Fracture of the endodontic instrument. 5th lecture in the series of lectures on endodontic preparation.
In: Lopes et al. Fracture of endodontic instruments. Clinical recommendations. Rev. bras. odontol., Rio de Janeiro, v. 68, n. 2, p. 152-6, jul./dez. 2011 Introduction
Fracture resistance is one of the main mechanical properties of endodontic instruments that should be informed to the user for guidance in clinical practice. During the chemical-mechanical preparation of a root canal, endodontic instruments undergo extremely adverse stresses that vary with canal anatomy, instrument dimensions, and practitioner skill. These adverse stresses continuously modify the resistance to torsion and rotational bending of endodontic instruments during instrumentation of a root canal. Instrument fracture can occur by twisting, rotary bending (tensile and compressive stresses), and combinations thereof. Fracture of Instruments by Torsion To occur the fracture by torsion is necessary that the tip of the endodontic instrument is immobilized and the other end (handle) is applied a torque exceeding the limit of resistance to fracture of the instrument.
It can occur for stainless steel and NiTi endodontic instruments, driven manually or by mechanized devices. If the tip of the instrument is not immobilized during instrumentation of a root canal, regardless of the value of the torque applied, torsion fracture of the endodontic instrument will not occur. Torque (T) can be defined as the rotatory effect created by a force (F) away from the axis of rotation of an object. It is calculated by the equation: Torque = F.R where, R (radius) is the distance between the point of force application (F) and the object's axis of rotation. Force in the International System of Units is expressed in Newton (N) and torque is expressed by the unit of force multiplied by the unit of radius length (newton x meter). For force the units kilogram (kgf) and gram (gf) are also used, and for length the centimeter and millimeter.
There are the relations between the units: 1 kgf = 1000 gf = 9,807 N 1 cm = 10 mm Clinical Recommendations Immobilization of the Instrument Tip When the tip of an endodontic instrument is immobilized inside a root canal and the rotation is to the right, there will be an initial plastic deformation (distortion) of its propellers. The presence of plastic deformation of the helixes observed when removing the endodontic instrument from a root canal during instrumentation, gives an alert that a torsion fracture is imminent. Thus, during instrumentation of a root canal, it is important that the practitioner remove the instrument from the interior of a canal more frequently and examine it carefully. Deformed endodontic instruments should be discarded before failure (fracture) occurs.
The plastic deformation also allows the professional to perform corrections and adjustments in the advancement of the instrument within the canal and in the torque to be applied to a new instrument used in instrumentation of the root canal. These measures aim to avoid immobilization and plastic deformation of the new endodontic instrument used in instrumentation. Immobilization of an endodontic instrument operated manually or by mechanical devices (motors) within a root canal can be minimized by reducing the loading and advancement of the instrument in the apical direction. The cutting action of endodontic instruments by means of the widening movement (alternating or continuous) is done by advancing the instrument 1 to 3 mm in the apical direction of the root canal, interleaved with withdrawals.
Higher feeds increase the contact area and the resistance to cutting of the dentin wall, which may cause total or partial immobilization of the instrument tip and induce loading beyond its limit of resistance to torsion fracture. For manually operated instruments, the advancement is controlled by the right-hand rotation angle applied to the handle of the endodontic instrument. For instruments with small diameters, the angle of right-hand rotation should not be greater than 45°. For larger diameter instruments, the angle of rotation can vary from 90 to 120 degrees. The smaller the angle of rotation applied to the handle of the endodontic instrument, the smaller will be its advance inside the root canal. It should also be noted that the advancement of an instrument within a root canal depends on the angle of inclination of the helix of the instrument. The smaller the angle, the greater the advancement. Fracture of an endodontic instrument.
