title: Investigation of dentin hardness in roots exhibiting the butterfly effect.
abstract: BACKGROUND
  Most vertical root fractures occur in root canal treated teeth, and they usually run in a buccolingual direction. The butterfly effect is an optical phenomenon seen in some sections of tooth roots. The aim was to investigate the microhardness of dentin in mesiodistal and buccolingual cross sections of roots exhibiting the effect.
  
  
  METHODS
  Thirty extracted single-rooted teeth were allocated according to patient age: group 1, 15-24 years; group 2, 25-44 years; and group 3, 45 years and older. Roots were embedded in acrylic and cut into ten 1-mm-thick cross sections. Sections were viewed under a light microscope and coded (1 or 2) according to presence or absence of the butterfly effect. A root scored 20 when all levels featured the butterfly appearance. The 2 teeth with the highest score from each group and 2 control teeth with the minimum score (10) were selected. Two adjacent, consecutive cross sections were chosen from the middle of the roots. Vickers microhardness testing was carried out on the dentin walls.
  
  
  RESULTS
  Mean hardness scores were highest mesiodistally (83.7 kgf/mm(2)) and lowest buccolingually (56.4 kgf/mm(2)), a significant difference (P = .028). This trend was found across all age groups.
  
  
  CONCLUSIONS
  Root sections with the butterfly effect are harder mesiodistally. This might explain the high prevalence of vertical root fractures that run buccolingually.
authors.affiliation: Sir John Walsh Research Institute, Faculty of Dentistry, University of Otago, Dunedin, New Zealand.
authors.forename: Assil A
authors.initials: AA
authors.lastname: Russell
authors.affiliation: Sir John Walsh Research Institute, Faculty of Dentistry, University of Otago, Dunedin, New Zealand.
authors.forename: Li Hong
authors.initials: LH
authors.lastname: Chris He
authors.affiliation: Sir John Walsh Research Institute, Faculty of Dentistry, University of Otago, Dunedin, New Zealand. Electronic address: nick.chandler@otago.ac.nz.
authors.forename: Nicholas P
authors.initials: NP
authors.lastname: Chandler
mesh_terms: D000293:Adolescent; D000328:Adult; D000465:Algorithms; D019412:Anatomy, Cross-Sectional / Q000379:methods; D003804:Dentin / Q000033:anatomy & histology* / Q000648:ultrastructure; D006244:Hardness; D006801:Humans; D008875:Middle Aged; D014092:Tooth Root / Q000033:anatomy & histology* / Q000648:ultrastructure; D055815:Young Adult
keywords: Dentin; endodontics; vertical root fracture