Shaping ability of Lightspeed rotary nickel-titanium instruments in simulated root canals. Part 2

0099-2399/97/2312-0742503.00/0 JOURNAL OF ENDODONTICS Copyright©1997 by The American Association of Endodontists

Printed in U.S.A. VOL. 23, NO. 12, DECEMBER1997

CLINICAL ARTICLES Shaping Ability of Lightspeed Rotary NickelTitanium Instruments in Simulated Root Canals. Part 2, S. A. Thompson, BDS, MPhil, and P. M. H. Dummer, BDS, MScD, PhD

The aim of this laboratory based study was to determine the shaping ability of Lightspeed nickel-titanium rotary instruments in simulated root canals. A total of 40 canals with four different shapes in terms of angle and position of curve were prepared w i ~ Lightspeed instruments using the stepback technique recommended by the manufacturer. This report describes the efficacy of the instruments in terms of prevalence of canal aberrations, the amount and direction of canal transportation, and overall postoperative shape. Preoperative and postoperative images of the canals were taken using a video camera attached to a computer with image analysis software. The preoperative and postoperative views were superimposed to highlight the amount and position of material removed during preparation. Only one (2.5%) zip and one elbow were created, with no ledges, perforations, danger zones, or blockages being produced. At specific points along the canal length there were highly significant differences (p < 0.001) between the canal shapes in total canal width and in the amount of resin removed from the inner and outer aspects of the curve. The direction of canal transportation at the end point of preparation was most frequently toward the outer aspect of the curve; although in half of the canals, transportation was either directed toward the inner aspect or not observed. At the, apex of the curve, the beginning of the curve, and halfway to the orifice, transportation was reversed with the majority of canals being transported toward the inner aspect of the curve. Mean absolute transportation was small and was below 0.06 mm at every position except the orifice. Overall, Lightspccd rotary instruments prepared canals well and would appear to be a valuable addition to the endodontic armamentarium.

Shaping the canal to receive a root filling is the most timeconsuming and difficult aspect of root canal therapy. Many techniques, instruments, and devices have been described but few appear capable of consistently producing the appropriate conical flared foru~ demanded of modern obturation techniques. In fact, the literature is replete with articles describing how shaping procedures can produce aberrations such as zips, elbows, danger zones, perforations, and ledges that compromise the integrity of the root itself and lead to difficulties during obturation. Recently, nickel-titanium hand and rotary instruments have been developed and marketed in the hope that their increased flexibility will reduce the prevalence of aberrations leading to canals with improved form suitable for filling. To date few investigations into the shaping ability of nickel-titanium files have been carried out, particularly those designed for use in rotary handpieces. Those that have appeared have concluded that canal shape was maintained by rotary nickel-titanium files and was significantly faster than hand preparation (1, 2). The aim of this study was to assess the ability of the Lightspeed rotary instruments to shape simulated canals in clear resin blocks.

M A T E R I A L S AND M E T H O D S Construction of Simulated Canals A range of simulated root canals in clear resin blocks was constructed as described previously (3, 4) to provide an experimental model with mild to severe conditions. A total of 40 root canals were produced, with either 20 ° or 40 ° curves and with a straight portion before the curve of either 8 mm or 12 ram. The radius of the arc making up the curved aspect of the canal was 16 mm.

ln~rume~s

Lightspeed instruments are made from nickel-titanium and have a flexible nontapered 16 mm shaft with a short cutting head with U-file blade design having a neutral rake angle and a noncutting

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Vol. 23, No. 12, December 1997 pilot (4). The complete set of 22 Lightspeed instruments are manufactured in sizes 20 to 100 with half sizes between 20 and 65.

Shaping Ability of Lightspeed Instruments

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Preparation of Simulated Canals All canals were prepared by one operator with Lightspeed root canal instruments using a 16: I reduction handpiece powered by an electric motor with a NT-Matic control box (NT Company, Chattanooga, TN). The box features a digital display indicating RPM, which allows maintenance of a constant speed, while compensating for load. Ten canals of each shape were prepared using a constant speed of 750 rpm. All canals were prepared to a working distance of 16 mm and to a Lightspeed size 35 master apical file. Each instrument was used four times before being replaced. Copious irrigation with water was performed before preparation and after the use of each instrument using disposable syringes (Monoject, Ballymoney, N Ireland) and 27 gauge irrigating tips (Endo-Tips, Ultradent Products Inc., Utah). Approximately 20 ml of water was used per block. Before use, each file was coated in Hibiscrub (Zeneca, Cheshire, UK) to act as a lubricant. Files were wiped regularly on a sponge to remove resin debris. The instrumentation sequence for Lightspeed instruments was the same regardless of shape or position of curve in the simulated canal (4). In the first instance, this entailed using a size 20 Lightspeed instrument to the full working length of 16 mm using light apical pressure with an "advance and withdraw" motion. Subsequent preparation was carried out through all the sizes to a master apical file size 35. Canal flare was achieved using a step-back procedure through the range to a size 70 instrument. A final recapitulation with the master apical file was carried out to the full working distance. As it was difficult initially to negotiate the size 20 Lightspeed into the orifice, without binding, the canal orifice was opened minimally before instrumentation using a Canal Master Rotary instrument (Brasseler USA, Savannah, GA); a size 70 CMR was used to a depth of 3 mm at a rotational speed of 1300 rpm.

J il

!:iL ii '

FIG 1. Composite image of a simulated canal with apical zip and elbow. The white region (arrowed as A) defines the image of the canal before preparation and the black region (B) the canal following enlargement. The apical zip is arrowed (Z) toward the end point of preparation and the elbow (E) more coronally.

Assessment of Canal Preparation An A101 Sonata image analysis system with H000 TPL VII image analysis software (Seescan, Cambridge, UK) was used to assess the results of canal preparation. Images of preoperative and postoperative canals were taken in a standardized manner using a C000 monochrome camera with a F010 zoom macro lens and F012 × 2 extender. A composite image was produced of the preoperative and postoperative images of each canal and superimposed using the software. The position and amount of resin removed as a result of preparation was detailed on the composite image. The computer images were standardized by securing the camera at a fixed distance (32 cm) from a microscope stage. Superimposition of preoperative and postoperative specimens was aided through orientation holes placed in the sides of the resin blocks. A magnification scale was calibrated for the image analysis software by grabbing an image of a 5 cm long graticule and measuring between two known points; the image of the graticule was stored in the software for recall. Canal aberrations: An assessment was made of the presence and position of several types of canal aberration (5, 6): 1. An apical zip was defined as an irregular widened area created by the master apical file near the end point of prep-

aration where resin had been removed excessively from the outer aspect of the canal (Fig. 1). 2. Elbows occurred concurrently with an apical zip and formed a narrower region, more coronally (Fig. 1). 3. Ledges were present when an irregular area of resin was removed from the outer aspect of the curved portion of the canal not associated with preparation at the end point. Ledges were always associated with a narrow region more coronally (Fig. 2). 4. Perforations occurred as separate and distinct false canals towards the end point, along the outer aspect of the curve not confluent with the original canal (Fig. 3). 5. Danger zones were defined as the area coronal to the elbow where excess resin had been removed from the inner aspect of the curve (Fig. 4). Danger zones were always associated with a narrower coronal region. Canal width: The composite images enabled assessment of the resin removed by preparation (5, 6). Eleven positions were assessed along the canal length, using a modification of the method described by Alodeh et al. (5). All measurements were carried out perpendicular to the axis of the original canal to the nearest 0.001 ram, using the image analysis software. The positions were:

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Thompson and Dummer

FIG 2. Composite image of a simulated canal with ledge. The ledge is arrowed (L) on the outer aspect of the curve some distance from the end point.

Position 1 (fixed), the end point of preparation. Position 2 (variable), the apical zip at its widest point. Position 3 (variable), the apical elbow at its narrowest point. Position 4 (variable), the ledge at its widest point. Position 5 (variable), the narrowest point of the canal associated with a ledge. Position 6 (fixed), the apex of the curve of the original canal. This was determined by the intersection of two lines, one drawn along and beyond the outer border of the coronal aspect of the original canal and the second drawn along the outer border of the apical aspect of the canal. Position 7 (variable), the widest point coronal to the elbow associated with excessive removal of tissue from the inner aspect of the curve. This defect has been termed the danger zone. Position 8 (fixed), the beginning of the curve. This position was determined as the point along the outer aspect of the original canal, where the canal deviated from the straight line. Position 9 (variable), the narrowest point of the coronal constriction, coronal to the curve and always associated with a danger

Journal of Endodontics

FIG 3. Composite image of a simulated canal with perforation. The perforation is arrowed (P) near the end point of preparation.

Recording, Storage, and Analysis of Data Data were recorded directly on coding sheets and then stored in a PC. Following error and range checks, the data were analyzed using MINITAB (Minitab Inc., State College, PA), an interactive statistics package.

RESULTS

Canal Aberrations

Zips and elbows. Only one zip and elbow was created in the 40 specimens. This occurred in a 20 °, 12 mm canal. Ledges. No ledges were created. Perforations. No perforations occurred during preparation. Blockages. No canals became blocked during preparation. Danger zones. No danger zones were observed and, consequently, no coronal narrowing.

zone.

Position 10 (fixed), a point halfway from the beginning of the curve (Position 8) to the orifice (Position 11). Position 11 (fixed), the canal orifice. The amount and direction of canal transportation was determined from the inner and outer width measurements.

Width Measurements The mean total widths of the prepared canals are shown in Table 1, together with the average amount of resin removed from the

Vol. 23, No. 12, December 1997

Shaping Ability of Lightspeed Instruments

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Transportation

FIG 4. Composite image of a simulated canal with danger zone and narrow region more coronally. The danger zone is arrowed (D) along the inner aspect of the curve with the narrow region (N) toward the orifice.

inner and outer aspects of the canal for the fixed measurement positions only. At the end point of preparation, the 20 °, 12-mm canals were widest followed by the 40 °, 8-ram canals. The narrowest canals were those with a 40 °, 12-ram curve. The differences between the mean total canal widths was significant (p < 0.005). At the apex of the curve 40 °, 8-mm canals were the widest, followed by the 40 °, 12-mm canals. At this position the narrowest canals were the 20 °, 12-mm shapes. There were highly significant differences (p < 0.001) between the canal shapes for total width. At the beginning of the curve, the widest canals were those with 40 ° , 8-ram curves, followed by the 20 °, 8-mm canals; the narrowest canals were those with the 20 °, 12-mm curves. There were highly significant differences (p < 0.001) between the canal shapes for mean total width at the beginning of the curve; at this position significant differences (p < 0.005) were also noted for the inner width measurements. The same pattern for total width measurements was noted halflvay to the orifice. Highly significant differences (p < 0.001) were noted in both the total and inner width measurements between the canal types. At the orifice, the widest canals were those with the 20 °, 12-mm curves, followed by the 40 °, 12-ram canals. There were highly significant differences (p < 0.001) between the canal shapes at both the inner and outer width measurements but not between the mean total widths.

Direction. At the end point of preparation, 31 out of 40 canals were transported (Table 2); 11 to the inner and 20 to the outer. There was a relatively even balance between inner and outer movement in 40 °, 8-mm and 20 °, 12-mm canals but a higher incidence of transportation toward the outer aspect of the curve with the other two canal shapes. Overall, there were no significant differences between the canal types in terms of direction of transportation. At the apex of the curve, 12 canals remained centered, while 20 transported to the inner and 8 to the outer. There was a higher incidence of transportation toward the inner aspect of the curve in both the 40 ° canal types with generally a more even balance between the inner and outer in the 20 ° canals. At the beginning of the curve, 10 canals remained centered, while 25 were transported to the inner and 5 to the outer aspect of the curve. Transportation occurred more often toward the inner aspect of the curve in canals with 40 ° curves and those with 20 ° , 8-mm curves. There were no significant differences between the canal types for transportation at either the apex or the beginning of the curve. Along the straight portion of the canal (halfway to the orifice) 7 canals remained centered; 21 canals were transported to the inner aspect of the curve with 12 canals transported to the outer. Transportation occurred more often toward the inner in all canal shapes except those canals with 20 °, 12-mm curves, where more transportation was apparent toward the outer aspect of the canal. Again, no significant differences were noted between the canal types. At the orifice, no transportation occurred in 3 out of the 40 canals; 22 canals were transported toward the inner and 15 to the outer. Generally, there was a higher incidence of transportation toward the inner aspect of the curve except for canals with 40 °, 8-mm shapes where 9 out of 10 canals were transported toward the outer wall. There were significant differences (p < 0.01) between the canal shapes for the direction of transportation at this position. Absolute amount of transportation. The degree of absolute transportation irrespective of direction is detailed in Table 3. At the end point of preparation, the magnitude of transportation was small (range: 0.004 to 0.011 mm) with no significant differences between canal shapes. There were no significant differences between the canal shapes for absolute magnitude of transportation at the apex of curve, beginning of the curve, or at the orifice. There was a trend, however, for transportation to increase toward the orifice. Along the straight portion of the canal (halfway to the orifice) there were significant differences between the canal shapes (p < 0.001) with greater values occurring in canals with 40 °, 8-mm curves.

DISCUSSION It is important during instrumentation of root canals to maintain the original canal curvature to produce a continuously tapering and conical form with the smallest diameter at the end point of the preparation (7). The tendency for straightening, which can compromise the integrity of the canal, especially at the apex (8) and inner (furcal) aspect of the root canal (9), must be overcome. Stainless steel instruments have been used in a variety of preparation techniques (7, 9 - 1 3 ) in an attempt to create the appropriate shape. However, studies have shown that procedural incidents occur commonly, producing aberrations such as hourglass-shaped

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Thompson and Dummer

Journal of Endodontics

TABLE 1. Mean total width (mm) of canals and the amount of resin removed from the inner and outer aspect of the curve (mm) by canal shape 20 °, 8 mm inner total outer

Canal shape 40 °, 8 mm 20 °, 12 mm inner total outer inner total outer

inner

end point apex of curve beginning of curve

0.057 0.082 0.099

0.361 0.465 0.550

0.064 0.072 0.074

0.065 0.097 0.137

0.384 0.534 0.646

0.068 0.070 0.083

0.059 0.077 0.086

0.387 0.428 0.479

0.072 0.075 0.078

0.053 0.086 0.102

0.359 0.471 0.541

0.065 0.068 0.073

halfway to orifice orifice

0.119 0.223

0.677 0.881

0.109 0.146

0.194 0.139

0.737 0.886

0.091 0.269

0.102 0.220

0.637 0.930

0.115 0.200

0.102 0.217

0.658 0.911

0.095 0.153

Position

i -

inner, t = total, o -

outer

40 °, 12 mm total outer

p value <0.005 <0.001 <0.005 <0.001 <0.001 <0.001

(t) (t) (i) (t) (i, t) (i,o)

widths.

TABLE 2. Number of canals transported toward the inner and outer aspect of the curve by canal shape Position end point apex of curve beginning of curve halfway to orifice orifice

20 °, 8 mm inner none outer 1 4 6 5 8

3 2 3 1 1

6 4 1 4 1

Canal shape 40 °, 8 mm 20 °, 12 mm inner none outer inner none outer 4 8 9 9 1

3 2 0 0 0

3 0 1 1 9

4 3 4 3 5

2 5 4 3 2

4 2 2 4 3

inner

40 °, 12 mm none outer

2 5 6 4 8

1 3 3 3 0

7 2 1 3 2

p value ns ns ns ns <0.01

TABLE 3. Mean distance of absolute transportation (mm) by canal shape Position end point apex of curve beginning of curve halfway to orifice orifice

20 °, 8 mm

40 °, 8 mm

0.006 0.011 0.017 0.025 1.027

0.004 0.013 0.027 0.055 1.216

canals (5, 6, 8) and transportation of the canal at the curve toward the inner aspect (5, 6, 9). These aberrations can be reduced but not entirely eliminated through the use of instruments having greater flexibility and noncutting tips (6, 12, 14). The development of the Canal Master system (14) and its subsequent marketing (Brasseler USA, Savannah, GA) provided a unique alternative to the traditional hand instrument through the elimination of the cutting flutes along the majority of the instrument shaft. Studies on the Canal Master design demonstrated the instrument produced round preparations with little transportation (15-20). It is clear that the noncutting pilot tip does not predispose to zip formation or transportation toward the outer aspect of the curve at the end point. Furthermore, as the smooth shaft does not cut there is no tendency for excessive preparation along the inner aspect of the curve. The advent of nickel-titanium allowed the construction of endodontic instruments with a low modulus of elasticity, increased flexibility, and superior resistance to fracture. However, few studies have evaluated the efficacy of either hand or rotary instruments made from this alloy. The aim of this study was to assess the shaping ability of nickel-titanium Lightspeed rotary instruments during the preparation of four differently shaped simulated canals. Since simulated canals were used as the model, a degree of caution should he used in the interpretation of the data and the extrapolation of the results to the clinical situation; however, variables usually encountered in real canals can he eliminated using this approach, and the experimental model has been shown to allow valid comparisons of preparation techniques and instruments (5, 8).

Canal shape 20 °, 12 mm 0.011 0.004 0.008 0.013 1.050

40 °, 12 mm

p value

0.009 0.011 0.015 0.016 1.174

ns ns ns <0.001 ns

Only one zip and elbow were created during preparation to suggest that Lightspeed instruments, like the Canal Master stainless steel hand instruments, were able to negotiate and prepare at the end-point even those canals with severe acute curves. Just as with the Canal Master system, the safe design of the cutting head with the noncutting pilot and the narrower flexible instrument shaft presumably contributed to the lack of canal irregularities in the apical region as well as along the canal length. The results suggested that Lightspeed instruments were superior to stainless steel hand files when investigated under identical conditions (5, 6). The lack of aberrations tbund with Lightspeed instruments (1, 2) and other nickel-titanium rotary devices has been reported previously, and the results of this study once again confirm the potential of this new generation of instruments to shape canals safely and in the appropriate manner. The differences between the canal types in terms of total width, although often statistically significant, were small in real terms and of little practical importance. This reflects the fact that Lightspeed instruments were not affected unduly either by the degree of curvature or by the position of curvature. Previous work with stainless steel hand files (6) has shown that canals were invariably wider as a result of removal of material on the outer wall of the canal at the end point of preparation and on the inner wall at the curve as instruments tended to straighten, an effect particularly noticeable in 40 ° canals. Clearly, the flexibility of the nickeltitanium instruments overcame this tendency, but the unique design of the Lightspeed instruments with their noncutting shaft inust also have been of importance.

Voh 23, No. 12, December 1997 Unfortunately, the limited total width of the canals from end point to orifice tended to produce a general lack of taper and flow, an aspect of shaping ability noted in the three-dimensional form of the canals (4). The narrowness of the canals and lack of taper would tend to create difficulties with some obturation techniques as spreader or plugger penetration could be compromised. Clearly, a more radical stepback phase would overcome this problem or, alternatively, an instrument with increased taper could be used to refine the canal shape at the end of the procedure. The direction of transportation at the various positions along the canal length reflected the picture noted previously with stainless steel files (5, 6) and stainless steel endosonic devices, that is, a tendency to transport toward the outer at the end point of preparation and a tendency to transport toward the inner at and around the curve. However, this trend was not entirely consistent as many canals displayed either no transportation or transportation in the opposite direction. Investigation of nickel-titanium files has produced only limited data, but similar trends have been reported by other groups (l, 2) when investigating Lightspeed instruments. Overall, the degree of absolute wansportation was small with no significant differences between the canal shapes in the region apical to the curve. Thus, although it could be argued that transportation was common, its magnitude was so small as to be unimportant with the result that the original shape of the canal was largely maintained. Drs. Thompson and Dummer are affiliated with the Department of Restorative Dentistry, University of Wales College of Medicine, Cardiff, UK. Address requests for reprints to S. A. Thompson, Department of Restorative Dentistry, Dental School, University of Wales College of Medicine, Health Park, Cardiff CF4 4XY, UK.

References 1. Esposito PT, Cunningham CJ. A comparison of canal preparation with nickel-titanium and stainless steel instruments. J Endodon 1995;21:173-76. 2. Glosson CR, Hailer RH, Dove SB, del Rio CE. A comparison of root canal preparations using Ni-Ti hand, Ni-Ti engine-driven and K-Flex endodontic instruments. J Endodon 1995;21:146-51.

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3. Dummer PMH, Alodeh MHA, AI-Omari MAO. A method for the construction of simulated canals in clear resin blocks. Int Endod J 1991 ;24:63-6. 4. Thompson SA, Dummer PMH. Shaping ability of Lightspeed rotary nickel-titanium instruments in simulated root canals. Part 1. J Endodon 1996; 23:698-702. 5. Alodeh MHA, Dummer PMH. A comparison of the ability of K-files and Hedstrom files to shape simulated root canals in resin blocks. Int Endod J 1989;22:226-35. 6. AI-Omari MAO, Dummer PMH, Newcombe RG. Comparison of six files to prepare simulated root canals. Part 2. Int Endod J 1992;21:67-81. 7. Schilder H, Yee FS. Canal debridement and disinfection. In: Cohen S, Burns RC, eds. Pathways of the Pulp. 3rd ed. St. Louis: The CV Mosby Company; 1984:175. 8. Weine FS, Kelly RF, Lio PJ. The effect of preparation procedures on original canal shape and on apical foramen shape. J Endodon 1975;1:255-62. 9. Abou Rass M, Frank AL, Glick DH. The anticurvature filing method to prepare the curved root canal. J Am Dent Assoc 1980;101:792-94. 10. Ingle Jl. A standardized endodontic technique utilizing newly designed instrument and filling materials. Oral Surg Oral Med Oral Path 1961 ;14:83-91. 11. Goerig AC, Michelich RJ, Schultz HH. Instrumentation of root canals in molars using a step-down technique. J Endodon 1982;8:550-54. 12. Roane JB, Sabala CL, Duncanson MG. The "balance force" concept of instrumentation of curved canal. J Endodon 1985;11:203-11. 13. Saunders WP, Saunders EM. Effect of noncutting tipped instruments on the quality of root canal preparations using a modified double-flared technique. J Endodon 1992;18:32-36. 14. Wildey WL, Senia ES. A new root canal instrument and instrumentation technique: a preliminary report. Oral Surg Oral Med Oral Path 1989;67:198207. 15. Leseberg DA, Montgomery S. The effects of Canal Master, Flex-R and K-Flex instrumentation on root canal configuration. J Endodon 1991;17:5965. 16. Baumgarter JC, Martin H, Sabala CL, Strittmatter EJ, Wildey WL, Quigley NC. Histomorphometric comparison of canals prepared by four techniques. J Endodon 1992;18:530-34. 17. Briseno BM, Kremers L, Hamm G, Nitsch C. Comparison by means of a computer-supported device of the enlarging characteristics of two different instruments. J Endodon 1993;19:281-87. 18. Camps J, Macouin G, Pertot W. Effects of Flexogates and Canal Master U on the root canal configuration in simulated curved canals. Int Endod J 1994;27:21-5. 19. Roig-Cayon M, Brau-Aguade E, Canalda-Sahli C, Moreno-Aguado V. A comparison of molar root canal preparations using Flexofile, Canal Master U and Heliapical instruments. J Endodon 1994;20:495-99. 20. Zmener O, Spielberg C, Olmos J. Effectiveness of two different methods for preparing curved root canals. Endod Dent Traumatol 1994;10: 215-19.