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Dental Lab Products | April 2009
Pizzi on fixed: Working with metal

Metal Here and Now

 
A metal's purity and makeup makes a difference in the esthetic outcome.

When esthetics in dentistry is the topic of discussion, veneers, pressable materials, and zirconia dominate the conversation and the advertising media. Seldom do we hear or see metal mentioned as an “esthetic” substructure. Manufacturers would have us believe that if one of the newer technology markets is not in play, then we are missing out on something. In reality, approximately 70% of all restorative work done today is still processed on a metal substructure.

I would never deny that these “newer” materials have great value in the marketplace. After all, there truly has been an increase in the number of restorations fabricated onto zirconia and other all-ceramic frameworks. However, the true perspective is that the number of dental restorations fabricated overall has increased, and porcelain-fused-to-metal still holds the predominate percentage in the market. This includes single units, small and large frames, metal inlays and onlays, full cast/metal occlusal veneers, and let’s not forget implant single units, multi-unit frames, and combination cases where metal implant-supported prostheses are used next to natural abutments.

Each restorative solution has its strengths and weaknesses. If we follow the basic principles of working with metal-based restorative processes, then esthetics can be a strength in our metal-based products.

WORKING WITH METAL: SLIDESHOW
(refer to slideshow for figures) 

 
Metal Selection

What alloy properties most influence your decision to use a particular metal? Is it the coefficiency to your ceramic material, or is it price? Does the strength or workability of the metal further define your choice? Or, does your dentist request a particular formulation? One factor rarely mentioned is metal purity (trace elements) and coloration (oxides). Without getting into the material science of alloys, we can at least take a look at the importance of these two factors.

All alloys have a certain percentage of gold, palladium, platinum and/or silver in their makeup. And, all alloys contain a small percentage of “trace” metals. Trace metals such as indium, ruthenium, zinc, and copper can aid in increasing or decreasing the melting temperature of an alloy, help the bond strength, and influence color appearance. Metallurgists spend a great deal of time trying to control and/or optimize each of these properties. One of the more visible and common effects of a trace metal’s influence on restorative outcome is the oxides that form on the alloy. These dark oxides challenge the technician to mask them, especially those that are black, dark brown, and dark blue. Even though we opaque the metal, the oxides can still affect our final outcome.

Sprue and Cast

The type of spruing system and investment used also play an important role in the quality of the casting and in achieving optimal esthetics. All investment materials should be placed in a pressure pot that can apply at minimum 40 to 50 pounds of pressure (Fig. A). This creates a density in the ring that aids with the overall smoothness of the casting and prevents micro air bubbles.

An indirect sprue system allows the molten metals to be drawn from a molten source that is usually in the heat zone of the casting ring. The larger the mass of the casting, the larger the feeder sprue should be. Accessory venting for larger cases helps create a smoother cast (Figs. B, C, D, E, and F).

Good Oxide Colors

To fully understand the affect oxidation has on the final outcome, it’s important to realize the enormous strain we put our alloys through during the fabrication process. Think about how many times we fire that single restoration and what that final oxide color looks like after the glaze bake. A simple test would be to run the raw alloy with no ceramic applied through all of your normal firings: 1. Oxidation; 2. First opaque; 3. Second opaque; 4. First dentin bake; 5. Enamels; 6. Correction; and 7. Glaze. I also usually do two margin bakes.

Regardless, the bottom line is the metal underlying each restoration gets fired at least six to nine times. Think of a B1 porcelain and the affect a “black” oxide would have on the appearance of the color throughout all of your firings (Figs. G, H, and I).

Frame Support

Metal ceramic materials work best in 1.2 to 1.5 mm of space. Your framework design should support this principle. Building the proper frame support in posterior restorations increases the surface area onto which we bond our porcelain, and that in turn helps to keep the ceramic under a more compressive force (Figs. J, K, and L). Today, I don’t believe metal collars are as critical as the support from the actual build of the underlying metal structure.

Connector size

Metal connectors allow the most amount of support with the least amount of metal mass. For metal frameworks, manufacturers recommend at least 2x2 mm; 3x3 mm with zirconia and 4x4 mm with alumina. The influence of the preparation’s axial walls plays a role in how the connector is created as will the number of abutments versus the span of the bridge (Figs. M, N, O, and P).

Light transmission

Obviously, light cannot pass through metal, but ceramic can manipulate light to appear as if it can. The use of ceramic margins and/or creating warmth in the cervical area through opaque modifications can help absorb light to create a more realistic restoration (Figs. Q, R, S, T, U, V, and W).

Firing parameters are also critical. We have to remember that an over-fired restoration looks good in our hands, but allows too much light to penetrate it intraorally causing the opaque layer to stand out. Seeing your work in the oral environment is truly the only way to understand what works and what doesn’t. To that end, photography can be your best friend in pushing yourself to a better level.