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 »  Home  »  Endodontic Articles 16  »  Factors influencing the fracture of nickel-titanium rotary instruments
Factors influencing the fracture of nickel-titanium rotary instruments
Introduction - Materials and methods.

B. Martin, G. Zelada, P.Varela, J. G. Bahillo, F. Magaan, S. Ahn & C. Rodriguez
Division of Dental Pathology and Therapeutics, Department of Stomatology, School of Dentistry, University of Santiago de Compostela, Spain.
Department of Computation, Faculty of Computer Science, University of A Coruna, Spain.

There is a potential risk of rotary nickel-titanium instruments fracturing within the canals. This is a major concern, since it can jeopardize the success of treatment. In most situations, fracture occurs in the apical third of the canal and the remaining portion is often difficult to remove, especially if the canal is narrow (Serene et al. 1995, Pruett et al. 1997, Schrader et al. 1999). The fragments that remain block the root canal system and impede adequate cleaning, shaping and sealing (Haikel et al. 1999, Cohen & Burns 2000).
The manufacturers of rotary instruments recommend that they be constantly checked for defects that might alert the user prior to fracture. However, it remains a concern that rotary nickel-titanium instruments might break without warning, that is without there being any previous, permanent, visible defect or deformation. Further, unlike their steel counterparts, early signs of metal fatigue are not usually detected in nickel-titanium instruments (Marending et al. 1998). Therefore, although visible inspection is to be commended, it would not seem to be the ideal way of evaluating nickel-titanium instruments in order to prevent fracture.
Fracture of rotary instruments takes place in two different ways: due to torsion or due to fatigue through flexure (Serene et al. 1995). Fracture due to torsion occurs when the tip or any other part of the instrument binds in the canal whilst the handpiece keeps turning. When this occurs and the elastic limit of the metal is exceeded, fracture of the instrument becomes inevitable. This type of fracture has been associated with the application of excessive apical force during instrumentation (Sattapan et al. 2000a, b).
Fracture due to fatigue through flexure occurs because of metal fatigue. The instrument does not bind in the canal but rotates freely until the fracture occurs at the point of maximum flexure (Sattapan et al. 2000b). This type of failure is believed to be an important factor in the fracture of nickel-titanium rotary instruments in clinical usage and may be due to their use in curved canals (Pruett et al. 1997).
Torque is another parameter, which might influence the frequency with which instruments break. When a motor that generates a high degree of torque is used, it is possible to exceed the instrument’s fracture point within the canal. A potential solution would be to use a low-torque endodontic motor, which would run within the maximum permissible torque limit for each rotary instrument. Low-torque motors stop rotating and then begin to rotate in the opposite direction when the instrument has to withstand levels of torque equivalent to those produced by the motor, thus preventing fracture (Gambarini 2000).
There are a number of factors that are associated with fracture of engine-driven rotary instruments including the speed of rotation, and the angle of curvature and the radius of the canals in which the yare used. However, few studies have attempted to explain the extent to which each of these factors is important. The aim of this project was to evaluate the effect of speed, and the angle of curvature and the radius of root canals on the fracture of nickel-titanium rotary instruments.

Materials and methods.
Two hundred and forty canals from extracted human maxillary and mandibular molars were used in this study. Those molars whose apices were not completely closed, those that had extensive caries or whose roots were dilacerated or bayonet shape were rejected. Mesial-distal and buccal-lingual directional radiographs were taken of all the teeth and the angle of curvature of each root canal was ascertained by following the methodology of Pruett et al. (1997). Two parameters were taken into account when determining the curvature of each root canal: the radius and the angle of curvature. Measurements were made by drawing a straight line along the main axis of the coronal part of the root canal and a second line along the main axis of the apical portion of the canal (Fig. 1). The angle of curvature may also be defined as the angle formed by the perpendicular lines traced from the points of deviation, a and b, that intersect in the centre of the circle. The radius of the curvature (r1 and r2) represents the severity with which the canal deviates from a straight line.
The parameters of the angle and the radius of curvature are mutually independent in such a way that even if two canals have the same angle of curvature they may have different radii of curvature, which indicates that some curves are sharper than others.

Figure 1. Determination of the angle (a) and radius (r) of curvature of the root canal.

Determination of the angle and radius of curvature of the root canal

After determining the angle of curvature, the teeth were divided into two groups of 120 canals according to whether the angle of curvature was less than 308 (group A), or greater than 30 degrees (group B). The most severe angle in each root was used to categorize the canals.
Once the canals had been classified according to their angle of curvature, both groups A and B were then divided into three subgroups of 40 canals, which subsequently underwent instrumentation at different constant rotational speeds: 150, 250 and 350 r. p. m. Each file was used at one rotational speed only. The instrumentation was carried out using two types of nickel-titanium endodontic rotary files: K3 (Kerr Europe, Herts, UK) and ProTaper (Dentsply Maillefer, Ballaigues, Switzerland). Thus, a total of 120 canals (60 canals from group A and 60 canals from group B)were instrumented using each type of file. The files were mounted on a low-speed, high-torque electric motor handpiece (TC Motor 3000, Nowag, Goldach, Switzerland) with a contra-angle 16 :1 reduction (WH 975, Dental Work, Burmoos, Austria).
The root canal orifices were made accessible and the entrance to each root canal located with an endodontic explorer. The canals were then made patent using a size 08K-file. The same operator prepared all canals and used a crown-down technique using sequence recommended by the manufacturers.
Light pressure during the instrumentation procedure was used together with back and forth movements of an amplitude of between 2 and 3 mm. The usage time for each instrument was maintained between 5 and10 s.
The root canals were irrigated, frequently with copious quantities of 5. 25% sodium hypochlorite and a water-soluble preparation containing 15% EDTA and urea peroxide (Dentsply GlydeTM file prep) in order to reproduce normal clinical conditions. All files were inspected and sterilized after each use. Each file was used to a maximum of 20 times; when fractures occurred, the instruments were replaced. K3and ProTaper files were used in the order shown in Tables 1and 2.

Table 1. Instrument sequence for the K3 files.

Instrument sequence for the K3 files

Table 2. Instrument sequence for the ProTaper files.

Instrument sequence for the ProTaper files