the lengthy sintering process began. The introduction of pre-shaded blocks (eg, Zirlux®, Zahn Dental, www.zirlux.com) has helped to lessen the time required to produce an esthetic monolithic zirconia restoration.
Taking the pre-shaded concept even further, there are now multi-shaded zirconia milling disks that simulate the color gradients that occur in the natural tooth. Kuraray introduced polychromatic multi-layered zirconia milling disks, which eliminate the coloring and drying process.
Translucent Full-Contour Zirconia
The goal of ceramic manufacturers is to produce a restorative material that approaches the optics found in nature. The level of translucency in dental porcelains is determined by the size, chemical nature, and number of crystals in the matrix.14 The translucency in zirconia is derived by the level of alumina particles, impurities, and structural defects that cause the light to scatter. By decreasing the source of light-scattering particles, the level of translucency increases.15
The thickness of ceramic material also affects the translucency. Wang and colleagues16 reported that as the thickness of the ceramic material increases, the degree of translucency decreases. Therefore, the type of zirconia used for full-contour restorations, which has a greater thickness than the veneered porcelain-to-zirconia, must be modified to create the translucency required.
The strength of zirconia is increased with the addition of alumina.17 However, having a different refractive index18 than zirconia, the addition of alumina to the formulation reduces the translucency.
3M ESPE (www.3mespe.com) changed the distribution and content of aluminum oxide in its original Lava™ product to 0.1wt%, which creates a more translucent version and at same time maintains the strength of the original. The new Lava™ Plus High Translucency Zirconia still has less translucency than glass ceramics but four times its strength.19
Sagemax Bioceramics Inc. (http://sagemax-dental.com) introduced NexxZr-White in 2011, a material that has a biaxial flexural strength of 1150 MPa. In 2013, it launched a pre-shaded version called NexxZr-19 Pre-shaded, which has a flexural strength of 1300 MPa and is available in 19 different shades. The position of the restoration can be adjusted in the milling disk depending on the amount of color required throughout the restoration. This year, it has introduced NexxZr®T, which has a biaxial flexural strength of 1150 MPa, a 35% 1-CR translucent factor (1 mm), and a 50% light transmission at 0.6-mm thickness. This version is most translucent in the incisal area.
Zenostar® Full Contour Zirconia (Ivoclar Vivadent) is available in six different pre-shaded milling disks. The Zenostar Zr Translucent has a 40% light transmission at 0.6 mm, with less than 0.1% aluminum oxide and a flexural strength of 1200 MPa.
BruxZir® (www.bruxzir.com, www.bruxzir.com) is another zirconia restorative that can be utilized as a full-contour material. Having an average flexural strength of 1200 MPa, BruxZir milling disks are also available in pre-shaded versions.