Article written by Otto Sabo, RDT, President Second Nature. Published in Oral Health magazine, Canada’s leading dental journal, July 2006.
The recently released CAD/CAM technology has had a huge impact on the dental laboratory industry. Dental professionals can use the system for designing a product and for controlling the manufacturing process. For example, once a design has been produced with the CAD component the design itself can control the machines that construct the final product.
This technology is quite an achievement – eventually it will replace the lost wax technique that has been in use for the past 100 years. Invented by the ancient Greeks and Romans in the late archaic period (ca. 500-480 B.C.) the lost wax technique was used for producing bronze statuary. At this time the lost wax technique replaced “sphyrelation”, a technique which means literally “hammer-driven”.
In dentistry “the lost wax-technique” has and is being used to produce full metal crowns and metal copings for dental porcelain, and since the mid 1980’s full ceramic (pressable) restorations. It’s a labour intensive technique which involves making a die model, which is mounted on an articulator.
At this time hand application of wax by melting wax, electronically or by flame, the wax is then incrementally built to desired form and function to full contour or framework substructure for veneering ceramics.
Once the wax pattern is complete, it’s removed from the die model, invested to create a mold of the pattern. The mold (investment) is then placed in a high heat burnout furnace to eliminate the wax from the investment. Once the burnout of the wax is achieved either molten metal is cast into the mold or molten ceramic is pressed into the mold. The investment is then removed from the structure by sand or glass bead blasting. The restorations are then fit and trimmed by a technician.
Cad/Cam’s arrival and its rate of progression within this technology will forever redefine the way we do things, minimizing hands-on techniques. The process takes less time and produces precise and predictable machined results. Although computers will inevitably replace some technicians, this may be welcome considering the fact that most dental laboratories have been plagued with a lack off of qualified or skilled dental technicians. As per Judy Fishman (LMT) 40% of laboratory owners are over the age of 55 and 68% of employees are over 35. Due to low starting wages, there’s been a failure to bring in enough new students.
As laboratory technicians and laboratory owners alike are reduced in numbers Cad/Cam systems are a great technological aide to help offset the impending imbalance, since an increased production by a decreased number of technicians will be achieved.
So far various Cad/Cam systems have been able to make frameworks for single crowns, bridges, inlays and onlays as well as custom implant abutments, and temporaries. These various Cad/Cam systems utilize materials such as aluminous oxide, zirconium oxide, lithium disilicate, titanium, and resin.
Until recently noble alloys were not being done since the CAD/CAM technology in dental manufacturing has been utilizing milling. Milling requires a large block of a material being milled down to a crown or bridge. Therefore there was a large amount of trimmings which could not be practically re-used. Taking into account the cost of semi-precious and high gold alloys this needs to be worked out.
At least two companies (Bego and Hint-Els) are promising a solution (by the end of the year 2006). It’s called Rapid Prototyping or Laser Sintered Layering. Our CAD data will guide a laser while it builds up the frame, layer by layer, using the material of our choice with extreme precision. This advancement will allow us to utilize CAD/CAM technology cost effectively for noble alloys. computer technologies also require less equipment than older ones, therefore less clutter and dust, making for more environmentally friendly work places for technicians and technologists.
Dental laboratories can now be set up to look more like dental offices, with a cleaner and a more stylish elegance. Since a laboratory is an extension of the dental practice, I feel this is a very positive step.
To illustrate the steps of a CAD/CAM made framework, we’ll describe the Lava system by 3M ESPE, which utilizes Zirconia as a substructure to veneering ceramics. We begin by making precise die models and articulations as we would conventionally, then we proceed with non contact photo optical scanning of the model with the Lava scan. At this stage the cores and regions of the connecting parts (for bridges) are scanned several times from different sides. The scans of different model positions are the basis for calculating the 3D image data. The bite record and tissue areas (for pontic areas for bridges) are also scanned. The CAD must also calculate 20% enlargement to compensate for their shrinkage. The system is extremely precise with an average marginal gap of 50 microns. In short, once all data is calculated, the data is transferred to the Lava Cutting Unit.
A pre-sintered zirconia oxide blank, in a chalk like green state is placed in the magazine (which can hold up to 21 blanks, all with individual bar-codes for milling). At this point the zirconia is soft and easy to cut utilizing metal burs by the cutting or milling unit. For a single coping it takes about 35 minutes milling. A bridge is completed in approximately 75 minutes. The milling could be done overnight since 21 blocks with their individual designs may be cut. At times we must combine cases of natural dentition, PLZ (porcelain laminated zirconia), pressed ceramics, and PFM’s (porcelain fused to metal). For combinations cases there is a much greater level of difficulty for balancing light passage due to differences in translucency levels from each ceramic system and problematic reflectivity of PBM’s. Therefore those types of cases require more advanced techniques and effort alike by clinicians and technologists to make them a success. Once the milling is ready, sharp edges are rounded off using rubber polishers. An important advantage of the Lava system is the pre-sintered cores can be dyed with 7 shade choices so the base shade is already in the core, instead of pure white as it would be normally with some of the other systems.
Once dyed (2 minute immersion), sintering takes 7 hours in the Lava Therm Sintering Unit (1500 degree Celsius). Only then will the frame have its full strength (over 1000 mpa).Since the cores already have a base shade, the ceramists do not need to use opaquers or liner materials which reduce the translucency of the restorations. Instead you may apply dentin and translucent ceramic powders directly for higher esthetic results. Clinicians may use translucent cements instead of high value cements unless trying to block out dark abutment colour.
Today’s esthetic requirements demand that a ceramist must have precise internal shade control of dental ceramics. The same control is needed with Zirconia substructures. Internal shading with stains and coloured ceramics is vital. On many occasions restorations seem lifeless due to not having enough layers of ceramic applied. Multilayering which is the incremental layering of different shades and translucencies of ceramic assist in light diffusions necessary to duplicate the light passage in natural dentition, PLZ (porcelain laminated zirconia), pressed ceramics, and PFM’s (porcelain fused to metal).
For combinations cases there is a much greater level of difficulty for balancing light passage due to differences in translucency levels from each ceramic system and problematic reflectivity of PBM’s. Therefore those types of cases require more advanced tech-niques and effort alike by clinicians and technologists to make them a success. Although CAD/CAM is quite a boost for the dental industry, ultimately it is teamwork which will be our blueprints in allowing us to integrate them into our practices in smooth synchronization.
Acknowledgement: I would like to extend special thanks to Dr. Bruce Glazer and to the dental professionals at Emerald Works.
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