The effect of hydration on the stiffness of intervertebral discs in an ovine model

Objective. To determine the hydration-over-time behaviour of ovine intervertebral discs and intact joints in a saline bath at body temperature and the effect this has on their stiffness compared to air at ambient temperature. Design. The hydration-over-time behaviour and stiffness of the ovine functional spinal unit and disc were quantified. Background. The fluid content of an intervertebral disc is not constant but varies with external load and load history. The stiffness of ovine functional spinal units in a hydrated environment and how this compares to testing in air have not been quantified. Methods. Intervertebral discs and functional spinal units were weighed and soaked in a saline water bath at 37 °C and reweighed each hour for 6 h. They were then allowed to stand in air at room temperature while the time to return to initial weight was recorded. Functional spinal units were randomly assigned to two groups. Axial compression, flexion, extension, lateral bending and axial torsion tests were performed on both the intact functional spinal unit and isolated disc. Group 1 was tested in air then in a saline water bath at 37 °C with the testing order reversed for Group 2. Results. Hydration of the disc reached a plateau after an average 3–4 h of soaking with the largest increase seen in the first hour. Four hours, standing in air at room temperature, was required to return specimens to their initial weight. The functional spinal unit stiffness was significantly lower for those specimens tested in the bath compared to air. Conclusions. Ovine intervertebral discs show similar hydration-over-time behaviour when compared to human discs. Stiffnesses in different modes of loading were significantly different when tested in a hydrated environment compared with the standard method of testing in air.