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

Introduction.
The unique metallurgical properties of nickel-titanium have made it possible to develop relatively safe rotary instruments. All the new systems incorporate instruments with a taper greater than the ISO standard 0.02 design; indeed rotary nickel-titanium instruments are available with tapers ranging from 0.04 to 0.12 (Thompson & Dummer 1997a). These greater taper instruments have been introduced to improve the relatively low cutting efficiency (Tepel et al. 1995) of nickel-titanium instruments, to reduce the incidence of instrument failures and to enhance canal shape.
Recently, there have been many reports on the effectiveness of rotary nickel-titanium instruments while shaping either simulated root canals in resin blocks (Tepel1997,Thompson & Dummer1997a,b, Baumann & Roth 1999, Schafer & Fritzenschaft 1999, Kum et al. 2000,Thompson&Dummer 2000a) or extracted human teeth with curved root canals (Kosa et al. 1999, Jardine & Gulabivala 2000).These studies corroborate the ability of rotary nickel-titanium instruments to maintain the shape of even severely-curved canals. Furthermore, in most of these studies, it has been pointed out that canal enlargement was significantly faster with rotary nickel-titanium instruments compared to the hand preparation. However, other aspects of the root-canal preparation with rotary nickel-titanium instruments, such as their increased cost and their susceptibility to corrosion under clinical conditions (Thompson 2000) are controversial (Schafer1998,Thompson 2000).
To date, little information on the mechanical properties and shaping ability of nickel-titanium FlexMaster instruments is available. The FlexMaster instruments are made of 55-nitinol (Lohmann in press), with cutting edges machined in to a round blank, resulting in a convex cross-section that is characterized by three equally spaced cutting edges, which are very similar to K-type blades (Fig.1).The instrument does not have radial lands or U-shaped blades. The FlexMaster instruments have ￡attened, non-cutting tips and a rounded transitional angle (Fig. 2). Thirteen FlexMaster instruments are available: sizes 20, 25 and 30 have three different tapers (0.02, 0.04 and 0.06), size 35 has two different tapers (0.02 and 0.06) and sizes 40 and 45 have only a 0.02 taper. The manufacturer recommends that root canals be enlarged with these instruments using the crown-down technique. Depending on the clinical situation, all instruments are not needed for canal enlargement and the instrumentation sequence can be modified according to the degree of curvature and the diameter of the root canal to be prepared. In curved and/or narrow canals, the instruments with greater tapers are designed to initially enlarge the coronal and middle portion of the canal, whilst the 0.02-tapered instruments are used to ￠finish the apical area and to merge the apical and coronal preparations.

Figure 1. Scanning electron microscope image of the cross-section of a FlexMaster instrument (0.06 taper size 30; original magnification 80x) showing three equally spaced K-type cutting edges. The cross-section of a K-type ?le is indicated by dotted lines.



Figure 2. Scanning electron microscope images of the tip region of FlexMaster instruments showing a non-cutting, flattened tip with rounded transitional angles (0.06 taper size 20; original magnification 320x).

The purpose of this study was to compare automated FlexMaster rotary nickel-titanium instruments with stainless steel hand K-Flexofiles (Dentsply Maillefer, Ballaigues, Switzerland) used in a reaming motion during the shaping of simulated curved root canals in resin blocks.

Simulated canals.
Simulated canals made of clear polyester resin (Viapal uP 004/64, Vianova Resins, Hamburg, Germany) with coloured canal walls were used to assess instrumentation. The degree of curvature was either 288 or 358. The diameter and the taper of all simulated canals were equivalent to an ISO standard size15 root-canal instrument. The 288 canals were13-mmlong, the straight part being 5.5 mm and the curved part 7.5 mm. The curvature was defined mathematically with a radius of 7.5 mm resulting in an angle of 288 according to the Schneider method (Schneider1971).The 358 canals were 13-mm long, the straight part being 5 mm and the curved part 8 mm. The radius of the curvature was 6.5 mm.

Preparation of simulated canals.
The simulated canals were prepared either with the Flex- Master rotary nickel-titanium instruments or the stainless steel hand K-Flexo￠les. The transparent blocks were covered with adhesive tape during the preparation phase. All instruments were used to enlarge two canals only. Prior to use, each instrument was coated with glycerine to act as a lubricant, and copious irrigation with water was performed repeatedly after the use of each instrument. All canals were enlarged by an operator experienced in both manual instrumentation with stainless steel K-Flexo￠les and automated preparation with FlexMaster instruments. Measurement of the canals were carried out by a second examiner who was blinded with respect to all the experimental groups. A randomly laid down sequence was used to avoid bias towards one of the two instrumentation techniques, that is rotary instrumentation with the nickel-titanium Flex- Master instruments or manual reaming with the stainless steel K-Flexo￠les. Only six resin blocks (three with 288 curves and three with 358 curves) were instrumente data time to minimize operator fatigue and familiarity. These six resin blocks were defined as a set. The order of use of the two instrument types within a set was rotated.
All canals were enlarged to an apical size 35. The following instrumentation sequences were used with the different instruments:

GroupA: FlexMaster 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 as stated by the manufacturer to be equivalent to a torque limitation of 1.5-1.7 Ncm. Instrumentation was completed in a crown-down manner according to the manufacturer’s instructions using a gentle in-and-out motion. Instruments were withdrawn when resistance was felt and changed for the next instrument. The preparation sequence was slightly modified from that recommended by the manufacturer, because in a pilot study the following instrumentation sequence allowed preparation of the different canals without difficulties:

1. A 0.06 taper size 20 instrument was used to 7 mm.
2. A 0.04 taper size 30 instrument was used to 9 mm.
3. A 0.04 taper size 25 instrument was used to11mm.
4. A 0.04 taper size 20 instrument was used to 13 mm, the full length of the canal.
5. A 0.02 taper size 25 instrument was used to 13 mm, the full length of the canal.
6. A 0.02 taper size 30 instrument was used to 13 mm, the full length of the canal.
7. A 0.02 taper size 35 instrument was used to 13 mm, the full length of the canal.

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

Group B: hand instrumentation with the K-Flexo￠le stainless steel instruments with non-cutting tips was performed using a reaming motion. All canals were sequentially prepared from size 15 up to 35 without pre-bending the instruments, which were used to 13 mm, the full length of the canal.
In each of these two test groups, 24 canals with 288 and 24 canals with 358 curves were enlarged. Thus, a total of 96 canals were prepared.

Assessment of canal preparation and analysis of data.
The time for canal preparation was recorded, and the total active instrumentation, instrument changes with in the described instrumentation sequence and irrigation was included. Changes of working length were determined by subtracting the ￠final length (measured to the nearest 0.5 mm) of each canal after preparation from the original length (13 mm). The preparation time and the change of working length were statistically analyzed using the Mann-Whitney U-test at a significance level of P < 0.05. The number of fractured and permanently deformed instruments during enlargement was also recorded. A w2-test was used to determine whether there were significant differences between the two instruments concerning the instrument failure and the deformation of instruments.
The assessment of preparation shape was carried out with the computer program Image 1.41 (National Institutes of Health public domain program). Therefore, the pre- and post-instrumentation canal shapes were taken in a standardized manner and magnified 40 times by means of a charged coupled device (CCD) camera (SSC-M370CE, Sony Corporation, Tokyo, Japan) and stored in the computer (Macintosh Quadra 660 AV, Apple Computer, Ismaning, Germany). Thereafter, a composite image of the pre- and post-instrumentation images was produced and superimposed (Fig. 3). The amount of resin removed, e.g. the difference between the canal configuration before and after instrumentation was determined bothfor the innerand the outer side of the curvature in1mmsteps using the Image1.41 program. The amount of resin substance removed in all canals was measured one-dimensionally with a precision of x0.01mm. The ￠first measuring point was 1mm away from the apical ending of the canal, the last measuring point was 10 mm from the apical ending, resulting in10 measuring points at the outer and10 points at the inner side of the canal, and thus, in a total of 20measuring points (Schafer et al. 1995, Tepel 1997). All measurements were made at right angles to the surface of the canal. The data were analyzed by the Mann-Whitney U-test, because for some measuring points the data was not distributed normally according to the Kolmogorov- Smirnov test.

Figure 3. Composite print of a simulated 358-curved canal after instrumentation with stainless steel K-Flexofiles (white region) and before preparation (black region). The 20 positions of measurement are outlined by the concentric circles.

Furthermore, based on the superimposition of pre and postoperative images, assessments were made according to the presence of different types of canal aberrations, such as apical zip associated with elbow, ledge, and perforation. These different types of canal aberration were defined according to the detailed descriptions published recently (Thompson & Dummer 2000b).

During the preparation of the 96 canals, a total of two instruments separated. Therefore, the following results were based on the remaining 94 canals. Two canals with 358 curves were excluded.

Instrument failure.
Table 1details the number of deformations and fractures of instruments which occurred during the study. Independent of the curvature of the canals, none of the stainless steel K-Flexo￠les separated. In the 288-curved canals, no fracture of the FlexMaster instruments occurred but in the 358-curved canals two nickel-titanium instruments (both 0.04 taper size 20, and used for the second canal) separated. The number of separated instruments (w2 ј0.511, P ј 0.478) and the number of permanently deformed instruments (w2 ј0.475, P ј 0.492) was not significantly different between the instrument types.

Preparation time.
The mean time taken to prepare the canals with the different instruments is shown in Table 2. Independent of the curvature of the canals, the shortest mean preparation time was recorded when FlexMaster instruments were used. Both in 288- and 358-curved canals, FlexMaster was significantly faster than K-Flexo￠ les (P < 0.001).

Table 1. Number of fractured and permanently deformed instruments.



Table 2. Mean preparation time (min) and SD with the two different instruments.



Table 3. Mean loss of working length (mm) and SD with the two different instruments.



Table 4. Incidence of canal aberrations by instrument types.

Change of working length.
All canals remained patent following instrumentation, thus, none of the canals became blocked with resin shavings. None of the canals showed overextension of preparation, whereas a loss of working distance was found in several canals. The mean loss of working length which occurred with the different instruments is listed in Table 3. The differences between the two instrument types were not statistically significant, in either the 288-curved (P ј 0.828) or 358-curved canals (P ј 0.628).

Canal shapes.
The results concerning the assessment of canal aberrations are summarized in Table 4. With respect to the different types of aberrations evaluated, both in canals with 288 and with 358 curves, there were no significant differences between the two instrument types (w2-test, P > 0.05), but, more zips and ledges were created with K-Flexo￠les. Only one perforationwas created by K-Flexo￠ le in a 358-curved canal.
On average, in the canalswith 288 curves, FlexMaster instruments removed material more evenly on the outer, as well as on the inner side of curvature (Fig. 4a).With the exception of three measuring points, significant differences (P < 0.05) occurred between resin removal achieved with the two different instruments (Table 5). The canals prepared with FlexMaster instruments remained better centred compared with those enlarged with K-Flexo￠les (Fig. 4).
In the canals with 358 curves, the FlexMaster instruments removed more material on the outer side of the curvature (Fig. 5a). On average, only limited material removal occurred at the inner side of the curvature in the apical part of the canals (Table 6). This elect was not significantly different compared with the material removal achieved with K-Flexo￠les at these measuring points (P > 0.05). Canals shaped with K-Flexo￠les had material removed mainly in the last 1-4 mm along the outer side of the curvature, resulting in moderate outer widening of the canal (Fig. 5b). In general, the FlexMaster instruments showed a more centred enlargement compared with the K-Flexo￠les (Fig. 5).

Figure 4. Mean changes in the canal shape of 288-curved canals as the result of instrumentation with (a) rotary nickel-titanium FlexMaster instruments and (b) stainless steel hand K-Flexo?les (n ¼ 24 canals in each case).



Table 5. Mean material removed (mm) and SD at the different measuring points after instrumentation of simulated 288-curved canals.



Figure 5. Mean changes in the canal shape of 358-curved canals as the result of instrumentation with (a) rotary nickel-titanium FlexMaster instruments and (b) stainless steel hand K-Flexo?les (n ¼ 24 canals in each case).



Table 6. Mean material removed (mm) and SD at the different measuring points after instrumentation of simulated 358-curved canals.

Discussion.
The purpose of this study was to compare the relative efficiency and shaping ability of rotary FlexMaster nickel- titanium instruments with stainless steel hand K-Flexofiles. K-Flexofiles were chosen as controls, because it is well-known that out of all hand instruments used in a rotary motion, these instruments displayed the greatest cutting efficiency (Tepel et al.1995) and most ideal shape (Al-Omari et al.1992, Schafer et al. 1995). Moreover, like the FlexMaster instruments (Fig. 2) the K-Flexofiles also possess a non-cutting tip without transition angle (Scha- fer et al.1995, Bishop & Dummer1997). Because there is no evidence in the literature to suggest that K-Flexofiles are better used in a linear or rotational working motion (Bishop & Dummer 1997), these instruments were used ina reaming motionas the present K-filehas a triangular cross-section. Indeed, several authors have pointed out that the K-files used in a reaming working motion were more efficient and their use resulted in less canal transportation compared to the K-type instruments in a filing motion (Jungmann et al.1975, Roane et al.1985).
The present study described the shaping abilities of the instruments under strictly controlled laboratory conditions, using clear resin blocks. The use of simulated canals in resin blocks does not re￡ect the action of the instruments in root canals of real teeth. However, resin blocks allow a direct comparison of the shaping ability of different instruments (Schafer et al. 1995). A major draw back off using rotary instruments in resin blocks is the heat generated, which may soften the resin material (Kum et al. 2000) and lead to binding of cutting blades, and separation of the instrument (Thompson&Dummer 1997c, Baumann&Roth1999).Thus, owing to the nature of the resin, care should be exercised in the extrapolation of the present results on failure of the use of the FlexMaster instruments in real root canals, where dentine is involved (Thompson & Dummer1997c).
In comparison with the stainless steel K-Flexofiles, rotary FlexMaster instruments achieved better canal geometry, showed less canal transportation and straightening (Figs 4 and 5), and created fewer canal aberrations (Table 4), both in canals with 288 and 358 curves. The ability of rotary nickel-titanium instruments to maintain the original shape of curved canals has been confirmed by several studies (Thompson & Dummer 1997a,b,c,d, Baumann & Roth 1999, Kum et al. 2000). However, similar to other rotary nickel-titanium instruments the FlexMaster instruments created a slight canal transportation toward the outer aspect of the curvature in the apical region of the canals, especially in those having 358 curves (Thompson &Dummer1997d,Thompson & Dummer 1998, Baumann & Roth 1999, Thompson & Dummer 2000b). This canal transportation may be owing to root-canal preparation with instruments of greater taper, because these are stiler compared with those of ISO taper (Kum et al. 2000, Thompson & Dummer 2000b). In contrast, canals enlarged manually with K-Flexofiles showed a more marked transportation toward the outer aspect of the curves and, although not statistically significant (Table 4), a higher incidence of canal aberrations (Al-Omari et al. 1992, Schafer et al. 1995, Bishop & Dummer1997, Kum et al. 2000).
In several studies, the shaping ability of different rotary nickel-titanium instruments and ￡flexible stainless steel hand K-files have been compared, using either simulated canals in resin blocks or extracted human teeth. Kosa et al. (1999) compared canal transportation in moderately curved mesial roots of mandibular molars using rotary ProFile Series 29 instruments and stainless steel Flex-R files in a contra-angle handpiece.According to their results, the re was no difference in the magnitude of absolute transportation between these systems. Luiten et al. (1995) and Esposito & Cunningham (1995) found no significant differences between NiTiMatic rotary nickel-titanium instruments and stainless steel K-Flex hand files. However, the results of two more recently published studies (Schafer & Fritzenschaft 1999, Kum et al. 2000) corroborate the fifindings of the present investigation, in that in severely curved simulated canals the use of rotary nickel-titanium instruments resulted in less canal transportation, fewer canal aberrations, less instrumentation time and better maintenance of working distance compared with stainless steel K-Flexofiles.
In the present study, none of the canals became blocked with resin shavings and none of the canals showed over-extension of preparation. Thus, the only changes of working length was a decrease of working distance. In general, it was possible with both types of instruments to control the working distance adequately (Table 3). This finding is in agreement with the observations of several other studies, in which only small mean changes in working distance occurred with rotary nickel- titanium instruments (Kum et al. 2000, Thompson & Dummer 2000a). On the whole, it is questionable whether the small changes of working length observed in the present study may have any clinical significance. These changes may be owing to minor canal straightening during canal enlargement or the lacko f length control by the operator (Thompson & Dummer 2000a).
The mean time for the canal preparation was recorded and instrument changes within the described instrumentation sequences were included. Both, in the 288- and 358-curved canals, the FlexMaster instruments were significantly faster than the hand preparation with K-Flexofiles (Table 2).This is in agreement with the fifindings of several authors that instrumentation times with rotary nickel-titanium instruments are substantially faster than with stainless steel hand instruments (Thompson & Dummer 1997a,b,c, Kum et al. 2000, Thompson & Dummer 2000a).
During the present study, no fractures occurred with K-Flexofiles, whereas two FlexMaster instruments separated, both in canals with 358 curves. It is worth emphasizing, that these both were 0.04 taper size 20 instruments and were used for the instrumentation of the second canal. To date, no data are available on the torsional properties of the FlexMaster instruments, therefore, no explanation can be given why this particular instrument was susceptible to separation. Related to the total number of FlexMaster instruments used a fracture rate of approximately1% (2 out of168 the Flex- Master instruments used when all instruments were used to enlarge two canals) resulted. This separation rate is in agreement with previously reported fracture rates of newer rotary nickel-titanium instruments (Thompson & Dummer 1997a, Baumann & Roth 1999, Kum et al. 2000,Thompson & Dummer 2000a).

References.

Al-Omari MAO, Dummer PMH, Newcombe RG (1992) Comparison of six files to prepare simulated root canals. Part 2. International Endodontic Journal 21,67-81.
Baumann MA, Roth A (1999) Effect of experience on quality of canal preparation with rotary nickel-titanium files. Oral Surgery 88,714-8.
Bishop K, Dummer PMH (1997) A comparison of stainless steel Flexofiles and nickel-titaniumNiTiFlex files during the shaping of simulated canals. International Endodontic Journal 30, 25-34.
Esposito PT, Cunningham CJ (1995) A comparison of canal preparation with nickel-titanium and stainless steel instruments. Journal of Endodontics 21,173-6.
Jardine SJ, Gulabivala K (2000) An in vitro comparison of canal preparation using two automated rotary nickel-titanium instrumentation techniques. International Endodontic Journal 33,381-91.
Jungmann CL, Uchin RA, Bucher JF (1975) Effect of instrumentation on the shape of the root canal. Journal of Endodontics 1,66-9.
Kosa DA, Marshall G, Baumgartner JC (1999) An analysis of canal centering using mechanical instrumentation techniques. Journal of Endodontics 25, 441-5.
Kum K-Y, SpOngberg L, Cha BY, II-Young J, Seung-Jong L, Chan-Young L (2000) Shaping ability of three ProFile rotary instrumentation techniques in simulated resin root canals. Journal of Endodontics 26,719-23.
Lohmann D (in press) Untersuchungen zur Ezienz desmaschinellen Wurzelkanalaufbereitungssystems FlexMaster auf der Basis permanent rotierend angetriebener Nickel-Titan- Instrumente. Dissertation. Munster, Germany: University of Mu? nster, in press.
Luiten DJ, Morgan LA, Baumgartner JC, Marshall JG (1995) A comparison of four instrumentation techniques on apical canal transportation. Journal of Endodontics 21, 26-32.
Roane J, Sabala CL, Duncanson MG (1985) The balanced force concept for instrumentationof curved canals. Journal ofEndodontics 11, 203-11.
Scha? fer E (1998) Metallurgie und Eigenschaften von Nickel- Titan-Handinstrumenten. Endodontie 7,323-35.
Scha? fer E, Fritzenschaft F (1999) Vergleichende Untersuchung zweier permanent rotierender Wurzelkanalaufbereitungssysteme auf Nickel-Titan-Basis. Endodontie 8, 213-26.
Scha? fer E,Tepel J,HoppeW(1995) Properties of endodontic hand instruments used in rotary motion. Part 2. Instrumentation of curved canals. Journal of Endodontics 21, 493-7.
Schneider SW (1971) A comparison of canal preparations in straight and curved canals. Oral Surgery 32, 271-5.
Tepel J (1997) Experimentelle Untersuchungen Uber die MaschinelleWurzelkanalaufbereitung (Habilitationsschrift). Munster, Germany: University of Munster.
Tepel J, Schafer E, HoppeW(1995) Properties of endodontichand instruments used in rotary motion. Part1. Cutting efficiency. Journal of Endodontics 21, 418-21.
Thompson SA (2000) An overview of nickel-titanium alloys used in dentistry. International Endodontic Journal 33, 297-310.
Thompson SA, Dummer PMH (1997a) Shaping ability of NT engine and McXim rotary nickel-titanium instruments in simulated root canals. Part1. International Endodontic Journal 30, 262-9.
Thompson SA, Dummer PMH (1997b) Shaping ability of Lightspeed rotary nickel-titanium instruments in simulated root canals. Part1. Journal of Endodontics 23,698-702.
Thompson SA, Dummer PMH (1997c) Shaping ability of ProFile .04 taper Series 29 rotary nickel-titanium instruments in simulated canals. Part1. International Endodontic Journal 30, 1-7.
Thompson SA, Dummer PMH (1997d) Shaping ability of ProFile .04 taper Series 29 rotary nickel-titanium instruments in simulated root canals. Part 2. InternationalEndodontic Journal 30,8-15.
ThompsonSA,DummerPMH(1998) ShapingabilityofMity roto 3608 and Naviflex rotary nickel-titanium instruments in simulated root canals. Part 2. Journal of Endodontics 24, 135-42.
Thompson SA, Dummer PMH (2000a) Shaping ability of Hero 642 rotary nickel-titanium instruments in simulated root canals: Part1. International Endodontic Journal 33, 248-54.
Thompson SA, Dummer PMH (2000b) Shaping ability of Hero 642 rotary nickel-titanium instruments in simulated root canals: Part 2. International EndodonticJournal 33, 255-61.