student research day - Case Western Reserve University School of ...

Alex Davis
Investigation of the biomechanical consequences of sub-fracture damage of cancellous
bone
Alex J. Davis, II, Seetha Kummari and Christopher J. Hernandez
Mechanical and Aerospace Engineering
Case Western Reserve Univesity
To explain the observation that almost half of osteoporosis-related fractures in the spine are not related to a spontaneous
loading event, it has been proposed that vertebral mechanical damage occurs due to multiple or prolonged loading events.
These events are believed to lead to microscopic cracks in otherwise healthy bone and ultimately reduce bone stiffness and
strength. This experiment was designed to begin investigating the characteristics of loads necessary to generate
microscopic tissue damage in cancellous bone without causing an overt fracture in the cortical shell. Using caudal vertebra
(7-9) from adult female Sprague Dawley rats, each vertebrae was potted in bone cement and mounted in a material testing
device using custom fixtures. Once mounted in the testing device, a sinusoidal cyclic load ranging from 0N to 260N was
applied at 2Hz. Loading was stopped prior to failure based on the rate of change of compliance and if this criteria was not
met within 5 hours of loading. All bones were stained for microscopic damage and were embedded undecalcified in methyl
methacrylate. Standard histomorphometrical analyses were used to determine bone volume fraction and amount of
microscopic cracks, trabecular microfractures , and cortical shell cracks. All loaded specimens displayed considerable diffuse
damage in the epiphyseal regions. Microfractures were observed in nine out of ten specimens loaded into the tertiary
phase and were fewer in number in specimens loaded in the secondary phase. Very few microscopic cracks or diffuse
damage were observed in the metaphyseal regions or in the cortical shell and no macroscopic or microscopic cracks were
observed in the cortical bone of any of the specimens. This present investigation demonstrates that microscopic tissue
damage in cancellous bone of the rat caudal vertebrae can be generated using a cyclic loading profile. The observation that
trabecular microfracture was the predominant form of damage raises further questions about the failure and repair
processes of trabecular bone.
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