Different effects of static versus cyclic compressive loading on rat intervertebral disc height and water loss in vitro.

The intervertebral disc plays the essential biomechanical roles of supporting load and permitting motion in the spine.1 The disc is a heterogeneous structure composed of a central nucleus pulposus surrounded by a highly organized fiber-reinforced anulus fibrosus. The nucleus pulposus is a hydrated gelatinous tissue composed of negatively charged glycosaminoglycans, collagens, and noncollagenous proteins.2,3 The glycosaminoglycan-associated negative charges in the nucleus pulposus produce a swelling force.4,5 Degeneration of the disc results in a loss of both water and proteoglycan contents2,3 as well as alterations in its elastic and viscoelastic behaviors.1,6 The hydration of the disc varies according to the condition of loading; fluid is expelled when the disc is loaded over its baseline swelling pressure and absorbed when it is unloaded.7–10 Several in vivo magnetic resonance imaging studies have indicated load-induced changes in the water content of the disc.11–14 Loading conditions on the motion segment affect the degree of hydration of the intervertebral disc and, in particular of the nucleus pulposus, under loading, and alterations in hydration subsequently also affect the elastic and viscoelastic mechanical behaviors of the intervertebral disc.1,7,15–17 There is a limited understanding of the time-dependent association between water loss and viscoelastic creep in the disc, and this requires additional experiments in a controlled in vitro environment that permits inferences regarding volume loss and mechanisms of viscoelasticity. There are very few studies on effects of cyclic loading on fundamental motion segment creep and hydration and improved understanding of the motion segment behaviors under cyclic or repetitive loading conditions is a priority for defining safe loading regimens. A clear understanding of the unique interaction between the solid porous matrix and the fluid can only be obtained using appropriate analytical models along with in vitro and in vivo experimental methods.18 The purpose of this study is to investigate the effects of variations in the load duration, load magnitude, and loading mode (i.e., cyclic or static) on rat caudal motion segment height and water loss in the nucleus and anulus. These data have implications for mechanobiology studies, and are important for improved understanding of mechanisms of viscoelasticity. It was hypothesized that: There is no difference in height loss and fluid (volume) loss of motion segments loaded in compression under cyclic (0.15–1.0 MPa) and static conditions with the same time average magnitudes (0.57 MPa). After initial disc bulge, tissue water loss is directly proportional to height loss under static loading.