Vol.:(0123456789) 1 3 Odontology https://doi.org/10.1007/s10266-019-00450-6 ORIGINAL ARTICLE Impact of new cross‑sectional designs on the shaping ability of rotary NiTi instruments in S‑shaped canals David Donnermeyer 1  · Anne Viedenz 2  · Edgar Schäfer 3  · Sebastian Bürklein 3 Received: 8 May 2019 / Accepted: 31 July 2019 © The Society of The Nippon Dental University 2019 Abstract The aim was to compare the shaping ability of diferent rotary nickel–titanium instruments in simulated S-shaped canals. One hundred S-shaped canals in resin blocks were prepared to an apical size 25 using F6 SkyTaper (Komet), Silk Files .04, Silk Files .06, Prototypes .04, and Prototypes .06 (all Mani) (20 canals/group). Material removal was measured at 20 measuring points, beginning 1 mm from the endpoint of preparation. Incidence of canal aberrations, preparation time, and instrument failures were also recorded. Statistical analyses were performed using ANOVA, Student–Newman–Keuls and Chi-square test. Pooled data of all measuring points revealed that canals instrumented with Prototypes .04 were signifcantly better cen- tered than those prepared with all other instruments, while Prototypes .06 and both Silk Files performed signifcantly better than F6 SkyTaper (p < 0.05). The preparation time difered signifcantly between all groups and Prototypes .04 allowed the fastest preparation (p < 0.05). There was no statistically signifcant association between type of instrument and incidence of aberrations (p > 0.05). The cross-sectional design had a marked impact on the shaping ability of the instruments, and less tapered instruments maintained the original canal curvature better than instruments having greater tapers. Keywords Canal transportation · Centering ability · Cross-sectional design · Metallurgy · Taper Introduction Several factors have been identifed that exert an impact on the incidence of canal transportation [1] such as design features of the instruments used for canal preparation (heli- cal angle, pitch, cutting angle, rake angle, radial lands, tip design, cross-sectional design, and taper), kinematics (full rotary or reciprocating motion), and metallurgical properties (alloy, electropolishing, heat treatment) [2, 3]. However, also root canal anatomy (degree and radius of curvature, double curvatures) displays a relevant impact on the outcome of root canal preparation [1]. The more severely curved and the smaller the radius of curvature of a root canal, the more distinctive is the risk of canal straightening. Certainly, one of the most challenging canal confgurations regarding pres- ervation of the integrity of the root canal anatomy and main- tenance of the location of the apical foramen is S-shaped canals. The confguration of S-shaped root canals is not uncom- mon in maxillary and mandibular molars. About 30–35% of mesiobuccal canals of maxillary and mandibular frst molars show a double curvature and even 59% of mesiobuc- cal canals of second mandibular molars display an S-shaped confguration [4]. Therefore, several studies used S-shaped canals, mostly simulated canals in resin blocks, for assessing either the shaping ability of diferent instruments [5–12] or the experience of dental students with diferent instruments [13]. Moreover, some case reports are available illustrating the complexity of treating this particular canal confguration [14, 15]. To improve the performance of root canal instruments with respect to their shaping ability, several innovations have been made during the last years, mostly regarding modifca- tions of the cross-sectional design [16–19] and novel heat- treated nickel–titanium (NiTi) alloys [2, 3]. The Silk Files (Mani, Tochigi, Japan) are available with a constant .04 and .06 taper and are characterized by a * Edgar Schäfer eschaef@uni-muenster.de 1 Department of Periodontology and Operative Dentistry, University of Münster, Münster, Germany 2 Münster, Germany 3 Central Interdisciplinary Ambulance in the School of Dentistry, University of Münster, Waldeyerstr. 30, 48149 Münster, Germany