Disc hydration measured by magnetic resonance imaging in relation to its compressive stiffness in rat models

The in vivo effects of body dehydration on the water content and compressive stiffness of rat intervertebral disc (IVD) and vertebral body were investigated. Thirty-one rats were divided into three dietary groups: water restricted ( WR), pair fed (PF) and ad libitum (AL). The in vivo water content of L4-5 IVD was measured using magnetic resonance imaging (MRI). Water contents of L4-5 IVD were also determined using the wet/dry weight method after killing. Compressive stiffness of the L5 vertebral body and the L3-4 motion segment were determined. Results showed that the water content of L4-5 IVD was significantly less in the WR group than in the PF and AL groups as measured by the wet/dry weight method; likewise, MRI water content was also less in the WR group. The material stiffness of the IVD was significantly greater in the WR than in the PF and AL groups. However, there was no significant difference in material stiffness of the L5 vertebral body for the WR group in comparison with the PF and AL groups. It was also found that the stiffness of the IVD was significantly lower than that of the vertebral body. This study suggests that the dehydrated matrix of the IVD may be responsible for a stiffer disc that may lead to a less flexible spine.