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WHOLE BONE MECHANICS

Motivated by the need to better understand the etiology of osteoporotic bone fractures, one major goal is to determine the effects of damage to trabecular bone - which may occur during a non-fracturing fall or very strenuous activities - on the strength of whole bones such as the proximal femur and vertebral body. A unique aspect of this work is our ability to incorporate our constitutive data for the mechanical behavior of intact and damaged trabecular bone tissue - measured using our accurate testing techniques described above - into these whole bone structural analyses, enabling us to develop highly accurate models of the real whole bone. In this way, there is a synergistic effect between our work on bone at the tissue and organ levels.

In collaboration with researchers at the Orthopaedic Biomechanics Laboratory (Beth Israel Hospital, Boston) we have refined our finite element modeling capability of the human spine. Comparing bone-specific finite element models with the real structure tested to failure mechanically, we found that locations of trabecular failure corresponded closely with regions of high strain (as opposed to high stress). We have also shown that the role of the cortical shell is minimal in the vertebral body, but that the anisotropy of the trabecular centrum is important to consider in finite element modeling of these complex systems. The results from these fundamental studies, in addition to developing our intuition for the complex mechanics of the vertebral body, have provided the framework from which we can now construct three-dimensional models of the whole spine from computed tomography scans with confidence of valid model predictions. Cuurent work in our laboratory is directed towards refining these analyses and developing methods to include the effects of damaged bone in our models. This work is funded by an ongoing NIH FIRST award.

Similar studies have been performed for the proximal femur and are now being performed for the facial skeleton (Figure). Using these models, for example, we have quantified stress distributions in the facial skeleton for various habitual loads, and have determined the effects of various surgical procedures and pathologies on these stresses. We are also initiating a collaboration with researchers from San Francisco General Hospital to incorporate aspects of fall dynamics into models of the proximal femur in order to better identify patients at risk of osteoporotic fracture. This work combines our expertise in bone mechanics with that of our collaborators in energy absorption and fall dynamics for the proximal femur.


ABOVE: Human facial skeleton
model used in finite element
modeling.

Relevant Publications

  1. Mizrahi J, Silva MJ, Keaveny TM, Edwards WT, and Hayes WC: Finite element stress analysis of the normal and osteoporotic lumbar spine. Spine, 18:2088Ð2096, 1993.abstract
  2. Silva MJ, Wang C, Keaveny TM, and Hayes WC: Direct and computed tomography thickness measurements of the human vertebral shell and endplate. Bone, 15:409-414, 1994.abstract
  3. Ford, CM , Keaveny, TM and Hayes WC: The effect of impact direction on the structural capacity of the proximal femur during falls, Journal of Bone and Mineral Research, 11:377-383, 1996.
  4. Keaveny TM, Jaloszynski RL, and Murphy SB: The importance of stresses in the etiology of slipped capital femoral epiphysis. Trans. Orthopaedic Research Society 1993, 704.
  5. Silva MJ, Wang C, and Keaveny TM: Direct thickness measurements of the vertebral body shell and endplate. Trans. Orthopaedic Research Society 1993, 401.
  6. Silva MJ, Keaveny TM, and Hayes WC: Load sharing between cortical and trabecular bone in the lumbar vertebral body. Trans. Orthopaedic Research Society 1994, 426.
  7. Silva MJ, Keaveny TM, and Hayes WC: CT-based finite element analysis predicts failure loads and fracture patterns for vertebral sections. Trans. Orthopaedic Research Society 1996, 273.
  8. Klisch SM, Duncan NA, Keaveny TM, and Lotz JC: The relative effects of disc water content and bone modulus on vertebral body stresses. Proc. ISSLS (Int Soc. for the Study of the Lumbar Spine), June 25-29, Burlington VT, 1996, 68.

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