Gianluca Gambarini
University of Siena,Dept. of Dental Materials

Flexibility of profile nickel-titanium rotary instruments.

Abstract
The purpose of the present study was to evaluate the bending properties of Profile .04 and .06 tapered, nickel-titanium rotary endodontic files ( Maillefer, Baillagues, Switzerland). Mechanical properties of the instruments were evaluated according to test procedures described by ANSI/ADA Specification no.28. Comparison of the results with the above mentioned Specification showed that both series of istruments satisfied and far exceeded the standard values. Despite greater diameters due to the increased tapers, mean values were much less than the maximum values set forth in the Specification. Bending moments were significantly greater for Profile .06 than for Profile .04 in all sizes. Values increased regularly with increased instruments size for both tapers. These excellent mechanical properties can be very useful for instrumentation of curved canals.

Fig.1: Test apparatus

Fig.2: Some of the tested instruments

Introduction
Mechanical methods of root canal preparation including rotational, reciprocating and hybrid devices have been promoted to shape canals and much research has been undertaken to assess the efficiency and safety of these devices. Overall, most studies have concluded that the problems created by these mechanical devices were not necessarily a consequence of the action of the handpieces, but rather the fact that the instruments were invariably made by stainless steel, an alloy which is not the most suitable for manufacturing rotary instruments.

The dimensional and physical performance requirements for endodontic instruments designed to prepare the root canals are listed in ANSI/ADA Specifications Nos. 28-58 and ISO 3630/1 (1,2). For some decades manufacturers have also marketed a great number of new instruments, based on different cross-sectional shapes and design concept,to alleviate some of the clinical difficulties encountered in instrumentation procedures, such as ledging,canal transportation (zip and elbow) or straightening the curved canals.Many of these procedural errors were caused by the stiffness of the alloys used,which increased with increasing instrument size (3). The majority of these modifications have consisted in changes in design,which also had a significant effect on instrument's physical and mechanical characteristics (4).

A new generation of endodontic instruments has been developed recently by the replacement of stainless steel (SS) with a nickel-titanium alloy (NiTi), which has a very low modulus of elasticity. Fabrication of endodontic files from nickel-titanium was investigated initially by Walia et al.(5), who demonstrated that NiTI files have a greater degree of elastic flexibility in bending and torsion, as well as superior resistance to torsional fracture,when compared with SS files manufactured with the same process (5).It was suggested that these characteristics should give NiTi endodontic files a greater ability to negotiate curved canals,to reduce the tendency of iatrogenic errors and to allow larger apical preparations of curved canals while maintaining the original path (6).

Thanks to the favourable mechanical properties of the alloy, NiTi instruments are rapidly becoming very popular. New, innovative engine-driven rotary systems using NiTi instruments of various design have been developed during recent years to facilitate cleaning and shaping procedures (7-9). Maillefer ProFile .04 and .06 Series (Maillefer SA, Baillagues,Switzerland) are nickel-titanium rotary instruments with an increased taper (double or triple the standard 0.02 mm),and a unique geometry specifically designed to minimize instrument stress and therefore maximize efficiency ( 8). These instruments have been developed in the hope that the greater flare along the instrument shaft would create automatically the flare required in canal shape. Moreover,the use of radial lands prevents the rotary instruments from cutting into the canal walls in an uncontrolled fashion and causing unwanted transportation.

When new root canal instruments with a unique design which differs markedly from conventional files are produced, several characteristics need to be investigated and tested to allow an efficient and safe clinical usage. On these basis, the aim of the present study was to evaluate the bending properties of Maillefer ProFile .04 and .06 instruments , according to testing procedures (Fig.1) described in ADA Specification no.28.

Materials and methods
Maillefer ProFile .04 and .06 Tapers,NiTi instruments in sizes 15 through 40 were investigated and compared based on ANSI/ADA Specification No.28 for K-type instruments.Ten instruments of each size were tested for stiffness by bending (Fig.2).Overall, one hundred and twenty endodontic instruments were evaluated. All instruments were examined under a measuring microscope at D3 and D16 to ensure uniformity of dimensions ( according to the tolerance indicated by ANSI/ADA Specification No.28), and under a stereo microscope ( magnification x20) to ensure uniformity of cutting flutes and defect free surfaces. All defective instruments were eliminated from the study.

Bending moment was measured when the instrument attained a 45° bend. Experimental procedures were based on testing methodology that has been developed for ANSI/ADA Specification No.28. ANSI/ADA No.28 standards (minimum values for torque and angular deflection at failure and maximum values for bending test) are shown in Table 1.

For the bending test instrumets were inserted in a chuck connected to an electric motor (Fig.1), revolving at 2.0 rpm ( Baure CM 2024, St.Aubin,Switzerland). Three millimiters of the tip were clamped in a chuck connected to a digital torque meter memocouple ( Maillefer SA,Baillagues,Switzerland ) and to a strip chart for recording . The amplifier was set at an angular deflection of 45 degrees, at which point the test stopped automatically. The bending moment was then measured and recorded by the memocouple and the permanent deformation angle was read directly on the strip chart.All data were recorded and subjected to statistical evaluation (p<.05) using ANOVA test.

Table 1
ANSI/ADA specification n. 28 standard value

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