Different effects of static versus cyclic compressive loading on rat intervertebral disc height and water loss in vitro

STUDY DESIGN. In vitro biomechanical study on rat caudal motion segments to evaluate association between compressive loading and water content under static and cyclic conditions. OBJECTIVE. To test hypotheses: 1) there is no difference in height loss and fluid (volume) loss of discs loaded in compression under cyclic (0.15-1.0 MPa) and static conditions with the same root-mean-square (RMS) magnitudes (0.575 MPa); and 2) after initial disc bulge, tissue water loss is directly proportional to height loss under static loading. SUMMARY OF BACKGROUND DATA. Disc degeneration affects water content, elastic and viscoelastic behaviors. There is limited understanding of the association between transient water loss and viscoelastic creep in a controlled in vitro environment where inferences may be made regarding mechanisms of viscoelasticity. METHODS. A total of 126 caudal motion segments from 21 Wistar rats were tested in compression using 1 of 6 protocols: Static loading at 1.0 MPa for 9, 90, and 900 minutes, Cyclic loading at 0.15 to 1.0 MPa/1 Hz for 90 minutes, Mid-Static loading at 0.575 MPa for 90 minutes, and control. Water content was then measured in anulus and nucleus regions. RESULTS. Percent water loss was significantly greater in nucleus than anulus regions, suggesting some water redistribution, with average values under 1 MPa static loading of 23.0% and 14.9% after 90 minutes and 26.9% and 17.6% after 900 minutes, respectively. Cyclic loading resulted in significantly greater height loss (0.506 ± 0.108 mm) than static loading with the same RMS value (0.402 ± 0.096 mm), but not significantly less than static loading at peak value (0.539 ± 0.122 mm). Significant and strong correlations were found between percent water loss and disc height loss, suggesting water was lost through volume decrease. CONCLUSION. Peak magnitude of cyclic compression and not RMS value was most important in determining height change and water loss, likely due to differences between disc creep and recovery rates. Water redistribution from nucleus to anulus occurred under loading consistent with an initial elastic compression (and associated disc bulge) followed by a reduction in disc volume over time.