Y. N. Yeni

The Influence of Bone Morphology on Fracture Toughness of the Human Femur and Tibia

The influence of porosity, osteon density, osteonal area, osteonal lamellar area, osteon size, and haversian canal size on the tension and shear fracture toughness, that is, the mode I and mode II strain energy release rate (GIc and GIIc), respectively, were investigated for the human femur and the tibia. The results suggest that porosity and osteon density were the best explanatory morphological parameters for GIc and GIIc. Both GIc and GIIc significantly decrease with increasing porosity. They also increase with increasing osteon density, the increase being significant for the femur only. Morphological parameters, altogether, can explain 49%-68% of the variation in fracture toughness. We concluded that, although there must be other factors such as biochemical components and microdamage, osteon morphology has an important influence on fracture resistance of the cortical bone.

Influence of microdamage on fracture toughness of the human femur and tibia

The relationship between microdamage accumulation and bone fragility is not well understood. Previous work has demonstrated a positive relationship between microdamage and age in human cortical bone. Prior investigations have also demonstrated that fracture toughness decreases with age in the same bone. Based on these findings, the objective of this study was to test the hypothesis that a decrease in fracture toughness can be attributed to an increase in microdamage density. Microdamage parameters (density, surface density, and average crack length) were measured from bone taken from the shaft of the human femur and tibia and correlated with results from fracture toughness tests of the same bone. Results indicated that there was a weak but significant inverse relationship between fracture toughness and microdamage parameters for tension loading of the femur. These findings suggest that in vivo microdamage observable at the transmitted light level (100x) plays a secondary role to other contributory factors to decreased fracture toughness with age. Results also indicate that this relationship depends on bone ductility that apparently differs between the femur and the tibia. This study, in addition to prior investigations, suggests that crack size (microscopic vs. submicroscopic) and crack origin or type (in vivo vs. stress induced de novo) may influence the relationship between microdamage and bone toughness.

Calculation of porosity and osteonal cement line effects on the effective fracture toughness of cortical bone in longitudinal crack growth

Based on the microscopic analyses of cracks and correlational studies demonstrating evidence for a relationship between fracture toughness and microstructure of cortical bone, an equation was derived for bone fracture toughness in longitudinal crack growth, using debonding at osteonal cement lines and weakening effect of pores as main crack mechanisms. The correlation between the measured and predicted values of fracture toughness was highly significant but weak for a single optimal value of matrix to cement line fracture toughness ratio. Using fracture toughness values and histomorphometrical parameters from an available data set, matrix to cement line fracture toughness ratio was calculated for human femoral bone. Based on these calculations it is suggested that the effect of an osteon on fracture toughness will depend on the cement lines ability to compensate for the pore in an osteon. Matrix to cement line fracture toughness ratio significantly increased with increasing age, suggesting that the effectiveness of osteons in energy absorption may be reduced in the elderly due to a change in cement line properties.