Predicting in vivo failure of rotary nickel-titanium endodontic instruments under cyclic fatigue

Abstract

The aim of this study was to examine the lifespan or number of cycles to failure of tapered rotary nickel-titanium (Ni-Ti) endodontic instruments. Simulated root canals with different curvatures were used to determine a relation between canal curvature and instrument lifespan. Using a novel mathematical model for the deformation of pseudoelastic Ni-Ti alloy, it was shown that maximum stress need not necessarily occur at the outer layer. On the basis of this observation, the Coffi n-Manson relation was modifi ed with parameters determined from this experiment. Results showed that the number of cycles to failure was infl uenced by the angle and radius of canal curvature and the size of instrument at the beginning of canal curvature. The resulting quantitative mathematical relation could be used to predict the lifespan of rotary Ni-Ti endodontic instruments under clinical conditions and thereby reduce the incidence of instrument failure in vivo.

Deformation of Nickel-Titanium (NiTi) rotary instruments in cyclic loading – in vitro study

“…kada je vrh instrumenta veći od promera kanala korena, ili kada se vrši prekomeran pritisak na instrument u toku rada [13]. Ukoliko je vrh instrumenta blokiran u kanalu korena, a endo-motor, koji rotira instrument, nastavi da isporučuje obrtni momenat, dolazi do akumulacije torzionog stresa unutar materijala, što nakon prekoračenja kritične tačke opterećenja, dovodi prvo do plastične deformacije, a posledično i loma instrumenta [14].…”

“…Torsional stress usually occurs when part of the instrument is screwed into the root canal wall, when the tip of the instrument is greater than diameter of the root canal, or when there is excessive pressure on the instrument during operation [13]. If the tip of the instrument in the root canal is blocked, and endo-motor that rotates the instrument continues to deliver torque, there is an accumulation of torsional stress within the material, which after exceeding the critical load points, consequently leads to plastic deformation and fracture of the instrument [14].…”

Cyclic fatigue resistance of ProTaper Gold and comparison with ProTaper Universal instruments

“…3,6,7 Apparently, this is one of the major reasons for instrument breakage during treatment and can be measured by the number of cycles to fracture (NCF). 4,8 NCF may be affected by type of wire, surface finish, thermal treatments, cross section and geometry of the instrument. 5,[9][10][11][12][13] ProTaper ® Universal (PTU; Dentsply/Maillefer, Ballaigues, CH) is a widely-described system of instruments, with progressive taper over the cutting blades, triangular cross section and center of rotation equal to center of mass.…”

Cyclic fatigue resistance of OneShape and WaveOne instruments using different angles of curvature

“…On the other hand, they are prone to unpredictable fractures due to cyclic fatigue, a result of the alternating tension-compression cycles which they are subject to when rotated near the apical third of root canals with a severe curvature [1][2][3][4][5] . Different manufacturing strategies have been employed to enhance the flexibility and fatigue fracture resistance of NiTi rotary endodontic instruments.…”

“…Rotary nickel-titanium (NiTi) endodontic instruments have become popular due to their increased flexibility, better cutting ability, and higher success rates in root canal preparation [1][2][3] . On the other hand, they are prone to unpredictable fractures due to cyclic fatigue, a result of the alternating tension-compression cycles which they are subject to when rotated near the apical third of root canals with a severe curvature [1][2][3][4][5] .…”