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Monday, September 08, 2008



Peter Pizzi
CDT, MDT

The use of milled all-ceramic CAD/CAM materials has increased dramatically in the past three years, particularly the use of zirconia as a substructure material. As a result, there is a need for us, as technicians, to rethink our normal layering protocol.
For almost 40 years, ceramic layered onto metal substructures has been the standard of restorative treatment in dentistry. During that time, we have witnessed many changes in the layering materials as well as the techniques used to build our crowns and bridges. Today, through material and technique evolution, porcelain-to-metal has reached an all-time esthetic high.
We have strived to understand the material science behind what we do in order to achieve this level of esthetic excellence. To be clear, let’s understand that metal and porcelain do not like each other all that much. Yet, we have managed to make them work together quite well. To achieve esthetic excellence we learned that we had to mask out the metal substructure with opaque. By masking out the metal, we create a base for our overlay porcelain that will give us the proper chroma and value we need to achieve the esthetics we desire.

CHALLENGES OF ZIRCONIA
Material science has now stepped up and brought us zirconia substructure material and, with it, different challenges and/or advantages. Unlike metal, zirconia is a more translucent material that allows light transmission. As a result, our mindset and layering techniques have to change if we want to maintain esthetic excellence. The old standard of opaquing the substructure will not produce the best results when working with zirconia. Our goal now is to create the proper base chroma and value with a material that allows light transmission. Even though the ability of zirconia to allow light transmission is a huge advantage, how we use this characteristic is yet another.
The biggest challenge we must face when planning our layering strategy is zirconia’s non-fluorescent nature. To achieve the fluorescence of natural teeth, we must first treat the zirconia core with a fluorescent material. Although light transmits well with zirconia materials, the ability of this material to fluoresce is non-existent.
 

POWDERS THAT BOOST FLUORESCENCE

All ceramic systems differ in some manner, so understanding your system’s strengths and weaknesses is a must. Using a fluorescent light in a very dark environment will help you determine which powders have the ability to fluoresce (Fig. A).
Fluorescent materials tend to boost the amount of light transmission throughout a zirconia restoration. To achieve this “boost” from the coping, margin materials and stains should be used (Fig. B). Liner systems also have fluorescence and can sometimes be considered for application much like layering an opaque material on the coping.
Be forewarned that applying the liner too thick or too thin affects the base shade and results in considerably different outcomes. For this reason, unless masking is needed, liners can be counterproductive. As with most “all-ceramic” restorations, the underlying shade must be identified (Fig. C).
Finish and treat the zirconia framework as recommended by the manufacturer. Although it may be slightly controversial, I believe that a light sandblasting (not more than two bars) will improve the bond of the overlay material. Over-blasting could be detrimental to the material and its CTE.
The use of a margin material is a key factor and should be used either as the final margin or as an overlay on the zirconia (Figs. D & E). The margin material will help bring light into the cervical area and will allow the restoration to appear more natural (Figs. F & G).
With some systems, the CTE of the margin material is slightly different than the building powders. For the purposes of this article, I used the Creation ZI material. Its CTE matches well and, in theory, could easily be used to cover my entire coping. Most Internal stains also are highly fluorescent and can be used to cover any other exposed parts of the coping.
Different colored materials help manipulate light and create contrast under the final overlay. Once the substructure has been treated, we can build the restoration with a similar mindset as with metal ceramic (Figs. H & I). 
 
 

 
Fig. A Margin materials will have the most amount of fluorescence, but all powders can be checked under black light.


Fig. B As with most powders. some stains have more fluorescence than others.

 


Fig. C Unlike PFM cases, a preparation shade for zirconia must be communicated to the technician.

 
Fig. D At this point, zirconia has no ability to fluoresce on its own.
 
Fig. E Margin materials can be used either as a true shoulder or as an overlay material.

Fig. F Under black light, it becomes obvious how we can improve the light transmission.
 
Fig. G Fluorescent stains can be added on the copings
for effect.

Fig. H Ceramic can then be added to our frame.


Fig. I The final restoration shows the desired level of
natural-looking fluorescence.

 

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