(a) Preoperative radiograph of a mandibular right first molar with a fragment in the middle third of the mesial root. (b, c) Operative microscope photographs of the pulp chamber floor. (d, e) Working length radiographs with different angulations to aid in the localization the fragment. (f, g) CBCT images of the fragment present in the isthmus. (h) Operative microscope photograph after exposure of the coronal part of the fragment. (i) Master cone radiograph. (j) Immediate postoperative radiograph. (k) Two-year follow-up radiograph (Courtesy Dr. K. Kalogeropoulos)
Cross section of the distal and mesial root of a lower first molar. The cross-sectional shape of the root canal determines to a great extend the available space between the fragment and the canal walls. The space around a fragment in an oval root canal provides access for manipulations as opposed to the available space in case of fragments in narrow canals with round cross section
5.2 Localization of the Fragment
It is widely recognized that the location of the fragment in relation to the curvature of the root canal is the main determinant, rather than the method used to retrieve it. Fragments in the coronal third that is, more or less, the straight visible portion of the root canal are managed more easily and with a higher success rate compared to the corresponding rates for fragments in the middle or apical third (Souter and Messer 2005; Madarati et al. 2008; Tzanetakis et al. 2008; Cuje et al. 2010). As a rule of thumb, what cannot be seen cannot be properly managed. When a fractured instrument lies partially around the canal curvature, and particularly when it lies totally beyond the curvature, safe nonsurgical management usually cannot be accomplished unless a straight-line access can be established to their most coronal portion (Ruddle 2002). This straight-line access might be possible in cases of fragments lying partially around curvatures (Ruddle 2002). There are several success rates reported in in vivo and ex vivo studies using a variety of techniques in different modifications for the removal of fractured instruments located partly or totally beyond the curvature (see Tables 6.1–6.9 in Chap. 6).
In conclusion, tooth factors and fragment location factors favorable for retrieval are straight and wide canals, localization in the coronal or mesial third of the root canal and localization prior to the curvature.
5.3 Fractured Instrument Factors
The material. The fragment is made of influences the likelihood of retrieval. Fragments of rotary nickel-titanium (NiTi) endodontic instruments are more difficult to remove in the vast majority of cases compared to stainless steel (SS) instruments. This can be attributed to the fact that:
SS fragments absorb the ultrasonic energy bodily during retrieval efforts and thus move early on in contrast to NiTi fragments which absorb the energy at and/or near the point of contact with the tip (Cohen et al. 2005).
NiTi instruments have also a greater tendency to fracture repeatedly, becoming smaller and smaller when ultrasonic energy is applied to them, whereas SS instruments are more robust (Ruddle 2002; Cohen et al. 2005; Suter et al. 2005; Madarati et al. 2008). The presence or absence of supporting dentinal wall as a contributing factor affecting the tendency of a file fragment to undergo secondary fracture has been investigated (Terauchi et al. 2013). It was concluded that whenever the coronal portion of NiTi fragments lacked dentin wall opposite to the area where the ultrasonic tip was applied, it tended to break significantly faster than those with dentinal wall support (Terauchi et al. 2013). It is possible that ultrasonic energy applied to the fragments was transmitted to the dentin, dissipating some of the stress that would have contributed to file breakage. Therefore, this secondary fracture of fragments can be reduced by applying the ultrasonic tip to the inner curvature of the canal (Terauchi et al. 2013).
Diameter differences 3 mm from the tip of instruments with various tapers
The type of the instrument. This is another largely unaddressed issue in the literature, and the available information is inconclusive. It has been found that the instrument type does not affect the removal success rate (Shen et al. 2004; Suter et al. 2005) and that it also influences removal efficacy (Hülsmann and Schinkel 1999). Thus, Lentulo spirals, for example, have been found to be easier to remove than reamers or Hedstroem files, with which the lowest success rate was recorded (Hülsmann and Schinkel 1999). In the case of Lentulo spiral fillers, this could be attributed to their ability to be bypassed via their empty centers (Hülsmann and Schinkel 1999) and, of course, to the fact that these instruments are used in completely instrumented and thus wider canals where easier manipulations are allowed.