title: Mechanical properties of ProTaper Gold nickel-titanium rotary instruments.
abstract: OBJECTIVE
  To evaluate and compare the resistance to cyclic fatigue and torsional stress, flexibility and surface microhardness of ProTaper Gold (PTG; Dentsply, Tulsa Dental Specialties, Tulsa, OK, USA) system with ProTaper Universal (PTU; Dentsply Maillefer, Ballaigues, Switzerland).
  
  
  METHODS
  PTG and PTU instruments were rotated in simulated canals and the number of cycles to failure was recorded to assess their cyclic fatigue resistance. Torsional strength was measured using a torsiometer after fixing firmly the apical 3 mm of the instrument. A scanning electron microscope was used to characterize the topographic features of the fracture surfaces of the broken instruments. The instruments were tested for bending resistance using cantilever-bending test. Vickers microhardness was measured on the cross section of instruments with 300 g load and 15 s dwell time. Data were analysed statistically using independent t-tests. Statistical significance was set at P < 0.05.
  
  
  RESULTS
  PTG instruments had a significantly higher resistance to cyclic fatigue and flexibility than PTU (P < 0.001). The fractured cross-sectional surfaces revealed typical features of cyclic fractures, including crack origins, fatigue zones and overload fast fracture zones. On the other hand, PTU instruments were associated with higher resistance to torsional stress and microhardness than PTG instruments (P < 0.001). After torsional tests, the fractured cross-sectional surfaces revealed skewed dimples near the centre of the fracture surfaces and circular abrasion streaks.
  
  
  CONCLUSIONS
  The PTG instrument had improved resistance to cyclic fatigue and flexibility compared with PTU. PTU instruments had improved resistance to torsional stress and microhardness compared with PTG.
authors.affiliation: Department of Endodontics, Mansoura University, Mansoura, Egypt. aelnaghy@mans.edu.eg.
authors.forename: A M
authors.initials: AM
authors.lastname: Elnaghy
authors.affiliation: Department of Dental Biomaterials, Faculty of Dentistry, Mansoura University, Mansoura, Egypt.
authors.forename: S E
authors.identifier: http://orcid.org/0000-0002-7525-1379
authors.initials: SE
authors.lastname: Elsaka
mesh_terms: D003748:Dental Equipment; D004867:Equipment Design; D004868:Equipment Failure; D008422:Materials Testing; D009532:Nickel; D013314:Stress, Mechanical; D014025:Titanium; D054159:Torsion, Mechanical
chemical_list: C040654:titanium nickelide; D009532:Nickel; D014025:Titanium
keywords: cyclic fatigue; flexibility; microhardness; thermal treatments; torsional resistance