(geometry) and composition (material) of the tissue. Thus, one cannot directly compare the ultimate force of vertebrae from different regions in the spine because the size of the cervical, thoracic, and lumbar vertebrae varies appreciably from each other and from one level to the next. For this reason, material property measurements, such as stress or force/area are often preferred, since they account for geometry variations of different size struc­tures. An estimate of the apparent stress at failure (σ

reduction in bone mass (and thus density) ensuring an increased rate of fracture.

bone from nonosteoporotic bone tissue, the fracture itself defines pathology. Since the occurrence of a fracture is not only the result of the mechanical properties of the bone, but is also a function of the fracturing trauma, both factors must be considered when defining osteoporosis. In the presence of a patient with a recent fracture, know­ing nothing or very little about the patient’s bone quality or the forces involved in the trauma, the most practical way for clarifying whether a fracture is osteoporotic or not is Harold Frost’s criterion of the “everyday trauma.” Frost stated that a fracture occurring because of an every­day trauma indicates that the patient has osteoporosis or bone fragility. Even if current technology allows us to determine, for example, the amount of bone mineral in different parts of the human skeleton, we still lack practi­cal techniques for measuring the fracture-generating forces. Therefore the “e veryday trauma” definition is still a practical measure for estimating bone fragility (27). Hence, development of models that can simulate both the loading and structural (trabecular) damage behavior of vertebral bodies are important for understanding clinical pathologies (e.g., osteoporosis) and for predicting bone fragility and its risk for fracture.