Finishing the Apical One Third: Endodontic Considerations

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The purpose of this series of articles is to provide useful information that will enable dentists to predictably shape root canals for three-dimensional obturation. The information in these articles is intended to help clinicians better understand how to safely use nickel-titanium (NiTi) rotary files. The author’s articles previously published in Dentistry Today described the concepts that form the foundation for successfully preparing root canal systems.1-4 Recently, a new generation of files for shaping root canals called the ProTaper system (DENTSPLY Tulsa Dental) has become available. ProTaper’s unique geometries and directions for use have been described (Figures 1 and 2).3-6 This article will focus on the various considerations that will influence predictability and success when finishing the apical one third. Where appropriate, the technique principles specific to the ProTaper system will be described; however, many of the principles discussed are applicable to all NiTi rotary systems.

FINISHING THE APICAL ONE THIRD

Figure 1. The ProTaper NiTi rotary set of files is comprised of just three shaping and three finishing instruments. Figure 2. The canals of this endodontically treated mandibular second molar were shaped with ProTaper files and demonstrate smooth flowing, multiplanar curvatures. (Courtesy of Dr. Dominique Martin; Paris, France)
Figure 3. A photo demonstrates the visual advantage using an abrasively coated ProUltra ENDO-3 ultrasonic instrument without water (DENTSPLY Tulsa Dental) to progressively sand, smooth, and finish the access preparation. Figure 4. Three-dimensional endodontic treatment is the foundation of perio-prosthetics.

Root canal preparation procedures are simplified and improved when there is straight-line access to the orifice(s) (Figure 3). Additionally, apical one-third finishing procedures are enhanced when the coronal two thirds of a canal is pre-enlarged. Once the coronal two thirds of a canal has been optimally prepared, the clinician has significantly improved radicular access to the apical one third of the root canal system for scouting and finishing procedures. The apical one third of a root canal can be finished once it has been negotiated to length, patency has been established, and the foramen has been enlarged to at least a size 15 hand file. The sine qua non of excellence in canal preparation is consistently finishing the apical one third. The ideal “finished” preparation will always exhibit a uniform taper that progressively narrows toward its terminal extent.7,8 Importantly, canals that have been optimally shaped promote both three-dimensional “cleaning” and “obturation” (Figure 4).

HAND VS ROTARY FINISHING

Figure 5a. The potential problem of immediately trying to negotiate tighter, more restrictive canals to length. Figure 5b. A pre-enlarged canal can more readily accommodate a precurved file and improve the predictability of achieving length.
Figure 5c. A 15 file can be utilized to determine if there is a smooth, reproducible glide path to safely accommodate a rotary shaping file.

When a 15 or larger file is confirmed at length, a decision on whether to use hand or rotary instruments to finish the apical one third can be made. It is wise to finish a root canal preparation with hand instruments when a system exhibits difficult anatomy, certain resorptive defects, or an iatrogenic problem. Small flexible files may be utilized to scout the apical one third and provide information regarding canals that abruptly merge, excessively curve, significantly recurve, dilacerate, or divide. In general, certain canals exhibit anatomical challenges that necessitate a reciprocating handle motion in order to move a precurved 15 file to length (Figures 5a and 5b). In these instances, the apical one third of a canal should be finished with hand instruments. However, if a straight, flexible, stainless steel 15 file can be gently pushed over a few millimeters and can passively slide to length, then rotary NiTi instruments can he employed to “finish” the apical one third of the canal. At times, difficult canals can be progressively enlarged with hand files until a smooth and predictable glide path to the terminus can be established, in which case rotary instruments may become suitable for finishing (Figure 5c).

IRREGULAR GLIDE PATH: HAND FILE

When there is an irregular glide path, regardless of etiology or size of the foramen, then 0.02 tapered hand instruments should be used to prepare and finish the apical one third of a canal. When the length of the canal has been carefully scouted and fully negotiated to its radiographic terminus with sizes 10 and 15 files, then the apical one third can be finished with 0.02 tapered 20 through 60  hand files. Sizes 20, 25 and 30 flexofiles (DENTSPLY Maillefer) may be selected to initiate apical one-third procedures, as these stainless steel files have improved flexibility, and their noncutting tips allow them to safely follow an existing pilot hole. All stainless steel instruments that are carried into the apical one third must be precurved to simulate the anticipated curvature of the canal based on well-angulated preoperative radiographs and clinical information, NiTi hand files sizes 35 through 60 (DENTSPLY Maillefer) may be advantageously selected because their flexibility provides greater safety and control in creating deep shape and finishing the apical one third of the canal. As an alternative to finishing the apical one third with 0.02 tapered hand files, a knurled mandrel (DENTSPLY Maillefer) can be placed over the handle of any of the three ProTaper finishing files so they can be utilized manually in a rotary motion. If there is a canal wall irregularity, then depending on its location and extent it may be necessary to curve the larger NiTi files to promote the mechanical shaping and cleaning objectives.

Figure 6a. Gauging and tuning procedures verify deep shape and confirm a “finished” preparation. Figure 6b. If the 20 file is at length, then each consecutively larger instrument uniformly steps out of the canal.

Hand files may be utilized to progressively enlarge, blend, and create uniform taper in the apical one third of the canal. Specifically, gauging and tuning with ISO 0.02 tapered instruments is a technique directed toward confirming a uniform taper in the apical one third of the root canal preparation.2,7,9 The clinician begins to gauge the most apical cross-sectional diameter of the canal when a hand or rotary instrument just snugs into the terminus and resists further apical travel. In order to prove with certainty that the diameter of the file at length represents the true size of the foramen, the clinician must then tune. Tuning is the clinical activity of recapitulating through a series of successively larger instruments and working them until they are observed to uniformly back out of the canal (Figure 6a). Clinically, the file that snugs in at length represents the true most apical cross-sectional diameter of the canal if, and only if, each successively larger instrument uniformly backs out of the canal in 1/2-mm intervals (Figure 6b).

Figure 7. The canals of this endodontically treated mandibular first molar were gauged and tuned, and the pack demonstrates the uniform and fully tapered shapes.

If increasingly larger instruments do not uniformly step back from the canal terminus, then “finish” by re-utilizing all the instruments in the series that worked short of the foramen.8 In more difficult cases, recapitulate through the entire series again, and perhaps yet again, to create an extraordinarily clean, smooth flowing, and evenly tapered preparation. Many times when performing shaping procedures, the clinician is just one recapitulation away from excellence. Gauging and tuning verify “deep shape,” and create the opening for three-dimensional cleaning and obturation (Figure 7).9

SMOOTH GLIDE PATH: PROTAPER ROTARY FILES

When a smooth, reproducible glide path to the terminus is confirmed with hand files, and after the ProTaper Shaper 1 (S1) and Shaper 2 (S2) have been carried to length, then the apical one third can be finished. In many cases, only one ProTaper finishing file will be required to create the optimal deep, fully tapered shape. Specifically, following the use of the ProTaper 20/0.07 F1 to the terminus, the size of the foramen is gauged using a 20/0.02 hand file (Figure 8). When gauging the size of the foramen, passively slide the hand file to length and gently tap on the handle, in an apical direction, to determine if the file is snug or loose at length. When gauging the foramen with a hand file, never rotate or reciprocate its handle, as this motion will serve to engage the cutting blades into dentin, screw the file long, and needlessly over-enlarge the foramen.

Figure 8. Following the use of the 20/0.07 F1 rotary file to length, the foramen is gauged using a 20/0.02 hand file.
Figure 9. Following the use of the 25/0.08 F2 rotary file to length, the foramen is gauged using a 25/0.02 hand file. Figure 10. Following the use of the 30/0.09 F3 rotary file, the foramen is gauged using a 30/0.02 hand file.

If the 20 hand file is snug at length, then the canal is fully shaped and, if irrigation protocols have been followed, ready to pack. If the 20 hand file is loose at length, only then should the clinician proceed to the next sequentially larger ProTaper finishing instrument, which is the 25/0.08 F2. Following the use of the F2 to length, the size of the foramen is gauged by using a 25/0.02 hand file (Figure 9). If the 25 hand file is snug at length, then the canal is fully shaped and, again, if irrigation protocols have been followed, ready to pack. If the 25 file is loose at length, then and only then should the clinician proceed to the next sequentially larger ProTaper finishing instrument, which is the 30/0.09 F3. Following the use of the F3 to the terminus of the canal, the foramen is gauged utilizing a 30/0.02 tapered hand file (Figure 10). If the 30 hand file is snug at length, then the canal is optimally shaped and, again, if irrigation protocols have been followed, ready to pack. If the 30 hand file is loose, then the next section will address how to finish these more open and typically straightforward canals.

FINISHING LARGER SYSTEMS

Figure 11. An endodontically treated maxillary central incisor demonstrates that a shaped canal provides resistance form to achieve three-dimensional obturation.

In instances where the foramen is gauged and determined to be greater than 0.30 mm, and a smooth glide path has been verified, then an alternative NiTi rotary file line may be used. The decision as to which specific line of instruments to select should he based on cutting efficiency, flexibility, and safety. Importantly, the instruments chosen and the techniques employed must create deep shape and the resistance form to hold gutta-percha during three-dimensional obturation (Figure 11). Research evaluating canal cleanliness compared with apical one-third shape has clearly shown that preparations need to taper at least 0.06 mm/mm to ensure that a sufficient volume of irrigant over an adequate interval of time can efficaciously circulate and clean into the canal anatomy.7-10 Except in larger and straighter canals, rotary files that have D0 diameters, greater than 0.30 mm and tapers greater than 6% are frequently too stiff to safely place into the apical one third of a more curved root canal. As such, NiTi rotary 0.02 tapered files or 0.04 tapered ProFiles (DENTSPLY Tulsa Dental) will provide the flexibility to safely shape the apical extent of these larger, more open canals. As noted with hand files, NiTi rotary 0.02 and 0.04 tapered files can be employed in a step-back technique to create virtually any tapered shape that is desired.

EXPAND THE DEEP SHAPE

There are cases when a ProTaper 20/0.07 finishing file is snug at length, the foramen is confirmed to be 0.2 mm after gauging with a 20/0.02 hand file, yet the clinician may find it advantageous to expand the deep shape of the canal. In the instance where the F1 was at length, then a fuller shape can be easily and safely accomplished by carrying the F2 2-mm short, and the F3 3-mm short of the working length. This clinical method of carrying each larger ProTaper finishing file progressively shorter than the previous one will maintain the size of the foramen while expanding the overall shape in the middle and apical one thirds of the canal.6 This step-back method is not done routinely and is only appropriate when different, well-angulated radiographs confirm that the dimensions of a root can safely accommodate a fuller shape.

TROUBLESHOOTING THE APICAL ONE THIRD

Figure 12. This endodontically treated mandibular second bicuspid demonstrates a smooth flowing dilacerated preparation, apical bifidity, and the ProTaper advantage. (Courtesy of Dr. Fabio Gorni; Milano, Italy)

Many endodontic complications can potentially occur when preparing the apical one third of a canal, and as such, many techniques have been developed and clinically shown to manage these iatrogenic events.11 The apical one third of the root canal system is typically a more challenging region to prepare. Anatomically, this region of the canal is tighter and more curved, and divisions are commonly present (Figure 12). Traditionally, most canals were prepared using files and gates glidden drills, and as such, the potential for iatrogenic events, including blocks, ledges, apical transportations, perforations, and broken instruments was ever present.12 Fortunately, NiTi rotary shaping files have virtually eliminated most of these iatrogenic events. However, some of this euphoria has been offset by the potential for NiTi rotary files to break, or to a lesser extent, transport the foramen. The remaining portion of this article will focus on the factors influencing NiTi breakage, apical transportations, and how to prevent these mishaps.

Factors influencing  NiTi Breakage

There are many factors that will individually and in combination influence file breakage.13 The clinician who understands the causes of file separation will have a greater appreciation for how to prevent breakage.

File Design

There are numerous design features that directly impact the performance of any given NiTi rotary file line. Some of the more important mechanical features include the percentage taper of an instrument, whether a file is fixed versus variably tapered, and if a file has a negative, neutral, or positive rake angle. Other critical features that influence performance are cross-sectional geometry, tip configuration, the length of cutting blades, and the helical angle and pitch of the cutting flutes. These design features influence flexibility, cutting efficiency, and safety. Additionally, the design of an instrument will dictate whether a file is too stiff to follow a glide path, or has a tendency to screw and get inadvertently pulled into a canal. Further, the design will influence how many recapitulations are required for a file or a series of files to safely achieve length.

Recommended Rpm

Each brand of NiTi rotary files has a recommended speed to ensure optimal performance. Clinicians need to select an electric motor that will allow them to utilize the rotary files they choose at the recommended rpm. When a NiTi rotary file is used at a speed slower than recommended, it tends to lose cutting efficiency and bogs down during use. Another potentially negative result of using a speed lower than recommended is increased friction, which can lead to rotary file failure. On the contrary, when a NiTi file is used at a speed faster than recommended, then it tends to become more dangerous as it can grab and screw into the canal, predisposing to breakage.6 The ProTaper files should be run at a speed of 300 rpm to ensure optimal performance.

Torque

Torque is the amount of rotation that a force exerts on an object and is measured in gram centimeters (g cm). The “rotation to failure” torque is measured by turning a file that has been grasped and immobilized at D3 at a constant rpm.14 The “mean working” torque may be defined as the torque required for any given NiTi file to cut dentin. The safety quotient (SQ) is a ratio where the rotation to failure torque is divided by the mean working torque, and ideally, should provide values greater than 1.15 The SQ formula has been typically used to evaluate fixed tapered files that oftentimes engage dentin toward their smaller, weaker, and less active cutting blades. On the contrary, the new ProTaper progressively tapered shaping files are designed to engage and cut dentin on their larger, stronger, and more active blades. As such, if the SQ formula is to provide useful information, then the data used for calculation must be indexed to the specific location where each ProTaper instrument has been designed to cut dentin.16 Torque control electric motors are intended to optimize file performance and safety, and have generally been programmed for fixed tapered files. When a smooth, reproducible glide path has been confirmed, research has shown that the ProTaper shaping files in combination with the Tecnika motor (DENTSPLY Tulsa Dental) can work safely at maximum torque and provide SQ values greater than 1.16

Stress and Strain

Figure 13. This endodontically treated mandibular second molar demonstrates that ProTaper files can smoothly and safely shape difficult, apical one-third curvatures. (Courtesy of Dr. Edmond Koyess; Beirut, Lebanon)

The strength of a file may be described by how much force it can resist without permanently changing its shape. Endodontically, when a rotating file engages tooth structure, stress within the file results and can be defined as the internal force within the instrument to resist the external force to cut dentin. If the file cannot resist an external force, then the NiTi metal changes its shape or uniformity, and this is known as strain. Specifically, when a rotary file is working in a curved canal, the engaged portion of the instrument is subject to stresses. Tensile stress occurs on the file’s outer curvature, and the metal stretches. At the same time, compressive stress occurs on the file’s inner curvature, resulting in the metal compressing. These combined stresses unite and make the instrument bend. The ProTaper shaping files have increasingly larger percentage tapers over the length of their blades, whereas the ProTaper finishing files have decreasingly smaller percentage tapers from D4 through D14. This design feature was intended to increase flexibility and decrease both the frictional stress and the potential for file breakage (Figure 13).16

Reproducible Glide Path

Figure 14. A photograph showing ISO 0.02 tapered 10, 15, and 20 files. Each instrument tapers 0.32 mm over its 16 mm of cutting blades.

Nickel-titanium rotary files should only be used in portions of the canal that have a confirmed, reproducible glide path. To assure there is available space within a canal for rotary instruments to follow, stainless steel 0.02 tapered sizes 10 and 15 hand files should be used to gather reconnaissance information and, when necessary, to create sufficient space prior to using more efficient rotary NiTi instruments.3,4 Appreciate that the 10 and 15 hand files have Dl6 diameters of 0.42 and 0.47 mm, respectively, and they can create considerably more space than the numerical number on their handles would suggest (Figure 14). The secret for safely using NiTi rotary instruments is to thoughtfully, deliberately, and, in more calcific canals, repeatedly use smaller-sized hand files until sufficient space exists for rotary files to follow.9 A NiTi rotary file is designed to “follow” a canal as its noncutting and most flexible working end is guided by a pilot hole of circumferential dentin. When it is confirmed that a 15 file can predictably slide through any part of a root canal, then this “secured” region can be safely prepared using NiTi rotary files.

Vertical Force/Pressure

Because each different brand of files has its own specific designs and geometries, different techniques have been recommended to maximize safety and cutting efficiency. Taper lock results when the entire length of a file’s cutting blades inadvertently engages dentin. Taper lock contributes to greater torsional stress, alloy fatigue, and the potential for file breakage. Research has shown that fixed tapered instruments with radial lands have a greater potential for taper lock and require four times more vertical force to cut dentin than progressively tapered files without radial lands.16 Fixed tapered instruments are oftentimes used in a pecking or pumping action to avoid being inadvertently pulled into the canal. On the contrary, a progressively tapered shaping file engages a smaller zone of dentin and is used with a lateral brushing action with virtually no apical pressure. Importantly, this method of use reduces the potential for taper lock, torsional stress, and the potential for breakage.9

Speed of Penetration

Another variable in NiTi rotary file breakage is the speed of penetration in which an instrument is deployed into a canal. The speed of penetration is influenced by the insertion pressure and the design of the file. As an example, instruments with full or relieved radial lands have a greater surface area that contacts dentin, as compared with files with a convex triangular cross-section. Additionally, files with fixed tapers generally have a greater surface area that contacts dentin compared with variably tapered files. As such, if the speed of penetration is too quick, then there is increased friction, and as the blades clog, the displacement of cut dentin is diminished and the potential for breakage is increased.

Root Canal System Anatomy

Figure 15. An endodontically treated maxillary first molar demonstrates five treated canals and that each system exhibits various anatomical configurations.

It must be recognized that certain root canals exhibit anatomical configurations that preclude the safe use of NiTi rotary files. Clinicians need to appreciate the five commonly encountered anatomical forms, which include canals that merge, curve, recurve, dilacerate, or divide (Figure 15). Scouter files provide information regarding the anatomy and give important feedback regarding the canal’s degree of curvature, recurvature, or if there is a dilaceration. Further, before introducing rotary instruments, clinicians need to know if a single canal coronally subsequently divides, or if two or more systems within a root merge along their length.

Lubrication

No file should be introduced into the root canal space until the appropriate irrigant is placed into the pulp chamber.17 Typically, before beginning NiTi rotary procedures, clinicians fill the chamber with sodium hypochlorite (NaOCl). To further reduce the friction between the rotary instrument and the wall of the canal, some clinicians advocate filling the pulp chamber brimful with NaOCl, then additionally dipping the active portion of a NiTi rotary file in a viscous chelator. Lubricating agents reduce friction and promote the maintenance of the glide path for rotary files to follow.

Directions for Use

Figure 16a. Furcated teeth have internal triangles of dentin that should be removed during the initial stages of treatment to facilitate shaping procedures. Figure 16b. The ProTaper SX file is used in a brushing motion to cut dentin and safely relocate canals away from external root concavities.

Each brand of instruments has its own specific directions for use. These guidelines are intended to minimize clinical breakdowns, such as a broken file. Typically, the work of a rotary NiTi instrument can continue as long as the file will passively feed deeper into the canal. When a file will not passively advance, then it should be immediately withdrawn, and the clinician should recognize that there may be insufficient canal diameter, intracanal debris, intrablade debris, or difficult anatomy. In addition, a rotary file may not be able to passively advance deeper into a canal if the distal extent of the file is too stiff to follow the anatomical configuration of the canal. One of the critical distinctions of the NiTi ProTaper system is that the three shaping files (ie, SX, S1, and S2) were designed to rapidly cut dentin using a deliberate brushing action on the withdrawal stroke.4,6,9 Specifically, the ProTaper technique is to let the shaping files passively float int

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