The New Science of Strong Endo Teeth

INTRODUCTION
Are today’s typical endodontic accesses as outdated as the typical G. V. Black cavity preparations that remain the current standard? Sadly, yes.
There are several impediments that have stopped progress and left endodontic access in the dark ages. Whether gouged by general dentists (Figure 1) or “gutted” by endodontists (Figure 2), the damage is seemingly everywhere and it is irreversible. Similar to outdated restorative cavity preparations, there are scant resources to devote to the monumental task of changing the way that we cut teeth. The money is in products. Witness the incredible advance in implants. When billions are at stake, change happens. In contrast, dentin and enamel have no advocates, no budget, and no slick advertising campaigns.
SS White Burs president, Tom Gallup, has made a generous donation to begin a new textbook, Contemporary Endodontic Access (Figures 3 and 4). This article will contain excerpts from the upcoming textbook.

Figure 1. Upper and lower incisors badly gouged by round burs; exacerbated by cingulum access.	Figure 2. Somehow pleasing to the eyes of many endodontists, these pre-op and post-op radiographs demonstrate the “superhighway to the apex” philosophy. As a restorative dentist, these hollowed-out teeth seen in the radiographs should elicit an immediate visceral response; nervous about any hope for longevity, and a sick feeling for the tooth and the patient.

Why Does Endo Access Remain in the Dark Ages?
Endodontics was first recognized as a specialty by the ADA in 1963. From that point on, endodontists were essentially handed the keys to the kingdom, and were charged with endodontic shaping design. Interestingly, the cavosurface portion of G. V. Black style accesses that endodontists inherited remain largely unchanged. Endodontists have instead been very apex-centric in their focus of endodontic shaping. Most troubling has been the recent shaping change; grotesque straight-line access, carving a superhighway to the apex. Sacrificing and obliterating massive amounts of tooth structure is in vogue today. This machined, man-made shape is referred to as “the look.” This was done without outcome studies that validated this extravagant expenditure of tooth structure. In a landmark literature review, apical shaping size and canal taper had no significant relation to better outcomes.¹ To summarize: Big shapes don’t create better outcomes. They do, however, weaken the tooth badly.

THE NEW LEXICON OF ENDODONTICS
As we seek to redesign endodontic access, it is crucial to create a new lexicon (an endodontic dictionary of sorts). In this article, we will discuss the new terms: soffit, pericervical dentin (PCD), orifice enhancement, and dispense with an oft abused term, endodontic shaping, and replace it with endodontic instrumentation.

The Pulp Chamber Soffit
In architecture, a soffit is described as the underside of a ceiling, at the corner of the ceiling and wall. The chamber of a young molar tooth is bounded by a roof, 4 walls, and a floor with small orifices that are arranged along the edges of the floor like pockets along a pool table. One of the silly rules (taught for decades) is that the entire roof of a chamber needs to be removed during endodontic access; or, you are a sloppy and lazy dentist. We absolutely refute that position. Today, when considering endodontic access, a worthwhile goal is to maintain a small border amount of the chamber roof; near the point where it curves 90° and becomes the wall. This tiny “lip” or “cornice” could be as small as 0.5 mm, or as large as 3.0 mm in some cases (where extra strength is needed, or when the anatomy allows it). There are several reasons why an increasing number of clinicians are embracing this concept (Table).

Figure 5. Dotted line shows the typical cut made to remove the entire pulp horn. Area between the lines should be maintained and is referred to as the soffit.

Figure 8. Endodontists invited to the SS White Modern Endodontic Consortium. Doctor participant names are listed in reference section. Their recognition of site-specific dentin preservation and commitment to long-term tooth retention, as evidenced in Figure 9, a glowing example of excellence trumping expedience.

Soffit (as used here within the context of describing dental access procedures) is a term that Dr. Khademi coined, and is an excellent descriptor (Figure 5). The preservation of the soffit and its comparison to the second moment of area is shown in Figures 6 and 7.

The Science of Bending
The second moment of area (also referred to as second moment of inertia or mass moment of inertia) is a mathematical representation of an I-beam’s resistance to bending. The value relies on the cross-sectional area and the location of the centroid. The design of an I-beam resists bending, even though it has huge hollow areas. When the flange of the I-beam is a good distance from the centroid, the second moment of area is great and the I-beam resists bending. Roughly speaking, the further away the tip of the flange from the center, the stronger the I-beam.
We do not want our teeth to bend and flex at the cervical area, which for demonstration purposes is identified as the centroid of the tooth. It (the cervical) is the most common area of occurrence for fracture failures in endodontically treated teeth (endo teeth). When brittle items such as teeth start to bend, they easily break. The myth of endodontic teeth being brittle may finally be explained in over-torqued dentin that slowly degrades until it finally fractures years (or decades) later. The endo tooth doesn’t dry out, but a hollowed out endo tooth is constantly bending. When robust coronal dentin is maintained good distances away from the tooth’s centroid, the tooth is stiffened, resists bending, and should resist fracturing. An additional component of the soffit, aside from a second moment of area, is the strength that is inherent in a continuous ring of dentin that can act like a metal barrel ring around an oak barrel.

Figure 9. This mural features post-op radiographs, demonstrating the preservation of the roof-wall interface (or soffit). These cases are a small sample of the exquisite work done by the endodontists in Figure 4. Notice the small shapes, and preservation of key dentin.

	Figure 10. If the pulp chamber is indeed sufficiently large enough, then a round bur can truly “drop in” to the pulp chamber; as shown here with a No. 6 round bur superimposed on the lower molar of this 11-year-old child. How often does that happen?
Figure 11. SS White High-Speed Tapering Diamond Burs; some in surgical length, designed for endodontic access. These replace round burs.	Figure 12. Especially safe and useful for general dentists is the slow-speed surgical length latch grip diamond bur which is compared to old fashioned No. 4 round bur. (left) It is a perfect replacement for iatrogenic latch grip surgical length round burs. (The kit is shown in Figure 13.)
Figure 13. Clark Khademi Endo Bur Kit (SS White) with burs from Figures 12 and 13.	Figure 14. Pericervical dentin is the most common area for catastrophic root fractures and catastrophic restorative failures. It is also an area that is gouged, abused, and often mutilated by clinicians who use the wrong burs (round burs and square-ended 557 carbide burs) and overuse of outdated burs (Gates Glidden burs).

Figure 15. An uncalcified tooth such as this has a natural “funnel” and often needs no orifice enhancement. A calcified tooth, on the other hand, will benefit from the creation of a small polished cone shape, which is best created with a conical carbide such as the Endoguide bur (SS White).

Figures 16 and 17. Figure 16 shows the sequence of tapering diamonds and then the EndoGuide Bur (SS White). Figure 17 demonstrates just touching the canal orifice to flare it slightly; then the transition into the access prep is made smooth by wiping the bur up with lateral pressure, away from the furcation. This enhancement will make the rest of the procedure easier and worth the time that it takes to create.

A talented group of endodontists, assembled at the behest of the 3 authors of this article, recently met at the first SS White modern Endodontic Consortium2 (Figure 8). There was an overwhelming consensus on the issue of preservation of the soffit. A mural of their cases is presented in Figure 9.
Multiple studies have shown that removal of dentin weakens both the root and the crown, leading to lower load failures. Further studies examining site-specific dentin conservation are warranted. However, until that time, we must use common sense and accepted engineering principles when deciding which dentin to sacrifice, and which dentin to conserve.

Dr. Khademi: Why Round Burs are so Dangerous
The traditional way of performing endodontic access is predicated on mental models derived from chalkboard constructs that no longer represent the day-to-day clinical realities presenting to the clinician, if they ever represented those realities. Yet text after text continues to show the same chalkboard constructs in the form of drawings of round bur access technique relying on tactile feedback as the round bur “drops in” to the chamber.
I was always baffled by these kinds of drawings because they didn’t seem at all like the cases I was seeing. Clearly, from a strict geometric argument standpoint, for the bur to truly “drop in” to the chamber, the chamber height needs to be larger than the height of the bud of the bur, just as they have been drawn (ie, made-up) here. While that happens routinely when drawing these things on a chalkboard or PowerPoint presentation, it hardly seems to happen on real patients.
It wasn’t until I lectured with Dr. Clark and the 2007 New Jersey Association of Endodontists Meeting when I saw this case of his (Figure 10) that I realized from where these bankrupt mental models might have come. Images like these, so frequently shown in dental schools, textbooks, and lectures are predicated on mental models based on occlusal decay in children.
Presuming one could drop into the pulp chamber in the way drawn and described in Figure 10, the chamber roof is now to be removed by scooping it up and away with the backside of the round carbide. A 2-dimensional (2-D) drawing with the relatively small size of the bur and chamber roof overhanging a large pulp chamber makes this seem like a reasonable proposition. The chamber walls are always drawn flat even though they are point-cut by a round bur.
In reality, it is truly impossible to cut flat walls in 3 dimensions with a round instrument. The difficulty is demonstrated by attempting a simpler 2-D version of flattening the occlusal surface of a tooth in-hand with a round bur, then examining it under magnification. If we wanted to flatten the occlusal surface, it would make much more sense to use an instrument that already has a flat shape, like a wheel diamond.
What happens clinically is that the chamber is not unroofed in some areas, leaving pulpal and necrotic debris (which is not such a big deal), but that was the purpose of the procedure; and the walls are overextended and gouged in other areas. Further, the internal radius of curvature at many of the pulpal line angles is simply too small for all but the smallest of round burs. As an endodontist seeing cases for retreatment, a feature of the previous access attempts with these instruments and mental models was the consistent gouge-up of the chamber walls and floors, and chambers full of debris. So the technique didn’t work anyway.
In the final analysis, round burs point cut in an endodontic access application, when instead what is needed is planing.
What is needed is a new set of mental models based on vision, and a new set of instruments reflective of the task at hand and the desired shaping outcomes. The new vision-based mental model is look, groom, and follow. The new instruments are all round-ended tapers (Figures 11 to 13).
The rounded ends are to increase the radii of the gouges and nicks that can act as stress concentration points. The flat sides help create smoother, flatter walls and minimize the gouges and dings that inevitably occur even with the most careful technique.

Pericervical Dentin
Pericervical dentin is a term that we first described in 2008. Pericervical dentin is an area roughly 4 mm coronal to crestal bone and 6 mm apical to crestal bone (Figure 14). It acts as the “neck” of the tooth and is key because it transfers occlusal forces to the root. In the end, strong, unmarred PCD trumps just about everything else in long-term retention of the tooth.

Endodontic Instrumentation Versus Endodontic Shaping
The problem, according to one of the endodontists in the MEC group (Dr. Steve Baerg of Tacoma, Wash), is that the term endodontic shaping is a poor choice of words and should be replaced with the term endodontic instrumentation. Ideally, we would prefer to just suck out the pulp, irrigate, and then obturate. An unenlightened work ethic demands that we need to grind and reshape the tooth. Simply massaging a small hand file in a young patient with a naturally ideal canal space, say a maxillary central incisor, then irrigating and obturating may feel like cheating.

Dr. Herbranson: Orifice Enhancement
We prefer the term orifice enhancement to orifice enlargement. The transition between the pulp space and the canal system in a young uncalcified tooth forms a natural “funnel” into the canal system (Figure 15). This makes finding the canal fairly easy and allows endodontic files to easily slide into the canal. It is one of the main reasons endodontic procedures are easier on young people. However if the canal space is calcified, which is too often the reality, this does not exist. In that case, it is prudent to recreate the funnel mechanically. It will make your life easier for the rest of the procedure. This is a delicate operation and must be done with the understanding that any dentin removal should be minimal and away from the furcation. The dentine is thin toward the furca and robust toward the external surface in this area. Traditionally, this has been done with Gates Glidden burs, but it is easy to overdo it with that system, and we see lots of examples where their use was too aggressive. Today, a better solution is found in using a small, cone-shaped, low-speed bur (such as the EG2 [SS White]) (Figures 16 and 17).

IN SUMMARY
Clinical Recommendations Two questions invariably come up:
How do you get the pulp out of the soffit/pulp horn? The answer is a back action explorer, with a little patience. Worst case, we have disassembled dozens of teeth where scraps of pulp were inadvertently left in chambers and it has not seemed to affect the tooth adversely, even after decades.
How much is too much soffit? In our hands-on courses, we have seen a tendency to leave excess soffit in one area, and gouging in another. It is better to leave a tiny 360° soffit and make a safe environment for a GP, than to be a “hero” and overdo it, compromising with gouging elsewhere and creating impossibly difficult working conditions within the tooth.
When it comes to endodontic access and directed dentin conservation, each case must be titrated. Opportunistic access, creativity, and a long view of the structural integrity of the dentin must be foremost in our minds. The time has come to integrate modern engineering to help design a strong endo tooth that will withstand the forces of occlusion and the detrimental effects of disease.

References

1. Ng YL, Mann V, Rahbaran S, et al. Outcome of primary root canal treatment: systematic review of the literature—Part 2. Influence of clinical factors. Int Endod J. 2008;41:6-31.
2. Honored KOL Endodontists at the SS White Modern Endodontic Consortium: Drs. Steven Baerg, Marc Balson, David Clark (GP), Robert Corr, Mitchell Davich, Glen Doyon, Eric Herbransen, John Khademi, Jihyon Kim (GP), Charles Maupin, Pushpak Narayana, Jeffrey Pafford, Michael Trudeau, Scot Weed, Rahim Karmali. November 30 to December 1, 2012. Half Moon Bay, Calif.

Dr. Clark founded the Academy of Microscope Enhanced Dentistry, which is an international academy formed in 2002 to advance the art and science of microdentistry, microendodontics, microperiodontics, and dental microsurgery. He has also developed the Bioclear Matrix System, a comprehensive, tooth specific, clear anatomic matrix and interproximal restorative system. He can be reached at drclark@microscopedentistry.com.

Disclosure: Dr. Clark is the owner of Bioclear Matrix Systems and is a consultant to SS White Burs.

Dr. Khademi is an endodontist and pioneer of restoratively driven micro-endodontics. He can be reached at jkhademi@mydurango.net.

Disclosure: Dr. Khademi is a consultant for SS White Burs and Caresteam Dental.

Dr. Herbranson earned a BS from La Sierra College, a DDS from Loma Linda University, and an MS in endodontics from Loma Linda University. Dr. Herbranson is a cofounder and, until recently, was the executive director of Brown & Herbranson Imaging, a company that develops dental and human anatomy education software under the eHuman moniker. He is also the developer of the Xmount series of microscope camera mounts. With more than 30 years in practice, Dr. Herbranson is a dedicated clinical endodontist. He has made a significant contribution during the last 20 years as clinical assistant professor at the University of the Pacific School of Dentistry, where he lectures to students and special interest groups on endodontics, technology in dentistry, and microscope photography. His study of physics and 40 years of experience in film and digital imaging provide him with an educated understanding of macro- and microphotography, and afford him a unique vision of endodontic education and image production. With his innovative approach and advanced imaging skills, Dr. Herbranson developed the unique processes and methodology for capturing images of human and dental anatomy now used as the basis for Brown & Herbranson Imaging’s educational technology. Dr. Herbranson is the coauthor of the chapter on tooth anatomy in Pathways of the Pulp, and is a frequent speaker and educator at universities and conferences on the subjects of integration of new technology into dentistry, the use of software and computers in presentations, and surgical operation microscope photography. He can be reached at eherbran@hotmail.com.

Disclosure: Dr. Herbranson is a consultant for eHuman and SS White Burs.