Image showing H-files discarded after clinical use due to plastic deformation of the cutting tip
Distribution of fractured and plastically deformed instruments within different ISO sizes of H-files after clinical use
The analysis of fractured H-files by means of optical microscopy, scanning electron microscopy (SEM) and micro-X-ray computerized tomography (micro-XCT) provided substantial information on the fracture mechanisms. SEM analysis of clinically fractured files revealed the presence of striations, which is the characteristic pattern of fatigue fracture. A crack originated from the cutting surface propagates during clinical use and causes final fracture of H-file (Fig. 3.3). The longitudinal cross-sectional analysis provided additional information on the location and orientation of these cracks in other parts of the fractured H-file. Figure 3.4 shows an optical microscope image of polished cross sections of an as-received and a multiple times used clinically fractured H-file (Fig. 3.4a, b, respectively). The as-received file is free of any internal cracks, pores or other manufacturing defects, while the in vivo fractured file presents many cracks located at the flute regions (Zinelis and Margelos 2002; Kosti et al. 2004). These cracks vary in size and are oriented perpendicular to the long axis of the file. Similarly, the analysis with the micro-XCT demonstrates no evidence of internal defects for the as-received file. However, the clinically fractured file shows extensive secondary cracking (cracks close to fracture plane) and also cracks located in the flute region (Fig. 3.5). Given that unused files are free from defects, it is clear that these cracks have originated and propagated during extensive clinical use. The origin of these cracks could be attributed to two proposed mechanisms. The first explanation suggests that the machining grooves developed during the manufacturing process with milling provide a myriad of sites for crack initiation (Luebke and Brantley 1991; Brantley et al. 1994; Luebke et al. 1995). However, the development of cracks in the flute region does not support this theory. Surface cracks from milling during the manufacturing process extend to all cutting surfaces of H-files, and thus the cracks must originate uniformly from the cutting surface and extend towards the centre of the instrument. In a second scenario, the location of cracks deep in the flutes should be attributed to the abrupt decrease in the cross-sectional diameter which has been introduced to facilitate loading of dentin debris (Zinelis and Margelos 2002). However, it seems that this acts as a stress concentration factor facilitating the initiation of surface cracks and their propagation perpendicularly to the long axis of the instrument (Zinelis and Margelos 2002; Kosti et al. 2004).
(a) Secondary electron image (SEI) with the characteristic striations of fatigue fracture on the surface of clinically fractured H-file. The asterisk indicates the origin of fracture, while the end of the fracture is indicated on the opposite side by the shear lip. (b) The characteristic striations near the origin of the fracture at higher magnification
Optical microscope photograph of the surface of an as-received (a) and a clinically fractured H-file (b). Multiple cracks (the origin is indicated by the numbers) are located on the flutes. The longest crack (#3) extends along the 73% (148.07/202.37) of the bearing cross section(4) (original magnification ×20)
Two-dimensional micro-XCT images of an unused and a clinically broken H-file. The as-received file is free of any internal defects, while the clinically fractured file shows many cracks close to the fracture plane and also in the flute regions (some of them are indicated by asterisks)
The determination of fatigue fracture as the main failure mechanism based on clinical data has important clinical and technological implications. From a clinical standpoint, it means that fracture will occur after a period of in-service use. However, cracks propagate without any macroscopic sign to warn the clinician about the deterioration of the file’s mechanical properties and the upcoming fracture. Although the crown-down technique slows down the crack propagation compared to step-back technique (Kosti et al. 2004), further research is required to estimate a safe number of instrumentation procedures with H-files in such a multivariant environment as the root canal.
Experimental data based on torque testing may provide information on the resistance of H-files to plastic deformation. However, they have failed to provide any clue as to the fatigue resistance of these instruments, and thus it is proposed that the evaluation of fatigue properties should be included in future specifications. Based on the aforementioned failure mechanisms, the lifespan of H-files could be elongated by introducing different alloys in the manufacturing process. In particular the small-sized files which failed as a result of plastic deformation would benefit if they were made of an alloy with higher yield strength and increased resistance to plastic deformation, without compromising the hardness and corrosion resistance of used SS austenitic grades AISI 303 and 304 (Darabara et al. 2004). Similarly, the larger sizes which failed as a result of the fatigue mechanism might benefit from the introduction of a more fatigue-resistant alloy and the reduction of the stress concentration factor due to the abrupt decrease in the cross-sectional diameter in the flute region. However, any change in the manufacturing process should not jeopardize other desirable file properties, such as cutting efficiency, rigidity and loading of dental debris.
3.2.2 Failure Mechanisms of Stainless Steel K-Files
Although both triangular and rectangular K-files are made from the same SS alloys as H-files (Darabara et al. 2004), the ratio between fractured and deformed discarded files is completely different. K-files are discarded in huge numbers due to plastic deformation, and only a fraction of them has been fractured intraorally. Sotokawa (1988) tested 2328 discarded K-files of rectangular and triangular cross section and found a fracture rate of less than 2%. Unpublished data of our group have recorded a similar fracture rate (<3%), although in a much smaller sample size (100 approximately) of K-files discarded after clinical use. The difference from H-files could be attributed to the higher rigidity, bending and torsional resistance, because of the thicker cross section of K-files. Experimental findings showed that K-files are more resistant to torsion (higher torque resistance) and demonstrate higher angular deflection with higher twist angles before fracture (Krupp et al. 1984), although both K-files and H-files are made from the same alloys (Darabara et al. 2004).
Figure 3.6 shows representative fracture surfaces from two retrieved K-files. Both files were fractured due to overloading under torsion (Fig. 3.6a, d) although fatigue striations were also observed (Fig. 3.6b). Small flat regions were also identified in the four corners of Fig. 3.6d, although the analysis at higher magnification determined a featureless flat surface, a finding that may be attributed to the rubbing action of mutual crack surfaces. In both cases the fatigue part occupies a small portion of the cross-sectional area denoting low stress concentration and high overloading. Testing the K-files under simulated conditions, Sotokawa (1988) concluded that fatigue cracks originating in the corners of the cross section decrease the bearing area, leading to the catastrophic fracture of K-files. However, the limited knowledge on this topic cannot provide a conclusive fracture mechanism, and extensive further research based on clinical fractured files must be carried out in order to elucidate the fracture mechanism under clinical conditions. Given that K-files fail due to plastic deformation, the increase of yield stress in torsion would have a beneficial effect on their clinical longevity.
SE images of the surface of clinically failed K-files. (a) A rather smooth fracture surface with shear tongues (indicated by the arrows). (b) Higher magnification of the upper right corner where a few fatigue striations (indicated by the arrows) have originated from the right angle of the cross section. (c) Higher magnification of the central area of (a) with the presence of shear tongues (black arrows) and skewed dimples (white arrows), a typical pattern of shear overloading. (d) A typical fracture surface due to shear under torsion with shear tongues (white arrows). The four corners (black arrows) show small flat regions that may be attributed to fatigue striations, although imaging at higher magnification showed featureless flat surfaces
3.3 Failure Mechanisms of NiTi Files
Currently the “fatigue resistance” of NiTi endodontic files has attracted a lot of attention as it is considered a good criterion for comparing the in-service life of different brands (Cheung 2007). The term “fatigue resistance” stands for the number of revolutions, an instrument can sustain before fracturing and thus it is supposed that a file with a higher fatigue resistance would last longer before fatigue fracture and from this standpoint is a safer instrument. Indeed, the fatigue mechanism fits well with the intracanal conditions as the files function under bending with hundreds of revolutions per minutes (rpm) and this is a perfect environment for fatigue phenomena. However, as it was mentioned in the introductory comments, it is widely accepted that experimental findings have limited clinical relevance and therefore are incapable of foreseeing what really happens in clinical conditions.
NiTi files are also discarded from dental clinics due to plastic deformation or fracture. A number of studies have recorded the failure rate of discarded NiTi files from different dental clinics, but their results are not directly comparable due to differences in their classification protocol and other uncontrolled variables (Sattapan et al. 2000; Al-Fouzan 2003; Arens et al. 2003; Parashos et al. 2004; Alapati et al. 2005; Cheung et al. 2005, 2007; Peng et al. 2005; Spili et al. 2005; Di Fiore et al. 2006; Iqbal et al. 2006; Shen et al. 2006; Spanaki-Voreadi et al. 2006; Wolcott et al. 2006; Wei et al. 2007). Thus there is no clear picture of the incidence of plastic deformation and fracture among discarded files. However, the findings of these studies provide additional information about the nature of fracture mechanisms in vivo. The most important information is that the fracture incidence is independent of the number of uses. Testing 930 instruments of different brands, Parashos et al. (2004) found that fracture incidence is irrelevant to the number of uses (Fig. 3.7). This finding is in accordance with the outcome of a large cohort study showing that the incidence of fracture did not significantly increased if ProTaper files (Dentsply Maillefer, Ballaigues, Switzerland) were reused up to four times (Wolcott et al. 2006). The findings of both studies contradict the involvement of fatigue mechanism, as the continuous degradation of the mechanical properties of instruments should provide an increased fracture rate over successive uses.