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Variation in long bone cross-sectional geometry can be given a more precise functional interpretation using engineering beam theory. However, difficulties in measurement technique have generally prevented studies of large samples of cross sections in this way. In the present study, an automated system utilizing an electronic digitizer and computer software was used to analyze cross-sectional geometric properties of 11 femoral and tibial locations in 119 individuals from the Pecos Pueblo, New Mexico site. The data generated allow identification of clear differences in geometric properties between different regions of the femur and tibia. These differences appear to be related to specific in-vivo mechanical loadings of the lower limb bones, serving to reduce stress and strain under these loadings. The data are also used to investigate possible differences in loading of the femur and tibia in the Pecos and modern samples, and between humans and a nonhuman primate sample.

Summary:Bone marrow contains a population of rare progenitor cells capable of differentiating into bone, cartilage. tendon, and other connective tissues. These cells. referred to as mesenchymal stem cells, can be purified and culture-expanded from animals and humans and have been shown to regencrate functional tissue when delivered to the site of musculoskeletal defects in experimental animal$. To test the ability of purified human mesenchymal stem cells to heal a clinically significant bone defect, mesenchymal stem cells isolated from normal human bone marrow were culture-expanded, loaded onto a ceramic carrier, and implanted into critical-sized segmental defects in the femurs of adult athymic rats. For comparison, cell-free ceramics were implanted in the contralateral limb. The animals were euthanized at 4,8, or 12 weeks, and healing bone defects were compared by high-resolution radiography, immunoliistochemistry. quantitative histomorphometry. and biomechanical testing. In mesenchymal stem cell-loaded samples, radiographic and histologic evidence of new bone was apparent by 8 weeks and histoniorphometry demonstrated increasing bone formation through 12 weeks. Biomechanical evaluation confirmed that femurs implanted with mesenchymal stem cell-loaded ceramics were significantly stronger than those that received cell-lree ceramics. These studies demonstrate that human mesenchymal stern cells can regenerate bone in a clinically significant osscous defect and may therefore provide an alternative to autogcnous bone grafts.

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The compressive strength of bone is proportional to the square of the apparent density and to the strain rate raised to the 0.06 power. This relationship is applicable to trabecular and compact bone, and provides clinical guidelines for predicting bone strength on the basis of x-ray and densitometric examination.