Figure 1: There it is: The broken PFM bridge that has been cut off and sent in by the dentist. Look at the connector size between the pontic and the abutment. That will come into play, but that did not break; the dentist cut through the connection between the pontic and the mesial abutment.

Figure 2: What you see going on there is pretty tiny. That’s definitely one of the benefits of metal. You’ve probably heard me say that I haven’t done a single-unit PFM since we introduced BruxZir Solid Zirconia in 2009, but I still have a use for PFM bridges because you can make these connectors as small as you like due to the strength of metal. So even though we can take a single BruxZir unit and hammer it into a piece of wood, when it comes to a bridge and a connector, that’s a different story. I don’t do single-unit PFMs anymore, but I still do metal bridges.

Figure 3: So let’s take a look at what the nice, conscientious, wonderful dentist sent: a full-arch lower impression for this 3-unit bridge. A double-arch tray is, in fact, contraindicated for a 3-unit bridge, so sending in a full-arch impression like this is fantastic …

Figure 4: … and a full-arch upper to go with it, with a separate bite registration. That’s absolutely wonderful!

Figure 5: When we pour up this case and take a look at it, things look pretty good. Looking at those two teeth, the preps are pretty close to ideal. Of course, the question we always have as a laboratory comes to mind: “What does our occlusal reduction look like?” So we’ll take a look.

Figure 6: And here’s the challenge. Let’s pretend it’s a real patient. We’re looking at the teeth from this angle probably, pulling the cheek back and trying to get the best look we can get. Maybe we get the patient to turn a little this way, a little that way, but it’s hard to do. This is one of the reasons why I love a depth-cut-based preparation sequence: Because you always know exactly where you are.

Figure 7: If we look at just the solid model for a second, we’ll notice that we have a little bump of soft tissue. Our mesial margin on the molar and our distal margin on the bicuspid are both below that, so any bridge framework is going to have to come up and over that tissue. There hasn’t been any recontouring done to that tissue.

Figure 8: Again, we have a wonderful impression and wonderful preparations, but any bridgework is going to have to come up and over that soft tissue, which might make you question if zirconia is the right choice and whether some of that tissue could have been removed.

Figure 9: Gone are the days of trying to visually assess and figure out what’s going on. This is one of the bigger benefits of CAD/CAM. When you send these two impressions to us, the upper and lower models get poured, scanned and then articulated with the bite. Now, with CAD/CAM, all of a sudden we’re able to bring the case into the digital realm, and this is where things really get interesting.

Figure 10: It’s no longer about guessing. We can try in our virtual bridge and see what it would look like, what the proper anatomy would be as we put the two abutments and the pontic in place with designs from our library. When I got into dentistry 26 years ago, there weren’t a lot of times where you’d have technicians necessarily carve anatomy like this for you, especially on abutment teeth; you’d have to give them enough room to be able to do it, and you’d probably pay a lot of money, too. The designs that get pulled from the library now pretty much have ideal anatomy, and then we can see areas where we have contact with the opposing teeth.

Figure 11: Let’s take a look now and see what we have in terms of vertical height. The Rule of 27 says that on a zirconia 3-unit bridge, we need to have 27 mm2 as our connector between the pontic and either of the abutments. It’s the height numeral squared times the measured width. So essentially, we’re looking at something that would be 3 mm high (32 = 9) multiplied by 3 mm wide, and that would be our 27 mm2. As long as we have 27 mm2 — 3 mm across and 3 mm vertically — we’ll have the appropriate strength.

Figure 12: You can fudge it a little bit. If it’s 2.6 mm vertically, you can make it slightly wider buccolingually. You can’t flatten it all the way, of course; you can’t vertically have 1 mm and then have 27 mm wide. That’s not going to work. But you can play around with it a little bit.

Figure 13: Notice that between #18 and #19 — between the bicuspid and the first molar — the height is 1.65 mm. We’d like that to be 3 mm, but this takes into consideration coming up and over the tissue to get to the pontic, so 1.65 mm is pretty narrow vertically. That’s at the boundary of being insufficient.

Figure 14: Back a tooth, between the pontic and the distal abutment, we have 1.63 mm. That means that we’re going to have to make it much wider than we ordinarily would if we had 3 mm.

Figure 15: When we start to look at the width of this, we can see that we have 2 mm. We’re not even getting to 3 mm between the bicuspid and the pontic …

Figure 16: … and we’re at 2.66 mm between the pontic and the molar. To take this out to 4 mm, where it would need to be to support the decrease in the vertical height of the connector, would result in a block of teeth there that is not hygienically acceptable, a bad periodontal situation, and esthetically unpleasing, even though it’s in the posterior. It’s starting to look like we’re definitely not going to be able to do a full-contour BruxZir bridge.

Figure 17: Let’s look at some reduction through the bicuspid area. You can see where this slice is taken — a 2D cross-section of the three-dimensional model — and you can see from the preparation out to the opposing tooth, we have 1.21 mm. That’s plenty for BruxZir Solid Zirconia. We prefer 1 mm as the ideal thickness of BruxZir zirconia, but anything above that is even better. We often say the minimum thickness of a BruxZir restoration is 0.6 mm, and it is, but that’s really not desirable. We always want to go more than that if we can because it allows you to make any adjustments that you need to make.

Figure 18: In terms of margins, you’ll see nice chamfer margins on both of these preparations. We’ll also often say that BruxZir restorations can tolerate a feather-edge margin much like cast gold, and that’s true, but that doesn’t mean it’s preferred. You will always be better off having something a little bit thicker so it doesn’t chip and break at any point, either during the finishing process, after sintering, or when it’s in the mouth. So a chamfer margin is preferable.

Figure 19: As we look at the molar just mesial to the midpoint, you can see we have 0.72 mm. So we’re getting pretty close to that minimum thickness of 0.6 mm. This will work, but a note to the doctor will be necessary, letting him know that if anything needs to be adjusted — if the bite happens to be high because of the temporary bridge or whatever — it should be adjusted on the opposing tooth, not on the restoration itself, because we do not want BruxZir restorations to be less than 0.6 mm.

Figure 20: As we go just distal to that, we’ll see that we have 0.95 mm, where we’re much closer to that ideal thickness of 1.0 mm. As I said, BruxZir Solid Zirconia likes it even thicker than that; all the way up to 2 mm, it continues to gain strength. The strength for zirconia really drops off at around 0.6 mm, which is why we consider that the minimal.

Figure 21: Looking at the models, it’s difficult to gauge clearance. Often, we can cheat in the laboratory and look from the lingual side and say, “Oh, yeah, that is a little close.” And if it is, it’ll be the lingual cusp of that upper tooth. It always is. If I’m placing only a couple of depth cuts, I’ll place them in the central groove and on the two lingual cusp tips, because those are always the ones on these posterior teeth that cause the problems.

Figure 22: So we have 0.72 mm and 0.95 mm, which means we actually have enough thickness on both of those teeth. And 1.2 mm on the bicuspid, which will be fine, too. So in terms of occlusal reduction, we actually have enough room to do the BruxZir bridge, but in terms of following the Rule of 27, we do not have the proper amount of space. Even though the occlusal clearance is there for the BruxZir bridge, because of the fact that the Rule of 27 would be violated, we’re not able to use the material.

Figure 23: We called the dentist and let him know that a PFM bridge is going to be the way to go here. There’s not going to be a way to predictably have a zirconia bridge here that will not fracture. It will fracture at those dimensions. It’s simply not going to be able to work, and so for this dentist and for this patient, a PFM bridge is the right treatment choice.

Figure 24: Not a bad-looking PFM bridge. Does it look as good as BruxZir Solid Zirconia? Maybe not. But as you can see, the doctor prepared enough so that no metal margins are visible anywhere on the bridge. Hopefully, the patient will be happy with the result, even though a BruxZir bridge wasn’t used.

Figure 25: When the bridge goes into place, you can see now that we have room to come up and over that tissue, and, of course, we have room on the occlusal as well. They didn’t even have to do a metal occlusal there.

Figure 26: Which brings me back to my initial comment about how small the connector was on the original PFM bridge. So do not throw PFMs away yet; certainly not for multiple-unit restorations like this, because they still play a very valuable role. There simply are only two choices here: It’s a PFM bridge or a cast-gold bridge, the only two materials that can have a connector small enough to get up and over that tissue and connect the pontic and abutment, but still have enough strength not to break. I’ve only seen one metal framework break in about 26 years of dentistry.