You walk into your office on Monday morning, happy to be alive, happy to be a dentist…and then it happens! Your receptionist tells you Mrs. Jones has just walked in with “a hole” in her new crown. You know she doesn’t really have a hole in her crown, but you have an idea what the problem is. You quickly recall that a lot of adjustment was necessary to get her occlusion right, and now that thin layer of remaining porcelain has worn off and a dark area of metal—“the hole”—is showing (Figure 1).
|Figure 1. The “black hole” syndrome with overcontouring.|
Now you are left with the financial-ethical dilemma…do you replace the crown at no charge to keep Mrs. Jones happy, or do you tell her “the tooth is still protected and nothing needs to be done” and run the risk of losing a patient as well as your reputation as a “quality” dentist. Regardless of your decision, you lose!
The single crown is the most common indirect restoration dentists place, yet it is the one we have the most problems with. Lab technicians tell us that 20% to 35% of the crown preparations they receive are under-reduced. With the myriad of new ceramic-to-metal, ceramic-resin, and all-ceramic systems—each with their own preparation requirements—our life has become even more complicated.
Improper or inadequate tooth preparation is the leading cause of most restoration failures. If the occlusal reduction is inadequate, there will not be enough room for restorative material, which leads to the “black hole,” fractured porcelain, or occlusal trauma to the tooth. If the axial preparation is not anatomically reduced adequately, the opaquer will show through or the crown must be overcontoured. This can lead to an increase in the occlusal table, which creates excessive force on the tooth and periodontium. If the gingival shoulder is not at least 1 mm wide and carried interproximally, the unsupported cusp or marginal ridge porcelain will fracture.
All of these common but disastrous problems could be prevented by simply reducing the tooth properly. However, we know that the intraoral environment is a difficult and deceiving place to work. What appears to be “enough reduction” without overcutting in the mouth will often be inadequate when viewed on stone models. And while a reduction matrix can be helpful, this, too, is subject to an intraoral judgment call. What we dentists need is a method to provide precise reduction that follows the intrinsic contours and anatomy of the tooth…any tooth! This would allow us to create that ideal crown prep for which all dentists have strived.
In reviewing the literature, there seems to be a consensus on what the ideal crown prep should look like: an occlusal/incisal reduction of 1.5 to 2 mm and an axial reduction of 1 to 1.5 mm with a 1-mm gingival shoul-der/chamfer margin, all of this to be accomplished following the natural contours and anatomy of the uncut tooth.
The problem seems to arise in how to accomplish this in a precise, accurate, and predict-able manner. Past techniques involve using a bur with a known diameter to make orientation depth cuts to be used as reduction guides. The dentist is told to “sink the bur three quarters of the diameter” into the labial surface; or to “cut grooves on the occlusal that are one and one-half” the diameter of the prescribed bur. While some very skilled clinicians can sink a torpedo diamond bur that is 1.25 mm wide at the tip and 1.6 mm wide at the base into the occlusal of a molar tooth to a uniform depth of 2 mm and into the axial surface of the tooth to a uniform depth of 1 mm, most dentists simply cannot accomplish this feat. All of these established reduction techniques depend solely on the skill and persistence of the clinician to establish the uniform anatomical reduction at the required depth.
The “wheel”-type depth-cutting burs do very well on the labial reduction for veneers. However, on crown preps we seem to be locked into the paradigm of making vertical cuts at an “estimated” depth that do not actually follow the contours of the tooth. There is a Zen saying by Basho: “Do not seek to follow in the footsteps of the wise. Seek what they sought!”
The following case demonstrates a technique using a new series of self-limiting depth-cut burs that will allow every dentist—regardless of experience or skill level—to produce a crown preparation with the exact amount of reduction the lab technicians need to do their jobs without compromise.
|Figure 2. Tooth No. 30 after sterilized core buildup prior to crown preparation.|
The patient presented with a large amalgam restoration in tooth No. 30, recurrent decay, and multiple radiating fractures, indicating the need for a full-coverage crown restoration. The old restorative material and decay were removed, and the tooth was checked for decay and deep fractures using a decay detector (Cari-D-Tect, Gresco). Finally, the tooth was sterilized using the Strupp technique and sealed with a resin-bonded core build-up material (Core Paste XP, Den-Mat, Figure 2).
Bur Selection for Crown Preparation
|Figure 3. Axial Reduction Burs.|
Axis Dental Company has produced a set of self-limiting, precise depth-cut burs called NTI Axial Reduction Burs (ARB). Their small size and rounded depth stops allow the clinician to cut safely into any groove or surface of any tooth to the desired depth. The burs are available in 4 depths: 0.6 mm (NTI MADC-006), 1 mm (NTI MADC-010), 1.5 mm (NTI MADC-015), and 2 mm (NTI MADC-020, Figure 3). These are the depth reduction requirements of all the crown and onlay systems available today. When used in the prescribed manner, any tooth can be quickly prepared to the exact specifications of the restoration of choice: PFM, PJC, all-ceramic, pressed porcelain, inlays, onlays, and any hybrid systems. There is absolutely no guesswork or “eyeballing” required on any surface reduction on any tooth. While this clinical case involved a lower molar PFM crown with a porcelain aesthetic margin, this technique can be applied to any tooth for any restorative system in use today. (All burs used in this technique are from Axis.)
|Figure 4. Take care to keep the bur perpendicular to the inclined surface.|
|Figure 5. The 847KR-016C bur is used to reduce the occlusal surface uniformly by 2 mm.|
|Figure 6. Completed occlusal reduction following the anatomical contour of the tooth.|
The preparation was started by using the 2.0-mm NTI MADC-020 to make a cut in the central groove from the distal toward the mesial. Next, a similar cut was made on the buccal incline and the lingual incline about halfway between the central groove and the cusp tip (Figure 4). While this represents a different approach from past techniques, these depth cuts actually follow the cusp ridges and grooves and allow for a more precise anatomical reduction (Figures 5 and 6).
|Figure 7. The 1.0 ARB carves the facial aesthetic shoulder into the interproximal area.|
After the occlusal surface was reduced using a football-shaped diamond (NTI C379-023), the 1.0 Axial Reduction Bur (NTI MADC-010) was used to make a depth cut at the level of the gingival crest from the distal around to the mesial (Figure 7). This was done on the facial and lingual of the tooth, keeping the bur perpendicular to the surface of the tooth. These 2 cuts will almost complete the marginal preparation.
|Figure 8. Always keep the ARB perpendicular to the surface of the tooth as all cuts are made.|
Next, the 1.5-mm Axial Reduction Bur (NTI MADC-015) was used to make a similar depth cut at the height of contour on the buccal and lingual surface. A third depth cut was made between the height of contour cut and the previously reduced occlusal surface (Figure 8). (Due to Wilson’s Curve, the lingual cusps are shorter than the buccal cusps, so there may not be room for this third cut on the lingual.)
|Figure 9. Note the anatomical reduction and minimal convergence taper in the gingival one third of prep.||Figure 10. Note adequate interproximal reductions and absence of tissue trauma.|
At this point I recommend using the NTI C847KR-016 bur to reduce the occlusal, buccal, and lingual surfaces quickly to the bottom of the depth cuts. The same bur or the smaller C847KR-014 bur can be used to carry the preparation carefully through the interproximal surfaces, leaving a 1-mm shoulder without scarring the adjacent teeth. By holding the bur parallel to the long axis of the tooth, the finished prep will have the ideal 6° convergence taper in the gingival one third of the preparation (Figure 9). If carefully done, this technique for axial reduction will produce a tooth that has been anatomically reduced 2 mm occlusally, 1.5 mm on the working cusp, and 1 mm around the gingival one third (Figure 10).
|Figure 11. Adequate occlusal clearance in all excursive movements.|
A NTI F847KR-016 fine bur was now used to smooth and round all surfaces and internal line angles. A visual inspection was carried out to ensure occlusal clearance in all lateral and protrusive movements (Figure 11). Final polishing and margin placement will depend on the type of restorative system to be used. Since the KR burs and the Axial Reduction Burs produce a rounded or radial shoulder (the type of margin preferred for all-ceramic systems or PFM with a porcelain margin), only a light smoothing of the margin was now necessary. A chamfer or beveled shoulder can be easily accomplished at this time in the nonaesthetic zones. The goal is to keep the margin at the level of the gingiva.
|Figure 12. Temp in place. Note contour, occlusion, and gingival health.|
The gingival tissue was carefully packed away from the margin using No. 7 Racord (Pascal). An Impregum (3M ESPE) impression was taken and a Luxatemp (Zenith/DMG) temporary was fabricated and cemented with Durelon (Primer Dental Products, Figure 12).
|Figure 13. Porcelain margin prepared for cementation.|
Once the lab returned the PFM, it was tried on the tooth to verify fit, occlusion, contacts, and proper marginal adaptation. This being accomplished, the PFM was prepared for seating. Since this PFM had a porcelain margin, it was etched with hydrofluoric acid and treated with silane to achieve a good bond (Figure 13). The remainder of the inside of the PFM was sandblasted and treated with Alloy Primer (Kuraray). The preparation was scrubbed with Consepsis Scrub (Ultradent), rinsed, and isolated. The PFM was bonded to the tooth using Panavia 21 with ED primer (Kuraray). After 10 minutes, the excess cement was removed, the occlusion and contacts were verified, and the patient was dismissed.
|Figure 14. Postoperative photo of aesthetic margin and health of gingival tissue.||Figure 15. Note the occlusal table and contours of restoration and adjacent natural teeth.|
In the postoperative photos, note the aesthetic, atraumatic margin (Figure 14) and the buccal contour, lingual contour, and occlusal table (Figure 15), all of which mimic those of the adjacent teeth. Thus, by providing our lab technicians with properly prepared teeth, they can provide us with restorations that both look and function like a natural tooth.
The technique described can at long last provide what our mentors had been seeking: an efficient and predictable way of producing the ideal crown preparation that will ultimately result in a higher quality restoration for the patient and a less stressful, more productive practice for the dentist. Truly a win-win situation.
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Special thanks to Rob White, the ceramist at Root Lab in Leawood, Kan.
Disclosure: Dr. Caldwell is the developer of the Axial Reduction Bur.