On the Relative Relevance of Subject-Specific Geometries and Degeneration-Specific Mechanical Properties for the Study of Cell Death in Human Intervertebral Disk Models

Capturing patient- or condition-specific intervertebral disk (IVD) properties in finite element models is outmost important in order to explore how biomechanical and biophysical processes may interact in spine diseases. However, disk degenerative changes are often modeled through equations similar to those employed for healthy organs, which might not be valid. As for the simulated effects of degenerative changes, they likely depend on specific disk geometries. Accordingly, we explored the ability of continuum tissue models to simulate disk degenerative changes. We further used the results in order to assess the interplay between these simulated changes and particular IVD morphologies, in relation to disk cell nutrition, a potentially important factor in disk tissue regulation. A protocol to derive patient-specific computational models from clinical images was applied to different spine specimens. In vitro, IVD creep tests were used to optimize poro-hyperelastic input material parameters in these models, in function of the IVD degeneration grade. The use of condition-specific tissue model parameters in the specimen-specific geometrical models was validated against independent kinematic measurements in vitro. Then, models were coupled to a transport-cell viability model in order to assess the respective effects of tissue degeneration and disk geometry on cell viability. While classic disk poro-mechanical models failed in representing known degenerative changes, additional simulation of tissue damage allowed model validation and gave degeneration-dependent material properties related to osmotic pressure and water loss, and to increased fibrosis. Surprisingly, nutrition-induced cell death was independent of the grade-dependent material properties, but was favored by increased diffusion distances in large IVDs. Our results suggest that in situ geometrical screening of IVD morphology might help to anticipate particular mechanisms of disk degeneration

The Hipparcos Input Catalogue. Volumes 1 - 7.

Vol. 1 - 5: The Hipparcos Input Catalogue. Vol. 6: Annex 1. Double and multiple stars. Vol. 7: Annex 2. The atlas of identification charts for faint stars. Annex 3. Identification charts for stars in galactic open clusters. Annex 4. Identification charts for stars in the Magellanic Clouds. The Hipparcos Input Catalogue was constructed as the observing programme for the European Space Agency's Hipparcos astrometry mission. The requirements of the project in terms of completeness, sky coverage, astrometric and photometric accuracy, as well as the necessary optimisation of the scientific impact, resulted in an extended effort to compile and homogenize existing data, to clarify sources and identifications and, where needed, to collect new data matching the required accuracy. This has resulted in an unprecedented catalogue of stellar data including up-to-date information on positions, proper motions, magnitudes and colours, and (whenever available) spectral types, radial velocities, multiplicity and variability information. The catalogue is complete to well-defined magnitude limits, and includes a substantial sampling of the most important stellar categories present in the solar neighborhood beyond these limits. The magnitude limits vary from 7.3 to 9 mag as a function of galactic latitude and spectral type, and there are no stars fainter than about V = 13 mag