Effect of loading frequency on trans-endplate nutrition across the intervertebral disc: A force-controlled unconfined compression experiment

Degeneration of the intervertebral disc (IVD) is affecting nearly 80% of the population at some point during their life. This degeneration is a cascading event and a primary cause of lower back pain. A healthy IVD primarily depends on diffusion through the cartilage endplates for its nutrition, however, transport is reduced if the vertebral endplates become sclerotic or proteoglycan content in the disc is diminished, causing cell apoptosis. It has been shown previously that loading frequency has a positive effect on the total nutrition transport for a displacement-controlled confined compression model. However these results are difficult to compare with in-vivo experiments. To closely mimic the natural state of IVD, a force-controlled unconfined compression model is implemented in COMSOL Multiphysics using the biphasic swelling theory. A comparative study between a model with heterogeneous and one with homogenous material properties has been presented. Both models were subjected to a square wave loading of 30 kPa where the frequency was varied from 0.1 to 5 Hz for a period of 300 seconds. Next, both models were subjected to loads that generated a 10% strain within the tissue. The heterogeneous model showed lower nutritional transport for both experiments, however, the observed trend was the same for both models. At higher frequencies the disc reported lower transport values.