Debris extrusion and foraminal deformation produced by reciprocating instruments made of thermally treated NiTi wires

Abstract Objective To evaluate the amount of apically extruded debris, percentage of foraminal enlargement and apical foramen (AF) deformation that occurred during root canal preparation with different reciprocation systems: Reciproc, WaveOne (M-Wire), and ProDesign R (Shape Memory Technology Wire) at two different working lengths (WLs): 0.0 and 1.0 mm beyond the AF. Material and methods The AF of 120 root canals in 60 mesial roots of mandibular molars were photographed with stereomicroscope and randomly assigned into four groups: manual, Reciproc (REC), WaveOne (WO), and ProDesign R (PDR); subsequently, they were further subdivided according to the WL (n=15). Teeth were instrumented, coupled to a dual collecting chamber, and then another photograph of each AF was captured. Extrusion was analysed by determining the weight of extruded debris. Each AF diameter was measured in pre- and post-instrumentation images to determine deformation, which was analysed, and afterwards the final format of AFs was classified (circular/oval/deformed). Results We found no significant differences when analysing each system at different WLs. When considering each WL, REC and WO showed highest extrusion values (P<.05); for AF enlargement, differences were observed only for WO, when it was used beyond the AF; differences were observed among M-Wire groups beyond the AF (P<.05). AF deformation was observed in all groups; PDR showed the lowest AF deformation values at both WLs; M-Wire groups showed 50% strain beyond the AF. Conclusion Authors concluded that beyond the apical limit, the alloy and taper are important aspects when considering extrusion and deformation.


Introduction
Since their introduction by Walia, Brantley, Gerstein 24 (1988), nickel-titanium (NiTi) alloy endodontic instruments have undergone several changes to produce further improvement in their properties 4,13 . Because NiTi is very sensitive to thermal and mechanical treatments, different manufacturing strategies are capable of producing alloys with differentiated superelastic, resistance and memory characteristics 12  Another clinically relevant aspect of these instruments/kinematics is their greater tendency to extrude debris through the apical foramen (AF) during the mechanical preparation of the root canal system, which can lead to postoperative pain 5,23 . This finding is not unanimous in the literature; however, it seems to be related to the design of the instruments (larger or smaller area for debris accumulation between the coils) and kinematics (release of the scrapings collected when the direction of movement is reversed) 1 Regarding the definition of the apical limit of instrumentation, Endodontics has been dedicated to investigating possible variations, and understanding the need to disinfect the entire root canal system, not only to a historically predetermined limit (i.e., 1.0 mm short of the AF), but throughout its entire extension, which means right up to the AF. Thus, apical limits considering the root canal length (RCL) of the tooth, However, a major concern about extending the apical limits (i.e., beyond AF) is the possibility that larger quantities of debris, bacteria and irrigators could be extruded through the AF compared with those that could occur during conventional instrumentation 1,17,20 . This extrusion has commonly been associated with postoperative pain and/or delay Instrumentation of the apical third was carried out with hand K-Flexofile (#50 -#25, Dentsply-Maillefer), using instrument #25 as apical file; instruments were used with balanced force motion.

G2.1 and G2.2 -Reciproc groups
Reciproc R25 instruments driven by motor VDW Silver (VDW GmbH) in "Reciproc All" function were used with gentle in-and-out movements (pecking); the range of motion was limited to 3.0 mm. After each sequence of three pecks, the instrument was completely removed from the root canal and cleaned with gauze.

G3.1 and G3.2 -WaveOne groups
In this group of root canals, preparation was performed similarly to that described for the Reciproc groups; however, we used the electric motor function "WaveOne All".

G4.1 and G4.2 -ProDesign R groups
This group of root canals was prepared in a manner similar to that described for the other reciprocating files; however, we used the electric Endo Easy SI motor (Easy Dental Equipment) in the "ProDesign R" function.  Table 1     WLs (P>.05). When we analysed the percentage of enlargement produced with the apical limit in the AF, we found no differences between techniques; the control group (manual) had the lowest percentages of enlargement. However, considering the preparation beyond the AF, significant differences were observed between control and G2 and G3 (P<.05); there were no differences between G4 and the other groups studied.

Discussion
Null hypotheses tested were both partially rejected, since we observed significant differences in apical debris extrusion and foraminal enlargements.
However, for foraminal deformation, these differences were not statistically significant.

Conclusions
Under the conditions of this study, the authors were able to conclude that all the instrumentation systems produced apical debris extrusion and foraminal deformation; however, rather than the apical limit used, the NiTi-based alloy and the taper were the factors that influenced the results of the reciprocating instruments. The ProDesign R system, made with shape-memory technology, and the .06 taper, showed the best results.