title: Prediction of bone strength by μCT and MDCT-based finite-element-models: how much spatial resolution is needed?
abstract: OBJECTIVE
  Finite-element-models (FEM) are a promising technology to predict bone strength and fracture risk. Usually, the highest spatial resolution technically available is used, but this requires excessive computation time and memory in numerical simulations of large volumes. Thus, FEM were compared at decreasing resolutions with respect to local strain distribution and prediction of failure load to (1) validate MDCT-based FEM and to (2) optimize spatial resolution to save computation time.
  
  
  METHODS
  20 cylindrical trabecular bone specimens (diameter 12 mm, length 15-20mm) were harvested from elderly formalin-fixed human thoracic spines. All specimens were examined by micro-CT (isotropic resolution 30 μm) and whole-body multi-row-detector computed tomography (MDCT, 250 μm × 250 μm × 500 μm). The resolution of all datasets was lowered in eight steps to ~ 2,000 μm × 2000 μm × 500 μm and FEM were calculated at all resolutions. Failure load was determined by biomechanical testing. Probability density functions of local micro-strains were compared in all datasets and correlations between FEM-based and biomechanically measured failure loads were determined.
  
  
  RESULTS
  The distribution of local micro-strains was similar for micro-CT and MDCT at comparable resolutions and showed a shift toward higher average values with decreasing resolution, corresponding to the increasing apparent trabecular thickness. Small micro-strains (εeff<0.005) could be calculated down to 250 μm × 250 μm × 500 μm. Biomechanically determined failure load showed significant correlations with all FEM, up to r=0.85 and did not significantly change with lower resolution but decreased with high thresholds, due to loss of trabecular connectivity.
  
  
  CONCLUSIONS
  When choosing connectivity-preserving thresholds, both micro-CT- and MDCT-based finite-element-models well predicted failure load and still accurately revealed the distribution of local micro-strains in spatial resolutions, available in vivo (250 μm × 250 μm × 500 μm), that thus seemed to be the optimal compromise between high accuracy and low computation time.
authors.affiliation: Department of Radiology, Technische Universität München, Munich, Germany; Department of Radiology, University of California, San Francisco, CA, United States; Max Planck Institute for Extraterrestrial Physics, Garching, Germany. Electronic address: jsb@tum.de.
authors.forename: Jan S
authors.initials: JS
authors.lastname: Bauer
authors.affiliation: Max Planck Institute for Extraterrestrial Physics, Garching, Germany.
authors.forename: Irina
authors.initials: I
authors.lastname: Sidorenko
authors.affiliation: Department of Radiology, Universität Köln, Germany.
authors.forename: Dirk
authors.initials: D
authors.lastname: Mueller
authors.affiliation: Department of Radiology, Technische Universität München, Munich, Germany; Department of Radiology, University of California, San Francisco, CA, United States; Max Planck Institute for Extraterrestrial Physics, Garching, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Baum
authors.affiliation: Department of Radiology, University of California, San Francisco, CA, United States; Department of Radiology, Charite, Berlin, Germany.
authors.forename: Ahi Sema
authors.initials: AS
authors.lastname: Issever
authors.affiliation: Institute of Anatomy and Musculoskeletal Research, Paracelsus Medical University, Salzburg, Austria.
authors.forename: Felix
authors.initials: F
authors.lastname: Eckstein
authors.affiliation: Department of Radiology, Technische Universität München, Munich, Germany.
authors.forename: Ernst J
authors.initials: EJ
authors.lastname: Rummeny
authors.affiliation: Department of Radiology, University of California, San Francisco, CA, United States.
authors.forename: Thomas M
authors.initials: TM
authors.lastname: Link
authors.affiliation: Max Planck Institute for Extraterrestrial Physics, Garching, Germany.
authors.forename: Christoph W
authors.initials: CW
authors.lastname: Raeth
mesh_terms: D015502:Absorptiometry, Photon / Q000379:methods*; D000368:Aged; D000369:Aged, 80 and over; D015519:Bone Density / Q000502:physiology*; D019245:Compressive Strength / Q000502:physiology; D003198:Computer Simulation; D055119:Elastic Modulus / Q000502:physiology; D005260:Female; D020342:Finite Element Analysis; D006801:Humans; D008297:Male; D008954:Models, Biological; D061330:Multidetector Computed Tomography / Q000379:methods*; D011856:Radiographic Image Enhancement / Q000379:methods; D011857:Radiographic Image Interpretation, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013314:Stress, Mechanical; D013718:Tensile Strength / Q000502:physiology; D013904:Thoracic Vertebrae / Q000000981:diagnostic imaging* / Q000502:physiology*
keywords: Bone; Finite element analysis; Multidetector computed tomography; Osteoporosis; Spine; X-ray microtomography