I have virtually eliminated the porcelain-fused-to-metal (PFM) crown from my practice.
Over the past few years, my confidence in all-ceramic crowns has skyrocketed to the point that it is pretty much all I do for crown and bridge restorations. I’ve written about the differences between the new ceramic materials and have written a few posts about my two favorites: lithium disilicate and zirconia. I use these as full-contour restorations in the posterior, that is, there is no veneering feldspathic porcelain on top. I only apply a layer of feldspathic in the anterior, but more on that later.
So what makes me decide which one to use? Sometimes it’s obvious to me and other times it can be a little tricky. Here are my criteria…
(1) Strength of bite and presence of bruxism
Picture a crown on the mandibular second molar on a bruxer. Yup, that sucker is going to get a lot of force in it’s lifetime. For heavy bruxers I will only do full contour zirconia crowns. But I also look for signs of wear on adjacent teeth. If I see wear facets or fractured porcelain from old PFMs, then I’m leaning towards full contour zirconia. Remember, the further posterior we go, the greater the bite forces are.
Fractured lithium disilicate on a mandibular second molar. A deeper preparation would have allowed for greater thickness of material and would have probably avoided the fracture. However zirconia would have likely lasted better at the same thickness.
(2) Surface area of the prepared tooth
Sometimes we prep a tooth and we realize there isn’t much left. It’s not always because we overprepared, rather, some teeth just done’t have much of a clinical crown to begin with. Mandibular second molars are infamous for this. When I don’t have much to grab onto, I prefer lithium disilicate since I can truly bond this material to the tooth. There are some products coming out that claim to be able to bond to zirconia, but I believe that bond is not as strong as the bond to lithium disilicate. I’m sure some studies will be coming out soon on this.
Here’s a photo of two, full-contoured crowns. One is made completely from lithium disilicate and one fully from zirconia:
Left: lithium disilicate, full-contour. Right: zirconia, full-contour.
I actually prefer the zirconia restoration because there was stain applied to it. If stain had been applied to the lithium disilicate, it would have looked fine. But this is how full contour restorations will look when they return from the lab. Lithium disilicate looks good enough that typically no stains are applied unless you ask for them. Zirconia, on the other hand, is bright white and will routinely have stain applied so that it looks more natural.
But keep in mind we’ve only been talking about full-contour restorations thus far. If we only make a coping out of the material and then add a layer of feldspathic porcelain for better esthetics, then both crowns can look great. I prefer lithium disilicate with feldspathic veneering for its translucency. However if I am trying to cover up a metal core or dark dentin, I prefer zirconia with feldspathic veneering since the zirconia will block out the ugly underneath.
So I use mostly full contour ceramics unless I’m working in the anterior and I need some enhanced cosmetics. In the posterior, I’ve had great success with both lithium disilicate and zirconia as full contour restorations. Which one I select depends mostly on the force factors and the amount of available tooth structure.