A comparative analysis of fit and a look into the pros and cons of best practice for die spacer is something no one seems to discuss. This may seem to be a small detail but in reality this is one of the great advancements in CAD CAM technology. Before I start, when working in an analog workflow, have you ever had a dentist tell you your crowns have become too loose or too tight and you have not changed anything? Die spacer may have been the culprit.
First, let’s discuss painting a die for relief to create room for cement. There are several companies that manufacture paint-like material that technicians paint on to stone dies to facilitate relief for space for cement. The general concept is to apply about 60 microns of material to the die prior to waxing. It is also noted that one will leave the paint 1-2 mm away from the margin. Many of these analog spacers come with multiple colors as it takes multiple coats to acquire the correct thickness. The multiple colors aid in seeing where subsequent coats are applied. Looking more closely, when using a new bottle of spacer (or paint) the viscosity of liquid is quite thin, almost water-like. By the time you get to the bottom half of the bottle, the viscosity is much thicker. Therefore, dies spaced from a new bottle result in thinner layers than those from a less than half full bottle. This leads to an obvious inconsistency of the cement gap. In addition, die spacers are a liquid. That said, we all know that liquids will flow more heavily into concavities and be much thinner on sharp line angles. In reality, when working in an analog workflow the opposite is probably more desirable. Lastly, if we determine 60 microns is the desired cement gap, how do we really know if we are even close? We don’t!
When looking at CAD software, many systems enable the designer to plan on an exact amount of die relief. In fact there are generally several different settings that enable one to adjust the thickness, distance from margin and areas for extra die relief as needed. One can preset these to create a laboratory standard for all cases or adjust based on a single die geometry. Doing this digitally provides an accurate, consistent, simple solution to what was once a common variable in analog workflows. The digital die spacer is also very consistent in thickness regardless of the shape of the die. In addition to being much more accurate, we eliminate the painting and dry time of the analog workflow. In digital this simply happens automatically. Once again, digital is more accurate, faster and less expensive.
In the 3Shape screenshot (below) I have performed a cut plane where you can see the measurement of the die spacer. The line on the right is the digital die, the left is the inside of the crown.
Thanks for reading,
Bob Cohen, CDTBack to All Posts