Tuesday, February 19, 2013
When deciding which restorative material is best suited for a particular patient, a dentist has a lot to consider these days.
Strength vs. esthetics, and metal-based vs. all-ceramic restorations are only a couple examples of the choices to be made when restoring natural dentition. As we transition away from the more traditional types of restorations such as full gold crowns and PFMs to some of the popular all-ceramic materials today such as Ivoclar Vivadent’s e.Max lithium disilicate and full zirconia restorations, more restorations are being digitally designed and milled as part of the fabrication process in the lab.
This transition provides a number of benefits, but as clinicians switch to new materials, it’s important to understand that a prep design that worked great for a full gold crown is not always the best prep design for a full contour CAD/CAM restoration.
Careful consideration must be given to the design of the margin and the amount of reduction provided when selecting a full contour milled restoration. In these instances a lab technician is not blocking out undercuts on the stone die manually, applying some sort of die spacer for cement room to the die, and then waxing directly onto that die, cutting and feathering the margin by hand in wax. Instead, the digitally workflow means that once the die and model are scanned and imported into the design software, the restoration is completely CAD designed and milled without coming in contact with the stone die. Any undercuts are virtually blocked out by the CAD software, and space is built into the inside of the restoration for cement.
CAD design software such as 3Shape’s Dental System allows technicians to set parameters for these details individually in order to fine tune the fit of their restorations. The software also has to compensate for the thickness of the milling bur—this is referred to as bur relief—which is usually around .2-.5mm and build that into the scheme.
In this article I show actual CAD screenshots from Ziemek Laboratories’ 3Shape design software demonstrating margin designs that are preferred, acceptable, and not preferred. These are affectionately coined, The Good, The Bad, and The Ugly. While we can and do fabricate crowns on preps with margins like all of these examples every day, the perception of the margin quality will vary with each different margin design.
In my experience, a dentist’s perception is that a margin is “perfect” or “like glass” when he runs an explorer over it and does not feel the “click” of the explorer tip over the margin of the crown. A margin can be 100% accurate and completely sealed (as verified by an X-Ray), but if the restoration’s margin is bulky or the emergence profile of the crown is over-contoured, this “click” will always be felt, and the perception of the clinician will be that the margin quality is poor.
An over-contoured margin is not hygienic and not good for the health of the gingival tissue. A shoulder or chamfer margin prep provides the space needed to mill a full contour restoration’s margin at the minimum material thickness of around .3-.5 mm and provide a margin that is not bulky. A knife edge margin prep on a CAD milled full zirconia restoration will usually yield the “click” or bulky appearance of the margin, as adequate room for the material was not provided, and the emergence profile at the margin has nowhere to go but “out” .3-.5mm from the existing prep.
The other consideration a dentist has to make when prepping a tooth for a CAD milled restoration is the amount of prep reduction, especially on the occlusal. Quite often I am chatting with a dentist about this subject and he or she tells me; “But I heard that I don’t have to reduce as much for a full zirconia crown as for a PFM because there’s no ceramic material layered on a milled full contour restoration, so you don’t need as much room.”
Although there is no feldspathic ceramic layered on a full zirconia crown, the minimum material thickness is around .5 mm. That means .5 mm for the minimum material thickness, plus room for cement gap, and the room needed for the bur relief which is all accounted for in the design software.
If a dentist gives us a crown prep with .5 mm of occlusal clearance and I attempt to design, mill, and finish a crown that will fit in that amount of space, how good do you think that restoration is going to look? It will have flat cusp tips and a very flat occlusal table with shallow anatomy. It won’t look like any of the patient’s natural teeth and it won’t look as good as any PFM’s in the patient’s mouth. The patient’s perception will most likely be that the crown is of a lesser quality. Not the desired outcome for anyone involved. 1.5 mm of reduction provides enough room to fabricate a great looking full contour milled restoration.
The Good
A 360° shoulder or chamfer margin will work well for virtually any type of CAD milled restoration and allows adequate room for the minimum material thickness of the zirconia at the margin, thus providing a proper emergence profile and no “click.”
The Bad
A nice shoulder margin on 50% of the prep changing into a knife edge on the other 50% will produce a restoration with an inconsistent margin as you move around the prep with an explorer. The tissue could also be affected differently from mesial to distal with this margin design.
The Ugly
A 360° knife edge margin does not usually provide enough room for the minimum material thickness of the zirconia without leaving the emergence profile and margin bulky.
Have a conversation with your lab about prep design for the CAD material you are requesting. Better yet, visit the lab and sit with the CAD designer who creates your restorations and watch them work with the design software. Ask them all about the design and milling process, because with this type of communication and information exchange, both the lab and dentist acquire a better understanding of the process and can provide each other with better results, which means better results for the patient.