Variation of Lumbar Spine Stiffness With Load

[+] Author and Article Information
W. T. Edwards, W. C. Hayes, I. Posner, A. A. White

Orthopaedic Biomechanics Laboratory, Department of Orthopaedic Surgery, Charles A. Dana Research Institute, Beth Israel Hospital and Harvard Medical School, Boston, MA

R. W. Mann

Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA

J Biomech Eng 109(1), 35-42 (Feb 01, 1987) (8 pages) doi:10.1115/1.3138639 History: Received December 23, 1985; Revised October 09, 1986; Online June 12, 2009


Mechanical studies of the Functional Spinal Unit (FSU) in-vitro have shown that the slopes of the load-displacement curves increase with load. This nonlinearity implies that the stiffness of the FSU is not constant over the range of physiologic loads, and that measurements obtained for FSU specimens through the application of individual loads cannot be summed to predict the response of the specimens to combined loads. Both experimental and analytical methods were developed in the present study to better quantify the nonlinear FSU load-displacement response and to calculate the coupled stiffness of FSU specimens at combined states of load reflecting in-vivo conditions. Results referenced to the center of the vertebral body indicate that lumbar FSU specimens are stiffer in flexion than in extension, and that FSU specimens loaded in flexion are stiffer at high loads than at low loads. The importance of combined load testing and a nonlinear interpretation of load-displacement data is demonstrated.

Copyright © 1987 by ASME
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