Effect of load frequency on trans-endplate transport of nutrients to the intervertebral disc

Abstract

There exists a strong correlation between disc degeneration and low back pain. The augmentation of nutrient transport through non-invasive treatment protocols has clinical significance. Results of this study show that it is possible to improve nutrient transport to the disc at every stage of degeneration thereby mitigating, stopping or potentially reversing disc degeneration through the application of dynamic, mechanical stimuli with appropriate frequencies. The findings of the study lay the foundation to develop a patient-specific model, capable of prescribing custom treatment regimens to patients

Lower back pain is affecting nearly 80% of the population at some point during their life and is the second most common reason for doctor visits in the United States of America, resulting in diagnosis costs of nearly 50 billion dollars a year. Lower back pain is closely related to the degeneration of the intervertebral disc (IVD). The intervertebral disc is the body's largest avascular tissue structure providing stability and flexibility to the spine. Sclerosis and loss of proteoglycan content are hallmarks of a degenerated disc. Both effects can reduce nutrient transport to the avascular disc and result in cell apoptosis and matrix degradation, further exacerbating disc degeneration.

We hypothesize that the cyclic loading can augment the nutrient transport to the disc, thereby mitigating or reversing disc degeneration. This hypothesis is computationally investigated by evaluating the tissue's mechanical response to displacement controlled confined compression and force controlled unconfined compression testing protocols. An augmented biphasic swelling model is developed to investigate the effects of dynamic mechanical loading on small molecule nutrient transport in the intervertebral disc. A sensitivity analysis is conducted to better understand the impact of disc degeneration on nutrient transport. Finally, an optimization protocol aims to augment the total nutrient transport in the degenerated disc was implemented.

The model shows that the total nutrient transport to the disc can be augmented by a dynamic load. For healthy and degenerated discs, the maximum total nutrient transport is observed when the loading frequency of the applied mechanical load is approximately 0.5 Hz. As the frequency of loading increased past 0.5 Hz, tissue stiffening limits the total nutrient transport to the disc.

Under identical loading conditions, the total nutrient transport to the disc decreases as the disc degenerates. Factors such as tissue porosity, proteoglycan content, and tissue permeability play an important role for the nutrient transport. By altering the magnitude of the applied mechanical load, we successfully optimized the total nutrient transport to the degenerated disc without overstressing the tissue.