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Johnson, Amy E.

Medical Laboratory Science Program

2006

M.S.

Progressive vertebral body height loss associated with aging and osteoporosis of the spine is most likely caused by microdamage initiated strain accumulation related to mean compressive cyclic loading. Osteoporotic vertebral compression fractures (OVCF) are routinely treated with polymethylmethacrylate (PMMA) cement in a minimally invasive procedure (vertebroplasty) that injects cement into the cancellous bone of the vertebral centrum. While previous studies have reported the fatigue behaviour of trabecular bone, none have investigated the influence of cement augmentation on fatigue behaviour, including endurance limit, effective strain range and strain accumulation behaviour. To study the fatigue behaviour of cement-augmented vertebrae, synthetic thoracic vertebrae were constructed using commercially available open-cell polyurethane foam to simulate trabecular bone and fiberglass resin to simulate cortical bone. Synthetic bone used for biomechanical tests are less variable and easier to handle than natural bone. Two hypotheses related to cement augmentation were examined: 1) cement augmentation will increase the endurance limit, increase mean effective strain and decrease cyclic creep of synthetic vertebrae; and 2) reduction of vertebral centrum porosity (via cement fill) will alter fatigue behaviour analogous to a low porosity solid such as PMMA-only. A morphology analysis and static mechanical tests were conducted to assess the use of polyurethane open-cell foam as a trabecular bone substitute. Compression fatigue tests were conducted under load control on two dissimilar porosity (92.1% and 89.4%) opencell foam based synthetic vertebrae with and without cement augmentation. Since cement augmentation effectively creates a more solid-like composite (filling voids in the foam), fatigue tests were also carded out on PMMA-only samples. The results show that the foam material had similar morphology and porosity as human vertebral cancellous bone, but had a lower material density (40%) and concomitant lower strength (69%) and stiffness (48%) in comparison to human bone. The open-cell foam based synthetic vertebrae, however, exhibited effective strain versus logarithm of cycles to failure (S-N curve), initial apparent modulus, progressive modulus reduction and strain accumulation behaviour comparable to human vertebral cancellous bone. These results indicate that synthetic open-cell foam vertebra is a reasonable alternative to human vertebral bone for both static and fatigue mechanical experiments. Based on the fatigue analysis of synthetic thoracic vertebrae constructs, cement repair improves the fatigue resistance of synthetic vertebrae. Cement augmentation tripled the endurance limit of synthetic vertebrae. As a result fatigue failure is not predicted to occur at effective strains that would otherwise cause fatigue failure in un-cemented vertebrae. The S-N slope of cement-augmented vertebrae showed no significant difference in comparison to the PMMA-only samples, indicating that PMMA cement dominates the fatigue process. Cement augmentation was also found to reduce the mean proportion of cyclic creep (progressive shortening) in total strain accumulation by 8.6%. Comparison of S-N curves and strain accumulation behaviour between the 92.1% and 89.4% porosity synthetic vertebrae showed no significant differences.