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Lubinski, Jacob R.

Mechanical Engineering

2007

MS

Disc degeneration has a high incidence and is associated with low back pain. One of the earliest signs of disc degeneration is a loss of proteoglycans and their associated swelling pressure. The overall purpose of this study is to gain an improved understanding of the role of swelling pressure in determining the water content and biomechanical behaviors of human intervertebral discs using in vitro biomechanical testing. We hypothesized that as salt concentration is increased the swelling pressure will decrease along with fluid content and equilibrium and dynamic stiffness. The first aim of this study was to measure the swelling pressure and transient viscoelastic behaviors of the disc under an axially compressive load in three separate saline bath concentrations (0.015 M, 0.15 My and 1.5 M). The second aim was to determine the cyclic biomechanical behaviors of the intervertebral disc when loaded axially at a frequency of 0.005 Hz and 1 Hz, under the same three saline concentrations. The third aim was to theoretically model the disc using a 1D poroelastic model with osmotic pressure effects. Results indicated that as the saline bath concentration was increased from 0.01 5 M to 1.5 M the swelling pressure and equilibrium stiffness decreased. Under these isometric conditions, there were no changes in water content indicating volume was held constant. There was no significant effect of frequency on dynamic modulus or hysteresis area. The main finding was that the increase in salt concentration resulted in a substantial decrease in total stress and nucleus pressure, with the most significant decrease at 1.5 M NaC1. This decrease was expected to be associated with a reduction in osmotic pressure as increased positive ions diffused into the disc, counteracting the high negative charge density of the proteoglycans. High NaCl concentrations also affected material behaviors with a decrease in dynamic modulus and in hysteresis area. The theoretical model was matched to experimental, time-dependent results and was able to specifically quantify the osmotic, fluid, and solid contributions to the total stress. In conclusion, this study quantified NaC1 concentration effects on biomechanical behaviors under transient and cyclic conditions towards developing an integrated understanding of the relationship between osmotic pressure and biomechanical behaviors.