Potential relationship between design of nickel-titanium rotary instruments and vertical root fracture.
BACKGROUND
Nickel-titanium (NiTi) rotary files can produce cleanly tapered canal shapes with low tendency of transporting the canal lumen. Because NiTi instruments are generally perceived to have high fracture risk during use, new designs have been marketed to lower fracture risks. However, these design variations may also alter the forces on a root during instrumentation and increase dentinal defects that predispose a root to fracture. This study compared the stress conditions during rotary instrumentation in a curved root for three NiTi file designs.


METHODS
Stresses were calculated using finite element (FE) analysis. FE models of ProFile (Dentsply Maillefer, Ballaigues, Switzerland; U-shaped cross-section and constant 6% tapered shaft), ProTaper Universal (Dentsply; convex triangular cross-section with notch and progressive taper shaft), and LightSpeed LSX (Lightspeed Technology, Inc, San Antonio, TX; noncutting round shaft) were rotated within a curved root canal. The stress and strain conditions resulting from the simulated shaping action were evaluated in the apical root dentin.


RESULTS
ProTaper Universal induced the highest von Mises stress concentration in the root dentin and had the highest tensile and compressive principal strain components at the external root surface. The calculated stress values from ProTaper Universal, which had the biggest taper shaft, approached the strength properties of dentin. LightSpeed generated the lowest stresses.


CONCLUSIONS
The stiffer file designs generated higher stress concentrations in the apical root dentin during shaping of the curved canal, which raises the risk of dentinal defects that may lead to apical root cracking. Thus, stress levels during shaping and fracture susceptibility after shaping vary with instrument design.
Kim
Hyeon-Cheol
HC
Department of Conservative Dentistry, School of Dentistry, Pusan National University, Yangsan, Gyeongnam, Korea. golddent@pusan.ac.kr
Lee
Min-Ho
MH
Yum
Jiwan
J
Versluis
Antheunis
A
Lee
Chan-Joo
CJ
Kim
Byung-Min
BM
D001696:Biomechanical Phenomena; D003198:Computer Simulation; D003722:Dental Alloys / Q000737:chemistry*; D003786:Dental Pulp Cavity / Q000473:pathology; D003804:Dentin / Q000473:pathology; D004198:Disease Susceptibility; D004867:Equipment Design; D020342:Finite Element Analysis; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000379:methods; D008422:Materials Testing; D008954:Models, Biological; D009532:Nickel / Q000737:chemistry*; D018915:Root Canal Preparation / Q000295:instrumentation*; D013314:Stress, Mechanical; D013499:Surface Properties; D014025:Titanium / Q000737:chemistry*; D019227:Tooth Apex / Q000473:pathology; D014082:Tooth Fractures / Q000209:etiology*; D014092:Tooth Root / Q000293:injuries* / Q000473:pathology; D055114:X-Ray Microtomography / Q000379:methods
D003722:Dental Alloys; C040654:titanium nickelide; D009532:Nickel; D014025:Titanium