CAREER: Modeling the Intervertebral Disc Using Quantitative MR Imaging

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The soft discs between the bones in the human spine fail from mechanical causes often during life, causing a great deal of pain. How fibrous composites fail is not understood either for engineering materials of this type or, of course, for the more complex material in the human spine. This Faculty Early Career Development Program (CAREER) project will contribute to our understanding of the mechanical behavior of these complex material composites. Fiber-reinforced composites provide materials with greater stiffness due to the fibers and large deformations from the softer extrafibrillar matrix. Fiber composites are important in commercial and military applications, but also to understanding the mechanical properties of human bodies. Many body tissues are fibrillar composites. Understanding the mechanical behavior of the material under its working environment and how load is transferred between neighboring tissues is a significant challenge to understanding multiple diseases. In particular, "slipped disc" of the human spine is a failure of the fibrillar composite structure that we don't understand. Traditional material testing techniques require specimens to be removed from their working position, significantly altering the loading environment and boundary conditions. High-resolution magnetic resonance imaging provides a noninvasive approach for determining the composition of a spinal disc in a live person without injury. This project will support fundamental research on how biological tissues change their mechanical properties as a result of changes in composition. The work will be based on magnetic resonance imaging methods because, later, the results of the project can be used in living people as a noninvasive, safe way to assess changes in mechanical behavior of body tissues. The project will help to advance the field of disc mechanics, and the research results should apply to other soft tissues of the body. Outreach to the community includes a plan to enhance STEM engagement for girls based on the Principal Investigator's ongoing efforts in local Oakland, CA and Berkeley, CA K-12 schools. Involvement of undergraduate and graduate students in the outreach activities substantially amplifies the impact.Noninvasive magnetic resonance imaging in combination with finite element modeling addresses many limitations with current approaches for understanding the mechanical behavior of composite structures, such as the intervertebral disc. The disc includes a soft gel-like material surrounded by a fiber-reinforced material. Hydration and swelling are important for the mechanical function of the disc and changes with degeneration are known to alter disc composition and mechanics. Changes in tissue composition result in altered residual stresses, tissue anisotropy, and failure mechanics. The disc provides an excellent model for understanding changes in stress distribution between two distinct materials. The research team will use quantitative magnetic resonance imaging to directly measure special changes in water and proteoglycan composition, which will be used to develop a subject-specific computational model that will be validated through joint- and tissue-level experiments. The knowledge gained will be important for 1) understanding changes in soft tissue mechanics with injury and degeneration, 2) developing innovative tools for studying soft material mechanics, and 3) understanding the underlying mechanisms that govern load distributions in complex soft materials.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.