By Paul Feuerstein, DMD
Computers seem to design many of the products that we use on a day-to-day basis, many through the world of CAD/CAM (Computer Assisted Design/ Computer Assisted Manufacturing). Most of the tool and die industry makes its parts via computer. Of course some of the machinery involved runs into the hundreds of thousands of dollars as parts have to be made out of various materials such as metal and plastic. The tolerances are within hundred-thousandths of an inch as these parts find their way into rockets and heart valves. As we think of this, I am reminded of Arsenio Hall on his old talk show. He would look at this, look at your laboratory procedures, and say, "Hmmm."
When we fabricate a crown, we go through many steps. The preparation design is mentally planned and thought out. In some instances, a model is pre-prepared, and the potential end result is waxed up and a template made. The idea is followed as the patient's tooth is prepared, then an impression is made in order to have a copy of what we just did. The lab is brought in to make a coping out of wax or ceramic and then analyzed it for a good fit. Hmmm.
Two products came in a few years ago and tried to add the CAD/CAM to this process. Procera came out with a scanner that would go over the die, feed that information to the computer, and software would then make a coping out of a block of pre-hardened ceramic. The costs were very high, so the best a lab could hope for was to get a scanner and then send the electronic information to the Procera computers for fabrication. The lab time was significantly less than waxing and casting, and once the learning curve was finished, the fit was remarkable. Sirona took this a step further and tried to get the entire process into the dental office. The early CEREC systems required a lot of dentist design and input and, in order to get the cost to a "reasonable" level, some sophistication had to be left out. However, the lab (and lab fee) was eliminated and impressions were done electronically. Still, the results paled in the face of a good technician.
Over the past couple of years, the arena has changed. Computers have become far more powerful and costs have dropped. New software has been developed that can make up a coping that has close to a zero micron gap. The software, in fact, has to build in the preferred spacing to allow room for cement. New ceramic materials have come on the scene that can be milled extremely thin, yet are strong enough to withstand the high forces generated by occlusion. Some of the new units can even create copings and frameworks out of blocks of metal such as titanium. In addition, CEREC has recently introduced new 3D software which now puts single units completely in the hands of the dentist with remarkable ease and accuracy.
The lab systems are still fairly costly and many of the companies maintain systems at the corporate areas and subcontract the frameworks to the labs. As the labs use more of this service, it may become cost-effective for them to purchase a unit for themselves. The ROI of this becomes positive if a larger number of units are created. In addition, if a lab increases its business, it does not have to hire additional employees. The units come out far faster than waxing and casting although, as we will explore in some systems, wax-ups are still done.
The software is becoming more sophisticated and, in many cases, little operator input is necessary to design the frames or copings. Having said that, they all have options for user input and modification, depending on the case. There are several different systems and varied methods of producing the frames. Some scan a model and have the computer design the coping or frame, while others require the lab to do a wax-up which is scanned. Although this may initially seem the same as before, you have to realize that, once the wax-up is complete, it takes only four minutes to scan it and construct it — much faster and easier than lost wax. In addition, there is no problem with dimensional change, casting errors, or porosity.
With most of these systems, traditional cements can be used. Many other ceramic systems do not allow glass ionomer cements due to their expansion and potential for internal fracture. These copings are so strong that sandblasting only gives a nominal advantage. 3M suggests that if using resin cements, sandblasting with products that embed particles of silica in the material allow for a better bond. Rocatec is for lab use while CoJet is for in-office. Due to the density, silanation only may not enhance the bond. It is also suggested that zinc phosphate cements should be avoided.
In 1994, Procera was introduced which promised a new level of coping accuracy. An impression is sent to the lab and, if the lab has a scanner, it is done there and the scanned data is sent to the central Procera lab in Fairlawn, N.J., (above) for the coping manufacture. This is done via modem, and the coping is actually mailed back to the lab. The coping is made of sintered aluminum oxide. Procera has recently introduced the Piccolo scanner (left), a smaller less expensive version.
Dentsply introduced the Cercon system (right) in early 2002. This is an affordable lab unit that can make individual copings and frames. These are milled out of a block of zirconium oxide. According to Dentsply, the material has a unique crystalline structure. Even if a fracture does occur, it does not "spider" as you see in windshield glass. This system has the models prepared and the copings waxed up in the traditional manner. It is these wax-ups that are scanned, then copied in the ceramic. The advantage, according to Dentsply, is that the lab does not have to learn complex design software and the technician can create and customize the frameworks as he or she always has. Thus this system is the CAM part. This also cuts the lab's initial costs and allows a lab, if they did not want to buy a system, to have complete controls of the casting design if they desire to send the case to a lab with the system.
DCS system of Switzerland (left, distributed in the U.S. by Popp Dental) is a CAD/CAM system where the design is done with the computer. This system differs in that castings can be made of multiple materials — zirconium, inceram, glass, high-strength composite, or titanium. There are at least 13 labs in the U.S. with the systems that can scan and make the copings for your lab or, if the lab has a scanner, the information can be sent to one of these centers for fabrication. This allows any lab to get involved with the system at multiple comfort and economic levels.
Sirona recently introduced CEREC In Lab (right). The unit does not look much different from the in-office product, however this produces only the copings that the lab then builds traditionally. The copings are made from VITA In-Ceram materials — Spinell for high aesthetics, Alumina for aesthetics and more strength for anterior bridges, and Zirconia for posterior restorations. It is currently designing up to three unit frames, although that can change. It still depends on reading the poured impressions, unlike the in-office use of a scanner. Having said that, it seems like only a matter of time before the impression will become optical.
Kavo's Everest (left) takes a slightly different approach in that it can use different materials for various applications. Using reinforced glass porcelain, the system can manufacture inlays, onlays, and veneers. Reinforced glass porcelain and titanium can be made into copings and frames. This system allows the lab to wax up the desired result which can be further refined if desired with the software.
3M has had a large introduction of the Lava system. There are numerous clinical studies underway and initial reviews from Gerry Kugel at Tufts as well as Tom Trinkner and Matt Roberts are impressive. The system uses pre-sintered zirconia for the frameworks. The preparations do not have to be aggressive as the frame only requires a thickness of .5 mm. The frames can be made in different shades, so there is no additional room needed for masking or shade-modification. The Lava system creates the copings and frames directly from scans of the models. No waxup is necessary.
The copings are made from blocks of zirconia. Some of these blocks are "sintered" (heated under great heat and pressure) before milling, while others are baked after the castings are milled from "semi-sintered" blocks. The debates among the companies can be simplistically compared to new versus prewashed jeans. The fracture resistance and longevity shown in studies will ultimately determine if one is better than the other, both being quite strong.
John Sorensen explains that the traditional all-ceramic systems suffer from being prone to fracture when used in the posterior areas. He states that the zirconia products actually strengthen at a point of fracture by "transformation toughening." As strange as this sounds, the pressure from the fracture changes the crystal structure ahead of it and stops it.
According to Larry Borman of TetraDynamics lab on New York's Long Island, there are several advantages to these systems. There is less labor for copings and frameworks, and fewer raw materials needed (gold and other alloy, wax, sprues, etc.). The higher initial costs could be a negative, although with more use, the ROI becomes better. He does note that if adjustments are necessary on the frameworks, they are not easy.
David Block of Aesthetic Porcelain Studios in Los Angeles notes that a lab has to make a careful decision about which system will produce excellent long-term products. He has many large and expensive machines sitting around idle that were once touted to be the latest and greatest, only to yield failed restorations over time. Many labs have been burned by big corporate promises. He points out that a failed bridge, even if warranted by the manufacturer, still costs time and aggravation to dentist, lab, and patient. As others stated, he will send out frames if requested by the dentist and will make his ultimate purchase decision of a system after the hard long studies are in.
Mark Jackson of Precision Prosthetics brings up another interesting point. The new materials are just that to the practitioners who were trained and developed a comfort level with porcelain-to-metal. Today's PFMs can be made beautiful and lifelike. The new materials are certainly strong, biocompatible, esthetic, and, very important, somewhat radiolucent. The practitioners will initially have to carefully select cases to gain confidence with these restorations.
In the May "CRA Newsletter," it was reported that the zirconia substructure offers the highest strength. The report looked at 16 different all-ceramic systems, including some of the CAD systems. The evaluators commented that some of the copings have a "bright white appearance that can be challenging to mask if the preparation is too shallow." Dr. Mike DiTolla, director of clinical education and research at Glidewell Laboratories, concurs that the practitioner has to be very careful with preparation design. He also notes that if the dentist underprepares the chamfer, the coping can show through the stacked porcelain material. He goes on to mention that a new (non-CAD, although electronically deposited alumina oxide) core material, Wol-Ceram, overcomes these issues due to variable shades and the ability to fabricate very thin cores.
There are other companies, such as Cynovad, that not only do copings, but can do wax-ups, with WaxPro 50 putting out 40 wax-ups an hour. Ivoclar, Degussa, and other lab-based companies all have either existing or upcoming systems. Any of the companies mentioned can be contacted easily thought their respective Web sites. These include enormous additional information, cases, preparation considerations, as well as names of labs and references of current users.
Should lab techs be shaking in their shoes? Until the computers go to school for advanced occlusion courses, and learn about form, function, and esthetics, there is plenty of time for the great lab artists to continue in our profession. Also, as CRA states in the May report, "No metal-free prostheses to date have data to demonstrate durability and longevity equal to porcelain-metal." It may be time, however, to take a harder look at this new technology and put it to the test in your daily practice.
Dr. Paul Feuerstein installed one of dentistry's first computers in 1978. For more than 20 years, he has taught courses on technology throughout the country. He is a mainstay at technology sessions, including annual appearances at the Yankee Dental Congress, and has been a part of the ADA's Technology Day since its inception. A general practitioner in North Billerica, Mass., since 1973, Dr. Feuerstein maintains a Web site (www.computersindentistry.com) and can be reached by email at [email protected].