The effect of cyclic compression on the mechanical properties of the inter-vertebral disc: An in vivo study in a rat tail model

Objective. To assess the changes in the mechanical properties of inter-vertebral discs in vivo following static and cyclic compressive loading of different frequencies. Design. An in vivo biomechanical study using a rat-tail model of the inter-vertebral disc. Background. Mechanical loading has been suggested as playing a major role in the etiology of disc degeneration, but the relationship is still not fully understood. Methods. Sixty Sprague-Dawley rats were subject to daily compressive stress via pins inserted in the 6th and 7th caudal vertebrae over a two-week loading period. Animals were randomly divided into a sham group (pin insertion, no loading), a static loading group, or cyclic loading groups of 0.5, 1.5, or 2.5 Hz. Loading was applied for 1 h each day from the 3rd to 17th day following pin insertion, and the angular compliance, angular laxity, and inter-pin distance were measured in vivo at days 0, 3, 10 and 17. Results. Changes in the inter-vertebral disc height depended on the frequency of loading, with the decrease in disc height in the static compression group significantly greater than that in all other groups, whereas the decrease in the 1.5 Hz cyclic compression group was significantly smaller than that in all other compression groups. Conclusions. Changes in disc properties depend on both the total load exposure and the frequency of loading. Cyclic loading in general produced less marked changes than static loading, but loading at particular frequencies may result in more severe changes. Relevance. Previous studies have shown the in vivo changes in the mechanical properties of inter-vertebral discs to depend on the magnitude and duration of loading. In this study, a frequency dependent response to cyclic loading is also demonstrated.