The Effects of Variation in Collagen D-spacing on Compact Bone Viscoelasticity: a Finite Element Analysis

The D-spacing that is characteristic of collagen and its structural arrangement was previously thought to be a constant value. Much research is revealing that it is actually a distribution of values in biological tissues. Recent ovine experimentation has also shown that the D-spacing distribution is significantly altered following estrogen depletion. While ewes contain some major biological differences between their human counterparts, they are an economical and robust large animal model for postmenopausal osteoporosis. So, the exploration of the possible implications that D-spacing has on the mechanical properties of the whole bone utilizing animal models and computational methods is warranted. Six Warhill ewes were used in this experiment and were either ovariectomized or underwent a sham surgery. The animals were sacrificed after 3 years and the radius and ulna bone were harvested for further analysis. Rectangular beams of compact bone tissue were machined from six different sectors in the whole bone and dynamic mechanical analysis tests were performed on the 24 specimens. The viscoelastic property, tangent delta, was measured from each test at varying frequencies. A composite arrangement of collagen and hydroxyapatite were then computationally modeled utilizing finite element analysis to observe the effects of altered D-spacing on the mechanical properties. Jager and Fratzl's staggered array model allowed the inclusion of a D-spacing configuration as well as the simplified 2 dimensional plane strain analysis. Hydroxyapatite was modeled as a perfectly elastic material, while the hydrated collagen component was linear viscoelastic through the use of the standard linear solid model. The main finding of the work is that D-spacing only significantly altered the tangent delta of the computational model when the mineral volume fraction changed. Since the composite model analyzed the structural arrangement of compact bone at such a small scale, the change in mineral volume fraction could only be realistically attributed to intrafibrillar mineral. The results of this preliminary analysis are v promising and warrant the continued investigation of D-spacing and mineral content and their significance in the osteoporotic condition. vi ACKNOWLEDGMENTS First, I would like to thank my advisor Dr. Scott Hazelwood for the opportunity to work on this project. The courses he taught were instrumental in finding a discipline that I'm fascinated and passionate about. I would also like to thank Dr. Clifford Les for his vast knowledge in this field and invaluable guidance throughout. His ability to question and understand various processes definitely helped shape the outcome of …