Tunnel Preps: Tooth Strength Preservation

For 43 years, I have observed teeth fracture in ways that surprised me—teeth that had comparative small restorations (Figure 1). It is obvious that the profession works hard to obtain the best outcome for each patient. Now comes an article, “Fracture Resistant Endodontic and Restorative Preparation,”¹ in the February 2013 issue of this magazine authored by Drs. Clark, Khademi, and Herbranson, advocating that “interproximal cavity preparations should be disconnected from occlusial restorations.” A further quote was, “The most predictable way to make a strong tooth is not to weaken it in the first place.”¹

These statements make great sense and are further supported with the work of Drs. Tim Rainey and Graeme Milicich concerning their work on the “peripheral rim” concept of tooth structure as reported in their article, “Stress Distribution in Teeth and the Significance in Operative Dentistry.”² Recently discovered structures within the occlusal surface of molars^3-4 indicate that conventional cavity designs are disharmonious with the tooth’s natural mechanical stress distribution system.^5-8 A simple tin can is also valuable in this discussion. Untouched, it can support significant vertical and lateral compressive loads. Even with the lid removed, it can still support reasonable vertical compressive loads. When lateral compressive loads are applied to the rim, however, it distorts easily. This is a simplified image, but the effects can be observed clinically in teeth that are failing due to the presence of restorations. An occlusal cavity in a posterior tooth can cause it to flex under compressive loads on external cusp planes to an extent that the distortion in the tooth results in structural failure of the peripheral rim.^9-10 When a tooth has the peripheral rim (Figure 2) sectioned for a proximal restoration, the peripheral rim is severed, thus facilitating the potential fracture of the tooth. A procedure that eliminates proximal decay without damage to the marginal ridge (peripheral rim) would help to preserve tooth strength.

THE TUNNEL PREP
Dr. Frank Spear stated, “My personal bias is that to change what you’re doing to something new, it needs to be more efficient, cost effective, and provide a better result. If it’s not, you have to ask yourself why you’re doing it.”

Figure 1. Broken cusp from a small preparation.	Figure 2. The peripheral rim sustaining strength.
Figure 3. Proximal decay in tooth No. 13 benefiting from a tunnel prep.	Figure 4. End-cutting carbide bur (SS White Burs 957 or DENTSPLY Midwest 956 or 957).

When I learned from Dr. Milicich that I could create a preparation from the buccal of any posterior tooth to eliminate decay exhibited on an x-ray (Figure 3), I added this procedure to my repertoire, and have used it continuously because it is efficient, cost effective, and the tooth gets a better outcome. Throughout time, I have used 3 restorative materials: composite or glass ionomer or self-etching self-priming cement. The procedure can be carried out with air abrasion (which I highly recommend), a Waterlase (BIOLASE), or an end-cutting carbide bur (ie, SS White Burs 957 or DENTSPLY Midwest 956 or 957) (Figure 4).

PREPARATION TECHNIQUE
When a radiograph exhibits a proximal carious lesion, after anesthesia is administered, the proximal is prepped straight and parallel with the interproximal line, carefully avoiding the next tooth. Figure 5 shows a molar prior to preparation, and Figure 6 shows the prep. Once the area has been opened up, a caries indicator dye is used to ensure that all the decay is removed (Figure 7). I must emphasize that good magnification is essential in this endeavor; I use either a 3.8 Orascoptic or 6.0 Surgitel with an attached light source for this confirmation. The tunnel prep procedure allows for the use of slow speed and a round bur of the best size to eliminate any infected dentin.

Figure 5. Before tunnel prep on tooth No. 3 with interproximal decay.	Figure 6. Tunnel prep before disclosing solution.
Figure 7. Tunnel prep with disclosing dye.	Figure 8. Toffelmire Matrix Band (HO Bands [Young Dental]) in place before injection of material of choice.
Figure 9. Injection into tunnel prep with small tip while backing out.	Figure 10. Purposeful overfilling of the tunnel prep (to be contoured).
Figure 11. The contoured filled tunnel prep of self-etching self-priming cement (Breeze [Pentron]).	Figure 12. An example of multiple tunnel preps.

The isolation for the restoration phase is a simple .001-inch Tofflemire Matrix Band (HO Bands [Young Dental]) (as in Figure 8) or the small sectionals (Garrison Dental Solutions) so that the placement (injection) of the filling material can be made with confidence (Figure 9). The final restoration should be overfilled (Figure 10) to be thorough. Then it can be trimmed back with an appropriate finishing bur (DENTSPLY Midwest No. 9103) to look like Figure 11. Multiple tunnel preps can be done, as seen in Figure 12.

THE RESTORATIVE MATERIAL
When it comes to the choices in restorative materials, as mentioned earlier, I have used composite resin. However, I have concerns about the assurance that it was all fully cured. Glass ionomer (such as GC Fuji II or GC Fuji IX [GC America]) is perfectly fine to inject, but I now believe that I have discovered a more exciting way to place the restoration. Many of us are cementing crowns with self-etching, self-priming resin cements, such as Breeze (Pentron), seT (SDI [North America]), RelyX Unicem (3M ESPE), and G-CEM (GC America). I decided to try this in my practice and think it is an excellent material choice for an assured cure, bond, and strength for the area filled. Figure 13 shows the various choices for this technique. What you see in this photo are 2 glass inomers and 4 self-etching, self-priming cements. Each will do the job. If, however, you believe that your prep will be too close to the occlusal of the proximal ridge (such as seen in Figure 14), you need to decide if you believe the required amount of support is present.

Figure 13. The choices of tunnel prep filling materials such as GC Fuji II, GC Fuji IX, and G-CEM from GC America; seT from SDI; RelyX ARC and RelyX Unicem from 3M EPSE; and Breeze (Pentron).	Figure 14. Tunnel prep near marginal ridge that is fillable.
Figure 15. Adequate radiopacity of the filling material is required.	Figure 16. The previously shown Figures 12 and 15, filled and completed composite resin restorations.

In this case, I did, and it continues to be successful. Figure 15 shows the needed radiopacity for the filling done for 2 of the teeth in Figure 16. Radiopacity of the filling material is very critical at recall appointments at which radiographs are taken, so there is no question about what has been accomplished and what is working.

CLOSING COMMENTS
After reading a previous article in Dentistry Today¹ concerning preservation of the strength of a tooth, it immediately followed that the tunnel prep was the required way to deal with interproximal decay rather than sectioning of the marginal ridge in a classic G. V. Black prep. The preparation into the mesial or distal of the subject tooth is rather straightforward; carefully executed so as to not damage the neighboring tooth. The only reason to question the use of the tunnel prep is when the radiograph reveals decay so close to the occlusal surface allowing for collapse of the marginal ridge. I have never had this happen.

I have found that the ideal restorative material for a tunnel prep is a self-etching/priming composite resin cement. These are the same resin cements that are frequently used for cementing porcelain restorations. These materials have strength enough to support the marginal ridge if needed, are radiopaque enough to be easily seen on recall radiographs, and they are easy to inject against a interproximal matrix band.

The issue of broken teeth has not been addressed adequately in our training until minimally invasive dentistry became the standard of care in the last decade. We can all eliminate the potential for fracturing teeth by taking more care in how we remove decay and restore the teeth.

References

1. Clark D, Khademi J, Herbranson E. Fracture resistant endodontic and restorative preparations. Dent Today. 2013;32:118-123.
2. Milicich G, Rainey JT. Clinical presentations of stress distribution in teeth and the significance in operative dentistry. Pract Periodontics Aesthet Dent. 2000;12:695-700.
3. Rainey JT. A sub-occlusal oblique transverse ridge: identification of a previously unreported tooth structure: the Rainey Ridge. J Clin Pediatr Dent. 1996;21:9-13.
4. Rainey JT. The maxillary molar mesial sub-occlusal enamel web: identification of a previously unreported tooth structure: the maxillary Rainey web. J Clin Pediatr Dent. 1998;22:195-198.
5. Osborne JW, Gale EN. Failure at the margin of amalgams as affected by cavity width, tooth position, and alloy selection. J Dent Res. 1981;60:681-685.
6. Larson TD, Douglas WH, Geistfeld RE. Effect of prepared cavities on the strength of teeth. Oper Dent. 1981;6:2-5.
7. Mondelli J, Steagall L, Ishikiriama A, et al. Fracture strength of human teeth with cavity preparations. J Prosthet Dent. 1980;43:419-422.
8. Grimaldi J. Measurement of the Lateral Deformation of the Tooth Crown Under Axial Compressive Cuspal Loading [thesis]. Dunedin, New Zealand: University of Otago; 1971.
9. Panitvisai P, Messer HH. Cuspal deflection in molars in relation to endodontic and restorative procedures. J Endod. 1995;21:57-61.
10. Re GJ, Norling BK. Forces required to crack unfilled and filled molar teeth. J Dent Res. 1980;59(special issue A). Abstract 344.

Dr. Whitehouse was a founding member and past president of the World Congress of Minimally Invasive Dentistry (WCMID), a Diplomate of that organization, and a Fellow in the International Congress of Oral Implantology. He is the past editor of the WCMID column in Dental Products Report. He practices in Castro Valley, Calif, and can be reached at (510) 881-1924 or cvdental@aol.com.

Disclosure: Dr. Whitehouse reports no disclosures.