Three-dimensional Analysis of Cutting Behavior of

Basic Research—Technology

Three-dimensional Analysis of Cutting Behavior of Nickel-Titanium Rotary Instruments by Microcomputed Tomography Ya Shen, DDS, PhD, and Markus Haapasalo, DDS, PhD Abstract The cutting behavior of nickel-titanium rotary instruments with and without irrigation was evaluated in a bovine bone model. Six brands of NiTi rotary instruments were constrained into a curved trench. The tips of the instruments were bent to create a 1-mm long initial contact with the floor of the trench. After a series of 100, 200, 300, 400, and 500 (1,500 total) push-pull strokes on each rotating instrument, the grooves were scanned by microcomputed tomography. The volume of removed material and the maximum depth of the cut groove were measured. Irrigation increased the cutting efficiency of the instruments significantly, except for Liberator (Li). There was a significant correlation between the extracted volume and the maximum depth.

The volume removal rate was highest with K3 and Li (dry) and with K3 and FlexMaster (FM) (irrigation group). The maximum cutting depth was highest with FM and K3 in both dry and irrigation groups. The cutting behavior of NiTi rotary instruments depends both on experimental setup, instrument design, and cutting condition. (J Endod 2008;34:606 – 610)

Key Words Cutting behavior, instrument, microcomputed tomography scan, nickel titanium

S

ince the introduction of the nickel-titanium (NiTi) rotary endodontic instruments in 1991, endodontic therapy has coupled 360° rotary instrumentation with the new technology of the NiTi alloy. Using NiTi rotary instruments for root canal instrumentation has enabled clinicians to more predictably and efficiently create consistently tapered preparations while minimizing procedural mishaps, especially in curved canals (1). Although these instruments have been applied with great success, progress in instrument design and surface treatment continues to be made even after sixteen years.

In addition to shaping ability, flexibility, resistance to breakage and cutting efficiency are the most desirable mechanical properties for endodontic instruments. Cutting dentin is an essential step during root canal treatment. It contributes greatly to the removal of infected dentin and provides an adequate funnel-shaped preparation (2). However, because of their pseudoelastic properties, NiTi instruments must be machined rather than twisted, which may lead to surface defects within the cutting surfaces and affect their cutting efficiency (3).

Currently, there are approximately 30 different brands and designs of NiTi rotary endodontic instruments on the market. Although the advertising literature is rich in claims of superiority of various NiTi rotary instrument designs, few of these claims can be supported by well-designed objective studies in endodontic literature. No standards exist for either the cutting or machining effectiveness of NiTi rotary instruments, nor have clear requirements been established for resistance to wear. Thus, the goals of this investigation were to (1) examine cutting behavior using a standardized approach and (2) determine the effect of irrigation on the above parameters of NiTi rotary instruments. The null hypothesis was that NiTi rotary instruments of different types have a similar cutting behavior when used dry and with sodium hypochlorite (NaOCl) irrigation.

Materials and Methods From the Department of Oral Biological & Medical Sciences, Division of Endodontics, Faculty of Dentistry, University of British Columbia, Vancouver, Canada.

Address requests for reprints to Dr Markus Haapasalo, Division of Endodontics, Oral Biological & Medical Sciences, UBC Faculty of Dentistry, 2199 Wesbrook Mall, Vancouver, BC, Canada V6T 1Z3. E-mail address: markush@interchange. ubc.ca.

0099-2399/$0 - see front matter Copyright © 2008 by the American Association of Endodontists. doi:10.1016/j.joen.2008.02.025

Dentin-Substitute Specimens Bovine femur bone was selected as the testing material (4). The microhardness of the specimens was 50 HV (Vickers hardness), which compares well with dentin (5).

The bones were cleaned with 5.25% NaOCl (90 seconds) and processed into 10 12 5 mm slabs with a band saw under cooling water. A 0.8-mm diameter trephine drill was used to cut a camber trench of 8.0 0.8 mm (length width). The dimensions of the trench were enough to keep the file from slipping and binding. At the top, the chamber trench was 1.5 mm deep. The vertical height from top to bottom of the chamber trench was 7.5 mm, and the chamber trench had a radius of about 14.5 mm.

Five parallel trenches at intervals of 1 mm were prepared on a slab. The bottom of the slab was 15 degrees from the horizontal level (Fig. 1). The slabs were ultrasonically cleaned in tap water for approximately 120 seconds before the experiment.

Cutting Behavior Test Device The instruments were mounted on an electric motor with a 1:16 reduction contraangle handpiece (ATR Tecnika Vision, Dentsply Maillefer, Tulsa, OK). The contra-angle was attached to the descending crosshead of an Instron Testing Machine (Mechanical Tester 8841; Instron Corp., Canton, MA). The slab was glued onto the Instron. The crosshead testing was performed at 3 Hz (36 mm/s). The instrument was operated as push-pull on the trench, at a constant speed of 350 rpm. The instrument was perpen-

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Basic Research—Technology strokes (for 66, 100, 133, and 166 seconds) in the second, third, fourth, and fifth trench. Each instrument was tested under dry conditions and with continuous NaOCl (fresh preparation) irrigation, respectively. Five and a quarter percent NaOCl was used at a flow rate of 20 mL/min. A total of 10 selected instruments from each brand were tested: 5 without irrigation (dry) (PFD, FMD, K3D, HSD, LiD, and AlD) and 5 with NaOCl irrigation (PFH, FMH, K3H, HSH, LiH, and AlH).

Evaluation Procedures A microcomputed tomography system (␮CT-80; Scanco Medical, Bassersdorf, Switzerland) was used to scan the bone specimens after 100, 200, 300, 400, and 500 strokes. Four hundred slices with a voxel size of 30 30 30 ␮m were acquired with each scanning procedure and a three-dimensional (3D) surface model was reconstructed. The series of images of each bone specimen after the experiments was imported into the image processing software package Amira 4.0 (Mercury Computer Systems, Chelmsford, MA). The image stacks were segmented by grayscale, and the bone specimen surface models were reconstructed after the experiments, and the surface model was transformed into a 3D model with the polygon file format using Amira software on a workstation. Volumes of interest were selected extending from the top to the bottom of the trenches. Extracted volumes were measured at 100, 200, 300, 400, and 500 strokes, respectively. The volume removal rate per unit time of instruments at 100, 300, 600, 1,000, and 1,500 strokes (or at the time 33.3, 100, 200, 333.3, and 500 seconds) was calculated. (The volume removal rate per unit time added extracted volume/time; time the number of strokes/frequency).

After preparation, the polygon file format surface model was imported into software program (Geomagic Qualify, Raindrop Geomagic, Research Triangle Park, NC, USA) for surface editing. The cut grooves were filled and erased. The surface models were generated before cutting. Finally, matched images of the groove before and after cutting were examined to evaluate the maximum depth of the cut.

One-way analyses of variance with Scheffe’s post hoc tests were used to compare means among various groups. Comparisons were then made by general linear model (GLM) tests among different types of instruments used dry and with NaOCl irrigation on the five grooves.

Pearson correlation coefficients were calculated between the maximum depth and the extracted volume. For all tests, the alpha-type error was set at 0.05. All statistical analyses were performed with the statistical package SPSS version 11.0 (SPSS for Windows; SPSS Inc, Chicago, IL).

Figure 1. Test equipment for measuring the cutting efficiency of NiTi rotary instruments.

dicular to the ground, starting from the top of the trench. The tip of the instrument was bent 1 mm in contact with the floor of the trench from 1 mm below the trench top (Fig. 1). The Instron chart recorded the force generated, which included compression and tension during the use of each instrument. The compression force was analyzed as a function of time generated by test instruments during cutting at a constant rate of descent.

Six brands of #30, 0.06 NiTi 21-mm long instruments were tested: ProFile (Dentsply Maillefer, Ballaigues, Switzerland), FlexMaster (VDW, Munich, Germany), K3 (SybronEndo, Orange, CA), Hero Shaper (Micro-Mega, Bensancon, France), Liberator (Miltex Inc, York, PA), and Alpha (Brasseler, Lemgo, Germany) (Table 1). This specific size was used because it ensured a measurable volume, and it is commonly used clinically. Every instrument was run on five trenches and underwent 100 strokes (33 seconds) in the first trench. Subsequently, the same instrument underwent a new series of 200, 300, 400, and 500

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Results The maximum compression forces for the instruments during five subsequent runs are shown in Table 2. The compression force generated in dry use was higher than under irrigation. The compression force was highest with HS and lowest with PF instruments both with and without irrigation.

The cutting efficiency of the instruments was higher under irrigation than when used dry (Fig. 2). Without irrigation, K3 removed the TABLE 1. Six Brands of NiTi Instruments (#30, 0.06 taper, 21 mm) Tested Group

Brand Name (manufacturer)

PF

ProFile (Dentsply Maillefer, Ballaigues, Switzerland) K3 (SybronEndo, Orange, CA, USA) HERO Shaper (Micro-Mega, Besancon, France) FlexMaster (VDW, Munich, Germany) Liberator (Miltex, Inc., New York, PA) Alpha (Brasseler, Lemgo, Germany)

K3 HS FM Li AI

Batch Numbers 5254370 03J215J 051006 0404310285 397P0604 099487

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Basic Research—Technology TABLE 2. The Maximum Compression Force (N) of the Six Brands of NiTi Instruments during Five Subsequent Runs Dry NaOCl

HS*

FM

K3

Li*

Al*

PF*

4.08 ! 0.38 3.30 ! 0.06

3.08 ! 0.20 2.46 ! 0.06

2.94 ! 0.05 2.24 ! 0.07

2.26 ! 0.11 1.90 ! 0.09

1.92 ! 0.14 1.22 ! 0.02

1.30 ! 0.05 0.64 ! 0.05

HS, HERO Shaper; FM, FlexMaster; Li, Liberator; AI, Alpha; PF, ProFile *There was a significant difference between groups in dry or NaOCl conditions (post hoc test, p

0.05).

maximum volume of material per unit time followed by Li, FM, HS, Al, and PF. With NaOCl irrigation, K3 again removed the highest volume followed by FM, HS, Li, Al, and PF (Fig. 2). The volume removal rate per unit time was significantly higher when irrigation was used for all instruments (post hoc test, p 0.05), except for Li instruments, which showed no significant differences. Compared with dry instrumentation, the initial volume removal rate for five of the instruments under NaOCl irrigation increased between 30% and 172% depending on the instrument. For the Li instrument, the increase was only 14%. The volume removal rate for all instruments decreased rapidly during continued use following a similar pattern, with the exception of the Al instrument, which showed a lower reduction in volume removal rate under NaOCl irrigation during the entire study.

The highest cutting depth was achieved by FM instruments followed by K3, HS, Li, PF, and Al when used without irrigation (Fig. 3 and Table 3). Under NaOCl irrigation, the order was almost similar: FM, K3, Al, HS, Li, and PF. For all instruments, the cutting depth was higher under irrigation (post hoc test, p 0.05), although Li instruments showed no statistically significant differences. The increase of depth between dry and wet (NaOCl) cutting was from 20% (Li) to 162% (Al) when measured at the first 100 strokes.

Within each instrument, there was a significant difference on the volume removal rate per unit time between dry and NaOCl conditions during the five subsequent runs (GLM test, F 54.50, p 0.001). Also, there was a significant difference in the maximum cutting depth (GLM test, F 346.41, p 0.001). High coefficients of correlation were detected between the maximum cutting depth and the extracted volume

in FMD, FMH, PFD, PFH, K3D, K3H, HSD, HSH, LiD, LiH, AlD, and AlH (r2 0.96, 0.95, 0.92, 0.83, 0.99, 0.86, 0.99, 0.93, 0.95, 0.96, 0.98, and 0.92, respectively; p 0.001).

Figure 2. The volume removal rate per unit time ("V/t, mm3/s) of the six brands of NiTi instruments during five subsequent runs.

Figure 3. Different grooves were created by the different instruments after 500 strokes with NaOCl irrigation: Li (left) and FM (right).

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Discussion Few studies in the past have reported on the cutting behavior of NiTi rotary instruments. Plexiglas (6) and extracted teeth have been used for the evaluations (7), measuring the weight loss of each sample after cutting. However, Plexiglas may not be suitable for an objective study of the cutting efficiency of NiTi instruments (8). Lugassy (9) has shown that according to the orientation of its structure, bovine bone better simulates dentin as an experimental model. The availability of multiple samples from one location allows various experimental instruments to be used on near identical samples. The use of bovine bone, therefore, eliminates the problem of the variability in hardness from teeth taken from multiple sources. Some researchers used bovine bone to simulate dentin during instrumentation (5, 10, 11). It is not easy to accurately measure the amount of dentin removed by some NiTi rotary instruments via weight measurements because of their relatively low cutting efficiency. Three-dimensional surface profilometry and ␮CT are common methods applied to reconstruction of 3D surface contour information (11, 12). The ␮CT can provide a 3D image over the entire cut area at a high resolution. During this study and pilot experiments, it was noticed that the maximum depth of cut by some instruments was less than 100 ␮m over 100 strokes. Therefore, to obtain sufficient material removal for reliable measurements, the first run by each instrument before measurement was 100 strokes (33 seconds).

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Basic Research—Technology TABLE 3. The Maximum Cutting Depth (mm) on the Six Brands of NiTi Instruments During the Series of 100, 200, 300, 400, and 500 Strokes File *FMD FMH *K3D K3H *HSD HSH *PFD PFH LiD LiH *AID AIH

Strokes 100

200

300

400

500

0.19 ! 0.06 0.26 ! 0.01 0.17 ! 0.01 0.25 ! 0.01 0.13 ! 0.01 0.23 ! 0.01 0.11 ! 0.01 0.14 ! 0.01 0.10 ! 0.01 0.13 ! 0.01 0.08 ! 0.01 0.21 ! 0.01

0.23 ! 0.02 0.35 ! 0.01 0.19 ! 0.01 0.26 ! 0.01 0.14 ! 0.01 0.24 ! 0.01 0.11 ! 0.01 0.15 ! 0.01 0.13 ! 0.01 0.14 ! 0.01 0.09 ! 0.01 0.30 ! 0.01

0.26 ! 0.10 0.37 ! 0.01 0.19 ! 0.01 0.26 ! 0.01 0.16 ! 0.01 0.25 ! 0.01 0.11 ! 0.01 0.15 ! 0.01 0.13 ! 0.01 0.15 ! 0.01 0.09 ! 0.01 0.31 ! 0.02

0.32 ! 0.05 0.41 ! 0.01 0.21 ! 0.01 0.27 ! 0.02 0.17 ! 0.02 0.25 ! 0.01 0.12 ! 0.03 0.15 ! 0.01 0.15 ! 0.01 0.19 ! 0.01 0.10 ! 0.01 0.32 ! 0.01

0.42 ! 0.10 0.49 ! 0.02 0.23 ! 0.03 0.31 ! 0.02 0.19 ! 0.02 0.29 ! 0.01 0.13 ! 0.03 0.16 ! 0.01 0.17 ! 0.01 0.24 ! 0.03 0.10 ! 0.01 0.32 ! 0.01

FMD, FlexMaster (dry); FMH, FlexMaster (with hypochlorite); K3D, K3 (dry); K3H, K3 (with hypochlorite); HSD, HERO Shaper (dry); HSH, HERO Shaper (with hypochlorite); PFD, ProFile (dry); PFH, ProFile (with hypochlorite); LiD, Liberator (dry); LiH, Liberator (with hypochlorite); AID, Alpha (dry); AIH, Alpha (with hypochlorite) *There was a significant difference in the same type of instrument showing the maximum cutting depth between dry and NaOCl groups during the five subsequent runs (post hoc test, p 0.05).

The canal instrumentation is partly characterized by the selfthreading effect. The dentin removal process by rotary instruments is a complex phenomenon that includes abrasive processes. In these processes, the dentin layer at the top is formed into a chip by a shearing process in the primary shear zone. The chip slides up the rake face, undergoing secondary plastic flow because of the forces of friction. The dentin is removed as small chips produced by a combination of cutting, plowing, and friction mechanisms. The rake angle may be an important variable in the mechanism of chip formation (13). Although no study has shown that the rake angle of K3 instruments is positive, it is at least close to neutral (14). Therefore, it may not be surprising that the volume removal rate per unit time was highest with K3 instruments. However, it is important to notice that, because of its relatively high stiffness, the force by which HS and other stiff files were pressed against the bone surface was higher than with files which are more flexible, such as PF and Al (Table 2). Obviously, different forces used in instrumentation are a limitation in the present study, and this has to be kept in mind when drawing final conclusions. However, standardizing of the force to be identical for all six instrument brands would have resulted in great variation in the length of the initial contact area (1 mm), which would have been a major confounding factor in the study. In curved canals, which are the prime target and reason to use NiTi rotary files, the degree of curvature and the stiffness of the file are major factors determining the force the individual files are pressed against the canal wall. Therefore, obtaining contact of same length (1 mm) for all files at the beginning of the instrumentation in fact helps simulate the true in vivo situation. However, it is obvious that a fully satisfactory experimental design to test the cutting effectiveness of the NiTi instruments still remains a challenge for future developments.

There is no consensus as to the definition of cutting efficiency on endodontic instruments (6, 11, 15-17). Theoretically, there are several parameters to evaluate the cutting tool. Besides the material removal rate per unit time, the major parameters in measuring the behavior of the cutting tool include the cutting speed, feed rate, the cutting depth, and the cutting environment (9). In addition to the two separated variables “depth of cut” (18) and “extracted volume” (11) that have been used previously to describe the cutting efficiency of endodontic file, the integrated 3-dimensional evaluation of cutting condition was introduced to more detailed understanding of cutting behavior. Recently, more suitable sophisticated measurement software has become available on prototype basis to allow measurement of basic geometric parameters such as volume as well as additional descriptors such as the maximum cutting depth. However, the precision of the calibration pro-

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cedure may be biased by imperfections during by true measurement errors. Under the conditions of the current study, there was a strong correlation between the extracted volume and the maximum depth.

Obviously, the relation between extracted volume and the maximum depth depends on the interaction of a number of factors, such as crosssectional configuration of shaft, sharpness of flutes, flute design, tip design, and forces. The results here indicated that the maximum cutting depth was achieved by FM. According to the beam theory, the stiffness of a structure is determined by the moment of inertia of the cross-section.

Generally speaking, a large cross-sectional area carries a large moment of inertia, which makes a structure stiff and difficult to bend (19). This was confirmed by the present findings that, as the area of the inner core of the cross-section increased (data not shown), the instrument tended to encounter more compression force.

Cutting fluids provide lubrication between the tool, chip, and workpiece at low cutting speeds. Lubrication causes the friction coefficient to change between the chip and the tool (13). The resultant forces are reduced with the decrease in frictional force. In addition, chemical additives may act on the root canal dentin to facilitate instrumentation.

The use of NaOCl to irrigate root canals is currently the gold standard to achieve tissue dissolution and disinfection (20). Sodium hypochlorite is present in root canals before inserting any rotary instrument to provide disinfection as well as lubrication (21). Some studies showed that NaOCl attacked the organic dentin matrix (22) and reduced root dentin microhardness (23). Yguel-Henry et al. (11) evaluated the effects of lubrication on cutting efficiency of Hedstrom and K-files and determined that tap water and 2.5% NaOCl solutions increased the cutting efficiency compared to dry tests. As expected, the current study showed that irrigation does in fact increase the cutting efficiency of NiTi rotary instruments. However, it has been shown that high concentrations of NaOCl profoundly affect mechanical dentin properties within the time frame of endodontic treatment, whereas low concentrations (1%) do not (24). More research is needed to identify different concentratious of NaOCl and different irrigants that interact with the cutting efficiency of instruments.

The changes on the torque and force were different on the different types of instruments when the same irrigation liquid was applied to them (21). The relief or clearance angle provides potential access to the cutting zone for lubrication (13). Li instruments are nonhelical files, which reduce the possibility of debris clogging the flutes. Hence, it was clearly shown in this study that under the dry conditions, the volume removal rate per unit time was higher with Li instruments than with

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Basic Research—Technology other instruments, but the rate with Li under dry conditions was not significantly different from that with irrigation.

Although not reported here, the instrument surface wear, as a correlation with the cutting behavior, was evaluated at the end of the total 1,500 strokes by scanning electron microscope. All instruments showed wear in the form of pitting defects and blunting of the cutting edges. The fact is that friction is higher when the instrument has a high cutting efficiency. The instruments with more wear had a higher initial efficiency than the instruments with less wear. This was in accordance with the comment by Kazemi et al. (25).

The combination of surface wear and low microhardness can decrease the cutting efficiency of NiTi instruments (26). Recently, some studies have examined the possibility of improving the cutting efficiency of NiTi instruments, specifically focusing on surface treatment techniques. These include the implantation of boron ions (27), a thermal nitridation process (6), physical vapor deposition of titanium nitride particles (26), and cryogenic treatment (28). All of these studies have yielded promising results, although the evaluation methods still have limitations.

Similarly, the Al instrument surface with a titanium nitride coating has been claimed to guarantee excellent cutting efficiency and to prevent the instrument from losing its sharpness. The present study clearly showed that the reduction in cutting efficiency in the process of the cutting was less with Al than with other instruments during irrigation. This indicates that less instrument wear occurred in the Al instruments.

This study presents a standardized method to evaluate the cutting effectiveness of NiTi rotary instruments. In the clinic, however, canal curvatures, instrument sequences, and multiple other factors affect the mechanical stress of rotary instruments. Furthermore, the dynamic analysis of the cutting efficiency is complicated by the physical complexity of machine tool systems and cutting processes and the fact that the measurements are time dependent because the components move relative to each other during the process. Further work is required to predict the static and dynamic behavior of NiTi rotary instruments under different conditions by finite-element analysis. Within the limitations of this study, irrigation greatly improved the cutting effectiveness of NiTi rotary instruments. The cutting behavior of NiTi rotary instruments depend both on experimental setup, instrument design, and cutting condition.

Acknowledgments The authors would like to thank Professor Dorin Ruse of theDivision of Biomaterials, Faculty of Dentistry, University of British Columbia for technical assistance with data analysis.

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