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 »  Home  »  Endodontic Articles 10  »  Smooth flexible versus active tapered shaft design using NiTi rotary instruments
Smooth flexible versus active tapered shaft design using NiTi rotary instruments
Introduction - Materials and methods.

L. Bergmans, J.Van Cleynenbreugel, M. Beullens, M.Wevers, B.VanMeerbeek & P. Lambrechts
Departments of Conservative Dentistry, Leuven BIOMAT Research Cluster,
Radiology and Electrical Engineering, ESAT,
Informatics and Telematics, LUDIT,
Metallurgy and Materials Engineering, MTM, Catholic University of Leuven, Belgium.

Root-canal instrumentation should provide a tapered canal form with adequate deep shape to allow three dimensional obturation (Schilder & Yee1984). However, realizing this objective in small curved canals is often difficult when using traditional instruments. In recent years, nickel-titanium (NiTi) rotary techniques have been developed to improve root-canal preparation (Glosson et al. 1995). In fact, owing to the unique properties of the alloy (e.g. shape memory, superelasticity and superior resistance to torsional fracture), NiTi files were able to improve both the morphological characteristics and safety of root-canal preparation (Walia et al. 1988). Various NiTi file designs for taper, blades, grooves and tip have been suggested. In general, neutral groove angles and radial land areas allow a continuous reaming motion. In addition, the Lightspeed system (Lightspeed Technology Inc., San Antonio, TX, USA) incorporates a smooth flexible shaft with a short cutting head (Fig.1). It represents a design that was originally developed to make stainless steel hand files more flexible (Wildey & Senia 1989). Disadvantages of this system include the fact that it tends to produce a round parallel preparation; flare (taper) can be achieved only using a step-back sequence with numerous instrument sizes (Thompson & Dummer1997a). On the other hand, the GT-rotary system (Dentsply Maillefer, Ballaigues, Switzerland) has an active shaft with greater taper (Fig.1). The increased flare along the active shaft makes the instrument less flexible but it produces the flare required in the final canal shape more efficiently. In addition, this feature allows the practitioner to manage a variably tapered file concept (i.e. changing taper either larger or smaller in the sequence of canal preparation).
In conclusion, one may question whether the existence of a smooth flexible shaft design is still beneficial when using Ni Ti rotary instruments during root-canal preparation. Indeed, the inherent flexibility of the NiTi alloy may reduce the need for a smooth and flexible design. In that case, an active tapered but less flexible design that allows a faster realization of the shaping objective might be favorable if other factors (e.g. debris removal, resistance to deformation and/or fracture and production costs) are not considered.
The purpose of this study was to evaluate the influence of the smooth flexible (Lightspeed) versus active tapered (GT-rotary) shaft design on canal preparation by NiTi rotary instruments in curved human root canals. The comparison was done using high-resolution X-ray microfocus computed tomography (XMCT) and custom- made software, according to a method described by Bergmans et al. (2001).

Figure 1. Upper: Lightspeed shaft design.
Lower: GT-rotary shaft design.

Lightspeed shaft design. GT-rotary shaft design.

Materials and methods.

Specimen selection and preparation.
Ten extracted mandibular molars, stored in 0.5% chloramine in water, were selected for the present study, based on their morphological appearance (i.e. fully formed apices and a similar degree of mesial root curvature on visual inspection).Access openings were made, occlusal surfaces flattened and distal roots removed. Next, ISO 10 K-Files (Dentsply Maillefer) were inserted into the mesial canals so that their tips were just visible at the apical foramina. Individual working lengths were calculated 1.0 mm short of these positions. Finally, the teeth were mounted in sample holders, placed into a sodium hypochlorite solution (2.5%) for 30 min, and again stored in 0.5% chloramine in water.

Scanning of uninstrumented teeth (PRE).
The hardware device used in this study was a desktop X-ray microfocus CT scanner (SkyScan 1072, SkyScan b.v.b.a., Aartselaar, Belgium) (Fig. 2). A first scanning procedure was completed for all teeth using9 0.7 kV, 300 mA and13x magnification resulting in a pixel size of 30x30 mm. The scanning procedure contained two stages and approximately 3.5 h were needed for one scanning. During acquisition, hundreds of two-dimensional projections through 1808 of rotation were saved in digital form in a computer disk. In order to gain the third dimension, the data stored as projections were then transformed into 250 new two-dimensional images (cross-sections).This data was stored for later use by software. After scanning, the samples were replaced into a 0.5% chloramine in water solution for 24 h to recover from dehydration.

Figure 2. SkyScan1072 X-ray microfocus CT scanner.

SkyScan1072 X-ray microfocus CT scanner

Root-canal instrumentation.
Each mesial root (n =10), containing two similar canals, was instrumented by both systems (randomly distributed to the buccal and lingual canals) and groups were defined as ‘Smooth’and ‘Active’. ‘Smooth’ used the Lightspeed system (Fig.1). The files have a smooth flexible shaft (nontapered), short cutting heads (0.254-1.75 mm in length) with blades of U-design (neutral rake angle), and a noncutting tip. The complete set incorporates 22 instruments of ISO sizing2 0-100, with half sizes between 20 and 65. ‘Active’ used the GT-rotary system (Fig.1). The files are made by grinding three equally spaced U-shaped grooves around the shaft of a tapered NiTi wire. The instruments have flutes with flat outer edges, known as radial land areas and a noncutting pilot tip. The standard set contains four taper sizes (.12, .10, .08, and.06). In addition, a series of .04 tapered files, with variable tip sizes (15, 20, 25, 30, 35 and 40) and16-mm cutting surface, is available for a second-stage instrumentation after gauging of the original apical constriction diameter.
All canals were instrumented by the same operator (LB) and according to the manufacturers’ instructions (see below). No attempt was made to create coronal preparing with Gates-Glidden burs because it would have reduced the magnitude of apical transportation (Montgomery1985). Cleaning and shaping was initiated using a size no.10 K-File followed by a size no.15 K-File, both in a watch-winding motion. All files were used with the pulp chamber containing a combination of 2.5% sodium hypochlorite and Glyde (Dentsply Maillefer). Irrigation was delivered with a 27-gauge needle (Monoject, Sherwood Medical, St Louis, MO, USA) and2 mL of irrigation were used between each file size. All files were used with light apical pressure and constant speed in a torque- control handpiece powered by an electric stepper motor (Endostepper,  SET, Germany). This motor provided presettings for each instrument type and size for optimal performance. In addition, new files were used for each canal.

Group ‘Smooth’- Lightspeed system (LS).
Canals assigned to group‘Smooth’ (n =10) were prepared with LS-instruments at a speed of1300 r.p.m. Apical instrumentation was initiated with a size 20 LS-instrument and continued with sequentially larger sizes until reaching size 45 (MAR). If any instrument failed to go to the working length, then the previous one was reused. Apical preparation was completed with a stepback of1.0 mm at a time with the next four larger instruments. Finally, mid-root instrumentation was continued by continuing the step-back ‘by feel’, as described by the manufacturer, to a size 70; recapitulation to working length was completed with the MAR to confirm canal patency.

Group ‘Active’- GT-rotary system (GT).
Canals assigned to group‘Active’ (n =10) were prepared with GT-instruments in a modified double flare concept (Fava1983) at speeds of 350 r.p.m. First, the sequence of four variably tapered instruments (.12/20, .10/20, .08/ 20 and .06/20) was repeated in a crown-down manner until the canals were instrumented to an apical size 20 with taper varying from .06 for the smallest canals to .08 for the larger ones. Canal preparation was completed by using. 04 tapered instruments with increasing tip sizes at full working length for apical preparation to a size 35 or 40. After mechanical preparation, the samples were irrigated with10% citric acid solution, placed into an ultrasound bath for 5 min to remove dentine chips and debris, and again stored in 0.5% chloramine in water.

Scanning of instrumented teeth (POST).
After repositioning, a second scanning procedure was completed for all teeth. Previous parameter settings (90.7 kV, 300 mA and 13_ magnification) were applied, thereby providing data that were stored for later use by software.

Image analysis.
Medical image volume fusion software described by Maes et al. (1997) was employed to geometrically register the image volumes resulting from both scanning procedures (PRE and POST). Next, a previously described methodology was applied on the registered image volumes (Bergmans et al. 2001). According to this method, qualitative analysis was done by visual inspection (3608 rotation) and quantitative analysis was performed at five perpendicular reslices. A first level was taken just below the furcation, and four more slices were made at equally spaced intervals (Fig.3a). The most apical slice was within 1-2 mm of the apical terminus. Values for transportation (Fig. 4a) and net transportation (Fig. 4b) were automatically (i.e. operator has no influence on the results) calculated in 36 directions and these values were grouped to reduce the amount of information. This procedure resulted in values for the buccal, mesial, lingual and distal direction (each representing the mean of three of the original 36 measurements), and values for the directions in between (each representing the mean of six of the original measurements). Those eight directions were coded as illustrated in Fig.3b. Centring ability (Fig. 4b) of the instrument towards the original canal was calculated by the ratio of net transportation and canal diameter after preparation (i.e. centring ratio). To compare rootcanal curvature, a quantitative description of the global root-canal anatomy was provided by the ‘smoothed out’ version of the PRE-axis, and the association of a Frenet-Serret coordinate frame a long the mid-point of the curve as described by Bergmans et al. (2001). Finally, subvolumes (i.e. in between the different horizontal reslices) were calculated, and by subtracting the PRE-subvolumes from the POST-subvolumes, the volumes of removed dentine for the coronal, middle-coronal, middle, middle-apical and apical 1/5th of the root-canal region was known.
The Shapiro Wilk test was used to test the assumption of normality of the data. The multiway factorial anova was then used to test for significant differences between means. When the overall F-test indicated a significant difference, the multiple comparison Tukey-Kramer procedure was taken to investigate which means differ from which other means. Regarding volumes and curvatures, the t-test or Wilcoxon test procedure was used.

Figure 3. (a) Measurements were made at five horizontal levels (perpendicular reslices).
(b) Representation of eight horizontal directions.

Measurements were made at five horizontal levels. Representation of eight horizontal directions

Figure 4. (a) Definition of transportation (T =d1 _d2).
(b) Definition of net transportation (NT + (T _ T0)) and centring ability (ratio = NT/D).

Definition of transportation