Access to the Root Canal System: Coronal Cavity Preparation

Access to the Root Canal System: Coronal Cavity Preparation

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Careful cavicy preparation and obturation are the keystones to suc­cessful root canal treatment. As in restorative dentistry, the final restoration is no better than the initial cavity preparation. Endodontic cavity preparation begins the instant the tooth is approached with a cutting instrument (Figure 1 ). Hence, it is important that adequate access be developed to properly clean and shape the canal system and obturate the space. When first approaching the tooth, one must have in mind the three-dimensional anatomy of the pulp chamber about to be entered, not the two-dimensional image revealed by the radiograph {Figure 2). It is this chamber outline that is to be "projected" out onto the occlusal or lingual surface of the crown (Figure 3).


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Figure 1: A-C, Entrance is always gained through the occlusal surface of posterior teeth and the lingual surface of anterior teeth B. Specialized end-cut­ting, fissure burs are used for the initial entrée through enamel or gold A, and a special end-cutting, amalgam bur is used to perforate amalgam fillings C. The same burs and stones may be used to extend the walls to gain full access. bearing in mind, that they are end cutting and can and can damage the chamber floor.





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Figure 2 : A standard radiograph (left) in buccolingual projection provides only a two-dimensional view of what is actually a three-dimensional problem. The maxillary second premolar is actually an ovoid ribbon, rather than a round thread.





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Figure 3 To gain adequate access to both canals, it is this broad ovoid outline that is projected out onto the occlusal surface, rather than a round “hole” in the central pit.





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Figure 4 : Black’s principles of cavity preparation—outline, convenience, retention, and resistance forms—apply to endodontic preparations as they do for coronal preparations: to the former to gain unlimited access to the canal orifices, to the latter as “extension for prevention.”



Endodontic cavity preparation is separated into two anatomic entities: coronal preparation and radicular preparation. Coronal preparation is discussed in this chapter. In doing so one may fall back on Black’s principles of cavity preparation—outline, convenience, retention, and resistance forms—admittedly developed by Black
for extracoronal preparation but just as applicable to intracoronal preparation (Figure 4). One may add removal of remaining carious dentin (and defective restorations) as well as toilet of the cavity (both of which are necessary) to make Black’s principles complete. Outline form is often thought of as only the coronal cavity. But actually the entire preparation, from enamel surface to the apical terminus, is one long outline form (Figure 5). On occasion outline may have to be modified for the sake of convenience to accommodate the unstressed use of root canal instruments and filling materials (Figure 6). In other words, to reach the apical terminus without interference from overhanging tooth structure or ultra-curved canals, one must extend the cavity outline.
Also, on occasion the canal may be prepared for retention of the primary filling point (see Figure 5). However, more important is resistance form, to prepare an “apical stop” at the canal terminus against which filling materials may be compressed without overextending the filling (see Figure 5). Proper preparation of the apical one-third of the canal is crucial to success.


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Figure 5 : Concept of total endodontic cavity preparation, coronal and radicular
as a continuum, based on Black’s principles. A, Radiographic apex. B,
Resistance form at the “apical stop.” C, Retention form to retain the primary
filling material. D, Convenience form subject to revision to accommodate larger,
less flexible instruments. E, Outline form with basic preparation throughout its
length, crown to apical stop.





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Figure 6 A, Obstructed access to a mesial canal. The overhanging roof misdirects
the instrument mesially, with a resulting ledge formation. B, Completely
removing the roof brings canal orifices into view and allows immediate access
to each canal.



Power-driven rotary instruments are used to penetrate the crown. Round carbide burs commensurate in size with the chamber, as seen on a radiograph, or round-tip, end-cutting tapered burs (Transmetal-
Caulk/Maillefer) or diamond stones (EndoAccess-Caulk/Maillefer, Tulsa, OK) are best for perforating enamel and entering the chamber (Figure 7). Enamel and precious metals are easily removed with carbide burs, but extra-coarse, round-tip diamonds are best for penetrating porcelain- fused-to-metal and all-porcelain restorations. Once the chamber is entered, the remaining tooth structure covering the chamber is removed with round or tapered burs or stones (Figure 8). Initially, high-speed rotary instruments are used. But once the chamber is entered, the neophyte is advised to use slower-speed instruments so that tactile sensation may, in part, guide the removal of the remaining structure. To overcut the crown only weakens it more.


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Figure 7 : According to the size of the chamber, a no.4 or no.6 round bur may
be used to remove the roof of the pulp chamber.






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Figure 8 : A, Final finish of the convenience form. In the case of a lower molar, the entire cavity slopes to the mesial aspect allowing easier access to the mesial canal orifices. B, Pulp-Shaper bur with noncutting tip (Dentsply/Tulsa) used to extend the access.



As soon as the canal orifices have been exposed, they may be entered with endodontic files to determine whether the instruments are either “under stress” or free of interfering tooth structure (Figure 9). If binding, convenience form dictates that the coronal outline form be extended to free up the shaft of the file. There should be unobstructed access to the canal orifices and direct access to the apical foramen. This is done with high-speed tapered burs or stones, preferably with noncutting tips (see Figure 8). Warning: fissure burs often “chatter,” distressing to the patient.


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Figure 9 : Unobstructed access to the canal orifices and down the canals to the “apical stop” area.





MAXILLARY ANTERIOR TEETH
Maxillary anterior teeth are entered from the lingual aspect. The enamel is perforated with a round carbide bur, an end-cutting, tapered bur, or a diamond stone held parallel to the long axis of the tooth (Figure 10). As soon as the pulp chamber is entered, the tapered bur is used to remove tooth structure incisally (Figure 11). This convenience removal allows adequate room for the shaft of burs that will enter deeper into the pulp chamber to remove its “roof” (Figure 12). Once the preparation is completed on the incisolabial surface, a tapered stone is used to remove the lingual “shoulder” (Figure 13).


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Figure 10 : Maxillary anterior teeth. Entrance is always gained through the lingual surface. Initial penetration is made at the exact center of the lingual surface. The bur should be held approximately parallel to the long axis of the tooth. This initial cut may be made with high-speed instruments, but the neophyte is warned to use slower speeds in proceeding toward the canal.





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Figure 11 : Maxillary anterior teeth. The preliminary cavity outline is funneled and fanned from the
incisal edge to allow room for the shaft of the bur to follow and penetrate
the chamber.






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Figure 12 Maxillary anterior teeth. Use of a slower-speed round bur is suggested to enter the pulp chamber, keeping in mind the verticality of the tooth. The roof of the pulp chamber is removed toward the incisal surface in a convenience extension.





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Figure 13 Maxillary anterior teeth. A fissure-style bur or diamond is used to remove the lingual shoulder and prepare the incisal extension to allow unobstructed access to the entire canal.
The final outline form on the lingual surface should reflect the size and shape of the pulp chamber, usually dictated by age (Figure 14). The entire outline form, from incisal to apex, must be free of any encumbrances that would interfere with cleaning, shaping, and obturation (Figure 15).





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Figure 14 Maxillary anterior teeth. The lingual outline form reflects the size of the pulp chamber—a larger, fan-shaped outline in youngsters and a long, ovoid outline in older patients. Be sure to remove any pulpal remnants to the mesial or distal aspects to prevent future staining.





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Figure 15 Maxillary anterior teeth. Final preparation with the instrument in place. The instrument shaft clears the incisal cavity margin and the reduced lingual shoulder, allowing an unstrained approach, under the complete control of the clinician, to the apical third of the canal.




Virtually the same procedures and precautions apply to maxillary lateral incisors and canines as to anterior teeth.
Operative Errors
The common error of perforating or badly gouging the gingivolabial aspect (Figure 16) is usually due to two factors: not allowing adequate access toward the incisal aspect of the preparation (see Figure 11) or not properly aligning the bur vertically with the long axis of the tooth. Another common failure is not providing adequate access or removal of the lingual shoulder (see Figures 11 and 13). Loss of control of the instrument results in a pear-shaped and inadequate preparation of the apical third (Figure 17). A similar failure, the result of inadequate access, diverts instruments from the canal lumen, as illustrated here in a canine with a labial root curvature, undetectable in a standard labial-lingual radiograph (Figure 18).


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Figure 16 Operative error—maxillary anterior teeth. Perforation of the labiocervical aspect caused by failure to complete the convenience extension toward the incisal prior to entrance of the shaft of the bur. This also can be caused by a failure to align the bur parallel to the long axis of the tooth.







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Figure 17 Operative error— maxillary anterior teeth. Pear-shaped apical preparation caused by a failure to complete the convenience extensions. The shaft of the instrument rides on the cavity margin and the lingual shoulder that direct the control of the instrument. Inadequate debridement and obturation ensure failure.






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Figure 18 Operative error—maxillary anterior teeth. Ledge formation at the apicolabial curve (not discernible in a radiograph) caused again by a failure to complete the convenience extension at the incisal surface and the lingual shoulder.





MANDIBULAR ANTERIOR TEETH
Mandibular anterior teeth also are entered from the lingual surface. The enamel is perforated with a round carbide bur, an end-cutting tapered bur, or a diamond stone, held parallel to the long axis of the tooth (Figure 19). As soon as the pulp chamber is entered, the bur/stone is used to remove tooth structure toward the incisal aspect (Figure20). This convenience removal allows adequate room for the shaft of burs that will enter deeper into the pulp chamber to remove its roof (Figure 21).


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Figure 19 Mandibular anterior teeth. Entrance is always gained through the lingual surface. Initial penetration is made in the exact center of the lingual surface. An endcutting, fissure bur is turned to cut at right angles to the lingual surface.







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Figure 20 Mandibular anterior teeth. As soon as the enamel is penetrated, the bur is turned vertically, beginning the convenience cut toward the incisal. Once enough overhanging structure has been
removed, the pulp chamber may be entered vertically with a no. 4 round bur in a slow handpiece.





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Figure 21 Mandibular anterio teeth. The orifice is widened with a fissure bur or a diamond toward the incisal and the shoulder toward the lingual to give a smooth-flowing preparation.





The final outline form on the lingual surface should reflect thesize and shape of the pulp chamber, usually dictated by age (Figure 22). The entire outline form, from incisal to apex, must be free of any encumbrances that would interfere with cleaning, shaping, or obturation (Figure 23).

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Figure 22 Mandibular anterior teeth. Again, the shape of the lingual outline form reflects the size of the pulp chamber, which in turn reflects the age of the patient.






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Figure 23 Mandibular anterior teeth. Final preparation with the instrument in place. The instrument shaft clears the incisal cavity margin and the reduced lingual shoulder, and penetrates unimpeded to the apex, under complete control of the clinician. Always search for a second canal to the labial or the lingual in lower incisors. Virtually the same procedures and precautions apply to mandibular lateral incisors and canines as to anterior teeth.




Operative Errors
The common error of gouging or perforating at the incisogingival aspect (Figure 24) is usually due to two factors: not allowing adequate access toward the incisal of the preparation (see Figures 20 and 21) or not aligning the bur vertically with the long axis of the tooth (see Figure 19). Inadequate access leads to the inability to explore, débride, and obturate the second canal, often not seen in a standard labiolingual radiograph (Figure 25).
Never enter the pulp chamber from a proximal surface (Figure 26). As inviting as it might appear in some situations, total loss of control of enlarging instruments is the result. Failure looms!




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Figure 24 Operative errors— mandibular anterior teeth. Inadequate lingual access controls the shaft of the bur and misdirects it to the labial. Cervical perforation can result.




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Figure 25 Operative errors— mandibular anterior teeth. Again, inadequate access prevents the exploration for a second canal toward the labial. Straight-on radiographs do not reveal this common error.





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Figure 26 Operative errors— mandibular anterior teeth. Never attempt to enter the canal from the proximal surface. A total loss of instrument control leads to ledging and/or perforation.




MAXILLARY PREMOLAR TEETH
Entrance is always gained through the occlusal surface of all posterior teeth. The enamel or restoration is perforated in the exact center of the central groove with a round carbide bur, an end-cutting, tapered bur, or a diamond stone held parallel to the long axis of the tooth (Figure 27).
Once the chamber is entered, an explorer or endodontic file is used to explore the orifices of the labial and lingual canals of the first premolar or the central canal (or possibly additional canals) in the second premolar (Figure 28). From this exploration one learns the necessary extension of the buccolingual outline form. Always probe for the possibility of additional canals in either premolar.


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Figure 27 Maxillary premolar teeth. Access to all posterior teeth is through the occlusal surface. Initial penetration is made parallel to the long axis of the tooth in the exact center of the central groove. A no. 4 round bur may be used to penetrate into the pulp chamber. The bur will be felt to “drop” when the pulp chamber is reached. If the pulp is well calcified, the drop will not be felt, so the bur penetrates until the nose of the handpiece touches the occlusal surface—9.0 mm. The orifice is then widened to allow exploration for the canal orifices.



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Figure 28 Maxillary premolar teeth. An endodontic explorer is used to locate the orifices of the buccal and lingual canals of the first premolar or the central canal of the second premolar. Always search for extra canals—the third canal in the first premolar or a second canal in the second premolar.



Buccolingual cavity extension is best done with tapering fissure burs or stones (Figure 29). Adequate buccolingual endodontic cavity outline form is in contrast to the mesiodistal restorative outline form (Figure 30). The final preparation should provide adequate, unimpeded access to all canal orifices (Figure 31). Cavity walls should not impede complete authority over the enlarging instruments.


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Figure 29 Maxillary premolar teeth. The cavity outline is then extended buccolingually with a
tapered, fissure bur or diamond


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Figure 30 Maxillary premolar teeth. The buccolingual preparation reflects the internal anatomy of the pulp chamber and the entrance to the  canal orifices


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Figure 31 Maxillary premolar teeth. Final preparation should provide unobstructed
access to the canal orifices.


Operative Errors
An error that occurs in maxillary premolar teeth is overextended preparation in a fruitless search for a receded pulp (Figure 32). The white color of the roof of the chamber is a clue that the pulp has not been reached. The floor of the chamber is a dark color. Failure to observe the mesiodistal inclination of a drifted tooth leads to a gingival perforation owing to the misaligned bur. The receded pulp is missed completely (Figure 33).



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Figure 32 Operative errors—maxillary premolars. Overextended preparation from a fruitless search for a
receded pulp. The enamel walls have been completely undermined and the tooth hopelessly weakened. Gouging relates to a failure to refer to the radiograph, which clearly shows the depth of pulp recession.



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Figure 33 Operative errors—maxillary premolars. Perforation at the mesiocervical indentation caused by failure to observe the distoaxial inclination of the tooth. The receded pulp is also completely bypassed. The maxillary first premolar is one of the most frequently perforated teeth.

An instrument can be broken or twisted off in a “crossover” canal (34). Failure may be obviated by extending the internal preparation to straighten the canals. Inadequate occlusal access leads to the failure to explore, instrument, and obturate a third canal (Figure 35).


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Figure 34 Operative errors—maxillary premolars. A broken instrument fractured in a “crossover” canal. This frequent occurrence may be obviated by extending the internal preparation to straighten the canals access (dotted lines).


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Figure 35 Operative errors—maxillary premolars. Inadequate occlusalaccess leads to the failure to explore  débride, and obturate the third canal
     second premolar (24% of the time).(6% of the time), and to find and instrument the second canal in the





MANDIBULAR PREMOLAR TEETH
Entrance is gained through the occlusal surface of all posterior teeth. The enamel or restoration is perforated in the exact center of the central ridge with a round carbide bur, an end-cutting, tapered bur, or a diamond stone held parallel to the long axis of the tooth (Figure 36). Once the chamber is entered, an explorer or endodontic file is used to explore the canal and to search for a second canal (Figure 37). From this exploration one learns the needed extent of the outline form. Additional canals in lower premolars are more prevalent in black patients. Removal of the roof of the pulp chamber and expansion of the occlusal outline form is best done with tapering fissure burs or stones (Figure 38).


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Figure 36 Mandibular premolar teeth. Initial penetration is made with a no.4 round bur in the central groove of mandibular premolars. When the chamber is entered, the bur is felt to “drop” into the space. If the pulp has receded, the bur should cut vertically until the nose of the contra-angle touches the occlusal surface—9.0 mm. In removing the bur, the orifice is widened buccolingually.




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Figure 37 Mandibular premolar teeth. An endodontic explorer is used to locate the direction and the extent of the chamber and the central canal. One should also be cautious to search for additional canals, particularly in black patients, in whom 32.8% of first mandibular premolars have two canals and 7.8% of mandibular
second premolars have two canals
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Figure 38 Mandibular premolar teeth. Buccolingual extension is completed with a tapered fissure bur or a diamond.


The ovoid outline form reflects the shape of the pulp chamber and must be extensive enough to accommodateinstruments and filling materials (Figure 39). Search for a second canal, especially in the first premolar. The final preparation should provide access from the occlusal surface to the apex (Figure 40). Cavity walls should not impede complete authority over the enlarging instruments.


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Figure 39 Mandibular premolar teeth. Buccolingual ovoid outline form reflects the internal anatomy of the pulp chamber and orifice to the  canal
 



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Figure 40 Mandibular premolar teeth. Final preparation is a tapered funnel from the occlusal surface to the canal, providing unobstructed access to the apical third of the canal





Operative Errors
An error that occurs in mandibular premolar teeth is perforation at the gingiva owing to the failure to recognize the distal tilting of the tooth that often follows extraction of the lower first molar (Figure 41). The pulp is missed m entirely.

Never enter the pulp from the buccal aspect (Figure 42). Total loss of instrument control and imminent separation of the file follows. Bifurcation of the canal is missed owing to the failure to thoroughly explore the canal in all directions (Figure 43). Perforation at the apical curvature owing to the failure to recognize by exploration the curvature to the buccal aspect (Figure 44). A standard buccolingual radiograph does not reveal buccal or lingual curvatures
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Figure 41 Operative errors—mandibular premolars. Perforation at the distogingival aspect caused by a failure to recognize that the premolar has tilted distally. The same error can
occur with a mesial tilt








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Figure 42 Operative errors— mandibular premolars. Never enter from the buccal aspect! The instrument is immediately under stress, will cause ledging, develop a pearshaped preparation, or fracture



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Figure43 Operative errors— mandibular premolars. Bifurcation of the canal is completely missed owing to a failure to adequately explore the canal with a curved instrument.
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Figure 44 Operative errors— mandibular premolars. Perforation at the apical curvature is caused by a failure to recognize, by exploration, buccal curvature. A standard buccolingual radiograph does not show buccal or lingual curvature. One should also be wary of perforating the apical foramen in perfectly straight canals, a common cause of acute apical periodontitis



MAXILLARY MOLAR TEETH
Molar teeth are always entered through the occlusal surface. Enamel or restorations are best perforated with a round carbide (no. 4) bur, an end-cutting, tapering fissure bur, or a diamond stone. The point of entrance should be the central pit, and the bur should be aimed at the orifice of the palatal canal, the largest canal (Figure 45). The same instrument can be used to remove the remaining roof of the pulp chamber


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Figure 45 Maxillary molar teeth. Molar teeth should all be opened through the occlusal surface. The initial cut is made in the exact center of the central pit. After perforating the enamel or restoration, the bur should be directed toward the palatal canal orifice, the largest canal. Using a round or tapered fissure bur, the occlusal opening should be enlarged so that an endodontic explorer can be used




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Once the opening is large enough, the orifices of the canals should be explored with explorers or files (Figure 46). In this manner, convenience extension is established. Final convenience extension is best done with a non-end-cutting, tapering fissure bur or stone so as not to nick the floor of the chamber (Figure 47).





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Figure 46 Maxillary molar teeth. Using the explorer the floor of the chamber is carefully explored to locate the canal orifices and the direction of the canals so that one knows in which direction to enlarge the outline form.






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Figure 47 Maxillary molar teeth. A tapered fissure bur or diamond is used to remove all the overhanging roof of the chamber and extend the outline form so that enlarging instruments can be used unimpeded.




The final occlusal outline form reflects not only the size and shape of the pulp chamber but the convenience extensions necessary to free the shafts of the enlarging instruments (Figure 48). Do not assume there are only three canals. One must always probe for extra canals! Keep in mind, however, that some maxillary second molars may have only two canals.
Outline form, from occlusal to apex, must be free of any encumbrance,
allowing unimpeded use of the enlarging instrument (Figure 49).


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Figure 48 Maxillary molar teeth. The outline form can usually be confined to the mesial half of the occlusal aspect and should reflect the internal anatomy of the chamber and the direction of the canals. The preparation is sloped to the buccal aspect to give easier access. One must also search carefully for the possibility of a fourth canal, which is present in half of cases. On the other hand, a few second molars may have only two canals.




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Figure 4-49 Maxillary molar teeth. Since all instrumentation is introduced from the buccal aspect, the cavity is sloped buccally for easier access. Each endodontic instrument, when inserted down the canal, must be unimpeded, free from any interfering wall



Operative Errors
One of the most common errors that occurs in maxillary molar teeth is perforation into the furcation, using a surgical-length bur and unknowingly passing through the narrow pulp chamber (Figure 50). The depth to the chamber should be measured on the radiograph and marked with Dycal on the shaft of the bur. In an underextended preparation, only the pulp horns are nicked. White-colored dentin is a clue to the underextension (Figure 51). The true floor of the chamber is marked by dark-colored dentin. An imperfect vertical preparation can occur in a molar tipped to the buccal aspect (Figure 52). The preparation should be parallel to the true long axis of the tooth.
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Figure 50 Operative errors—maxillary molars. One of the most common errors is perforation of the furcation while searching for a receded pulp with a surgical-length bur. Wider access helps prevent these accidents, as does measuring the depth on the radiograph. These perforations maybe repaired with placement of mineral trioxide aggregate (MTA, Dentsply/Tulsa).





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Figure 51 Operative errors—maxillary molars. A, Underextended preparation. The roof of the pulp chamber has not been removed. The white color of the dentin in contrast to the dark color of the dentin on the floor of the chamber should be the clue. The pulp horns have barely been nicked, and the clinician has assumed that the canal orifices have been located. Total control of the instruments will be lost if instrumentation proceeds through tiny orifices. B, Example of the failure to remove the roof of the pulp chamber. One can easily visualize how the interfering tooth structure will control the path of the instrument
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Figure 52 Operative errors—maxillary molars. Inadequate vertical preparation related to a failure to recognize the severe buccal inclination of an unopposed molar
A final error in maxillary molars is perforation of the palatal canal, commonly caused by the assumption that the palatal canal is straight (Figure 53). In more cases than not, the palatal canal curves to the buccal aspect, a fact that does not appear in the standard buccolingual radiograph. Careful exploration with fine-curved files should reveal the presence and direction of the curve.




MANDIBULAR MOLAR TEETH
All mandibular molar teeth are entered through the occlusal surface. Initial penetration is made in the exact center of the mesial pit, aimed for the orifice of the distal canal, the largest canal. Round carbide burs (no. 4), end-cutting, tapering fissure burs, or diamond stones are used to enter the chamber and to remove the roof of the chamber as well (Figure 54).
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Figure 53 Operative errors—maxillary molars. Perforation of a palatal root is commonly caused when the
clinician assumes the canal is straight and fails to explore and enlarge the canal with a fine, curved instrument
Remember, roots that curve buccally appear to be straight in buccolingual 
radiographic projections.

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Figure 54 Mandibular molar teeth. Entrance is always gained through the occlusal surface of posterior teeth. Initial penetration is made in the exact center of the mesial pit, with the bur directed distally.
Once the chamber has been entered, the bur is used to enlarge the opening orifice by cutting away the roof of the pulp chamber. This allows for the entrance of exploring instruments.
Endodontic explorers or files are used to locate the orifices of the canals and to determine the direction convenience extensions must be made (Figure 55). One must carefully explore for a possible fourth canal in the distal root
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Figure 55 Mandibular molar teeth. An endodontic explorer is used to locate the orifices of the distal, mesiobuccal, and mesiolingual canals. Special care is taken to explore for a possible fourth canal in the distal root. Tension on the explorer indicates how much of the walls must be removed to gain unchallenged access to the canals’ full length.
Non-end-cutting, tapering fissure burs or stones are best used to expand the outline form to accommodate unimpeded use of the enlarging instruments (Figure 56). The final outline form is dictated by the size and shape of the pulp chamber plus the convenience extensions needed to free the enlarging instruments from interference (Figure 57). Severe extensions to the mesial are not uncommon. The final outline form, from occlusal to apex, provides unobstructed access for all enlarging instruments and filling materials (Figure 58).

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Figure 56 Mandibular mol teeth. Final finishing and funneling of the cavity walls are completed with a fissure bur or a diamond.
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Figure 57 Mandibular molar teeth. The “square” outline form reflects the anatomy of the pulp chamber modified for convenience form, mostly flared to the mesial to allow easier access to the mesial canals. Four canals are shown in this illustration.
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Figure 58 Mandibular molar teeth. The final preparation provides unobstructed access to the canal orifices and should not impede the enlarging instruments; there should be a free flow to the apical third of all canals


Operative Errors
Perforation into the furcation is commonly caused by using a surgical- length bur and unknowingly passing through the narrow pulp chamber (Figure 59). Depth to the pulp chamber should be measured on the radiograph and marked on the shaft of the bur with Dycal. Perforation at the mesiogingival aspect is caused by the failure to orient the bur to the long axis of a mandibular molar severely tipped mesially (Figure 60).
Failure to locate a second distal canal occurs because of a lack of exploration for a fourth canal hidden by inadequate outline form (Figure 61).
Perforation of a curved, distal root is caused by using a large, straight instrument in a severely curved canal (Figure 62). Such a curve should be observed in a radiograph, and fine, curved or flexible instruments should be used.

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Figure 59 Operative errors— mandibular molars. Perforation into the furcation is caused by the use of too long a bur and a failure to recognize on the radiograph the depth of recession of the pulp. The bur should be measured against the radiograph and the depth marked on the shaft. This is a common error.

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Figure 60 Operative errors— mandibular molars. A common error is perforation at the mesiogingival aspect caused by a failure to recognize the severity of the tilt of the lower molar. The bur must be oriented with the long axis of the tooth
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Figure 61 Operative errors— mandibular molars. Failure to locate a second distal canal is caused by the
overhanging dentin wall and a lack of exploration for a second canal


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Figure 62 Operative errors mandibular molars. Perforation of the curved, distal root is caused by the use of a large, straight instrument in a large but severely curved canal




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