In spite of the plethora of considerable metallurgical improvements in instrument design, alloy composition, and manufacturing process, file failure during instrumentation remains a primary concern. Endodontic instruments are the foreign objects most frequently found in the root canal either in retreatment cases or as a mishap in initial treatments. A literature review revealed a prevalence of retained fractured instruments of between 0.7 and 7.4% in teeth undergoing root canal treatment (RCT) (Crump and Natkin 1970; Hülsmann and Schinkel 1999; Spili et al. 2005; Iqbal et al. 2006; Parashos and Messer 2006; Cheung et al. 2007).

Instrument fracture is an undesirable and troublesome incident during RCT that frustrates both practitioners and patients. It can happen even to experienced clinicians following the most appropriate preventive measures. Instrument fracture may occur in both anterior and posterior teeth, but it is most frequently reported in molars (Iqbal et al. 2006; Wu et al. 2011; Ungerechts et al. 2014), with similar instrument fracture rates for the maxilla and the mandible (Iqbal et al. 2006; Tzanetakis et al. 2008; Ungerechts et al. 2014). Among molars, it is particularly reported as occurring in the mesial roots of mandibular molars (Molyvdas et al. 1992; Hülsmann and Schinkel 1999; Ward et al. 2003).

The vast majority of instrument fracture occurs in the apical third of the root canal (Molyvdas et al. 1992; Yared et al. 2000; Al-Fouzan 2003; Ankrum et al. 2004; Di Fiore et al. 2006; Iqbal et al. 2006; Tzanetakis et al. 2008; Ungerechts et al. 2014; Wang et al. 2014). The probability of file fracture in the apical area was estimated to be 33 times greater compared to the coronal third of the canal and almost six times greater when compared to the middle third of the root canal (Iqbal et al. 2006).

The incidence of endodontic instrument fracture is still an area of uncertainty, firstly because the numerous studies that have assessed this phenomenon offer varying and sometimes conflicting results and, secondly and most importantly, because incidence rates result from studies with several noncomparable methodologies.

The overall reported incidence rate of fractured hand instruments range between 0.25 and 6% (Crump and Natkin 1970; Hülsmann and Schinkel 1999; Spili et al. 2005; Iqbal et al. 2006). The incidence of SS hand instrument fracture among undergraduate students has been reported to be 1.8% (Kerekes and Tronstad 1979) on a tooth level and 1.3% on a root level (Sjogren et al. 1990). A lower percentage of 1% on a tooth level was reported in a retrospective investigation of the incidence of hand instrument (SS and NiTi) fracture during conservative RCT performed by undergraduate dental students over a 10-year period at the University of Bergen in Norway (Ungerechts et al. 2014). The introduction of NiTi instruments, which nowadays have become a mainstay in the vast majority of endodontic and general practices and have added a new dimension to the practice of endodontics, despite their undeniably favorable qualities, has not resulted in an elimination of the problem. The common perception is that NiTi rotary instruments have a higher failure incidence than SS hand instruments (Barbakow and Lutz 1997; Cheung et al. 2005; Iqbal et al. 2006; Wolcott et al. 2006). In contrast (Parashos and Messer 2006), based on the best available clinical evidence, they state that the frequency of fracture of rotary NiTi instruments may actually be lower than that for SS hand files. The incidence rate of fractured rotary NiTi instruments varies greatly according to the type of instrument (brand, size, taper, cross-sectional shape, and instrument design), the assessment of fracture incidence, the operator, the methodology used, and several other variables that differ among the experimental works. These differences are clearly evident in studies that have investigated the fracture incidence of rotary instruments after clinical use (Table 1.1), as well as in ex vivo studies (Table 1.2). A very low fracture incidence was found with instruments with a reciprocation motion, namely, the reciprocating WaveOne files (Cunha et al. 2014) and the Reciproc instruments (Plotino et al. 2015).