Biologic response of the intervertebral disc to static and dynamic compression in vitro

Study Design. This study attempts to determine the biologic response of the intervertebral disc to static and dynamic compression in vitro. Objective. To quantitate and compare the changes of histology, Type I and II collagens, aggrecan, interleukin (IL)-1 beta, and tumor necrosis factor (TNF)-beta expression, and apoptosis in rabbit intervertebral discs following static and dynamic compression in vitro. Summary of Background Data. Compressive stimuli associated with weight-bearing and loading of the intervertebral disc are thought to be important regulators of disc cell metabolism. Very little is known about the different types of mechanical stimuli that may achieve regulation of intervertebral disc cell metabolism. We examined the biologic response of the rabbit intervertebral discs when exposed to static and dynamic compression in vitro. Methods. The rabbit intervertebral disc explants were in vitro subjected to unconfined uniaxial compression. Static compression of 0.5 and 1 MPa and dynamic compression of 0.5 and 1 MPa were applied at the frequency of 0.1 and 1 Hz for 6 hours, respectively. After loaded with different types of mechanical stimuli, disc explants were examined for histologic changes, collagen I, collagen II, aggrecan, IL-1 beta and TNF-beta expression, and apoptosis. Results. The static compressive load was found to suppress gene expression for collagens and aggrecan in the disc, whereas the disc under dynamic compression exhibited significant anabolic change with increase in gene expression for Type I and II collagen and aggrecan. The regional difference of the responses to mechanical loading in vitro was found between the anulus fibrosus and nucleus pulposus. All loading conditions caused marked histologic changes, up-regulation of IL-1 beta and TNF-alpha expression, and increase in TUNEL-positive cells in the intervertebral discs, with the most significant from control was that when statically loaded. Conclusion. Mechanical loading is involved in the physiology and pathology of disc degeneration. Static and dynamic compression may induce different biologic response of the intervertebral disc: static compression has catabolic role on the disc, whereas the dynamic load at appropriate level may benefit the synthetic activity and anabolic response of the disc.