Endodontic Working Width: Current Concepts & Techniques

MinIWW(N) Minimal initial horizontal dimension N-mm short of working length (WL) MinIWW0 Minimal initial horizontal dimension at WL MinIWW1 Minimal initial horizontal dimension 1-mm short of WL MinIWW2 Minimal initial horizontal dimension 2-mm short of WL MaxIWW(N) Maximal initial horizontal dimension N-mm short of WL MaxIWW0 Maximal initial horizontal dimension at WL MaxIWW1 Maximal initial horizontal dimension 1-mm short of WL MaxIWW2 Maximal initial horizontal dimension 2-mm short of WL MinFWW(N) Minimal final horizontal dimension N-mm short of WL MinFWW0 Minimal final horizontal dimension at WL MinFWW1 Minimal final horizontal dimension 1-mm short of WL MinFWW2 Minimal final horizontal dimension 2-mm short of WL MaxFWW(N) Maximal final horizontal dimension N-mm short of WL MaxFWW0 Maximal final horizontal dimension at WL MaxFWW1 Maximal final horizontal dimension 1-mm short of WL MaxFWW2 Maximal final horizontal dimension 2-mm short of WL

- Endodontic Working Width : Current Concepts & Techniques In a relatively round canal, MinIWW and MaxIWW are approximately the same. In an oval, long oval, or flat canal, MaxIWW is several times larger than MinIWW at different levels of the canal.

In a maxillary cuspid (before instrumentation), MinIWW0 may be the same as MaxIWW0, whereas MaxIWW12 may be three to four times larger than MinIWW12. On the other hand, after instrumentation, MinFWW0 may the same as MaxFWW0 (considering no significant transportation). However, the ratio between MinFWW12 and MaxFWW12 may be altered by mechanical preparation of the canal.

Current Descriptions of the Horizontal Dimensions (Cross Sections) of the Root Canal

1. Round (circular) canal: MaxIWW equals MinIWW.

2. Oval canal: MaxIWW is greater than MinIWW (up to two times more).

3. Long oval canal: MaxIWW is two or more times greater than MinIWW (up to four times more).

4. Flattened (flat, ribbon) canal: MaxIWW is four or more times greater than MinIWW.

5. Irregular canal: It cannot be defined by list points 1–4.

Determination of Initial Working Width at Working Length

1. First determine the preoperative canal diameter by passing consecutively larger instruments to the WL until one binds. This initial apical file estimation is referred to as the determination of MinIWW0.

2. Then, the master apical file size (MaxFWW0) is suggested to be three ISO file sizes larger than that initial binding file.

But this concept is being questioned as recent studies suggest that the first k file that binds at the WL did not accurately reflect the diameter of the apical canal. The inaccuracy and discrepancy is the result of various morphologic and procedural factors as each of the factor discussed further can affect the clinician’s tactile sense.

FACTORS AFFECTING THE DETERMINATION OF WORKING WIDTH AT WORKING LENGTH

CANAL MORPHOLOGY

Canal morphology is a critically important part of root canal therapy. Various in vitro studies have recorded the scales and average sizes of root canals (14), but only few clinical attempts have been made to determine the working width (WW). Sectioning of all levels of the teeth and making section plane exactly perpendicular to the canal curvature is a difficult task. Therefore, most morphometric studies cannot exactly depict the horizontal dimensions of root canal system. Until recently, most investigations have involved counting the number of canals and foramina and categorizing the canals based on the manner in which they join or split.

Current studies focus more on the shape of the canal systems and its clinical implications than on the actual, preoperative size of the canal (8). The horizontal dimension of the root canal system is not only more complicated than the vertical dimension but also more difficult to investigate because it varies greatly at each vertical level of the canal as shown in Figs. 6.1–6.3. The round canal can be measured more easily because the minimum initial working width (MinIWW) and maximum initial working width are the same. However, other factors make initial working width (IWW) determination difficult, even in straight canals. To determine the MinIWW of the oval, long oval, and flat canals, we may need proper instrument and tactile sensation.

The presence of undetected lingual canal or an untreated isthmus may be responsible for endodontic failure of lower incisors. The prevalence of two canals in mandibular incisors has been has been found to be 11.5%–44.1%, although many merge into single canal in the apical 1–3 mm of the root.

Factors Affecting the Determination of Working Width at Working Lenght Sectioning of root is the most common method for evaluating the shape of the canal. Cross sections at different levels in a root allow direct viewing of canal shape and position with regard to the borders of the root surface.

When using an instrument to measure working length (WL), the longer the canal, the higher the friction resistance. In a very long canal (>25 mm), the friction resistance may increase to affect the tactile sense of the clinician to determine the IWW correctly. In addition, if the coronal flare is too conservative or limited to the coronal third of the canal, then shaft of the instrument may engage the canal wall and lead to a false/premature conclusion regarding WW.

CANAL TAPER

Any discrepancy between the gauging instrument and canal due to tapering can cause an early engagement of the canal wall with the instrument, leading to a false sensation of apical binding. The tapering of the canal may increase due to early coronal flare; however, the tapering discrepancy between the gauging instrument and canal wall may decrease. The last 3–5 mm of the canal has parallel walls, making accurate determination of IWW. The root canal content may be fibrous or calcified material (calcific metamorphosis) creating different degrees of frictional resistance against the gauging instrument. This eventually affects the tactile sense of the clinician to determine the IWW accurately.

CANAL CURVATURE

Curved canals can deflect the gauging instrument and increase friction resistance. The root canal curvature can be categorized as two dimensional, three dimensional, with small radius, with large radius, and double curvature (S- shaped, Bayonet shaped), and with different degrees of severity. Each of these curvatures has a different effect on the tactile sense of a clinician. The combination of these curvatures makes determination of IWW a difficult task, if not possible. The study by Wu et al. (10) indicated that in curved mandibular premolars, the first K-file and the first Light Speed instrument that bound at the WL failed to accurately reflect the diameter of the apical canal.

Success of the root canal therapy depends on careful canal preparation. The main aim of endodontic instrumentation is to enlarge a canal without deviating from the original canal curvature.

It has been stated that ―The final preparation should be an exact replica of the original canal configuration in shape, taper and flow, only larger.

CANAL CURVATURE

Curved canals can cause deflection of the gauging instrument and increase friction resistance. The curvature of the root canal can be categorized into two dimensional, three dimensional, small radius, large radius, and double curvature (S-shaped, Bayonet shaped) and with different degree of severity. Each of these curvatures has a different effect on a clinician’s tactile sense. The combination of these curvatures make correct determination of Initial Working Width extremely difficult, if not possible. In curved mandibular premolars, the study by Wu et al indicated that the first K file and the first Light speed instrument that bound at the Working Length failed to accurately reflect the diameter of the apical canal. .

Careful canal preparation is an important part of successful root canal therapy. The ability to enlarge a canal without deviation from the original canal curvature is a primary objective in endodontic instrumentation. It has been stated that ―The final preparation should be an exact replica of the original canal configuration in shape, taper and flow, only larger. After studying the effects of several instrumentation techniques, it is noted that every file, whether precurved or straight, tended to straighten within the canal . They reported that the largest amount of apical canal preparation occurred at the outer portion of curvature, away from the furcation.

An attempt to solve this problems has led to the development of various instrumentation technique like step back, crown down, balanced force, anti curvature filling etc in addition several instruments like K flex, flex arc, flex- o, Protaper, race files, light speed, hero shaper-hands and rotary files have been designed. This instruments aim at alleviating procedural difficulties at coronal, middle, apical regions

.The Schneider method is the primary technique used to measure Canal wall irregularities

Attached pulp stones, denticles and reparative dentin can create convexities on the canal wall surface. Resorption can produce concavities on the canal wall surface. These phenomena can serve as an impacting factor that induces a false estimation of the true canal dimension at working length and other levels.

Instrument for determining initial working width

The rigidity and tapering of the instrument used for determining IWW can affect accuracy. Any tapering discrepancy between the gauging instrument and canal may lead to an early instrument engagement of the canal wall, altering the tactile sensation. The rigid instrument in a curved canal also lead to a false tactility.

ELIMINATING THE INFLUENCE OF AFFECTING FACTORS

Prior to determining the initial working width (IWW), the orifices should be widened for early coronal flaring (crown down, double flaring), which ensures effective irrigation and minimizes any interferences with tactile sensation. To avoid interference and to achieve better results, an adequate instrument with maximal flexibility and minimal taper should be used. The exact outline of the horizontal dimensions of the root canal should be followed by root canal preparation at every level of the canal. To minimize incomplete cleaning of the root canal system, circumferential preparation or instrumentation need to be taken into consideration. A continuous reaming action of most of the nickel–titanium (NiTi) rotary instruments makes the canal relatively circular. Incomplete cleaning of the root canal system may be a result of imprudent use of the NiTi rotary instruments alone, which leads to failure of the endodontic therapy.

Determination of the Minimal and Maximal Final Working Width at Working Length

A final working width is required for bacteria and their substrates removal, and also for dead pulp tissue removal. Further, it is necessary to increase the capacity of the root canal to retain a larger amount of sterilizing agent and also to prepare the tooth to receive the canal filling.

To increase the success rate of the treatment, it is essential to remove the infected dentin. For this purpose, the instruments and techniques used should help in retaining the original shape of the canal to maximize the cleaning effectiveness and minimize unnecessary weakening of tooth structure.

How Do We Know When The Final Apical Instrument Size Is Reached?

By instrumenting and cross sectioning many teeth, it has been concluded that no technique is perfect, and a correct WW is a clinical judgment. However, instrument that meets resistance 4–5 mm short of working length (WL) and then requires a firm push to reach WL closely approximates the correct WW.

The instrument that approximates the correct WW the final apical size. Rotary system has been found to be very important for achieving larger apical preparations safely. In its original form (stainless steel and hand driven), NiTi rotary system evolved through the 1990s and 2000s from improvements made to the earlier versions.

Majority of rotary systems comprise a very short cutting blade, a non-cutting pilot tip, and a smooth flexible taperless shaft.

This provides it maximum flexibility to negotiate curves and cut dentin from canal walls, maintaining canal anatomy without the need for excessive mid-root or coronal over enlargement. Because only the very tip of the instrument comes in contact with canal walls, the tactile sensation is incomparable. Rotary system is extremely safe because of its safety release feature that aids the instrument to separate at the handle instead of at the tip when excessive twisting forces are encountered.

HOW DO WE KNOW WHEN THE FINAL APICAL INSTRUMENT SIZE IS REACHED?

By instrumenting and the cross- sectioniong many teeth it has been concluded that no technique is perfect, and a correct working width is a clinical judgement. However, instrument that meets resistance 4 to 5 mm short of working length and then requires a firm push to reach working length closely approximates the correct working width. The instrument that approximates the correct working width the Final Apical Size (FAS). To achieve larger apical preparations, safely, Rotary system has been found to be invaluable. In its original form (stainless steel and hand driven), NiTi rotary system, evolved through the 1990s and 2000s from improvements made to earlier versions.

Majority of rotary systems have :  A very short cutting bladeï€ ï€ ïƒ¼ A non-cutting pilot tipï€ ï€ ïƒ¼ A smooth flexible taperless shaft.

This affords it maximum flexibility to negotiate curves and cut dentin from canal walls maintaining canal anatomy without the need for excessive mid- root or coronal over enlargement. Tactile feel is unequalled because only the very tip of the instrument comes in contact with canal walls. Rotary system is extremely safe due to its safety release feature which causes the instrument to separate at the handle instead of at the tip when excessive twisting forces are encountered.