Abstract
Introduction
Sustained loading on the intervertebral disc leads to loss of disc height. The generally accepted explanation for this is that the disc loses height due to an unbalance between the external load on the disc and the osmotic pressure in the disc. Consequently, water is expelled from the disc until the osmotic attraction reaches an equilibrium with the pressure applied. In this study, we compared the time course of loss of disc height with loss of pressure in the nucleus. We expected to see a similar time course of disc height and intra-discal pressure.
Methods
Fifteen caprine lumbar discs were tested in a saline bath. Of each motion segment both vertebral bodies were cut-off close to the endplate. After a preload of 6 hours at 10N, an axial compressive load of 150N was applied to the discs for 18 hours by an Instron testing device. An 1.33mm pressure needle was inserted in the nucleus to measure hydrostatic pressure. Both change of disc height and change of nucleus pressure were measured at 2 samples/s. A double Kelvin–Voigt model was fitted to estimate the time constants of both hydrostatic pressure and disc height loss. The model comprises two time constants: the first modelling a fast change, the second a slow change. A paired t-test was used to compare the time constants of both the pressure and the disc height.
Results
The slow time constant of the change of disc height (3.4 hours, sd=0.8) was larger (p=0.0006) than the slow time constant of the change of nucleus pressure (2.4 hours, sd=0.8). The difference between the time constants of the fast regime (0.33 hours resp 0.27 hours) was not significant (p=0.27).
Discussion
In contrast to our expectations, we found a difference between the slow time-dependent behavior of the change of disc height and nucleus pressure. The discs reached an equilibrium between internal and external pressure well before the change of disc height came to a stop. This indicates that the change of disc height depends on more variables than mechanical equilibrium alone. Likely, viscoelastic properties of the annulus fibrosis have an important role as well. If confirmed, regenerative therapies should not only focus on restoring nucleus pulposus pressure to regain disc height, but also consider annulus properties.
