title: Potential relationship between design of nickel-titanium rotary instruments and vertical root fracture.
abstract: 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.
authors.affiliation: Department of Conservative Dentistry, School of Dentistry, Pusan National University, Yangsan, Gyeongnam, Korea. golddent@pusan.ac.kr
authors.forename: Hyeon-Cheol
authors.initials: HC
authors.lastname: Kim
authors.forename: Min-Ho
authors.initials: MH
authors.lastname: Lee
authors.forename: Jiwan
authors.initials: J
authors.lastname: Yum
authors.forename: Antheunis
authors.initials: A
authors.lastname: Versluis
authors.forename: Chan-Joo
authors.initials: CJ
authors.lastname: Lee
authors.forename: Byung-Min
authors.initials: BM
authors.lastname: Kim
mesh_terms: 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
chemical_list: D003722:Dental Alloys; C040654:titanium nickelide; D009532:Nickel; D014025:Titanium