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 »  Home  »  Endodontic Articles 6  »  Efficiency of rotary nickel-titanium FlexMaster instruments compared with stainless steel hand K-Flexofile - PART 2
Efficiency of rotary nickel-titanium FlexMaster instruments compared with stainless steel hand K-Flexofile - PART 2
Cleaning effectiveness and instrumentation results in severely curved root canals of extracted teeth



E. Schafer & D. Lohmann
Department of Operative Dentistry, University of Munster, Munster, Germany.

Introduction.
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. Scanning electronic microscopy is useful in examining root-canal cleanliness after preparation with different techniques or instruments. Several studies have concluded that none of the instrumentation techniques or devices completely clean root canals, especially when curved (Bolanos&Jensen1980, Haikel&Allemann1988, 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 & Geurtsen 1996, Hulsmann et al. 1997). However, it was recently shown by several investigations that automated devices using rotary nickel-titanium instruments with various tapers led to good instrumentation results, even in severely curved root canals (Thompson & Dummer 1997, Kum et al. 2000). Unfortunately, little is known about the cleaning effectiveness of these systems, but most recently it has been observed that the different rotary nickel-titanium instruments showed inconsistent results (Hulsmann et al. 2000, Schafer & Zapke 2000).
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 FlexMaster (VDW, Antaeos, Munich, Germany) 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 48 extracted human teeth 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 size10 up to the intact root tip, and whose root-canal width near the apex were approximately compatible with size 15 were included. This was checked with silver points sizes 15 and 20 (VDW, Antaeos, Munich, Germany).
Standardized radiographs were taken prior to instrumentation with the initial root-canal instrument of size 15 inserted into the curved canal. The tooth was attached to a Kodak Ultra-speed ¢lm (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 ¢lm. The X-ray tube, and thus the central Xray beam, was aligned perpendicular to the root canal. The exposure time (0.12 s;70 kV,7 mA) was same for all radiographs with a constant source-to-¢lm distance of 50 cm and an object-to-¢lm distance of 5 mm. The ¢l ms were developed, ¢fixed, and dried in an automatic processor (Durr-Dental XR 24 Nova, Durr, Bietigheim-Bissingen, Germany).

Table 1. Characteristics of curved root canals (n= 24 teeth per group).

Characteristics of curved root canals
* n= 24 canals in each group.

The degree and the radius of canal curvature were determined using a computerized digital image processing system(Schafer et al. in press).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 24 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 re determined on the basis of a radiograph with the ¢final root-canal instrument inserted into the canal using the same technique (Schafer et al. in press) in order to compare the initial curvatures with those after instrumentation. Only one canal was instrumented in each tooth.

Root-canal instrumentation.
The working length was obtained by measuring the length of the initial instrument (size10) 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 two canals 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 NaCl using a plastic syringe with a closed-end needle.
The following instrumentation sequences were used with the two instruments:

Group A
FlexMaster: these instruments were set into permanent rotation (250 rpm) with a 8 :1 reduction handpiece (Type 5059, Nouvag, Goldach, Switzerland) powered by a torque-limited electric motor (TCM Endo 2, Nouvag, Konstanz, Germany) using torque setting 2,which is stated to be equivalent to a torque limitation of 1.5- 1.7 Ncm by the manufacturer. Instrumentation was completed using the crown-down technique 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 (smaller) instrument in the sequence. The preparation sequence was the same as described in part 1 of this two-part report (Schafer & Lohmann 2002):

  1. A 0.06 taper size 20 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 0.04 taper size 25 instrument was used to two thirds of the working length.
  4. A 0.04 taper size 20 instrument was used to the full working length.
  5. A 0.02 taper size 25 instrument was used to the full working length.
  6. A 0.02 taper size 30 instrument was used to the full working length.
  7. A 0.02 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. All canals were sequentially prepared from size15 up to 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 sodium chloride and dried with adsorbent paper points. Roots were split longitudinally, prepared for SEM investigation and examined under the SEM (Philips PSEM 500_, Eindhoven, the Netherlands) at 20-2500_ 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 analyzed statistically. Owing to the ordinal nature of the scores, the data were subjected to the Wilcoxon test (P < 0.05).

Instrumentation results.
Based on the canal curvatures assessed prior to and after the instrumentation, canal strength 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 the total active instrumentation, instrument changes within the sequence and irrigation was included. 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 change of working length were analyzed 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 and permanently deformed instruments during the enlargement was also recorded.

Table 2. Summary of scores for debris.

Summary of scores for debris
* Listed are the number of canal areas evaluated as scores 1-5 (n= 24 teeth per group). Three canal areas (coronal, middle and apical thirds) have been evaluated per tooth, thus resulting in a total of 72 canal areas per group. Score1indicates the best and score 5 theworst canal cleanliness.