So what in the heck do we do with cusp tips on crown preps? Are we “just” prepping them and hoping for the best? Or are we trying to accomplish something? And if we are trying to accomplish something what the heck are we trying to accomplish? In a nutshell cusp tips need to end up exactly where they used to be (and, of course, they’ll be a bit shorter: 2mm for porcelain fused to metal/all ceramic and 1.5mm for full cast) so that we can achieve perfect clearance for long term success of our crowns (including esthetics). And how do we put the cusp tips back where they used to be? I’m glad you asked (well, maybe you didn’t ask…and maybe you don’t care…which seems odd if you’re still reading this posting). We “move” the cusp tips into place through our prepping functional and nonfunctional cusp bevels. Huh? Yes! Those bevels are not just something we have to do just because we were told to do it back in our dental school crown and bridge class days. Functional and nonfunctional cusp bevels serve a…function (no pun intended…actually, yes, pun intended).
So how do we get those functional and nonfunctional cusp bevels to “function” properly. What sequence of steps do we follow? I like to start things off by prepping the occlusals of my crown preps first. Why? Is it just because I was told to do it that way back in dental school in my crown and bridge class? Yes and no. Yes, because I was taught to do it that way in my crown and bridge class (at the lovely University of…yeah, you all know where I went to dental school). And no, because there is more insulating tooth structure between bur and pulp on the occlusal versus the axial surfaces of the tooth…the more occlusal tooth structure I can eliminate upfront means less axial height/tooth structure I have to cut away which usually means a little less insult to the pulp (please don’t insult the pulp as pulps are very sensitive to insults which will likely lead to patient annoyance which may end up in your patient insulting you).
And how do we get the perfect amount of occlusal reduction? There are a number of ways we can bulls-eye our occlusal reduction. In the Simulation Lab we can and should use a putty matrix (preferred method as it gives an exact/objective measurement AND is how preps will be graded during the Capstone exam AND BOARDS). On a human we need to adjust the amount of occlusal reduction based upon clearance between our prep and the opposing tooth before we prep. It’s important to remember that on a HUMAN the amount of clearance between the occlusal part of the prep and the opposing tooth IS key. Remembering that we need 2mm clearance for PFM/ACC and 1.5mm clearance for FCC we remove only enough tooth structure to achieve that ideal clearance (for instance, if we have, say, 1 mm of clearance between opposing teeth before we prep anything then we only need to reduce 0.5mm to 1.0mm on our crown prep depending on the type of crown we’re doing). And there are various tools at our disposal to measure the clearance between our prep and the opposing tooth: wax (my method of choice); occlusal reduction gauges; ball burnisher; etc. In the Simulation Lab we are keeping things SIMPLER (if you can believe that) by disregarding the occlusal clearance between the unprepared teeth before we prep (we are actually assuming that our unprepared tooth is in exact occlusion with its opposing partner) and use STRICTLY the objective preparation measurements given to us by the putty matrix. And when we are done with our occlusal reduction we should have some semblance of occlusal anatomy remaining (same shapes just shorter) AND our inner cusp slopes of our prep SHOULD BE PARALLEL to the cusp slopes of the opposing tooth (that way there will be the exact amount of clearance throughout the entirety of the occlusal part of our prep). Further, our prep will NOT have any cusp tips at this point and our occlusal should look like Pac Man with its mouth open way too wide.
Now we have an important decision to make: do we prep our functional/nonfunctional cusp bevels first or do we prep our axial walls instead? If we decide to prep our functional/nonfunctional cusp bevels first it will give us less axial tooth structure to prep which might help the pulp a bit BUT it might be easier to overtaper the prep when it comes time to prep the axial walls. If we decide to prep our axial walls first we might be able to bulls-eye the taper on the nonfunctional cusp but not necessarily on the functional cusp (because it’s SO curvy) and we might insult the pulp a bit more. So how about a compromise? How about prepping the functional cusp bevel before axial reduction and then prepping the nonfunctional cusp bevel after axial reduction?
So let’s try prepping that functional cusp bevel before we do any axial reduction. And how do we prep that functional cusp bevel? Bur angle is KEY. And how do we get that angle? Line up your bur with the occlusal third of the functional cusps on neighboring teeth AND/OR the opposing cusp slope on the tooth opposing our prep (our functional cusp bevel will end up being parallel to the opposing cusp slope just like we did with the inner slopes of the occlusal part of our prep). Bur angle will be anywhere between 25 and 45 degrees and it HAS TO MATCH the angle of neighboring and opposing tooth structure (because exact clearance is key to crown success). And now with exact bur angulation we start digging first into the occlusal and then the middle third of the outer slope of that functional cusp. By doing so we create a whole new outer slope plane on that functional cusp that matches the angulation of the teeth next door AND ESPECIALLY the OPPOSING CUSP SLOPE. And as you’re creating that plane you will begin to see CUSP TIPS reappear. And as those cusp tips reappear we “move” them closer and closer to where they used to be. And when do we stop prepping that functional cusp bevel? At that exact moment when our cusp tips end up directly in line with neighboring cusps (or when the cusp tips are directly under their original position in the putty matrix). So now we should have three prepped planes: one inner cusp slope (buccal side of the occlusal); a second inner cusp slope (lingual side of the occlusal); and a functional cusp bevel (within the occlusal and middle third of the outer slope of the functional cusp). Those three planes should all be exactly parallel to their opposing cusp slopes (it will look like a capital N that is stretched/elongated with obtuse angles rather than ninety degree angles). There should now be a significant/prominent line angle where the functional cusp bevel prep meets the unprepared axial surface. Further, I haven’t taken the functional cusp bevel interproximally/gone past the MB, DB, ML, or DL line angles into the mesial and distal (I will connect/taper the functional cusp bevel into the central groove on the mesial and distal AFTER I have prepped the mesial and distal axial walls). And oh, lest I forget, the central groove will also be exactly where it was before prepping (just shorter).
Now what? Let’s prep our buccal and lingual axial walls. So now we need to be very cognizant of getting the right amount of reduction moving from gingival third toward the occlusal to give us JUST the right amount of taper to: have excellent retention AND NOT produce an undercut; avoid over prepping/over tapering. In doing so we really have to avoid having the axial reduction result in the loss of the functional cusp bevel that we worked so hard to make (in other words we have to make sure that our axial prep doesn’t go directly from gingiva to the cusp tips we worked so hard to produce/move into place by creating that functional cusp bevel…ya gotta keep a functional cusp bevel or ya gonna be over prepped/over tapered). So we always try to keep a bit of a line angle/break where the axial part of the prep meets the functional cusp bevel. And how do we do this? How do we get an outstanding overall axial reduction? We have to limit the amount of axial reduction occlusally to no more than 0.75mm greater depth than where we started out gingivally. In other words, a 1mm gingival chamfer will end up tapering to a depth of (ideally) around 1.5mm of reduction at the occlusal most part of our axial preparation. Or, a 0.5mm gingival chamfer will end up tapering to a depth of (ideally) around 1.0mm of reduction at the occlusal most part of our axial preparation. That’s not much (we can always make further adjustments as needed later on based upon individual/unique patient needs). And how does the proper amount of axial reduction look occlusally in relation to our functional cusp bevel and nonfunctional cusps (remembering that we haven’t placed a nonfunctional cusp bevel yet). Depth cuts from gingival to functional cusp bevel LINE ANGLE will result in a depth cut that is 0.5mm deeper than the gingival axial depth (so if we start at, say, 1mm depth axially at the gingiva then we should have a depth cut of around 1.5mm deep at the functional cusp bevel line angle). AND, depth cuts from gingival to the terminus of the nonfunctional cusp at the occlusal surface preparation (in other words, the LINE ANGLE where the occlusal prep meets the unprepared outer slope of the nonfunctional cusp) will also result in a depth cut that is 0.5mm deeper than the gingival axial depth (so if we start at, say, 1mm depth axially at the gingiva then we should have a depth cut of around 1.5mm deep at the occlusal/nonfunctional cusp line angle). And now we connect all of our depth cuts AND prep our mesial and distal axial surfaces (I describe my technique for prepping mesials and distals in a previous posting).
And now for the nonfunctional cusp bevel. Looking at the angle of the occlusal-most 2mm of the outer slopes of neighboring teeth’s nonfunctional cusps (which can be anywhere from 65 to 85 degrees) I can then use that angle and prep into the outer slope of the nonfunctional cusp/s on the tooth I’m prepping and “move” the cusp tips in line with the cusp tips on neighboring teeth (or prep until the cusp tips line up with where they used to be in the putty matrix). The process is very similar to what we did with the functional cusp bevel with the exception that there is no cusp slope opposing that outer slope of our nonfunctional cusps. Also, the nonfunctional cusp bevel on mandibular molars will be minimal due to those cusps having little to no contour. Further, I may have to “move” those nonfunctional cusp tips just a bit more than their original position if I find that there isn’t enough space on the occlusal most 1 to 1.5mm of the outer slope of that bevel to completely accommodate the space needed for proper crown thickness (1.5mm for FCC and 2.0mm for PFM/ACC) and this is usually readily/measurably apparent using a putty matrix (in the Sim). So now we should have a proper nonfunctional cusp bevel which should have proper clearance for metal or metal/ceramic or ceramic. And finally I will bring the functional and nonfunctional cusp bevels around the MB, DB, ML, and DL line angles and taper/connect them to the central groove on the mesial and distal.
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