Lecture six--------------------------------------------------------------------------------- د.احمد غانم
Master apical file:
In addition to determining the correct length for canal preparation, a method of for calculating the correct degree of enlargement must be clarified. The optimal enlargement of each canal should be calculated separately; there is no hard fast rule that is universally acceptable. However, the initial size of file that binds at the apical portion of the canal is of great helpful in the determining the final canal length.
Master apical file (MAF) MAY BE DEFINE AS A THREE SIZES LARGER THAN THE FIRST FILE THAT BINDS AT THE FULL WORKING LENGTH PASSIVELY AFTER STRAIGHT LINE ACCESS. ALSO CAN BE DEFINDED AS A LARGEST FILE USED AT THE WORKING LENGTH PSSIVELY AFTER STRAIGHT LINE ACCESS.
In the other word, if a size 10 file put into a relatively small canal but is loose and does not remove dentin, it does not bind. In the same canal a size 20 or 25 files does work against the canal walls but does not reach the apical portion of the canal. A size 15 does reach the apical portion and the file cut against the wall; thus it is the initial size of that binds at the full working length, and the master apical file for this canal was size 30. The classic test for determining correct width of any canal preparation has been that finding of clean white dentin shavings on the flute of the reamers and files. This test is relatively incorrect. Many canals are oval or ribbon shaped in cross section. Clean white dentin shavings are attainable from walls close to each other, but the far walls may be completely untouched while this sign is obtain.
Root Canal Preparation Techniques
Currently employed methods of canal preparation can be divided into two distinct approaches: those which prepare the coronal section of the canal system with large instruments and progress towards the apex (coronal-apical technique) and those which start at the apex with fine instruments and progress back towards the cervical orifice with larger instruments (apical-coronal technique). Furthermore, a hybrid technique might be used combining more than one technique.
1. Standardized Technique.
2. Step-back Technique
3. Balance Force.
(Also known as flaring or serial preparation).
The "Step Back" method indicates that the dentist works from the bottom of the canal back towards the crown.
General guide lines
Establish working length using small flexible file and locate the apex.
Step back from the apex, while increasing file size accompanied by reduction in the instrument length (0.5mm).
Once we get to the middle portion of the canal, you may switch back to Gates Gliddens , Peeso Reamers or Hedström files.
Your goal is to achieve a funnel shape.
The step-back technique done, by dividing the preparation into two phases.
Phase I: is the apical preparation starting at the apical constriction. Apical area is prepared up to #30 file with recapitulation by using prior size files and irrigation with 2ml after each instrument .
Phase II is the preparation of the reminder of the canal, gradually stepping back while increasing in size. The preparation step back up the canal in 0.5mm increments from #30 file through #45 file. Recapitulation is with #30 file to full working length.
The completion of the preparation is the refining phase IIA and IIB to produce a perfect continuing taper from the apex to coronal area.
Refining IIA done using Gates-Glidden drills 2, and 3 to create coronal and middle root preparation.
Refining phase IIB is done by using #30 file, through circumferential filing smoothes step back to produce a perfect continuing taper from the apex to the cervical portion of the canal.
1. Step-down Technique.
2. Double-flare Technique
3. Continuous Wave Shaping.
4. Crown-down Pressureless Technique
In "Crown-Down Pressureless technique". the dentist essentially works from the crown of the tooth, shaping the canal as he or she moves towards the apex.
General guide lines:
Begin at the coronal portion, The first instruments are Gates Gliddens The instruments are used in a large to small sequence.
The Peeso Reamers or Hedstrom files follow in the mid-root region
Continue down the canal, reducing the size of each file as you get closer to the apex
Your goal is to achieve a funnel shape.
Finally, progressively smaller files take the dentist towards the apex.
Preparation of the coronal part of the canal system first is now considered to be superior as these results in a number of significant advantages:
Most of the microorganisms will be in the coronal third of the root canal system .Hydrostatic pressure can occur within the root canal if working length confirmation or apical preparation is initiated at the start of preparation because the file will act like a piston in a cylinder and this pressure may force pulp debris, dentine chips, irrigant solution and microorganisms through the apical foramen.
Early coronal flaring allows better penetration of the irrigant solution.
Also, early flaring of the coronal part of the canal system prevents binding of the instruments as they are unencumbered throughout most of their length
And also give better access to the apical part of root canal.
These concepts lead to the advocating of the coronal-apical preparation. When a dentist instruments a molar with a hand file and feels the #25 or #30 binding, the impression historically has been that the file is binding in the apical few millimeters. In actuality the file is not binding in the apical third. It is binding in the coronal half of the tooth. Therefore, when we use a crown down technique, we eliminate the cervical constricture and by opening up the coronal half first, we now let a file proceed unhindered into the apical third of the canal.
The result of this is an increased tactile awareness in the apical third of the canal. By having a file that is not binding in the coronal half of the tooth, we now have a file that possesses exquisite proprioceptive abilities. Additionally, the crown down technique enhances the efficacy of our irrigation agent, which we recommend as either 2.6% or 5.25% sodium hypochlorite. The result of this is an increased tactile awareness in the apical third of the canal. By having a file that is not binding in the coronal half of the tooth, we now have a file that possess exquisite proprioceptive abilities
Method of Crown-down Pressureless Technique
As described by Morgan and Montgomery (1984). A #35 file was inserted or glide to the point of first resistance without using apical pressure. This file was used as a depth guide to use files of larger sizes and the Gates-Glidden drills. Files #40 through 60 and #2. #3 Gates-Glidden drills were then used to the same length by outward stroking of the instruments without using apical force (each instrument used should be followed by 2ml irrigation). Next a provisional working length (PWL) is established from pre-operative x-ray film at a point 3mm short of radiographic apex starting with the insertion of a #35 file. When the binding of the file against the canal walls was noticed, the file was rotated clockwise two full revolutions (720 degree) without apical pressure. The sequence was repeated with successively smaller files until the PWL was reached. The canal is prepared the same way with size 40 and 45 files until FWL was reached. The full working length (FWL) is then verified with a X-ray. Then a #40 file was inserted and rotated clockwise two full revolutions without apical pressure. This is repeated with successively smaller files until the FWL was reached. Finally the apical cleaning and shaping is completed with #15 through #30 files by using a push-pull circumferential motion.
Mechanized Technique of Root Canal Preparation
In an attempt to speed up and facilitate root canal preparation, automated techniques have been developed. These include ultrasonic and sonic, vibratory and rotary system.
1. Ultrasonic Technique.
2. Sonic Preparation
3. Rotary Nickel-Titanium Techniques
Most experienced operators mix the desirable aspects of different techniques used for instrumentation of the root canal. The most techniques are differ little from any other, the important aspect being that the operator is aware of the limitation of each technique to suit individual needs and the instructions of each technique should be not followed as
‘cook-book’ style, except in the early stages of learning.
Other Methods of Preparation
Rotary Nickel-Titanium Techniques
Iatrogenic complication arising from cleaning and shaping
Blockage: the canal may suddenly lose patency during a cleaning and shaping process. This can be result of tissues compression, debris accumulation, wall perforation, or instrument separation.
furcal perforations (strip): a furcal perforation is a mid-curvature root perforation into the periodontal ligament and is the worst possible out come, especially as the closers to the crown (why). Anticurvature pressure is extremely effective when used Gates-Glidden drills in early radicular access preparations. Another way to prevent furcal perforation is to never take large Gates-Glidden drills or pesos deep into the root canal.
Apical perforation: when the apical third of is curved (almost all canals are curved) there is a risk of carving out a new portal of exit. This portal of exit is most often, a result of uncontrolled transportation and ledge formation. Attempts to reestablish the length past a ledge result in the file tip cutting straight through the root structure and into the periodontal ligament.
Altered foramina: Rip or Zip (tear drop transportation) when files are passed through the apical foramen they can change its shape very rapidly and irreversibly. A few in-and-out movements can convert a round apical foramen into a delta shape (zipped). This apical transportation may severely compromised apical repair. Apical foramen and canals may be transported (apical transportation) externally or internally. External transportation is caused by failing to precurved files, using large instruments, or being too heavy handed. The original apical foramen is torn. When the instrument overused, the elastic memory of the instrument may create the teardrop and tearing of the apical foramen. Many endodontic failures result from external transportation due to lake of properly Obturation. The second form of external transportation is direct perforation. This egregious error usually begins with a ledge or apical blockage. The deflected instrument continues its misdirection until it perforates the root surface. Internal transportation occurs when the foramen becomes clogged with dentin mud or denticles. The particles may irritate the attachment apparatuses after root canal filling or the particles may prevent Obturation of other apical foramen that branch off the main canal. Finally, this internally transported foramen may perforate the external root surface through a false path.