endodontics Editor: SAMUEL
SELTZER,
DDS
Department of Endodontology School of Dentistry Temple University 3223 North Broad Sfreet Philadelphia, Pennsylvania 19140
Another look at root canal instrumentation William L. Wildey, DDS, BS,a E. Steve Senia, DDS, MS, BS, FACD,b and Steve Montgomery, DDS, FACD,” San Antonio, Tex. THE
UNIVERSITY
OF TEXAS
HEALTH
SCIENCE
CENTER
AT SAN
ANTONIO
Several aspects of root canal instrumentation need additional research. Various factors must be considered in an analysis of instrumentation of the root canal system: the dentin that is cut; the technique used to cut it; the design of the instruments; the material and manufacturing process used to make the instruments; the irrigant used during the procedure; and the anatomic configuration of the root canal system. An analysis of these factors clearly indicates that existing root canal instruments and techniques are less than ideal and, in fact, do not accomplish what is expected of them. Root canals must be properly, but, at the same time, destructive and unnecessary iremoval of dentin should be kept to a minimurn. The Flex-R and Canal Master instruments were developed to address some of the shortcomings of existing instruments and techniques. More scientifically based research is needed to fully evaluate these new instruments and techniques and to develop future instruments, (OR&LSURCORALMEDORALPATHOL~~~~;~~:~~~-~O~)
redictably successful endodontic treatment reP quires accurate diagnosis and proper cleaning, shaping, and obturation of the root canal system. Although this may not always be attainable, striving for the ideal should be the goal of every endodontic professional. The ideal objective of instrumentation is to clean and shape canals with minimal dentin removal and transportation. In this context, transportation means the undesirable, unwanted eccentric and excessive removal of root dentin surrounding the original canal. However, this objective is easier said than done because the mechanics involved are complex, especially in curved canals. Unfortunately, existing root canal instruments may produce undesirable effects such as ledges, apical zips, and strip perforations.
7/15/40259
These occur because of aggressive tips on instruments, indiscriminate cutting by the 16 mm cutting blade, and the rapid increase in rigidity with each increase in instrument size. In an analysis of root canal instrumentation, at least six factors should be considered: 1. The material that is cut (dentin) 2. The anatomic configuration of the root canal (straight, curved, smooth, or rough) 3. The material of the cutting instrument (stainless steel or nickel-titanium) 4. The manufacturing process used to make endodontic instruments (grinding or twisting) 5. The design of the instrument (file, reamer) and the technique used (file or ream) 6. The fluid used during the procedure (lubricant or chelating agent) The purpose of this article is to amplify these six factors with emphasis on the mechanics involved in
499
500
Wildey,
Senia,
and Montgomery
ORALSURGORAL
?dEDoRALPATHOL
October1992
Fig. 3. Optimum tip configurationlieswithin lined area. Exact shapedependson size and rigidity of instrument. MATERIAL THAT IS CUT-DENTI GRADUAL
CURVE
ABRUPT
CURVE
Fig. 1. Instrumentexertslessforce againstdentin wail in gradual curve than abrupt one (large arrows indicate greater force than small arrows).
............................................................................................................... ......................................................... .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . .....
Standard
.......................................................................................................................... .................................................................................................................... .......................................................... ‘.:.:.~.......:.:.:.:.:.:.:.:.....:.:.:.:.~~:.:.~.:.:.:.:.:.:.:...:.:.:.:.:.:.:...:.....:.:.:
tip
Dentin is a hard material that is difficult to cut. Once cut, it must be removed. Fortunately, the dentin chips that are cut are small, which makes removal easier. If removal is not adequate, clogging of the cutting blades and eventually the canal itself will occur. Clogging reduces cutting efficiency and increases stress on the instruments. ANATOMIC CONFIGURATION OF THE ROOT CANAL-STRAIGHT, CURVED, SMOOTH, ROUGH
liritiiiiiri~~~~~~~~~~~~~~:~ .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . . . . . . . . . . . . . . . . . . . .
.............
:.:
..
The anatomy of the root canal is so diverse that each canal is unique. Canal curvature is variable and so is the extent of canal wall surface roughness. Some canals are minimally curved, whereas others are moderately to severely curved. Also, and perhaps more importantly, curves may be gradual or abrupt. During instrumentation, the more abrupt the curve is, the more concentrated the forces against the dentin wall will be (Fig. 1). Also some root canals curve in more than one direction at the same time. For example, the mesiobuccal canal of a mandibular molar frequently curves both distally and lingually, and 38% of the lingual curves are equal to or greater than the distal curve. 1 ..................................................................................... .................................................................................... ..................................................................................... .................................................................................................................. .................................................................................................................. ..............................................................................
. .. ,
‘~~~Canal
Master tiPi~~~~~ :.~:.:.:.~~:.:.:.:.~:.:.~.~ ...................... ;‘....................... .............. ............... ................................................ ................................................ ............................................... .‘.~.‘.‘.~.~.‘i.‘.‘.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~ .......................... .................................................... ..> ........................
Fig. 2. Three tip configurationsusedon root canal instruments.Standard tip (arrows) cuts more aggressively than modified(Flex-R-file) one. Canal Master is leastaggressive. cleaning and shaping the root canal system. Another purpose is to encourage new instrument design and techniques basedon research that is focused on these six factors.
MATERIAL OF THE CUTTING INSTRUMENT-STAINLESS STEEL
The material from which root canal instruments are made is critical. The material must be hard enough to hold a sharp edge but flexible enough to follow curves and resist breakage. Stainless steel is commonly used today. The percentage of carbon is the most important factor relative to the physical properties of stainless steel, although other materials such as silicon and manganese are also present. Stainless steel can be made hard enough to scratch glass or soft enough that it can be easily scratched. Whereas there are other materials available with better qualities,
cost becomes a significant factor in the manufacturing of root canal instruments. Nickel-titanium is now
Volume 74 Number 4
Another Hedstrom
look at root canal instrumentation
501
K Type
Fig. 4. In curved canal, rounded noncutting tip of Hedstrom file may advance beyond small obstruction (A) but not large one (B). K-type tip can advance beyond large obstructions with gradual slope (C) because of its inclined plane and cutting ability. K-type tip may not advance beyond large obstruction with abrupt slope (D) because its inclined plane and cutting ability cannot be put into function. Arrow indicates force vector from canal curve.
being used for some endodontic instruments because of its flexibility. Claims of longer usage and less breakage have yet to be studied. MANUFACTURING PROCESS USED TO MAKE ENDODONTIC INSTRUMENTS-GRINDING OR TWISTING
The physical properties of the stainless steel used to make endodontic instruments can easily be changed by the manufacturing process. For example, minimum grinding without significant temperature changes produces minor strains in the metal simply as a result of physically working the metal. Considerable grinding causes intense localized heat, which is rapidly absorbed by the surrounding metal and coolant. With expansion and contraction caused by the rapid heating and cooling, severe strains are produced in the metal. Under some grinding conditions, particularly when the design involves sharp edges, even higher localized temperatures are produced and greater strains in the metal result. When a deep cut is made, very small cracks called grinding cracks form on the surface of the metal. These grinding cracks are the result of greater temperature changes in addition to the irregular physical disruption of the metal itself. In all of the above events, it may or may not be helpful to relieve these strains and cracks by retempering the metal (reheating and controlled cooling) after the grinding operation. Retempering of the metal will change the properties of the metal.2 The object of retempering is to
make the steel better suited, structurally or physically, for the specific application. These grinding effects also occur when instruments are made by twisting because they are first ground to obtain their basic cross sectional shapes-triangular or square. Subsequent twisting will add additional strains to the metal (work hardening). INSTRUMENT REAMER-AND USED-FILING TIP DESIGN
DESIGN-FILE OR THE TECHNIQUES OR REAMING
Both the design of a root canal instrument and the way it is used are important. The critical areas of the file or reamer are the tip and the cutting portion or blade. The tips of three current instruments are shown in Fig. 2. The standard tip can be too aggressive because the first flute makes the initial cut in canal transportation.3 The modified (Flex-R file) and Canal Master tips are less aggressive. The ideal configuration of the tip probably should vary according to the size and rigidity of each instrument. The ideal tip should be within the borders of the lined area in Fig. 3. The design of the tip has a bearing on whether the instrument can reach the foramen, especially in a curved canal. For example, there are times when a no. 15 Hedstrom will not advance to the foramen, but a K-type will. This is because the tip of a Hedstrom is essentially rounded and noncutting, whereas the K-type tip has both an inclined plane and cutting ability (Fig. 4). Therefore when these instruments are
502
Wildey, Senia, and Montgomery
ORAL
SURG
ORAL
MED
O~LPATHOL
October 1992 There are instances when even a K-type instrument
will not advance because of an exaggerated roughness or abrupt curve (Fig. 4, 0). In this case, prebending the instrument
Force of instrumen
may allow
it to advance apically
beyond this point. There are also situations when no existing instrument can reach the apical foramen. Another problem may arise from a rounded noncutting tip design. If the canal diameter is smaller than the diameter of the rounded noncutting tip, burnishing of the canal wall will occur rather than cutting. This will place an increased load on the instrument and limit its ability to advance apically.
C ............................ :::;:: ..: ;~::~:.: .....::......:Li::.:.;: i.);. ........... .y.,.:> :,I.:.;;.I......... ::.:,:.~.:(y.~:~: $.;;;i.y;,:: .... ........ ..x.......... .......... :~:: .......... ...... ........ ................ ........... ........ .................... :.::; ..~~~ :,~: ,,,,) ‘.““.::‘I-::.:‘.:::.~.::: ;:.:~ ........................... .... ..~..i ..................... . .......... ..~ ... :::.:,, ..... ............................................ ,,, y:.:::i;:...~.:::.~ ..................... . ......... ..~.~....~. > ............... ‘.~.~.~:~~~~:,~~iii:.;,:~,, ):{.;y; .... ... . .;,:,.,:,,: ,.,:,.,:., :~, ::,; :’ ..;... ................ ;y,; I,: ..... ........................ ..:...................... ............................. CUTTING BLADES ..~.i..... .:. .....y......... :.:;I;......... ~..~..:.....~: ............... ::...:.:.:::...:.:.::.:....~.:..~: i............ ~:.‘:.“:::~:~::.: .......................................... ...... ::;$
SELTZER,
DDS
Department of Endodontology School of Dentistry Temple University 3223 North Broad Sfreet Philadelphia, Pennsylvania 19140
Another look at root canal instrumentation William L. Wildey, DDS, BS,a E. Steve Senia, DDS, MS, BS, FACD,b and Steve Montgomery, DDS, FACD,” San Antonio, Tex. THE
UNIVERSITY
OF TEXAS
HEALTH
SCIENCE
CENTER
AT SAN
ANTONIO
Several aspects of root canal instrumentation need additional research. Various factors must be considered in an analysis of instrumentation of the root canal system: the dentin that is cut; the technique used to cut it; the design of the instruments; the material and manufacturing process used to make the instruments; the irrigant used during the procedure; and the anatomic configuration of the root canal system. An analysis of these factors clearly indicates that existing root canal instruments and techniques are less than ideal and, in fact, do not accomplish what is expected of them. Root canals must be properly, but, at the same time, destructive and unnecessary iremoval of dentin should be kept to a minimurn. The Flex-R and Canal Master instruments were developed to address some of the shortcomings of existing instruments and techniques. More scientifically based research is needed to fully evaluate these new instruments and techniques and to develop future instruments, (OR&LSURCORALMEDORALPATHOL~~~~;~~:~~~-~O~)
redictably successful endodontic treatment reP quires accurate diagnosis and proper cleaning, shaping, and obturation of the root canal system. Although this may not always be attainable, striving for the ideal should be the goal of every endodontic professional. The ideal objective of instrumentation is to clean and shape canals with minimal dentin removal and transportation. In this context, transportation means the undesirable, unwanted eccentric and excessive removal of root dentin surrounding the original canal. However, this objective is easier said than done because the mechanics involved are complex, especially in curved canals. Unfortunately, existing root canal instruments may produce undesirable effects such as ledges, apical zips, and strip perforations.
7/15/40259
These occur because of aggressive tips on instruments, indiscriminate cutting by the 16 mm cutting blade, and the rapid increase in rigidity with each increase in instrument size. In an analysis of root canal instrumentation, at least six factors should be considered: 1. The material that is cut (dentin) 2. The anatomic configuration of the root canal (straight, curved, smooth, or rough) 3. The material of the cutting instrument (stainless steel or nickel-titanium) 4. The manufacturing process used to make endodontic instruments (grinding or twisting) 5. The design of the instrument (file, reamer) and the technique used (file or ream) 6. The fluid used during the procedure (lubricant or chelating agent) The purpose of this article is to amplify these six factors with emphasis on the mechanics involved in
499
500
Wildey,
Senia,
and Montgomery
ORALSURGORAL
?dEDoRALPATHOL
October1992
Fig. 3. Optimum tip configurationlieswithin lined area. Exact shapedependson size and rigidity of instrument. MATERIAL THAT IS CUT-DENTI GRADUAL
CURVE
ABRUPT
CURVE
Fig. 1. Instrumentexertslessforce againstdentin wail in gradual curve than abrupt one (large arrows indicate greater force than small arrows).
............................................................................................................... ......................................................... .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . .....
Standard
.......................................................................................................................... .................................................................................................................... .......................................................... ‘.:.:.~.......:.:.:.:.:.:.:.:.....:.:.:.:.~~:.:.~.:.:.:.:.:.:.:...:.:.:.:.:.:.:...:.....:.:.:
tip
Dentin is a hard material that is difficult to cut. Once cut, it must be removed. Fortunately, the dentin chips that are cut are small, which makes removal easier. If removal is not adequate, clogging of the cutting blades and eventually the canal itself will occur. Clogging reduces cutting efficiency and increases stress on the instruments. ANATOMIC CONFIGURATION OF THE ROOT CANAL-STRAIGHT, CURVED, SMOOTH, ROUGH
liritiiiiiri~~~~~~~~~~~~~~:~ .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . . . . . . . . . . . . . . . . . . . .
.............
:.:
..
The anatomy of the root canal is so diverse that each canal is unique. Canal curvature is variable and so is the extent of canal wall surface roughness. Some canals are minimally curved, whereas others are moderately to severely curved. Also, and perhaps more importantly, curves may be gradual or abrupt. During instrumentation, the more abrupt the curve is, the more concentrated the forces against the dentin wall will be (Fig. 1). Also some root canals curve in more than one direction at the same time. For example, the mesiobuccal canal of a mandibular molar frequently curves both distally and lingually, and 38% of the lingual curves are equal to or greater than the distal curve. 1 ..................................................................................... .................................................................................... ..................................................................................... .................................................................................................................. .................................................................................................................. ..............................................................................
. .. ,
‘~~~Canal
Master tiPi~~~~~ :.~:.:.:.~~:.:.:.:.~:.:.~.~ ...................... ;‘....................... .............. ............... ................................................ ................................................ ............................................... .‘.~.‘.‘.~.~.‘i.‘.‘.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~ .......................... .................................................... ..> ........................
Fig. 2. Three tip configurationsusedon root canal instruments.Standard tip (arrows) cuts more aggressively than modified(Flex-R-file) one. Canal Master is leastaggressive. cleaning and shaping the root canal system. Another purpose is to encourage new instrument design and techniques basedon research that is focused on these six factors.
MATERIAL OF THE CUTTING INSTRUMENT-STAINLESS STEEL
The material from which root canal instruments are made is critical. The material must be hard enough to hold a sharp edge but flexible enough to follow curves and resist breakage. Stainless steel is commonly used today. The percentage of carbon is the most important factor relative to the physical properties of stainless steel, although other materials such as silicon and manganese are also present. Stainless steel can be made hard enough to scratch glass or soft enough that it can be easily scratched. Whereas there are other materials available with better qualities,
cost becomes a significant factor in the manufacturing of root canal instruments. Nickel-titanium is now
Volume 74 Number 4
Another Hedstrom
look at root canal instrumentation
501
K Type
Fig. 4. In curved canal, rounded noncutting tip of Hedstrom file may advance beyond small obstruction (A) but not large one (B). K-type tip can advance beyond large obstructions with gradual slope (C) because of its inclined plane and cutting ability. K-type tip may not advance beyond large obstruction with abrupt slope (D) because its inclined plane and cutting ability cannot be put into function. Arrow indicates force vector from canal curve.
being used for some endodontic instruments because of its flexibility. Claims of longer usage and less breakage have yet to be studied. MANUFACTURING PROCESS USED TO MAKE ENDODONTIC INSTRUMENTS-GRINDING OR TWISTING
The physical properties of the stainless steel used to make endodontic instruments can easily be changed by the manufacturing process. For example, minimum grinding without significant temperature changes produces minor strains in the metal simply as a result of physically working the metal. Considerable grinding causes intense localized heat, which is rapidly absorbed by the surrounding metal and coolant. With expansion and contraction caused by the rapid heating and cooling, severe strains are produced in the metal. Under some grinding conditions, particularly when the design involves sharp edges, even higher localized temperatures are produced and greater strains in the metal result. When a deep cut is made, very small cracks called grinding cracks form on the surface of the metal. These grinding cracks are the result of greater temperature changes in addition to the irregular physical disruption of the metal itself. In all of the above events, it may or may not be helpful to relieve these strains and cracks by retempering the metal (reheating and controlled cooling) after the grinding operation. Retempering of the metal will change the properties of the metal.2 The object of retempering is to
make the steel better suited, structurally or physically, for the specific application. These grinding effects also occur when instruments are made by twisting because they are first ground to obtain their basic cross sectional shapes-triangular or square. Subsequent twisting will add additional strains to the metal (work hardening). INSTRUMENT REAMER-AND USED-FILING TIP DESIGN
DESIGN-FILE OR THE TECHNIQUES OR REAMING
Both the design of a root canal instrument and the way it is used are important. The critical areas of the file or reamer are the tip and the cutting portion or blade. The tips of three current instruments are shown in Fig. 2. The standard tip can be too aggressive because the first flute makes the initial cut in canal transportation.3 The modified (Flex-R file) and Canal Master tips are less aggressive. The ideal configuration of the tip probably should vary according to the size and rigidity of each instrument. The ideal tip should be within the borders of the lined area in Fig. 3. The design of the tip has a bearing on whether the instrument can reach the foramen, especially in a curved canal. For example, there are times when a no. 15 Hedstrom will not advance to the foramen, but a K-type will. This is because the tip of a Hedstrom is essentially rounded and noncutting, whereas the K-type tip has both an inclined plane and cutting ability (Fig. 4). Therefore when these instruments are
502
Wildey, Senia, and Montgomery
ORAL
SURG
ORAL
MED
O~LPATHOL
October 1992 There are instances when even a K-type instrument
will not advance because of an exaggerated roughness or abrupt curve (Fig. 4, 0). In this case, prebending the instrument
Force of instrumen
may allow
it to advance apically
beyond this point. There are also situations when no existing instrument can reach the apical foramen. Another problem may arise from a rounded noncutting tip design. If the canal diameter is smaller than the diameter of the rounded noncutting tip, burnishing of the canal wall will occur rather than cutting. This will place an increased load on the instrument and limit its ability to advance apically.
C ............................ :::;:: ..: ;~::~:.: .....::......:Li::.:.;: i.);. ........... .y.,.:> :,I.:.;;.I......... ::.:,:.~.:(y.~:~: $.;;;i.y;,:: .... ........ ..x.......... .......... :~:: .......... ...... ........ ................ ........... ........ .................... :.::; ..~~~ :,~: ,,,,) ‘.““.::‘I-::.:‘.:::.~.::: ;:.:~ ........................... .... ..~..i ..................... . .......... ..~ ... :::.:,, ..... ............................................ ,,, y:.:::i;:...~.:::.~ ..................... . ......... ..~.~....~. > ............... ‘.~.~.~:~~~~:,~~iii:.;,:~,, ):{.;y; .... ... . .;,:,.,:,,: ,.,:,.,:., :~, ::,; :’ ..;... ................ ;y,; I,: ..... ........................ ..:...................... ............................. CUTTING BLADES ..~.i..... .:. .....y......... :.:;I;......... ~..~..:.....~: ............... ::...:.:.:::...:.:.::.:....~.:..~: i............ ~:.‘:.“:::~:~::.: .......................................... ...... ::;$
