Effect of Loading Frequency on Intervertebral Disc Cells.

Cells of the intervertebral disc have a limited capacity for matrix repair and may contribute to the onset and progression of degenerative disc changes. Disc degeneration contributes to declining nutrition, loss of viable cells, cell senescence, post-translational modification of matrix proteins, accumulation of degraded matrix molecules, and fatigue failure of the matrix. During daily activities intervertebral discs are exposed to oscillatory hydrostatic loads. It is known that dynamic loads with critical frequencies close to that of the human spine resonant frequency (4-8 Hz) have a destructive effect on disc tissue properties. Whether this destructive effect is purely mechanical due to loading magnification, or biological affecting cell metabolism is unknown.

In understanding this phenomenon, we implemented a mechanical testing system efficient to distribute a range of loading frequencies with specified amplitude of hydrostatic pressure to 3-D alginate cultured nucleus pulposus and transition zone cells of the IVD. The 3-D alginate cultures were divided into six loading scenarios, five of which were subjected to the 1, 3, 5, 8, and 10 Hz loading at 1 MPa. The sixth group, being the control, did not undergo any loading. Each scenario was loaded for 3 days, 30 minutes daily, in a hydraulic chamber filled with culture media. The effect of the loading frequency on the collagen metabolism of each scenario of nucleus pulposus and transition zone was contrasted by calculating the incorporated [³H]-proline into the collagen of the media and cell extracts. The results of the nucleus pulposus exhibited a prominent destructive effect occurring in the proximity of 5 Hz, with respect to the incorporated and released collagen, into and from the IVD cells, respectively. The transition zone exhibited a slight destructive effect around 3 and 8 Hz, but at 5 Hz reported increasing incorporated and released collagen. The findings are significant because 5 Hz is within the resonant frequency of the human spine. These results indicate frequencies close to 5 Hz may be the cause of degeneration in the nucleus tissue, but may in fact be a range that the transition zone requires for maintaining a healthy cellular environment.