Rotary Biomechanics: Reality or Future?

 

 

Jesus Djalma Pécora

Alexandre Capelli

Fábio Heredia Seixas

Melissa Andréia Marchesan

Danilo Mathias Zanello Guerisoli

 

 

            A major concern regarding rotary instruments is their unexpected fracture while in operation. It can occur without any visible sign of permanent deformation, and the two most important causes of fractures are torsional and flexural failures (SERENE et al., 1995). Torsional fractures occur when the tip or any other part of the instrument engages the root canal while the other part still rotates. Flexural failures are those caused by metal fatigue, when the instrument is used in a canal with a small curve radius. The limit of flexibility of the instrument is exceeded, thus resulting in fracture by cyclic fatigue (PRUETT et al., 1997; LOPES et al., 1999).

            New procedures, able to reduce the occurrence of instrument fractures, have been researched. Many techniques of root canal preparation, with different instruments and engines, have been recommended by authors and manufacturers (LEONARDO & LEONARDO, 2002).

            This study suggests a new method for root canal preparation with rotary Ni-Ti instruments that can reduce the rate of fractures, using files from any manufacturer and either electric or pneumatic engines.

 

 

THE FREE-TIP PREPARATION TECHNIQUE

 

First considerations

            Perform access surgery, irrigate pulp chamber and root canals copiously with sodium hypochlorite, leaving it inundated with the solution. Initial exploration of the orifices must be performed with an endodontic explorer or manual files of compatible diameter.

            Nickel-titanium rotary instruments of any manufacturer can be used. It is possible to combine different brands, choosing the most suitable for each step of treatment.

            The engine can be either electric or pneumatic, from any manufacturer. This technique suggests the use with a speed between 250 and 350 rpm.

 

Cervical preparation

            A Ni-Ti #25 taper .06 instrument must be attached to the handpiece and the engine speed must be set between 250 and 350 rpm.

            Start the motor and using a smooth pecking motion, initiate the instrumentation of the root canal following its long axis. Avoid eccentric movements of the file, neither try to force it laterally.

            Continuing with the instrument inside the root canal longer than the necessary to reach working length (around 2/3 of the total length at this stage) must be avoided. At 300 rpm, the instrument will perform 5 turns per second around its axis.

            Irrigate with sodium hypochlorite and alternate it with EDTA. Tissue dissolution is directly proportional to sodium hypochlorite concentration.

            A new Ni-Ti instrument must be chosen, with greater taper (.08, .10 or .12) than the previously used (Orifice Shaper, Dentsply-Maillefer; GT Accessories, Dentsply-Maillefer; Flare, Quantec). Tip diameter (D₁) must be about 25 – 40 (Figure 1).

 

Apical preparation

            Following cervical preparation, a minor taper (thus, more flexible) instrument must be chosen for apical preparation (15/.04 or 20/.02). As a result, the file will operate without cervical interferences and will access curves due to its flexibility, reaching the temporary working length. The working length must be then determined by an X-ray.

            The root canal preparation must be continued with the following instruments: 20/.02; 20/.04; 25/.04 or 15/.04; 15/.06; 20/.04 and 25/.04 until working length is reached.

            If any instrument does not reach the working length, irrigate the root canal with sodium hypochlorite and use one or two instruments with greater taper than the used previously. Then try to use the instrument that did not reach the working length. Always remember to irrigate copiously the root canal with sodium hypochlorite (Figures 2 and 3).

 

Finalization

            Choose an instrument with intermediary taper than the previously used for apical preparation and tip diameter equal or smaller than that.  This instrument will smooth the irregularities and promote a conical and continuous shape of the root canal (Figure 4).

            The files used at this stage are:

 

• 15/.06 – for narrow canals, sharp or double curvatures, curves with small radius.
• 20/.06 – for slightly narrowed canals, moderate curves (40 to 70º).
• 25/.06 – for smooth curves (up to 40º)

 

Discussion

            There are many reason for instrument fractures. One of them is the radius of the curvature and its location (PRUETT, 1997). The smaller the radius, higher will be the stress submitted to the instrument (LOPES, 1999). Clinically, the small radius curves are located at the apical third of teeth, which favors fractures near the tip of the file. Instruments with greater taper are more prone to fractures when used in sharp curves.

            Another factor that raises the incidence of fractures is the increase of pressure applied by the operator to the apex (BLUM et al., 1999). The speed of rotation of the instruments was also reported as directly responsible for fractures (DIETZ et al., 2000).

            Pressure control and movement applied to instruments, as well as the use of engines especially developed for this aim with speed reduction, help to avoid instrument fractures.

            The risk of fractures is always increased when dentine cutting is performed with the tip of the instrument (BLUM et al. 1999). With excessive apical pressure and high torque, the instrument ruptures.

            The free-tip preparation technique aims to prepare the root canal with areas of the instrument with greater taper first, leaving the tip free. This reduces dramatically the occurrence of fractures and was described in a similar way by other authors (McSPADDEN, 1996; BASSI apud LEONARDO & LEONARDO, 2002). The majority of instruments break at their most fragile portion, i.e., the tip or near it.

            In order to avoid such problem, preparation can be initiated with a smaller taper instrument and facilitate the way for the next file, which will have its tip working freely inside the canal, acting just as a guide. Thus, the root canal will be prepared in a crown-down way. In order to reach the working length, the instrument must prepare the cervical portion of the canal first, enlarging it up to the apex. The areas of the instrument with greater metallic structure will receive the forces during biomechanical preparation.

            The knowledge from the manual files does not apply to the rotary files, since stainless steel is very different from nickel titanium in metallurgic aspects. Thus, the files have a different mechanical behavior.

 

Conclusion

            Root canal preparation with Ni-Ti rotary files is a worldwide reality, including Brazil. This can be confirmed by the number of courses focusing this new technology and its introduction in many universities as part of the discipline of Endodontics.

 

References

1.              AHLQUIST, M.; HENNINGSSON, O.; HULTENBY, K. & OHLIN, J. The effectiveness of manual and rotary techniques in the cleaning of root canals: a scanning electron microscopy study. I Endod J, v. 34, p. 533-537, 2001

2.              BLUM, J.Y.; COHEN, A.; MACHTOU, P.; MICALLEF, J.P. Analysis of forces developed during mechanical preparation of extracted teeth using ProFile NiTi rotary instruments. I Endod J, V.32, p. 24-31, 1999

3.              BORTNIK, K.L.; STEIMAN, H.R.; RUSKIN, A. Comparison of nickel-titanium file distortion using electric and air-driven handpiece. J Endod v.27, n.1, p.57-59, 2001

4.              BUCHANAN, L. S. The standardized-taper root canal preparation - Part 1. Concepts for variably tapered shaping instruments. Int Endod J, v.33, p.516-529, 2000.

5.              BUCHANAN, L. S. The standardized-taper root canal preparation - Part 2. File selection and safe handpiece - driven file use. Int Endod J, v.34, p.63-71, 2001.

6.              DEUS, Q.D. de. Endodontia. 5a ed., Medsi, Rio de Janeiro, 1992. 695p.

7.              DIETZ, D.B.; DI FIORE, P.M.; BAHCALL, J.K.; LAUTENSCHLAGER, E.P. Effect of rotational spesd on the breakage of Nickel-Titanium rotary files. J Endod, v.25, n.2, p. 68-71, 2000

8.              HÜLSMANN, M.; SCHADE, M.; SCHÄFERS, F. A comparative study of root canal preparation with HERO 642 and Quantec SC rotary Ni-Ti instruments. I End Journal, v.34, n.5, p.538-546, 2001.

9.              LEONARDO, M.R. LEONARDO, R.T. Sistemas Rotatórios em Endodontia-Instrumentos de Níquel-Titânio, Artes Médicas, 2002.

10.         LOPES, H.P., SIQUEIRA JR, F., ELIAS, C. Instrumentos Endodônticos. In: LOPES, H.P., SIQUEIRA JR, F. Endodontia. Biologia e Técnica. Rio de Janeiro: Medsi, p.279-318, 1999 (a).

11.         LUMLEY, P. J. Cleaning efficacy of two apical preparation regimens following shaping with hand files of greater taper. Int Endod J, v.33, p. 262-2, 2000

12.         MacSPADDEN, J.T. Advanced geometries in endodontic micro files: The rationale Chattanooga, The NT Company (1996).

13.         PETERS, O. A.; BARBAKOW, F. Effects of irrigation on debris and smear layer on canal walls prepared by two rotary techniques: a scanning electron microspic study. J Endod, v.26, n.1, p.6-10, 2000.

14.         PRUETT, J.P.; CLEMENT, D.J.; CARNES,D.L. Ciclic fatigue testing of nickel-titanium endodontic instruments. J Endod, v.23, p.77-85, 1997.

15.         SATTAPAN,B.; NERVO, G.J.; PALAMARA, J.E.A.; MESSER, H.H. Defects in rotary Nickel-titaium files after clinical use. J Endod, v.26, n.3, p.161-165, 2000

16.         SCHÄFFER, E. Root canal instruments for manual use: a review. Endod Dent Traumat, v. 13, p. 51-64, 1997.

17.         SERENE, T.P.; ADAMS, J.D., SAXENA, A. Nickel-Titanium Instruments: Applications in endodontics. St. Louis Missouri, USA: Ishiyaku Euroamerica, Inc., 112p, 1995.

18.         SIQUEIRA, J.F.; ARAÚJO, M.C.; GARCIA, P.F.; FRAGA, R.C.; DANTAS, C.J. Histological evaluation of the effectiveness of five instrumentation techniques for cleaning the apical third of root canals. J Endod, vol.23, n.8, p.499-502, 1997.

19.         THOMPSON, S. A.; DUMMER, P. M. Shaping ability of ProFile .04 taper series 29 rotary nickel-titanium instruments in simulated root canals: Part 1. Int Endod J, v.30, n.1, p.1-7, 1997a.

20.         THOMPSON, S. A.; DUMMER, P. M. Shaping ability of ProFile .04 taper series 29 rotary nickel-titanium instruments in simulated root canals: Part 2. Int Endod J, v.30, n.1, p.8-15,1997b.

21.         WALIA, H.; BRANTLEY, W.A.; GERSTEIN, H. – An initial investigation of the bending and torsional properties of nitinol root canal files. J Endod vol.14, nº. 7, p.346-51, 1988.

22.         YARED, G.M.; BOU DAGHER & MACHTOU, P. Cyclic fatigue of ProFile rotary instruments alter clinical use. I End Journal, v.33, p.204-207, 2000.

23.         YARED, G.M.; BOU DAGHER & MACHTOU, P. Failure of Profile instruments used with high and low torque. I End Journal, v.34, p.471-475, 2001.

24.         YARED, G.M.; BOU DAGHER & MACHTOU, P. Inflense of rotacional speed, torque and operator’s proficiency on ProFile failures. I End Journal, v.34, p.47-53, 2001.