Clinical StudyShort applications of very low-magnitude vibrations attenuate expansion of the intervertebral disc during extended bed rest
Introduction
The loss of functional weightbearing encountered during bed rest (BR) or spaceflight initiates catabolic changes throughout the musculoskeletal system. These periods of reduced loading lead to increased incidence of low back pain (LBP) as associated with abnormal spinal lengthening [1], atrophy of the spinal musculature [2], increased intervertebral disc (IVD) height and area [1], [4], and altered IVD composition [3]. Even restoration of full function may not remedy LBP. More than 50% of astronauts complain of LBP during space missions as well on returning to earth [1]. On earth, LBP is an epidemic with more than 80% of the adult US population experiencing a severe episode during their lifetime [5]. As remedies are mostly palliative once a person experiences LBP on earth or in space from a morphologically altered disc, interventions that can prevent or attenuate long-term morphological IVD changes are critical.
The role of weightbearing in the retention of IVD form and function is not entirely clear. Because of its lack of vascular supply, loading, and load-induced fluid flow are certainly critical to the exchange of nutrients and waste. The extent by which the IVD will alter its morphology on removal of ground reaction forces and muscle forces is evident even over a 24-h diurnal cycle during which the spine can elongate 2 cm [6]. Spaceflight exacerbates crew member height change by as much as 6 cm [1]. Although factors unrelated to the IVD also are certain to play a role in LBP, IVD degeneration and expansion are considered primary etiologic factors in this debilitating disorder [3], [4]. Indeed, IVD expansion alone can exacerbate LBP through processes, such as increased strain of neighboring facet joint capsules [7], fracture of innervated vertebral end plates, or stimulation of nociceptors in the posterior annulus fibrosus arising from radial fissures or protrusions.
Currently, no prophylaxes or treatments for IVD pathology exacerbated by extended periods of nonweightbearing are available. Walking exercises with and without loaded backpacks showed that the application of even large mechanical loads during phases of unloading do not return IVD volume to normal levels [8], [9], [10]. In a previous study, daily running routines on a lower body negative pressure treadmill partially negated altered disc morphology during spine deconditioning over a 28d unloading period but the daily 45 minutes protocol failed to reduce expansion of the lower lumbar IVDs [11].
Because fluid motion and displacement are related to the frequency of the applied mechanical signal [12], [13], it is possible that increasing the loading frequency may increase the ability of the countermeasure to exude fluid from the disc and therefore attenuate IVD expansion [14]. Higher-frequency mechanical signals can be transmitted into the musculoskeleton via whole body vibrations. Although vibrations may improve neuromuscular function [15], reinforce lumbar proprioception [16], and potentially pose a treatment for LBP [17], they need to be approached with caution as they can cause the back pain they are intended to prevent [18]. Safety concerns for the musculoskeletal system arise when whole body vibrations exceed accelerations of 1 g [19], while the International Organization for Standardization has identified no evidence of acute or chronic complications of 20–90 Hz vibrations when exposure falls below 0.56 g (often termed low-magnitude vibrations). In the current study, controls (CTRs) subjected to 90 days of restricted BR were compared with those confined to BR but also subjected to short bouts (10 min/d) of high-frequency (30 Hz), low-magnitude (0.3–0.5 g) vibrations to evaluate the potential of these signals to slow IVD expansion during nonweightbearing and a subsequent 7d recovery period.
Section snippets
Experimental design
This study was reviewed and approved by the Committee on Research in Human Subjects of Stony Brook University, the University of Texas Medical Branch at Galveston, and NASA's Johnson Space Center. Healthy human subjects of astronaut age (35±7 y, 18 males, 11 females) and without a history of back pain were randomly assigned to a CTR (n=11) or a low-magnitude mechanical signal (LMMS) intervention (n=18) group. All subjects underwent continuous 6° head-down BR for 90d followed by a 7d reambulation
Results
At baseline, there was no difference in age, body mass, or height between CTR (35±8 y, 74±1 kg, 171±1 cm) and LMMS (35±7 y, 75±9 kg, 172±7 cm) subjects. Coefficients of variation for the individual measurements were 0.6% for spinal length, 1.8% for IVD volume, 2.4% for NP volume, 5.0% for IVD convexity, and 1.5% for muscle volume. None of the subjects experienced any complications such as deep vein thrombosis or sequelae. Because there were no significant differences in any variable between the 0.3 g
Discussion
Deconditioning because of nonweightbearing typically results in elongation of the spine and a volumetric increase in the size of the IVD [1], [4], outcomes that can directly promote LBP [1]. Here, we show that short daily exposure to low-magnitude vibratory stimuli, given to volunteers subject to 90d of restricted BR will attenuate deleterious changes in IVD morphology, particularly in the most caudal IVDs, and serve to retain the convexity of the disc, a key indication of disc health. The
Acknowledgments
We would like to thank Dr Janice Meck, Sherlene Dryer, Robert Pietrzyk, and the entire NASA JSC and UTMB bed rest team for their invaluable contributions. Technical assistance with the MRI and CT measurements by Dr. Mark Wagshul, Dr. Harlan Evans, and Dennis Dornfest was greatly appreciated. We are also grateful for the financial support from NSBRI (BL00802) and NASA (NNJ04HI06G), and the NASA Flight Analogs/Bed Rest Research Project.
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Cited by (0)
Institutional Review Board Approval: This study was reviewed and approved by the Committee on Research in Human Subjects of Stony Brook University, the University of Texas Medical Branch at Galveston, and NASA's Johnson Space Center.
FDA device/drug status: not approved; assessed as no significant risk.
Author disclosures: CR (financial relationship, Stockholder and consultant for Juvent Inc.).