March 17, 2016
by Dr. Horst Michael Birkhoff
For many years, PFM crowns and bridgework have been the backbone of prosthodontic dentistry all over the world. While having its limitations and problems, it has been a reliable way to restore our patient’s teeth for decades.
About 12 years ago, with the advent of more modern CAD/CAM systems, and patient’s desire to get metal-free restorations, zircon started to replace the PFM’s metal frame. Usually the veneering material would still be fieldspathic ceramic.
Right from the beginning, these restorations showed a very high “chipping” rate, something also known with PFMs, but not in such high rates. Strangely enough, these fractures would usually happen within the fieldspathic veneering, not between the zircon framework and the fieldspathic veneering.
Many different attempts have been taken to minimize the risk of chipping, for example, creating an “anatomically reduced” framework instead of a “flat base”, wide step or champhered prep margins, and bonding instead of cementing. At least in this author’s experience, none of that has really helped in the long term. Everything larger than three units would sooner or later show chipping, no matter what lab was used, no matter what prep margin, etc.
Because pure zircon ceramic, while being extremely strong, has the esthetics of a toilet bowl, manufacturers started to develop “coloring” of the framework to prevent the dead-white look. First used for frameworks only, at the beginning of this decade Glidewell presented their “Bruxzir” ceramic as a “translucent” zircon that would not require veneering, but only coloring of the material and as a result could be milled fully anatomically in the final tooth’s shape. Many other manufactures followed and presented their own “translucent” zircon, amongst them Sirona with their InCoris TZI blocks that could be milled in the CEREC MCXL milling unit. The big hope was… no veneering, no chipping!
In 2011, being the proud owner of a CEREC BlueCam and MCXL, I aquired a Vita MS 6000 fast-sintering oven and started to use InCoris TZI. Being pretty happy with the traditional fieldpathic or lithiumdisilicate blocks for single unit restorations, I wanted to extend my CEREC production to multi-unit restorations in order to avoid the dreaded chipping problem and to get more independent from my lab.
To make a long story short, I was not disappointed. Starting with small bridges, we have since done large multi-unit restorations including attachment-denture-bar combos, etc.
There were three major concerns when starting with this material:
1. Would it be strong enough so that even larger bridges in the molar region would not fracture? Yes, it is, if the connectors between the units are large enough and a maximum of two
molar pontics was used. Both of the two restorations (out of more than 200) that fractured since 2011 were large-span bridges with more than two pontics and at the same time had a very narrow profile with little vertical height in the connectors.
2. Would the material (because it is so hard) abrade the antagonists once the glaze would be chewed off? No, it does not. Both Glidewell’s research on Bruxzir and my own experience with InCoris TZI show that this new material is extremely antagonist-friendly, as long as it is polished (or glazed) to a smooth surface. Interestingly, I could not find any abrasion of glaze or shading colors during those five years.
3. Would these restorations, due to their hardness, push the patient’s chewing function out of their “comfort zone” into TMJ- like problems? No, they do not. With these large restorations, I have experienced significantly less “occlusal problems” than with single unit restorations, especially single unit lithiumdisilicate crowns. My personal explanation for this phenomenon: The use of the CEREC “biocopy” function whenever possible, which basically duplicates the patient’s teeth. So as long as the patient has a functional occlusal surface BEFORE treatment, he will receive a 99 percent identical surface AFTER treatment.
With single units, however, it is usually more convenient and faster to use the “biogeneric” mode, which lets the computer calculate the tooth’s optimal surface based on algorithms. Unfortunately, Mother Nature is sometimes smarter than algorithms (and the virtual articulator is still a very arbitrary one). As a result, the individual patient may not be as happy with his brand new computer-calculated biogeneric crown surface, but rather prefer a copy of his original tooth surface. This may change in the future when the “biogeneric” mode and virtual articualtor will be further improved.
The only real problem with the new material is esthetics. TZI restorations cannot compare to fieldspathic restorations or natural teeth in translucency. After all, the blocks have a basic shading (either by dipping “white” blocks into a shading fluid or by using pre-colored blocks) and are then painted with staining colors and glazed. However, this does not mean that their use is strictly limited to the posterior. They match pretty well to older PFMs and also to older patient’s teeth (see pix of a 70-year old patient’s bridge). InCoris TZI is certainly not the optimal material to restore a single anterior tooth in an otherwise perfect dentition. But for anything in the posterior, and even for some anterior cases it is a wonderful material.
As an anecdote, I have never had a patient complain because his restorations are not “translucent” enough but I have had lots of patients complain when their veneered zircon bridges or PFMs showed chipping!
Additional benefits: this material can be milled (or cut) to very thin dimensions; half a mm is usually enough in minimal thickness. So it is possible to use straightedge prep margins and little occlusal reduction, similar to pure metal (gold) restorations.
Improvements: since IDS last year, Sirona has introduced a new set of carbide burs for the MCX/L. Now the material can not only be “milled” with diamond burs but also be “cut”, which allows for nicer and thinner margins and even better occlusal anatomy, upgrading older MCXLs to the new burs is possible but sometimes costly.
At first, TZI blocks were only available in plain white and the restorations needed to be colored by dipping into a “coloring liquid”. This turned out to be very tricky, because the resulting shade would vary by the “dipping time” as well as by the age of the coloring fluid. Also, the color would only get about 0.5-1 mm into the block. This meant that when having to adjust the final restoration in the mouth, the original white color would show through pretty soon.
To overcome these problems, Sirona has come out with pre-colored TZI blocks in many popular Vita shades. These blocks are colored through and through, so the original shade will remain even after major occlusal adjustments.
At IDS last year, some manufacturers showed multiple shaded blocks, allowing a shade transition from cervical to occlusal–similar to the old VITA Triluxe forte or Ivoclar’s Multi blocks. These blocks were even shown in a shape to fit the CEREC MCX/L. Unfortunately, legal problems seem to prohibit the sale of these blocks and Sirona so far does not offer multi-shaded blocks themselves.
But even with these limitations, InCoris TZI finally allows you to make maximum use of your CEREC machine, for the benefit of both your patients and your office’s financial situation. Take the plunge!
Consequences For Your Dental Office:
At least with bridgework, the whole procedure is now chairside, eliminating all the problems that are related with impressions and labs.
No conventional impressions are needed any more–maybe with one exception: This author found it more convenient to create the temporary with the help of an alginate impression. But even the temp could be milled out of a plastic block if one wanted to eliminate conventional impressions completely.
After a learning curve, integration into the office workflow is easy. It now takes us about the same time to take optical impressions and do the virtual design that it took us just to do conventional impressions before! We are actually saving time! Once the machine starts milling, the rest is easy: taking the milled product out of the mill, cutting off the sprue, 30 minutes dry oven, then 80 minutes sintering. Finally add some characterization/stain and a few layers of spray glaze. None of these steps takes more than a few seconds and can easily done between patients.
With bridgework and individual implant abutments, production (and financial rewards) stays 100 percent in the dental office (except for supplies like blocks, milling burs and implant ti-bases). Only for attachment/denture combos the denture still has to be produced in a lab.
Once the learning curve is mastered, I don´t think that there is a more profitable technology available for dentists than extending their Cerec machine indications with InCoris TZI.
So, What Do You Need?
1. Hardware: First, you need either a CEREC BlueCam or OmniCam. As for the milling machine, Sirona has replaced the older MCXL with the newer MCX for the regular dentist’s package. Unfortunately, the MCX can only mill blocks of max. 40mm size. Because TZI shrinks about 30 percent during sintering, this results in max. 3 unit bridges that is possible with this machine. Ironically, the older MCXL is better! It can mill the same 85 mm blocks like the brand new four-motor MCXL PREMIUM. The PREMIUM, however, having four motors, saves you changing instruments between milling TZI and fieldspathic/lithiumdisilicate/composite blocks. But otherwise the older MCXL is just as good!
2.: Software: At least CEREC version 4.0 and Inlab version 4.0 are needed. Within a few weeks of this writing, Sirona will release version CEREC PROFESSIONAL 4.4. Reportedly, this version will integrate all the conventional INLAB options (of versions 4.0 and 4.2) into the CEREC software, so you will no longer need two different programs (CEREC and INLAB).
3. Sintering oven: Basically, any regular zircon oven will do (Zycomat, etc.) but these usually have to run overnight for a complete firing cycle. For faster service, invest in one of the new fast-sintering ovens offered by Vita, Sirona and other manufacturers.
4. Staining/glazing: You can use any products you want. This author has used Vita’s shading paste & spray glaze, simply because it has been on the market since CEREC day one and we are used to it.
So, if you already have a CEREC BlueCam or OmniCam with MCXL, your additional investment can be limited to a low four-digit number by purchasing the necessary software licenses, a used standard zircon firing oven and a few supplies.
Disclaimer: CEREC, INLAB, InCoris TZI are registered trademarks of Sirona, Bruxxzir i.e. of Glidewell labs.
Dr. Horst Michael Birkhoff. Grevenbroich/Germany. Dr. Birkhoff graduated from Duesseldorf University in 1986 and has a private practice since 1988.
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