A new technique for the in vitro measurement of nucleus pulposus swelling pressure

Swelling of the intervertebral disc nucleus pulposus may be a contributing factor in lower back pain syndromes. We have designed and tested a new osmometer for in vitro determination of nucleus pulposus swelling pressure. The functional principle of the osmometer involves compressing a sample of nucleus pulposus with nitrogen gas until saline pressure gradients across a 0.45-micron Millipore filter are eliminated. Swelling pressures of both pooled dog and pooled pig lumbar disc nucleus pulposus were measured on the new osmometer and were compared with swelling pressure determined using the equilibrium dialysis technique. The osmometer measured swelling pressures comparable to those obtained by the dialysis technique. This osmometer provides a rapid, direct, and accurate measurement of swelling pressure of the nucleus pulposus.