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Proper Root Canal Access

Requirements and Techniques

Contemporary endodontic therapy emphasizes the removal of infected soft and hard tissue from the root canal system and then sealing it to prevent re-infection. Both steps (ie, disinfection and filling) depend on access of the root canal system. Access cavities should follow the specific anatomy of the tooth under treatment, allow unimpeded cleaning and shaping, and should not overly weaken root structure to retain restorability. This editorial reviews basic principles of access cavity preparation and illustrates clinical steps for optimized and simplified procedures.

Tooth development occurs in pre- and postnatal stages in a tightly regulated relationship between odontoblast and ameloblast layers; dental morphogenesis is a genetically controlled process and the arrangement of odontoblasts and ameloblasts leads to a striking resemblance of pulp space and outer tooth contour (Figure 1). This relationship allows certain landmarks to be used when accessing root canal systems clinically. For instance, if one canal is found in a maxillary premolar far to the buccal, it is very likely that a second canal more to the palatal exists and needs to be accessed. This basic law of symmetry holds true in most cases, for instance, when looking for additional canals in the mesiobuccal or distobuccal root of maxillary molars (Figure 2). The presence of carious lesions or restorations, however, may lead to tertiary dentin deposition and thus alter the shape of the pulpal space, in particular in the most coronal aspects. Additionally, aging reduces overall pulp size and may complicate access cavity preparation. Thus, dental and pulpal anatomy is highly complicated and variable.1 

As an indispensable part of pre-endodontic diagnostics, preoperative radiographs (preferably periapical exposures in different angulations and a bitewing) should be taken to glean as much information as possible about coronal and radicular pulp space, its size, and potential complications during access (Figures 3-4-5-6 ).2 Obviously, the presence of crowns or large restorations makes radiographic interpretation difficult, but in most cases, an estimate of pulpal space position and size can be made. It has been proposed for one to initiate access prior to rubber dam placement;3 this can both help to prevent opening of the wrong tooth and also afford a better judgment with regard to the long axis of the tooth.

The initial penetration into enamel or restorative material is then made with a suitable high-speed bur, angled toward the largest expected pulp space as detected on the preoperative radiograph. Upon entry into the pulp space, a non-end cutting bur is selected in order to avoid gouging of the cavity floor. Overpreparation should be avoided in order to retain as much structural strength as possible while exposing the pulpal cavity in its entirety. Underpreparation restricts access to the root canal system with the potential consequence of preparation errors (eg, apical zips, strip perforation). Furthermore, success with rotary instruments depends strongly on unimpeded straight-line access into the coronal and middle root canal thirds (Figure 1).

Straight-line access is accomplished with several other instruments, most notably Gates Glidden drills, various nickel-titanium instruments and ultrasonically energized inserts. Root canal orifices, however, are not always immediately accessible. An endodontic probe (ie, DG 16 or similar) has to be rigid but still sufficiently fine to probe into potential canal orifices. Obstructing calcification may be removed under the operating microscope with the aid of excavators or ultrasonic instruments. Even finer hand instruments allow tactile exploration under magnification and entry into obstructed root canal spaces.

With this armamentarium, a detailed understanding of dental anatomy, clinical expertise, and a degree of luck, access into the pulpal cavity and the root canal system will be successful in most cases. In a maxillary first molar (Figures 7 and 8), the mesiobuccal pulp horn is typically the highest in relation to the occlusal plane, while the palatal may be the most voluminous. After initial penetration into either one of the two, the outline is shaped and refined to remove undercuts. The goal is to have slightly diverging walls, similar to a Class I inlay cavity. After further refinement and following the landmarks on the cavity floor, all four canal orifices are clearly visible and ready for further enlargement.

 

Conclusion 

In summary, access to the root canal space has to be performed with the “do no harm” principle in mind. When procedural mishaps such as perforations are avoided and straight-line access to the middle root canal third is gained, a procedural foundation for a successful root canal treatment is laid. The importance of this step cannot be overstated and developments continue to make access safer and more efficient.

 

*Assistant Clinical Professor, Division of Endodontics, School of Dentistry, University of California San Francisco, San Francisco, CA.

References 

  1. Brown P, Herbranson E, eds. Dental anatomy & 3D interactive tooth atlas Version 2.0. Carol Stream, IL: Quintessence Publishing, 2005.
  2. Vertucci FJ, Haddix JE, Britto LR. Tooth morphology and access cavity preparation. In: Cohen S, Hargeaves KM, eds. Pathways of the Pulp, 9th ed. Philadelphia, PA: Mosby, 2005.
  3. Carrotte P. Endodontics: Part 6. Rubber dam and access cavities. Brit Dent J 2004;197(9):527-534.
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