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Nickel titanium rotary instrumentation in the coronal root third of curved canals Karkan, Mandana

Abstract

One of the desired outcomes of root canal instrumentation is to stay centered in the root canal and avoid stripping of the walls which could lead to perforation. Previous studies have shown that nickel titanium (NiTi) instruments stayed centered in the root canal system to a greater degree than stainless steel instruments. However, in cases such as mesial roots of mandibular molars, where root canals lie closer to the furcation side or the inner part of curved roots (danger zone), root canal instrumentation should be directed away from this region. This type of instrumentation, anticurvature filing technique, has not been reported utilizing rotary NiTi files to determine if removal of dentin during instrumentation can be directed away from the danger zone. It is of clinical significance to determine if rotary NiTi files can be directed away from the danger zone in order to avoid perforation and canal stripping which can lead to endodontic failure. The aim of this study was to investigate whether rotary NiTi Orifice Shapers™ (ProfileR, Tulsa Dental, Dentsply, USA) can be directed away from the danger zone, into the bulky or safety zone of the root dentin during instrumentation of the coronal portion of mesial canals of mandibular molars. For studying the anatomical morphology of root canals before and after instrumentation teeth were mounted in a modified muffle block. The modified muffle block allowed for removal and exact repositioning of the complete tooth block after tooth sectioning. Teeth modified muffle blocks were sectioned at 3 different levels, around furcation and orifice. The mesial canals were divided into 2 groups. Group A (Force group) where force was applied 90 degree to the long axis of the root while instrumenting with NiTi Orifice Shapers™ (Profile[sup R]). Group B (no force group) functioned as control where no lateral force was applied during the instrumentation. Prior to instrumentation using NiTi rotary Orifice Shapers™ (Profile[sup R]), the canals in both groups were enlarged with K-files hand-instruments (Union Broach) up to size 25 as a pre-rotary instrumentation step. The first rotary instrumentation of canals was done using NiTi Orifice shapers™ (Profile[sup R]) according to the manufacturers suggested sequence sizes 30, 50, 40. The second rotary instrumentation of canals involved size 40 Orifice Shapers™ (Profile[sup R]) only. The third instrumentation of canals was done with Gates- Glidden bur #2 (Dentsply, Oklahoma, USA) as a positive control. The modified muffle block sections were scanned before and after each instrumentation. The images were superimposed in Corel Photopaint™ (Corel, Ottowa, CA) and transferred to Scion NIH image 1.62 (Scion Corp, Maryland, USA). Utilizing this software the X -Y centre point coordinates and the area of each canal space were computed. The X -Y centre point movement was calculated after the first rotary instrumentation, after the second rotary instrumentation and finally after Gates-Glidden instrumentation. The lateral force was applied 90 degrees to the long axis of the tooth, and was measured with an Instron Universal Testing Machine (Instron, Massachuset, USA), at all times. No significant difference in canal centre movement was found between force and no force groups after first and second rotary instrumentation. However, a significant difference (p=0.007) was seen in canal centre movement between force and no force groups after Gates-Glidden instrumentation (positive control). It was concluded that with the amount of force (3-5.5N) and the time period (12-16sec) under which the force was applied, it is not possible to direct NiTi Orifice Shapers™ away from the danger area in the coronal root third of the mesial root of mandibular molars.

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