A Methodology for Mechanical Measurements of Technical Constants of Trabecular Bone

Vertebral trabecular bone was tested by non-destructive uniaxial and triaxial loadings with the purpose of investigating the orthotropic properties of bone. A triaxial testing apparatus using hydrostatic pressure was developed and allowed to characterise the bony tissue in a three-dimensional stressed state. Thirty specimens, in the form of 10 mm cubes, were tested. The Young's moduli obtained in this study for the trabecular bone of human lumbar vertebrae are found to be in agreement with the values obtained by ultrasonic methods. Analyses of triaxial compressive tests provided, for the first time, the Poisson's ratios of vertebral trabecular bone. These values are found to satisfy thermodynamic restrictions established by Cowin and Van Buskirk (1986). Finally, no significant differences in the material properties were found for segment level (L3-L4).

In Vitro Fluid Dynamics of the St Jude Valve Prosthesis in Steady and Pulsatile Flow

The flow downstream from the St Jude valve prosthesis is studied in vitro in both steady and pulsatile flow. Conditions for the pulsatile flow studies approximate the in vivo resting state. Laser-Doppler anemometry (LDA) is used to measure instantaneous point velocities at numerous locations pre-selected to conform to a systematic, flow-mapping measurement methodology. Overall, the results for the St Jude valve indicate a relatively undisturbed forward flow through the prosthesis. The acceleration ratios, defined as the maximum mean axial velocity for the St Jude valve divided by that for no valve obstructing the flow, are small, being 1.2 and 1.4 for steady and pulsatile flow, respectively. Both the secondary flow structure and the flow disturbance produced by this valve are minimal. Maximum mean velocities in the radial and azimuthal directions are only about 5–10 per cent of that in the axial direction. There is, however, notable evidence of flow wakes generated by the valve's opened leaflets. Velocity defects caused by these wakes are found to be as much as 50 per cent. The bulk flow through this valve is in this sense quite unlike that reported by others for the natural aortic valve studied in vitro.