title: Microfracture and changes in energy absorption to fracture of young vertebral cancellous bone following physiological fatigue loading.
abstract: METHODS
  Fifty-five human thoracolumbar vertebrae were randomly fatigue loaded and analyzed.
  
  
  OBJECTIVE
  The purpose of this study was to explore the relationship between fatigue loading, trabecular microfracture, and energy absorption to fracture in human cadaveric thoracolumbar vertebrae.
  
  
  BACKGROUND
  Although trabecular microfractures are found in vivo and have been produced by fatigue loading in vitro, the effect of the level of physiologic fatigue loading on microfracture and energy absorption has not been investigated.
  
  
  METHODS
  Fifty-five human thoracolumbar vertebrae (T11-L4) were randomly divided into 5 groups: 1) control (no loading, n = 6); 2) axial compression to yield (n = 7); and 3-5) 20,000 cycles of fatigue loading at 2 Hz (each n = 14). The level of fatigue loading was determined as a proportion of the yield load of Group 2 as follows: 10% (Group 3), 20% (Group 4), and 30% (Group 5). Half of the specimens in groups 3 to 5 were used for radiographic and histomorphometric analysis to determine microfracture density and distribution, whereas the other half were tested to determine the energy absorption to yield failure.
  
  
  RESULTS
  No radiographic evidence of gross fracture was found in any of the groups following fatigue loading. A mean 7.5% increase in stiffness was found in specimens subject to cyclic loading at 10% of yield stress (Group 3). Fatigue at 20% (Group 4) and 30% of yield stress (Group 5) caused significantly higher (P < 0.05) increases in mean stiffness of 23.6% and 24.2%, respectively. Microfracture density increased from 0.46/mm in Group 3 to 0.66/mm in Group 4 and 0.94/mm in Group 5 (P < 0.05). The energy absorbed to failure decreased from 21.9 J in Group 3 to 18.1 J and 19.6 J in Groups 4 and 5, respectively (P < 0.05).
  
  
  CONCLUSIONS
  Fatigue loading at physiologic levels produced microfractures that are not detectable by radiography. Increased fatigue load results in an increase in microfracture density and decrease energy absorbed to fracture, indicating a reduced resistance to further fatigue loading.
authors.affiliation: Department of Orthopaedic Surgery, The University of Hong Kong, Hong Kong, PR China. wwlu@hkusua.hku.hk
authors.forename: W W
authors.initials: WW
authors.lastname: Lu
authors.forename: K D K
authors.initials: KD
authors.lastname: Luk
authors.forename: K C M
authors.initials: KC
authors.lastname: Cheung
authors.forename: Qiu
authors.initials: Q
authors.lastname: Gui-Xing
authors.forename: J X
authors.initials: JX
authors.lastname: Shen
authors.forename: L
authors.initials: L
authors.lastname: Yuen
authors.forename: J
authors.initials: J
authors.lastname: Ouyang
authors.forename: J C Y
authors.initials: JC
authors.lastname: Leong
mesh_terms: D015502:Absorptiometry, Photon; D000328:Adult; D001696:Biomechanical Phenomena; D015775:Fractures, Stress / Q000000981:diagnostic imaging* / Q000209:etiology / Q000473:pathology*; D006801:Humans; D008297:Male; D016103:Spinal Fractures / Q000000981:diagnostic imaging* / Q000209:etiology / Q000473:pathology*; D016474:Weight-Bearing