Shaping and Centering Ability, Cyclic Fatigue Resistance and Fractographic Analysis of Three Thermally Treated NiTi Endodontic Instrument Systems

Abstract

The better understanding of the clinically important behavioral features of new instrument systems has an important significance for the clinical endodontics. This study aimed to investigate the shaping and centering ability as well as cyclic fatigue resistance of HyFlex CM (CM), HyFlex EDM (EDM) and EdgeFile (EF) thermally treated nickel-titanium (NiTi) endodontic instrument systems. Sixty curved root canals of the mesial roots of mandibular molars were randomly assigned into three groups (n = 20) and shaped using CM, EDM and EF files up to the size 40 and taper 04 of the instruments. µCT scanning of the specimens before and after preparation was performed and the morphometric 2D and 3D parameters were evaluated in the apical, middle and coronal thirds of root canals. In each group, 40.04 instruments (n = 20) were subjected to the cyclic fatigue resistance test in artificial root canals at 37 °C temperature until fractures occurred, and the number of cycles to failure (NCF) was calculated. The fractographic analysis was performed using a scanning electron microscope, evaluating topographic features and surface profiles of the separated instruments. The one-way analysis of variance with post hoc Tuckey's test was used for statistical analysis of the data; the significance level was set at 5%. All systems prepared the comparable percentage of root canal surface with the similar magnitude of canal transportation in all root thirds (p > 0.05), but demonstrated significantly different resistance to cyclic fatigue (p < 0.05). The most resistant to fracture was EF, followed by EDM and CM. The length of the fractured fragments was not significantly different between the groups, and fractographic analysis by SEM detected the typical topographic features of separated thermally treated NiTi instrument surfaces.

Keywords: cyclic fatigue; fractographic analysis; micro-computed tomography; nickel–titanium; root canal shaping; rotary movement; thermal treatment; transportation.

Figure 1

Cross-sectional images representing reference points of measurements before (greenish) and after (reddish) root canal shaping at the level of 9 (A), 6 (B) and 3 mm (C) from the apex subsequently. X1 represents the shortest distance from the mesial edge and the Y1 from the distal edge of the unprepared root canal to the appropriate edge of the root. X2 and Y2 represent the shortest distance from the mesial and distal edge of the prepared root canal to the relevant edge of the root [19].

Figure 2

Two adjustable metal blocks (A,B) were screwed (1) to make a cyclic fatigue testing device. The tested instrument was inserted into the simulated canal via “orifice” (2) up to the terminal point (3) and subjected to rotation [19].

Figure 3

3D illustrative examples of the shaping ability of the thermally treated instrument systems: preoperative (A), postoperative (B) and superimposed reconstructions (C).

Figure 4

Micrographs of new thermally treated EF (a,d), CM (b,e) and EDM (c,f) endodontic instruments, demonstrating significant topographic differences of the surfaces (magnification ×200).

Figure 5

The SEM analysis under ×200 magnification revealed significant differences of the integrity of the instrument surfaces after cyclic fatigue test. The microscopic defects (white rows) were extensively evident on EF instruments (a,d,g); the CM files demonstrated minimal distortions (b,e,h), while the EDM instruments did not demonstrate any visible defects (c,f,i).

Figure 6

Representative SEM micrographs of the cross-sectional surfaces of fractured files at the ×500 (upper row), ×1000 (middle row) and ×3000 (lower row) magnifications. The typical fatigue propagation zone (dotted line) and dimpled area (white rows) were observed on EF (a), CM (b) and EDM (c) instruments. On higher magnifications, the detailed features of the dimpled area of EF (d,g), CM (e,h) and EDM (f,i) thermally treated files are evident.