Frequency dependent effects of chronic cyclic loading on the lumbar spine in-vivo

The purpose of this study was to characterize the effects of cyclic mechanical loading at different frequencies on the intervertebral disc and vertebral endplates. Static, 0.5 Hz or 5 Hz loading at 5 × BW was applied to the rabbit lumbar spine in vivo 2 hours/day, 5 days/week for up to 30 weeks. Following the loading period, MRI, μCT and histology analyses were performed. Results indicate an increase in subchondral bone density in cyclically loaded animals which was loading rate-dependent. There was also a decrease in trans-endplate diffusion at the loaded levels in animals exposed to high frequency loading. The trend of increased bone density at the loaded level was not seen in statically loaded animals. The extent of disc degeneration was also dependent on the frequency of loading, with decreases in disc height, fibrosis and matrix disorganization occurring at high frequency and static loading. The differences observed between loading frequencies may be related to the rate dependant, viscoelastic mechanical properties of the intervertebral disc.