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Azerbaycan Saytlari

 »  Home  »  Endodontic Articles 16  »  Factors influencing the fracture of nickel-titanium rotary instruments
Factors influencing the fracture of nickel-titanium rotary instruments
Statistical analysis - Results - Discussion - References.



Statistical analysis.
The following variables were included in the analysis:

  • File type: K3 and ProTaper. 
  • Speed of instrumentation:150, 250 and 350 r. p. m.
  • Angle of curvature of the canals.
  • Radius of curvature.

Given that instruments in canals with curvature <308 did not fracture, the baseline variables of the two groups (fracture yes or no) were compared using the chi-square test for all the samples of an angle of  >308. Values of  95% CI and P < 0. 05 were applied for all of the tests. For the multivariate analysis, in order to estimate the factors influencing file fracture a binary, nonconditional, logistic regression model was used with the fracture as dependent variable and file design, rotational speed, angle and radius of curvature as independent variables.

Results.
From a total of 240 root canals, instrumented using two different designs of files, 22 files fractured in a total of 22 canals: 12 ProTaper and 10 K3. All fractures occurred in group B canals whose curvature was greater than 308. There were no file fractures in the group A teeth, i. e. those specimens with slightly curved or relatively straight canals.
At a rotational speed of 150 r. p. m. , five of the instrument fractured (two K3 and three ProTaper). At 250 r. p. m. , seven instruments fractured (four K3 and three ProTaper). At 350 r. p. m. , 10 instruments fractured (four K3 and six ProTaper) (Tables 3 and 4).
All the fractures occurred in the apical third of the canal, a few millimeters from the tip of the instruments. The fractures occurred in mesial-buccal and distal-buccal canals in maxillary molars and mesial-buccal and mesial-lingual canals in the mandibular molars.
The logistic regression model found fracture statistically associated with rotational speed and angle of curvature. The instruments, which were used at a speed of 350 r. p. m. , fractured more often than those used at 250 r. p. m. (OR: 1113. 88; 95% CI: 2. 36-526420. 05; P < 0. 05) and more often than those used at 150 r. p. m. (OR: 13531. 33; 95% CI: 5. 37-34120254. 00; P < 0. 05). A reduction in the angle of curvature produced a significant decrease in the incidence of fracture (OR: 0. 2083; 95% CI: 0. 068-0. 6502; P < 0. 01). These relationships remained significant after their adjustment in order to assimilate the potential interactions between variables. No significant differences were found with respect to the file design or with respect to the radii (Table 5).

Table 3. K3 fractures and their relationship to rotational speed, and angle and radius of the curvature of the canal.

K3 fractures and their relationship to rotational speed, and angle and radius of the curvature of the canal

Table 4. ProTaper fractures and their relationship to rotational speed, and angle and radius of the curvature of the canal.

ProTaper fractures and their relationship to rotational speed, and angle and radius of the curvature of the canal

Table 5. Logistic regression analysis for ?le fracture (only significant variables are shown).

Logistic regression analysis for ?le fracture (only significant variables are shown)

Discussion.
There are many factors that influence the fracture of nickel-titanium rotary files. Rotational speed is not generally considered to be a significant factor with respect to the fracture of nickel-titanium endodontic rotary instruments (Glickman 1997, Pruett et al. 1997). Some studies, however, have shown that rotational speed does indeed influence instrument fracture in curved canals (Gabel et al. 1999, Dietz et al. 2000), and this might be explained by the fact that the contact between the file and canal walls may cause sufficient stress to cause fracture. Increased rotational speeds augment the rubbing of the file within the canal, and thus these files break more readily than those, which are used at lower speeds (Gabel et al. 1999, Sattapan et al. 2000a). In our study, the files which were used at150 and 250 r. p. m. fractured less frequently than those used at 350 r. p. m.
The life expectancy of an instrument is related to a specific number of rotary cycles (Yared et al. 1999). A lower rotational speed, therefore, should mean that the life span of an instrument is prolonged, breaking only after reaching a specific number of rotations.
Clinically, the fatigue of an instrument may be related to the degree of flexure that the instrument undergoes when placed in a curved root canal. When the curvature of canals is more pronounced, the cyclical fatigue that the instrument undergoes is greater, and thus its life expectancy is lower (Pruett et al. 1997). In this study, all instrument fractures occurred in canals with accentuated angles. The radius of curvature, however, was not a factor that influenced instrument fracture significantly, a fact which contradicts the findings of other studies in which both the angle and radius were found to be significant.
In a study conducted by Sattapan et al. (2000a), it was observed that the files that broke due to excessive torsion exhibited signs of deterioration above the point of fracture. On the other hand, the files that broke due to fatigue through flexure did not exhibit defects linked to their subsequent fracture.
It would appear that the resistance of files differs depending on whether the canals are relatively straight or slightly curved or, conversely, whether the curvature of the canals is pronounced and acute. In the first type of canal, it was possible to work using high rotational speeds whilst using each file at least 20 times without fear of fracture. The second type of canal, on the other hand, demands that files should be used at minimum speed.

References.

Cohen S, Burns RC (2000) Vas de la Pulpa, 7th edn. Madrid, Spain: Mosby-Harcourt.
Dietz D, Di Fiore P, Bahcall J, Lautenschlager E (2000) Effect of rotational speed on the fracture of nickel-titanium rotary files. Journal of Endodontics 26, 68-71.
GabelWP, HoenM, Steiman HR, Pink FE, Dietz R (1999) Effect of rotational speed on nickel-titanium file distortion. Journal of Endodontics 25, 752-4.
Gambarini G (2000) Rationale for the use of low-torque endodontic motors in root canal instrumentation. Endodontics and DentalTraumatology16, 95-100.
Glickman G (1997) Niquel-titanio en endodoncia. Operatoria Dental Endodoncia1, 3-8.
Haikel Y, Serfaty R, Bateman G, Senger B, Allemann C (1999) Dynamic and cyclic fatigue of engine-driven rotary nickeltitanium endodontic instruments. Journal of Endodontics 25, 434-40.
Marending M, Lutz F, Barbakow F (1998) Scanning electron microscope appearances of Lightspeed instruments used clinically: a pilot study. International Endodontic Journal 31, 57-62.
Pruett JP, Clement DJ, Carnes DL Jr (1997) Cyclic fatigue testing of nickel-titanium endodontic instruments. Journal of Endodontics 23, 77-85.
Sattapan B, Nervo G, Palamara J, Messer H (2000a) Defects in nickel titanium endodontic rotary files after clinical usage. Journal of Endodontics 26, 161-5.
Sattapan B, Palamara J, Messer H (2000b) Torque during canal instrumentation using rotary nickel-titanium files. Journal of Endodontics 26, 156-60.
Schrader C, Ackermann M, Barbakow F (1999) Step-by-step description of a rotary root canal preparation technique. International Endodontic Journal 32, 312-20.
Serene TP, Adams JD, Saxena A (1995) Nickel-titanium instruments: applications in endodontics. St Louis, MO, USA: Ishiyaku EuroAmerica.
Yared GM, Bou Dagher FE, Machtou P (1999) Cyclic fatigue of ProFile rotary instruments after simulated clinical use. International EndodonticJournal 32, 115-9.