title: Micro-MR imaging-based computational biomechanics demonstrates reduction in cortical and trabecular bone strength after renal transplantation.
abstract: OBJECTIVE
  To examine the ability of three-dimensional micro-magnetic resonance (MR) imaging-based computational biomechanics to detect mechanical alterations in trabecular bone and cortical bone in the distal tibia of incident renal transplant recipients 6 months after renal transplantation and compare them with bone mineral density (BMD) outcomes.
  
  
  METHODS
  The study was approved by the institutional review board and complied with HIPAA guidelines. Written informed consent was obtained from all subjects. Micro-MR imaging of distal tibial metaphysis was performed within 2 weeks after renal transplantation (baseline) and 6 months later in 49 participants (24 female; median age, 44 years; range, 19-61 years) with a clinical 1.5-T whole-body imager using a modified three-dimensional fast large-angle spin-echo pulse sequence. Micro-finite-element models for cortical bone, trabecular bone, and whole-bone section were generated from each image by delineating the endosteal and periosteal boundaries. Mechanical parameters (stiffness and failure load) were estimated with simulated uniaxial compression tests on the micro-finite-element models. Structural parameters (trabecular bone volume fraction [BV/TV, bone volume to total volume ratio], trabecular thickness [TbTh], and cortical thickness [CtTh]) were computed from micro-MR images. Total hip and spine areal BMD were determined with dual-energy x-ray absorptiometry (DXA). Parameters obtained at the follow-up were compared with the baseline values by using parametric or nonparametric tests depending on the normality of data.
  
  
  RESULTS
  All mechanical parameters were significantly lower at 6 months compared with baseline. Decreases in cortical bone, trabecular bone, and whole-bone stiffness were 3.7% (P = .03), 4.9% (P = .03), and 4.3% (P = .003), respectively. Decreases in cortical bone, trabecular bone, and whole-bone failure strength were 7.6% (P = .0003), 6.0% (P = .004), and 5.6% (P = .0004), respectively. Conventional structural measures, BV/TV, TbTh, and CtTh, did not change significantly. Spine BMD decreased by 2.9% (P < .0001), while hip BMD did not change significantly at DXA.
  
  
  CONCLUSIONS
  MR imaging-based micro-finite-element analysis suggests that stiffness and failure strength of the distal tibia decrease over a 6-month interval after renal transplantation.
authors.affiliation: Department of Radiology, University of Pennsylvania, 1 Founders, 3400 Spruce St, Philadelphia, PA 19104, USA. chamith@mail.med.upenn.edu
authors.forename: Chamith S
authors.initials: CS
authors.lastname: Rajapakse
authors.forename: Mary B
authors.initials: MB
authors.lastname: Leonard
authors.forename: Yusuf A
authors.initials: YA
authors.lastname: Bhagat
authors.forename: Wenli
authors.initials: W
authors.lastname: Sun
authors.forename: Jeremy F
authors.initials: JF
authors.lastname: Magland
authors.forename: Felix W
authors.initials: FW
authors.lastname: Wehrli
mesh_terms: D015502:Absorptiometry, Photon; D000328:Adult; D001696:Biomechanical Phenomena; D015519:Bone Density; D012080:Chronic Kidney Disease-Mineral and Bone Disorder / Q000000981:diagnostic imaging / Q000473:pathology*; D005260:Female; D020342:Finite Element Analysis; D006801:Humans; D007090:Image Interpretation, Computer-Assisted; D021621:Imaging, Three-Dimensional; D016030:Kidney Transplantation; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D013977:Tibia / Q000000981:diagnostic imaging / Q000473:pathology*; D051598:Whole Body Imaging