Determining the Modulus of Intact Bovine Vertebral Cancellous Bone Tissue: Development and Validation of a Protocol
Engbretson, Andrew Craig
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Cancellous, or spongy, bone accounts for nearly 80% of the human skeleton’s internal surface area, despite comprising only 20% of its mass. It is made up of a network of struts and plates that provide lightweight internal support to mammalian bones. In addition, it often serves as the main interface between the skeletal system and implanted devices such as artificial hips, knees, and fracture fixation devices. However, hip arthroplasties can succumb to loosening of the implant due to bone resorption, which is thought to be caused by a mismatch in both apparent and real stiffness between the device and the surrounding bone. Many studies have attempted to determine the Young’s modulus of cancellous bone tissue, but the results are far from being in agreement. Reported values range from less than 1 to nearly 20 GPa. In addition, the small size of trabeculae has made dissection and testing a challenge. In this thesis, whole individual trabeculae from a bovine lumbar spine were tested in three-point bending to determine their Young’s modulus using custom-made equipment to fit a miniature single-axis testing device. The device itself was validated by testing materials with moduli ranging from 1 to 200 GPa. The structure of the cancellous bone and the morphology of the individual struts were determined using micro x-ray computed tomography (µXCT). Individual struts were manually isolated from slices made using a low-speed saw under constant lubrication and measured under a stereomicroscope. Samples exhibiting no machined surfaces (and thus deemed to be whole, or “uncut”) were compared to struts that had been cut by the saw during sectioning. Validation showed that the system was capable of determining the modulus of materials that were approximately five times stiffer than the expected cancellous modulus (copper, at 115 GPa) to within 10% of published values. This gave confidence in the results for bone. The modulus of the “uncut” specimens was found to be 15.28 ± 2.26 GPa, while the “cut” specimens had a significantly lower modulus (p = 1.665 × 10-6) at 2.63 ± 2.65 GPa. The lower modulus for “uncut” specimens may be due to microdamage that occurred during machining and dissection.