Endodontic Access Preparations: Dentin Conservation and the Anatomical Danger Zones

Eric Herbranson, DDS, MS, FDIC

Dr. Eric Herbranson, DDS, MS, FDIC

The author and his practice partner, Dr. Paul Brown, have spent the last 30 years in clinical endodontic practice. As established specialists, they have handled their share of difficult cases and in that time they became aware that most of the iatrogenic treatment errors in endodontic therapy were the result of an incomplete understanding of dental anatomy. Their frustration with how to address this lack of understanding and how to teach the principles of anatomy were answered with a serendipitous visit by Dr. Brown to a computer laboratory at Stanford University, California. The Stanford/NASA Biocomputational Laboratory was developing interactive viewing software for three-dimensional computer models and applying it to medical computerized tomography (CT) images. Dr. Brown realized that this technology could be applied to dental anatomy education. With funding from the National Institutes of Health, the authors have spent the last 10 years developing a unique educational product using this software and the power of the modern computer to teach dental anatomy. Called the “3D Interactive Tooth Atlas,” the current version, 5.1, contains more than 450 high-resolution interactive computer models of real human teeth derived from research-grade micro CTs (mCTs).1 The authors are pleased that this unique product is starting to impact the anatomy education of dental students in the United States. While the author has always been a student of dental anatomy,2 this work has dramatically increased his awareness of the nuances and clinical ramifications of the teeth that we treat. As his understanding has increased, the awareness of the complexity of our treatment task became more obvious.

Figure 1 – An upper first molar with seven canals. Image was taken with an intraoral video camera.

At the same time, endodontics has seen significant changes. For example, the wide adoption of the surgical operating microscope has allowed dentists to visualize more complex anatomy.3,4 When the author started practice in the 1970s, the assumption was that the upper first molar had a second canal in the mesiobuccal (MB) root about 50% of the time.5 Currently, endodontists are reporting fourth canals in the MB anywhere from 70% to 95% of the time in clinical practice, and teeth with five and six canals are seen with some frequency.6 The author’s practice partner, Dr. John Jaber, recently treated an upper first molar with seven canals (Figure 1). All of this variation and complexity that dentists are seeing is the result of the increased understanding of the potential for complexity and the ability to visualize details with the microscope. This has created better treatment outcomes because if a clinician understands and can see the anatomy, they can treat optimally.

Endodontics also saw the introduction and adoption of engine-driven nickel-titanium (NiTi) rotary instruments for shaping the canals. They have increased the efficiency of treating endodontics cases, especially difficult ones with curved canals. As one might expect, there is no universal answer to dental problems and there occasionally exist incompatibilities between the file system being used and the anatomy being addressed. This can be a subtle problem, and this article will point out some of the anatomical danger zones and the shaping strategies needed to address them. The author does not attempt to provide a comprehensive review of all the file systems on the market, but seeks to identify the features of which the clinician needs to be aware when selecting instruments for a specific situation.

The Problem: There Is Less Dentine than We Think

Figure 2 – A lower first molar mesial root. The left image is a voxel model from the mCT scan, the middle image is a radiographic derived from the mCT scan, and the right image is a slice from the mCT data, at a level just below the furcation.

This situation is illustrated by Figure 2, showing the mesial root of a lower first molar that was extracted because of a distal root fracture. The mesial root, along with some attached bone was mCT scanned to evaluate the clinical fill requirements. This tooth was shaped with a popular file system that produces a final, predefined shape with one instrument. The clinician found and treated a middle mesial canal. The interesting feature of this tooth is the presence of deep concavities on both the mesial and distal sides of the root, which produced a narrow isthmus in the middle of the root. While there is some minimal transportation of the canals toward the furcation in the MB and distobuccal (DB) canals, the file used is appropriate to the amount of dentine in these canals. This is not the case with the middle mesial. The minimal amount of dentine, combined with some transportation of the canal, resulted in a possible lateral strip perforation. A more appropriate shaping strategy would have been to use a file with a smaller diameter at this level. Interestingly, this did not result in destruction of the bone and was not the cause of the extraction. Middle mesial canals are reported to occur in approximately 13% of lower first molars7 and they may be common with another canal. The clinician should assume that the middle mesial canal does not join another canal until proven otherwise. The clinician should also assume that the root has a furcation-side concavity, resulting in a thin layer of dentine over that canal. A more conservative shaping preparation should be routine to prevent a lateral strip perforation.

Figure 3 – A polygon model of a five-canal lower first molar with the mesial root apex transparent. Note the complex anatomy with three canals and five portals of exit.

Concavities in the roots of lower molars are more common than is appreciated and are not obvious in radiographs. These concavities tend to be on the furcation side of the root but can occur in the outside root surface also (Figure 3).

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