Low-Level Vibrations Maintain the Intervertebral Disc During Unloading

Changes in intervertebral disc (IVD) biochemistry, morphology and mechanics have been characterized only incompletely in the rat hindlimb unloading (HU) model. Although exposure to chronic vibrations can be damaging, low-magnitude vibrations can attenuate the geometric changes of the IVD due to altered spinal loading. Here, we tested the hypothesis that lowmagnitude, high-frequency vibrations will mitigate the hypotrophy, biochemical degradation and deconditioning of the IVD during HU. When applied as whole-body vibrations through all four paws, Sprague-Dawley rats were subjected to HU and exposed to daily periods (15min/d) of either ambulatory activities (HU+AMB) or whole body vibrations superimposed upon ambulation (HU+WBV; WBV at 45Hz, 0.3g). After 4wks and, compared to age-matched control rats (AC), the lumbar IVD of HU+AMB had a 22% smaller glycosaminoglycans/collagen ratio, 12% smaller posterior IVD height, and 13% smaller cross-sectional area. Compared to HU+AMB rats, the addition of low-level vibratory loading did not significantly alter IVD biochemistry, posterior height, area, or volume, but directionally altered IVD geometry. When subjected to upright vibrations through the hindpaws, rats were HU for 4wks. A subset of HU rats stood in an upright posture on a vertically oscillating plate (0.2g) at 45- or 90-Hz (HU+45 or HU+90). After 4wks, regardless of sham (HU+SC) loading (HU±SC) and, compared to AC, IVD of HU±SC had 10% less height, 39% smaller nucleus pulposus area, less glycosaminoglycans in the nucleus pulposus (21%), anterior annulus fibrosus (16%) and posterior annulus fibrosus (19%), 76% less tension-compression neutral zone (NZ) modulus, 26% greater compressive modulus, 25% greater initial elastic damping modulus, 26% less torsional NZ stiffness, no difference in collagen content and a weaker relationship between tension-compression NZ modulus and posterior height change. Exogenously introduced oscillations maintained the morphology, glycosaminoglycan content and axial elastic properties of IVD. Compared to HU±SC, the IVD of HU+90 had 8% larger average height, 35% greater nucleus pulposus area, more glycosaminoglycans in the nucleus pulposus (24%), anterior annulus fibrosus (17%) and posterior annulus fibrosus (19%), 339% greater tension-compression NZ modulus, 18% smaller compressive modulus, and maintained the relationship between tension-compression NZ modulus and posterior height change, but no difference in torsional NZ stiffness or initial elastic damping modulus. In summary, very brief, small mechanical signals partially protected the IVD during hindlimb unloading.

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Year

Entry

1999

Buschmann MD, Kim Y-J, Wong M, Frank E, Hunziker EB, Grodzinsky AJ. Stimulation of aggrecan synthesis in cartilage explants by cyclic loading is localized to regions of high interstitial fluid flow1. Arch Biochem Biophys. June 1, 1999;366(1):1-7.

2006

Palmer AW, Guldberg RE, Levenston ME. Analysis of cartilage matrix fixed charge density and three-dimensional morphology via contrast-enhanced microcomputed tomography. Proc Natl Acad Sci USA. December 19, 2006;103(51):19255-19260.

1995

Ohshima H, Urban JPG, Bergel DH. Effect of static load on matrix synthesis rates in the intervertebral disc measured in vitro by a new perfusion technique. J Orthop Res. January 1995;13(1):22-29.

2001

Pfirrmann CWA, Metzdorf A, Zanetti M, Hodler J, Boos N. Magnetic resonance classification of lumbar intervertebral disc degeneration. Spine. September 1, 2001;26(17):1873-1878.

1999

Sato K, Kikuchi S, Yonezawa T. In vivo intradiscal pressure measurement in healthy individuals and in patients with ongoing back problems. Spine. December 1, 1999;24(23):2468-2474.