Modern Endodontic Access and Dentin Conservation, Part 2

Together, the authors explore the Endodontic-Endo-Restorative-Prosthodontic (EERP) continuum. This 2-part article will focus on the pervasive endodontic problems vexing patients, restorative dentist, and endodontists. The authors provide alternative models and thought processes to treat the tooth in a nontraditional approach—from cusp tip to apex. In addition, they will propose immediate tools to implement these important changes.

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During patient treatment, the clinician needs to consider a multitude of factors that will affect the ultimate outcome. In simple terms, these factors can be grouped into 3 categories: the operator needs, the restoration needs, and the tooth needs. The operator needs being conditions the clinician needs to treat the tooth. The restoration needs being the prep dimensions and tooth conditions for optimal strength and longevity. The tooth needs being the biologic and structural limitations for a treated tooth to remain predictably functional. In this article we want to discuss failures of endodontically treated teeth that occur, not because of chronic or acute apical lesions, but because of structural compromises to the teeth that ultimately render the tooth useless. We want to shift the coronal focus to the cervical area of the tooth and to create awareness for an endo-restorative interface. In part 2 of this article series, we will introduce a set of criteria that will guide the clinician in treatment decisions to maintain optimal functionality of the tooth.


Endodontic accesses are traditionally conservative to the occlusal/incisal tooth structure. However, with the changes that have occurred in restorative dentistry, this technique is unnecessarily restrictive for the operator and potentially damaging to the more critical cervical area of the tooth. 

A NEW MODEL FOR ENDODONTIC ACCESS
As we deconstruct endodontic access, it is crucial to understand the “Big 5” catalyst forces that will change the future of endodontic access and coronal shaping. They are: (1) implant success rates (The bar is raised); (2) operating microscopes and microendodontics; (3) biomimetic dentistry; (4) minimally invasive dentistry; and (5) aesthetic demands of patients, combined with manufacturer recommendations for axial reduction for all-ceramic crowns.


In both of our practices, our endodontic goals and armamentarium have been in a constant state of flux for nearly a decade as we have collaborated to bring the EERP continuum to maturity. The goal? To satisfy the demands of the above-mentioned Big 5 forces for change. In so doing, when preparing endodontic access, we have come to realize that our previous needs as dentists were often in conflict with the needs of the tooth.

The Hierarchy of Tooth Needs

Table 1. The Hierarchy of Tooth Needs for Anterior Teeth
Extremely High Pericingulum
Dentin
Pulp in Immature
Teeth
High Cingulum Enamel
Axial Wall DEJ
Cervical Enamel
Medium Peri-incisal Enamel
Low 2º Dentin
No Value or Liability 3º Dentin
Inflamed Pulp in
Mature Teeth
Exposed Dentin in
Incisal Area

The following tables (Tables 1 and 2) represent the hierarchy of needs to maintain optimal strength, fracture resistance, along with several other characteristics needed for long-term full function of the endodontically treated tooth. This brief article is designed to simply introduce the reader to the reshuffling of the values assigned to different tooth structures, and to the nuanced role of the importance of regional tissues. A full explanation of the “new hierarchy” will be presented in future articles.


The brevity of this article precludes a full definition for all of the terms of the glossary. However, there are 4 terms that will be explained below. Others will be mentioned in the context of the 3 featured cases included below. 

The Inverse Funnel and Blind Tun­neling are demonstrated by the 2 endodontic accesses performed on my younger brother, Tom, who occasionally bumps his teeth while playing on the ski slopes and soccer field (Figure 1). A round bur was used by his general dentist as he labored to discover the canal systems in these calcified incisors. Note that as the access goes deeper into the tooth, it becomes wider internally, hence the term inverse funnel. In the new approach advocated by Clark/Khademi (CK), the access and new bur selection should allow for the formation of a true funnel; wherein the narrow portion of the funnel is in the pericervical dentin zone, and the cavosurface has a 45º angle with an infinity edge margin becoming a generous “mouth” or “top” of the funnel. Models contrasting the CK funnel, the inverse funnel, and the blind tunnel are shown in Figures 2 to 3e. The stark difference between the tip size of the new CK endodontic-ac­cess/endo-exploration bur and a comparable round bur is shown in Figure 4.

Table 2. Glossary of Terms for Modern Endodontic Access and Acronyms
Note: The red text indicates a nondesirable outcome, or technique.
Glossary of Terms Acronym
The endodontic-endorestorative-prosthodontic continuum EERP
Three-Dimensional ferrule 3-D Ferrule
Peri-Cervical dentin PCD
Peri-Cingulum dentin
 
The inverse funnel  
Blind tunneling  
Blind funneling  
Partial de-roofing  
Soffit  
Stepped access  
Secondary dentin 2º Dentin
Tertiary dentin 3º Dentin
Biomimetic endodontic shaping BES
Arbitrary round shaping ARS
The dentinal map  
The dentino-enamel junction DEJ
The junction of primary and secondary dentin D²J
The junction of primary and tertiary dentin D³J
Pulp tissue remnant PTR
Points of negotiation PON

Peri-Cervical Dentin (PCD) is the dentin near the alveolar crest. While the apex of the root can be amputated, and the coronal third of the clinical crown removed and replaced prosthetically, the dentin near the alveolar crest is irreplaceable. This critical zone, roughly 4 mm above the crestal bone and extending 4 mm apical to crestal bone, is sacred. There are 3 reasons for this: (1) ferrule, (2) fracturing, and (3) dentin tubule orifice proximity from inside to out. The research is unequivocal: long-term retention of the tooth and resistance to fracturing are directly relational to the amount of residual tooth structure.1,2 The more dentin we preserve, the longer we keep the tooth.

Peri-Cingulum Dentin: In the instance of incisor ac­cess, the research done by Magne and Belser3 in regards to the importance of the cingulum directly conflicts with the traditional cingulum-positioned endodontic access technique currently taught. There are severe tensile forces concentrated at the cingulum when the maxillary anterior teeth are functionally loaded. These forces can lead to structural break­down when the peri-cingulum dentin is compromised during traditional access near the cingulum. The situation is further exacerbated by deep axial reduction when a crown preparation is performed and a deep margin is also cut in the palatal area. For that reason, CK accesses are positioned closer to the incisal edge. In the instance of the worn tooth with exposed dentin, the access includes this landmark as the incisal-facial border of the access (Figures 5 to 7).


Gouged Access Collage First Row—Maxillary Anteriors: These anteriors represent a spectrum of gouging typically seen in anterior teeth. The first case shows very common occult mild cervical gouging stemming from an access that was placed too far cervically with a round bur. Stress-focusing mesial and distal gouges nearly eliminating ferrule quality resulted from using square-ended carbides as shown in the second case. Round burs used both cervically and deeper in the root system have severely compromised the PCD. This eventually results in perforation (in red) of the root system in the fourth case. Note that the access gets wider (inverse cone) as it progresses apically in this lateral incisor. Keep in mind that the gouging is usually more severe in the bucco-lingual plane. Correct incisally placed access maximally preserving the irreplaceable PCD.

Second Row—Mandibular Incisors: The sequence of lower incisors show the same types of errors as seen in the first row, starting with mild occult gouging and over-enlargement of access that was placed too far cervically, ending with a perforation in the fourth case. As the earlier drawings show, these teeth are invariably gouged to the buccal as well. The para­dox of these case types is perhaps best illustrated by the third case with the traditional cingulum style access and the extensive cervical gouging: the more calcified the case, the more incisal the access must be placed. In fourth case, the access was extended completely to the incisal edge, re-oriented and the canal was located. The perforation was repaired with mineral trioxide aggregate and Ca(OH)2 was placed. The canals were obturated at a subsequent visit. The “Monday morning test” is that the correct access is invariably farther incisal than traditionally de­scribed, and in the calcified case, may go straight through the incisal edge (as shown in the earlier drawings).

Why Are Round Burs So Destructive?

Figure 1. My younger brother, Tom, received trauma to both his upper and lower central incisors. The teeth subsequently underwent dystrophic calcification, and although still in function, they were badly weakened. His dentist lacked the proper tools and followed an access form that is no longer appropriate.

Figure 2. A new model for lower incisor access is depicted, along with the new Clark/Khademi (CK) endodontic access bur. Note the access has been moved away from the cingulum and towards the incisal edge. The delicate tip size of the bur and its conical shape are helpful to both visual and tactile endodontics.
Figure 3a. Blind Tunneling: Gouging that is common with round burs and cingulum access. Buccal-lingual gouging (not easily seen in x-rays) occurs in nearly every traditionally-accessed case. Figures 3b to 3e. The Inverse Funnel: As the access grows internally, an inverse funnel is created. Precious peri-cervical dentin (PCD) is lost each time the bur enters the tooth.

In reality, it is truly impossible to cut flat walls in 3 dimensions with a round instrument. In reality, with the use of a round bur, the chamber is unroofed in some areas leaving pulpal and necrotic debris, and the walls are overextended and gouged in other areas. Furthemore, the internal radius of curvature at many of the pulpal line angles is simply too small for all but the smallest of round burs.


In the final analysis, round burs point cut in an endodontic access application, when instead what is needed is planing. What is required 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 of a rounded-ended taper design.

FEATURED CASE 
The Calcified Incisor (Clark) 

Figure 4. Illustration comparing the CK endodontic access bur to the corresponding round bur. The tip size of these burs is less than half as wide as the corresponding round bur. One of the prototype CK endodontic access burs (right) is shown and contrasted with the corresponding surgical length round bur (left). The access burs, designed by Drs. Clark and Khademi, will be available in early 2009 from SS White Burs.

Figure 5. Lingual view of the CK model of ideal mandibular anterior access. This extremely calcific tooth shows the ideal cavity outline to satisfy operator, restorative, and tooth needs.

Figure 6. Facial view of the access and the tiny lingual notch. In a case with significant wear and significant exposed dentin, the access will go directly through the incisal. The facial extension of the exposed dentino-enamel junction becomes the facial margin of the access.

Figure 7. Invisible restoration of the CK access and tooth at 3-year recall. The margins were heavily beveled before restoration (not pictured). This is the “Infinity Edge” margin (introduced by Dr. Bob Margeas). The access was closed with Filtek Supreme Plus (3M ESPE). Our SEM evaluation of this technique, combined with the unique properties of such composite resin materials, shows ideal wear and microleakage resistance.

The maxillary left central incisor (tooth No. 9) in a 21-year-old female was undergoing dystrophic calcification (Figures 8 and 9). For such teeth, a cingulum positioned and round bur or fissure bur driven access, the risk of gouging is high. When the access is moved toward the incisal edge, there are many benefits. Additionally, with the use of the CK access burs, the conical shape encourages the bur to follow a truer course. Because this tooth had a failing composite restoration on the incisal edge, I had the luxury of moving the access through the incisal edge and with a generous cone (Figures 10 and 11). The patented shape derived from the original SS White Fissurotomy (Figure 12) bur is ideal for large incisors. A conical carbide has many advantages over other modalities. For example, ultrasonic tips allow good visualization, but do not end cut well. Also, they do not leave a polished dentinal surface. The rough surface left by the diamond is much harder to “read” than a polished surface when studying the nuanced differences in color, opacity, and texture of the dentin. Access prepared with a tapered diamond will share the same problems found with ultrasonic tips. A carbide bur has the advantage here as it is superior in end cutting, and also leaves a polished dentinal surface allowing ideal visual clues.

Gouged Access Collage
Collage. (Mural is described in the text.) Note: Blue arrows indicate gouges. Red arrows indicate perforations. “JK” indicates that case was done by Dr. John Khademi with adherence to the modern model of directed dentin conservation.
Figures 8 and 9. Feature Case: This 21-year-old female patient was engaged to be married and had requested comprehensive aesthetic treatment. The first treatment planned for the left central incisor was elective/proactive removal of the degenerating pulp, followed by internal bleaching, and then placement of a porcelain veneer.
Figures 10 and 11. Following the new hierarchy of tooth needs, the preservation of PCD dictates that incisal composite be sacrificed, an easy compromise.

Even after the access is placed essentially through the incisal edge, there remains slight binding of the file (Figures 13 to 15). As the feature case progresses, the file is allowed to enter the tooth without binding, and there was no deviation as the bur discovered the tiny thread of residual pulp tissue. (Figures 16 to 18). When the useless tertiary dentin is engaged and removed at the incisal, and carefully followed into the cervical zone, the perfect orientation of a long trajectory creates a safe guide, just like a surgical stint can guide the drill and placement of an endosseous implant. Incisal access is superior to cingulum access in the same way that a rifle is more accurate than a pistol; the barrel is much longer and therefore the trajectory is much easier to control. When combined with the operating microscope, the properly equipped clinician can confidently access the canal system early on in the incisal-apical direction.

Figure 12. The large CK access bur is shown. This bur is appropriate for larger incisors. The tip size is actually more delicate than a No. 2 round bur and creates an ideal cone-shaped access.
Figures 13 to 15. Although the access was positioned through the incisal edge, the file is actually binding slightly against the incisal portion of the access. The series of radiographs depicts the “dead on” discovery of an extremely calcified pulp. No unnecessary removal of the most crucial dentin (peri-cervical and peri-cingulum) occurred.

Clark Sequence for Large Incisor Access
1. Begin with the friction grip original SS White Fissurotomy bur or the new surgical length large CK bur. Start with the cavosurface design. Create a beveled margin as you begin the access, instead of later. You will get better light for vision, and the smaller internal shape will be compensated by a better funnel shape externally as we insert instruments and gutta-percha into the tooth. In the words of the great John Stropko, “Don’t fight the case!”
2. In a calcific case, switch later to the CK NTF (Narrow Taper Fissurotomy, latch grip) as you get deeper into the tooth. Constant visualization of the D2J will guide your bur orientation as you stay dead center in the bulls-eye of the dentinal map.

Figure 16. Midtreatment radiograph shows that the file has encountered the pulp chamber dead on, and “early.”

Figure 17. Midtreatment radiograph with file to length.

Figure 18. Final radiograph showing adequate shape and obturation for a nonlesion case.

Clark Sequence for Small Incisor Access
The sequence for small incisors, typically the lower incisor, should begin with an ovoid shape and utilizing the more delicate SS White NTF friction grip bur or the surgical length friction grip CK NTF or the Super NTF.

FINAL RECOMMENDATIONS AND CLOSING COMMENTS
It may seem odd at first, but put away your round burs and Gates Glidden burs, and your square-ended 556 fissure burs. Move your anterior accesses away from the cingulum, as close to the incisal edge as possible. For worn anteriors, go right through the incisal edge with your access. Then, take your fissurotomy bur along the incisal edge to remove a millimeter thickness of dentin. Generate a long bevel on enamel. Then as you close the access with a good microfilled composite you will cover all of the ugly and porous exposed dentin with at least a millimeter thickness of composite. The color of the tooth will immediately improve and the incisal of the tooth will resist future staining and wear.


Thus you “bless the tooth” as you create endodontic access, as opposed to “cursing the tooth” with traditional burs and techniques. In vital (nonlesion) cases, you can confidently make very small endodontic shapes that will be more consistent with lateral condensation techniques. Get a microscope today, and take courses in microscope-assisted access techniques.

Acknowledgement
The authors would like to thank Dr. Jihyon Kim and Dr. Eric Herbransen for their contributions.


References

  1. Lertchirakarn V, Palamara JE, Messer HH. Patterns of vertical root fracture: factors affecting stress distribution in the root canal. J Endod. 2003;29:523-528.
  2. Tamse A, Fuss Z, Lustig J, et al. An evaluation of endodontically treated vertically fractured teeth. J Endod. 1999;25:506-508.
  3. Magne P, Belser U. Bonded Porcelain Restorations in the Anterior Dentition: A Biomimetic Approach. Chicago, Ill: Quintessence Publishing; 2002.

Dr. Clark is a general dentist and pioneer in Biomimetic Microendodontics and Minimally Traumatic Restorative Microdentistry. He can be reached at he can be reached via e-mail at drclark@microscopedentistry.com or
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