Multiaxial fatigue characterization of anisotropic bone-implant interfaces

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Metallic implants, used in various types of orthopaedic surgery, are critical to reducing pain and restoring mobility for millions of individuals worldwide. The success of these procedures depends on the mechanical stability of the heterogeneous interface established between the bone tissue and implant, forming the bone-implant system. In spine surgery, for example, pedicle screw instrumentation systems establish rigid stabilization of the spinal column until long-term bony fusion is achieved. However, in the period prior to fusion, these technologies are susceptible to fatigue failure resulting from non-uniform load variations during activities of daily living. This can lead to screw loosening and loss of spinal column stabilization; a significant cause of failure in fusion procedures at a high clinical and societal cost. Yet, the biomechanics of this interface failure mode in the spine are not well understood or properly accounted for in the design and pre-clinical evaluation of implant devices. The proposed research program aims to improve the structural and functional performance of bone-implant systems by advancing our fundamental understanding of the biomechanical factors and implant design considerations that influence the bone-implant interface strength. Through the next five years my team will systematically quantify differences in how, when, and where pedicle screw loosening occurs in comparison of simulated uniaxial and multiaxial cyclic loading modes. The how will identify relevant interactions between loading modes and resulting interface damage using a Design of Experiments approach. When screw loosening occurs will use machine vision and intelligent sensors to measure relative interface movement. Where screw loosening occurs will examine cumulative bone damage using state-of-the-art microscale computed tomography imaging. We will also develop a new, adaptive loading method to improve anisotropic characterization of multiaxial fatigue strength in pedicle screw fixation. This approach will better evaluate interface fatigue strength in the context of bone microstructure variability and build towards new testing standards of the pedicle screw-bone interface. Leveraging outcomes from this research, my team will evaluate design improvements in a 3D printed pedicle screw expansion mechanism based on assessment of multiaxial fatigue strength. Integrating computational topology optimization of the implant design combined with new test methods developed for multiaxial fatigue will establish a new pre-clinical pipeline to evaluate and enhance bone-implant system performance. Ultimately, this research program will improve our understanding of bone-implant interface mechanics and lead to new innovations in spine surgery technologies to help avoid costly revision procedures. This will further strengthen Canada's leadership in producing highly-skilled engineers ready to address global challenges in the medical device industry and beyond.