Change of intradisc pressure versus volume change

To gain further insight into the biomechanics of the intervertebral disc and determine a potential mechanism for causation and relief of symptoms related to a herniated disc, the pressure-volume relationship was determined within the nucleus pulposus (NP). In 17 intact human cadaver lumbar discs, pressure was measured continuously within the NP by means of a miniature strain gauge at the tip of a size 4 French (1.3 mm) catheter inserted into the NP. The volume of the NP was increased at the slow, continuous rate of 0.034 ml/min by the pump-regulated infusion of saline colored with methylene blue. In 12 unloaded discs, NP pressure rose in a linear fashion (linear r2 = 0.96) from an initial mean pressure of 174 +/- 81 kPa. The mean rate of pressure rise was 327 +/- 109 kPa/ml of volume increase. The peak pressure reached was limited to 550 kPa by the capacity of the strain gauge. Similar linear relationships were obtained during saline infusion into 5 vertically loaded discs. The data define the pressure-volume relationship within the disc and show that the NP, surrounded by the relatively inelastic annulus and the solid vertebral end-plates, has the properties of a tight hydraulic space in which a large pressure rise will regularly result from a small increase in volume. Presumably, the opposite is also true. The data may provide a biomechanical basis for variation in symptoms related to physiological changes in disc volume, and for any benefit obtained from interventions designed to remove disc tissue.