Numerical Simulation On The Effects Of Age-related Bone Degeneration On Bone Mechanical Properties At Macroand Micro-levels

As an important part in human body, bone plays a crucial role in support and protection. The load-bearing characteristic of bone may lead to more frequent degeneration compared with other tissues. Meanwhile, the load-bearing capability of bone may be seriously impaired during the aging process. With the aggravation of aging problem, the number of people influenced by bone degeneration is increasing. However, the mechanism of age-related bone degeneration on the mechanical properties is still largely unknown. In this study, the effects of age-related bone degeneration on the mechanical properties of whole-bone were identified. Since the variations in bone macro-level mechanical properties were significantly affected by micro-level mechanical properties, the effects of age-related cortical and trabecular bone degeneration on the micro-level mechanical properties were also investigated. To investigate the mechanism of age-related degeneration and fracture in the whole-bone, cortical bone, and trabecular bone, variations in bone mechanical properties at macro- and micro-levels with age were synthetically analyzed, which provided an efficient strategy for prevention and therapy of age-related bone diseases such as fracture in the aging population. This thesis can be divided into the following three sections:Aging and weight-bearing could decrease the load-bearing capability of whole-bone. The effects of working loads on the mechanical properties of whole-bone have been extensively studied, however, little is known about the influences of resting loads on the mechanical properties of whole-bones with different degrees of degeneration. In the first section, four different poroelastic finite element models of lumbar spine with different degrees of degeneration were set up to investigate the responses of different resting modes on stress distribution and recovery of lumbar models. By changing resting time and resting frequency within the fixed total resting time, it was shown that the axial effective stress and fluid exchange rate in lumbar spine gradually decreased with the increase of resting frequency, whereas the pore pressure in nucleus and radial displacement in intervertebral disc gradually increased in this process. The changing rate of biomechanical parameters in lumbar spine also showed significant differences under different resting frequencies. The results of the normal, mildly, and moderately degenerated lumbar spines indicated that the greatest changing rates of the biomechanical parameters occurred in the one-time and three-time rests; for the severely degenerated lumbar spine, the greatest changing rate was observed in the five-time rests. Based on the study in this section, it can be concluded that the increase of resting frequency can promote the recovery of damaged lumbar spine in the fixed total resting time. This study proposed promising resting modes for bone recovery in different degrees of degenerated lumbar spines.A close relationship exists between macro- and micro-level mechanical properties in whole-bone degeneration, and cortical bone plays a dominant load-bearing role in the whole-bone, therefore, the strength of whole-bone is largely determined by cortical bone micro-level mechanical properties. In the second section, the micro-mechanical properties were analyzed for rat femoral cortical bones with different ages to quantify the relationship between rat age and cortical bone mechanical properties at micro-level. The micro-finite element models of rat femoral cortical bones with different ages were established based on the Micro-CT images. Then, an approach combining micro-finite element analysis and macro-compressive test was used to simulate the fracture processes of rat femoral cortical bones under compressive load, so as to predict the micro-level failure strains of rat femoral cortical bones with different ages. The results indicated that the age-related micro-level failure strain of rat femoral cortical bone showed a biphasic behavior with a gradually increased trend within the age of 7 months and a sharply decreased trend beyond 7 months. The predicted results in combination with the macro-experimental data showed that in the period of skeletal maturity(1-7 months of age), both the macroand micro-level mechanical properties showed a large promotion; in the period of skeletal ageing(9-15 months of age), the micro-level mechanical properties sharply deteriorated, however, the macro-mechanical properties only slightly deteriorated. In this study, the variations in macro- and micro-levels mechanical properties of rat femoral cortical bones with different ages were revealed through the analysis of cortical bone mechanical parameters at macro- and micro-levels, which provided a theoretical basis for prevention and therapy of age-related fracture in clinics.Compared with cortical bone, the porosity of trabecular bone is larger, and trabecular bone loss caused by age-related degeneration is more serious. Therefore, for whole-bone containing more trabecular bone the strength is not only related to the mechanical properties of cortical bone, but also significantly affected by the mechanical properties of trabecular bone. In the third section, the variations in mechanical properties of trabecular bone were investigated for the influences of different bone loss locations and patterns in the age-related trabecular bone degeneration. Bone loss caused by age-related degeneration was simulated by trabecular thinning and trabecular loss. Trabecular bone loss could be observed at low strain locations or random sites. Based on these several aged trabecular bone models were set up. The models were solved by extended finite element method, and the fracture processes from crack initiation to complete failure were simulated. The results indicated that, when the decrease in trabecular bone mass was the same, trabecular bone loss at low strain locations generated more damage to the mechanical properties of trabecular bone than that occurred at other random sites. The decrease of trabecular bone mechanical properties caused by trabecular loss was more severe than that caused by trabecular thinning. In addition, loss of vertical trabeculae was more likely to result in fracture of trabecular bone compared with loss of horizontal trabeculae. In this study, the effects of different locations and patterns of trabecular bone loss on the mechanical properties were quantitatively investigated, and the key factors influencing the mechanical properties of trabecular bone were obtained.In this study, the effects of age-related bone degeneration on macro- and micro-mechanical properties of whole-bone, cortical bone, and trabecular bone were investigated by using macro- and micro-finite element analyses in combination with macro-experimental data. The biomechanical mechanisms of age-related bone degeneration and fracture were explored, which provided a theoretical basis for prediction, prevention, and therapy of age-related fracture.