The mechanical and thermal effects of focused ultrasound in a model biological material.

This paper is motivated by possible medical applications of focused ultrasound in the minimally invasive treatment of a variety of musculoskeletal disorders that are responsive to thermal treatment. A model-based analysis of the interaction of high-intensity focused ultrasound with biological materials is carried out in an effort to predict the path of the sound waves and the temperature field in the focal region. A finite-element-based general purpose code called PZFlex is used to determine the effects of nonlinearity and geometrical complexity of biological structures. It was found that at frequencies of interest in therapeutic applications, the nonlinear effects are usually negligible and the geometrical complexities can be handled through a substructuring procedure. An approximate analytical method with acceptable accuracy is developed as an alternative to the purely numerical approach used in PZFlex. The mechanical and thermal effects in two-layered fluid material systems induced by high-frequency focused ultrasound are calculated through this analytical method. The results are compared with those obtained using PZFlex as a benchmark.

Healing of critically sized femoral defects, using genetically modified mesenchymal stem cells from human adipose tissue.

The FDA has approved the clinical use of recombinant bone morphogenetic proteins (BMPs). However, the use of recombinant BMPs in humans has required large doses of the proteins to be effective, which suggests that the delivery method of bone morphogenetic proteins needs to be optimized. Gene therapy is an alternative method to deliver such recombinant proteins, and gene transfer techniques have been tested on a variety of cell types including bone marrow cells, skin fibroblasts, peripheral blood monocytes, and muscle-derived cells. In this study, we sought to determine the ability of BMP-2-producing human adipose-derived mesenchymal stem cells to heal a critically sized femoral defect in a nude rat model. After approval by the human subjects protection committee, human adipose tissue was obtained from healthy donors. The lipoaspirate was processed as previously described (De Ugarte, D.A., et al. Cells Tissues Organs 174, 101, 2003). Cells were grown in culture and infected with a BMP-2-carrying adenovirus. Five million cells were applied to a collagen- ceramic carrier and implanted into femoral defects as previously described (Zuk, P.A., et al. Mol. Biol. 13, 4279, 2002). All animals were killed at 8 weeks. Femora were dissected out and underwent radiographic, histologic, and biomechanical analysis. Eleven of the 12 femora in the group treated with human processed lipoaspirate (HPLA) cells genetically modified to overexpress BMP-2 had healed at 8 weeks. This was assessed by radiographs, by mechanical testing, and by histology. The one femur that did not heal had a subacute infection. All eight of the femora treated with the rhBMP-2-impregnated collagen-ceramic carrier healed. No statistically significant difference was detected between these two groups. Evaluation of the control groups: group II (collagen- ceramic carrier with HPLA cells) and group III (collagen-ceramic carrier alone) showed that none of the femora had healed by 8 weeks. Our results indicate that HPLA cells genetically modified by adenoviral gene transfer to overexpress BMP-2 can induce bone formation in vivo and heal a critically sized femoral defect in an athymic rat. The HPLA cells alone did not induce significant bone formation. However, when combined with an osteoinductive factor these cells may be an effective method for enhancing bone healing and the tissue engineering of bone.