| The ceramic crown continuum | 
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| Fig. 5 The most esthetic crown (IPS Empress) is given an arbitrary esthetic value of 10 and a well-done PFM a value of 6.5. Zirconia-cored crowns are in between the two, with a value of 7.5. The esthetic scale moves from right to left and the longevity scale from left to right. |
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There are several zirconia-cored systems on the contemporary market that show considerable promise as PFM replacements. This may become more desirable as the price of gold escalates, driving the laboratory costs for PFM restorations higher. However, it is critical to understand that this technology is essentially in its infancy.
Not all zirconia-cored restorations are the same. Zirconia cores milled in the green state and then sintered to “heal” any defects created by the milling process are more desirable than cores that are high-temperature milled. Near-surface damage results from high-temperature milling.8 Systems with several core colors are preferred over those that only have one or two (Fig. 6).
Although few long-term studies on zirconia-cored restorations have been published, it is clear that the cores are strong. They also possess a property called “transformation toughening.” This occurs when a defect begins to propagate, and the core material in the area of the defect changes from the tetragonal to the monoclinic form. This change is accompanied by a slight expansion, which places the defect under compression and prevents propagation. This prolongs the life-span of these all-ceramic restorations.
But there is a downside. Many procedures, including extensive occlusal adjustment, autoclaving, and internal air abrasion can, under certain circumstances, cause most of the core to undergo the transformation. This dramatically weakens the core, and increases the system’s inherent technique sensitivity.
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| Fig. 6 Zirconia systems that provide multiple core shades and shades of dentin and enamel porcelain provide optimal esthetic results. |
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While zirconia cores are strong, a high incidence of ceramic veneer chipping has been reported. A review of on-going trials of zirconia-cored restorations found the incidence ranges between 8% and 50% at one and two years.9 This is in stark contrast to reported veneer fracture rates for PFMs, which is between 4% and 10% at 10 years. The etiology of this high fracture rate is speculative and may be a result of the elastic modulus of the core, a lack of understanding of the basic mechanism of bonding of the ceramic veneer to the core, or lack of support of the veneering porcelain by the core.
Zirconia cores are milled from a die of the preparation to a uniform thickness of 0.3 mm (anterior teeth) or 0.5 mm (posterior teeth). Most clinical preparations have one or more variables from the ideal, so this may result in uneven thicknesses of the veneering porcelain. If this exceeds the industry standard of 2 mm, such unsupported porcelain is likely to fail. This lack of support, if identified, can be modified before milling by altering the computer program (Fig. 7).
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