![]() do NOT think that smaller equals better! Burs bend during grinding! This is compensated by the parameter in CEREC called “occlusal offset.” (I will discuss this in another article.) Do you think smaller burs bend more, or less, than larger diameter burs? Does the material “hardness” have an impact? Think about those things very hard before you just jump on to a smaller diameter bur bandwagon.ĭentists have been trained that “4-6 degree taper” is the holy grail of vertical walls. Will your sharp transitions fit what the step bur could do in the mirrored image of your preparation? YOU are a multi-axis milling unit! The MCXL and other chairside milling units are limited to 4 axes! (I will discuss the pros/cons of 4 axis milling later).īut…. It is like a mirror image to your own handpiece if you were PREPPING the tooth completely vertical. Imagine the step bur that is in your milling unit while you are prepping the tooth. Because of it’s large diameter, it tended to over mill a lot more than what we see today with our smaller diameter burs. Many years ago in the history of CEREC, there was ONLY a cylinder bur that was 1.6mm in diameter. If the rounded transition fits into the geometry of the bur milling the surface, you will have little to no over milling. Rounded transitions are the key to reducing over milling. The main thing that the clinician can control is the preparation surface. These variables can be dependent on each other. The “over mill” is dependent on the prep surface, the thickness of the spacer, the bur geometry, and the angle for which the bur can mill. “Over milling” is where the burs remove more material on the intaglio surface so that the restoration will seat without binding. This is ideal when moisture tolerance is needed.In all of the years of CAD/CAM dentistry, this discussion has always happened. Remember: Zirconia can be seated conventional for many clinical situations. For additional content on this topic, give this roundtable discussion a read. That’s why it’s vital to clean the intaglio after try-in either by sandblasting or by wiping the inside of the restoration with gauze soaked in 5% sodium hypochlorite, rinse with water and air dry. These phosphate groups from the saliva will grab on to the oxide groups on the zirconia and make it impossible for bonding to take place. Why is this so crucial? Zirconia has oxide groups which bond to phosphate groups in your dental cement However, when you try-in the crown, that process brings the zirconia into contact with saliva, which just so happens to contain a good deal of phosphate as well. Initially, with sandblasting but also important after try-in to clean saliva. ![]() Such monomers are used in cement or bonding systems developed for zirconia.įurther, a key to getting strong zirconia bonding is proper preparation of the intaglio surface. Silanes do not bond to zirconia, but phosphate monomers or polyacids bond very strong to zirconia (and do not to glass ceramics!). Important to know: The zirconia surface is chemically very different to silica based glass ceramics. In fact, bonding of zirconia restorations is easy, but different to glass ceramics. There are myths that you can’t bond to zirconia.
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