E. Schafer & R. Schlingemann
Department of Operative Dentistry, University of Munster, Munster, Germany.

Introduction.
In order to facilitate the irrigation process during root-canal preparation and to simplify filling, adequate shaping of the root canal is considered as a key requirement for successful root-canal treatment. Whilst numerous studies have shown that nickel-titanium rotary instruments can effectively produce a well-tapered root canal form sufficient for obturation, with minimal risk of transporting the original canal (Thompson & Dummer 1997, 1998, Bertrand et al. 2001, Hulsmann et al. 2001, Schafer & Lohmann 2002), concern has been expressed about the comparatively high incidence of fractures of rotary Ni-Ti instruments. In addition, only limited scientific data regarding the cleaning ability of these new rotary systems has been reported. Most recently it has been observed that the different rotary nickel-titanium instruments produced inconsistent results (Hulsmann et al. 2000, Schafer & Zapke 2000, Hulsmann et al. 2001, Gambarini & Laszkiewicz 2002, Schafer & Lohmann 2002, Versumer et al. 2002).
The quality guidelines of the European Society of Endodontology (1994) state that the elimination of residual pulp tissue, the removal of debris and the maintenance of the original canal curvature during enlargement are the main objectives of root-canal instrumentation. Several studies have concluded that none of the instrumentation techniques or devices completely clean root canal, especially in curved canals (Bolanos & Jensen 1980, Haikel & Allemann 1988, Hulsmann et al. 1997). Most of these authors also indicate that the cleaning ability of manual root-canal instrumentation is superior to automated devices (Mizrahi et al. 1975, Schwarze & Geurtsen1996, Hulsmann et al.1997).
The aim of this investigation was to compare the cleaning efficacy (residual debris, quality of the smear layer) after preparation of severely curved root canals with rotary nickel-titanium K3 files (SybronEndo, West Collins, CA, USA) or with stainless steel hand K-Flexofiles (Dentsply Maillefer, Ballaigues, Switzerland). Moreover, another purpose of this study was to assess whether instrumentation had an effect on canal curvature.

Materials and methods.

Selection of teeth.
A total of 60 extracted human maxillary and mandibular molars with at least one curved root and curved root canal were selected for this investigation. Coronal access was achieved using diamond burs. Only teeth whose clinical crowns were largely intact, whose root canals were freely accessible with a root-canal instrument size 10 up to the intact root tip, and whose root-canal width near the apex was approximately compatible with size 15 were included. This was checked with silver points sizes15 and 20 (Antaeos, Munich, Germany).
Standardized radiographs were taken prior to the instrumentation with the initial root-canal instrument of size 15 inserted into the curved canal. The tooth was placed in a radiographic mount made of silicone based impression material (Silaplast Futur, Detax, Ettlingen, Germany) to maintain constant position. The radiographic mount comprised a radiographic paralleling device embedded in acrylic resin. This device was attached to a Kodak Ultra-speed film (Kodak, Stuttgart, Germany) and was aligned, so that the long axis of the root canal was parallel and as near as possible to the surface of the film. The X-ray tube, and thus the central X-ray beam was aligned perpendicular to the root canal. The exposure time (0.12 s; 70 kV, 7 mA) was the same for all radiographs with a constant source-to-film distance of 50 cm and an object-to-film distance of 5 mm. The films were developed, fixed, and dried in an automatic processor (Durr-Dental XR 24 Nova, Durr, Bietigheim-Bissingen, Germany).
The degree and the radius of canal curvature were determined using a computerized digital image-processing system (Schafer et al. 2002). Only teeth whose radii of curvature ranged between 4 and 9 mm and whose angles of curvature ranged between 258 and 358 were included (Table 1). On the basis of the degree and the radius of curvature, the teeth were allocated into two identical groups of 30 teeth. The homogeneity of the two groups with respect to the degree and the radius of curvature was assessed using a t-test (Table 1). At the end of canal preparation, the canal curvatures were redetermined on the basis of a radiograph with the final root-canal instrument inserted into the canal using the same technique (Schafer et al.2002) in order to compare the initial curvatures with those after instrumentation. Only one canal was instrumented in each tooth.

Table 1. Characteristics of curved root canals (n = 30 teeth per group).

Root-canal instrumentation.
The working length was obtained by measuring the length of the initial instrument (size 10) at the apical foramen minus 1mm for all groups. The canals of all teeth were prepared with instruments up to size 35; instruments were used to enlarge one canal only. After each instrument, the root canal was flushed with 5 mL of a 2.5% NaOCl solution and at the end of instrumentation with 5 mL of saline using a plastic syringe with a gauge 30 closed-end needle (Hawe Max-I-probe, Hawe-Neos, Bioggio, Switzerland). The needle was inserted as deep as possible into the root canal.
The following instrumentation sequences were used with the two instruments:

Group A.
K3 instruments.
These instruments were set in to permanent rotation (250 rpm) with a 18:1 reduction handpiece (K3 handpiece, W&H, Buermoos, Austria) powered by a torque-limited electric motor (K3etcm, Kerr, Karlsruhe, Germany) using torque setting 3, which is as stated by the manufacturer to be equivalent to a torque limitation of 1.2 N cm. Instrumentation was completed in a crown-down manner according to the manufacturer’s instructions using a gentle in-and-out motion. Every instrument was withdrawn when resistance was felt and replaced by the next instrument in the sequence. The preparation sequence was the same as described in Part 1 of this two-part report (Schafer & Florek2 002):

1. A 0.06 taper size 30 instrument was used to one-half of the working length.
2. A 0.04 taper size 30 instrument was used to one-half to two-thirds of the working length.
3. A 0.06 taper size 25 instrument was used to one-half to two-thirds of the working length.
4. A 0.04 taper size 25 instrument was used to two thirds of the working length.
5. A 0.04 taper size 20 instrument was used to the full working length.
6. A 0.04 taper size 25 instrument was used to the full working length.
7. A 0.04 taper size 30 instrument was used to the full working length.
8. A 0.04 taper size 35 instrument was used to the full working length.

Once the instrument had negotiated to the end of the canal and had rotated freely, it was removed.

Group B.
K-Flexofile.
Hand instrumentation with these stainless steel instruments with non-cutting tips was performed using a reaming motion. Thus, the instruments were manipulated in a clockwise rotation of about 90-1208 until it reached the full working distance. A step-back method of instrument manipulation was not used. All canals were sequentially prepared from size 15 up to size 35 without pre-bending the instruments, which were used to the full working length.

Evaluations.
All root-canal preparations were completed by one operator, whilst the scanning electron microscope (SEM) evaluations and the assessment of the canal curvatures prior to and after instrumentation were carried out by a second examiner who was blind with respect of all to the experimental groups.

Canal cleanliness.
After preparation, all root canals were flushed with saline and dried with adsorbent paper points. Roots were split longitudinally, prepared for SEM investigation and examined under the SEM (Philips PSEM 500X, Eindhoven, the Netherlands) at 20-2500x magnification.
Separate evaluations were recorded for debris and smear layer. The cleanliness of each root canal was evaluated in three areas (apical, middle, and coronal third of the root) by means of a numerical evaluation scale (Hulsmann et al.1997). The following scheme was used:
Debris (dentine chips, pulp remnants, and particles loosely attached to the canal wall):

• Score 1: clean canal wall, only very few debris particles.
• Score 2: few small conglomerations.
• Score 3: many conglomerations; less than 50% of the canal wall covered.
• Score 4: more than 50% of the canal wall covered.
• Score 5: complete or nearly complete covering of the canal wall by debris.

Smear layer (dentine particles, remnants of vital or necrotic pulp tissue, bacterial components, and retained irrigant):

• Score 1: no smear layer, orifice of dentinal tubules patent.
• Score 2: small amount of smear layer, some open dentinal tubules.
• Score 3: homogenous smear layer along almost the entire canal wall, only very few open dentinal tubules.
• Score 4: the entire root-canal wall covered with a homogenous smear layer, no open dentinal tubules.
• Score 5: a thick, homogenous smear layer covering the entire root-canal wall.

The data established for scoring the debris and the smear layer were separately recorded and analysed statistically. Owing to the ordinal nature of the scores, the data were subjected to Wilcoxon’s test (P < 0.05).

Instrumentation results.
Based on the canal curvatures assessed prior to and after instrumentation, canal straightening was determined as the difference between canal curvature prior to and after the instrumentation. The t-test was used for comparison of the two groups. The level of statistical significance was set at P < 0.05.
The time for canal preparation was recorded and included total active instrumentation, instrument changes with in the sequence and irrigation. The change of working length was determined by subtracting the final length (measured to the nearest 0.5 mm) of each canal after preparation from the original length. The preparation time and the loss of working length were analysed statistically using the t-test (preparation time) and the Mann-Whitney U-test (change of working distance) at a significance level of P < 0.05. The number of fractured instruments during enlargement was also recorded. A w2-test was used to determine whether there were significant differences between the two instruments. The number of deformed instruments was not recorded.