< First Part

Torque at Failure
Means and standard deviations for torque at failure in clockwise rotation are shown in Table 2. The mean maximum torque values at failure regularly increased as the instruments size increased. Both series of instruments satisfied and far exceeded Specification No.28 minimum values shown in Table 1. Torque at failure was significantly greater for Profile .06 than Profile 04 for all sizes.

Angular Deflection at Failure
Means and Standard deviation values are illustrated in Table 3. Both instruments series far exceeded ANSI/ADA requirements. Minimal or no statistically significant differences were found between the two series. No correlation was found between revolution at failure and either instruments size or taper.

Cyclic Fatigue Test
Cycles to failure were recorded at the moment the instruments fractured. Mean values and SD are shown in Table 4. The mean maximum values generally decreased as the instruments size increased, with the exception of Profile .04 n.35 and Profile .06 n.30,which fractured at higher mean values than the preceding instruments sizes. Profile .06 Tapers exhibited significantly fewer cycles to failure than Profile .04 for all sizes. Profile 06 size 40 showed a more relevant decrease in cycles to failure than the other instruments.

Discussion
Torsional performance,bending properties and cyclic fatigue of NiTi endodontic instruments have been reported in some previous studies (11-16), 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. The above mentioned results are consistent with the findings of the present study.

Camps and Pertot reported that CMU-NiTi (Canal-Master U) showed significantly lower torque at failure than CMU-SS in all sizes (11). CMU-NiTi also exhibited high angular rotation at failure, showing increasing values with increased instruments sizes,whereas with CMU-SS the opposite occurred.

Tepel et al.(12) reported that NiTi K-files displayed lower resistance to bending and torque values than conventional SS K-files.Rowan et al. (13) showed that despite differences in rotation to fracture ( SS files had a significantly greater rotation to failure in the CW direction,whereas the NiTi had a significantly greater rotation to failure in the CCW direction), there was essentially no difference between SS and NiTi instruments in the torque that it took to cause failure in both directions.

Kazemi et al. found that NiTi instruments were more resistant to wear than their stainless steel counterparts (15). Marsicovetere et al. (16) showed that NiTi Lightspeed instruments far exceeded the values of the ANSI/ADA Specification No.28 for revolution to failure. On the other hand,torque to failure tests showed that instruments size 30 through 50 were below the minimum values.

All these controversial results are not easy to explain. It seems clear that file design has a significant, and not yet totally understood,role in the performance and durability of NiTi endodontic instruments under stress.Different manufacturing process may also result in different mechanical behaviour. Furthermore,it was stated (7)that some mechanical properties of NiTi alloys do not comply with convensional rules of metallurgy. Properties such as flexibility and hardness for stainless steel are known,therefore other properties such as resistance to failure during stress are quite predictable. Such simple relationships do not exist for NiTi alloys,which undergo a deformation of the crystal structure when suBjected to mechanical stresses. During these structural changes,the resistance to failure may decrease dramatically,particularly when stresses are sharply changed, and unexpected breakage can occur.

The results of the present study indicated that all measurements exceeded the ANSI/ADA Specification no.28 minimum average values for resistance to failure by twisting tests. Both Profile .04 and .06 instruments showed consistency of mechanical properties among different sizes. Moreover, values increased regularly with increased instrument sizes for both stifness and torque at failure tests. The angular deflection test can give some information about the risk that an instrument which is binding at its tip will fractured if it is rotated any further.However, under clinical conditions the operator has the potential to release the load on the tip of the instruments. On these bases, this parameter does not necessarily correlate with the clinical experience and therefore,does not allow the complete evaluation of endodontic instruments from a clinical perspective.

It is important to understand that the design, tapers, and mode of action of these new NiTi engine-driven instruments clearly place them in a new category of endodontic instruments, which is completely different from conventional K- and H-type files.A new specification which provides a document to identify methods for size and products designation, safety considerations ( for example minimum requirements for flexibility and fracture forces) is therefore needed. Furthermore, test detailed in ANSI/ADA Specification No.28 are conducted in a static mode,which is probably not the most appropriate way for estabilishing the dynamic characteristics of engine-driven rotary instruments.

According to Pruett et al. (14),to test rotary instruments the tip should not be statically locked,but allowed to rotate freely. Moreover,as the phenomenon of repeated cyclic fatigue was consideredby the authors the most important factor in instrument separation and such property was accurately evaluated with a cyclic fatigue test.Results showed that NiTi rotary instruments with larger diameter shafts failed under significanly fewer cycles than did smaller instruments under identical test conditions. Serene had also done a preliminary study on rotation to breakage (cyclic fatigue) of hand instruments, reporting a significant difference between SS and NiTi files (9).

A cyclic fatigue test, which is more similar ( speed, curved canals ) to the manner that rotary instruments are used clinically ,was performed in this study. Data obtained from the present investigation indicated that increased taper and larger diameters resulted in higher instrument stress and,consequently, fewer cycles to failure. Profile .04 instruments failed after significantly greater number of cycles in all instruments sizes. Understanding these limitations should allow the practitioner to take advantage of the superior qualities of NiTI alloy, by minimizing mechanical stress and reducing fractures during clinical use. Unfortunately ,it is very difficult to correlate the in vitro results (number of cycles to failure) with the actual in vivo resistance to failure, because anatomic complexities are extremely different and irregular.It is extremely complicated to understand and/or reproduce experimentally all the different types of mechanical stress that may occur when an endodontic instruments is rotated inside a curved,irregular root canal.

No correlation was noted between data recorded from static (ANSI/ADA) and dynamic (cyiclic fatigue) tests. For example, Profile .06 instruments showed greater torque values at failure, but fewer cycles to failure than Profile .04 for each instruments size. These findings confirm the hypothesis that specific tests, which should include dynamic operation in a flexed state,are needed to evaluate the mechanical properties of this new class of instruments.A new specification is therefore needed to control the quality, dimensions and mechanical properties of NiTi,variable taper, rotary instruments, in which minimum strength requirements should be precisely estabilished.

Bibliografy

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