By knowing your product and standardizing your techniques—and sticking to them—you can be successful in processing titanium restorations.
A variety of factors are contributing to titanium's rising popularity: it's now classified by the ADA between a noble and high noble alloy and, as the precious metal market continues to hit record prices, titanium is an attractive alternative. In addition, CAD/CAM techniques have eliminated the problems associated with casting titanium.
Our lab has been working with titanium for almost three years and, during that time, we have studied clinical data and manufacturer recommendations, tested various techniques and finishing burs and discs, and conducted R&D on CAD-designed titanium frames. The following standardized techniques for processing porcelain-fused-to-titanium (PFT) restorations have yielded consistent results in our lab.
Standardize Firing Parameters
All temperatures for firing titanium have to be below 883oC. Firing to and/or above this temperature will cause failure of your restoration, including distortion, margin creep and expansion of the alloy. Here's why: In its metallic form, titanium has a hexagonal, close-packed crystal lattice Alpha (a phase), which transforms into a body-centered cubic form Beta (b phase) at 883oC.
In addition, porcelain calibration is a crucial step for successful porcelain firings and needs to be done before firing bonder or any porcelain. We use GC Initial Ti Clear Windows (CL-W)—a highly transparent shade—to standardize our firing parameters. Fabricate a shade tab approximately 1-mm thick and place it on platinum foil or master ceramic adjustable tip pegs. As a starting point, fire it at approximately 40o higher than the manufacturer's recommended starting point; a properly fired CL-W tab is clear and retains sharp edges (see Photo #1).
If you get the results you want, raise all firing parameters by the same amount (40o); this is the proper maturation point in your furnace. If not, adjust the temperature up or down in 10o increments until you get the specified firing results, then raise or lower the rest of your firing parameters by the same amount.
CAD Designing Titanium Frameworks
Properly designed frames ensure proper porcelain support, bridge joint size and virtually eliminate metal finishing. Our laboratory is currently using the 3Shape scanner to scan and design titanium frameworks and I've partnered with Ryan Gillespie, Production Manager at Precision Milling, to establish default settings that enable us to design successful frameworks. Here are a couple of settings I'd like to share that, when used singularly or in combination, can help you design better-fitting cases. Experiment with these recommendations to get your desired result:
In the software under "Coping Design," coping wall thickness should always be .35mm or greater. If you design your copings with a wall thickness of .4mm or greater, you can set your margin line offset at settings as low as .1mm giving you a knife-edge margin.
Offset angles of an average 75o are recommended on most cases. Raising the offset angle thins out the facial wall above the margin, while lowering the offset angle thickens the wall above the margin. You can also raise and lower the extension offset angle to thicken or thin out the angle of the margin.
There's no limitation to the size of case you can design and mill in titanium (see Photo #2); we have designed and milled 14-unit bridges. However, the fit of the frame is determined by the accuracy of the master model. I recommend Amann Girrbach's Giroform® system, formerly called the Kiefer model system, which offers no linear discrepancy even after separating the dies. The linear measurement of a model fabricated with this system will be the same as a solid model pour from the master impression.
Finish down sprue sites with titanium carbide cutters and always cut in one direction using light pressure and a maximum speed of 15,000RPM (see Photo #3). Komet or Renfert Dynex discs can be used at the same handpiece speed to thin areas around the margin as needed.
There are three critical points to sandblasting:
One- Never use a particle size less than 120-150 microns of pure aluminum oxide abrasive. The larger particle size leaves a rougher surface which promotes a better bond; substituting with 50 microns will lead to failure of the porcelain.
Two- Never exceed 2.5 bars or 35 PSI of pressure when blasting the surface of titanium and blast at a 45o angle (see Photo #4).
Three- After sandblasting, steam clean the frame using distilled water, blot dry and lay on a tissue for five minutes to oxidize the titanium frames. Passivation
This natural oxidation process is crucial to the success of your bonder and porcelain application. Frames need five minutes to oxidize; if you exceed 30 minutes, you'll need to re-sandblast, steam clean and start this process over.
GC Ceramic Bonder This is a crucial procedure and cannot be skipped. The GC Bonder seals the surface of the titanium alloy and prohibits formation of excess oxidation which can weaken the ceramic bond.
One- Mix the GC Initial titanium bonder with the GC titanium bonder liquid in a very thin consistency. Apply a thin layer with a glass rod or brush being careful to avoid pooling of the bonder. When applied correctly, the coping will shine through. Thicker areas of gray or white indicate you've applied too much bonder.
Two- Fire at 810oC and follow GC's chart for additional details. Properly fired bonder surface is dark and shiny (see Photo #5).
Apply a very thin slurry of opaque on the titanium-treated surface and then fire it; the fired surface should be shiny. Apply additional needed coats of opaque and fire again; surface should still be slightly shiny.
We use the GC Initial Titanium Porcelain System and the spray opaque technique which yields a thin, finished opaque layer as well as complete coverage of dark oxide margins. Any overspray on the inside of the crown or framework can be wiped clean with a damp brush.
We use the GC Initial porcelain systems for all of our products. Because they are compatible systems, we can fabricate restorations of different substructures adjacent to each other, layer the porcelain with the same application techniques and same internal and external modifiers, and achieve esthetic results.
Here are some guidelines:
Apply various porcelains and modifiers to achieve desired esthetic results and fire to predetermined parameters for titanium (see Photo #6).
Finish contours and glaze (see Photo #7). Oxide formation inside restorations should be removed using 50-micron aluminum oxide at no more than 2.5 bars or 30 PSI of pressure. A larger abrasive grit size coupled with increased air pressure can cause damage to margins.
Use rubber wheels and polish finish lines with polishing compounds made for titanium. Let the restoration set and passivate for 10 minutes before steam cleaning. This is necessary to retain the high luster achieved when polishing.
Never use stripping solutions to remove porcelain from titanium frames as they will cause distortion. Sandblast with 125 microns at 2 bars or 30 PSI of pressure. Avoid Cross Contamination
Our lab has separate titanium processing areas--like dedicated porcelain furnaces as well as separate burs and finishing points--to avoid cross contamination with other alloys.
Author's Note: I recently worked with GC to develop a two-page guide on getting started with titanium. To download a copy, visit www.gcamerica.com/lab/products/GC_Initial/ and click on "Quick Start Guide (GC Initial Ti)" in the list on the right.