Journal of Endodontics Research - http://endodonticsjournal.com
Efficiency of rotary nickel-titanium K3 instruments compared with stainless steel hand K-Flexofile. Part 2. Cleaning effectiveness and shaping ability in severely curved root canals of extracted teeth
http://endodonticsjournal.com/articles/154/1/Efficiency-of-rotary-nickel-titanium-K3-instruments-compared-with-stainless-steel-hand-K-Flexofile-Part-2-Cleaning-effectiveness-and-shaping-ability-in-severely-curved-root-canals-of-extracted-teeth/Page1.html
By JofER editor
Published on 03/2/2009
 
E. Schafer & R. Schlingemann
Department of Operative Dentistry, University of Munster, Munster, Germany.

Aim.
To determine the cleaning effectiveness and the shaping ability of K3 nickel-titanium rotary instruments and stainless steel hand K-Flexofiles during the preparation of curved root canals in extracted human teeth.  

Conclusions.
Under the conditions of this study, K-Flexofiles allowed significantly better removal of debris than K3 instruments. K3 files maintained the original curvature significantly better. A number of K3 instruments fractured.

Introduction - Materials and methods.
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).

Characteristics of curved root canals

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.

Results.
Instrument failure.
During the preparation of the curved canals, none of the stainless steel K-Flexofiles, and five K3 (two 0.04 taper size 30 and three 0.04 taper size 35) nickel-titanium instruments separated (w2 = 1.733, P = 0.191). All nickel-titanium instruments separated at the tip region (Fig.1).

Canal cleanliness.
The scores for debris and smear layer are detailed in Tables 2 and 3. Completely cleaned root canals were never found. On average, more effective cleaning was observed in the coronal and middle thirds of canals (Fig. 2).
In general, the use of K-Flexofiles resulted in significantly less debris (P < 0.001) compared to canal preparation with K3 instruments (Table 2). In terms of smear layer (Fig. 3), the K-Flexofiles resulted in 24.4% and the K3 system in 17.3% of specimens having scores 1 and 2 (Table 3); no statistically significant differences were apparent (P > 0.05).

Instrumentation results.
The mean time taken to prepare the canals with the two types of instruments is shown in Table 4. There were no statistical significant differences between the two instruments (P = 0.274).
All canals remained patent following instrumentation, thus none of the canals were blocked with dentine. With both types of instruments, one canal showed overextension of preparation, whereas a loss of working distance was found in two canals prepared with K3 and four canals enlarged with K-Flexofiles. The mean changes of working length that occurred with the different instruments are listed in Table 4. The differences between the two instrument types were not statistically significant (P = 0.544).
The mean straightening of the curved canals is shown in Table 5. The use of K3 instruments resulted in significantly less straightening (1.368) during instrumentation (P < 0.0001) compared to the K-Flexofiles (6.918; Fig. 4).

Figure 1. Separated K3 file in the apical portion of a curved canal. Notice the agglomeration of debris (original magnification 40x).

Separated K3 file in the apical portion of a curved canal. Notice the agglomeration of debris

Table 2. Summary of scores for debris.

Summary of scores for debris

Table 3. Summary of scores for smear layer.

Summary of scores for smear layer

Figure 2. Canal wall after preparation with K3 rotary nickel-titanium instruments.
(a) Nearly clean canal wall with small agglomerations of debris particles in the middle portion of the prepared canal (score2, original magnification 40x).
(b) Apical portion of the canal: complete or nearly complete covering of the canal wall by debris after preparation (score 5, original magnification 40x).

Mean changes in the canal shape of 288-curved canals as the result of instrumentation

Figure 3. Canal wall after preparation with K3 rotary nickel-titanium instruments: small amount of smear layer and some open dentinal tubules (score 2, original magnification 640x).

Canal wall after preparation with K3 rotary nickel-titanium instruments: small amount of smear layer and some open dentinal tubules

Table 4. Mean preparation time (min) and SD and mean changes of working distance (mm) and SD with the two different instruments.

Mean preparation time (min) and SD and mean changes of working distance (mm) and SD with the two different instruments

Table 5. Mean degree of straightening of curved canals (8) and SD after canal preparation with the two different instruments (n = 30 canals in each group).

Mean degree of straightening of curved canals (8) and SD after canal preparation with the two different instruments

Figure 4. Straightening of the curved canals after preparation with the two different instruments (n = 24 canals in each group): combined box-and-whisker and dot plot, each dot represents a reading of the difference between canal curvature prior to and after instrumentation.

Straightening of the curved canals after preparation with the two different instruments


Discussion - References.
Discussion.
One of the most important objectives during root-canal instrumentation is the removal of vital and/or necrotic pulp tissue, infected dentine, and dentine debris in order to eliminate most of the microorganisms from the root-canal system (European Society of Endodontology 1994, American Association of Endodontists 1998). The ability to achieve some of these objectives was examined in this investigation on severely curved root canals, involving K3 rotary nickel-titanium instruments and stainless steel hand K-Flexofiles.
Debris was defined as dentine chips, and residual vital or necrotic pulp tissue attached to the root-canal wall which in most cases is infected (Hulsmann et al. 1997).
Thus, debris might prevent the efficient removal of microorganisms from the root-canal system. Moreover, debris may occupy part of the root-canal space, and thus may also prevent complete obturation of the root canal (Wu et al. 2001). The smear layer is a surface film of a thickness of approximately 1-2 mm (American Association of Endodontists1998). Smear layer, which is mainly inorganic, is produced every time a canal is instrumented (Grandini et al. 2002), no smear layer is found on areas that are not instrumented (West et al. 1994). The smear layer contains dentine particles, residual vital or necrotic pulp tissue, bacterial components, protein agglomerates, blood cells as well as retained irrigants, and it blocks the openings of the dentinal tubules (West et al.1994, Grandini et al. 2002). In this way, a thick and non-homogenous smear layer can prevent the efficient elimination of intracanal microorganisms, and compromise the complete sealing of the root canal (Petschelt et al.1987, West et al.1994).
Although it is recommended to use antibacterial irrigants in combination with chelating agents in order to remove debris as well as the inorganic/organic smear layer (West et al.1994, Hulsmann et al. 1997, Gambarini 1999, Grandini et al. 2002), in the present study sodium hypochlorite alone was used as an irrigant. This solution would appear the best available canal irrigant owing to its antibacterial and organic tissue dissolving properties (Spongberg et al. 1973, Turku n & Cengiz 1997), but it is not possible to remove the smear layer with NaOCl (Yamada et al. 1983, Grandini et al. 2002, Guerisoli et al. 2002). Nevertheless, considering the major objective of the present investigation (to solely compare the cleaning effectiveness of the two instrumentation techniques under identical conditions) a simple irrigation technique was used, avoiding any associations of different irrigation solutions. Since it has been recently shown by Hulsmann et al. (2001) and Grandini et al. (2002) that EDTA containing chelating agents (e.g. RC-Prep, Premier, PA, USA, or Glyde File Prep, Dentsply Maillefer, Ballaigues, Switzerland) may be partially responsible for a good cleanliness of the root-canal walls after instrumentation with rotary nickel-titanium instruments, it has to be taken into consideration that the cleaning efficiency of the two instrumentation techniques evaluated in the present study might be further improved using a combination of NaOCl and EDTA containing chelating agent.
In the present study, the cleaning efficacy of two instrumentation methods was examined on the basis of a separate numerical evaluation scheme for debris and smear layer, by means of an SEM-evaluation of the coronal, the middle and the apical portions of the canals (Mizrahi et al. 1975, Bolanos & Jensen 1980, Haikel&Allemann 1988, Hulsmann et al.1997).With both instrumentation techniques, partially uninstrumented areas with remaining debris were found in all canal sections. This finding has also been described by other authors (Bolanos & Jensen 1980, Schwarze & Geurtsen 1996, Hulsmann et al. 1997). Moreover, the present results indicate that on average the apical third of the canals was less clean than the middle and coronal thirds regardless of the instrument used. This observation is also in agreement with other studies (Wu & Wesselink1 995, Hulsmann et al.1997, Schafer & Zapke 2000, Hulsmann et al. 2001, Gambarini & Laszkiewicz 2002).
In general, the use of stainless steel K-Flexofiles resulted in significantly less remaining debris (Table 2) compared to canal shaping with rotary nickel-titanium K3 instruments, whereas for smear layer no significant differences between these instruments occurred (Table 3, P = 0.329). Whilst these results corroborate a previous report, in that stainless steel hand instruments proved to be superior to ProFile rotary nickel-titanium instruments as far as cleaning efficacy is concerned (Schafer & Zapke 2000), in other studies no significant differences between the cleaning efficacy of stainless steel hand and rotary nickel-titanium instruments (Lightspeed, ProFile .04, Rotofiles) were observed (Kochis et al. 1998, Bechelli et al. 1999, Hulsmann 2000). More recently it has been shown that some other modern rotary nickel-titanium instruments, such as Hero 642 and Quantec SC instruments (Hulsmann et al. 2001), GT rotary files (Gambarini & Laszkiewicz 2002) or FlexMaster instruments (Schafer & Lohmann 2002) showed a good cleaning ability. Generally, the canals prepared with these rotary instruments showed only minimal amounts of remaining debris and in many specimens only a thin smear layer was detected with many open dentinal tubules.
With the reservation that the comparability of these results obtained in different studies is limited, it can be concluded that obviously, even different rotary nickel- titanium instruments vary in their debris removal efficiency, possibly due to their flute design (Gambarini 1999,Hulsmannet al.2000). For instance, ProFile instruments have radial lands and it has been shown that this file design was less efficient in debris removal compared to rotary instruments having a positive rake angle (Schafer& Zapke2000, Versumer et al.2002). Instruments possessing U-shaped blades with radial land perform a planing action on the root-canal walls rather than a cutting action, which could to be the main reason for the inferior cleaning ability of these instruments (Versumer et al.2002). The K3 files also posses radial lands and this design feature might explain their poorer cleaning efficiency compared to the results obtained with FlexMaster instruments under identical experimental conditions (Schafer & Lohmann 2002).
Summarizing these aspects, it is open to question whether the differences in the cleaning effectiveness of K3 instruments and K-Flexofile observed in the present study has any clinical significance in term of successful canal debridement, particularly as the ability of K3 instruments to maintain the original canal curvature was significantly superior compared with that of K-Flexofiles.
The teeth in all experimental groups were balanced with respect to the apical diameter of the root canal. Furthermore, based on the initial radiograph the teeth were also balanced with respect to the angle and the radius of canal curvature. To achieve this a computerized digital image-processing system was used to determine both the angle and the radius of curvature (Schafer et al. 2002). The homogeneity of the two groups with respect to the defined constraints was examined using a t-test. According to the P-values obtained (Table 1), the groups were well balanced. The curvatures of all root canals ranged between 258 and 358 and the radii ranged between 4.2 and 8.1 mm (Table 1). Thus, the curvatures of the human root canals were comparable to those of the simulated canals in resin blocks used in the first part of this two-part report (curvatures: 288 and 358; radii: 6.5 and 7.5 mm), allowing a comparison of the results obtained in simulated and in human root canals (Schafer & Florek2 002).
Concerning the ability of the two instruments tested to maintain original root curvatures even in severely curved canals, better compliance with original canal shape was obtained using the K3 system compared to hand instrumentation using K-Flexofiles (Fig. 4). A previous study conducted on curved canals in extracted human teeth also demonstrated that the original canal shape was maintained better when using rotary nickel-titanium instruments compared to a hand-preparation technique with stainless steel K-Flexofiles (Bertrand et al. 2001). In general, the results of the present study using extracted human teeth confirm the findings obtained in the first part of this two-part report after preparation of simulated canals (Schafer & Florek in press), in that the use of K3instruments resulted insignificantly less canal transportation than K-Flexofiles. In simulated canals K3 instruments were significantly faster than K-Flexofiles. Certainly, K3 files needed less time to prepare the root canals of real teeth than K-Flexofiles, but this differences was not significant, in contrast to the results obtained in simulated canals.
During the present study, no fractures occurred with K-Flexofiles, whereas five K3 instruments separated, these allwere0.04 taper instruments. Related to the total number of K3 instruments used a fracture rate of approximately 2.1% resulted and related to the total number of real canals enlarged with these instruments a separation rate of approximately 16.7% resulted. Summarizing these data and the findings obtained in the first part of this two-part report after preparation of simulated canals, it seems that during the crown-down technique great caution should be exercised with the use of 0.04 taper K3 files, as already reported for other rotary nickel-titanium instruments (Blum et al. 1999). Certainly, it is important to note that the K3 rotary instrumentation sequence used in our studies is based on the findings obtained in a pilot study, but is not the one recommended by the manufacturer, because up to now, a particular preparation sequence for severely curved canals recommended by the manufacturer were not available. The manufacturer should recommend a well-defined instrumentation sequence for canal preparation which can be modified according to the difficulty of the canal to be enlarged and to the diameter of the original canal in order to minimize the separation rate. The authors acknowledge that the preparation sequence used in the present study may contribute to the separation rate, and therefore, further research seems to be necessary to investigate the influence of different instrumentation sequences on the separation rate of K3 instruments.

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