Rheology of Intervertebral Disc

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In Brief Study Design. An ex vivo biomechanical study on the rheological properties of healthy porcine and degenerated human intervertebral disc. Objective. To quantify the effect of loading history, loading magnitude, fatigue loading, and degeneration on disc rheology. Summary of Background Data. Disc rheological parameters, i.e., the aggregate modulus (HA) and hydraulic permeability (k) regulate the mechanical and biologic function of disc. The knowledge of effects of loading condition and degeneration on disc rheology can be beneficial for the design of new disc/nucleus implants or therapy. Methods. The following 4 phases of experiments were conducted to find the changes of disc rheological properties: (1) Effect of loading history during 1-hour creep (640 N) and relaxation (20% strain) test. (2) Effect of loading magnitude (420 N vs. 640 N) during the creep test. (3) Effect of fatigue loading (420 N, 5 Hz for 0.5, 1, and 2 hours) on the creep loading behavior. (4) Difference of healthy porcine and degenerated human discs during creep loading. The experimental data were fitted with linear biphasic model. Results. The aggregate modulus increased but hydraulic permeability decreased during creep loading. The aggregate modulus decreased but the hydraulic permeability did not change significantly during relaxation loading. The higher creep loading increased the aggregate modulus but decreased the hydraulic permeability. The fatigue loading did not change the aggregate modulus significantly, but decreased hydraulic permeability. Comparing the degenerated human disc to the healthy porcine disc, the aggregate modulus was higher but the hydraulic permeability was lower. Conclusion. The external loading and degeneration induce disc structural changes, e.g., the disc water content and interstitial matrix porosity, hence affect the disc rheological properties. The increase of aggregate modulus may be due to the reduction of disc hydration level, whereas the decrease of hydraulic permeability may be because of the shrinkage of disc matrix pores. In this study, we tested the effect of loading history, loading magnitude, fatigue loading, and degeneration on disc rheological properties (i.e., the aggregate modulus and hydraulic permeability). We conclude that the disc rheology is affected by loading-induced structural changes and disc degeneration. The increased aggregate modulus may be due to the dehydration of disc, whereas the decreased hydraulic permeability may be because of the compressed pores within the disc matrix.