Anterior Access Cavity Preparations Many of the same steps are used in similar tooth types to prepare an access cavity. The following discussion outlines the steps for maxillary and mandibular anterior teeth. Tooth-specific access concerns are illustrated and discussed in the section Morphology and Access Cavity Preparations for Individual Teeth, later in the chapter. Removal of Caries and Permanent Restorations Caries typically is removed early, before the pulp chamber is entered. This minimizes the risk of contamination of the pulp chamber or root canal(s) with bacteria. Defective permanent restorations, whether amalgams, composite resins, or crowns, must be removed entirely to prevent coronal leakage from contaminating the pulp chamber, the root canal(s), or both after the endodontic appointment. Removal of defective permanent restorations also permits straight-line access and prevents restorative fragments from becoming lodged in the root canal system (see Fig. 7-74, D). If recurrent decay is detected or suspected, the permanent restoration must be removed entirely to prevent coronal contamination of the pulp chamber. The management of intact permanent restorations when recurrent caries is not present requires some judgment. Amalgam and composite restorations typically are removed entirely to improve visibility during the search for root canal orifices. As stated in an earlier study, 1 clinicians were about 40% more likely to miss fractures, caries, and marginal breakdown if restorations were not completely removed. However, the clinician may want to retain the proximal portion of a class II restoration that extends subgingivally to aid in rubber dam isolation. When parts of existing permanent restorations are not removed, the clinician usually can widen the access opening larger than is ideal at the expense of removing restorative material, not sound tooth structure. The remaining permanent restoration material is removed at the end of the appointment before the temporary restoration is placed. Often clinicians decide to perform endodontic therapy through intact crowns rather than removing or replacing them. These access cavities are repaired after completion of the root canal procedure. The patient and the clinician must realize that visibility can be compromised in these circumstances, particularly if a DOM is not used. The clinician can enhance visibility by beveling the crown's cavosurface margins and by making sure all axial walls are glassy smooth. Initial External Outline Form Once caries and restorations have been addressed, the clinician creates an initial external outline opening on the lingual surface of the anterior tooth. This step often is performed
FIG. 7-40 A, In anterior teeth the starting location for the access cavity is the center of the anatomic crown on the lingual surface (X). B, Preliminary outline form for anterior teeth. The shape should mimic the expected final outline form, and the size should be one half to three fourths the size of the final outline form. C, The angle of penetration for the preliminary outline form is perpendicular to the lingual surface. D, The angle of penetration for initial entry into the pulp chamber is nearly parallel to the long axis of the root. E, Completion of removal of the pulp chamber roof; a round carbide bur is used to engage the pulp horn, cutting on a lingual withdrawal stroke. during the removal of caries and restorations. For an intact tooth, the clinician should begin in the center of the lingual surface of the anatomic crown (Fig. 7-40, A). A #2 or #4 round bur or a tapered fissure bur is used to penetrate through the enamel and slightly into the dentin (approximately 1 mm). An outline form is created, similar in geometry to an ideal access shape for the particular anterior tooth (see Fig. 7-40, B); it is one half to three quarters the projected final size of the access cavity. Because most of this step involves removal of enamel, the high-speed handpiece is used for cutting efficiency. The bur is directed perpendicular to the lingual surface as the external outline opening is created (see Fig. 7-40, C). Penetration of the Pulp Chamber Roof Experienced clinicians can comfortably penetrate the pulp chamber roof with a high-speed handpiece, but less experienced clinicians may find the increased tactile sensation of a slow-speed handpiece a safer option. Continuing with the same round or tapered fissure bur, the clinician
changes the angle of the bur from perpendicular to the lingual surface to parallel to the long axis of the root (see Fig. 7-40, D). Penetration into the tooth is accomplished along this root's long axis until the roof of the pulp chamber is penetrated; frequently a drop-in effect is felt when this occurs. The clinician should measure the distance from the incisal edge to the roof of the pulp chamber on a dimensionally accurate pretreatment radiograph and limit penetration to this distance. If the drop-in effect is not felt at this depth, the clinician should evaluate the situation carefully to prevent a gouge or perforation. The depth and angle of penetration should be assessed for any deviation away from the long axis of the root in both the mesiodistal and buccolingual dimensions, and the penetration angle should be realigned if necessary. If all looks good, the clinician should probe the access opening with an endodontic explorer, using magnification and illumination. Often the sharp explorer tip penetrates through the pulp chamber roof with firm pressure. Angled radiographs should be taken to assess progress if any confusion or doubt exists. A little caution and concern at this stage can prevent an iatrogenic mishap (see Fig. 7-75, C). Complete Roof Removal Once the pulp chamber has been penetrated, the remaining roof is removed by catching the end of a round bur under the lip of the dentin roof and cutting on the bur's withdrawal stroke (see Fig. 7-40, E). Because each tooth has a unique pulp chamber anatomy, working in this manner allows the internal pulp anatomy to dictate the external outline form of the access opening. In vital cases, pulp tissue hemorrhage can impair the clinician's ability to see the internal anatomy. In such cases, as soon as enough roof has been removed to allow instrument access, the coronal pulp should be amputated at the orifice level with an endodontic spoon or round bur and the chamber irrigated copiously with sodium hypochlorite. If the hemorrhage continues, a tentative canal length can be established by measuring the pretreatment radiograph. A small broach coated with a chelating agent then can be introduced into the canal and rotated, thereby amputating the radicular pulp at a more apical level. This procedure is followed by irrigation with sodium hypochlorite. After hemorrhage has been controlled, allowing visibility, all of the pulp chamber roof, including the pulp horns, must be removed and all internal walls must be flared to the lingual surface of the tooth. Complete roof removal is confirmed with a #17 operative explorer if no catches are discovered as the explorer tip is withdrawn from the pulp chamber along the mesial, distal, and facial walls. Identification of all Canal Orifices After the pulp chamber has been unroofed, the canal orifices are located with an endodontic explorer (Fig. 7-41). This instrument is to the endodontist what a periodontal probe is to the periodontist. Used for reaching, feeling, and often digging at the hard tissue, it is the tactile extension of the clinician's fingers. Natural anatomy indicates the usual places for orifices, but restorations, dentinal protrusions, and dystrophic calcifications can dictate the actual configuration the clinician encounters. While probing the chamber floor, the explorer often penetrates or dislodges calcific deposits blocking an orifice. It also can be used to evaluate straight-line access. Positioning the explorer in an orifice allows the clinician to
FIG. 7-41 orifices. An endodontic explorer is used to search for canal check the shaft for clearance from the axial walls and to determine the angle at which a canal departs the main chamber (Fig. 7-42). The endodontic explorer is preferred over the rotating bur as the instrument for locating canal orifices. Not only is the explorer a safer option, its double-ended design also offers two angles of approach. The clinician should keep in mind the probability of finding additional canals in the tooth and the most likely anatomic location of these canals. In a study 94 involving maxillary molars, the authors looked at several pretreatment variables and their influence on the clinician's likelihood of detecting root canals. The results of this study indicated that the number of root canals detected varied according to age and caries. Age was a significant factor in detecting fewer canals, most likely due to the calcification and morphologic changes that occurs in root canals as a patient ages. The presence of caries at the time of treatment resulted in the detection of greater number of root canals. However, because caries was associated with younger individuals in this study, age seems to play the dominant role in the clinician's likelihood of detecting root canals.
Removal of the Lingual Shoulder and Orifice and Coronal Flaring Once the orifice(s) has been identified and confirmed, the lingual shoulder is removed. This structure is the lingual shelf of dentin that extends from the cingulum to a point approximately 2 mm apical to the orifice (Fig. 7-43). Its removal aids straight-line access and allows for more intimate contact of files with the canal's walls for effective shaping and cleaning. FIG. 7-42 Root canal orifices are explored to assess straight-line access and to determine the path of insertion for endodontic instruments.
FIG. 7-43 Lingual shoulder of the anterior tooth, extending from the cingulum to 2 mm apical to the orifice. The lingual shoulder can be removed with a tapered safety-tip diamond or carbide bur or with Gates-Glidden burs. The tip of a fine safety-tip diamond bur is placed approximately 2 mm apical to the canal orifice and inclined to the lingual during rotation to slope the lingual shoulder. The clinician must be careful when using this bur to avoid placing a bevel on the incisal edge of the access preparation (Fig. 7-44). When Gates-Glidden burs are used, the largest that can passively be placed 2 mm apical to the orifice is used first. During rotation, the bur is leaned against the lingual shoulder and withdrawn. The clinician can increase the size of these burs sequentially, depending on the size of the canal, and repeat the shaping of the lingual wall until the lingual shoulder of dentin has been eliminated. During this process the orifice should also be flared so that it is contiguous with all walls of the access preparation. This can be done with small to large Gates-Glidden burs. These burs are used in a circumferential filling motion, flaring each wall of the canal in sequence. To prevent iatrogenic mishaps on thin walls facing a root concavity, these burs should be placed passively into the canal and rotated as they are gently leaned against a canal wall and withdrawn. Another approach to flaring the orifice involves the use of rotary nickel titanium orifice openers; they should be used at slow speeds and low torque. Straight-Line Access Determination After the lingual shoulder has been removed and the orifice(s) flared, the clinician must determine whether straight-line access has been achieved. Ideally, an endodontic file can approach the apical
foramen or the first point of canal curvature undeflected. Unnecessary deflection of the file can result in numerous consequences related to loss of instrument control. Deflected instruments function under more stress than FIG. 7-44 Placing an incisal bevel on the lingual surface of a maxillary anterior tooth can lead to fracture of the permanent restoration during occlusal function. undeflected instruments and are more susceptible to separation during the shaping and cleaning process (Fig. 7-45). Deflected instruments also lack access to critical areas of the canal and therefore do not shape and clean effectively. Attempts to shape and clean without straight-line access often lead to procedural errors such as ledging, transportation, and zipping (Fig. 7-46). A ledge is an iatrogenically created root canal wall irregularity that may impede placement of an intracanal instrument to the apex. Transportation occurs in the portion of the canal apical to a curvature when canal wall structure opposite the curve is removed, tending to straighten the canal curvature. Zipping, or elliptication of the apical foramen, occurs when an overextended file transports the outer wall of the apical foramen. Conversely, undeflected instruments provide better tactile sensation, which is necessary for feeling the canal anatomy and feeling how the file is performing in the root canal system. Straight-line access is evaluated by inserting into the canal the largest file that fits passively to the apical foramen or the point of the first canal curvature. This internal length can be determined by measuring a diagnostic pretreatment periapical radiograph. The file is inserted gently and withdrawn as the clinician feels for canal binding or deflection. If deflection is detected, the
clinician must reevaluate the adequacy of lingual shoulder removal before changing the incisal edge position of the access preparation. Inadequate removal of the lingual shoulder causes the file to deflect in a facial direction, and an FIG. 7-45 Separation of a rotary endodontic instrument as a result of underextended access preparation rather than canal binding.
FIG. 7-46 Inadequate access preparation. The lingual shoulder was not removed, and incisal extension is incomplete. The file has begun to deviate from the canal in the apical region, creating a ledge. FIG. 7-47 A, The lingual ledge of dentin remains, deflecting the file toward the labial wall. As a result, portions of the lingual canal wall will not be shaped and cleaned. B, Removal of the lingual ledge results in straight-line access.
inexperienced clinician may overextend the incisal edge of the access preparation in an attempt to achieve straight-line access (Fig. 7-47). If the lingual shoulder has been adequately removed and the file still binds on the incisal edge, the access cavity should be extended farther incisally until the file is not deflected. The final position of the incisal wall of the access cavity is determined by two factors: (1) complete removal of the pulp horns and (2) straight-line access. Visual Inspection of the Access Cavity The clinician should inspect and evaluate the access cavity, using appropriate magnification and illumination. Although this can be done during any stage of the preparation, it should always be done at this point. The axial walls at their junction with the orifice must be inspected for grooves that might indicate an additional canal. The orifice and coronal canal must be evaluated for a bifurcation (see Fig. 7-14). Refinement and Smoothing of Restorative Margins The final step in the preparation of an access cavity is to refine and smooth the cavosurface margins. Rough or irregular margins can contribute to coronal leakage through a permanent or temporary restoration. Proper restorative margins are important because anterior teeth may not require a crown as the final restoration. Definite, smooth cavosurface margins allow the clinician to place and finish a composite resin final restoration with the precision necessary to minimize coronal leakage. Such leakage could jeopardize the success of the root canal procedure. Another factor the clinician must consider when finalizing the access margins of a maxillary anterior tooth is that the final composite resin restoration will be placed on a functional tooth surface. The incisal edges of the mandibular anterior teeth slide over these maxillary lingual surfaces during excursive jaw movement. Therefore the restorative margins of maxillary anterior teeth should be created to allow a bulk of restorative material at the margin. Butt joint margins are indicated rather than beveled margins, which produce thin composite edges that can fracture under excursive functional loads and ultimately result in coronal leakage. Obviously, if the anterior tooth requires a crown as the final restoration, the access cavosurface margin becomes a less critical factor. Hargreaves, Kenneth, Stephen Cohen. Cohen's Pathways of the Pulp, 10th Edition. Mosby, 2011. VitalBook file.