Finite Element Nonlinear Dynamic Response Analysis of the Human Knee Joint

Objective. To develop a detailed non-linear 3-D dynamic finite element model of the knee joint and perform preliminary analysis simulating different loading conditions to confirm reasonable function of the model. Design. Using ANSYS, a finite element model of the human knee was created and tested. Background. Finite element models of the knee have been developed in previous studies. However, no study has generated a detailed tissue level model, or performed dynamic testing on a model. Methods. Initial cartilage and ligament geometry was received from a previous study. A finite element mesh including bone was created. The model was constrained to simulate different experimental testing conditions by rigidly fixing the distal tibia and limiting the motion of the proximal femur. Free vibration and steady state analyses of the model were performed simulating experiments. Results. A detailed, highly nonlinear finite element model of the human knee was created in three dimensions. Axial compressive load and two constraint conditions were applied. Solution was performed to the point of easy convergence. The model was also examined through dynamic analysis to find the mode shapes. The model performed well under this initial analysis. Conclusion. The model was developed and tested successfully. The model needs further refinement and verification with experimental data to follow. The preliminary analysis of the model indicated that constraint conditions could significantly affect the magnitude and distribution of stresses within the different components of the knee joint. Mode shapes are also varied at different constraint conditions. The model is applicable to predict the vulnerable parts of the knee joint at different clinical situations as well as occupational conditions

Finite element modeling of 3D human mesenchymal stem cell-seeded collagen matrices exposed to tensile strain

The use of human mesenchymal stem cells (hMSCs) in tissue engineering is attractive due to their ability to extensively self-replicate and differentiate into a multitude of cell lineages. It has been experimentally established that hMSCs are influenced by chemical and mechanical signals. However, the combined chemical and mechanical in vitro culture conditions that lead to functional tissue require greater understanding. In this study, finite element models were created to evaluate the local loading conditions on bone marrow derived hMSCs seeded in three dimensional collagen matrices exposed to cyclic tensile strain. Mechanical property and geometry data used in the models were obtained experimentally from a previous study in our laboratory and from mechanical testing. Eight finite element models were created to simulate three-dimensional hMSC-seeded collagen matrices exposed to different levels of cyclic tensile strain (10% and 12%), culture media (complete growth and osteogenic differentiating), and durations of culture (7 and 14 days). Through finite element analysis, it was determined that globally applied uniaxial tensile strains of 10% and 12% resulted in local strains up to 18.3% and 21.8%, respectively. Model results were also compared to experimental studies in an attempt to explain observed differences between hMSC response to 10% and 12% cyclic tensile strain

Genetic background influences fluoride's effects on osteoclastogenesis

Excessive fluoride (F) can lead to abnormal bone biology. Numerous studies have focused on the anabolic action of F yet little is known regarding any action on osteoclastogenesis. Little is known regarding the influence of an individual’s genetic background on the responses of bone cells to F. Four-week old C57BL/6J (B6) and C3H/HeJ (C3H) female mice were treated with NaF in the drinking water (0ppm, 50ppm and 100ppm F ion) for 3 weeks. Bone marrow cells were harvested for osteoclastogenesis and hematopoietic colony-forming cell assays. Sera were analyzed for biochemical and bone markers. Femurs, tibiae and lumbar vertebrae were subjected to microCT analysis. Tibiae and femurs were subjected to histology and biomechanical testing, respectively. The results demonstrated new actions of F on osteoclastogenesis and hematopoietic cell differentiation. Strain specific responses were observed. The anabolic action of F was favored in B6 mice exhibiting dose dependent increases in serum ALP activity (p < 0.001); in proximal tibia trabecular and vertebral BMD (tibia at 50&100ppm, p = 0.001; vertebrae at 50&100ppm, p = 0.023&0.019, respectively); and decrease in intact PTH and sRANKL (p = 0.045 and p < 0.001, respectively). F treatment in B6 mice also resulted in increased numbers of CFU-GEMM colonies (p = 0.025). Strain specific accumulations in bone [F] were observed. For C3H mice, dose dependent increases were observed in osteoclast potential (p < 0.001), in situ trabecular osteoclast number (p = 0.007), hematopoietic colony forming units (CFU-GEMM: p < 0.001, CFU-GM: p = 0.006, CFU-M: p < 0.001), and serum markers for osteoclastogenesis (intact PTH: p = 0.004, RANKL: p = 0.022, TRAP5b: p < 0.001). A concordant decrease in serum OPG (p = 0.005) was also observed. Fluoride treatment had no significant effects on bone morphology, BMD and serum PYD crosslinks in C3H suggesting a lack of significant bone resorption. Mechanical properties were also unaltered in C3H. In conclusion, short term F treatment at physiological levels has strain specific effects in mice. The expected anabolic effects were observed in B6 and novel actions hallmarked by enhanced osteoclastogenesis shifts in hematopoietic cell differentiation in the C3H strain