52. can a division of the canal into zones of preparation be … · 2016-02-03 · the curvature...

52. Can a division of the canal into zones of preparation be beneficial?If all of the parameters listed above are to befollowed, dividing the canal into two zonescan simplify and expedite canal preparation,with each zone having a different techniqueapproach. These zones are most effectivelydifferentiated by the location of curvaturesor constrictions, which result in peaks ofincreased torsion or pressure duringadvancement into the canal. Two methodsare very useful in identifying a restriction:(1) If a file that is smaller than the canalmeets resistance as it is passively advancedinto the canal, the depth at which the resist-ance is met also denotes the terminus of the

ccoorroonnaall zzoonnee. (2) To advance a .06 tapered,.25 mm diameter file the first 1 mm into acanal after it becomes engaged, a specificpressure needs to be applied. If a depth isreached in a canal short of the workinglength that the determined specific pressureneeds to be exceeded in order for addition-al advancement to occur while using 1 mmincrements of insertions, that depth of pen-etration denotes the coronal zone. The .25tip is used to detect a constriction and the.06 taper has the rigidity that will resist acurvature.The most reliable files for follow-ing this procedure include the K-3, Quantecand the RaCe files.This second technique fordetermining the ccoorroonnaall zzoonnee requires alearning curve and should be employed onlyafter sufficient experience.

John T. McSpadden, D.D.S

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Table 122 A Quantec 25/06 file was inserted and withdrawn at a rate of ½ mm/s. It was inserted 2 mm andwithdrawn 1 mm with each cycle. The peak pressure in pounds per square inch was recorded with each cycle.The pressure for the 5th insertion (5th mm depth of insertion after engagement) was substantially higherthan any previous insertion. This level indicated the terminus of the coronal zone.

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The Zone Technique for canalpreparationThe zone technique was designed with twoobjectives for minimizing file stress for anytype of NiTi rotary file: One, the canal diam-eter should be large enough coronal to acurvature to prevent any engagement in thatportion of the canal when any file is beingused apical to the curvature. Two, the filediameter is not too large to rotate safely in acurvature.

The first step is to determine if there is acurvature of any significance and how far thecurvature is from the apex.Withdrawing the

file used to establish the working length, andpassively re-inserting will indicate a curva-ture if it meets any resistance short of theworking length since the canal is now largerthan the file.The canal portion short of theresistance defines the ccoorroonnaall zzoonnee and theportion beyond the resistance defines theaappiiccaall zzoonnee (Fig.124).The length of the canalto the curvature, the coronal zone, is meas-ured and recorded with the same importanceas determining the working length.The work-ing length minus the coronal zone lengthprovides the distance the curvature is fromthe apex, the apical zone length.

Mastering Endodontic Instrumentation

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Fig. 123

The red arrow indicates the type of anatomical restriction orcurvature represented in the chart above that would requirean increase in pressure to advance to a greater depth in thecanal. Assuming that this is at the 5mm depth after the filebecame engaged as in the chart above, this level would rep-resent the terminus of the coronal zone. Stress on the files isminimized of the apical zone beyond the curvature is pre-pared with a technique that considers the file diameter as ittransgresses the curvature.

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The second step is to determine the dis-tance each of the files having different sizesand tapers can safely be advanced aroundthe curvatures and which size file will needto be used in the coronal zone to prevent anysubsequent file from binding in the apicalzone. By using the parameters suggestedabove for diameter limitations ((..6600mmmm ffoorr aa..0022 ttaappeerr,, ..5555mmmm ffoorr ..0044 ttaappeerr,, ..5500mmmm ffoorr..0066 ttaappeerr,, aanndd ..3355mmmm ffoorr aa ..0088 ttaappeerr)), wecan calculate if the diameter of a selected filewould exceed our limitations.That determina-tion can be calculated by using the followingformula:

DDiiaammeetteerr lliimmiittaattiioonn** ((--)) ttiipp ssiizzee ((÷÷)) ttaappeerr ((==)) tthhee lleennggtthhtthhee ffiillee ccaann bbee pprroojjeecctteedd aarroouunndd aa mmooddeerraattee ccuurrvvaattuurree..*no more than .60mm for a .02 taper, .55mm for .04 taper,.50mm for .06 taper, and .35mm for a .08 taper

EExxaammppllee 11:: If we select a size .25/.04taper file to negotiate the canal illustrated inFFiigg..112255, the diameter limitation suggested inthe parameter is .55 mm.The .55 mm diame-ter limitation minus the tip size, .25 mm, is.30.The difference, .30, is divided by .04, the

taper, which equals 7. The number of mil-limeters a .25/.04 file can be advancedbeyond a moderate curvature is 7 mm andtherefore, can be used to the working lengthin this situation. A size .50/.04 or largerwould be required to enlarge the coronalzone to prevent any engagement in that por-tion of the canal while using a .25/.04 file tothe working length.

EExxaammppllee 22:: If we select a size .25/.06 taperfile to negotiate the canal illustrated in FFiigg.. 112255,the diameter limitation suggested in theparameter for a .06 taper is .50mm. The diam-eter limitation, .50mm, minus the tip size,.25mm, is .25. The difference, .25 divided by.06,the taper, is 4 and is the number of millime-ters a .25/.06 file can be advanced beyond amoderate curvature or into the apical zoneand, therefore, should be advanced no closerthan 2mm from the working length in this situ-ation.A size .50/ 06 or larger would be requiredto enlarge the coronal zone to prevent anyengagement in that portion of the canal whileusing a .25/.06 file to the working length.


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Fig. 124 The points of resistance files encounter, indicated by the arrows, when passively being inserted into canals havinga larger diameter, divides the canals into coronal and apical zones. The zones require different preparation approaches.


Once this procedure is followed, the ter-minus of the canal can easily be enlarged byusing .02 tapered files.For more severe curva-tures, the file sizes need to be reducedaccordingly. Although the zone techniquedoes require some mental exercise, hopefullywithin the capacity of any dentist doing rootcanals, its advantages can result in majorreductions in preparation time and file stress.

53. How do I prepare thecoronal zone?The coronal zone presents no particular prob-lems as long as it can be defined and its termi-nus is not violated.Essentially,the coronal zoneis a straight canal and the considerations forpreventing file breakage can easily be applied.The selection of the first file for preparing

the coronal zone should meet two objec-tives.The file tip should be small enough tonegotiate the entire coronal zone yet rigidenough to tactilely detect any point of curva-ture or a restriction that requires an increasein applied pressure for apical advancement.The final preparation of the coronal zoneshould be large enough to avoid or minimizeengagement of any subsequent file and pro-vide adequate access for obturation.A frequentmistake is to use tip diameters at the terminusof the coronal zone that are smaller at thatpoint than the diameters for files used in theapical zone. For instance, if a 25/.08 file iscarried to the terminus of the coronal zoneand a 25/.02 file is carried 5 mm apical tothat point, the 25/.02 will be engaged in thecoronal zone, since its diameter will be.35mm at the terminus of the coronal zone.


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Fig. 125 The recorded length at which the smaller file encounters resistance in a larger canal canbesubtracted from the working length to obtain the distance the curvature is from the working length.


When encountering a canal anatomy thatcannot be instrumented with conventionaltechniques while following the parameterslisted above,closely following an exercise fordesigning a technique modification that will

conform is an excellent mechanism for learn-ing efficiency and avoidance of file failure.Examples for technique procedures thatconform to the instrumentation rationalepresented thus far are as follows:


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Fig. 126 The tooth illustrated has been sectioned to expose the mesio-buccal canal.The curvature begins 6 mm from the apical foramen and its radius is slightly lessthan 8 mm. Adjacent to the canal are the canal diameters prior to instrumentationfrom the foramen to the canal orifice.



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SStteepp 11.. DDeetteerrmmiinnee tthhee tteerrmmiinnuuss ooff tthhee ccoorroonnaall zzoonnee..

Fig. 127 A 25/.06 Quantec SC is taken to the terminus of the coronal zone ( 6 mmfrom the working length). The file, rotating at 350 rpm, should be advanced intothe canal in 1 mm increments with insertion-withdraw movements. Each insertionshould not require any more pressure than was required for its first insertion onceengaged. When a position is reached in the canal that requires more pressure foradvancement, that position is a good indication of the terminus of the coronalzone. In order to ascertain the position of the terminus, a file smaller than the canalcan be advanced until resistance is met. For instance, if a 15/.02 is carried to theworking length and withdrawn and reinserted, and resistance is met at some point,then that point denotes the terminus of the coronal zone. The two positions shouldbe the same. The numbers in pink denote the levels of file engagement.



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Fig. 128 Defining the coronal zone. The area in the circle illustrates where the file meets the increasedresistance of a curvature and defines the terminus of the coronal zone. Images A and B illustrate dif-ferent views of the .25 SC tip; one rotated 90 degrees from the other. The Quantec SC 25/06 was select-ed as the first file to negotiate and to define the coronal zone for 3 reasons, (1) The two fluted spade-shaped Quantec SC tip will enter and enlarge a canal orifice as small as .10 mm. as illustrated in the Aand B images. (2) The Quantec SC .06 taper file had the most favorable safety ratio of any of the filestested with its dimensions. (3) The .06 taper is small enough to advance into the canal without requir-ing excessive pressure and it has sufficient rigidity to tactilely detect restrictive curvatures.



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22.. DDeetteerrmmiinnee tthhee ddiiaammeetteerr nneeeeddeedd iinn tthhee ffiinnaall eennllaarrggeemmeenntt aatt tthhee tteerrmmiinnuuss ooff tthhee ccoorroonnaall zzoonnee..

Fig. 129 55/.06 K-3. Having the same taper as the 25/.06 file, it is helpful to circum-ferentially file the canal during its advancement. This file was selected because ofits resistance to distortion and its high cutting efficiency. By cautiously removing thisfile to a position short of its maximum insertion, the speed of rotation can beincreased to 1,000 to 1,200 rpm to selectively reposition the canal orifice and taperof the coronal zone (to reduce the amount of engagement during its progress) tosufficiently enlarge in order that subsequent files will not become engaged, and toenlarge the canal access to conform to dimensions needed for ease of obturation.The Quantec/K-3 design is particularly effective for this type canal enlargement.Care must be taken not to engage the tip of the intrument by carrying it to agreater depth.



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SStteepp 33.. EEnnllaarrggee tthhee aappiiccaall zzoonnee ttoo aa ssiizzee 2255//..0022..

Fig 130 Advance the 15/.02 Quantec SC to terminus of apical zone (working length), followedwith a 20/.02 and a 25/.02 Quantec LX to working length. The Quantec .02’s were selectedbecause they have the highest safety ratio for .02 files and the greatest cutting efficiency duringpenetration. The SC tip was selected for the 15/.02 file because of the uncertainty of the canaldiameter and the reduced risks of canal transportation with its flexibility. Since the largest diam-eter at the terminus of the coronal zone was .55, the maximum diameters for the .02 files at theworking length are smaller and the files are only engaged for 6mm.



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Fig. 131 25/.04 K-3 taken to terminus of apical zone. Following this procedure the filetip remains passive until it reaches the working length and is only engaged for 6 mm.The largest diameter of this file to be engaged is .45. The K-3 file was selected becauseit demonstrated the highest efficiency of any of the files tested.

SStteepp 44.. EEnnllaarrggee tthhee aappiiccaall zzoonnee ttoo tthhee ddeessiirreedd ddiimmeennssiioonnss ppeerrmmiissssiibbllee..



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Fig. 132 25/.06 K-3 taken 1mm short of working length. Any additional taper or apicaldiameter should be provided by using .02 or .04 tapered files with a step-back tachniquein the apical zone. The enlargement of the coronal zone can be determined by therequirements of the operator. Each file in the sequence used conforms to the parame-ters dicussed above. The K-3 was selected for its high efficiency.


As discussed above, the K-3 files andQuantec files were used because results ofthe particular tests employed in our evalua-tions indicated either file provided a bettersafety ratio or greater preparation efficiency.Most other file series have the same file sizesavailable and can be used in the same mannerto conform to the prescribed parameters.TheProtaper Series is an exception that has fileswith unique accelerating and deceleratingtapers.Although the coronal zone can easilybe prepared with the Protaper Series, the api-cal zone cannot effectively be preparedwhile performing within the parameters,par-

ticularly without engaging more than 6 mmof working surface or without using largediameters in the curvature.

The above example was used to illustrateall of the limitations the parameters place oncanals that have curvatures that extend sixor more millimeters from the workinglength. Techniques for preparing mid-rootand coronal curvatures require more step-back approaches with .04 and .02 tapers toconform to the parameters.

If the parameter of taper and diameterlimitations for a canal having a 45-degreecurvature and a 8 mm radius is used as a


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Fig. 133 This illustration is another means of representing the case above indicating the instruments used and the depths ofadvancement. Using this type representation enables one to determine if all the parameters are followed. Note that the K-325/.06 required advancing 1 mm short of the working length in order to conform to the parameters. Depending on the tipdesign, the use of a size 25/1.0 or larger orifice opener could facilitate enlargement with less engagement if used before the55/.06 file was used in the coronal zone.

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reference, the limitations of file dimensionsin canals having different curvatures can beextrapolated mathematically.Two factors, theworking length and the curvature radius,need to be considered in determining the filetaper and diameter that can be used in theanatomy illustrated above while adhering tothe recommended parameters. For instance,if we want to know how far we can advancea 25/.06 file beyond the point of curvaturethe parameters state the diameter should notexceed a .50 mm diameter which is reachedwhen the file extends only 4 mm beyond thepoint of curvature. However, if the radius ofcurvature is16 mm or two times the amountstated in the parameters, the diameter usedcan be larger.The relationship is based on thesquare of the file diameter.The square root oftwo times the .50 mm diameter squaredequals a size .70 mm diameter which meansa 25/.06 file can be advanced to the workinglength without undue stress on the file.Thatdetermination assumes the other parametersare considered.Although the first impressionof this process may seem complicated, littleexercise is required to master this approach.The benefit is efficiency and the reduction ofthe threat of failure. If the clinician insistson following a routine technique sequencebecause consistency weighs heavily in theirpractice efficiency, then the sequencedescribed in Fig. 133 encompasses all of

our considerations for minimizing instru-ment stress while providing an efficientroutine for rroouuttiinnee ccaannaallss. That techniquesequence is as follows:

1. 25/.06 to curvature2. 55/.06 to curvature3. 25/.02 to working length4. 25/.04 to working length 5. 25/.06 1 mm short of working lengthThe exceptions to “routine” of course, is

any canal having an apical zone greaterthan 6 mm, a curvature less than 45degrees and a radius less than 8 mm.

54. Can files be designed so youdo not have to calculate how toavoid excessive stress? Excessive stress on files of current designscan be avoided.This can be done if the causeof stress is understood and calculations aremade as to where and how files can be used.Files can and are being designed that mini-mize mistakes in calculations and judgment.Since the two most frequent causes of filebreakage are the result of using files havingdiameters too large for the degree of canalcurvatures and engaging too much of thefile’s working surface, designs can be incor-porated to minimize these events.Addressingdesign changes for reducing potential stressis the purpose of the next section.


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52. can a division of the canal into zones of preparation be … · 2016-02-03 · the curvature...

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