Experimental factors governing the internal stress state of the intervertebral disc

This paper presents an experimental study of the response of the interverte bral disc to short-term static-axial loading within the conceptual framewor k of mechanical consolidation as applied to swelling materials. An experime ntal technique developed previously for articular cartilage was used to loa d the disc in compression and measure simultaneously the matrix internal ex cess pore pressure under both radially constrained and radially unconstrain ed conditions. Specifically the short-term pattern of development of the hy drostatic excess pore pressure was investigated. The average ratio of the m aximum excess pore pressure u(1) to the nominal applied stress for unconstr ained loading was approximately 1.9 while for constrained loading it was ab out 1.4. This behaviour is interpreted in terms of the generation of a pore pressure in the nucleus which reflects a non-uniform partitioning of the a pplied load across the inhomogeneous structure of the disc. These results p rovide a quantitative insight into the role that physico-chemically generat ed swelling plays in determining the internal mechanical response of the di sc to an externally applied load and suggest a quantitative means of invest igating the important influence of degenerative changes on the internal str ess state of the disc without needing to pierce the annular wall in order t o insert a pressure sensing device. (C) 1998 IPEM. Published by Elsevier Sc ience Ltd. All rights reserved.