The Role of Suppressed Bone Turnover in Cortical Bone Material Composition, Organization and Fracture Resistance
This NSF funded project ;focuses on advancing the understanding of the changes in material composition, organization, and fracture resistance of bone due to suppressed bone turnover via multiscale computational modeling.
Bisphosphonates are the most commonly used osteoporosis treatment that has been effective in preventing osteoporotic fractures by suppressing bone turnover. Despite the beneficial effects of bisphosphonates, there is accumulating evidence of a potential complication in the form of atypical femoral fracture. The recent reports of possible association of atypical femoral fracture with prolonged bisphosphonate use brought into attention the possibility of adverse mechanical modifications in bone due to extensive suppression of bone turnover. The project utilizes a new fracture mechanics-based finite element modeling approach to perform systematic and controlled evaluations at multiple scales to quantify the critical levels of material property changes that will impair the fracture resistance of bone. Specifically, the study willdetermine the effect of bone mineral and matrix heterogeneity on crack growth, quantify the influence of increased mineralization and accumulation of advanced glycation end products on crack propagation, and identify the critical level of microcrack accumulation in bone that will adversely affect the fracture toughening mechanisms in bone.
Noninvasive patient-specific fracture risk assessment
The diagnosis of fracture risk and osteoporosis has been traditionally done based on bone mass measurements. However, recent studies show that fracture incidence cannot be predicted by bone mass alone and factors such as bone geometry, microstructure, and bone’s material properties affect an individual’s fracture risk. Therefore, new fracture assessment tools that include other factors in addition to bone mass are needed. The overall goal of this research project is to develop a new and improved noninvasive patient-specific fracture risk assessment tool that utilizes computed tomography and fracture mechanics-based finite element modeling. Specifically, the project focuses on the fracture risk assessment of Colles’ fracture which is shown to be an early indicator of increased risk of future spine and hip fractures.
Our initial research efforts in this area focused on using idealized geometries of human radius bone to validate the computational method that we developed. This approach . The results obtained from this work identified the significant influence of the cortical bone geometry on Colles’ fracture load (Ural, 2009). In addition, the simulations showed that best fracture risk prediction can be obtained through combined evaluation of intrinsic properties of the bone and external factors during a fall (Buchanan and Ural, 2010).
Contour images of the fracture plane of the radius bone during Colles’ fracture
Following the establishment of the modeling approach, we are now applying this approach to actual human radius bone images obtained using HR-pQCT to establish a patient-specific fracture risk assessment. This project is in collaboration with Columbia University.
A sample HR-pQCT bone image
A sample HR-pQCT bone image of distal radius bone from a 63-year-old subject and the corresponding finite element model.
Evaluation of microscale fracture mechanisms in bone
The effect of microstructural features on the toughness and crack propagation behavior in human cortical bone has been demonstrated in the literature by experimental studies. However, despite a general understanding of the effect of bone microstructure and its properties on toughness and crack growth, a thorough assessment of the relationship between the bone microstructure, its mechanical properties, and microcrack formation and growth is not present in the literature. The aim of this research project is to provide a mechanical understanding that explains the effect of each microstructural component including osteons, cement lines, pores, and interstitial bone on cortical bone fracture.
Our research efforts in this area first focused on the finite element evaluation of the influence of cement lines on crack propagation behavior using a single microstructural unit. The new modelling approach that we developed utilizing a single microstructural unit was the first study in the literature that included both toughness and strength considerations in the assessment of the effect of cement line on bone fracture. This study demonstrated the significant role that the strength of the cement line plays in the crack deflection characteristics (Mischinski and Ural, 2011).
A sample finite element mesh demonstrating crack penetration and an crack deflection case (a), (c). Stress contours for crack deflection into a cement line and crack penetration into an osteon (b), (d).
Building up on this new approach and new findings, our subsequent study utilized actual human bone microstructure images and focused on developing more advanced modelling techniques such as arbitrary crack growth to assess bone fracture behavior. The results of this study provided additional insight into the influence of cement lines on crack propagation trajectory and fracture response and identified the effect of different microstructural arrangements on the microscale fracture processes. (Mischinski and Ural, 2011).
A microscopy image of human cortical bone, the corresponding finite element mesh, and crack growth trajectory based on variation in material properties of the cement line.
Modeling of strain rate effects on bone fracture
Bone is subject to a wide range of strain rates during daily activities or traumatic fracture events such as accidents or falls. Previous studies showed that the mechanical response of bone, including its modulus of elasticity, yield stress and strain, and ultimate stress and strain vary with the loading rate. A comprehensive understanding of traumatic fractures requires an investigation of bone’s resistance to fracture initiation and propagation under a variety of low and high strain rates. Most of the fracture toughness measurements reported in the literature under varying strain rates, however, corresponded to quasi-static conditions. In this study, we aim to evaluate the effect of strain rate on fracture toughness of human cortical bone during crack propagation using a finite element approach. The simulation results showed that bone’s resistance against the propagation of fracture decreased sharply with increase in strain rates up to a threshold level and attained an almost constant value for strain rates larger than this threshold (Ural et al, 2011). We also evaluated the effect of loading rate on distal forearm fracture. The simulations showed that the most drastic reduction in fracture load occurs at strain rates corresponding to the transition from controlled to impact falling. These results are particularly important for the improvement of fracture risk assessment in the elderly because they identify a critical range of loading rates that can dramatically increase the risk of distal forearm fracture (Ural et al., 2012). This project is in collaboration with RPI and Cranfield University.
Normalized R-curve slope vs. strain rate and normalized Kinit vs. strain rate for 2D compact tension specimens.