Gianluca Gambarini
University of Siena,Dept. of Dental Materials

Torsional and cyclic fatigue testing of profile niti rotary instruments.

Abstract
The purpose of the present study is twofold: first to test Maillefer Profile .04 and .06 instruments for number of revolution to separation and maximum torque at failure,according to testing procedures described in ANSI/ADA Specification no.28. Second,to compare results with those obtained using a cyclic fatigue test, which is considered a more appropriate method for evaluating the dynamic characteristics of engine-driven rotary instruments. 240 Maillefer Profile .04 and .06 Tapers,NiTi instruments in sizes 15 through 40 were investigated and compared . The results indicated that all measurements exceeded the ANSI/ADA Specification No.28 minimum average values for resistance to failure by twisting tests. Data obtained from the cyclic fatigue test showed that increased taper and larger diameters resulted in higher instrument stress and,consequently, fewer cycles to failure. No correlation was noted between data recorded from static (ANSI/ADA) and dynamic (cyiclic fatigue) tests. A new specification (No.101) is therefore needed to control the quality, dimensions and mechanical properties of NiTi rotary files, in which test methods and minimum strength requirements for instruments having design,dimension or materials which are not included in the above-mentioned Specification No.28 should be precisely estabilished.

Fig.1: Fatigue test device

Fig.2: Artificial canal

Introduction
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 all of these instruments have been fabricated from stainless steel.Meanwhile,manufacturers have 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). These modifications have consisted in changes in design,which also had a significant effect on instrument's physical and mechanical characteristics (4).

A more recent innovation has been the replacement of stainless steel (SS) with a nickel-titanium alloy (NiTi), which has a very low modulus of elasticity. NiTI files have been shown to exhibit 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).These characteristics 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 and 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 maximize efficiency ( 8).

When new root canal instruments with a 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. Although some mechanical testing have been previously performed on NiTi files, results have been controversial and dependent mainly on instrument's type and design (10-13). Moreover,ANSI/ADA Specification No.28 is not considered an appropriate test of the dynamic characteristics of engine-driven rotary instruments,because it does not consider fatigue and breakage of rotary instruments operated in flexed conditions while preparing curved canals (14). Therefore,there is a clear need for the development of a test protocol for these innovative NiTi rotary instruments.

A new ANSI/ADA Specification No.101 is currently being developped to specify requirements and test methods for mechanically operated instruments for root canal preparation having designs or materials which are not included within the provisions of the above-mentioned ANSI/ADA Specification No.28. Generally this specification will include root canal instruments having 2% tapers with diameter sizes not included within Specification Nos. 28 and 58, root canal instruments having tapers other than 2% and root canal instruments having other shapes.

On these bases, the purpose of the present study is twofold: first to test Maillefer Profile .04 and .06 instruments for number of revolution to separation and maximum torque at failure,according to testing procedures described in ANSI/ADA Specification no.28. Second,to compare results with those obtained using a new test method (resistance to cyclic fatigue ) , which is considered a more appropriate procedure for evaluating the dynamic characteristics of engine-driven rotary instruments.

Materials and methods
240 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 (Table 1) and on a new test method for evaluating resistance to fracture. Twenty instruments of each size were tested for resistance to fracture by twisting and for the cycic fatigue test. For the twist test ( maximum torque and angular deflection at failure), in order to insert the instruments into the apparatus, each handle was removed where it met the shaft. The shaft end was then inserted in a chuck connected to an electric motor , revolving at 2.0 rpm ( Baure CM 2024, St.Aubin,Switzerland). Three millimiters of the tip were clamped in another chuck connected to a digital torque meter memocouple ( Maillefer SA,Baillagues,Switzerland ) and to a strip chart for recording . The test apparatus was able to measure maximun torque with an accuracy of +/- 1 gm-cm and angular deflection with an accuracy of +/- 2 degrees.

Cyclic fatigue studies were performed using a specifically developped test apparatus ( Maillefer SA,Baillagues,Switzerland ),that allowed free rotation of the instruments within an experimental stainless steel canal. A variable speed motor was set to operate at 350 rpm and the shaft of the rotary instruments were gripped in the chuck of the motor (Fig 1). Ten instruments of each size were tested in a selected canal shape (90° canal curvature with a 5 mm. radius of curvature) at the selected speed Each instrument was placed at the same depth within the artificial stainless-steel canal (Fig.2) and operated to failure. The time of instrument failure was determined by visual inspection and recorded. The number of cycles to failure was calculated from the rpm data by multiplying the rpm by the time to failure.

All data were analyzed by analysis of variance (ANOVA). Significance was determined at the 95% confidence level.

Table 1: ANSI/ADA specification no.28 standard values

Table 2: Means and standard deviationsfor torque at failure (gm-cm)

Table 3: Means and standard deviations for angular deflection at failure ( in degrees)

Table 4: Means and standard deviations for cycles to failure

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