title: Controlled failure mechanisms toughen the dentino-enamel junction zone.
abstract: BACKGROUND
  The dentino-enamel junction (DEJ) durably unites dissimilar hard brittle enamel and tough flexible dentin. In contrast to artificial bonds between restorations and dentin, the DEJ rarely fails except when it is affected by inherited disorders. Knowledge of DEJ toughening mechanisms is important in understanding inherited disorders, in biomimetic engineering of junctions between artificial restorations and teeth, and in tissue-engineering a DEJ.
  
  
  OBJECTIVE
  The purpose of this study was to identify specific DEJ-zone failure mechanisms and to survey the fracture toughness of the human DEJ zone.
  
  
  METHODS
  Fracture toughness indentations were made at 3 sites across the DEJ zone of 10 human incisor teeth. Failure modes identified using optical microscopy and fracture toughness (MPa.m(1/2)) were calculated following Vickers microindentation. Site mean values were then calculated and compared using 1-way analysis of variance (alpha=.05).
  
  
  RESULTS
  The DEJ did not undergo catastrophic interfacial delamination; instead, damage was distributed over a broad zone. The primary damage mode involved cracking and damage dispersion in the specialized first-formed enamel close to the DEJ. Multiple, somewhat convoluted and sometimes branching, cracks spread and diffused damage over a wide area of adjacent enamel rather than producing catastrophic interfacial failure. Other secondary mechanisms included short microcracks in the DEJ adjacent dentin with possible cracked bridging, as well as plastic deformation of the DEJ without delamination. A DEJ-zone fracture toughness of approximately 0.8 to 0.9 MPa.m(1/2) was calculated.
  
  
  CONCLUSIONS
  DEJ-zone damage occurred primarily within the adjacent layer of specialized first-formed enamel, and the optical DEJ interface resisted delamination.
authors.affiliation: University of California-Los Angeles School of Dentistry, 10833 Le Conte Avenue, Los Angeles, CA 90095, USA. snwhite@ucla.edu
authors.forename: S N
authors.initials: SN
authors.lastname: White
authors.forename: V G
authors.initials: VG
authors.lastname: Miklus
authors.forename: P P
authors.initials: PP
authors.lastname: Chang
authors.forename: A A
authors.initials: AA
authors.lastname: Caputo
authors.forename: H
authors.initials: H
authors.lastname: Fong
authors.forename: M
authors.initials: M
authors.lastname: Sarikaya
authors.forename: W
authors.initials: W
authors.lastname: Luo
authors.forename: M L
authors.initials: ML
authors.lastname: Paine
authors.forename: M L
authors.initials: ML
authors.lastname: Snead
mesh_terms: D001696:Biomechanical Phenomena; D003743:Dental Enamel / Q000293:injuries / Q000503:physiopathology*; D003799:Dental Stress Analysis; D003804:Dentin / Q000293:injuries / Q000503:physiopathology*; D006244:Hardness; D006801:Humans; D007180:Incisor; D013314:Stress, Mechanical; D014082:Tooth Fractures / Q000145:classification / Q000503:physiopathology*