Written by Robert G. Ritter, DMD Monday, 16 September 2013 09:43
The ongoing evolution of restorative materials is bringing dentists more options than ever before to achieve the most desirable mix of properties. In the field of laboratory-fabricated restorations, clinicians and their lab partners have long been seeking to balance aesthetics, strength, and ease of use.
While the concepts of strength and ease of use are well understood, aesthetics are of course more subjective, yet can still be discussed in some objective terms. When we speak of aesthetics in this category, we typically mean 2 things: color and translucency. In order to best mimic natural tooth structure, a restoration must reflect, scatter, and absorb light similar to the way that a natural tooth does. Lithium disilicate has proven popular in recent years, thanks to its high performance in the aesthetic category. It has come with some tradeoffs, specifically its lower strength than zirconia and the inadvisability of traditional cementation (resin bonding is the preferred method). Meanwhile, in the zirconia field, recent advances have enabled dentists to achieve greater aesthetics than ever before, while maintaining the strength for which the material is known. A major advantage of utilizing zirconia is the ability to traditionally cement with resin-modified glass ionomers (RMGIs). An examination of some of the factors that impact the performance of zirconia will highlight what has changed with recent improvements.
As we know, translucency refers to the degree to which a material allows light to pass through it. In zirconia, the presence of impurities and/or structural defects can have an impact on the material’s translucency. Zirconia impurities can cause light absorption, obviously detracting from the material’s ability to allow light through. Additionally, when light encounters structural defects like pores, it scatters—again reducing the translucency.
An additional factor that can have a major effect on zirconia’s translucency is the content and distribution of alumina. Alumina is incorporated into some zirconia materials in order to increase their aging stability, among other benefits. The drawback, however, is that alumina has a different refractive index than the rest of zirconia material, which scatters light, resulting in similar translucency drawbacks as those caused by structural defects. To avoid this issue, some zirconia materials do not contain alumina, but—as could be predicted—this makes them vulnerable to problems with aging stability.
To understand how zirconia is colored, a brief review of principles surrounding color and light is necessary. Colors are created when substances absorb certain parts of the rainbow spectrum. For example, when we see a green leaf, it appears as such because the red and blue portions of the light spectrum have been absorbed.
In zirconia coloring, traces of ions are used to change the material’s light absorption properties; these ions are incorporated into the zirconia structure during sintering. Variables that may affect coloring include the ions used in this process, as well as their concentration and distribution. The microstructure of the zirconia can have an impact here as well, and as the chemical composition of each zirconia is different, shading liquids must be customized to each specific zirconia for best results. However, not all zirconia products are offered with dedicated shading systems.
Because of zirconia’s hardness, it is sometimes assumed that the material is abrasive to opposing tooth structure. This is incorrect, however, as the material’s smoothness is actually the factor that determines its abrasivity.1 Excessive abrasion can be avoided with a smooth zirconia surface because there is little mechanical interlocking between it and the opposing surface. However, other commonly used materials such as veneering porcelain and pressed glass ceramic have been found to be more abrasive to enamel than certain zirconias.2
A New High Translucency Option
A recently introduced zirconia material (Lava Plus High Translucency Zirconia [3M ESPE]) provides dentists with a new restorative option that combines zirconia’s well-known strength with improved aesthetics. Dentists are likely familiar with the original Lava Zirconia material from the same manufacturer, which has now been around for more than 10 years, and has amassed a significant clinical track record. The high translucency version can be used in either a monolithic or traditionally layered restoration, and is suitable for both anterior and posterior placements. Its translucency is enabled by high quality processing, which minimizes the effects of impurities and structural defects as described above. Alumina content for the material is reduced to a tenth of its weight, while the distribution of alumina is optimized. Both of these factors enable the material to maintain the aging stability that traditional Lava zirconia has achieved.3,4
In addition to translucency, the color capabilities of this material are unique as well. Eighteen dyeing liquids are available, giving labs the ability to match the 16 VITA classical A1-D4 shades plus 2 bleach shades. The dyes use ionic components for yellow, red, and gray colors, all customized specifically for the chemistry of the zirconia and engineered to ensure uniform distribution during drying and sintering.
The wear properties and strength of this new zirconia generation have been shown to be as reliable as its predecessor. Research shows that aging does not affect the smoothness of the material and therefore it is “antagonist friendly,” similar to Lava Zirconia.3,4 Additional data confirms that the new zirconia’s strength is equivalent to the previous generation. The same study on strength also examined glass ceramics and found that while they offered additional translucency, they demonstrated approximately two thirds less flexural strength than zirconia.5
The following case demonstrates the use of this material with overlying ceramic to create highly aesthetic anterior restorations.
Diagnosis and Treatment Planning
The patient was a 79-year-old woman who had been a long-time patient of the practice. Her existing PFM restorations had been placed more than 25 years ago, and she stated she was unhappy with the aesthetics, both due to the color of the restorations and the fact that recession on many of the teeth had exposed black triangles (Figures 1 and 2). The patient was also interested in slightly modifying the shape of her teeth. Her first premolars were missing, and she was interested in options to improve the appearance of the first molars that had moved into the places of the premolars (Figures 3 and 4).
|Figure 1. Patient was unhappy with the aesthetics of the existing restorations, including the color and the black triangles.||Figure 2. Recession throughout the years had caused black triangles and exposure of the PFM margins.|
|Figure 3. Molars had moved into the position of the premolars.||Figure 4. The molars were evident in the side view.|
The patient was informed that because the color of the existing restorations could not be modified, the most fitting treatment would be to remove the existing crowns and replace them. A total of 10 units would be placed, replacing the existing crowns on teeth Nos. 6 to 11 and 4 and 13, while additionally placing “bicuspidized” crowns on teeth Nos. 3 and 14 by bulking up the mesial buccal cusps and by minimizing the distobuccal cusps. The patient agreed to this plan.
A full series of photos was taken, as well as upper and lower preliminary impressions with a vinyl polysiloxane impression material (Imprint III [3M ESPE]). A stick-bite and Kois face-bow records were taken, and the case was mounted (Panadent articulator). Local diagnosis records were also taken before submitting the case to our dental laboratory team. The lab was instructed to create a diagnostic wax-up and to modify the shape of the teeth to “feminize” them, with particular attention to removing the black triangles, with the “bicuspidization” of teeth Nos. 3 and 14.
The patient returned to the office 3 weeks later. After the wax-up was reviewed and approved by the patient, she was anesthetized for the procedure. The anterior 8 crowns were removed and prepared, and the molars were prepped as well (Figure 5). A final polyether (Impregum Soft [3M ESPE]) impression was taken, and a bite registration (AFFINITY Quick Bite [CLINICIAN’S CHOICE]) was also taken. The wax-up was used to create a putty matrix, and 10 temporaries were fabricated (Protemp Plus [3M ESPE]) in shade A1. The provisionals were tried-in to verify their fit and contour, then polished outside of the mouth. The provisionals were then cemented with a resin-based temporary cement (TempBond Clear [Kerr]).
One week later, the patient returned for an appointment to adjust her occlusion and to ensure that the envelope of function was not altered. Final shades were taken for the prosthetics, and a final stick-bite and impression of the temporaries were taken and sent to the dental laboratory for final fabrication of the restorations.
The lab team poured up the impressions, mounted the resulting models, and scanned the case. Then, they milled 10 copings out of the Lava Plus zirconia. Next, porcelain was applied and the case was sent back to our dental office (Figure 6).
|Figure 5. The prepared teeth, shade recorded.||Figure 6. The bis-acryl provisional (shade A1).|
|Figure 7. Removal of gelled excess cement.||Figure 8. Occlusal view of the seated restorations.|
|Figure 9. Biscupidization of the molars created a more feminine appearance.||Figure 10. Final result.|
|Figure 11. The exceptional translucency of the zirconia enabled a lifelike result.|
Upon the patient’s return, the provisionals were carefully removed and all 10 units were tried in the mouth. The patient evaluated the size, shape, and color of the final units and approved them for final cementation. The restorations were removed and the inside of each unit was cleaned with a universal cleaning gel (Ivoclean [Ivoclar Vivadent]) to prepare them for traditional cementation. The crowns were then rinsed and dried, after which a zirconium primer (Z-Prime Plus [BISCO Dental Products]) (to optimize the bond of the resin cement to the zirconia) was applied to the inside of each restoration and air-dried.
Plain flour pumice with water was used to remove any surface contamination from the 10 prepped teeth, and the teeth were then rinsed and dried. A RMGI cement (RelyX Luting Plus [3M ESPE]) was applied in a thin layer to the inside of each crown, and the crowns were seated in place, starting with the central incisors. Each crown was spot tacked (a light-activated RMGI cement) for 5 seconds on the facial and 5 seconds on the lingual to set the cement to a gel stage to encourage easy removal of excess (Figure 7). The cement was allowed to set for an additional 5 minutes and the final removal of any excess was completed. No final adjusting or polishing was necessary (Figures 8 to 11).
The patient was very happy with the outcome of the procedure and was excited to have a prettier smile with which to enjoy the rest of her golden years.
In addition to the benefits in aesthetics and strength that have already been discussed, a key benefit of zirconia restorations is the practitioner’s ability to place them with conventional cements. In a multi-unit case such as this one, the additional ease of use of traditional cementation is much appreciated, making for a very easy delivery day.
As dentists continue to seek out ways to streamline their tools while providing patients with restorations that will look great and serve them well for years, high translucency zirconia provides them with a very helpful option.
The author would like to thank Bob Williams and the dental laboratory team at Synergy Ceramics.
- Oh WS, Delong R, Anusavice KJ. Factors affecting enamel and ceramic wear: a literature review. J Prosthet Dent. 2002;87:451-459.
- Sorensen JA, Sultan EA, Sorensen PN. Three-body wear of enamel against full crown ceramics. J Dent Res. 2011;90(special issue A). Abstract 1652.
- Dittmann R, Urban M, Schechner G, et al. Wear behavior of a new zirconia after hydrothermal accelerated aging. J Dent Res. 2012;91(special issue A). Abstract 1317.
- Dittmann R, Urban M, Braun P, et al. Wear behavior of zirconia after hydrothermal accelerated aging. J Dent Res. 2011;90(special issue B). Abstract 307.
- Wang F, Takahashi H. Translucency and biaxial flexural strength of dental ceramics. J Dent Res. 2012;91(special issue A). Abstract 422.
Disclosure: Dr. Ritter reports no disclosures.
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