Large MODB pin-retained amalgam restoration. The loss of original occlusal anatomy landmarks and pulp canal space calcification will make this a more challenging tooth to access for root canal therapy
In addition, a tooth that has been previously accessed, with or without complete treatment, presents the risk of preexisting perforation, canal blockage, or loss of normal pulpal floor anatomical landmarks. Gouging of the floor and walls of the pulp chamber during access can obliterate normal internal anatomy that could otherwise be used to help locate canal orifices.
4.3 Goals of Endodontic Access
4.3.1 Traditional Access Preparation Compared to Minimally Invasive Access
In the traditional approach to endodontic access, there are three primary goals: conservation of tooth structure, complete unroofing of the pulp chamber, and straight-line access to the apical third of the canal . While all three of these goals are reasonable and supported by decades of clinical practice, new technology in endodontics, such as super flexible nickel-titanium (NiTi) instruments and dental microscopes, has resulted in a shift in the primary focus of access to preservation of coronal tooth structure to avoid possible predisposition to future root fracture. It is still important to completely remove any remnants of pulp tissue from the pulp horns and to have an access opening large enough to locate all canals. Failure to remove all pulp tissue can cause discoloration of the coronal tooth structure and, in addition, provide a potential source of nutrition for any remaining microorganisms. Examples of a traditional molar access are shown in Figs. 4.8 and 4.9.
The concept of minimally invasive access and focus on preservation of coronal tooth structure, also referred to as directed dentin conservation  and conservative endodontic cavity , is a relatively new idea, and there are no long-term clinical studies to either support or reject this approach. However, anecdotally, there is a growing concern among many endodontists that root fracture of endodontically treated teeth, often occurring years after initial root canal treatment, is a significant cause of treatment failure and should be addressed to improve the probability of long-term survival of endodontically treated teeth. Some preliminary in vitro research suggests that a conservative endodontic cavity approach is probably worthy of additional study and consideration [4, 5]. Without the enhanced magnification and illumination offered by a dental operating microscope, a conservative endodontic access presents greater risk of perforation and missed anatomy than a traditional access preparation. Figures 4.10, 4.11, and 4.12a–c demonstrate variations of a minimally invasive endodontic access preparation.
4.3.2 General Rules for Establishing the Outline and Depth of an Endodontic Access Cavity
Krasner and Rankow  proposed three rules derived from the sectioning and observation of the pulp chambers of 500 extracted teeth:
Law of centrality
Law of concentricity
Law of the CEJ
The law of centrality states that the floor of the pulp chamber is located in the center of the tooth at the CEJ. This often varies from the center of the occlusal surface of the tooth, so it is important to be able to visualize the cross section of the tooth at the CEJ. The law of concentricity states that the walls of the pulp chamber are concentric with the external surface of the tooth at the CEJ (Fig. 4.13). That is, the occlusal outline of the access cavity should mimic the general shape, on a smaller scale, of a cross-sectional slice through the tooth at the CEJ. For example, a tooth that is wider in the facial-lingual/palatal dimension than mesial-distal at the CEJ (such as a mandibular incisor or maxillary premolar) should have a similar occlusal outline to assist in locating all canals. Krasner and Rankow  noted that canals were typically located at the angle formed by the junction of two canal walls in a multi-rooted tooth. Finally, the law of the CEJ states that the CEJ is the most predictable landmark for determining the depth of the pulp chamber (Fig. 4.14). Even in teeth with calcified and/or receded pulp chambers, the roof of the pulp chamber can be found at the level of the CEJ . This finding applies primarily to molars since the roof of the pulp chamber for anterior teeth and premolars often extends coronal to the CEJ in younger patients and can recede apical to the CEJ with increasing age and restorative treatment. The CEJ landmark can serve as an excellent guide to help prevent perforation through the pulpal floor or lateral surface of the tooth. If the pulp chamber has not been located after bur penetration to the estimated depth of the CEJ, it is appropriate to stop and expose a radiograph (a bitewing image is often the most useful) to determine the actual location of the pulp space in relation to the initial access cavity (Fig. 4.15). Another general rule to help prevent a pulpal floor perforation is to recognize that the dentin on the floor of the pulp chamber is usually darker in color than the more coronal dentin. It may be difficult to precisely determine the location of the CEJ in teeth with full occlusal coverage restorations. These teeth often present a greater risk for pulpal floor perforation (Fig. 4.16) and should be approached with caution. The enhanced magnification and illumination provided by a dental operating microscope is particularly important when performing endodontic access on teeth with full occlusal coverage restorations.