In Vivo Tibial Compressive Stiffness Variations after HR-pQCT Measurements of 60 Days Immobility during the Berlin Bed Rest Study II Using μfinite Element Analysis
As bones are able to adapt and optimize their internal architecture for supporting and transmitting physiological loads without suffering damage two differently controlled mechanical stimuli were analyzed and compared to test the hypothesis that vibration exercise could avoid loss of bone mass even during immobility. 24 subjects (including a control group) where maintained in BR during 60 days and 6° head tilt position. Two training regimes were tested during the BR phase: 1) a resistive compressive force plus vibration exercise (RVE) and 2) identical that 1, but without vibration (RE). In vivo effects of immobility on bone architecture in time and zones with high risk for suffering damage were estimated by determining stiffness variations of human tibial samples (μFE Analysis) before during and after BR using data generated after HR-pQCT. Histograms of the percental number of elements in specific ranges for stress/strain were compared after FEA. A major aim was to find threshold values for defining mechanical strains in the Frost curve from in vivo data, which can be used to adjust a previous developed computational model to simulate bone remodeling. In all cases reduced capacity for load transmission was quantified during BR. In average 1.8% for the RVE, 2.5% for the RE and 10% for the CTR-G was estimated after FEA. Regions with highest von Mises values were increased at br59 for the CTR-G. These were localized at the center of the cancellous and at the anteromedial region of the cortical bone with increased cortical porosity. The same specific bone regions appeared to be affected for RVE and the RE-G. For the first time changes in the architecture of bone and quantification of its capacity for load transmission and compressive stiffness in vivo were determined before, during and after a long period of immobility using μFE from HR-pQCT measurements.
KeywordsHR-pQCT FE Analysis Bed rest Compressive stiffness
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