E. Schafer & H. Florek Department of Operative Dentistry, University of Munster, Munster, Germany.Introduction.
Several nickel-titanium instruments have been developed recently for use in a rotary technique. All of the new systems incorporate instruments with a taper greater than the ISO standard 0.02 design; indeed, rotary nickel-titanium instruments are available with tapers ranging from 0.04 to 0.12 (Thompson & Dummer 1997 a, Bergmans et al.2001). These greater taper instruments have been introduced to improve the cutting efficiency of nickel-titanium instruments, to reduce the incidence of instrument failures and to enhance canal shape in order to allow more apical placement of the irrigant and to facilitate root canal filling, especially when using thermoplastic obturation techniques (Bergmans et al. 2001).
Besides variation in taper, nickel-titanium rotary instruments are characterized by different cross-sections and designs of blades (Bergmans et al. 2001). In general, rotary files having a positive rake angle differ from those possessing U-shaped blades with radial land areas (Bergmans et al. 2001). Due to these flat areas, the rake angle of these files is neutral or slightly negative; thus, canal enlargement is thought to occur through planing of dentine rather than a cutting action and chip dislodgement (Bergmans et al.2001). It is well known that the cutting efficiency of a file depends upon the rake angle of its cutting blades (Wildey et al. 1992). Since dentine is a dense and resilient material, instruments having a negative rake angle are less efficient and require more energy to cut dentine than files with a positive rake angle (Wildey et al.1992).
With regard to the cutting efficiency of rotary root canal instruments, a positive rake angle would tend to result in efficient chip dislodgement (Wildey et al. 1992, Bergmans et al. 2001). So, other rotary instruments, such as Hero 642 (MicroMe. ga, BesancÂ¸ on, France) or Quantec files (Tycom, Irvine ,CA, USA)have a helical cross-section resulting in a slightly positive rake angle (Bergmans et al. 2001). These instruments have no radial lands (Bergmans et al. 2001).
The K3 instrument (SybronEndo, West Collins, CA, USA) is reported to have a slightly positive rake angle in combination with so-called radial land relief and asymmetrical cross-sectional design (Bergmans et al. 2001) (Fig.1). The peripheral blade relief areas (Fig.1) are alleged to have two functions:
- to increase the peripheral mass in order to increase the instrumentsâ€™ resistance to fracture and
- to reduce the amount of area of the radial lands that comes in contact with the canal wall in order to reduce frictional resistance.
To date, little information on the mechanical properties and shaping ability of nickel-titanium K3 instruments is available. The K3 instruments are made of 55- nitinol (Florek 2003), and have flattened noncutting tips and a rounded transitional angle (Fig. 2). Twenty K3 instruments are available: sizes15-60 have two different tapers (0.04 and 0.06). The manufacturer recommends that root canals be enlarged with these instruments using the crown-down technique (Stock 2001).
The purpose of this study was to compare automated K3 rotary nickel-titanium instruments with stainless steel hand K-Flexofiles (Dentsply Maillefer, Ballaigues, Switzerland), used in a reaming motion during the shaping of simulated curved root canals in resin blocks.
Figure 1. Scanning electron microscope image of the cross-section of a K3 instrument (0.06 taper size 30; original magnification 160x) showing radial land relief areas (indicated by arrows). The cross-section of the K3 file is asymmetrical.
Figure 2. Scanning electron microscope images of the tip region of K3 instruments showing a noncutting, flattened tip with rounded transitional angles.
(a) 0.06 taper size 30; original magnification 160x.
(b) 0.06 taper size 30; original magnification 160x.