Straight Canals V.s. Curved Canals

Continuous Tapering Funnel

Development Of Preparation Techniques

Standardized Technique

Step-Back Technique

Step-Down Technique

Instrument Manipulation

Canal Irrigation

Working Length Determination

Precurving Manual Instruments

Negotiating Canals

Recapitulation

Canal Preparation Techniques Explained

STRAIGHT CANALS V.S. CURVED CANALS

Posterior teeth characterize by more than one root. These roots often exhibit a network system of fins, ramifications, and lateral canals, and most importantly curved canals.Research also has shown that anterior teeth often have curved canals, that directed bucally or lingually, so we see them as straight canals in the radiograph.In the past, all root canals treated as they were straight, and instrumentation was done using standardized tech-nique. This led to occurrences of procedural errors and often failure of treatment.Recent development in the past 60 has led to the inven-tion of new techniques that minimize time and proce-dural accidents, and guarantee a successful delivery of treatment

CONTINUOUS TAPERING FUNNEL

The goal of canal preparation is to clean root canal sys-tem and shape it as tapering funnel to receive obutation material.This tapering funnel, is wide coronally and start to nar-row as it move apically, and ends in the apical stop,

which represent the narrowest part of the canal system and where the obturation material should ends.

DEVELOPMENT OF PREPARATION

TECHNIQUES

In order to fully understand the preparation techniques that are written in textbooks, it is necessary to view them in their historical order, and view their draw-backs that led to development of new, less aggressive techniques.

ROOT CANAL PREPARATIONOsama Asadi, B.D.S, Published for Iraqi Dental Academy Blog

Success in endodontic treatment depend largely on shaping and cleaning of root canal system. In recent years, there has been a significant move away from the hand taper 2% standardized instrumentation into rotary Ni-Ti variable taper crown-down instrumentation.In addition to that the concept of cleaning and shaping has been turned into shaping and cleaning. As the shaping process proceed cleaning process and it determine the quality of root canal cleaning.

LECTURE OUTLINE

CHAPTER

Figure 1: Continuous tapering funnel

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Preparation techniques are (in historical order):• Standardized technique• Step-back technique• Step down technique (or crown-down)• Hybrid technique (or double-flare technique)

STANDARDIZED TECHNIQUE

This technique was used for many years and now it is outdated. It involve inserting each size of file to the full working length. The canal enlarged until white shaving dentin is seen at apical few millimeters of the file. The canal is then instrumented for further 2-3 files

The problem with this technique was:• It works fine for straight canals, but not for curved

canals• It led to procedural errors such as ledging, zipping,

perforation• Loss of working length sometimes due to packing

of debris

This led to development of step-back technique

STEP-BACK TECHNIQUE

This technique was created to overcome the problems in curved canals The root canal is prepared for the full working length to master apical file size 25 or 30.Then successively, each larger instrument is inserted 1 mm less than the previous instrument. Between each instrument, master apical file is inserted to the full working length to clear any debris. This is called reca-pitulation.Some modification of this technique is passive step-back technique, in which each successive file is inserted not 1 mm shorter, but to the distance where resistance is felt and rotated, then withdrawn.Step-back technique overcomed the procedural errors occurred with standardized technique in moderately curved canals. But procedural accidents still occur in severe curved canals. Also this technique is time con-suming. This led to development of step-down tech-nique

STEP-DOWN TECHNIQUE

This technique was first invented by Schilder in 1974, and described in detail by Goerig et al.The principle of this technique is that the coronal aspect of root canal is widened and cleaned first before the apical part. This has some advantages:• It allow straight access to the apical area of the root

canal.• It eliminate dentinal interference found in the cor-

onal two-thirds of the canal, which allow fast and efficient apical instrumentation.

• It remove the coronal pulp and debris first, which reduce the incidence of post-operative pain

• It allow irrigant solution to clean the canal more efficiently.

This technique is the most widely used preparation technique, and it is represent the gold-standard of root canal preparation.

INSTRUMENT MANIPULATION

During years of dentistry, many techniques for han-dling manual instruments has been introduced:• Watch-winding: it involve reciprocating clock-

wise, counterclockwise rotation of the instrument.• Reaming: it involve clockwise rotation of the in-

strument.• Filing: it involve up and down movement of the

instrument, while pressing the instrument against canal wall.

• Circumferential filing: it is the same as filing motion, but each mesial, distal, bucall, and lingual side of the wall is worked against the instrument successively.

• Anti-curvature filing: it involve filing of the in-strument away from the furcation area or what called danger zone, to avoid perforation.

• Balanced force: It is the best technique. It involve clockwise rotation of instrument for 90 degree, then counterclockwise rotation of instrument with apical pressure (you may hear clicking sound, which is natural)for 120 degree, then final clock-wise rotation and the instrument is withdrawn.

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The recommended technique in modern endodontics is the balanced force technique, and it should be fol-lowed.

Figure 2: Red area represent danger zone, which should be

avoided during preparation

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CANAL IRRIGATION

Practioner should have some basic knowledge of root canal irrigation. It has been described in previous lec-tures. Here are some facts about irrigation:• During preparation, root canals should be kept wet,

with copious irrigation used for each instrument, and recapitulation between each instrument is a must.

• It is established that sodium hypochlorite is the irrigation of choice

• Heating of irrigant can increase its efficiency• Diluting sodium hypochlorite will not reduce its an-

tibacterial action, but reduce the dissolving capacity• Care should be taken not to extrude irrigant beyond

the apex as it lead to unhappy events• EDTA should be used to remove smear layer and

opens dentinal tubules• Passive ultrasonic activation of irrigant is one of

modern advances in root canal cleaning.

WORKING LENGTH DETERMINATION

The working length can be determined radiographically or electronically using apex locater, or both for accurate results. First, measure the the length of the tooth from reference point to the apex by holding the ruler parallel to the preoperative radiograph. This is the initial work-ing length.

Then subtract 0.5 - 1 mm from the initial working length and insert small file inside the canal and take a radiograph.

Check the radiograph, if the file is short of the apex add this distance to the working length. The final working length should be 0.5-1 mm short of the apex.

PRECURVING MANUAL INSTRUMENTS

Each hand instrument should be precurved according to root canal curvature, regardless of the technique used in canal preparation. This is has an advantage. In non-pre-curved instruments when a file face an obstruction or curvature it will stuck in the way. But if we precurve the file and and insert it into the canal when it faces an

obstruction we can rotate it to set it free, which will not occur if we use straight files.

Figure 4: Two types of precurving. On the right,

precurving for canal preparation. On the left,

precurving for separated instrument bypass

Figure 3: Balanced force technique

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NEGOTIATING CANALS

After access opening and pulp extirpation, the canal is negotiated using fine files, not reamers, in filing motion only (up and down). This prevent instrument breakage.After few files, the balanced motion should be used af-ter the canal has been clean and wide enough. It worth mentioning that British Dental Journal do not recommend using reamers, as it lead to more procedur-al errors

RECAPITULATION

While the canal is full of irrigant, take a small file (#10, #15) and advance it to the full working length.It should be done between each successive files regard-less of the technique used. The purpose of recapitula-tion is to maintain the patency of the canal and avoid blockage of canal by debris.

CANAL PREPARATION TECHNIQUES

IN DETAIL

Here I will describe root canal preparation techniques that are available in the textbooks in summary.

Standardized techniqueSteps:

• Determine working length as described earlier• Precurve the file and insert it to the full working

length• Using balanced force or watch-winding technique

rotate the file inside the canal until it loose• Irrigate with sodium hypochlorite• Use next larger file to the full working length• Irrigate with sodium hypochlorite and recapitulate

with the first file• Repeat the steps with successively larger files to

the full working length until white shaving dentin is noticed at the apical few millimeter of the file, then use 2 or 3 more larger instrument while irrigat-ing and recapitulating. The canal now should have a continuous tapering funnel shape.

Worth-mentioning that this technique is no longer rec-ommended as it lead to procedural errors in curved ca-nals. A step-back or step-down technique is preferred.Step-back techniqueSteps:

• Determine working length as described previously• Precurve the the first file (e.g., #10) and insert it to

the full working length.

• Using watch-winding or balanced force technique rotate the file inside the canal until it is loose

• Irrigate and insert next file to the full working length• Insert successive files until one of the files can not

reach the full working length. The final file that reach to the full working length (usually #25 or #30) is called master apical file (MAF), and it is used to recapitulate the canal between each successive file.

• Irrigate the canal and insert next larger instrument than MAF, with 1 mm shorter of the working length

• And keep inserting files 1 mm shorter of the previ-ous file, with frequent irrigation and recapitulation

This technique has overcome the problem with stan-dardized technique and works fine in moderately curved canals. However, in severely curved canals it can lead to procedural errors. Also it is a time consum-ing procedure.

Step-down techniqueThis technique is divided into two phases:1. Coronal flaring2. Apical Preparation

Coronal flaring is done using Gates Glidden drill or ro-tary Orifice Shapers, but it also can be done using hand

Figure 5: Standarized Technique

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Figure 6: Step-back Technique

files if GG is not available. Apical preparation is done with hand files or rotary instruments

Coronal flaring

The purpose of this step is to enlarge the coronal two third of the canal so it can receive rotary instruments or hand files, also to permit efficient delivery of irrigant to the canal.This can be accomplished using Gates Glidden drills from #4 to #2 (some canals can accept #6 size drill, but most canals accept size #4). It is important to use hand files first before GG to avoid fracture of such instru-ments. GG should be used only in straight portion of the canals, and not passed beyond the canal curvature.Alternatively, rotary orifice shapers can be used instead of GG. If both are not present at current time, large hand files can be used instead.

Apical preparationAfter the process of coronal flaring is completed, further canal preparation is started. Hand file (or rotary) matching the size of last GG drill is used to prepare the canal. Then a smaller sizes are used to further works in the canal. Each files will go deeper into the canal. When reaching at 3 mm short of the estimated working length, the file is inserted into the canal and radiograph is taken. After confirmation of working length, smaller files are worked into the canal to the size #15. Frequent irrigation and recapitulating is a must.This is the best technique available and represent the modern practice of dentistry. It is also called crown-down technique.

Hybrid techniqueIt involve combination of coronal flaring, step-down technique, then working four files to the full working length, then step-back technique. It is also highly rec-ommended. Also many other combination of rotary instruments are available for the hybrid technique.

Temporary fillingIn multi-visit endodontic treatment it is important to seal the cavity with temporary filling material between visits. There are several temporary filling materials available in the market. Most common one is Cav-it. Although it has low wear resistance, it can be used in combination with GIC temporary filling to provide strong temporary filling that can lasts for several weeks and can withstand wearing action of oral environment. In this concept, Cavit is placed for the lower layers and GIC for the outer layers that faces oral environment.The proper way to place temporary filling is by layering technique shown in this picture:

REFERENCES

• PDQ Endodontics• Endodontics, Volume II, Arnaldo Castellucci• Cohen’s Pathways of Pulp• Pocket Atlas of Endodontics, Rudolf Beer• Endodontics Principles and Practice, Mahmoud

Torbinejad• A Clinical Guide to Endodontics, British Dental

Journal, P. Carrote

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Figure 7: Step-down technique