Morphometric analysis of shank-to-flute ratio in rotary nickel-titanium files

A comparative study of mechanical resistance of two reciprocating files

Background

To evaluate the cyclic fatigue resistance of two different Nickel-Titanium instruments, Reziflow (Komet, Brasseler GmbH & Co., Lemgo, Germany) and WaveOne Gold (Dentsply Maillefer, Ballaigues, Switzerland).

Material and Methods

Two groups of 20 different NiTi endodontic instruments of identical tip size of 0.25 mm were tested; Reziflow and Wave-One Gold primary. Cyclic fatigue testing was performed in a stainless steel simulated root canal manufactured by reproducing the instrument’s size and taper. A simulated root canal with a 90 degrees angle of curvature and 5 mm radius of curvature was constructed for the instruments tested. The centre of the curvature was 5 mm from the tip of the instrument and the curved segment of the canal was approximately 5 mm in length. Both the instruments were used in the same preset program specific for the WaveOne instruments. Each instrument was rotated until fracture occurred and the time to fracture (TtF) and the length of the fractured fragment were recorded. Means and standard deviations of TtF and fragment length were calculated and data were subjected to statical analysis (P<0.05).

Results

Statistically significant differences (P<0.05) were noted between Reziflow and WaveOne Gold instruments. There were no significant differences (P>0.05) in the mean length of the fractured fragments between the instruments.

Conclusions

Rezifllow instruments were associated with a significantly higher cyclic fatigue resistance than WaveOne Gold instruments.

Key words:Endodontic instruments, NiTi alloy, Reciprocating motion, Cyclic Fatigue, Heat treatment.

Cyclic fatigue of Reciproc and WaveOne reciprocating instruments

AIM

To evaluate the cyclic fatigue resistance of Reciproc(®) and WaveOne(®) instruments in simulated root canals.

METHODOLOGY

Two groups of 15 NiTi endodontic instruments of identical tip size of 0.25 mm were tested, group A; Reciproc(®) R25 and group B: WaveOne(®) primary. Cyclic fatigue testing was performed in a stainless steel artificial canal manufactured by reproducing the instrument's size and taper. A simulated root canal with a 60° angle of curvature and 5-mm radius of curvature was constructed for both the instruments tested. The centre of the curvature was 5 mm from the tip of the instrument and the curved segment of the canal was approximately 5 mm in length. The Reciproc(®) instruments were activated using the preset programme specific for the Reciproc(®) instruments, whilst the WaveOne(®) instruments were activated using the preset programme specific for the WaveOne(®) instruments. All instruments were rotated until fracture occurred and the time to fracture (TtF) and the length of the fractured tip were recorded and registered. Means and standard deviations of TtF and fragment length were calculated for each system and data were subjected to Student's t-test (P < 0.05).

RESULTS

A statistically significant difference (P < 0.05) was noted between Reciproc(®) and WaveOne(®) instruments. Reciproc(®) R25 instruments were associated with a significant increase in the mean time to fracture when compared with primary WaveOne(®) instruments (130.8 ± 18.4 vs. 97.8 ± 15.9 s). There was no significant difference (P > 0.05) in the mean length of the fractured fragments between the instruments.

CONCLUSIONS

  Reciproc(®) instruments were associated with a significantly higher cyclic fatigue resistance than WaveOne(®) instruments.

Rotary NiTi Instrument Fracture and its Consequences

The fracture of endodontic instruments is a procedural problem creating a major obstacle to normally routine therapy. With the advent of rotary nickel-titanium (NiTi) instruments this issue seems to have assumed such prominence as to be a considerable hindrance to the adoption of this major technical advancement. Considerable research has been undertaken to understand the mechanisms of failure of NiTi alloy to minimize its occurrence. This has led to changes in instrument design, instrumentation protocols, and manufacturing methods. In addition, factors related to clinician experience, technique, and competence have been shown to be influential. From an assessment of the literature presented, we derive clinical recommendations concerning prevention and management of this complication.

Cyclic fatigue resistance and three-dimensional analysis of instruments from two nickel-titanium rotary systems

AIM

To determine how instrument design affects the fatigue life of two nickel-titanium (Ni-Ti) rotary systems (Mtwo and ProTaper) under cyclic fatigue stress in simulated root canals.

METHODOLOGY

Cyclic fatigue testing of instruments was performed in stainless steel artificial canals with radii of curvature of 2 or 5 mm and an angle of curvature of 60 degrees . A total of 260 instruments were rotated until fracture occurred and the number of cycles to failure were recorded. The morphology of Ni-Ti rotary instruments was investigated by measuring the volume of millimetre slices of each instrument size starting from the tip to the shank by means of microCT analysis. The fracture surface of three representative samples of each size was analysed by scanning electron microscopy (SEM). Data were analysed by one-way anova, Holm t-test, paired t-test and linear regression; the significance was determined at the 95% confidence level.

RESULTS

Cycles to failure significantly decreased as the instrument volume increased for both the radii of curvature tested (P < 0.01). The radius of curvature had a statistically significant influence on the fatigue life of the instruments (P < 0.05). Larger instruments underwent fracture in less time under cyclic stress than smaller ones. SEM evaluation showed typical features of fracture through fatigue failure.

CONCLUSIONS

The metal volume in the point of maximum stress during a cyclic fatigue test could affect the fatigue life of Ni-Ti rotary instruments. The larger the metal volume, the lower the fatigue resistance.