|Irradiation Effects on Human Cortical Bone Fracture Behavior|
Human bone is strong but still fallible. To better predict fracturing in bone, researchers need a mechanistic framework to understand the changes taking place on different size scales within bone, as well as the role of sustained irradiation damage. Combining in situ mechanical testing with synchrotron x-ray diffraction imaging and/or tomography, is a popular method of investigating micrometer deformation and fracture behavior in bone. However, the role that irradiation plays in these high-exposure experiments, and how it affects the properties of bone tissue, are not yet fully understood. A team of researchers led by Robert O. Ritchie at the Lawrence Berkeley National Laboratory and the University of California, Berkeley used synchrotron radiation micro-tomography at Advanced Light Source Beamline 8.3.2 to investigate changes in crack path and toughening mechanisms in human cortical bone with increased exposure to radiation, finding that exposure to high levels of irradiation can lead to drastic losses in strength, ductility, and toughness.
Cortical bone is the hard outer layer of bone that is designed to resist fracture. It is a unique and highly complex biomaterial due to its being both light and tough. At the molecular level, bone is made up of fibrous polymer collagen and hard mineral nanoparticals of hydroxyapatite that reinforce it. At the micron level, bone contains osteons - bone cylinders ~100µm in diameter with a central, longitudinal, tubular cavity (Haversian canal), blood vessels, and nerves. Bone’s mechanical behavior is a function of this multi-scaled, hierarchical structure.
Human bone is exposed to irradiation for a wide range of medical and scientific research. For example, bones are sterilized through gamma source irradiation for bone allograft surgery, where the standard dose is between 25 kGy to 35 kGy (a fatal dose received by the body is 5 gray, where 1 gray= 1 J/kg). Despite this established standard dose, the effect of irradiation on the mechanical integrity of bone remains controversial.
Irradiation effects resulting from experiments using in situ testing with high-energy synchrotron x-ray diffraction and tomography imaging remain a concern for researchers. A typical tomography experiment involves irradiation at a rate of ~100 Gy/s, leading to as much as ~MGy of irradiation resulting from long exposure times.
Synchrotron radiation microtomography was employed to evaluate the changes in the fracture resistance of bone exposed to high levels of irradiation. Researchers collected three-dimensional images of the crack paths and microstructures in bone subjected to different degrees of irradiation (0 kGy, 50 Gy, 70 kGy, 210 kGy, and 630 kGy); the higher doses were comparable to the x-ray irradiation levels typically encountered during x-ray computed microtomography (µXCT) scans.
The effect of irradiation is to dramatically degrade the strength and toughness of bone together with any capacity for plastic deformation (which relates to bones’ ability to break and reform bonds). For example, bone toughness decreased by a factor of five after 210 kGy of irradiation. This is associated with degradation mechanisms at numerous length-scales. Specifically, at the nanoscale, irradiation leads to a marked increase in collagen cross-linking and molecular damage (assessed using Raman spectroscopy), resulting in a loss in strength and plasticity. Additionally, at length-scales above a micron, toughening mechanisms can be markedly changed. Cortical bones’ resistance to fracture in the transverse (breaking) orientation can be associated with radical changes in the crack path, such as crack deflection and twisting, as a growing crack encounters the boundaries of the osteons. The technique of X-ray microtomography has been utilized here to identify changes in these toughening mechanisms at this significant length scale.
The three-dimensional tomographic images show the crack path and the microstructure for both the non-irradiated human cortical bone and the bone irradiated with 210 kGy. It was observed that the crack path in cortical bone following significant levels of irradiation, still displayed crack deflection at the boundaries of the osteon; however, the frequency of such deflections increases with irradiation, leading to smaller amplitude deflections and less tortuous crack paths, both of which lessen the toughening from this mechanism.
This study shows that when biological tissue such as bone is exposed to high levels of irradiation, serious deleterious effects to the collagen can occur, leading to drastic losses in strength, ductility and toughness. It is therefore critical that studies on bone using in situ tests involving radiation, such as deformation and fracture testing coupled with x-ray diffraction and/or tomography, take this into careful consideration.
Research conducted by H.D. Barth and R.O. Ritchie (Berkeley Lab and University of California, Berkeley), M.E Launey, J.W. Ager III and A.A. MacDowell (Berkeley Lab).
Research funding: Laboratory Directed Research and Development Program of Lawrence Berkeley National Laboratory. Operation of the ALS is supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences.
Publication about this research: H.D. Barth, M.E Launey, A.A. MacDowell, J.W. Ager III and R.O. Ritchie, On the Effect of X-ray Irradiation on the Deformation and Fracture Behavior of Human Cortical Bone, Bone, Vol. 46 (6), 2010.
ALS Science Highlight #212