Stiffening of the nucleus pulposus upon axial loading of the intervertebral disc: An experimental in situ study

Mechanical loading is inherently related to the function and degeneration of the intervertebral disc. We present a series of experiments aimed at measuring the effect of a loading/unloading cycle of the intervertebral disc on the mechanical properties of the nucleus pulposus. The study relies on our new minimally invasive microindenter, which allows us to quantify the storage and loss moduli of the nucleus pulposus by inserting an optomechanical probe in an intact (resected) intervertebral disk through the annulus fibrosis via a small needle. Our results indicate that, under the influence of compressive loading, the nucleus pulposus exhibits a more solid-like behavior.