Disc Nucleus Cellular Response to Dynamic Pressures at Critical Frequencies: A Pig Model

Disc degeneration is a multifactor phenomenon. It has been found that intervertebral disc (IVD) cells respond to such factors as pH, osmotic pressure, genetic factors, and mechanical loading (Guilak, 1999). During daily activities the human intervertebral disc is exposed to oscillatory hydrostatic loads that produce pressures >2MPa in vivo (Nachemson, 1964 and 1979). It is known that dynamic loads with critical frequencies close to that of the in vivo human spine resonant frequency (4–5 Hz) have a destructive effect on disc tissue properties (Pope, 1993). Whether this destructive effect is purely mechanical, due to loading magnification, or biological, affecting cell metabolism, is unknown. Previous work (Merryman, 2002) showed that there was no significant effect upon monolayer IVD cells loaded at 15Hz, while lower frequencies (1 and 8Hz) altered collagen synthesis compared to control. To address this issue, we developed a mechanically active culture system capable of delivering a wide range of loading frequencies and amplitudes of hydrostatic pressure to cultures of disc cells. Nucleus pulposus cells of pig discs were isolated and suspended in alginate beads. Alginate cultures were divided into 6 groups; five groups were exposed to cyclic pressures of frequencies 1, 3, 5, 8, and 10Hz with the same amplitude of 1MPa, and group 6 was the control group (no loading). Cultures of different groups were loaded for 3 days (30 minutes daily) in a hydraulic chamber filled with culture media. The effect of loading frequency on collagen metabolism among different groups was compared by measuring incorporated [₃H]-proline into collagen for medium and total extracts. The results indicated a poor synthesis rate and more degradation near the 5Hz frequency.