Dr. Liang Chen, Dr. Douglas Brown, Dr. Byoung In Suh
Zirconia: the Future of Aesthetic Indirect Restorative Dentistry
Current dental all-ceramic systems can be classified into three categories: porcelains, glass ceramics (such as lithium disilicate), and polycrystalline ceramics (such as zirconia and alumina). Zirconia-based restorations and lithium disilicate ceramics are the fastest growing segment in the North American laboratory market. Zirconia’s high strength and fracture toughness are two of its unique advantages compared to current glass filled ceramic systems. Zirconia has a flexural strength (900-1100 MPa) that is twice as strong as alumina, and three times as strong as e.max lithium disilicates. Combined with a fracture toughness of 8-10 MPa (twice greater than alumina) and the ability to “transformational toughen”, zirconia can now be considered a universal indirect restorative material.
The computer-aided design/manufacturing (CAD/CAM) technology, together with excellent mechanical and aesthetic qualities, has expanded the clinical use of zirconia to include aesthetic conservative restorations and long span restorations. In addition, zirconia’s hard and dense surface is ideal for resisting wear damage to opposing dentition and making zirconia an attractive material for dental restorations. The applications of zirconia (e.g. Lava, 3M; Cercon, Dentsply; Procera, Nobel Biocare) in dentistry include: endodontic posts, implants, implant abutments, orthodontic brackets, cores for crowns, and fixed partial denture prosthesis frameworks. We will continue to see the growth of zirconia restorations due to its esthetic features, strong strength and conservative preparation with minimal reduction.
Zirconia Can’t be Bonded?
Ever since zirconia crowns have been commercialized in early 2000s, researchers and clinician have tried various methods to bond zirconia, such as hydrofluoric acid-etching followed by silanization, or silica-coating followed by silanization. Hydrofluoric acid-etching/followed by silanizaiton has been proved to be a predictable procedure for bonding glass-ceramics. However, it is not effective for zirconia bonding. Polycrystalline zirconia ceramics contain no glass compositions, so they can’t be either etched or silanized (Figure 1). Silica-coating is effective for the surface treatment of softer materials, such as metal. However, it is not effective for the treatment of zirconia, which has a much harder surface (Figure 2). Therefore, many clinician and researcher experienced failure in zirconia bonding in early 2000s, and they thus believed that zirconia can’t be bonded.
Figure 1: Hydrofluoric acid-etching and silane can’t improve resin-zirconia bonding
Figure 2: SEM images of zirconia surface after silica-coating with and without water-rinse (Left: before water rinse; Right: after water rinse). Nano-silica was “attached” on zirconia surface, but it was easily rinsed off from zirconia.
Zirconia Demands Adhesively Bonding
Research studies showed that the retentive zirconia restorations, such as full-coverage zirconia crowns, could be successfully luted (without the need of adhesively bonding) with RMGI or self-adhesive resin cements. However, the use of Zirconia in conservative aesthetic dentistry in combination with short, non-retentive preparation designs commonly seen in today’s complex restorative cases demands that chemistry be created to allow development of adhesive primers, specific to glass-free polycrystalline ceramic materials (zirconia). This chemistry enhances the adhesion between hydrophobic resin composites/cements and indirect polycrystalline ceramic substrates. BISCO’s goal was to create a cohesive interface between the zirconia and resin cement, improve initial bond strengths capable of resisting hydrolytic degradation, and allow for the use of stronger and more durable resin cements.
The Optimal Procedure for Zirconia Bonding
Numerous research studies showed the most reliable method to bond zirconia is sandblasting followed by priming with a phosphate monomer. Specifically, the optimal clinical procedure for zirconia bonding includes 4 steps:
1) Sandblast zirconia with alumina sand
Studies showed that sandblasting zirconia significantly improved the durability of resin-zirconia bond (With sandblasting: initial bond strength 40 MPa, bond strength after aging 30 MPa; Without sandblasting: initial bond strength 21 MPa, bond strength after aging 0 MPa). Other studies showed that the best way to sandblast zirconia is to use alumina sand (particle size 30-50 μm) at air pressure (15-45 psi). The bigger particle, such as 250μm, might decrease zirconia strength.
2) Prime zirconia with Z-Prime Plus
After zirconia is sandblasted, rinsed, and dried. Zirconia should be primed with a MDP-containing zirconia primer. Research showed phosphate-monomer chemically bonded to zirconia. Among various phosphate-monomers, MDP is the most effective one to bond zirconia. In 2009, Bisco invented the world’s first dedicated zirconia primer, Z-Prime Plus (Figure 3). Z-Prime Plus changed the zirconia bonding history. It made impossible mission of zirconia bonding become possible. Studies showed Z-Prime Plus formed chemical bond with zirconia, and helped to form strong (30 MPa) and durable resin-zirconia bond (Figure 4).
Figure 3: MDP-based Z-Prime Plus, the world’s first dedicated zirconia bonding agent
Figure 4: Z-Prime Plus helped to form strong and durable resin-zirconia bond strength. The bond strength didn’t decrease after 5 year-aging (30 MPa = 306 kg force/cm2).
3) Apply adhesive on tooth
Literature showed the best tooth treatment method is selectively-etching enamel followed by application of a universal adhesive. The reason is that self-etching dentin (instead of total-etching) would reduce the possibility of post-op sensitivities. Selectively-etching enamel would reduce marginal discoloration. It is important for clinician to know whether a dental adhesive is compatible with dual-cure resin cements. If the adhesive is not compatible, clinician should mix the adhesive with an Activator before its application on tooth. Bisco’s All-Bond Universal is universally compatible with all of the dual-cure, self-cure, and light-cure resin cements, without the need of a separate activator.
4) Seat restoration with a dual-cure resin cement
Zirconia is not as translucent as many glass ceramics. Light intensity may reduce substantially (80-90% reduction) when it goes through zirconia restoration and reaches to resin cement. It is important to use a dual-cure cement with a good self-curability. Bisco’s Duolink and Duolink Universal have a great self-curability, due to their unique initiator systems.
Zirconia is Special – Saliva Contamination
Unlike other indirect restorations, zirconia would react with phosphate-containing compounds, which include saliva and phosphoric acid etchant, resulting in adverse effect on zirconia bond. The adverse effect can’t be eliminated by simply ultrasonication cleaning with water or organic solvents. Only strong alkaline solution (such as potassium hydroxide solution) can clean the contaminant and eliminate the adverse effect. ZirClean, which is based on potassium hydroxide, has thus been developed (Figure 5 and 6).
Figure 5: Saliva contamination reduced zirconia bond substantially. Ultrasonication with water can’t eliminate the adverse effect of saliva contamination. Only alkaline gel ZirClean can effectively eliminate the adverse effect.
Figure 6: ZirClean, a strong alkaline gel, which is based on potassium hydroxide, has been launched by Bisco, to address the clinical issue of zirconia saliva contamination
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Recommended Clinical Procedure for Zirconia Bonding
For non-retentive zirconia restorations (crowns, short crowns, veneers, inlay, onlay, Maryland bridge, etc), an adhesively bonding protocol is required. The following is the recommended clinical protocol for zirconia bonding: