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ISB Student Dissertation Report, Wafa Tawackoli, Department of Bioengineering, Rice University

Research Topic: Vibrational analysis of multiple stages of trabecular bone loss using rapid prototype duplicates

Read full report here:  2005_Tawackoli_W.pdf 429.62 KB

I am grateful to ISB and my advisor (Dr. Michael Liebschner, Rice University) for kindly providing me each with US$2000 student dissertation award in 2005. This award allowed me to construct a reliable vibration testing system and rapid prototyping duplicates of trabecular bone models to explore the feasibility of using structural dynamics as a diagnostic tool for determining bone integrity.

Osteoporosis is a systemic skeletal disease which increases bone fragility and susceptibility to fracture. The purpose of this study was to explore the feasibility of using structural dynamics as a diagnostic tool for determining bone integrity. In order to isolate the effect of structural changes, the material properties must be controlled. I hypothesized that duplicate samples with the same density (volume fraction) but different micro-architecture will exhibit different frequency responses, hence different biomechanical properties.

Triangulated surfaces of a randomly selected bone cubes taken from the lateral aspect of a human lumber vertebra were generated using microCT scans. Using computer simulation, the overall bone mass of the normal bone cubes were reduced through surface erosion to three additional stages: an osteopenic, a moderate osteoporotic, and a severe osteoporotic. After scaling up all four computer models to a 12 cm edge length, digital files were exported to a rapid prototyping system for fabrication.

In our laboratory, we have constructed an active electro-dynamic shaker capable of producing mechanical vibration. The vibrational device produces a dynamic force operating on the mass reaction principle. A reaction force excites the structure under test. Both input signal (dynamic force) and output signal (oscillation / acceleration) are measured at point contact to the test structure using an impedance head. Vibrational analysis were carried out by applying a swept sine dynamic force and white noise to each bone models while simultaneously recording acceleration and dynamic force. The power spectra of the force and acceleration were computed. Velocity and displacement, dynamic stiffness, half-power bandwidth at peak, and damping ratio for both bone cubes were obtained using FRF measurements and mathematical integration over the frequency spectrum.

The normal bone model showed the weakest modal coupling in power spectrum and largest in the half-peak bandwidths, whereas the severe osteoporotic bone model showed the strongest modal coupling and smallest in the half-peak bandwidths. The dynamic stiffness of the normal bone model was about five time of the osteopenic bone model. Although at a preliminary stage, the results have shown a clear difference between these four bone stages. Currently, vibrational analyses in three axes are performed and vibrational response sensitivity due to loading condition will be investigated. Further studies are underway to ensure the feasibility and reliability of a structural dynamics approach to detect early stages of bone loss. I very much look forward to publish my results in the near future in one of the ISB-affiliated journals. Again, I would like to express my gratitude to the ISB council for funding and supporting my research.