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 »  Home  »  Endodontic Articles 7  »  Shaping ability of GT Rotary Files in simulated resin root canals
Shaping ability of GT Rotary Files in simulated resin root canals
Discussion - References.



Discussion.
The maintenance of canal curvature and the creation of a funnel shaped canal form with the smallest diameter at the apex and the widest diameter at the orifice are the objectives during instrumentation of root canals (Schilder & Yee 1984). The problem of straightening occurs mostly in severely curved canals and at the apex of the canal (Weine et al.1975). In recent years, a great number of reports described the advantages of nickel-titanium rotary instruments. These instruments have the potential to prepare root canals rapidly and safely to produce relatively few aberrations during the root canal preparation. Care, however, should be taken when canals with short curves beginning near the end-point are to be prepared (Bryant et al. 1999).
The aim of this study was to assess the shaping ability of GT Rotary Files in simulated root canals with a severe curve. The preparation technique that was used was a crown-down/stepback technique, as advocated by the manufacturer. To assess instrumentation of curved canals, clear resin blocks were used in this study. These were chosen because shape, size, taper, and curvature of the experimental canals are standardized. The credibility of resin blocks as an ideal experimental model for the analysis of endodontic preparation and preparation techniques has been validated by Weine et al. (1975) and Dummer et al. (1991). Because of slight differences in hardness between dentine and the experimental resin, care should be taken, however, in extrapolating the results to the clinical situation. Never theless, the use of simulated canals in resin blocks results in the opportunity to standardize the research method and to exclude parameters that could influence the preparation outcome.
The determination of the degree of curvature of root canals was based on data according to Schneider (1971), i.e. the degree of canal curvature is determined by only one parameter, the angle formed by the intersection of two lines. The first line is drawn parallel to the long axis of the canal, and the second line runs from the apical foramen to the point wherein the canal begins to leave the long axis of the canal. This method was refined by Alodeh & Dummer (1989) who added the radius of the curve as a second parameter in defining the degree of root curvature.
Nickel-titanium files have already shown their efficiency in shaping the root canal. Their flexibility assumes that they could be useful during preparation of curved canals by keeping the preparation path centred, and by creating a tapered preparation thanks to the specifically tapered design of consequent instruments. Aberrations of the root canal, still occur, albeit to a lesser degree than in non-mechanical instrumentation (Kum et al. 2000). Additional problems, such as instrument deformation and breakage still occur, especially in curved canals. The question remains as to what critical degree of curvature a nickel-titanium file can be used safely. Some manufacturers advise not to use rotary instruments in root canals with curves exceeding 608. The present data show that the presence of a 608 curve does not necessarily result in more aberrations than in canals with a 408 curve. In a study comparing the use of ProFile rotary nickel-titanium instruments Bryant et al. (1998) showed that aberrations occurred more often in canals where the curve began 12 mm from the orifice. In the present study, a similar finding was observed. When the number of separated or deformed instruments was compared, deformation of 0.04 tapered instruments occurred more frequently in a 608 curved canal. Instrument fracture was seen in 0.04/30 instruments in canals with a straight section of 8 mm regardless of the angle of curve. Bryant et al. (1999) stated in this respect when using 0.04 and 0.06 ProFiles that the use of 0.06 taper instruments in the crown-down technique appeared to be useful and was recommended. The crown-down approach with the GT Rotary Files with a taper of 6, 8, 10 and 12% is also highly recommended. Great care, however, should be taken in canals with 608 curves as instrument deformation is high and may result in consequent instrument fracture. Torque control as a safety measure has, therefore, been advocated by the manufacturer, although Yared et al. (2001) demonstrated that there was no difference with respect to the incidence of instrument failure between no-torque and high and low torque motors.
Canal types I and III showed a transportation towards the inner aspect of the canal, whereas transportation in canal types II and IV was to the outer aspect. Thanks to the direct visualization through the resin blocks during preparation it was seen that the canal aberrations especially became clear when enlarging the endpoint of preparation to a size 30 or 35. These size sare more rigid than the smaller ones, and tend to straighten the canal. This suggests that in canals with short acute curvatures, instrumentation of the apical part with GT Rotary Files should better not exceed the size 25 in narrow canals in order to obtain a tapered shape. These findings agree with those of Bryant et al. (1998) in their research on ProFile 0.04 and 0.06 tapered instruments. In this respect, it must also be said that there is no difference in design between the GT Rotary Files 0.04 and Pro-Files 0.04.
The phenomenon of ‘outer widening’ (Bryant et al. 1999) was only seen in a limited number of resin canals with a 608 curve. The use of the more tapered instruments initially to open the orifice allowed the 0.04 series to pass more easily to the end-point and to remain centered in the canal. The fire fore, a strict crown-down approach to avoid stress on instruments by early opening of the coronal part of the root canal when using nickel-titanium rotary instruments is essential.

References.

AlodehMHA, Dummer PMH (1989) Acomparison of the ability of   K-files and Hedstrom files to shape simulated root canals in resin blocks.   International Endodontic Journal 22, 226-35.
  Bryant ST, Dummer PMH, Pitoni C, Bourba M, Moghal S (1999) Shaping ability   of .04 and .06 taper ProFile rotary nickel-titanium instruments in simulated   root canals. International Endodontic Journal 32, 155-64.
  Bryant ST,Thompson SA, Al-Omari MAO, Dummer PMH (1998) Shaping ability of   ProFile rotary nickel-titanium instrumens with ISO sized tips in simulated root   canals. Part 2. International Endodontic Journal 31, 282-9.
  Dummer PMH, Alodeh MHA, Al-Omari MAO (1991) A method for the construction   of simulated canals in clear resin blocks. International Endodontic Journal   24, 63-6.
  Esposito PT, Cunningham CJ (1995) A comparison of canal preparation with   nickel-titanium and stainless steel instruments. Journal of Endodontics   21, 173-6.
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  Kum KY, Spangberg L, Cha BY, Il-Young J, Seung-Jong L, Chan- Young L (2000)   Shapingability of three ProFile Rotary instrumentation techniques in simulated   resin root canals. Journal of Endodontics 26, 719-23.
  Pruett JP, Clement DJ, Carnes DL (1997) Cyclic fatigue testing of nickel-titanium   endodontic instruments. Journal of Endodontics 23,77-85.
  Schilder H (1974) Cleaning and shaping the root canal. Dental Clinics   of North America 18, 269-96.
  Schilder H,Yee FS (1984) Canal debridement and desinfection. In: Cohen,   S, Burns, RC, eds. Pathwayso f the Pulp, 3rd edn. St. Louis: The CV Mosby Company,   p.175.
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  Thompson SA, Dummer PMH (1997) Shaping ability of ProFile .04 taper series   29 rotary nickel-titanium instruments in simulated root canals. Part1. International   Endodontic Journal 30, 1-7.
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  Yared GM, Bou Dagher FE, Machtou P (2001) Failure of ProFile instruments   used with high and low torque motors. International Endodontic Journal 34,   471-5.
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