Chronic back pain is a significant clinical problem, affectingapproximately 30% of all adults at some point during their lives and resulting in an estimated $90 billion in medical expenditures and lost wages in 1998 in the United States. Current treatments, such as muscle stimulation, fusions and total disc replacements (TDR), are mechanically-based and may be good at offering patients immediate pain relief. The problem is that pain relief is often not sustained, as the body adapts to the altered mechanical state caused by such a procedure. Assuming that the human body has evolved such that the healthy spine is an optimal mechanical design, new treatments should focus on preserving or restoring healthy spinal dynamics. The design of such devices, of which TDRs are one type, is a growing field in orthopaedics, but innovation is largely driven by trial-and-error.

To elaborate further, although TDRs are subject to elaborate clinical testing and, in the case of two particular designs, FDA approval, their effect on the overall dynamics of the spine is not fully known. Forinstance, existing designs mimic similar devices for the knee joint, and, in some respects, can be considered as poor replacements for the intervertebral disc. Further, the intervertebral disc is a viscoelastic body whose material properties show distinct temporal variations while TDRs have inert mechanical properties. To date, the field of spinal dynamics has been largely empirical; and although there is an abundance of experimental data from cadaveric testing, there is a lack of a governing theory for the evaluation of existing treatments. A major goal of the proposed research is to provide this theory. In so doing, we believe we can further the development of novel effective treatments for back pain.

The goal of our research is to develop a framework for analyzing the dynamics of the spine. To achieve this goal, we will apply concepts from mechanics which include bifurcation and pseudospectral analyses, continuum mechanics, and multibody system dynamics. A series of nonlinear models will be created from, and validated against, experimental data from in vivo (live subject) and in vitro (cadaver) studies of spinal motion in normal, pathological, and surgically-altered states. The data will be taken from numerous studies in the literature, the extensive database of experimental data at the Orthopaedic Biomechanics Laboratory at UC San Francisco, and ongoing work under the direction of Professors Jeffrey Lotz and Oliver O'Reilly.

Of central importance to the research is the resolution of the following questions:
A. How sensitive are the dynamics of the human spine to disc degeneration, fusion and TDRs?
B. How does the degree of accuracy used to experimentally measure the kinematics of the human spine correlate to characterizations of its dynamics?
C. How does the conditioning of muscle groups influence the functioning of the spine?
D. Is it possible to use dynamical systems theory to advance treatment of back pain and establish improved design guidelines for surgical systems such as TDRs?

As part of the proposed work, we will not only develop theoretical concepts that will advance the treatment of back pain; we will also actively engage numerous undergraduate, graduate, and medical students in learning spinal biomechanical concepts as well as foster interdisciplinary collaborations between faculty members in mechanical engineering, bioengineering, and medicine. Materials developed as part of this project will be integrated into graduate and undergraduate courses on rigid body dynamics and orthopaedic biomechanics at UC Berkeley as well as a lecture series for orthopaedic surgery residents and medical students at UC San Francisco. Two Ph.D. students and approximately six undergraduates will execute the proposed research under the direction of an extensive mentoring network in both of the PIs research groups. The individuals in these groups feature postdoctoral scholars and orthopaedic surgeons. There will be intensive collaboration between these students and researchers in the UC San Francisco Orthopaedic Bioengineering Laboratory who conduct complementary experimental studies of human spinal motion. Every effort will be made to attract and recruit students from underrepresented groups, and students will be given ample opportunity to present their research in departmental seminars, national and international conferences, and peer-reviewed publications.