Photo Essay: Ensuring Sufficient Reduction with the Reverse Preparation Technique
Chairside Magazine: Volume 10, Issue 1
If you skipped over my Editor’s Letter at the beginning of this issue, take a peek at it so I don’t sound like too much of a broken record. Using a prep technique based on depth cuts not only ensures that you have given the technician enough room for whatever restorative material you have prescribed, but it also fashions the preparation into the ideal shape without requiring the sculpting skills of Rodin.
Figure 1: I am going to prepare tooth #9 for a BruxZir® Solid Zirconia crown, but I am going to use depth cuts fit for the preparation of a bilayer crown. This amount of reduction works for all restorative materials; there is no such thing as a BruxZir or IPS e.max® crown (Ivoclar Vivadent; Amherst, N.Y.) that is “too thick.” Unfortunately, “too thin” crowns do exist, and this prep technique is an attempt to eliminate those.
Figure 2: Using an Ultradent syringe (Ultradent Products, Inc.; South Jordan, Utah), I place the PFG gel (Steven’s Pharmacy; Costa Mesa, Calif.) into the sulcus. I previously was in the habit of using a cotton-tipped applicator, but that made it difficult to get the PFG gel into the sulcus; because I am using single-tooth anesthesia for this case, the needle is going to go into the sulcus, so that’s where the topical needs to go. The tufted tip on the syringe makes it easy to sneak it subgingivally.
Figure 3: The 30-gauge extra short needle of my STA® (Single Tooth Anesthesia) System (Milestone Scientific; Livingston, N.J.) is placed in the sulcus, and a few drops of anesthetic are given as I penetrate the base of the sulcus and contact bone (for an in-depth look at this technique, take the free “Rapid Anesthesia Technique” course here). While this is a technique I began using for single mandibular molars to avoid blocks, I realized one day that patients hate anterior infiltrations just as much, and I began using single-tooth anesthesia here as well.
Figure 4: The first step of the Reverse Preparation Technique (which is also covered in the aforementioned course) is to break the proximal contacts. We are only preparing tooth #9, so we are going to do this with a thin 056 carbide bur on both the mesial and distal. The goal here is to break contact with the adjacent teeth just enough so we can place our first retraction cord. We need to place this cord now because we are going to create our gingival margin next.
Figure 5: The first cord is Ultrapak® Size #00 Cord from Ultradent. It is a hollow, braided cord that has no epinephrine and has not been soaked in any medicament. The cord is flossed into place on the mesial and distal, and the two loose ends are grabbed on the lingual and pulled until the cord rests against the facial surface of the tooth. A non-serrated cord-packer is used to pack the cord on the facial, and the two ends are cut on the lingual so that they are flush when they are in the lingual sulcus.
Figure 6: Now that the first cord has retracted the tissue about 0.5 mm, it’s time to prep the gingival margin. This is why I call it the Reverse Prep Technique: We prep the gingival margin first, not last like I was taught in dental school. We use an 801-021 bur (Axis Dental; Coppell, Texas) to trace around the gingival margin, taking this bur to nearly half its diameter in depth, about 0.8 mm. It cuts a perfect half-circle into the gingival third.
Figure 7: This gingival depth cut helps ensure we will deliver an esthetic crown, as most crowns look the most fake in the gingival third. Because this depth cut is a perfect half-circle, after we do our axial reduction we will be left with a perfect quarter circle, which is a precision deep chamfer or shallow shoulder. There is no easier way to prep an ideal margin.
Figure 8: Because we are restoring the tooth to its original length, I am preparing a 2 mm depth cut in the incisal edge. I typically place two of these cuts as it helps me quickly reduce the incisal edge while keeping it level. Under-reduction of incisal edges leads to crowns that are facially prominent in the incisal third that tend to look bulky and “bucky.”
Figure 9: With the depth-cut bur perpendicular to the facial surface of the tooth at the junction of the incisal and middle thirds, we make a 1.5 mm axial depth cut. This depth cut should be just gingival to the incisal-edge depth cuts. Ensuring that we get enough facial reduction to have an esthetically pleasing crown that is the same size as the natural tooth next to it is difficult to achieve without using this depth cut.
Figure 10: At this point, the depth cuts are all finished. This allows me to really fly through the rest of the prep, because the gingival margin is already essentially done, the incisal edge takes about 15 seconds, and the facial reduction is marked with a depth cut. There is no guessing about how much to reduce. It’s like having a GPS unit to guide you through your prep: When the depth cuts are gone, the prep is essentially done.
Figure 11: The 856-025 bur (Axis Dental) is really the workhorse of this technique. I find it to be such an easy bur to cut with because of its coarse grit and wide surface area. As I move the bur mesiodistally, I am doing the facial reduction based on the axial depth cut. I am really not doing any reduction in the gingival third; the tip of the bur is almost floating in space as I reduce to the facial depth cut and blend it with the gingival.
Figure 12: I turn the 856-025 bur perpendicular to the incisal edge and connect the two 2 mm depth cuts I made earlier. As the bur moves mesiodistally, it is pretty easy to make quick work of reducing the incisal edge. Because the tip of the bur is pointed at the lingual, I roll the tip of the bur about 20 degrees toward the lingual margin.
Figure 13: With the 379-023 football bur (Axis Dental), I now do the lingual reduction. This is a convex bur, so it cuts a concave surface, which is the shape of the lingual surface of a natural tooth. You don’t really need to place a lingual depth cut because you have the opposing tooth to use as reference, but you could certainly place a depth cut here if you wish. Mark with articulating paper where the opposing incisor contacts the lingual surface, and place the depth cut right in the blue mark to ensure adequate lingual clearance.
Figure 14: This picture shows why I will never switch from an electric handpiece: I am able to turn the speed all the way down to 2,000 rpm, which allows me to turn off the water. Because the bur is only spinning at 2,000 rpm, I will not generate excessive heat, even with the water off. This is the only way I can really dial in and smooth the margins. With the water off, it is simple to see what you are doing. Often, I will use an 856-025F fine-grit bur to smooth out the margin at this point, but in this case I will place the top cord first.
Figure 15: The prep is now essentially done, and it’s time to place the top cord, Ultrapak Size #2E Cord (Ultradent). The “E” stands for epinephrine. My experience has been that patients who can tolerate epi in the local can tolerate it in the cord as well. The first cord retracts the tissue vertically for margin placement, and the second cord provides all of the lateral retraction to get a great impression. A one-cord technique will not accomplish this.
Figure 16: The top cord is now fully in, and this cord is the one that displaces tissue laterally to make room for the impression material. This #2 cord can’t be used in all clinical situations; it is simply too big for many lower anteriors, or upper bicuspids with minimal attached tissue. A smaller top cord, such as a size #1 cord or even a size #0, can be used in these cases. Some retraction is always better than none.
Figure 17: With the top cord in place, you get one last opportunity to get a great look at the prep, so I will typically take about 45 seconds to polish the prep, especially the gingival margin. Again turning the handpiece down to 2,000 rpm with the water off, I will use a fine-grit (red stripe) 856-025 bur to give the prep a mirror-like finish.
Figure 18: Here is a look at the finished prep from the incisal view. You can see the top cord in place with just a small tail protruding on the lingual for easy removal. The gingival margin of the preparation is smooth and uniform all around the preparation, due to using the round bur early on in the procedure while all the rest of the hard-tissue landmarks are still in place.
Figure 19: The last step of the preparation sequence is to place the ROEKO Comprecap anatomic (Coltène/Whaledent Inc.; Cuyahoga Falls, Ohio) on the prep. It helps to slightly wet the inside of the Comprecap to keep the tooth moist and the cotton fibrils from sticking to the prep. The Comprecap helps to keep the retraction cord in place and the patient’s tongue from dislodging the cord, but more importantly drives blood from the surrounding capillary bed.
Figure 20: The patient bites down on the anatomically formed Comprecap for 8–10 minutes. This ensures you have plenty of retraction. You should really get up and go finish another procedure during this time; otherwise, there is no way you will actually wait the 8–10 minutes needed for optimal results. Go do a hygiene check, go log in to Dentaltown®, or update your Facebook status; just let the patient bite on the Comprecap for the full time.
Figure 21: The result of waiting 8–10 minutes is a sulcus that you cannot miss with your intraoral tip. I am pretty sure I could flip some alginate from the other side of the operatory into the sulcus and get a good impression. When your assistant pulls the top cord, look down from the incisal with a mirror and you will see what I am talking about. You will see the impression material flow ahead of the tip in the sulcus.
Figure 22: The impression has not been contaminated by blood or other gingival fluids because the first (bottom) cord was left in place during impression. As a result, you will also get an impression of the 1 mm of tooth structure apical to the gingival margin. This enables dental technicians to precisely see the exact gingival margin and allows them to build a proper emergence profile into the restoration — an important characteristic for whether or not an anterior crown will appear natural.
Figure 23: After we try in the BruxZir crown and find the fit to be acceptable, the patient approves the esthetics of the crown and we clean it out prior to cementation. I decided to cement the restoration rather than bond it into place because I had sufficient prep length and it was not over-tapered. I use Ceramir® Crown & Bridge Cement from Doxa Dental (Newport Beach, Calif.) because of its natural bond to zirconia and simple cleanup. A pinewood stick is used to provide pressure while the cement sets.
Figure 24: Here is the final BruxZir restoration on tooth #9, day of cementation. While it probably won’t be mistaken for a natural tooth, it blends in well with the adjacent natural tooth #8. When I compare it to the existing all-ceramic and PFM crowns in the anterior segment, I think it looks better, although those other crowns are a few years old. With adequate reduction as a result of the Reverse Prep Technique, we allow the technician to give us an optimal esthetic result.