What is the most suitable preference in NITIE

Root canal anatomy: Maximum control in the curve

Part 1: Use of a new NiTi file sequence

The internal anatomy of human teeth often consists of a complex network of multi-planar curved canals and anastomoses. In the case of strongly curved canals, the lifelike design of the canal course is a particular challenge. The first part of this article in this issue explains how the use of a new NiTi file sequence can help.

The primary goal of endodontics is the prevention of periradicular diseases and their cure. The basis for this is the mechanical instrumentation and chemical cleaning of the canal1The former largely determines the effectiveness of all subsequent steps.2 For fillings based on gutta-percha, the shape of the canal should meet the following criteria:

  • The main channel resembles a funnel that tapers continuously towards the apex.
  • The diameter of the main canals narrows increasingly apically.
  • The preparation follows the original form.
  • The apical constriction should not be shifted or widened.1,3

The objectives of the instrumentation from a biological point of view are:

  • Limitation of the instrumentation to the natural limits of the root,
  • no pressing of necrotic debris into the periradicular tissue,
  • complete removal of organic tissue from the main and side canals,
  • Creation of sufficient space for rinsing and medication with the best possible preservation of the surrounding dentine to ensure the functionality of the tooth.3

In straight canals, the implementation of these principles is uncomplicated, major problems only arise with strongly curved canals, ramifications or anastomoses (Fig. 1). In such cases, it can be difficult to follow basic endo techniques and treatment protocols. A new NiTi file sequence in the so-called TCA technique can now be used for safe, reliable instrumentation.

Management of curved canals

Based on their curvature, Nagy et al. Root canals in four categories4:

  • straight I-shape (28 percent of all root canals)
  • apically curved or J-shape (23 percent)
  • arched or C-shaped (33 percent)
  • multiple curved or S-shape (16 percent)

In studies by Schäfer et al. 84 percent of the root canals were curved, 17.5 percent of which were classified as S-shaped due to a double curve.5 Of the curved canals, 75 percent had an angle of curvature of less than 27 degrees, 10 percent an angle of 27 to 35 degrees, and 15 percent a sharp curvature of over 35 degrees.

Traditionally, the degree of curvature was also determined using the Schneider angle measurement method6 described: Root canals with an angle of five degrees or less can be classified as straight, canals with an angle of 10 to 20 degrees as moderately, and canals with a curve greater than 25 degrees as severely curved. Decades later, Pruett et al. points out that in two root canals with an identical angle, according to the Wein classification, curves can run abruptly in completely different ways.7 To define the abruptness of the curve, they introduced the radius of a curvature: that of a circle drawn on the curved section. With rotating instruments, the number of failure-free cycles until failure decreases significantly with decreasing radius of curvature and increasing angle of curvature.

Further attempts to mathematically describe curvatures in two-dimensional x-ray images led to the introduction of parameters such as the length of the curved section5 and the position defined by the height and distance of the curvature.8 Estrela et al. described a method for determining the radius of root canal curvatures using CBCT images9that are analyzed by software. All attempts to adequately describe curvatures had one goal: the preoperative assessment of the risk of canal displacement or instrument breakage.

Overflaring

Following the glossary of endodontic terms (AAE 2012)10, canal dislocation is the removal of canal wall structure on the outside of the curve in the apical half due to the tendency of the file to assume its original linear shape. In the case of hand files made of stainless steel and conventional hand or motor-operated NiTi files, the restoring force depends directly on the file size and taper. The larger the taper or length, the stronger the restoring effect due to the increased mass of metal. If channels were shaped exactly according to the dimensions of the instruments, displacements would not be an issue: the instruments would be optimally steered by the given course. Unfortunately, they can never be tailored exactly to the duct dimension. Because of the reset effect, each file therefore follows its own path within the curved canal and thus displaces it.11

If a strong enlargement of the apical approach is sought, the dentin removal to the outer apical curve usually increases excessively.12 The widening of the inner curve can also become excessive. To avoid such complications, dentists tend to increase flaring in curved canals while reducing the apical instrumentation size.13

This in turn often leads to a decrease in the angle of curvature, a shortening of the length, an increase in the radius and a shifting of the curve apically (Fig. 2). In strongly curved canals, a smaller apical preparation is preferable for two reasons:

  • First: preparations with a smaller diameter result in less abrasion on the canal wall, less friction and thus a lower likelihood of undesirable effects.
  • Second, smaller diameter files are more flexible and break-proof, which reduces the risk of canal dislocation.13

Unfortunately, flaring leads to unnecessary removal of irreplaceable dentin material for easier probing of the apical third. In addition, a smaller apical preparation can make it more difficult for the irrigation solution to penetrate to the appropriate depth of treatment. In highly curved canals, the ability of the irrigation solution to reach the critical apical third depends directly on the appropriate apical access and the appropriate administration technique.14 A sufficient apical preparation to disinfect the canal without overflaring the coronal section in the case of severely curved canals is one of the great endodontic challenges - especially in view of the current philosophy of maximum substance preservation and minimally invasive procedure.

In addition, the risk of unexpected instrument breakage with motorized NiTi files is a significant problem. Two factors have been identified: cyclical fatigue and torsional breakage. When a motorized instrument is activated in a curved canal, continuous tensile and compressive stress at the pivot point of the curve can lead to instrument breakage due to cyclic fatigue. If the tip of a motorized instrument becomes blocked in the canal while the shaft is moving, exceeding a certain shear torque can lead to torsional fracture. As the curvature becomes more complex, the number of trouble-free cycles before the break occurs decreases.

Use of NiTi files

Overall, nickel-titanium alloys are softer than stainless steel, have a low elasticity (about a quarter to fifth of stainless steel), but are stronger, more resistant, more elastic and show shape memory and superelasticity.15 The NiTi alloys used in endodontics contain around 56 percent nickel and 44 percent titanium.16 They can exist in two different temperature-dependent crystal structures, called martensite (low temperature phase) and austenite (high temperature phase). The lattice structure can be converted from austenitic to martensitic by adjusting the temperature or pressure. During the transformation in the opposite direction, the alloy goes through an unstable crystallographic intermediate phase (R phase).

A root canal treatment stresses NiTi files: with conventional NiTi files, stress-induced martensitic transformation occurs in no time at all. Changes in volume or density occur. The ability to withstand loads without permanent deformation is called superelasticity. This is strongest at the beginning, when an initial deformation of up to eight percent elongation can be completely overcome. After 100 deformations, the tolerance is around six percent and after 100,000 deformations around four percent. The so-called “memory effect” can be observed in this area.15 In 2011 COLTENE introduced “Controlled Memory” files. The files are manufactured in a thermo-mechanical process for a controlled restoring effect, which makes the files extremely flexible and break-proof (without the shape memory and restoring force of conventional files). Special NiTi file sequences such as the HyFlex ™ EDM Max Curve Set (Fig. 3) are recommended for the effective preparation of strongly curved canals.

Interim conclusion

Avoiding overflaring and canal displacements pose particular challenges, especially with curved canals. NiTi files with “Controlled Memory” effect are flexible and unbreakable at the same time due to their special material properties. With demanding anatomies, sequences such as the HyFlex ™ EDM Max Curve Set help endo experts “stay on the right path”.

In the second part of this specialist article, which appears in the 1/2020 issue of the Endodontie Journal, the efficient use of TCA technology in practice is explained.

The corresponding literature list is available here.

Author: Dr. Antonis Chaniotis

The technical article is in Endodontics Journal published.

Photo: author