I came up with the idea of using a sandblasting device as a method of material removal after I found out about Oliver Brix‘ brilliant technique of carving the occlusal surface of CAD/CAM manufactured lithium disilicate units via sandblasting.
My theory is based on experience: Lithium disilicate is mainly used for restorations on anterior teeth. Their anatomical shape means the preparation usually has a very narrow incisal area.
In many cases, the CAD-CAM milling or grinding tools in the milling devices cannot reach these areas well. If you allow the cutter or diamond bur to mill all the way to the end of the preparation, depending on the diameter of the bur, too much material could be removed. If more material than necessary is removed and the wall thickness is maintained, this reduces the space available for aesthetic individualization and characterization. Another option is to mill the material in the machine with very small instruments, but this process takes much longer.
Regardless of how a restoration is made, it is not usually necessary to alter or readjust the material by hand.
To ensure the restoration fits perfectly on the die, it is possible to grind the inner surface of the crown manually with a diamond bur and handpiece. If you want to be innovative, you can achieve an accurate fit by sandblasting the inner surface using a fine sandblasting device.
What is the procedure?
Before fitting the crown in place, coat the outer surface of the die with a surface marker (Fig. 1). Any interfering contacts can then be seen on the inner surfaces of the crown (Fig. 2). Then check the crown using the video microscope EASY view 3D with an enlargement factor of 15 or 20, in both 2D and 3D to view the starting situation.
Next, sandblast the individual areas with the fine sandblasting device. A prerequisite for this method is that the sandblasting device is equipped with an immediate stop function, because we need maximum precision in this material removal method. The nozzle should have a diameter of 0.40 mm and ensure that the blasting sand used is Cobra 25 mµ at 1.0 Bar.
Areas that used to be trimmed with a bur and handpiece are now processed using a nozzle and blasting sand: the principle is the same, the processing method is different: Coat the die with a surface marker, place the crown onto the die, check the fit visually (Fig. 3), remove excess material using the sandblasting device (Fig. 4, 5, 6, 7, 8). Repeat the process until the crown fits perfectly on the die (Fig. 9).
I check the accuracy of the margin on my restorations using the EASY view 3D Video Microscope (Fig. 10) and when the results are completely satisfactory, I continue with the following working steps in the conventional manner (Fig. 11, 12, 13, 14).
Why I changed my way of working?
Here are my reasons:
- I didn’t have to buy new equipment or materials
- I save a lot of time by using a sandblasting device to improve the internal fit of a crown
- I use less diamond grinding burs to adapt the veneers/crowns onto the dies
- I preserve my CAD/CAM milling machine and require less fine milling tools with small diameters, which are very expensive
- My crowns/veneers have an optimal wall thickness so that they can be correctly built-up with ceramic or characterized with stains
- And last but not least, the results are even better fitting accuracy of my crowns on the prepared die.
One small change of procedure created a lot of positive results, both in terms of savings and quality. I would like to thank Renfert, the manufacturer of the fine sandblasting device with immediate stop function and my colleague, Oliver Brix, for his suggestion to use devices creatively in order to improve the quality of our work.
Devices and materials used: