0099-2399/92/1811-0565/$03.00/0 JOURNAL OF ENDODONTICS Copyright © 1992 by The American Association of Endodontists
Printed in U.S.A.
VOL. 18, NO. 11, NOVEMBER1992
CLINICAL AID Endodontic Retreatment: A Rational Approach to Root Canal Reinstrumentation Ely Mandel, DCD, DSO, and Shimon Friedman, DMD
Technic Co., Tustin, CA) can be used to bypass gutta-percha root fillings, particularly in curved canals (8). It is also useful in bypassing and retrieving solid objects which resist removal attempts with hand instruments (9). Additional techniques are available for retrieving solid objects, using mainly various mechanical devices (10, 11). These techniques appear to be time consuming, unsuitable for use in curved roots, and require considerable sacrifice of root dentin (10). Regardless of the technique used, endodontic retreatment requires total regaining of canal patency. This is invariably accomplished by exerting pressure in the apical direction with rigid instruments, with the purpose of removing the root filling material. When executed undiscriminately this procedure may be ineffective and even harmful (2, 10). However, when it is performed systematically canal patency can be successfully regained in most retreatment cases. Subsequently, endodontic retreatment may be accomplished by using routine endodontic procedures. The purpose of this article was to discuss the mechanical considerations governing the regaining of canal patency and to present a rational step by step approach to reinstrumentation of the root canal during nonsurgical endodontic retreatment.
During nonsurgical endodontic retreatment, endodontic instruments are forced apically to remove the root canal filling material and regain canal patency. Undiscriminating burrowing down the canal in the apical direction may be fruitless and harmful. To avoid complications, the dentin overhanging the canal orifice must be removed and an unobstructed access established to the root filling material, so as to facilitate its removal. Reinstrumentation of the filled canal must take into consideration the nature of the filling matedal and the physical properties of endodontic instruments, as well as the dynamic aspects of canal preparation. This article discusses the mechanical considerations pertaining to root canal retreatment and outlines a step by step rationale approach to retreatment.
Nonsurgical endodontic retreatment consists of cleaning, shaping, and three-dimensional filling of previously obturated root canals. It is the treatment of choice for the management ofendodontic failures when access to the root canals is feasible (1). To successfully accomplish retreatment, all of the obstructions preventing a direct access to the root canals have to be removed (2). These include the intracoronal restoration (2) and frequently also dentin overhanging the canal orifice in teeth where the previous endodontic access cavity was underextended. Next, the obstructions within the canal are removed, i.e. root filling materials and foreign objects (2). The most frequently encountered root filling materials which have to be removed in retreatment are pastes and cements, semisolid and solid materials (2). In a recent study these were the filling materials in 20.6%, 53.6%, and 21.7%, respectively, of the 1300 retreated cases (3). Retreatment has been addressed frequently with respect to the techniques of removing the various filling materials from the root canal (4-10). Hard setting pastes can be retreated by ultrasonic endodontic instruments, although this procedure is time consuming (4, 5). Ultrasonic instrumentation also facilitates removal of solid objects (6, 7). The Canal Finder (Endo
MECHANICAL CONSIDERATIONS A rational approach to nonsurgical endodontic retreatment must take into consideration the existing obstructions of the root canal. These may be morphological, resulting from an underextended endodontic access cavity, or iatrogenic, consisting of filling materials and/or other foreign objects (2): In addition, the dynamic aspects of canal preparation must be considered, as they apply to instrumentation of fine, curved canals. To evaluate the obstructions and the canal shape a preoperative radiograph (R 1) is taken before retreatment. In radiograph R1 the iatrogenic obstructions are usually clearly evident, whereas the existence of morphological obstructions is only indicated. In addition, radiograph R1 is used to estimate the canal length which has to be retreated. The sequence in which the morphological and iatrogenic obstructions are removed depends on the nature of the iatrogenic obstructions, particularly the root filling material.
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F~G 2. The Endo-Z carbide tungsten bur has a noncutting tip. This bur is recommended for aligning the axial walls of the access cavity with the cave-canal line angle.
F~G 1. A schematic representation of an endodontic instrument inserted into a curved canal which is coronally confined by deposition of dentin. The file (F) is pressed against the canal wall opposite the dentin deposit (DD) (arrow). As a result the file is distorted and its tip is unable to negotiate the apical canal curvature (open arrow).
Removal of Morphological Obstructions The complexity of endodontic morphology is often the cause of erroneous root canal preparation. Failure to correctly address the pulp chamber morphology may compromise the subsequent endodontic procedures, and consequently treatment failure may result (12). Therefore, when retreatment is undertaken in a tooth in which endodontic therapy has failed, the already existing endodontic access cavity has to be suspect and addressed accordingly (2). The axial walls of the pulp chamber are probed for undercuts relative to the pathway of inserting instruments into the root canals. The removal of all such undercuts is a prerequisite to efficient endodontic retreatment. In multirooted teeth, the canals frequently emerge from the pulp chamber forming an angle with the axial pulp chamber walls. This angle may be further accentuated by the deposition of secondary dentin on the axial walls overhanging the canal orifices, noticed particularly in the mesial roots of mature mandibular molars (13). Clinically, these dentin deposits confine the canal orifice and interfere with instrumentation. Consequently, the instruments are pressed against one canal wall without contacting the walls circumferentially. Even if their tip is precurved, the instruments become distorted and fail to negotiate the apical canal curvature (Fig. 1). As a result, the canal walls are filed unevenly (16). Restriction of file movements may be avoided by straightening the cave-canal line angle as a preliminary step in retreatment, thus allowing an interference-free negotiation of the apical curvature. The cave-canal line angle may be
F~G 3. Radiographs of a mandibular first molar in two stages of endodontic retreatment. A, Length measurement radiography (R2) showing the K file in the mesial canal reaching the end of the existing obturation, referred to as "retreatment length" in the text. B, Control radiography (R3) showing the K file after it was advanced apically to the apical constricture, now extending to the "working length."
straightened by using long shank burs in a decreasing size sequence, from #8 to #2. The line angle is repeatedly probed with a DG-16 endodontic explorer (Hu-Friedy, Chicago, IL) until it is confirmed to be straightened. Then, Gates Glidden drills #2 and 3 (Caulk/Maillefer, Ballaigues, Switzerland) are used to drill out the filling material and remaining overhanging dentin from the coronal third of the canal (2, 8, 10). This procedure is similar to the step-down or crown-down tech-
Vol. 18, No. 11, November 1992
Nonsurgical Endodontic Treatment
567
FIG 4. Radiographs demonstrating stages of retreatment in a maxillary first molar. A, Preoperative radiograph (R1) indicating that a ledge may be present in the mesial root canal. B, Length measurement radiograph (R2) demonstrating the file stopping at the ledge. C, The file in the control radiograph (R3) shows that the ledge was bypassed and the apical curve of the canal negotiated. D, Final radiograph showing the root canals obturated with gutta-percha and sealer using the vertical condensation technique. Note the ledges that were bypassed in both the mesial and palatal roots (arrows).
niques ( 14, 15). The drills are operated mainly on the external canal wall opposite the pulpal chamber floor. By using Gates Glidden drills, (a) the coronal canal level is flared, (b) a receptacle is created for a solvent used later to dissolve the filling material, and (c) the coronal portion of the filling material is quickly removed, reducing the amount of material that has to be retreated by files (2, 10). Furthermore, the canal section which still remains to be treated can be accessed in a straight line and distortion of the instruments may be avoided. After straightening the cavo-canal angle, the outline of the access cavity is extended to establish a proper convenience form. An Endo-Z bur (Caulk/Maillefer) is used to align the axial walls of the access cavity with the straightened cavocanal line angle (Fig. 2). At this stage an additional radiograph may be taken in order to check the proper orientation of the access cavity in relation to the canal path which remains to be retreated. This radiograph will replace radiograph R I as an indicator of the estimated retreatment and working lengths, particularly when coronal restorations or tooth structure have been removed during access preparation.
Removal of latrogenic Obstructions The mechanical considerations governing reinstrumentation of the remaining portion of the root canal pertain to the iatrogenic obstruction of the canal by filling materials and foreign objects. In the following text the apical extent of the root canal filling will be referred to as "retreatment length"
(RL), as opposed to the "working length" (WL) extending to the apical constricture (Fig. 3). PASTES, CEMENTS, AND SEMI-SOLID MATERIALS A solvent is used to facilitate the removal of soluble root filling materials (10). Because the receptacle for the solvent was created at the coronal level of the canal, dissolving the material at the mid-canal level is usually simple. However, removal of the filling material from the apical portion of curved canals requires specific consideration, since forcing endodontic instruments into a curve involves the risk of ledge formation, transportation, and perforation (16, 17). In addition, the instruments become distorted and may be broken in the canal. The clinical approach suggested for negotiating a curved canal during retreatment is the following. The access cavity is irrigated with sodium hypochlorite, dried, and then flooded with solvent. Two millimeters are cut off the tip of a 21-mm ISO #15 K file to improve the manual control over the active part of the instrument. The shortened instrument is then uniformly precurved and passively introduced into the canal as far apically as possible with its curve corresponding to that of the canal. Then, the file is advanced apically using halfturn rotation movements and without exerting considerable pressure. Because K files are active both in rotational and longitudinal motions (18), they engage the root canal filling material mass. When the file is withdrawn from the canal the
568
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!:!il}~!;i
FIG 5. Radiographs demonstrating various stages in retreatment of a maxillary second molar. A, The silver point in the mesial root canal was inadvertently cut at the canal orifice during the removal of morphological obstructions. B, Control radiograph (R3) demonstrating that the silver point was removed and the file negotiated the canal to the working length. C, Final radiograph showing the root canals obturated with gutta-percha and sealer using the vertical condensation technique.
filling material is removed, which may be confirmed visually by observing that the file tip is covered with material. The file is advanced apically by repeating this procedure several times, each time cleaning the instrument and correcting its curve,
and adding solvent to the access cavity. As the file reaches the estimated RL, Gates Glidden drills #1, 2, and 3 are successively used in the canal without apical pressure along with copious irrigation with sodium hypochlorite. A length measurement radiograph (R2) is then taken with the file in the estimated RL position (Fig. 3A). Radiograph R2 confirms that the retreatment pathway conforms with the original shape of the filled canal. If the instrument tip is not distinguished from the filling material mass (8), an additional radiograph is taken after some further apical preparation. If there remains filling material apical to the instrument tip, the final RL is determined and the material is removed with a #10 K file in which 1 m m is cut off the tip. The previously described procedure for removing the filling material is repeated, carefully proceeding to obtain RL. Radiograph R2 is also used to determine the WL, which is the basis for further instrumentation instead of RL. In canals that were originally obturated short of the apex and have to be prepared further apically, WL is longer than RL (Fig. 3). In these cases the transition between the filled and unfilled portions of the canal is a critical point in retreatment. This point must be clinically determined with precision, since a ledge or even a perforation may accidentally occur there by pushing-rotating the file apically (2). In fact, a ledge may exist at the terminus of the canal filling independent ofretreatment attempts and may sometimes be the cause of too short canal fillings (Fig. 4, A and B). Therefore, during retreatment the possibility of a ledge at the end of the canal filling should always be taken into consideration (2). A similar situation is encountered in root canals from which a post was removed. To address this possibility, the position of the instrument tip in radiograph R2 is observed in relation to the remaining apical canal portion, and the potential risk of canal transportation during further instrumentation is evaluated. The first apical exploring must always be carried out only after most of the material has been removed from the filled portion of the root canal. When a ledge is suspected or confirmed, it may be bypassed successfully if both the mechanisms involved in its formation are properly addressed. First, the entire canal pathway coronal to the ledge must be relatively straightened to allow correct file operation. This is done by filing against the wall opposite the apical curvature with reamers used in serial sequence in retraction-rotation motions (16). This technique avoids pushing movements which would carry the ledge further and/or pack it with debris and dentinal chips. The same result may also be accomplished accurately by using the anticurvature filing technique (19). The second consideration in bypassing a ledge is the resistance of the endodontic instruments to deformation, referred to as elastic memory (16). By forcing a straight instrument into a curved path it tends to uncurve, and as a result the tip of the instrument cuts more into the wall opposite the curve. Because the tip displays a greater cutting efficiency than the flutes (18, 20), apical transportation or a ledge may occur. To prevent these undesirable consequences the instruments inserted apically in a curved canal should be deliberately precurved at the tip (17), so as to avoid the effects of elastic memory. Therefore, when bypassing a ledge is attempted in a curved canal, a precurved #08 K file is used to gently probe the apical curve for canal patency beyond the ledge. Subsequently, larger files are used to determine the direction of the apical curve and a control radiograph
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(R3) is taken to confirm the WL (Fig. 4C). Completion of the canal preparation is then carried out routinely (Fig. 4D).
remove it as they are withdrawn from the canal. Again, completion of canal preparation is then carried out routinely (Fig. 5C).
SOLID MATERIALS
This article is dedicated to the memory of Dr. Jacques Breiliat, who warmly encouraged its publication.
An accessible silver point which is tightly wedged between the root canal walls or embedded in a hard sealer may be separated by forceful pulling. According to Averbach (11), "the basic goal of any silver point removal technique is to break up any sealing materials surrounding the point." Therefore, breaking the seal of the silver points should be attempted first, followed by grasping and gentle pulling coronally (11). If this is not successful, bypassing of the point or other foreign objects should be attempted with hand instruments (10). The following guidelines may be of clinical assistance in bypassing a solid object. First, do not begin the retreatment by strongly pulling a well-retained silver point. Also, do not remove the morphological obstructions as would be done in the retreatment of pastes or semi-solid materials, because drilling with rotary instruments at the canal orifice will cut the silver point at that level and jeopardize its retrieval (Fig. 5A). Instead, create a nonconfining access coronal to the canal orifice. Then, probe the existing space between the canal walls and silver point at the coronal third of the canal by using the DG16 explorer to chip and tease out the pieces of sealer. Then the sealer in the middle third of the canal is removed in the same way as described above for the retreatment of pastes and cements. As soon as the files reach at the estimated RL, a radiograph (R2) is taken in order to determine the instrument position relative to the canal pathway, the apical end of the obstruction, and the root end. If the instrument is at or close to the apical tip of the solid object, stop the pushingrotating file motions. Copiously irrigate the access cavity and the canal with sodium hypochlorite. Bypass the remaining obstruction with a #08 K file with a precurved tip. Then, take an additional radiograph (R3) to confirm WL (Fig. 5B). Proceed with the removal of morphological obstructions as soon as the small K file fits freely in the canal. The last contacts between the solid object and the canal walls break during the subsequent cleaning and shaping of the canal, allowing its retrieval. Hedstrom files may be useful in performing this step. Caution is required to avoid pushing the object apically with the files (10). Rather than pushing the files, they should be gently threaded into the previously created space, so that they can possibly engage the solid object mass and
Dr. Mandel is a lecturer, Department of Endodontics, University of Paris Vii, Paris, France. Dr. Friedman is associate professor and head elect, Department of Endodontics, University of Toronto Faculty of Dentistry, Toronto, Canada. Address requests for reprints to Dr. Ely Mandel, 63 Ave Franklin Roosevelt, 75008 Paris, France.
References 1. Friedman S, Stabholz S. Endodontic retreatment--case selection and technique. Part 1. Criteria for case selection. J Endodon 1986;12:28-33. 2. Stabholz A, Friedman S. Endodontic retreatment--case selection and technique. Part 2. Treatment planning of retreatment. J Endodon 1988;14:60714. 3. Allen RK, Newton CW, Brown CE. A statistical analysis of surgical and nonsurgical endodontic retreatment cases. J Endodon 1989;15:261-6. 4. Krell KB, Neo J. The use of ultrasonic endodontic instrumentation in the retreatment of paste-filled endodontic tooth. Oral Surg 1985;60:100-2. 5. Jeng HW, El Deeb ME. Removal of the hard paste fillings from the root canal by ultrasonic instrumentation. J Endodon 1987;13:295-301. 6. Nagai O, Tani N, Kayaba Y, Kodan S, Osada T. Ultrasonic removal of broken instruments in root canals. Int Endod J 1986;19:298-304. 7. Krell KV, Fuller MW, Scott GL The conservative retrieval of silver cones in difficult cases. J Endodon 1984; 10:269-73. 8. Friedman S, Rotstein I, Shar-Lev S. Bypassing gutta-percha root fillings with an automated device. J Endodon 1989;15:432-7. 9. Hulsmann M. The removal of silver cones and fractured instruments using the Canal Finder system. J Endodon 1990;16:596-600. 10. Friedman S, Stabholz A, Tamse A. Endodontic retreatment--case selection and technique. Part 3. Retreatment techniques. J Endodon 1990;16:601-3. 11. Averbach RE. Retreatment of failures. In: Walton R, Torabinejad M, eds. Principles and practice of endodontics. Chicago: WB Saunders, 1989;32134. 12. Levin H. Access cavities. Dent Clin North Am 1967;11:701-10. 13. Baume LY. The biolcgy of the pulp and dentin. In: Myrrs H, ed. Monograph in oral science no. 8. Philadelphia: Karger, 1980. 14. Goerig AC, Michelich RJ, Schlitz HH. instrumentation of root canal in molar using the step down technique. J Endodon 1982;8:550-4. 15. Morgan LF, Montgomery S. An evaluation of the crown-down pressureless technique. J Endodon 1984;10:491-8. 16. Schilder H. Cleaning and shaping the root canal. Dent Clin North Am 1984;28:269-96. 17. Buchanan LS. Cleaning and shaping the root canal system. In: Cohen S, Bums RC, eds, Pathways of the pulp. St. Louis: Mosby Year Book, 1991:168-75. 18. Felt RA, Moser JB, Heuer MA. Flute design of endodontic instruments: its influence on cutting efficiency. J Endodon 1982;8:253-9. 19. Abou-Rass M, Frank AL, Glick DH. The anticurvature method to prepare the curved root canals. J Am Dent Assoc 1980;101:792-5. 20. Miserendino LJ, Moser JB, Heuer MA, Osetek EM. Cutting efficiency of endodontic instruments. Part I: A quantitative comparison of the tip and flutes region. J Endodon 1985;11:435-41.
Printed in U.S.A.
VOL. 18, NO. 11, NOVEMBER1992
CLINICAL AID Endodontic Retreatment: A Rational Approach to Root Canal Reinstrumentation Ely Mandel, DCD, DSO, and Shimon Friedman, DMD
Technic Co., Tustin, CA) can be used to bypass gutta-percha root fillings, particularly in curved canals (8). It is also useful in bypassing and retrieving solid objects which resist removal attempts with hand instruments (9). Additional techniques are available for retrieving solid objects, using mainly various mechanical devices (10, 11). These techniques appear to be time consuming, unsuitable for use in curved roots, and require considerable sacrifice of root dentin (10). Regardless of the technique used, endodontic retreatment requires total regaining of canal patency. This is invariably accomplished by exerting pressure in the apical direction with rigid instruments, with the purpose of removing the root filling material. When executed undiscriminately this procedure may be ineffective and even harmful (2, 10). However, when it is performed systematically canal patency can be successfully regained in most retreatment cases. Subsequently, endodontic retreatment may be accomplished by using routine endodontic procedures. The purpose of this article was to discuss the mechanical considerations governing the regaining of canal patency and to present a rational step by step approach to reinstrumentation of the root canal during nonsurgical endodontic retreatment.
During nonsurgical endodontic retreatment, endodontic instruments are forced apically to remove the root canal filling material and regain canal patency. Undiscriminating burrowing down the canal in the apical direction may be fruitless and harmful. To avoid complications, the dentin overhanging the canal orifice must be removed and an unobstructed access established to the root filling material, so as to facilitate its removal. Reinstrumentation of the filled canal must take into consideration the nature of the filling matedal and the physical properties of endodontic instruments, as well as the dynamic aspects of canal preparation. This article discusses the mechanical considerations pertaining to root canal retreatment and outlines a step by step rationale approach to retreatment.
Nonsurgical endodontic retreatment consists of cleaning, shaping, and three-dimensional filling of previously obturated root canals. It is the treatment of choice for the management ofendodontic failures when access to the root canals is feasible (1). To successfully accomplish retreatment, all of the obstructions preventing a direct access to the root canals have to be removed (2). These include the intracoronal restoration (2) and frequently also dentin overhanging the canal orifice in teeth where the previous endodontic access cavity was underextended. Next, the obstructions within the canal are removed, i.e. root filling materials and foreign objects (2). The most frequently encountered root filling materials which have to be removed in retreatment are pastes and cements, semisolid and solid materials (2). In a recent study these were the filling materials in 20.6%, 53.6%, and 21.7%, respectively, of the 1300 retreated cases (3). Retreatment has been addressed frequently with respect to the techniques of removing the various filling materials from the root canal (4-10). Hard setting pastes can be retreated by ultrasonic endodontic instruments, although this procedure is time consuming (4, 5). Ultrasonic instrumentation also facilitates removal of solid objects (6, 7). The Canal Finder (Endo
MECHANICAL CONSIDERATIONS A rational approach to nonsurgical endodontic retreatment must take into consideration the existing obstructions of the root canal. These may be morphological, resulting from an underextended endodontic access cavity, or iatrogenic, consisting of filling materials and/or other foreign objects (2): In addition, the dynamic aspects of canal preparation must be considered, as they apply to instrumentation of fine, curved canals. To evaluate the obstructions and the canal shape a preoperative radiograph (R 1) is taken before retreatment. In radiograph R1 the iatrogenic obstructions are usually clearly evident, whereas the existence of morphological obstructions is only indicated. In addition, radiograph R1 is used to estimate the canal length which has to be retreated. The sequence in which the morphological and iatrogenic obstructions are removed depends on the nature of the iatrogenic obstructions, particularly the root filling material.
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F~G 2. The Endo-Z carbide tungsten bur has a noncutting tip. This bur is recommended for aligning the axial walls of the access cavity with the cave-canal line angle.
F~G 1. A schematic representation of an endodontic instrument inserted into a curved canal which is coronally confined by deposition of dentin. The file (F) is pressed against the canal wall opposite the dentin deposit (DD) (arrow). As a result the file is distorted and its tip is unable to negotiate the apical canal curvature (open arrow).
Removal of Morphological Obstructions The complexity of endodontic morphology is often the cause of erroneous root canal preparation. Failure to correctly address the pulp chamber morphology may compromise the subsequent endodontic procedures, and consequently treatment failure may result (12). Therefore, when retreatment is undertaken in a tooth in which endodontic therapy has failed, the already existing endodontic access cavity has to be suspect and addressed accordingly (2). The axial walls of the pulp chamber are probed for undercuts relative to the pathway of inserting instruments into the root canals. The removal of all such undercuts is a prerequisite to efficient endodontic retreatment. In multirooted teeth, the canals frequently emerge from the pulp chamber forming an angle with the axial pulp chamber walls. This angle may be further accentuated by the deposition of secondary dentin on the axial walls overhanging the canal orifices, noticed particularly in the mesial roots of mature mandibular molars (13). Clinically, these dentin deposits confine the canal orifice and interfere with instrumentation. Consequently, the instruments are pressed against one canal wall without contacting the walls circumferentially. Even if their tip is precurved, the instruments become distorted and fail to negotiate the apical canal curvature (Fig. 1). As a result, the canal walls are filed unevenly (16). Restriction of file movements may be avoided by straightening the cave-canal line angle as a preliminary step in retreatment, thus allowing an interference-free negotiation of the apical curvature. The cave-canal line angle may be
F~G 3. Radiographs of a mandibular first molar in two stages of endodontic retreatment. A, Length measurement radiography (R2) showing the K file in the mesial canal reaching the end of the existing obturation, referred to as "retreatment length" in the text. B, Control radiography (R3) showing the K file after it was advanced apically to the apical constricture, now extending to the "working length."
straightened by using long shank burs in a decreasing size sequence, from #8 to #2. The line angle is repeatedly probed with a DG-16 endodontic explorer (Hu-Friedy, Chicago, IL) until it is confirmed to be straightened. Then, Gates Glidden drills #2 and 3 (Caulk/Maillefer, Ballaigues, Switzerland) are used to drill out the filling material and remaining overhanging dentin from the coronal third of the canal (2, 8, 10). This procedure is similar to the step-down or crown-down tech-
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Nonsurgical Endodontic Treatment
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FIG 4. Radiographs demonstrating stages of retreatment in a maxillary first molar. A, Preoperative radiograph (R1) indicating that a ledge may be present in the mesial root canal. B, Length measurement radiograph (R2) demonstrating the file stopping at the ledge. C, The file in the control radiograph (R3) shows that the ledge was bypassed and the apical curve of the canal negotiated. D, Final radiograph showing the root canals obturated with gutta-percha and sealer using the vertical condensation technique. Note the ledges that were bypassed in both the mesial and palatal roots (arrows).
niques ( 14, 15). The drills are operated mainly on the external canal wall opposite the pulpal chamber floor. By using Gates Glidden drills, (a) the coronal canal level is flared, (b) a receptacle is created for a solvent used later to dissolve the filling material, and (c) the coronal portion of the filling material is quickly removed, reducing the amount of material that has to be retreated by files (2, 10). Furthermore, the canal section which still remains to be treated can be accessed in a straight line and distortion of the instruments may be avoided. After straightening the cavo-canal angle, the outline of the access cavity is extended to establish a proper convenience form. An Endo-Z bur (Caulk/Maillefer) is used to align the axial walls of the access cavity with the straightened cavocanal line angle (Fig. 2). At this stage an additional radiograph may be taken in order to check the proper orientation of the access cavity in relation to the canal path which remains to be retreated. This radiograph will replace radiograph R I as an indicator of the estimated retreatment and working lengths, particularly when coronal restorations or tooth structure have been removed during access preparation.
Removal of latrogenic Obstructions The mechanical considerations governing reinstrumentation of the remaining portion of the root canal pertain to the iatrogenic obstruction of the canal by filling materials and foreign objects. In the following text the apical extent of the root canal filling will be referred to as "retreatment length"
(RL), as opposed to the "working length" (WL) extending to the apical constricture (Fig. 3). PASTES, CEMENTS, AND SEMI-SOLID MATERIALS A solvent is used to facilitate the removal of soluble root filling materials (10). Because the receptacle for the solvent was created at the coronal level of the canal, dissolving the material at the mid-canal level is usually simple. However, removal of the filling material from the apical portion of curved canals requires specific consideration, since forcing endodontic instruments into a curve involves the risk of ledge formation, transportation, and perforation (16, 17). In addition, the instruments become distorted and may be broken in the canal. The clinical approach suggested for negotiating a curved canal during retreatment is the following. The access cavity is irrigated with sodium hypochlorite, dried, and then flooded with solvent. Two millimeters are cut off the tip of a 21-mm ISO #15 K file to improve the manual control over the active part of the instrument. The shortened instrument is then uniformly precurved and passively introduced into the canal as far apically as possible with its curve corresponding to that of the canal. Then, the file is advanced apically using halfturn rotation movements and without exerting considerable pressure. Because K files are active both in rotational and longitudinal motions (18), they engage the root canal filling material mass. When the file is withdrawn from the canal the
568
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Journal of Endodontics
!:!il}~!;i
FIG 5. Radiographs demonstrating various stages in retreatment of a maxillary second molar. A, The silver point in the mesial root canal was inadvertently cut at the canal orifice during the removal of morphological obstructions. B, Control radiograph (R3) demonstrating that the silver point was removed and the file negotiated the canal to the working length. C, Final radiograph showing the root canals obturated with gutta-percha and sealer using the vertical condensation technique.
filling material is removed, which may be confirmed visually by observing that the file tip is covered with material. The file is advanced apically by repeating this procedure several times, each time cleaning the instrument and correcting its curve,
and adding solvent to the access cavity. As the file reaches the estimated RL, Gates Glidden drills #1, 2, and 3 are successively used in the canal without apical pressure along with copious irrigation with sodium hypochlorite. A length measurement radiograph (R2) is then taken with the file in the estimated RL position (Fig. 3A). Radiograph R2 confirms that the retreatment pathway conforms with the original shape of the filled canal. If the instrument tip is not distinguished from the filling material mass (8), an additional radiograph is taken after some further apical preparation. If there remains filling material apical to the instrument tip, the final RL is determined and the material is removed with a #10 K file in which 1 m m is cut off the tip. The previously described procedure for removing the filling material is repeated, carefully proceeding to obtain RL. Radiograph R2 is also used to determine the WL, which is the basis for further instrumentation instead of RL. In canals that were originally obturated short of the apex and have to be prepared further apically, WL is longer than RL (Fig. 3). In these cases the transition between the filled and unfilled portions of the canal is a critical point in retreatment. This point must be clinically determined with precision, since a ledge or even a perforation may accidentally occur there by pushing-rotating the file apically (2). In fact, a ledge may exist at the terminus of the canal filling independent ofretreatment attempts and may sometimes be the cause of too short canal fillings (Fig. 4, A and B). Therefore, during retreatment the possibility of a ledge at the end of the canal filling should always be taken into consideration (2). A similar situation is encountered in root canals from which a post was removed. To address this possibility, the position of the instrument tip in radiograph R2 is observed in relation to the remaining apical canal portion, and the potential risk of canal transportation during further instrumentation is evaluated. The first apical exploring must always be carried out only after most of the material has been removed from the filled portion of the root canal. When a ledge is suspected or confirmed, it may be bypassed successfully if both the mechanisms involved in its formation are properly addressed. First, the entire canal pathway coronal to the ledge must be relatively straightened to allow correct file operation. This is done by filing against the wall opposite the apical curvature with reamers used in serial sequence in retraction-rotation motions (16). This technique avoids pushing movements which would carry the ledge further and/or pack it with debris and dentinal chips. The same result may also be accomplished accurately by using the anticurvature filing technique (19). The second consideration in bypassing a ledge is the resistance of the endodontic instruments to deformation, referred to as elastic memory (16). By forcing a straight instrument into a curved path it tends to uncurve, and as a result the tip of the instrument cuts more into the wall opposite the curve. Because the tip displays a greater cutting efficiency than the flutes (18, 20), apical transportation or a ledge may occur. To prevent these undesirable consequences the instruments inserted apically in a curved canal should be deliberately precurved at the tip (17), so as to avoid the effects of elastic memory. Therefore, when bypassing a ledge is attempted in a curved canal, a precurved #08 K file is used to gently probe the apical curve for canal patency beyond the ledge. Subsequently, larger files are used to determine the direction of the apical curve and a control radiograph
Vol. 18, No. 11, November 1992
Nonsurgical Endodontic Treatment
569
(R3) is taken to confirm the WL (Fig. 4C). Completion of the canal preparation is then carried out routinely (Fig. 4D).
remove it as they are withdrawn from the canal. Again, completion of canal preparation is then carried out routinely (Fig. 5C).
SOLID MATERIALS
This article is dedicated to the memory of Dr. Jacques Breiliat, who warmly encouraged its publication.
An accessible silver point which is tightly wedged between the root canal walls or embedded in a hard sealer may be separated by forceful pulling. According to Averbach (11), "the basic goal of any silver point removal technique is to break up any sealing materials surrounding the point." Therefore, breaking the seal of the silver points should be attempted first, followed by grasping and gentle pulling coronally (11). If this is not successful, bypassing of the point or other foreign objects should be attempted with hand instruments (10). The following guidelines may be of clinical assistance in bypassing a solid object. First, do not begin the retreatment by strongly pulling a well-retained silver point. Also, do not remove the morphological obstructions as would be done in the retreatment of pastes or semi-solid materials, because drilling with rotary instruments at the canal orifice will cut the silver point at that level and jeopardize its retrieval (Fig. 5A). Instead, create a nonconfining access coronal to the canal orifice. Then, probe the existing space between the canal walls and silver point at the coronal third of the canal by using the DG16 explorer to chip and tease out the pieces of sealer. Then the sealer in the middle third of the canal is removed in the same way as described above for the retreatment of pastes and cements. As soon as the files reach at the estimated RL, a radiograph (R2) is taken in order to determine the instrument position relative to the canal pathway, the apical end of the obstruction, and the root end. If the instrument is at or close to the apical tip of the solid object, stop the pushingrotating file motions. Copiously irrigate the access cavity and the canal with sodium hypochlorite. Bypass the remaining obstruction with a #08 K file with a precurved tip. Then, take an additional radiograph (R3) to confirm WL (Fig. 5B). Proceed with the removal of morphological obstructions as soon as the small K file fits freely in the canal. The last contacts between the solid object and the canal walls break during the subsequent cleaning and shaping of the canal, allowing its retrieval. Hedstrom files may be useful in performing this step. Caution is required to avoid pushing the object apically with the files (10). Rather than pushing the files, they should be gently threaded into the previously created space, so that they can possibly engage the solid object mass and
Dr. Mandel is a lecturer, Department of Endodontics, University of Paris Vii, Paris, France. Dr. Friedman is associate professor and head elect, Department of Endodontics, University of Toronto Faculty of Dentistry, Toronto, Canada. Address requests for reprints to Dr. Ely Mandel, 63 Ave Franklin Roosevelt, 75008 Paris, France.
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