The choice is yours

March 21, 2012

THE SET-UP

THE SET-UP

“When considering material selection for porcelain veneer restorations, it is often a compromise between the strength of the material and the esthetic quality of the ceramics. This month, Dr. Lisa Kalfas discusses some of the options and indications for various ceramic materials currently used to create porcelain veneer restorations. New materials, such as lithium disilicate (IPS e.max, Ivoclar Vivadent), may offer the best of both worlds for these thin, esthetic indirect ‘enamel replacements."
-Dr. Robert Lowe, team lead

There are many options in the realm of “all porcelain” restorative materials available to today’s cosmetic dentist. But when it comes to veneers, where esthetics are paramount, some of these materials are best left to posterior applications.

In your veneer cases, strength is important, yet esthetic outcomes are the highest priority. Your goal is to achieve both. But with all the available options, it can be difficult to determine which veneer material will best meet this goal. This article will give you an overview of what’s available, making it easier for you to determine what works best for your patients and your practice.

It should look natural

When discussing esthetic properties, our goal with veneers is to mimic, as closely as possible, the properties found in natural enamel. Translucency tops the list of the most important esthetic properties. This quality separates the life-like veneer from the life-less PFM. The flow of light through the enamel layer and into the dentin truly defines vitality in the restoration. Blocking the light, even when a restoration is beautifully finished, is almost always apparent at the margin. The subsequent “dark root,” even though it may be subgingival, can be evident.

For that reason, I will eliminate opaque “all porcelain” cores, such as zirconia and alumina, from this discussion. And because I want to discuss veneers crafted by skilled technicians, I also will not include “no-prep” Cerinate veneers, whose chemistry is guarded and the work outsourced. This brings us to three choices worthy of examination: feldspathic (stacked) porcelain, leucite-reinforced porcelain and lithium disilicate.

What to consider first

Before discussing material options, we are pressed to identify the qualities important to our veneer goals. In veneering a tooth, we are replacing enamel, that layer of hard, translucent, shaded hydroxyapatite covering the dentin. We therefore need to consider strength, translucency and color management. Proper adhesive bonding of porcelain to dentin dramatically increases the restoration’s strength. However, while we have achieved fabulous esthetic results for years at the hands of highly skilled technicians, advances in technology are occurring mainly in the area of flexural strength.

 

Option No. 1: Feldspathic porcelain

Feldspathic porcelain has long set the standard for esthetics in dental restoratives. Feldspathic veneers are hand-crafted, meaning the esthetics are dependant entirely on the technician’s artistic skill. Powders of numerous shades and translucencies are brush-applied using either the refractory model or platinum foil techniques. Layering various opacities provides depth and the subsequent distinction of the dentin and enamel layers found in natural teeth.

The draw-back with feldspathic porcelain is two-fold. First is the lack of flexural strength. Although at 90 MPa it is stronger than enamel (50 MPa), it does not compare favorably with newer materials. The percentage of crystalline structure is an important factor in determining the strength of porcelains. The higher the crystalline structure, the more resilient to crack propagation the material will be. At only 20%, feldspathic porcelain is considerably weak, particularly at the thin marginal edges. The second drawback to feldspathic is its high fusing temperature. Subsequent firings of the finished restoration, to add stain or a contact for instance, almost certainly result in fracture.

Option No. 2: Leucite-reinforced ceramics

In the early 1990s we were introduced to leucite-reinforced ceramics. With a higher percentage of crystals (40%), the flexural strength nearly doubles to 140 MPa. The configuration of the leucite crystals inhibits crack propagation. This material comes in several forms: powders, blocks for pressing, and blanks for milling (CAD/CAM). The pressed technique uses the lost wax technique. The restoration is waxed, invested and burnt-out as with cast metal. However, the material, in the form of an ingot, is then melted and pressed into the void with a special plunger at pressure in a pressing furnace.

There are many ingot choices, with varying shades and translucency levels. The ingots are monochromatic, however, so characterization is added through cut-backs and layering and firing with powdered leucite-reinforced porcelain. While some prefer milled cores, the pressing procedure creates a slightly stronger restoration, eliminating porosity and inconsistencies.

Option No. 3: Lithium disilicate porcelain

The final material is lithium disilicate porcelain. Similar to the leucite-reinforced material, this may be pressed or milled. This ceramic’s crystals are needle-like, so when they are compressed they line up very tightly. This results in a much stronger restoration. The flexural strength is in the 360-400 MPa range, with a 70% crystalline structure composition. Again, the tight crystal alignment makes the material resilient to crack propagation, and the material’s edge-strength is extremely high. The material remains stable even after several firings, which means adding contacts or staining is not an issue.

As with the leucite material, the lithium disilicate ingots and blanks come in a full range of shades and opacities. Esthetic core cut-backs and layering also can be done with this material, and here a powdered fluorapatite ceramic is used.

The importance of ingots

The ingot options are an important feature in the pressed and milled materials. Again, we are trying to replace enamel and allow light to flow into the dentin and through the tooth. While a translucent shade may seem optimal, sometimes the more translucent shades inherently have a lower value. And of course there are times when we need more opacity to block a darker underlying dentin shade, whether it’s a result of tetracycline staining, a dark endodontically-treated tooth, or, as we see more and more, patients requesting “super” white bleach shades. In this case, more opaque cores not only block underlying tooth color, but offer a higher value. In any case, lab communication is very important, and it is especially important to communicate the “stump shade” of the tooth you’re restoring.

Getting there

With today’s materials there is no reason we cannot meet our patients’ esthetic desires and our own requirements for longevity in our restorations. Education is key to making this happen. There are excellent continuing education opportunities available for today’s dentist to learn more about cosmetic dentistry. Dental schools typically do not focus on this highly demanded area, making post-graduate courses a must. Proper knowledge of the materials and techniques, and selecting a skilled laboratory technician, are vital to successful veneer cases. And with the advances in material technology, success is that much more attainable.

Dr. Lisa Kalfas is a 1987 graduate of the University of Colorado School of Dentistry and has completed several post-doctoral programs in cosmetic dentistry at the Las Vegas Institute for Advanced Dental Studies. She joined the faculty in 1999 and teaches post-doctoral courses in cosmetic and advanced dentistry. Dr. Kalfas has practiced in Boulder, Colo. since 1989 and is a member of several dental associations.