Mechanical overloading initiates intervertebral disc degeneration, presumably because cells break down the extracellular matrix (ECM). We used Fourier Transform Infrared Spectroscopy (FTIR) imaging to identify, visualize and quantify the ECM and aimed to identify spectroscopic markers for early disc degeneration. In seven goats, one disc was injected with chondroitinase ABC (mild degeneration) and after three months compared to control.
Purpose of study and background
Methods and Results
Degeneration of the intervertebral disc is a strong contributor of low back pain. Studies have shown that both, mechanical unloading and overloading, lead to disc degeneration. This is intuitively clear if one considers that an intervertebral disc essentially is a poro-elastic material embedded with cells, which depend on fluid flow for the transport of nutrients and waste products. As such, mechanical loading is also required for regeneration. It is unclear, however, how much loading is beneficial or detrimental for the healthy or degenerated disc. We developed a loaded disc culture system for the long-term study of disc physiology. This way we could control both the mechanical and biochemical conditions. If no loading was applied, about half of the cells died within a week. Cells died under a low dynamic loading regime after three weeks. A diurnal loading regime rescued cell viability, gene expression profile and mechanical behavior of the discs. Both static and dynamic overloading induced damage to the discs and led to catabolic and inflammatory gene expressions.Purpose of study and background
Methods and Results
Daytime spinal loading is twice as long as night time rest, but diurnal disc height changes due to fluid flow are balanced. A direction-dependent permeability of the endplates, favouring inflow over outflow, has been proposed to explain this; however, fluid also flows through the annulus fibrosus. This study investigates the poro-elastic behaviour of entire intervertebral discs in the context of diurnal fluid flow. Caprine discs were preloaded in saline for 24 hours under different levels of static load. Under sustained load, we modulated the disc's swelling pressure by replacing saline for demi-water and back again to saline, both for 24h intervals. We measured the disc height creep and used stretched exponential models to determine the respective time constants. Reduction of culture medium osmolality induced an increase in disc height, and the subsequent restoration induced a decrease in disc height. Creep varied with the mechanical load applied. No direction-dependent resistance to fluid flow was observed. In addition, time constants for mechanical preloading were much shorter than for osmotic loading, suggesting that outflow is faster than inflow. However, a time constant does not describe the actual rate of fluid flow: close to equilibrium fluid flow is slower than far from equilibrium. As time constants for mechanical loading are shorter and daytime loading twice as long, the system is closer to the loading equilibrium than to the unloading equilibrium. Therefore, paradoxically, fluid inflow is faster during the night than fluid outflow during the day.
