Root canal preparation: Different techniques
- General definition of the concept:
Root canal preparation is the most important phase of endodontic treatment. Its main objective is to eliminate as completely as possible the organic content of the root canal system (pulp, necrotic debris, germs and degradation products, etc.) and possibly its pathological mineral content (pulpoliths and various calcifications, etc.)
This canal shaping is carried out mechanically using endodontic instruments, and chemically by the action of a disinfectant solution used and renewed permanently throughout the treatment.
Even after several decades of modern endodontics, there are still disagreements about the ideal shape to give to the canal in order to allow optimal cleaning and quality obturation. Since the 1970s, we have observed the simultaneous emergence of two schools… the American school defends its principle of conicity of the apical limit, the Scandinavian school advocates a more cylindrical canal shape with the formation of a wide apical box one or two mm from the radiographic apex.
The two main formatting concepts:
Scandinavian School, “Standardized” Approach 1960 by John Ingle:
The concepts of this method take into account a greater protection of the periapical periodontium.
In the standardized technique, the goal is to achieve apical widening and create an “apical box” or “apical stop” to block the root canal filling materials.
The Scandinavian school recommends stopping the preparation 1 or 2 mm from the radiological apex and instrumenting the canal up to a diameter of 60 to 80/100 in order to create an “apical box”.
In infected teeth, this school advises against endodontic TRT in a single session.
American School, Approach based on conicity = Schilder (1974):
The Schilderian technique advocates maintaining the apical foramen as narrow as possible and creating a taper from the apical preparation limit.
The apical part of the canal is funnel-shaped.
2) Objectives of shaping the canal system: (Schilder, 1974; Machtou et al., 1993
A) Biological objectives:
respect for the periapex : The periapex must not have undergone toxic irritation (propulsion of necrotic debris), mechanical (over-instrumentation) or chemical (irrigants).
Trimming : Trimming allows the elimination of all living, necrotic or potentially necrotic pulp tissue.
B) mechanical objectives:
Taper:
The canal should flare regularly from the apical orifice to the coronal orifice and merge with the walls of the access cavity.
Root canal preparation: Different techniques
Layer:
Except in the apical region, the canal anatomy reproduces the external contour of the root. For mechanical reasons, mainly concerning the resistance of the root, the cleaned and shaped canal must be modelled, but wider, on its original anatomy.
Maintaining the position of the foramen:
Realizing the importance of this goal and achieving it is the key to success in Endodontics. The apical foramen must be maintained in its original spatial position on the root surface. Under no circumstances should it be displaced, torn or lost during canal preparation.
Foramen diameter:
The diameter of the foramen should be kept as small as possible. It is biologically useless to unnecessarily widen the apical region in order to achieve excellent cleaning. Keeping the foramen narrow gives the canal the required resistance shape.
3) Catheterization:
Catheterization is the operator’s first exploration of the unknown world of the canal system. It is performed with a thin, flexible instrument; generally a K file no. 8/100 or 10/100, 15/100th of a mm pre-curved that is slid with the fingertips to the apical foramen. It informs us about the canal anatomy, the presence of curvatures, interference with the walls of the access cavity, ramifications and the canal volume.
Catheterization instruments:
Most authors agree to use the K type file n=° 6, 8, 10, 15
MMC instruments are particularly well suited for this function.
K pins can be used as an alternative.
Instrumental and technical sequence:
The canal system must be lubricated with irrigating products before penetration of the instrument.
All instruments must be pre-bent before being introduced into the canal.
Any initial penetration is done according to a continuous linear movement of the instrument in the apical direction, associated with a rotation of 1/8 turn clockwise with return to the starting position by a reverse 1/8 turn
Today, there are Nickel-Titanium (Ni-Ti) catheterization instruments used in continuous rotation such as the PathFile or the G-files. These instruments would allow a faster and safer passage up to the apical third (without risk of formation of a stop) while respecting the canal anatomy as much as possible thanks to their flexibility and low conicity.
4) Cleaning and shaping the canal:
These two stages, once very different, are now one and the same: the canal cleaning is done simultaneously with the shaping of this canal with the help of permanent antiseptic and solvent irrigation.
Root canal preparation should consist of connecting the access cavity to the canal so that the files slide along the walls, from one to the other without catching. This therefore assumes the creation of a continuous slope going from the chosen apical limit to the access cavity. The preparation will consist of obtaining
of an enlarged and sanitized root space (i.e. bacteriologically acceptable) with smooth walls that respect the original anatomy of the canal.
4-3-Working length and apical limit of preparation LT:
Working length (WLL) is the distance from the apical limit to a fixed coronal landmark. There are different ways to determine the WLL. WLL can be determined radiographically, electronically or by tactile sense. Incorrect determination of WLL leads to over-instrumentation leading to excess filling material or under-instrumentation leading to under-filling.
4-3-1-Apical anatomy:
Kuttler carried out an anatomical study of the apical region of the root, several structures are identifiable (Kuttler, 1955):
The apical constriction, which is the narrowest point apically, it opposes 2 cones opposable by their summit, one dentinal which narrows to the cemento-dentinal junction, the other cemental which widens towards the periodontium, forming the apical foramen.
Cemento-dentin junction (CDJ ) = place of confluence of dentin and cementum on the canal walls
-The anatomical apex : Also called the vertex of the tooth or even the apical dome, it is an anatomical structure.
-The radiological apex: This is the projected image of the most apical part of the tooth appearing on an X-ray taken without deformation .
-The apical foramen: This is the main exit of the canal towards the periodontium limited by the cementum walls, unlike the constriction, it is not clinically identifiable .
Apical constriction : the narrowest area of the canal. Its location is always very close to the foramen
The statistical data cross-referenced by Kuttler position the apical constriction on average at 0.5-0.1 mm from the apical foramen.
diagram of the apical region (after Kuttler) (Perard et al., 2009).
1: apical constriction. Above point 1, we observe the dentinal cone.
2: cementodentinal junction.
3: distance separating the center of the foramen from the cementodentinal junction (cemental cone),
4: foramen.
5: distance separating the center of the foramen from the vertex .
Choice of apical limit:
The canal preparation must be located at the CDJ and will have a pointed shape, this cone constitutes the dentin matrix intended to receive the obturation.
In relation to the radiographic apex it will be located:
At 0.5mm if we think that the canal is straight
At 1mm if the canal is suspected of being apically deviated (curvature)
At 1mm in the case of a root with resorption
Root canal preparation: Different techniques
4-3-2-Determination of the working length :
- Empirical methods:
Based on the operator’s tactile sense and experience or on the patient’s painful sensation when exceeding the apical limit.
Nowadays, this technique is no longer used due to the considerable scientific progress that has been made in the field of radiology and then apex locators.
B) The radiographic method:
For many years, the use of retroalveolar radiography to determine LT in endodontics has been considered a reliable and reproducible technique. The most widely used method of analyzing radiographic images, which we will retain, is that described by Beveridge in 1966. It includes two steps:
Analysis of the preoperative image :
Approximate measurement of the root length on this first image (estimated LT) can be facilitated by the use of radiopaque millimeter grids.
Analysis of the radiograph in place:
With the instrument (K file or pin) brought to the estimated LT minus 0.5 or 1mm, allowing adjustment of the measurement on an additional shot if necessary
C) Radiovisiography (RVG):
Allows to obtain a good quality image but above all easier to use clinically and to archive. Digital radiography has made it possible to lower the doses of irradiation, to significantly reduce the image acquisition time and to play on the variation of contrast in order to better identify the anatomical structures which interest us.
D) Apex locator:
After placing the lip electrode and turning on the localizer, the second electrode is brought into contact with the instrument, which is advanced into the canal until the signal indicates the “foramen.”
The silicone stop is adjusted relative to a stable horizontal coronal landmark and the indicated length is measured using a millimeter ruler. This measured length corresponds to that of the foramen.
Root canal preparation: Different techniques
4-3-3-Canal permeability :
The repeated passage of shaping instruments is accompanied by the production of debris both in the main canals and in the intercanal isthmuses. The latter promote the creation of real plugs that can obstruct the canal lumen and prevent instrumental progression. This is why the use of a so-called permeability instrument is recommended. This is a small-diameter file comparable to that used for catheterization, flexible and capable of slightly exceeding the apical foramen. Its use also promotes the maintenance of the canal trajectory
4 -3-4-The master apical file (LAM):
This is the largest caliber file reaching the working length without constraints and which determines the final apical diameter of preparation (at least -25/100 mm) and obturation of this canal. Anatomical or pathological considerations (infected tooth) can however lead the practitioner to increase the apical caliber of preparation or the apical conicity.
4-4-Root shaping techniques:
Standardized technique:
At the origin of the Scandinavian approach. Once the working length is determined upstream of 0.5 to 1 mm from the anatomical apex, the shaping of the canal is initiated by different instruments of increasing diameter brought up to the LT, leading to the formation of an “apical box”. The K files are used in ¼ turn-retraction to cut the dentinal chips, while the corresponding scrapers are used for the removal of the detached debris following the sequence:
- – K 08
- – K 10
- -K 15 ⇒ Scraper 15
- – K 20 ⇒ Scraper 20
- -K 25 ⇒ Scraper 25
Step back:
The apico-coronary technique involves progressive enlargement in successive and programmed stages. After catheterization and determination of the LT, under irrigation of 1 to 2 ml of 2.5% sodium hypochlorite between each instrument pass, the sequence is as follows:
File K or pins =15 20 25 at working length under irrigation with 2 mL of NaOCl
30 LT -1 mm, 35 LT -2, LT 40- 3…….
Passage of the K10 or K15 summary file
Serial technique:
The serial technique is of great interest when it is impossible to directly reach the working length using a 15 file (fine or curved canals). Machtou et al., (1993) recommend an initial shaping of the middle third by respecting the following sequence. Serial passage of pins 15 to 35, starting from the length initially reached by the K 15 file (“initial LT”). This length is of course less than the actual working length (“LT”):
- spindle 15 at working length
- pin 20 to length minus 1mm
- pin 25 to length minus 2mm
- pin 30 to length minus 3mm
- pin 35 to length minus 4mm
This sequence allows a coronal widening which frees the instruments from parietal constraints and then allows the K 15 file to descend deeper than previously. This new working length is the starting point for a new sequence of pins.
Balanced Forces Technique (Roane):
The aim of this technique was to limit the phenomenon of apical transport during manual instrumentation, particularly in traction. The movement is broken down as follows:
– ¼ clockwise turn associated with apical pressure, engagement of the edges of the instrument in the dentinal walls, by slight screwing effect.
– ¾ turn counterclockwise while holding the instrument in place, therefore cutting the walls.
Complete clockwise rotation, to load the debris into the coils, then withdraw to evacuate them and allow irrigation.
The author notes that the dynamics of the instrument, and its alternating rotational movement
clockwise/counterclockwise is the origin of a natural centering of the instrument in the canal and the maintenance of the initial trajectory.
Manual and rotary technique:
This is actually the Schilder “step back” technique, where part of the canal preparation is done manually as already described, another part using rotating instruments represented by drills, they are intended for widening the coronal part in order to straighten the curvature and eliminate interferences and thus promote instrumental access to the apical third. There are many varieties of drills, the most used are Gates Gilden drills and Largo drills. The use of drills must always remain limited to the straight portion of the canals given the major risk of perforation in the event of an attempt to cross a canal curvature.
Root canal preparation: Different techniques
4-5-Incidents during formatting:
Shaping incidents are often linked to canal transport phenomena, at more or less advanced stages.
Transport is defined as an iatrogenic intra-radicular deviation of the path of the main canal, due to the elastic memory of endodontic instruments. Transport is described as internal when the instruments are confined in the canal, and external when there is over-instrumentation, resulting in tearing of the foramen. Mild transport is most often unavoidable in the presence of curved canals.
Stripping:
Stripping is an iatrogenic lesion. It consists of over-preparation of the internal part of the canal curvature resulting in lateral perforation. This phenomenon is encountered more particularly on the distal face of the mesial roots of the mandibular molars. It is very difficult to find the original axis of the canal and the walls are therefore not instrumented.
Apical deformation:
As the diameter of the instruments (their rigidity) and the stress exerted increase, a straightening of the apical curvature is observed as well as a displacement of the foramen, responsible for an ovalization of the latter (called Zipping). This opening of the foramen in the shape of an “hourglass” makes the sealing of the obturation very difficult and predisposes to an excess of the obturation material.
False channel:
Stage following the stop when the forceful work continues, and characterized by the creation of an iatrogenic canal which can lead to perforation. The root is weakened by the creation of a zone of weakness. Resuming the original path is difficult due to the tendency of the instruments to lodge in the false canal.
Stop:
The stop results from forceful work by the operator and when the instrument lacks flexibility. It is a stripping in a narrow area, often located at the level of a significant canal curvature, which leads to deviation of the canal and blocking of the instrument.
Perforation:
Perforation is the iatrogenic consequence of the instrumental preparation of a false canal. The instrument then crosses the root dentine as well as the cementum and causes the canal network to communicate with the periodontal ligament, creating irritation, which can result in irreversible damage to the periapical tissues leading to extraction.
Crown-Down or corono-apical preparation (Papin and Marshall 1980):
Corono-apical shaping technique, which requires the use of instruments of decreasing diameter and/or taper in the apical direction.
Coronal parietal constraints are eliminated and then progressively in the apical direction, allowing instruments of smaller diameter or smaller taper to advance. In addition, the technique has the advantage of introducing instruments at the apex only after disinfecting the coronal and medial 1/3. This technique is generally performed with continuous rotation instruments.
Root canal preparation: Different techniques
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