Biomechanical Preparation of Root Canal

BIOMECHANICAL PREPARATION OF THE ROOT CANAL INTRODUCTION Predictable success of endodontic treatment requires accurate diagnosis, proper cleaning and shaping and hermetically obturation of the root canal. The technical demands and level of precision required for successful performance of treatment have traditionally been achieved by careful manipulation of instruments within root canal space and by strict adherence to biologic and surgical principle essential for disinfection and healing. Techniques for biomechanical preparation of the root canal differ in accordance with clinical observation and research discoveries. HISTORICAL PERSPECTIVE • Pierre Fauchard (1733), one of the founders of modern dentistry described instruments for trepanation of teeth, preparation of root canals and cauterization of pulp. • Edward Maynard (1838) has been credited with the development of the first endodontic hand instruments. Notching a round wire (in the beginning watch springs, later piano wires) he created small needles for extirpation of pulp tissue. • In 1852 Arthur used small files for root canal enlargement. • Ingle and Levine (1958) listed standardization for instruments. • Ingle (1961) described conventional technique/ standardized taper technique of root canal preparation. Biomechanical preparation The biomechanical preparation of root canal is the attainment of free access to the apical foramen, through the root canal, by mechanical means. The objectives of biomechanical preparation are 1. To cleanse the pulp chamber and root canal of the pulp tissue remnants, bacteria’s and their toxins, altered dentin, foreign debris, caries and saliva. 2. To remove obstruction 3. To enlarge and taper the canal to receive maximum amount of irrigating solution and medicament 4. To smoothen the canal wall in order to improve contact of medicament with the infected canal surface and 5. To prepare apical stop and canal walls so as to facilitate obturation of canal. In order to achieve these objectives, biomechanical preparation of root canal involves three separate procedures i.e. debridement or cleaning, shaping and apical preparation. Cleaning 1

BIOMECHANICAL PREPARATION OF THE ROOT CANAL Cleaning refers to the removal of all contents of the root canal system before and during shaping: organic substrates, microflora, bacterial by products, food ,caries, denticles, pulp stones, dense collagen, previous root canal filling material and dentinal filings resulting from root canal preparation. Objective: The objective of cleaning is to eliminate or at least to minimize existing or potential debris from the canal space. This debris can be tissue remnants, bacteria’s, altered dentin, food debris, caries and saliva. Since it is not always possible to remove all irritants completely, the attempt is made to reduce remaining debris to sub critical level or to seal these irritants within the canal space and prevent their escape into the periapical area. Techniques: It is a combined mechanical and chemical process. It begins with establishment of straight line access, which allows better visualization, control in instrumentation and loosening of debris and irritants. These irritant are further removed physically by flushing action or chemically by dissolution by irrigating solutions. Microorganisms are also destroyed by chemicals and then physically removed by flushing action. Although sound in theory, complete debridement is very difficult and in some cases impossible. Evaluation: Historically, presence of clean dentinal shavings were considered end point for cleaning. It is however, difficult see shavings on file at all. Research has also shown little relationship between clean shavings and quality of debridement. Another in accurate measurement is presence of clean irrigating solution. ‘Glassy smooth’ walls are preferred criteria indicating that dentin has been planed in as many areas as possible. Although not totally accurate, this is best in available lot. Shaping Shaping refers to preparing a specific cavity form with five design objectives. Objective: the purpose of shaping is to enlarge and taper the canal to receive obturating material. The selected obturation technique will thus dictate the approach and outcome of shaping. Techniques: It is a pure mechanical process. It is achieved by reaming and filing the canal by one of the various techniques using hand and /or rotary instruments. Evaluation: Historically, it was evaluated by measuring size of instrument which is first to show binding when inserted upto working length. Then enlargement is done which is two or three sizes beyond the binding instrument with minimum of upto file size no 25. The most accurate method is by using condensing instrument. The attainment of adequate canal enlargement and flaring is determined by the ability of the spreader or plugger to penetrate close to working length. The shaping always facilitates ‘cleaning’ by eliminating pulp, bacteria and their toxins; by removing restrictive dentin, which allows an effective volume of irrigant to work deeper and more quickly to potentially circulate into all aspects of the root canal system. Apical preparation

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BIOMECHANICAL PREPARATION OF THE ROOT CANAL It is critical to prepare an apical matrix or barrier at the proper working length. This barrier has two functions 1. To serve as stop against which obturating material can be condensed 2. To prevent materials, irrigants and instruments from extruding into the periapical tissue. Depending on the root canal morphology variations in apical preparation will occur. Most desirable is an apical stop, then an apical seat, as opposed to open apex. The apical stop is the creation of a complete barrier at the end of the preparation which will confine the materials to the canal. On the other hand, the apical seat configuration is such that there is not a complete barrier but a constriction of walls. PROECEDURAL TERMs: 1) MAF-Master apical file: It is the largest file that binds slightly at correct WL after straight line access. It is determined by passively placing the successively larger files at the C.W.L. until correct size is reached which binds at the tip. The file binding at first or smallest file to bind is initial apical file. 2) Pre curving of instruments: Precurving of stainless steel instruments is mandatory while negotiating curved canals. It is a valuable tool for feeling canal passages and for moving around calcifications, ledges and around curved foramina. It helps to prevent procedural problems and perform adequate shaping in curvatures. Precurving can be done either with cotton or gauze or using commercially available devices utilizing the diagnostic x-ray. 3) Recapitulation: An essential step especially in apical coronal techniques – it means the use of instruments in the correct size sequence smaller to larger and returning to smaller instruments from time to time before advancing to a larger size. E.g. after 15 no. 10mm them proceed to 20, then use 10 and 15 and proceed to 25 and so on. This helps prevent packing of dentinal filings and ensures patency of root canal through to the apical foramen. 4) Anticurvature filing-Filing away from curvatures and danger areas described in detail under curvatures. Basic terms of Motions of instrumentation – BMP is a dynamically delicate motion – flowing, rhythmic and energetic. Various motions involved are: Methods of Cleaning and Shaping Cleaning and shaping are dynamically delicate motions, flowing, rhythmic, and energetic. In order to use files and reamers efficiently, the movements require distinction. There are 6 distinctive motions of files and reamers. 3

BIOMECHANICAL PREPARATION OF THE ROOT CANAL A)

Follow:

Usually performed with files. Are used initially during cleaning and shaping or any time an obstruction blocks the foramen. Irrigating, precurving different kinds of curves, curving all the way to the tip of the instrument and multiple curves in multiple directions of the instrument are all part of follow. A)

Follow-withdraw

Files are used. This motion is used once the foramen has been reached and the next step is to create the path from access cavity to foramen. The motion is follow, then withdraw or “follow and pull” or “follow and remove”. It is simply an in – and – out passive motion that makes no attempt to shape the canal. B)

Cart

Refers to the extension of a reamer to or near the radiographic terminus. The reamer should gently and randomly touch the dentinal walls and “cart” away debris. C)

Carve Reamers are used for shaping. The key is not to press the instrument apically but simply to touch the dentin with a precurved reamer and shape on withdrawal randomly.

D)

Smooth Is accomplished with files. In the past, most endo procedures were performed with a smoothing or circumferential filing motion. If the previous four motions are followed smoothing is rarely required.

E)

Patency Is achieved with files/ reamers. It means that the portal of exit has been cleared of any debris in the path. Also included are 2 other terms given by Ruddle-Gauging and Tuning. Gauging refers to the knowing the cross sectional diameter of the foramen that is confirmed by the size of the instrument that “snugs in” at working length.

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BIOMECHANICAL PREPARATION OF THE ROOT CANAL Tuning is ensuring that each sequentially larger instrument uniformly backs out of the canal by 0.5 mm. Also included is scouting that refers to using instruments to gauge and estimate the root canal anatomy, form and variations and is same as follow. Motion of instrumentation / envelopes of motion: A) Filing: Indicates a push-pull motion of the instrument. The inward passage is powered by hand and file rigidity. Cutting is done during withdrawal or pull stroke. Done using files and usually in circumferential manner. B) Reaming Indicates clockwise / right-hand rotation of an instrument. The instrument must be restrained from insertion to generate a cutting effect. Instrument # is increased when this motion is employed. It is a rotating-pushing motion limited to a quarter to half turn. C) Turn-and-pull(Combination) Is a combination of reaming and filling, the file is inserted with a ¼ turn clockwise and inwardly directed hand pressure (i.e. reaming) positioned into the canal by this action, the file is subsequently withdrawn (i.e. filling). The rotation during placement sets the cutting edges of the file into dentin and the non-rotating withdrawal breaks local the dentin that has been engaged. Disadvantages: Hourglass canal shapes were observed by Weine. According to Schilder Clockwise rotation of a half-revolution followed by withdrawal. The file is not inserted towards the apex, rather, he gradually allows the preparation to progress out of the canal. ¼ turn to right followed by straight pull out

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BIOMECHANICAL PREPARATION OF THE ROOT CANAL D) Watch-winding Is the back-and-forth oscillation of a file (30-60°) right and left as the instrument is pushed into the canal. It is an expanded use of the “Vaiven” technique described by Ingle. This back and forth motion can be combined with a pull stroke and effectively planes walls. It has various advantages like canal centering, not necessitating precurving and balancing tooth structure cutting with instrument mechanics. This back-and-forth movement causes the files and reamers to plane the walls efficiently. In a way, this is a predecessor to the balanced force technique, as the 30-60° of clockwise rotation pushes the file tip and working edges into the canal and the 30-60° of counter clockwise motion partially cuts away the engaged dentin. E) Watch-winding and pull When used with H-files, watch winding cannot cut dentin with the backstroke. It can only wiggle and wedge the edges tightly into the wall. With each clockwise turn, the instrument moves apically until it meets resistance and must be freed with a pull stroke.

Rules governing biomechanical instrumentation During biomechanical preparation following rules should be followed. 1. Direct access should be obtained along straight lines. 2. Smooth instruments should precede barbed broach and rough instrument. 3. Narrow instruments should precede wide ones in sequential manner. 4. Reamer should precede files and should be given only ¼ to ½ turn at a time. 5. Files should be used with a pull stroke. 6. Instrument should be fitted with instrument stops. 7. Short handle instruments should be preferentially used in the posterior teeth and lower anteriors ; long handle instrument in maxillary anterior teeth 6

BIOMECHANICAL PREPARATION OF THE ROOT CANAL 8. No instrument should be forced when it binds. 9. the canal should be enlarged at least three sizes greater than the original size. 10. Instrument should be confined to root canal so as not to traumatize apical tissue. 11. Debris should not be forced through the apical foramen 12. All instrumentation should be done in wet canal. Schidler in 1967 gave concept of shaping and cleaning and gave five mechanical objectives and four biologic objectives for this procedure. Schilder’s mechanical objectives 1. Develop continually tapering conical form. This allows irrigants and instruments to clean all parts of the canal. By such shape hydraulic principle can operate by the restricted flow principle. So, during compaction as flow is restricted, the gutta percha and sealer take the path of least resistance; namely apical and lateral foramina. 2. Make the canal narrower apically, with the narrowest cross sectional diameter at its terminus. This allows creating control and compaction at every level of preparation., by harmonizing with thermo mechanical properties of gutta percha. Only exception to this objective is internal resorption. 3. Make the preparation in multiple planes. This allows in preserving natural curve or flow even in curved canals. 4. Never transport the foramen. This prevents delicate foramina from getting lost due to improper sequence of instrumentation. 5. Keep the apical foramen as small as possible. This prevents tearing of foramina, decrease contact of obturating material with periapical tissue and decrease in microleakage around margins. Schilder’s biologic objectives 1. Confinement of instrumentation to the roots themselves. 2. No forcing of necrotic debris beyond the foramen. 3. Removal of all tissue from the root canal space. 4. Creation of sufficient space for intra-canal medicaments. A. BIOMECHANICAL METHODS Techniques of Hand Instrumentation The various techniques have been proposed in cleaning and shaping of canals over the years.

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BIOMECHANICAL PREPARATION OF THE ROOT CANAL These were different techniques according to type of instrument movements, type and direction of instrumentation. Techniques according to type of Instrument movements 1. Circumferential filing technique 2. Balanced force technique. Techniques according to type and direction of instrumentation These can be divided firstly into two types depending upon direction of instruments and further by type of instrumentation. I. Apico - coronal methods 1. Conventional technique/ standardized taper technique. 2. Step back technique. 3. Ohio state technique 4. Southern California technique 5. Passive step back technique. 6. Progressive enlargement technique. 7. Incremental technique. 8. Anti curvature filing technique. 9. Reverse Flaring Technique II. Corono – apical methods 1. Step down technique. 2. Crown down pressure less technique. 3. Double flared technique. 4. Modified double flared technique. 5. Alternated rotary motion technique.

Techniques according to type of Instrument movements 1. Circumferential filing technique It was given by Lim and stock in 1987. It is a movement of file around the circumference of canal while moving it in small vertical movements of 1to 3 mm amplitude. It is a method of filing whereby K or H file is first placed on the buccal side of the canal, then reinserted and placed mesially, then lingually and then distally until all walls have received planing. It is used for enhancing flaring, in oval root canals. Advantages: • It maintains spatial relationship of root canal in root. • It achieves rapid cutting of dentin. 2. Balanced force technique. This technique was introduced by Roane & Sabala in 1985. It was originally associated with specially designed stainless-steel with non cutting tip or Ni-Ti K-type 8

BIOMECHANICAL PREPARATION OF THE ROOT CANAL instruments (Flex-R-Files) with modified tips in a step down manner. It is used when there is no curvature or generalized curvature in canal. The coronal and middle third are flared with Gates Glidden drills to improve radicular access. Flex R file is introduced into the root canal with a clockwise motion of 900 or less with slight apical pressure for apical advancement (placement phase), followed by counterclockwise rotation of 1200 or greater, with adequate apical pressure (cutting phase).This motion is repeated and instrument is advanced toward the working length at shallow steps. When working length is obtained, the instrument is freed by one or more counter clockwise rotations made while the depth is held constant. The file is removed with a slow clockwise rotation to load debris into flutes and is withdrawn from the root canal. Apical preparation is recommended to larger sizes than with other manual techniques, e.g., 1mm away from radiographic apex to size no 80 in straight canals and no 45 in curved canals. Roane and Sabala believed in carrying preparation to radiographic apex of teeth and purposely shape the foraminal area. This apical control zone of 1mm was prepared in step back manner in increment of 0.5 mm. Advantages: • Good apical control of the file tip as the instrument does not cut over the complete length, • Good centring of the instrument because of the non-cutting safety tip thus preventing transportation. • In preparation of curved canals occurs without or with only minimal straightening which was superior to step back technique. • No need to pre-curve the instrument • Less amount of apically extrusion of debris • Cleanliness was rated superior compared with the crown down pressure less and step back techniques. Disadvantages: • High incidence of procedural problems such as instrument fracture. It was reported because of overzealous clockwise rotation on insertion. • Varying results were reported for the amount of dentine removed in apical 1mm in curved canals. • The Balanced force technique required more working time Techniques according to type and direction of instrumentation I. Apico- coronal methods 1. Standardized taper technique Ingle (1961) described the first formal root canal preparation technique, which has become known as the ‘standardized technique’. This is a classical traditional technique. In it attempt is made to shape canal to the same size, taper and configuration of standardized instruments. In it one under copious irrigation, uses 9

BIOMECHANICAL PREPARATION OF THE ROOT CANAL successively larger instruments, each to the working length, starting with small sizes and enlarging, using combinations of reaming and circumferential filing. In straight canals enlargement was done till three sizes more than initial file which binds. However, in curved and narrow canals it was kept at maximum of file no 25. If canal cannot be prepared by reaming throughout its length, the remain canal is filed circumferentially and apical part (3 to 4 mm) is left as it was prepared into round and slightly tapering shape to receive initial canal material. The copious irrigation and recapitulation is performed to prevent clogging of apical part. The shape of final preparation resembles the “Washington Monument”, the silhouette of obelisk, tapering to end in a pyramidal shape with apex showing 750 point of the instrument. This technique was designed for single-cone filling techniques. Disadvantages: • This technique has shown accumulation of dentinal debris at apical area, • Formation of less defined taper in the preparation, • Occurrence of apical transportation and procedural errors. • Studies have shown that a standardized shape cannot be formed in curved canals. 2.

Step back technique It was first described by Clem (1969) and Martin (1974). It was also known as telescoping technique as final preparation resembles open telescope, its size increasing, section by section from apex to chamber. The other name for it was serializing technique. This method results in more defined taper and smaller apical preparation. It is divided in three phases by Mullaney (1979) Phase I Establishing apical stop and preparing the apical section Phase II Stepping back and cleansing the preparation A. Refinement with Gates glidden drills B. Apical stepping refinement After establishing straight line access and patency of canal, small K file (no 10/15) is used to determine working length. Under copious amount of irrigation, most apical portion of the canal is prepared with reaming action. This is done at only or two or three sizes larger than the first file which binds minimum of file size 25. When preparing curved canal at the apex, no 25 file is the maximum size used. With straight or slightly curved canals this size could be increased. If canal is larger than file size 25 at the apex, do not enlarge apical region more than the first file that shows the binding. The instrument last used to prepare the apical region is known as the master apical file (MAF). Once apical preparation is complete, the step back procedure begins. The taper is created by shortening the working length of each successively larger instrument by 1 mm using peripheral filing as opposed to reaming .The Walton et al (1976) suggested step back to be performed by shortening by 0.5mm as it allows for 10

BIOMECHANICAL PREPARATION OF THE ROOT CANAL preparation that allows maximum insertion of spreader. With the circumferential filing, the tip of the file is pressed against all walls of the canal on the outstroke. The successive increase in instrument size and the decreasing length of the file will create the step back taper. Generally, stepping back is continued to at least size of a no 60 or no 70 file. This generally gives adequate debridement as well as sufficient taper. Large canals will require flaring to larger sizes. Recapitulation is critical and necessary to maximize debridement and minimize procedural errors. After each step back file, return to full canal length with the instrument smaller than the master apical file. Spin and wriggle file to carefully loosen the debris and dentin chips that invariably become packed into the apical region of the canal. Frequent use of irrigating solution in copious amounts is also critical for debridement. Following each file size and after recapitulation, used at least 2 ml of irrigant to flush the loosened debris from the canal. To prevent forcing debris and irrigating solution beyond the apex, it is mandatory to inject with minimal pressure and to prevent binding of needle to canal walls. Completion of preparation is next step which consists of enlarging and cleaning the canal in the mid root and coronal areas and smoothening of canal walls. It has two components 1. Coronal and mid root preparation 2. Apical stepping refinement Then there is final irrigation and canal is dried with paper points. Dry filing is last step which is done using last size file used to the working length. The file is spun carefully to the length to remove dentin chips that have become packed during drying. Dry filing are not indicated if there is no apical stop, as this step will remove dentin chips which have formed a partial plug or seal or may actually open the apex further. The one of the modification of step back preparation advocate step back to be started 2 to 3 mm above working length after apical preparation is completed. This gives a short almost parallel retention form to receive filling material which shows better tug back. According to Lim (1985) step back is considered best in preparing curved root canal and is considered as better method to prevent complications such as formation of ledges, perforations. Some dentists believe that apex should be enlarged to minimum of a no 40 file regardless of canal curvature. The techniques given to achieve this were Ohio state technique and Southern California technique. 3.

Ohio state technique After establishing straight line access and patency of canal, small K file (no 10/15) is used to determine working length. Under copious amount of irrigation, most apical portion of the canal is prepared with reaming action. The enlargement of the 11

BIOMECHANICAL PREPARATION OF THE ROOT CANAL apex is done to a no 25 file size. Next no 2 Gates Glidden drill is used to flare coronal two thirds of the canal. This allows no 30 and no 35 file to be worked till working length. Then no 3 gates Glidden drill is used to flare coronal part to allow no 40 file to reach original working length. The final taper is created by using step back from the no 40 to no 70 size files. Disadvantage: • This produces greater apical deviations like ledges and zip. 4. Southern California technique This method consists of filing and enlarging the apex to a size no 40, using a procedure which applies mesial pressure is used on all files. This enlarges preparation towards mesial and, which tend to straighten the curvature of the original canal toward the greatest amount of structure in the curved roots of mandibular and maxillary molars. The final taper is created by using step back from the no 40 to 70 size files. Disadvantage: • This produces greater apical deviations like ledges and zip. • It involves cutting of excessive crown structure. 5. Passive Step back technique It was given by Torabinejad 1994. It is combination of hand instruments and rotary instruments as Gates Glidden drill and Peeso reamer to achieve adequate coronal flare before apical preparation. After establishing straight line access and patency of canal, small K file is used to determine working length. Under copious amount of irrigation, most apical portion of the canal is prepared with small file of no 10 /15 with very light 1/8 to 1/4 turn and push pull strokes to establish path to foramen with little or no resistance. With same motion subsequent larger files (no 20,25,30,35 and 40) are carried into the canal as far as they can be inserted passively. After their passage canal is irrigated with NaOCl. This step provides insight to internal canal anatomy, removes debris and create mildly flared canal for inserting Gates Glidden drill. Now Gates Glidden drill no 2 is inserted to point were it binds slightly then is pulled back by 1 to 1.5 mm and is used in up and down motion with light pressure. Similarly, Gates Glidden drill no 3 and 4 are used. Working length and patency is reestablished. The apical portion of the canal is then prepared with reaming action. This is done at only or two or three sizes larger than the first file which binds minimum of file size 25. When preparing curved canal at the apex, no 25 file is the maximum size used. With straight or slightly curved canals this size could be increased. The remaining root canal is prepared in step back with frequent recapitulation with files in circumferential filing method. Then apical stepping refinement is done with last apical file used in circumferential manner, followed by reestablishing patency and final irrigation. There is one modified version of technique where sequential use of hand and ultrasonic instruments for removal of coronal dentin is done. 12

BIOMECHANICAL PREPARATION OF THE ROOT CANAL After establishing path to foramen with little or no resistance with passive use of file till no 40, a size no 15 endosonic file is used. It is inserted to point of minimal resistance and is pulled back by 1mm and then is activated for 30 seconds. It is moved in up and down manner circumferentially. This is done to remove debris. Then similar normal technique, Gates Glidden drill no 2 and 3 are used passively to achieve straight line access. Working length and patency is reestablished. The apical portion of the canal is then prepared with reaming action. This is done at only or two or three sizes larger than the first file which binds minimum of file size 25. When preparing curved canal at the apex, no 25 file is the maximum size used. With straight or slightly curved canals this size could be increased. The remaining root canal is prepared in step back with frequent recapitulation with files in circumferential filing method. The refinement is now done by no 15 endosonic file 1 to 2 mm short of working length. According to Cameron and Archer et al (1994) this technique provides significantly cleaner canals and reduces procedural errors when compared to conventional step back technique. 6.

Progressive enlargement technique It was introduced by Oswald R J et al in 1987. It is a type of step back which is taught at University of Washington. After establishing straight line access and patency of canal, small K file (no 8/10) is used to determine working length. They are moved with 1/8 to 1/4 counterclockwise motions with apical pressure. The enlargement of canal is done in three phases: apical preparation, progressive filing and coronal 2/3 filing. In apical preparation, first file (no 15) which binds at working length is used in push pull and circumferential filing method. Then no 20 file is placed 0.5mm of working length and used similarly manner. The Gates Glidden drill no 2/3 are used for coronal preflaring. Then apical patency is confirmed. Pre curved file (no 15/20) is now worked apically in watch winding motion with light pressure in apical direction. When it reaches working length, it is withdrawn by several millimeters and then worked to length and withdrawn. This process is repeated till file can easily slide without watch winding motion. This is done at only or two or three sizes larger than the first file which binds minimum of file size 25 in straight canal and no 20 in curved canal. Now progressive filing phase starts, file one size larger to last file used in apical preparation is taken and is worked in same manner as above to length 0.5 m short of working length. Now files are used in step back manner at 0.5 mm increments, with subsequent recapitulations. The coronal two third is prepared with H file having same size as last file in progressive filing in circumferential filing technique. Advantages: • Reduces procedural errors.

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BIOMECHANICAL PREPARATION OF THE ROOT CANAL 7.

Incremental technique It was given by Weine et al in 1976. It was based on assumption that in severely curved or calcified canals the increment of 0.05mm between file sizes is too great which can stress instrument and lead to procedural errors. This technique is similar to step back and only vary in that, the serialization is done by new increments of files between the established width which are made by clipping established width files by 0.5, 1 or 2 mm. Standardized instrument show increase in diameter by 0.02mm/mm from D0 to D16. Therefore if 1mm is clipped from no 10 file it becomes no 12. He advocated in difficult cases the dentist should routinely trim size 10, 15, 20 and 25 to sizes 12, 17, 22 and 27. The cut tip is smoothened and reestablished with proper transitional angle by flat tip, a metal nail file.

8. Anti curvature technique It was introduced by Abou - Rass et al in1980. It was advocated for smaller curved canals with concavities on root surface so as to prevent strip perforation. Such canals are mostly seen on mesiobuccal canal of maxillary molar, mesial root of mandibular molar and mandibular incisors. In this method, access preparation is modified to achieve unobstructed approach h to canal as much as possible and then pressure is applied on instrument so that shaping will occur away from inside of the root curvature in coronal and middle third of canal i.e in bulky /safety zones. This pressure is applied as file is pulled from the root canal. Advantages: Prevent mid curvature straightening which prevents strip perforation in furcation area. 9. Reverse Flaring Technique It was given by Weine. In it minimal filing is done at tip, followed out by enlarge of coronal portion by 0.004/0.006 taper files or ultrasonic files or Gate Glidden drills or Peeso reamers. Then apical preparation is completed and finally apical flaring is done. It is used for curved canals mostly Advantages; • Remove dentin on orifice of canal, thus providing straight line access. • Prevents apical extrusion of debris. • Less amount of procedural errors as zips and perforations. II. Corono – apical methods 1. Step down technique. It was given by Marshall and Papin (1980) and Goerig et al (1982). After establishing straight line access and patency of canal, straight K file (no 35) is placed in straight portion canal with light apical pressure to the point where it 14

BIOMECHANICAL PREPARATION OF THE ROOT CANAL meets resistance (known as radicular access length). Now, Gates Glidden drill (2 and 3) is used to flare the canal. Now file no 30 is used in reaming motion from flared part of canal with apical pressure. The introduction of sequentially smaller size of files to a 1mm greater depth is done till provisional working length is reached which is 3mm short of radiographic apex. True working length is established on basis of the provisional working length radiographically. The introduction of sequentially smaller size of files to a 1mm greater depth is done till true working length, 1m short of radiographic apex, is reached. File which was one size larger than file last used from provisional working length is taken. is now used at provisional working length level and similar introduction of smaller size of files sequentially to a 1mm greater depth is done till true working length is reached. This is done till sufficient apical preparation is done. Advantages: • Good apical control of the file tip as the instrument does not cut over the complete length. • No need to pre-curve the instrument • Less amount of apically extrusion of debris. • Less amount of procedural errors as zips and perforations 2. Crown down pressure less technique. It was given by Morgen and Montgomery in 1984. After establishing straight line access and patency of canal, straight K file (no 35) is placed in straight portion canal with outapical pressure to the point where it meets resistance (known as radicular access length). Using this as depth guide files of larger sizes or Gates Glidden drill (2 and 3) is used to flare the canal. Now file no 30 is used in two full clockwise rotations 7200 from flared part of canal without apical pressure. The introduction of sequentially smaller size of files to a 1mm greater depth is done till provisional working length is reached which is 3mm short of radiographic apex. True working length is established on basis of the provisional working length radiographically. The introduction of sequentially smaller size of files to a 1mm greater depth is done till true working length, 1m short of radiographic apex, is reached. The final apical cleaning was completed with size15 to size 25 using circumferential filing. Advantages: • Less amount of apically extrusion of debris. • Less amount of procedural errors as zips and perforations. 3. Double flared technique. It was given by Fova (1983). This technique is hybrid of step down and step back techniques. After establishing straight line access, under copious irrigation, the 10/ 15 size file is inserted one third way into the canal and removed. A file size 80 is marked by stop and is inserted to same length without pressure and 15

BIOMECHANICAL PREPARATION OF THE ROOT CANAL canal wall is circumferently filed. Instrument is removed and canal is irrigated. The introduction of sequentially smaller size of files to a 1mm greater depth is done till coronal and middle levels are prepared. A file size 15/ 20 is gently placed to full working length and the true working length is established radiographically, then canal is prepared to this level similarly as given above till apical preparation is completed and finally canal is flared by step back preparation. Advantages: • Less extrusion of apical debris • Better control of file in apical area due to preflaring. 4. Modified double flared technique. It was given by Saunders and Saunders (1982).This technique is hybrid of step down, step back and balance force technique. After establishing straight line access, under copious irrigation, the 40 size Flex R file is inserted in middle third of the canal and is moved in balanced force technique. This area is then enlarged in sequential manner with larger sizes. Coronal area is then flared with Gates Glidden drill (2 and 3).Then no 20 file is taken till apex and working length is confirmed radiographically. The canal is prepared in sequential manner with larger sizes till apical preparation of size 40 is reached or clean dentin chips are seen. Then stepping back preparation is carried out with balanced force method to finish the canal preparation. Advantages: • Less extrusion of apical debris and less chances of ledging. 5. Alternated rotary motion technique (ARM) Canals were prepared by using Nitiflex files in the alternated rotary motions (ARM) technique, as described by Siqueira (1997). A no 25 Nitiflex file was inserted into the root canal to a point where it bound slightly and then turned clockwise with no more than quarter rotation. It was then turned counterclockwise with light apical pressure. Counterclockwise rotation was also no more than quarter. These motions were repeated continuously until the file reached the working length. ARM was maintained in this position for a few seconds. The file was withdrawn 1 to 2 mm, still oscillating, then replaced to the working length. This continuous oscillation associated with the up and down motion was repeated until the file was able to slide easily to the working length. Each sequentially larger file was worked in a similar fashion. Apical preparation was completed by enlargement through no 40 Nitiflex file. B. CHEMICAL METHODS In the past beside biomechanical preparation, chemical means were also applied in conjunction with instrumentation. They are no longer used and are only of academic interest. These were classified as 1. Acid method 16

BIOMECHANICAL PREPARATION OF THE ROOT CANAL 2.

Alkali method

1.

Acid method The acids which were used in root canal treatment for enlarging the canal and gaining access to the apical foramen were a. 30% hydrochloric acid (HCl), b. 50% Sulphuric acid (H2SO4), c. Phenolsulphonic acid and d. 50% reverse aqua regia. Purpose: The purpose of using acid was to dissolve inorganic structure of dentin and to make it soft which will facilitate its easy removal. This will assist root canal instrument in reaching apical foramina when canal is narrow or blocked Technique: The acid in small amount is pumped into the canal with instrument as far as it can go. It is allowed to remain there for few minutes. Instrumentation is then carried out until it reaches apical foramina or has widened the canal sufficiently. This technique resulted in quick corrosion of root canal instruments. a. Hydrochloric acid It was employed as 30% solution. It is more active than sulphuric acid and does not have self limiting action. (Grossman 1943). The dentin gets completely dissolved in it with no residue as the resultant calcium chloride is readily soluble in excess acid. It also interacts with sodium dioxide without leaving any residue It is desirable to neutralize acid in the root canal with a weak solution of sodium bicarbonate. b. Sulphuric acid It was introduced by Callahan in 1894. It is employed in concentrations varying from 30% to 50%. It destroys pulp tissue by precipitating proteins and abstracting water. Its solvent action on dentin is self limiting as it forms an insoluble calcium sulphate. This calcium sulphate can block the root canal. In order to prevent this canal is frequently irrigated with water or with weak solution of sodium bicarbonate. The bicarbonate neutralizes acid and also forms soluble salt sodium bisulphate. As a byproduct of this reaction, an effervescent solution is formed which helps in dislodging the organic debris. Sulphuric acid also interact with sodium dioxide but it leaves a residue. c. Phenolsulphonic acid It was recommended by Buckley in 1917, because it was not so destructive as sulphuric acid and because of its thick consistency which allowed it to be carried into root canal with ease. It is less potent than hydrochloric acid and sulphuric acid. If sodium bicarbonate is used to neutralize this acid, insoluble sodium phenolsulphonate is formed which can block the root canal. d. Reverse Aquaregia It was found that nitric acid was slightly more effective than hydrochloric acid. But it was not routinely used because of its highly irritating fumes. So in 17

BIOMECHANICAL PREPARATION OF THE ROOT CANAL cases with severe calcification where sulphuric acid and hydrochloric acid were unsuccessful, 50% solution of reverse aqua regia should be used, as suggested by Sintenis in 1925.This solution has highest solvent power on dentin than any other acid. This solution is composed of following: Hydrochloric acid 1part Nitric acid 4 parts Distilled water 5 parts It was carried in the canal with platinum wire. 2.

Alkali method The alkalis which were used in root canal treatment for dissolving organic material and enlarging the canal and gaining access to the apical foramen were a. Sodium potassium alloy b. Sodium dioxide Purpose: The purpose of using alkali was to disorganize and dissolve organic pulpal debris and dentin matrix or to render organic structure of dentin friable and soft which will facilitate its easy removal. This will assist root canal instrument in reaching apical foramina when canal is narrow or blocked. It is also used to render canal sterile. a. Sodium potassium alloy It was introduced by Schreier in 1892. It contains of following: Metallic sodium 2 parts Metallic potassium 1 part This is a very hygroscopic material which shows violent reaction with water. So it is kept in hermetically sealed(with wax) glass tubes or under non aqueous liquid as benzene. It has a silvery sheen. It is removed in very small quantity, about size of pinhead or less on a smooth broach and is transferred quickly to dried root canal. The violent reaction occurs between water in pulpal debris resulting in liberation of sodium and potassium hydroxides with evolution of hydrogen gas. These hydroxides combine with water to form caustics solution which attack organic pulpal material and dentin. These are also powerful caustic disinfectants. The hydrogen is liberated with much heat which sometimes incinerates some of the organic material and accounts for flash of light. It should never be used in canals with wide apical foramen so as to prevent it from coming in contact with periapical tissues. b. Sodium dioxide It was introduced by Kirk in 1893. It is yellowish, granular, highly hygroscopic powder. It is a powerful oxidizing agent and is highly destructive to organic tissues. It is however is not active as sodium potassium alloy. 18

BIOMECHANICAL PREPARATION OF THE ROOT CANAL CONCLUSION As it is seen not a single method is such that it can be universally used in all canal shapes, curvatures. All methods have their indications, advantages and disadvantages. So, proper examination of root canal for its shape size and curvature with understanding of mechanics of biomechanical techniques will only help operator to choose suitable technique for achieving high success. REFERENCES 1. Abou-Rass M, Frank AL, Glick DH. The anti curvature filing method to prepare the curved root canal. J Am Dent Assoc 1980: 101: 792– 794. 2. Backman CA, Oswald RJ, Pitts DL. A radiographic comparison of two root canal instrumentation techniques. J Endod 1992: 18: 19–24. 3. Fava L. The double-flared technique: an alternative for biomechanical preparation. J Endod 1983: 9: 76–80. 4. Goerig AC, Michelich RJ, Schultz HH. Instrumentation of root canals in molar using the step-down technique. J Endod 1982: 8: 550–554. 5. Gutman J L, Dumsa T. Cleaning and shaping of root canal system. In Pathways of the pulp by Cohen S and Burns R C, 4th ed. C V Mosby company, Missouri. 6. Hulsmann M, Peters O A et al. Mechanical preparation of root canals: shaping goals, techniques and means. Endodontic Topics 2005, 10, 30– 76. 7. Ingle JI. A standardized endodontic technique using newly designed instruments and filling materials. Oral Surg Oral Med Oral Pathol 1961: 14: 83–91. 8. Ingle J I, Himel V T et al. Endodontic Cavity preparation. In Endodontics by Ingle J I, Bakland L K, 5th ed. Lea and Febiger, Philadelphia. 9. Ingle J I, Mullaney T A et al. Endodontic Cavity preparation. In Endodontics by Ingle J I, Taintor J F, 3rd ed. Lea and Febiger, Philadelphia. 10. Lim S, Stock CJ. The risk of perforation in the curved canal: anti curvature filing compared with the step back technique. Int Endod J 1987: 20: 33–39. 11. Morgan L, Montgomery S. An evaluation of the crown-downpressure less-technique. J Endod 1984:10: 491–498. 12. Mullaney T P. Instrumentation of finely curved canals. Dent Clin North Am, 1979; 23(4): 575-592. 13. Roane JB, Sabala CL, Duncanson MG Jr. The ‘balanced force’ concept for instrumentation of curved canals. J Endod 1985: 11: 203–211.

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BIOMECHANICAL PREPARATION OF THE ROOT CANAL 14. Saunders WP, Saunders EM. Effect of noncutting tipped instruments on the quality of root canal preparation using a modified doubleflared technique. J Endod 1992: 18: 32–36. 15. Siqueira Jr JF, Rocas IN, Santos SR, Lima KC, Magalhaes FA, de Uzeda M. Efficacy of instrumentation techniques and irrigation regimes in reducing the bacterial population within root canals. J Endod 2002:28: 181– 184. 16. Torabinejad M. Passive step back technique. Oral Surg Oral Med Oral Pathol 1994: 77: 398–401. 17. Torabinejad M. Passive step back technique. A sequential use of ultrasonic and hand instruments. Oral Surg Oral Med Oral Pathol 1994: 77: 402–405. 18. West J D, Roane J B et al. Cleaning and shaping of root canal system. In Pathways of the pulp by Cohen S and Burns R C, 6th ed. C V Mosby company, Missouri. 19. Weine F S. Intracanal treatment procedures, basic and advanced topics. In. Endodontic therapy by Franklin S Weine, 5th ed. C V Mosby company, Missouri. 20. Weine F, Healeey, Gerstein H, Evanson L. Pre-curved files and incremental instrumentation for root canal enlargement. J Can Dent Assoc 1970: 36: 155–157.

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