< First part

Results
The results of stifness test are illustrated in Table 2. Despite greater diameters due to the increased tapers,the bending moment of both instruments in all sizes satisfied ANSI/ADA requirements. 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.

Table 2: means and standard deviations for bending moment (gm-cm) at 45 degrees.

Discussion
Mechanical properties of NiTi endodontic instruments have been reported in some previous studies), but none of them completely evaluated Maillefer ProFile rotary instruments. Only Walcott and Himel (10) investigated the torsional properties of NiTi,rotary, .04 tapered "U" files in sizes 15,25 and 35. They showed that instruments satisfied and far exceeded the standard for maximum torque and for angular deflection at maximum point. Higher flexibility of NiTi instrumets was also demonstrated by Tepel et al.(11), who reported that NiTi K-files displayed lower resistance to bending and torque values than conventional SS K-files.

These favourable properties are extremely useful in clinical practice. Nickel-titanium instruments have a great ability to recover from gentle distortions. They possess remarkable flexibility that can withstand the rapid,repeated distortions of rotation in curved canals. Moreover, a more flexible instrument can more easily follow the root canal if there is a curvature. A rigid file tends to create aberrations in curved canals including zips and canal transportation., which are a result of the inherent stifness of the metal.It is important to note that also the .06 tapered instruments shows excellent bending properties, which permit safe clinical use.

It has been shown that the NiTi alloy has excellent springback and superelastic properties (12). The super-elastic property can be described briefly. The stress value remains fairly constant up to a certain point of instrument deformation. At the same time,when the deformation rebounds, the stress value remains fairly constant. In clinical endodontics this typical behaviour reduces the tendency of straightening the curved root canals, and the risk of canal transportation. In addition these properties allowed the idea of a safe rotary file to be born. The stainless steel files did not have the flexibility to be used for rotary movements in curved canals, without significantly altering the canal configuration and perforating the canal walls.

The mean values of the tested files in the present study support this affirmation. With the advent of nickel-titanium it became practical to develop a file type of instrument that could be effectively and safely used as a rotary root canal instruments in curved canals.The results of the present study indicated that all measurements satisfied the ANSI/ADA Specification no.28 standard values for bending tests. Both ProFile .04 and .06 instruments showed consistency of mechanical properties among different sizes. Moreover, values increased regularly with increased instrument sizes..An improved quality of manufacturing process ( it is technically more difficult and complicated to fabricate NiTi than SS instruments) can be a possible reason for these constant results. On the contrary, inconsistencies in taper,flute number and design,and shape of the pilot tip could increase stiffness.

ProFile .04 instruments resulted most flexible,and therefore best suited for instrumentation of severely curved and narrow canals. The more flexible instruments tend to negotiate curved canals better and reduce the tendency of straightening, zipping, ledging or perforation of curved canals. The excellent flexibility of .06 ProFile instruments allows safe instrumentation of moderately curved root canals.

The result of the present study demonstrated that ProFile nickel-titanium, variable taper, instruments have excellent bending properties which let it possible to operate these instruments with a low-speed rotary handpiece, that potentially allows shaping of narrow curved root canals without aberrations.

Bibliografy

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