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Mechanical Engineering
BYU Applied Biomechanics Engineering Laboratory

The Balanced Spine


When removed from the body, the lumbar spine buckles at relatively small compressive loads (~90 N). These loads are substantially lower than those measured in vivo, even lower than the weight of the average adult torso. Patwardhan et al., showed that the inclusion of a compressive "follower load" that passes through the instantaneous center of each spinal segment substantially increases the load bearing capacity of the isolated spine. The physiological rationale behind the follower load is that the muscle architecture follows this path.

Our hypothesis is that the location of the aggregate abdominal center of mass also plays a critical role in the location and function of this follower load. Essentially, that the spine is passively controlled through a quasi-staticly balanced system that allows maintenance of erect posture with a minimum of muscle expenditure.

BalancedSpine.jpg
BalancedSpine.jpg

The figure above shows that the anterior-posterior location of the center of mass closely tracks with the vertebral centroid (an approximation of the instantaneous axis of rotation in flexion-extension). Thus, the natural curvature of the spine is optimized to minimize the applied moments induced by the paraspinal musculature.

This hypothesis has broad implications for the placement of spinal fusion systems, as well as for the design of motion-preserving and motion-restoring spinal implants. Failure of a surgical treatment to accurately reproduce the correct instaneous axes of rotation disturbs the passive control system, forcing the muscles to work harder to maintain erect posture. It also induces changes in the working lengths and loads experienced by the surrounding spinal tissues.

Related Publications


Jeffs, S., Bowden, A.E. (2013) “A New Paradigm on the Passive Stability of the Human Lumbar Spine,” Transactions of the Orthopaedic Research Society, Vol 38, Abstract 0815.

Shaun Jeffs, MS (2011), “Passive load-bearing capacity of the human lumbar spine in the neutral standing postures.” Brigham Young University MS Thesis.

Halverson, P., A.E. Bowden, E. Stratton, D. Pearsall, and L.L. Howell (2009). The balanced spine: a passive mechanism for maintaining erect posture. Proc 55th Annual Meeting of the Orthopaedic Research Society.